Business Case Scenarios In The Deployment Of A Wimax™ Network
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Business Case Scenarios in the Deployment of a WiMAX™ Network March 2009
Copyright 2009 WiMAX Forum. All rights reserved. “WiMAX”, “Mobile WiMAX,” “Fixed WiMAX,” “WiMAX Forum,” “WiMAX Forum Certified,” and the WiMAX Forum and WiMAX Forum Certified logos are trademarks of the WiMAX Forum. All other trademarks are the properties of their respective owners.
Business Case Scenarios in the Deployment of a WiMAX™ Network
Author’s Note Performance of wireless systems is highly dependent on the operating environment, deployment choices and the end-to-end network implementation. Performance projections presented in this paper are based on simulations performed with specific multipath models, usage assumptions, and equipment parameters. In practice, actual performance may differ due to local propagation conditions, multipath, customer and applications mix, and hardware choices. The performance numbers presented should not be relied on as a substitute for equipment field trials and sound RF analysis. They are best used only as a guide to the relative performance of the different deployment alternatives reviewed in this paper as opposed to absolute performance projections. About the Author Doug Gray is a Telecommunications Consultant and is currently under contract to the WiMAX Forum®. Gray has had extensive experience in broadband wireless access systems in engineering and management positions at HewlettPackard, Lucent Technologies and Ensemble Communications. Acknowledgements The author would like to acknowledge the contributions of the many WiMAX Forum® members who have taken the time to review and provide comments and insights regarding the contents of this paper and the conclusions drawn.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Copyright Notice, Use Restrictions, Disclaimer, and Limitation of Liability Copyright 2009 WiMAX Forum. All rights reserved. The WiMAX Forum® owns the copyright in this document and reserves all rights herein. This document is available for download from the WiMAX Forum and may be duplicated for internal use, provided that all copies contain all proprietary notices and disclaimers included herein. Except for the foregoing, this document may not be duplicated, in whole or in part, or distributed without the express written authorization of the WiMAX Forum. Use of this document is subject to the disclaimers and limitations described below. Use of this document constitutes acceptance of the following terms and conditions: THIS DOCUMENT IS PROVIDED “AS IS” AND WITHOUT WARRANTY OF ANY KIND. TO THE GREATEST EXTENT PERMITTED BY LAW, THE WiMAX FORUM DISCLAIMS ALL EXPRESS, IMPLIED AND STATUTORY WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF TITLE, NONINFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE WiMAX FORUM DOES NOT WARRANT THAT THIS DOCUMENT IS COMPLETE OR WITHOUT ERROR AND DISCLAIMS ANY WARRANTIES TO THE CONTRARY. Any products or services provided using technology described in or implemented in connection with this document may be subject to various regulatory controls under the laws and regulations of various governments worldwide. The user is solely responsible for the compliance of its products and/or services with any such laws and regulations and for obtaining any and all required authorizations, permits, or licenses for its products and/or services as a result of such regulations within the applicable jurisdiction. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES WHATSOEVER REGARDING THE APPLICABILITY OR NON-APPLICABILITY OF ANY SUCH LAWS OR REGULATIONS OR THE SUITABILITY OR NONSUITABILITY OF ANY SUCH PRODUCT OR SERVICE FOR USE IN ANY JURISDICTION. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES WHATSOEVER REGARDING THE SUITABILITY OR NON-SUITABILITY OF A PRODUCT OR A SERVICE FOR CERTIFICATION UNDER ANY CERTIFICATION PROGRAM OF THE WiMAX FORUM OR ANY THIRD PARTY. The WiMAX Forum has not investigated or made an independent determination regarding title or non-infringement of any technologies that may be incorporated, described or referenced in this document. Use of this document or implementation of any technologies described or referenced herein may therefore infringe undisclosed third-party patent rights or other intellectual property rights. The user is solely responsible for making all assessments | 3
Business Case Scenarios in the Deployment of a WiMAX™ Network
relating to title and non-infringement of any technology, standard, or specification referenced in this document and for obtaining appropriate authorization to use such technologies, technologies, standards, and specifications, including through the payment of any required license fees. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES OF TITLE OR NONINFRINGEMENT WITH RESPECT TO ANY TECHNOLOGIES, STANDARDS OR SPECIFICATIONS REFERENCED OR INCORPORATED INTO THIS DOCUMENT. IN NO EVENT SHALL THE WiMAX FORUM OR ANY MEMBER BE LIABLE TO THE USER OR TO A THIRD PARTY FOR ANY CLAIM ARISING FROM OR RELATING TO THE USE OF THIS DOCUMENT, INCLUDING, WITHOUT LIMITATION, A CLAIM THAT SUCH USE INFRINGES A THIRD PARTY’S INTELLECTUAL PROPERTY RIGHTS OR THAT IT FAILS TO COMPLY WITH APPLICABLE LAWS OR REGULATIONS. BY USE OF THIS DOCUMENT, THE USER WAIVES ANY SUCH CLAIM AGAINST THE WiMAX FORUM AND ITS MEMBERS RELATING TO THE USE OF THIS DOCUMENT. The WiMAX Forum reserves the right to modify or amend this document without notice and in its sole discretion. The user is solely responsible for determining whether this document has been superseded by a later version or a different document. “WiMAX,” “Mobile WiMAX,” “Fixed WiMAX,” “WiMAX Forum,” “WiMAX Certified,” “WiMAX Forum Certified,” the WiMAX Forum logo and the WiMAX Forum Certified logo are trademarks of the WiMAX Forum. Third-party trademarks contained in this document are the property of their respective owners.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Table of Contents Business Case Scenarios in the Deployment of a WiMAX™ Network ................. 7 1.0 Introduction ................................................................................................... 7 2.0 Spectrum Choices......................................................................................... 8 2.1 A Comparison of 700 MHz, 2500 MHz, and 3500 MHz: ............................. 9 2.2 Amount of Usable Spectrum: .................................................................... 10 2.3 Frequency Division or Time Division Duplexing ........................................ 12 3.0
Reuse 1 or Reuse 3 ................................................................................... 15
4.0
Alternative Usage Models .......................................................................... 18
4.1 Mobility with Reliable Indoor Coverage ..................................................... 18 4.2 Mobility with “Best Effort” Indoor Coverage ............................................... 18 4.3 Fixed Usage Model ................................................................................... 19 4.4 Usage Models: Summary .......................................................................... 19 5.0
WiMAX Base Station Antenna Configurations ........................................... 20
6.0
Conclusion ................................................................................................. 22
References.......................................................................................................... 23
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figures Figure 1: Range and Path Loss Comparison ........................................................ 9 Figure 2: Meeting Capacity Requirements.......................................................... 11 Figure 3: Upper 700 MHz Band in the US .......................................................... 13 Figure 4: Relative Number of Base Stations Required for TDD or FDD to Meet a Specific DL Data Density Requirement ....................................................... 15 Figure 5: Frequency Reuse Factor of 1 .............................................................. 16 Figure 6: Frequency Reuse Factor of 3 .............................................................. 17 Figure 7: Varied Usage Models Result in Wide Variation in Coverage............... 20 Figure 8: Advanced Antenna Systems Lowers the Cost/Mbit ............................. 21
Tables Table 1: Summary of Comparative Attributes of TDD and FDD ......................... 14
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Business Case Scenarios in the Deployment of a WiMAX™ Network 1.0 Introduction It is very important for the operator or network planner to consider a variety of factors before forging ahead with WiMAX™ equipment choices and deployment alternatives. This white paper is intended to provide the reader with a perspective on the implications that various deployment choices have on the complexity and cost-effectiveness of the final WiMAX network. From a business case perspective this paper does not go into a detailed financial analysis of the various alternatives discussed. A detailed study of this nature is best done on an operator-by-operator basis since it is not possible to generalize the wide range of variables required for this kind of analysis. One metric however that does seem to be universal in an attempt to quantify differences between deployment choices is base station count. Whether dealing with an overlay or a Greenfield deployment scenario the number of base stations required to achieve the necessary geographic coverage or to meet specific data density requirements represents the biggest contributor to the overall end-to-end network investment. Base station count therefore, is a good indicator for assessing the viability of the business case. In this context the base station not only includes the WiMAX equipment but the site development costs which include site acquisition, towers, weatherized electronic enclosures, cabling, stand-by power, backhaul, etc. Discussions with various operators and network planners indicate that the WiMAX equipment costs can range from as little as 15% to no more than 30% of the total base station cost for a facilities-based operator. The high base station infrastructure cost has caused many operators to follow a business model that shares the base station infrastructure among multiple operators. The WiMAX equipment cost in this case plays a more dominant relative role. Another business model that is prevalent is one in which the base station infrastructure is leased. This model simply translates base station infrastructure CAPEX to OPEX. Whichever model is followed, the costs associated with the base station access network will still have a major impact on the viability of the business case. From an investment perspective therefore, the key metric in evaluating the tradeoffs between the deployment choices discussed in this paper will be base station count. Many factors influence the number of base stations necessary to achieve either a specific data density required for dense urban or urban regions or to achieve adequate range and coverage over a less populated suburban or rural region. Factors that affect base station count that are discussed in this paper are: frequency band, amount of available spectrum, duplex choice, frequency reuse, usage model, and base station antenna configurations. | 7
Business Case Scenarios in the Deployment of a WiMAX™ Network
In Section 2.0 various spectrum band choices are considered ranging from 700 MHz to 3500 MHz. Closely aligned with the frequency band alternatives is the amount of usable spectrum associated with the different bands and the relative benefits of deploying with time division (TDD) or frequency division duplex (FDD). Frequency reuse factors are discussed in Section 3.0. If sufficient usable spectrum is available a conservative reuse factor will improve channel spectral efficiency and thus provide increased channel throughput but may not always be the best and most efficient use of the total spectrum assignment. Section 4.0 provides a discussion of the tradeoffs associated with the choice of which usage model to address. These choices can range from a mobile usage model addressing both in-building and outdoor coverage to a fixed usage model with the deployment of roof top subscriber units to maximize coverage area with a minimal deployment of base stations. Section 5.0 provides a brief discussion regarding the relative merits of the various base station advanced antenna systems supported by WiMAX. References listed at the end of the paper are cited throughout the discussion for the reader desiring to explore the various topics discussed in this paper in greater detail.
2.0 Spectrum Choices In many countries the regulatory regime has already determined or will determine the frequency band and channelization scheme for the licenses issued for WiMAX deployments. In many countries there may only be one choice available while in other countries there may be two or more frequency bands that operators can consider for the acquisition of spectrum for their WiMAX deployment. There may also be cases in which prospective operators will be in a position to provide inputs and suggestions to regulators as to spectrum and channelization schemes that are best suited for the delivery of broadband services over a WiMAX network. In this section we provide some insights as to the tradeoffs that must be considered with regard to spectrum choices. Specifically we look at: • Frequency band: 700 MHz vs. higher bands in the 2500 and 3500 MHz range • Amount of spectrum: 10 MHz vs. 30 MHz • Duplexing: FDD or TDD
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Business Case Scenarios in the Deployment of a WiMAX™ Network
2.1 A Comparison of 700 MHz, 2500 MHz, and 3500 MHz: For comparative purposes the range of country-by-country spectrum alternatives can be conveniently sorted into three groupings; 700 MHz1, 2500 MHz2, and 3500 MHz3. From a propagation range perspective the lower bands have an obvious advantage due to the lower path loss. This is illustrated in Figure 1 which shows the expected path loss versus cell radius for the three frequency bands [Ref. 1]. For a comparable system gain, a 700 MHz deployment will provide a considerable range advantage over a 2500 MHz or 3500 MHz deployment. This attribute makes 700 MHz especially attractive to cost-effectively cover sparsely populated regions [Ref. 2].
Figure 1: Range and Path Loss Comparison Lower building penetration loss in the 700 MHz band can greatly improve the quality of in-building coverage for mobile services and lower cable loss can also be used to advantage. Cable loss will be a consideration when base-mounted transmit power amplifiers are deployed in lieu of tower-mounted transmitters. In these cases the transmit power must be sufficient to overcome the cable loss. In the 2500 MHz or 3500 MHz band cable losses can range from approximately 2 dB for a high performance cable to almost 6 dB for a lower cost cable for a 32 meter tower height. For the same types of cable in the 700 MHz band these losses will range from 1 dB to about 3 dB. To achieve the same transmit power at 1
Includes bands between 450 MHz to 960 MHz also known as UHF
2
Includes bands between 2300-2690 MHz
3
Includes bands between 3300-3800 MHz
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Business Case Scenarios in the Deployment of a WiMAX™ Network
the base station antenna port, 700 MHz deployments can use lower power basemounted amplifiers or alternatively lower cost cable. Despite the above advantages, the 700 MHz band is not without its challenges. Due to the longer wavelength, antenna sizes will be larger to achieve antenna gains comparable to antennas in the higher frequency bands. This can add cost to the base station due to the requirement for more robust mounting structures to handle the higher wind loads. The longer wavelength in the 700 MHz band will also limit the use of some of the more advanced multiple antenna systems supported by WiMAX. For 2-element antenna systems polarization diversity can be employed. For higher order MIMO systems however, some environments will require an antenna spacing of five or more wavelengths to assure low correlation between antennas. To achieve a five wavelength separation the antenna spacing would have to be at least 2 meters. The ability to make use of higher order MIMO systems and beamforming in the higher bands can greatly narrow the range benefit of 700 MHz over 2500 or 3500 MHz. Another important consideration in the 700 MHz band is the amount of available spectrum available to individual operators. Spectrum assignments tend to be more limited in the lower bands than in the higher bands. Since deployments for broadband services in urban regions tend to be constrained by capacity requirements as opposed to range, limited spectrum dictates deployment of more closely spaced base stations. In these cases the amount of available spectrum becomes the more important factor to consider than the frequency band. This factor will be looked at in more detail in the following section. In any case the greater range capability in the 700 MHz band makes it especially well suited for rural deployments even if the assignment is for limited spectrum.
2.2 Amount of Usable Spectrum: Studies conducted by the WiMAX Forum® on projected traffic demands in urban centers concluded that 30 MHz of spectrum would be a minimal requirement to meet long term anticipated data density requirements [Ref. 3,4]. The traffic studies predict that downlink data density requirements in urban centers will range from 25-30 Mbps per sq-km to handle peak busy hour traffic requirements for broadband services. Although with any amount of spectrum additional base stations can be deployed to meet these high data density requirements it is important to recognize that one quickly reaches a point of diminishing returns. In addition to the added cost of deploying additional base stations, which can have significant detrimental effect on the viability of the business case, one must also contend with the limitations of cell to cell co-channel interference resulting from more closely spaced base stations. Increased interference lowers the spectral efficiency thus resulting in a lower channel capacity putting additional pressure on the need for a greater number of base stations.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
The chart in Figure 2 summarizes the impact of limited spectrum in an urban environment covering an area of 100 sq-km requiring a high downlink data density4.
Figure 2: Meeting Capacity Requirements Key lessons from the data presented in Figure 2 are: a) Having 3 times more usable spectrum, in this case, 30 MHz vs. 10 MHz, provides greater than 3 times benefit5 in base station infrastructure costs in capacity-constrained deployments that require data densities of 20 Mbps/sqkm or more. Data density requirements of this magnitude are projected to be typical in urban and dense urban deployments to meet peak busy hour traffic demands [Ref. 3]. b) With a limited amount of spectrum, 10 MHz in this case, there is no difference in base station requirements for 700, 2500, or 3500 MHz with data density requirements of 20 Mbps/sq-km or more. c) With a limited spectrum allocation, the range benefit of lower frequencies only comes into play in areas with lower population densities, in this example, regions requiring data densities of 5 Mbps/sq-km or less. With more and more countries moving towards the use of auctions for the allocation of spectrum licenses it is important to also consider the impact of license costs for a more complete picture of the business case trade-offs. Although it is not possible to accurately quantify the impact of spectrum costs since they will vary greatly from market to market and the number of competing 4
Assumes TDD with a channel BW of 10 MHz and a DL to UL traffic ratio of 3:2
5
The closer base station to base station spacing results in added cell to cell interference, therefore more than 3x base stations are required with one-third the spectrum.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
bidders participating in the auctions, it is possible to come up with some generalizations. • In any particular band spectrum license cost will be directly proportional to the amount of spectrum and the population in the geographic region covered by the license • Since lower frequencies are viewed as more favorable due to lower path loss and better building penetration, licenses in lower bands will generally cost more than licenses in the higher bands for the same amount of spectrum and geographic area.
2.3 Frequency Division or Time Division Duplexing Another important decision that some operators will face is whether to deploy WiMAX with Time Division Duplex (TDD) or with Frequency Division Duplex (FDD). The condition under which this choice has to be made is when: • The spectrum allocation consists of paired channels consistent with an FDD solution • Regulations do not restrict the use of TDD or FDD The Upper 700 MHz band allocation in the US is a good example of a spectrum allocation that is made up of licenses with paired channels consistent with an FDD solution. FCC regulations however, allow both FDD and TDD operation in this band thus giving the operator the flexibility to select the duplexing approach that best fits his requirements. The structure of this band is shown in Figure 3. The license denoted as C-C is especially interesting in that it will support a WiMAX solution with two TDD 10 MHz channels or an FDD WiMAX solution with a 10 MHz downlink channel paired with a 10 MHz uplink channel.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 3: Upper 700 MHz Band in the US When given the choice, generally the attributes of TDD make it the preferred duplexing approach for broadband services. This is especially true when traffic on the network is expected to be asymmetric and spectrum is limited [Ref. 5]. With asymmetric traffic, one of the channels will be underutilized with FDD whereas TDD can adapt DL and UL frames to match actual traffic conditions. Although some of this advantage is diminished due to the requirement for inter base station and inter-operator synchronization for interference control. There will, in most cases, still be a net gain since it is reasonable to expect that traffic conditions will be similar over a large group of users in the same geographic region. TDD also assures reciprocity between the DL and UL channels for easy channel quality estimation. This is an important attribute for the implementation of some of the more advanced antenna systems. With a spectrum assignment consisting of paired channels, which is the case for license C-C or D-D in Figure 3, TDD requires an outdoor transceiver unit for each of the paired channels whereas FDD is implemented with a single transceiver that covers both the DL and UL channels. Another aspect of FDD that can make it the favored choice in some deployment scenarios is the ability to deal with interference from high power transmissions from collocated or closely located wireless operations in adjacent bands. In these cases it will often be preferable to dedicate one channel exclusively to DL in the collocated equipment rather than running the risk of adjacent channel interference when the channel was operating in UL mode as would be the case some of the time with TDD. Table 1 provides a comparison of the attributes for the two duplexing approaches when working with a spectrum allocation comprising paired channels.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Table 1: Summary of Comparative Attributes of TDD and FDD TDD • Adaptive DL to UL ratio for better spectral efficiency with asymmetric traffic • Channel reciprocity for easy support of closed loop advanced antenna systems • Greater flexibility with frequency reuse schemes with two independent paired channels • Simple transceiver design
FDD • Dedicated DL and dedicated UL channel • Single transceiver to cover two paired channels • Avoidance of self-interference between DL and UL • Can mitigate interference from DL transmissions in adjacent channels with collocated base station equipment
Figure 4 provides a relative base station deployment comparison for TDD relative to FDD for a typical capacity-constrained environment. This analysis illustrates the TDD advantage for DL to UL traffic asymmetries ranging from 3:2 to 3:1. For a traffic asymmetry of 3:1, deploying with TDD to meet a specific DL data density will require almost 40% fewer base stations as compared to a deployment with FDD. DL to UL traffic ratios in the range of 3:1, and possibly higher, are expected to be typical as traffic becomes more and more data-centric with applications such as web browsing, streaming video and music, and location based services. On the other hand there may also be regional-specific deployment scenarios in which traffic is projected to be symmetric or nearly so. This would be the case with traffic dominated by voice services or possibly in a business-oriented environment where large file transfers are expected in both the DL and UL directions with roughly equal probability. Under these traffic conditions the difference in the required number of base stations with FDD or TDD would be insignificant resulting in FDD being the more cost-effective approach due to the reduced equipment requirements. The potential for interference from wireless applications in adjacent bands must also be considered. If the probability is high, FDD may prove to be the better deployment alternative for interference mitigation.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 4: Relative Number of Base Stations Required for TDD or FDD to Meet a Specific DL Data Density Requirement
3.0
Reuse 1 or Reuse 3
Traditional reuse patterns for conventional cellular deployments adopted cell frequency reuse factors as high as seven (7) to mitigate intercellular co-channel interference (CCI). These deployments assured a minimal spatial separation of 5:1 between the interfering signal and the desired signal but required seven times as much spectrum. With technologies such as CDMA, introduced with 3G, and OFDMA, introduced with WiMAX [Ref. 6], more aggressive reuse schemes can be employed to greatly improve overall spectral efficiency. Two common frequency reuse configurations for a multi-cellular deployment with 3-sector base stations and conventional sector antennas are a sector reuse of 3, i.e. (c, 3, 3)6 and a sector reuse of 1, (c, 1, 3) also referred to as universal frequency reuse. With a frequency reuse of 1 the same channel is deployed in each of the three (3) base station sectors7 as shown in Figure 58. This approach has the advantage of using the least amount of spectrum and in many cases, may represent the only deployment alternative due to limited spectrum 6
Nomenclature for describing the frequency reuse pattern in this paper is (c, n, s); where c is the number of base station sites in a cluster, n is the number of unique frequency channels required, and s is the number of sectors per base station site. 7
Another deployment alternative with a single channel is to “share” the channel over all 3 sectors. This approach effectively splits the channel into three sub-channels and assigns each subchannel to a specific sector making it comparable to a reuse of 3 with 1/3 the channel bandwidth. 8
Due to multipath, reflections, interference from neighboring operators, etc., interference patterns will be much more complex than depicted in Figures 5 and 6. These figures are meant only to provide a generalized picture to the reader as to where self-interference is most likely to occur.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
availability. It also eliminates the need for any frequency planning in the layout of base stations. With Reuse 1, a pseudorandom subcarrier permutation scheme along with channel segmentation is employed with OFDMA to mitigate cochannel interference (CCI) at the sector boundaries and at the cell-edge [Ref. 6]. As a result some channel capacity is sacrificed since some subcarriers will not be fully utilized throughout the entire cell coverage area.
Figure 5: Frequency Reuse Factor of 1 With reuse 3 a separate channel is deployed in each of the three sectors as shown in Figure 6. This alternative mitigates the interference at the sector and cell edges resulting in a higher channel throughput. Although a reuse of 3 can increase the channel spectral efficiency by 30-50% the overall spectral efficiency compared to reuse 1 will be lower since three times as much spectrum is required.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 6: Frequency Reuse Factor of 3 If sufficient spectrum is available to the operator, a potential deployment strategy that can be undertaken is to deploy initially with a reuse of 3 and as demand grows in the higher density, capacity-constrained environments, over-lay additional channels on a sector-by-sector basis to meet the growing demand. Long term the deployment in these areas will ultimately migrate to a more aggressive re-use scheme in which all three available channels are deployed in each sector. In the lower population density areas where capacity is not an issue however, a frequency reuse factor of 3 can be maintained. The discussion of frequency reuse and interference is not complete without mentioning beamforming. WiMAX supports higher order antenna arrays or “Adaptive Beamforming”. Adaptive beamforming enables beam adaptation based on both channel and interference conditions. This enables the antenna array to not only maximize signal strength to the desired user but also provides a mechanism to null out interference from unwanted sources. With Adaptive Arrays or Adaptive Beamforming, algorithms can be employed to constructively enhance both signal to noise and signal to interference ratios in all propagation environments to mitigate the effects of self-interference as well as interference from neighboring operators. Signal to Interference ratio increases up to 10 dB in an urban mobile environment have been reported using these techniques [Ref. 7].
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Business Case Scenarios in the Deployment of a WiMAX™ Network
4.0
Alternative Usage Models
In assessing the business opportunity for a Mobile WiMAX deployment an operator may also want to consider the relative value of alternative usage models. Usage models range from one that provides broadband services with full mobility and coverage for both indoor and outdoor environments to one which addresses only fixed broadband services.
4.1 Mobility with Reliable Indoor Coverage Due to the requirement for portability and long battery-life mobile handheld devices have limited transmit power and lower antenna gain. Additionally for indoor coverage one must deal with building penetration losses which can range from 10 to 20 dB depending on the frequency band and building characteristics. Despite these challenges the mobile usage model is most interesting in that has the greatest revenue potential. To address this market it is necessary to deploy a sufficient number of base stations to meet the capacity and most importantly the requirements for ubiquitous coverage over the geographic area of interest in indoor environments. With the availability of micro-cells, pico-cells, femto-cells, and repeaters the business case to achieve the desired coverage for this usage model is greatly improved. These self-contained, low-cost pole mounted units can be installed in selected locations to improve coverage at a fraction of the cost of additional large-scale macro base stations. Beamforming which benefits both the downlink as well as the uplink link budget can also be used to enhance inbuilding coverage. Generally this usage is the most attractive from a business case perspective since it provides for quality broadband connections for the maximum number of potential customers.
4.2 Mobility with “Best Effort” Indoor Coverage Some operators may elect to follow a deployment approach that phases the infrastructure investment over a longer period of time. Rather than deploying to ensure reliable indoor coverage throughout the coverage area, an operator can elect to initially deploy only the number of base stations required to ensure that mobile customers have good outdoor performance throughout the coverage area but only best effort service when located inside of buildings. With this scenario, mobile customers at the cell peripheries that are located in a building will be faced with “spotty” coverage and may have to move near a window or outside the building to get a quality connection. Mobile customers located closer to the base station site however, would experience a good connection regardless of their location. This deployment scenario can considerably reduce the required number of base stations compared to a deployment that takes into account building penetration loss throughout the entire coverage area thus reducing the initial infrastructure investment. The tradeoff, of course, is the risk of having some inconvenienced customers and the higher churn that may result from the affected customers switching to other operators. Longer term, additional base stations
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Business Case Scenarios in the Deployment of a WiMAX™ Network
together with pico-cells, and femto-cells can be deployed over a period of time to improve indoor coverage.
4.3 Fixed Usage Model Another market model that an operator can consider is one that provides a focus on fixed services. This usage model can take advantage of outdoor mounted subscriber stations with high gain directional antennas for increased base station range and coverage. This eliminates building penetration loss and improves the system gain with increased uplink transmitter power and higher antenna gain. The directional subscriber antenna also reduces interference leading to higher spectral efficiency. When considering this usage model an operator must also consider the added expense of a truck-roll and professional installation for the fixed outdoor subscriber terminals. It should be noted however, that this will only be necessary at the periphery of the cell coverage area and the cost is only incurred when a customer has signed on for service. Customers closer to the base station will be able to connect with self-installed indoor terminals and overthe-air-activation. This deployment option will significantly increase the range and coverage capability of a base station in any of the frequency bands being considered. In the 2500 MHz band for example, the outdoor fixed antenna option will provide 4 to 5 times the range and about 20 times advantage in coverage area over an indoor mobile device. With this deployment model the reduced infrastructure investment must be traded-off against the reduced addressable market due to the inability to adequately cover mobile users. This deployment approach can also prove to be the most cost-effective model for reaching the maximum number of customers with the minimal infrastructure cost in rural environments and can be especially appealing in emerging markets.
4.4 Usage Models: Summary The relative coverage area estimates for various usage models assuming a (2x2) MIMO base station sector antenna for each case is summarized in Figure 5. As described above the indoor mobile handset results in the least area coverage per base station while the roof-top mounted outdoor subscriber terminal provides the greatest coverage with other usage models falling in between.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 7: Varied Usage Models Result in Wide Variation in Coverage Deploying to ensure reliable coverage for indoor mobile applications requires the highest front-end capital investment but maximizes the addressable market for the operator. Any of the alternative usage models will reduce the addressable market but can nevertheless offer a viable business case for the operator due to the benefit of lower initial infrastructure cost to cover the geographical area of interest. As a market entry strategy, an operator may choose an alternative approach to gain a time-to-market advantage and build a core customer base. The operator can then deploy additional base stations over a period of time to improve coverage and expand the addressable market to include other usage models. This phased deployment approach can enhance the business case by spreading the infrastructure investment over a longer period of time.
5.0
WiMAX Base Station Antenna Configurations
WiMAX base station equipment is available today in a wide range of configurations and as new air interface releases become available the range of options available to the operator will increase. Many of the options have to do with the base station antenna configuration. Since Mobile WiMAX supports advanced antenna systems the operator or network planner will be faced with selecting the best option for any particular area of interest. Key performance benefits that higher order MIMO systems and beamforming offer are higher peak data rate performance and higher average channel spectral efficiency [Ref. 4, 8]. Under certain conditions range can also be enhanced with these systems. The capability for higher peak data rates enhances the customer experience for the delivery of broadband services and increased channel capacity or range helps to reduce base station requirements necessary to meet data density or coverage requirements. These added performance benefits however, do not come without some added base station complexity and cost | 20
Business Case Scenarios in the Deployment of a WiMAX™ Network
from both equipment as well as an installation perspective due to the additional antenna elements. Although complexity may be higher for base stations with higher order antenna systems they generally offer added value in that they result in a net lower cost per megabit (see Figure 8) and, potentially, reduce the number of required base stations to meet specific downlink data density requirements.
Figure 8: Advanced Antenna Systems Lowers the Cost/Mbit The tradeoff that an operator must consider is whether or not to incur the additional equipment and installation cost for a more advanced base station antenna option at the outset or opt, initially, for the lowest cost approach and upgrade in the future as warranted by the need for increased performance and/or capacity. Obviously in the higher density urban centers which will have the highest capacity demand during the peak busy hour, choosing the solution with the highest spectral efficiency and highest channel capacity would be a logical and sensible approach. This will provide a solution that will result in a lower deployment cost per megabit with a high probability that the base station capacity will be fully utilized in the near term to generate operator revenue. Compared to a (1x2) SIMO base station configuration, deploying with higher order (2x2) MIMO or Beamforming plus MIMO can reduce the number of required base stations to meet the data density requirements in a capacity-constrained environment by 7080% [Ref. 4]. In these cases the savings in base station infrastructure costs will greatly outweigh the increased WiMAX equipment costs. In the areas with lower population densities surrounding the city center which tend to be range-limited, the choice may not be as obvious. One must carefully assess the potential demand and project the growth over time and deploy accordingly. Investing in excess capacity can result in expenditures that may
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Business Case Scenarios in the Deployment of a WiMAX™ Network
never be recaptured by future revenue and under estimating capacity can result in costly and time-consuming truck-rolls in the future to install base station upgrades or alternatively, in dissatisfied customers arising from the service degradation caused by an overloaded network.
6.0
Conclusion
This paper has provided insights into the tradeoffs associated with some of the many decisions an operator or network planner must make when assessing the relative attributes of the various WiMAX deployment and spectrum alternatives that are available for consideration. Base station requirements are used as a means of comparing the relative merits of the various alternatives. This metric is generally agreed to be the major component of the total end-to-end network investment and hence a good indicator for assessing the relative viability of the business case when comparing two or more deployment alternatives. Some of the conclusions drawn are: • Deployments in the UHF (700 MHz) band are well-suited to lower density rural areas regardless of how much usable spectrum is available • For capacity-constrained environments requiring data densities in excess of 20 Mbps per sq-km it is desirable to have access to at least 30 MHz of usable spectrum, regardless of frequency band for a cost-effective deployment. • When one is faced with the option of FDD or TDD deployment, factors that must be considered include: DL to UL traffic asymmetry, need for interoperator synchronization, and the potential for interference from closely located operations in adjacent bands. • Although a more conservative reuse factor of 3 will result in lower interference and therefore, higher channel capacity the overall spectral efficiency will be reduced compared to reuse 1. • Base station solutions with adaptive beamforming offer the potential for both signal enhancement and interference mitigation to provide higher channel capacity and improved range. • Alternative usage models can greatly impact the base stations required to meet coverage requirements but lower initial deployment costs must be traded off against a reduced addressable market and the risk of adversely affecting performance for some customers. • Base stations with advanced antenna systems offer benefits that are highly likely to pay off in high density urban areas but may not always be the most cost-effective alternative in lower density environments that do not have high capacity requirements.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
References 1. “A Comparative Analysis of Spectrum Alternatives for WiMAX Networks with Deployment Scenarios Based on the U.S. 700 MHz Band”, WiMAX Forum website, June 2008. 2. “Forum® Position Paper for WiMAX™ Technology in the 700 MHz Band”, WiMAX Forum website, March 2008. 3. “A Review of Spectrum Requirements for Mobile WiMAX™ Equipment to Support Wireless Personal Broadband Services”, WiMAX Forum website, September 2007. 4. “Mobile WiMAX - A Comparative Analysis of Mobile WiMAX™ Deployment Alternatives in the Access Network”, WiMAX Forum website, May 2007. 5. “Mobile WiMAX-Part II: A Comparative Analysis”, WiMAX Forum website, May 2006 6. “Mobile WiMAX-Part I: A Performance and Comparative Summary”, WiMAX Forum website, September 2006 7. P.H. Lehne, O. Rostbakken, and M. Petersen, “Estimating Smart Antenna Performance from th Directional Radio Channel Measurements”, Proceedings 50 IEEE Vehicle Tech. Conf. Sept 1999, pp 57-61. 8. “Introduction to MIMO Systems, Rohde & Schwarz Application Note 1MA102”, June 2006, Available on WiMAX Forum website
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Copyright 2009 WiMAX Forum. All rights reserved. “WiMAX”, “Mobile WiMAX,” “Fixed WiMAX,” “WiMAX Forum,” “WiMAX Forum Certified,” and the WiMAX Forum and WiMAX Forum Certified logos are trademarks of the WiMAX Forum. All other trademarks are the properties of their respective owners.
Business Case Scenarios in the Deployment of a WiMAX™ Network
Author’s Note Performance of wireless systems is highly dependent on the operating environment, deployment choices and the end-to-end network implementation. Performance projections presented in this paper are based on simulations performed with specific multipath models, usage assumptions, and equipment parameters. In practice, actual performance may differ due to local propagation conditions, multipath, customer and applications mix, and hardware choices. The performance numbers presented should not be relied on as a substitute for equipment field trials and sound RF analysis. They are best used only as a guide to the relative performance of the different deployment alternatives reviewed in this paper as opposed to absolute performance projections. About the Author Doug Gray is a Telecommunications Consultant and is currently under contract to the WiMAX Forum®. Gray has had extensive experience in broadband wireless access systems in engineering and management positions at HewlettPackard, Lucent Technologies and Ensemble Communications. Acknowledgements The author would like to acknowledge the contributions of the many WiMAX Forum® members who have taken the time to review and provide comments and insights regarding the contents of this paper and the conclusions drawn.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Copyright Notice, Use Restrictions, Disclaimer, and Limitation of Liability Copyright 2009 WiMAX Forum. All rights reserved. The WiMAX Forum® owns the copyright in this document and reserves all rights herein. This document is available for download from the WiMAX Forum and may be duplicated for internal use, provided that all copies contain all proprietary notices and disclaimers included herein. Except for the foregoing, this document may not be duplicated, in whole or in part, or distributed without the express written authorization of the WiMAX Forum. Use of this document is subject to the disclaimers and limitations described below. Use of this document constitutes acceptance of the following terms and conditions: THIS DOCUMENT IS PROVIDED “AS IS” AND WITHOUT WARRANTY OF ANY KIND. TO THE GREATEST EXTENT PERMITTED BY LAW, THE WiMAX FORUM DISCLAIMS ALL EXPRESS, IMPLIED AND STATUTORY WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF TITLE, NONINFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE WiMAX FORUM DOES NOT WARRANT THAT THIS DOCUMENT IS COMPLETE OR WITHOUT ERROR AND DISCLAIMS ANY WARRANTIES TO THE CONTRARY. Any products or services provided using technology described in or implemented in connection with this document may be subject to various regulatory controls under the laws and regulations of various governments worldwide. The user is solely responsible for the compliance of its products and/or services with any such laws and regulations and for obtaining any and all required authorizations, permits, or licenses for its products and/or services as a result of such regulations within the applicable jurisdiction. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES WHATSOEVER REGARDING THE APPLICABILITY OR NON-APPLICABILITY OF ANY SUCH LAWS OR REGULATIONS OR THE SUITABILITY OR NONSUITABILITY OF ANY SUCH PRODUCT OR SERVICE FOR USE IN ANY JURISDICTION. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES WHATSOEVER REGARDING THE SUITABILITY OR NON-SUITABILITY OF A PRODUCT OR A SERVICE FOR CERTIFICATION UNDER ANY CERTIFICATION PROGRAM OF THE WiMAX FORUM OR ANY THIRD PARTY. The WiMAX Forum has not investigated or made an independent determination regarding title or non-infringement of any technologies that may be incorporated, described or referenced in this document. Use of this document or implementation of any technologies described or referenced herein may therefore infringe undisclosed third-party patent rights or other intellectual property rights. The user is solely responsible for making all assessments | 3
Business Case Scenarios in the Deployment of a WiMAX™ Network
relating to title and non-infringement of any technology, standard, or specification referenced in this document and for obtaining appropriate authorization to use such technologies, technologies, standards, and specifications, including through the payment of any required license fees. NOTHING IN THIS DOCUMENT CREATES ANY WARRANTIES OF TITLE OR NONINFRINGEMENT WITH RESPECT TO ANY TECHNOLOGIES, STANDARDS OR SPECIFICATIONS REFERENCED OR INCORPORATED INTO THIS DOCUMENT. IN NO EVENT SHALL THE WiMAX FORUM OR ANY MEMBER BE LIABLE TO THE USER OR TO A THIRD PARTY FOR ANY CLAIM ARISING FROM OR RELATING TO THE USE OF THIS DOCUMENT, INCLUDING, WITHOUT LIMITATION, A CLAIM THAT SUCH USE INFRINGES A THIRD PARTY’S INTELLECTUAL PROPERTY RIGHTS OR THAT IT FAILS TO COMPLY WITH APPLICABLE LAWS OR REGULATIONS. BY USE OF THIS DOCUMENT, THE USER WAIVES ANY SUCH CLAIM AGAINST THE WiMAX FORUM AND ITS MEMBERS RELATING TO THE USE OF THIS DOCUMENT. The WiMAX Forum reserves the right to modify or amend this document without notice and in its sole discretion. The user is solely responsible for determining whether this document has been superseded by a later version or a different document. “WiMAX,” “Mobile WiMAX,” “Fixed WiMAX,” “WiMAX Forum,” “WiMAX Certified,” “WiMAX Forum Certified,” the WiMAX Forum logo and the WiMAX Forum Certified logo are trademarks of the WiMAX Forum. Third-party trademarks contained in this document are the property of their respective owners.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Table of Contents Business Case Scenarios in the Deployment of a WiMAX™ Network ................. 7 1.0 Introduction ................................................................................................... 7 2.0 Spectrum Choices......................................................................................... 8 2.1 A Comparison of 700 MHz, 2500 MHz, and 3500 MHz: ............................. 9 2.2 Amount of Usable Spectrum: .................................................................... 10 2.3 Frequency Division or Time Division Duplexing ........................................ 12 3.0
Reuse 1 or Reuse 3 ................................................................................... 15
4.0
Alternative Usage Models .......................................................................... 18
4.1 Mobility with Reliable Indoor Coverage ..................................................... 18 4.2 Mobility with “Best Effort” Indoor Coverage ............................................... 18 4.3 Fixed Usage Model ................................................................................... 19 4.4 Usage Models: Summary .......................................................................... 19 5.0
WiMAX Base Station Antenna Configurations ........................................... 20
6.0
Conclusion ................................................................................................. 22
References.......................................................................................................... 23
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figures Figure 1: Range and Path Loss Comparison ........................................................ 9 Figure 2: Meeting Capacity Requirements.......................................................... 11 Figure 3: Upper 700 MHz Band in the US .......................................................... 13 Figure 4: Relative Number of Base Stations Required for TDD or FDD to Meet a Specific DL Data Density Requirement ....................................................... 15 Figure 5: Frequency Reuse Factor of 1 .............................................................. 16 Figure 6: Frequency Reuse Factor of 3 .............................................................. 17 Figure 7: Varied Usage Models Result in Wide Variation in Coverage............... 20 Figure 8: Advanced Antenna Systems Lowers the Cost/Mbit ............................. 21
Tables Table 1: Summary of Comparative Attributes of TDD and FDD ......................... 14
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Business Case Scenarios in the Deployment of a WiMAX™ Network 1.0 Introduction It is very important for the operator or network planner to consider a variety of factors before forging ahead with WiMAX™ equipment choices and deployment alternatives. This white paper is intended to provide the reader with a perspective on the implications that various deployment choices have on the complexity and cost-effectiveness of the final WiMAX network. From a business case perspective this paper does not go into a detailed financial analysis of the various alternatives discussed. A detailed study of this nature is best done on an operator-by-operator basis since it is not possible to generalize the wide range of variables required for this kind of analysis. One metric however that does seem to be universal in an attempt to quantify differences between deployment choices is base station count. Whether dealing with an overlay or a Greenfield deployment scenario the number of base stations required to achieve the necessary geographic coverage or to meet specific data density requirements represents the biggest contributor to the overall end-to-end network investment. Base station count therefore, is a good indicator for assessing the viability of the business case. In this context the base station not only includes the WiMAX equipment but the site development costs which include site acquisition, towers, weatherized electronic enclosures, cabling, stand-by power, backhaul, etc. Discussions with various operators and network planners indicate that the WiMAX equipment costs can range from as little as 15% to no more than 30% of the total base station cost for a facilities-based operator. The high base station infrastructure cost has caused many operators to follow a business model that shares the base station infrastructure among multiple operators. The WiMAX equipment cost in this case plays a more dominant relative role. Another business model that is prevalent is one in which the base station infrastructure is leased. This model simply translates base station infrastructure CAPEX to OPEX. Whichever model is followed, the costs associated with the base station access network will still have a major impact on the viability of the business case. From an investment perspective therefore, the key metric in evaluating the tradeoffs between the deployment choices discussed in this paper will be base station count. Many factors influence the number of base stations necessary to achieve either a specific data density required for dense urban or urban regions or to achieve adequate range and coverage over a less populated suburban or rural region. Factors that affect base station count that are discussed in this paper are: frequency band, amount of available spectrum, duplex choice, frequency reuse, usage model, and base station antenna configurations. | 7
Business Case Scenarios in the Deployment of a WiMAX™ Network
In Section 2.0 various spectrum band choices are considered ranging from 700 MHz to 3500 MHz. Closely aligned with the frequency band alternatives is the amount of usable spectrum associated with the different bands and the relative benefits of deploying with time division (TDD) or frequency division duplex (FDD). Frequency reuse factors are discussed in Section 3.0. If sufficient usable spectrum is available a conservative reuse factor will improve channel spectral efficiency and thus provide increased channel throughput but may not always be the best and most efficient use of the total spectrum assignment. Section 4.0 provides a discussion of the tradeoffs associated with the choice of which usage model to address. These choices can range from a mobile usage model addressing both in-building and outdoor coverage to a fixed usage model with the deployment of roof top subscriber units to maximize coverage area with a minimal deployment of base stations. Section 5.0 provides a brief discussion regarding the relative merits of the various base station advanced antenna systems supported by WiMAX. References listed at the end of the paper are cited throughout the discussion for the reader desiring to explore the various topics discussed in this paper in greater detail.
2.0 Spectrum Choices In many countries the regulatory regime has already determined or will determine the frequency band and channelization scheme for the licenses issued for WiMAX deployments. In many countries there may only be one choice available while in other countries there may be two or more frequency bands that operators can consider for the acquisition of spectrum for their WiMAX deployment. There may also be cases in which prospective operators will be in a position to provide inputs and suggestions to regulators as to spectrum and channelization schemes that are best suited for the delivery of broadband services over a WiMAX network. In this section we provide some insights as to the tradeoffs that must be considered with regard to spectrum choices. Specifically we look at: • Frequency band: 700 MHz vs. higher bands in the 2500 and 3500 MHz range • Amount of spectrum: 10 MHz vs. 30 MHz • Duplexing: FDD or TDD
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Business Case Scenarios in the Deployment of a WiMAX™ Network
2.1 A Comparison of 700 MHz, 2500 MHz, and 3500 MHz: For comparative purposes the range of country-by-country spectrum alternatives can be conveniently sorted into three groupings; 700 MHz1, 2500 MHz2, and 3500 MHz3. From a propagation range perspective the lower bands have an obvious advantage due to the lower path loss. This is illustrated in Figure 1 which shows the expected path loss versus cell radius for the three frequency bands [Ref. 1]. For a comparable system gain, a 700 MHz deployment will provide a considerable range advantage over a 2500 MHz or 3500 MHz deployment. This attribute makes 700 MHz especially attractive to cost-effectively cover sparsely populated regions [Ref. 2].
Figure 1: Range and Path Loss Comparison Lower building penetration loss in the 700 MHz band can greatly improve the quality of in-building coverage for mobile services and lower cable loss can also be used to advantage. Cable loss will be a consideration when base-mounted transmit power amplifiers are deployed in lieu of tower-mounted transmitters. In these cases the transmit power must be sufficient to overcome the cable loss. In the 2500 MHz or 3500 MHz band cable losses can range from approximately 2 dB for a high performance cable to almost 6 dB for a lower cost cable for a 32 meter tower height. For the same types of cable in the 700 MHz band these losses will range from 1 dB to about 3 dB. To achieve the same transmit power at 1
Includes bands between 450 MHz to 960 MHz also known as UHF
2
Includes bands between 2300-2690 MHz
3
Includes bands between 3300-3800 MHz
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Business Case Scenarios in the Deployment of a WiMAX™ Network
the base station antenna port, 700 MHz deployments can use lower power basemounted amplifiers or alternatively lower cost cable. Despite the above advantages, the 700 MHz band is not without its challenges. Due to the longer wavelength, antenna sizes will be larger to achieve antenna gains comparable to antennas in the higher frequency bands. This can add cost to the base station due to the requirement for more robust mounting structures to handle the higher wind loads. The longer wavelength in the 700 MHz band will also limit the use of some of the more advanced multiple antenna systems supported by WiMAX. For 2-element antenna systems polarization diversity can be employed. For higher order MIMO systems however, some environments will require an antenna spacing of five or more wavelengths to assure low correlation between antennas. To achieve a five wavelength separation the antenna spacing would have to be at least 2 meters. The ability to make use of higher order MIMO systems and beamforming in the higher bands can greatly narrow the range benefit of 700 MHz over 2500 or 3500 MHz. Another important consideration in the 700 MHz band is the amount of available spectrum available to individual operators. Spectrum assignments tend to be more limited in the lower bands than in the higher bands. Since deployments for broadband services in urban regions tend to be constrained by capacity requirements as opposed to range, limited spectrum dictates deployment of more closely spaced base stations. In these cases the amount of available spectrum becomes the more important factor to consider than the frequency band. This factor will be looked at in more detail in the following section. In any case the greater range capability in the 700 MHz band makes it especially well suited for rural deployments even if the assignment is for limited spectrum.
2.2 Amount of Usable Spectrum: Studies conducted by the WiMAX Forum® on projected traffic demands in urban centers concluded that 30 MHz of spectrum would be a minimal requirement to meet long term anticipated data density requirements [Ref. 3,4]. The traffic studies predict that downlink data density requirements in urban centers will range from 25-30 Mbps per sq-km to handle peak busy hour traffic requirements for broadband services. Although with any amount of spectrum additional base stations can be deployed to meet these high data density requirements it is important to recognize that one quickly reaches a point of diminishing returns. In addition to the added cost of deploying additional base stations, which can have significant detrimental effect on the viability of the business case, one must also contend with the limitations of cell to cell co-channel interference resulting from more closely spaced base stations. Increased interference lowers the spectral efficiency thus resulting in a lower channel capacity putting additional pressure on the need for a greater number of base stations.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
The chart in Figure 2 summarizes the impact of limited spectrum in an urban environment covering an area of 100 sq-km requiring a high downlink data density4.
Figure 2: Meeting Capacity Requirements Key lessons from the data presented in Figure 2 are: a) Having 3 times more usable spectrum, in this case, 30 MHz vs. 10 MHz, provides greater than 3 times benefit5 in base station infrastructure costs in capacity-constrained deployments that require data densities of 20 Mbps/sqkm or more. Data density requirements of this magnitude are projected to be typical in urban and dense urban deployments to meet peak busy hour traffic demands [Ref. 3]. b) With a limited amount of spectrum, 10 MHz in this case, there is no difference in base station requirements for 700, 2500, or 3500 MHz with data density requirements of 20 Mbps/sq-km or more. c) With a limited spectrum allocation, the range benefit of lower frequencies only comes into play in areas with lower population densities, in this example, regions requiring data densities of 5 Mbps/sq-km or less. With more and more countries moving towards the use of auctions for the allocation of spectrum licenses it is important to also consider the impact of license costs for a more complete picture of the business case trade-offs. Although it is not possible to accurately quantify the impact of spectrum costs since they will vary greatly from market to market and the number of competing 4
Assumes TDD with a channel BW of 10 MHz and a DL to UL traffic ratio of 3:2
5
The closer base station to base station spacing results in added cell to cell interference, therefore more than 3x base stations are required with one-third the spectrum.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
bidders participating in the auctions, it is possible to come up with some generalizations. • In any particular band spectrum license cost will be directly proportional to the amount of spectrum and the population in the geographic region covered by the license • Since lower frequencies are viewed as more favorable due to lower path loss and better building penetration, licenses in lower bands will generally cost more than licenses in the higher bands for the same amount of spectrum and geographic area.
2.3 Frequency Division or Time Division Duplexing Another important decision that some operators will face is whether to deploy WiMAX with Time Division Duplex (TDD) or with Frequency Division Duplex (FDD). The condition under which this choice has to be made is when: • The spectrum allocation consists of paired channels consistent with an FDD solution • Regulations do not restrict the use of TDD or FDD The Upper 700 MHz band allocation in the US is a good example of a spectrum allocation that is made up of licenses with paired channels consistent with an FDD solution. FCC regulations however, allow both FDD and TDD operation in this band thus giving the operator the flexibility to select the duplexing approach that best fits his requirements. The structure of this band is shown in Figure 3. The license denoted as C-C is especially interesting in that it will support a WiMAX solution with two TDD 10 MHz channels or an FDD WiMAX solution with a 10 MHz downlink channel paired with a 10 MHz uplink channel.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 3: Upper 700 MHz Band in the US When given the choice, generally the attributes of TDD make it the preferred duplexing approach for broadband services. This is especially true when traffic on the network is expected to be asymmetric and spectrum is limited [Ref. 5]. With asymmetric traffic, one of the channels will be underutilized with FDD whereas TDD can adapt DL and UL frames to match actual traffic conditions. Although some of this advantage is diminished due to the requirement for inter base station and inter-operator synchronization for interference control. There will, in most cases, still be a net gain since it is reasonable to expect that traffic conditions will be similar over a large group of users in the same geographic region. TDD also assures reciprocity between the DL and UL channels for easy channel quality estimation. This is an important attribute for the implementation of some of the more advanced antenna systems. With a spectrum assignment consisting of paired channels, which is the case for license C-C or D-D in Figure 3, TDD requires an outdoor transceiver unit for each of the paired channels whereas FDD is implemented with a single transceiver that covers both the DL and UL channels. Another aspect of FDD that can make it the favored choice in some deployment scenarios is the ability to deal with interference from high power transmissions from collocated or closely located wireless operations in adjacent bands. In these cases it will often be preferable to dedicate one channel exclusively to DL in the collocated equipment rather than running the risk of adjacent channel interference when the channel was operating in UL mode as would be the case some of the time with TDD. Table 1 provides a comparison of the attributes for the two duplexing approaches when working with a spectrum allocation comprising paired channels.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Table 1: Summary of Comparative Attributes of TDD and FDD TDD • Adaptive DL to UL ratio for better spectral efficiency with asymmetric traffic • Channel reciprocity for easy support of closed loop advanced antenna systems • Greater flexibility with frequency reuse schemes with two independent paired channels • Simple transceiver design
FDD • Dedicated DL and dedicated UL channel • Single transceiver to cover two paired channels • Avoidance of self-interference between DL and UL • Can mitigate interference from DL transmissions in adjacent channels with collocated base station equipment
Figure 4 provides a relative base station deployment comparison for TDD relative to FDD for a typical capacity-constrained environment. This analysis illustrates the TDD advantage for DL to UL traffic asymmetries ranging from 3:2 to 3:1. For a traffic asymmetry of 3:1, deploying with TDD to meet a specific DL data density will require almost 40% fewer base stations as compared to a deployment with FDD. DL to UL traffic ratios in the range of 3:1, and possibly higher, are expected to be typical as traffic becomes more and more data-centric with applications such as web browsing, streaming video and music, and location based services. On the other hand there may also be regional-specific deployment scenarios in which traffic is projected to be symmetric or nearly so. This would be the case with traffic dominated by voice services or possibly in a business-oriented environment where large file transfers are expected in both the DL and UL directions with roughly equal probability. Under these traffic conditions the difference in the required number of base stations with FDD or TDD would be insignificant resulting in FDD being the more cost-effective approach due to the reduced equipment requirements. The potential for interference from wireless applications in adjacent bands must also be considered. If the probability is high, FDD may prove to be the better deployment alternative for interference mitigation.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 4: Relative Number of Base Stations Required for TDD or FDD to Meet a Specific DL Data Density Requirement
3.0
Reuse 1 or Reuse 3
Traditional reuse patterns for conventional cellular deployments adopted cell frequency reuse factors as high as seven (7) to mitigate intercellular co-channel interference (CCI). These deployments assured a minimal spatial separation of 5:1 between the interfering signal and the desired signal but required seven times as much spectrum. With technologies such as CDMA, introduced with 3G, and OFDMA, introduced with WiMAX [Ref. 6], more aggressive reuse schemes can be employed to greatly improve overall spectral efficiency. Two common frequency reuse configurations for a multi-cellular deployment with 3-sector base stations and conventional sector antennas are a sector reuse of 3, i.e. (c, 3, 3)6 and a sector reuse of 1, (c, 1, 3) also referred to as universal frequency reuse. With a frequency reuse of 1 the same channel is deployed in each of the three (3) base station sectors7 as shown in Figure 58. This approach has the advantage of using the least amount of spectrum and in many cases, may represent the only deployment alternative due to limited spectrum 6
Nomenclature for describing the frequency reuse pattern in this paper is (c, n, s); where c is the number of base station sites in a cluster, n is the number of unique frequency channels required, and s is the number of sectors per base station site. 7
Another deployment alternative with a single channel is to “share” the channel over all 3 sectors. This approach effectively splits the channel into three sub-channels and assigns each subchannel to a specific sector making it comparable to a reuse of 3 with 1/3 the channel bandwidth. 8
Due to multipath, reflections, interference from neighboring operators, etc., interference patterns will be much more complex than depicted in Figures 5 and 6. These figures are meant only to provide a generalized picture to the reader as to where self-interference is most likely to occur.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
availability. It also eliminates the need for any frequency planning in the layout of base stations. With Reuse 1, a pseudorandom subcarrier permutation scheme along with channel segmentation is employed with OFDMA to mitigate cochannel interference (CCI) at the sector boundaries and at the cell-edge [Ref. 6]. As a result some channel capacity is sacrificed since some subcarriers will not be fully utilized throughout the entire cell coverage area.
Figure 5: Frequency Reuse Factor of 1 With reuse 3 a separate channel is deployed in each of the three sectors as shown in Figure 6. This alternative mitigates the interference at the sector and cell edges resulting in a higher channel throughput. Although a reuse of 3 can increase the channel spectral efficiency by 30-50% the overall spectral efficiency compared to reuse 1 will be lower since three times as much spectrum is required.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 6: Frequency Reuse Factor of 3 If sufficient spectrum is available to the operator, a potential deployment strategy that can be undertaken is to deploy initially with a reuse of 3 and as demand grows in the higher density, capacity-constrained environments, over-lay additional channels on a sector-by-sector basis to meet the growing demand. Long term the deployment in these areas will ultimately migrate to a more aggressive re-use scheme in which all three available channels are deployed in each sector. In the lower population density areas where capacity is not an issue however, a frequency reuse factor of 3 can be maintained. The discussion of frequency reuse and interference is not complete without mentioning beamforming. WiMAX supports higher order antenna arrays or “Adaptive Beamforming”. Adaptive beamforming enables beam adaptation based on both channel and interference conditions. This enables the antenna array to not only maximize signal strength to the desired user but also provides a mechanism to null out interference from unwanted sources. With Adaptive Arrays or Adaptive Beamforming, algorithms can be employed to constructively enhance both signal to noise and signal to interference ratios in all propagation environments to mitigate the effects of self-interference as well as interference from neighboring operators. Signal to Interference ratio increases up to 10 dB in an urban mobile environment have been reported using these techniques [Ref. 7].
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Business Case Scenarios in the Deployment of a WiMAX™ Network
4.0
Alternative Usage Models
In assessing the business opportunity for a Mobile WiMAX deployment an operator may also want to consider the relative value of alternative usage models. Usage models range from one that provides broadband services with full mobility and coverage for both indoor and outdoor environments to one which addresses only fixed broadband services.
4.1 Mobility with Reliable Indoor Coverage Due to the requirement for portability and long battery-life mobile handheld devices have limited transmit power and lower antenna gain. Additionally for indoor coverage one must deal with building penetration losses which can range from 10 to 20 dB depending on the frequency band and building characteristics. Despite these challenges the mobile usage model is most interesting in that has the greatest revenue potential. To address this market it is necessary to deploy a sufficient number of base stations to meet the capacity and most importantly the requirements for ubiquitous coverage over the geographic area of interest in indoor environments. With the availability of micro-cells, pico-cells, femto-cells, and repeaters the business case to achieve the desired coverage for this usage model is greatly improved. These self-contained, low-cost pole mounted units can be installed in selected locations to improve coverage at a fraction of the cost of additional large-scale macro base stations. Beamforming which benefits both the downlink as well as the uplink link budget can also be used to enhance inbuilding coverage. Generally this usage is the most attractive from a business case perspective since it provides for quality broadband connections for the maximum number of potential customers.
4.2 Mobility with “Best Effort” Indoor Coverage Some operators may elect to follow a deployment approach that phases the infrastructure investment over a longer period of time. Rather than deploying to ensure reliable indoor coverage throughout the coverage area, an operator can elect to initially deploy only the number of base stations required to ensure that mobile customers have good outdoor performance throughout the coverage area but only best effort service when located inside of buildings. With this scenario, mobile customers at the cell peripheries that are located in a building will be faced with “spotty” coverage and may have to move near a window or outside the building to get a quality connection. Mobile customers located closer to the base station site however, would experience a good connection regardless of their location. This deployment scenario can considerably reduce the required number of base stations compared to a deployment that takes into account building penetration loss throughout the entire coverage area thus reducing the initial infrastructure investment. The tradeoff, of course, is the risk of having some inconvenienced customers and the higher churn that may result from the affected customers switching to other operators. Longer term, additional base stations
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Business Case Scenarios in the Deployment of a WiMAX™ Network
together with pico-cells, and femto-cells can be deployed over a period of time to improve indoor coverage.
4.3 Fixed Usage Model Another market model that an operator can consider is one that provides a focus on fixed services. This usage model can take advantage of outdoor mounted subscriber stations with high gain directional antennas for increased base station range and coverage. This eliminates building penetration loss and improves the system gain with increased uplink transmitter power and higher antenna gain. The directional subscriber antenna also reduces interference leading to higher spectral efficiency. When considering this usage model an operator must also consider the added expense of a truck-roll and professional installation for the fixed outdoor subscriber terminals. It should be noted however, that this will only be necessary at the periphery of the cell coverage area and the cost is only incurred when a customer has signed on for service. Customers closer to the base station will be able to connect with self-installed indoor terminals and overthe-air-activation. This deployment option will significantly increase the range and coverage capability of a base station in any of the frequency bands being considered. In the 2500 MHz band for example, the outdoor fixed antenna option will provide 4 to 5 times the range and about 20 times advantage in coverage area over an indoor mobile device. With this deployment model the reduced infrastructure investment must be traded-off against the reduced addressable market due to the inability to adequately cover mobile users. This deployment approach can also prove to be the most cost-effective model for reaching the maximum number of customers with the minimal infrastructure cost in rural environments and can be especially appealing in emerging markets.
4.4 Usage Models: Summary The relative coverage area estimates for various usage models assuming a (2x2) MIMO base station sector antenna for each case is summarized in Figure 5. As described above the indoor mobile handset results in the least area coverage per base station while the roof-top mounted outdoor subscriber terminal provides the greatest coverage with other usage models falling in between.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
Figure 7: Varied Usage Models Result in Wide Variation in Coverage Deploying to ensure reliable coverage for indoor mobile applications requires the highest front-end capital investment but maximizes the addressable market for the operator. Any of the alternative usage models will reduce the addressable market but can nevertheless offer a viable business case for the operator due to the benefit of lower initial infrastructure cost to cover the geographical area of interest. As a market entry strategy, an operator may choose an alternative approach to gain a time-to-market advantage and build a core customer base. The operator can then deploy additional base stations over a period of time to improve coverage and expand the addressable market to include other usage models. This phased deployment approach can enhance the business case by spreading the infrastructure investment over a longer period of time.
5.0
WiMAX Base Station Antenna Configurations
WiMAX base station equipment is available today in a wide range of configurations and as new air interface releases become available the range of options available to the operator will increase. Many of the options have to do with the base station antenna configuration. Since Mobile WiMAX supports advanced antenna systems the operator or network planner will be faced with selecting the best option for any particular area of interest. Key performance benefits that higher order MIMO systems and beamforming offer are higher peak data rate performance and higher average channel spectral efficiency [Ref. 4, 8]. Under certain conditions range can also be enhanced with these systems. The capability for higher peak data rates enhances the customer experience for the delivery of broadband services and increased channel capacity or range helps to reduce base station requirements necessary to meet data density or coverage requirements. These added performance benefits however, do not come without some added base station complexity and cost | 20
Business Case Scenarios in the Deployment of a WiMAX™ Network
from both equipment as well as an installation perspective due to the additional antenna elements. Although complexity may be higher for base stations with higher order antenna systems they generally offer added value in that they result in a net lower cost per megabit (see Figure 8) and, potentially, reduce the number of required base stations to meet specific downlink data density requirements.
Figure 8: Advanced Antenna Systems Lowers the Cost/Mbit The tradeoff that an operator must consider is whether or not to incur the additional equipment and installation cost for a more advanced base station antenna option at the outset or opt, initially, for the lowest cost approach and upgrade in the future as warranted by the need for increased performance and/or capacity. Obviously in the higher density urban centers which will have the highest capacity demand during the peak busy hour, choosing the solution with the highest spectral efficiency and highest channel capacity would be a logical and sensible approach. This will provide a solution that will result in a lower deployment cost per megabit with a high probability that the base station capacity will be fully utilized in the near term to generate operator revenue. Compared to a (1x2) SIMO base station configuration, deploying with higher order (2x2) MIMO or Beamforming plus MIMO can reduce the number of required base stations to meet the data density requirements in a capacity-constrained environment by 7080% [Ref. 4]. In these cases the savings in base station infrastructure costs will greatly outweigh the increased WiMAX equipment costs. In the areas with lower population densities surrounding the city center which tend to be range-limited, the choice may not be as obvious. One must carefully assess the potential demand and project the growth over time and deploy accordingly. Investing in excess capacity can result in expenditures that may
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Business Case Scenarios in the Deployment of a WiMAX™ Network
never be recaptured by future revenue and under estimating capacity can result in costly and time-consuming truck-rolls in the future to install base station upgrades or alternatively, in dissatisfied customers arising from the service degradation caused by an overloaded network.
6.0
Conclusion
This paper has provided insights into the tradeoffs associated with some of the many decisions an operator or network planner must make when assessing the relative attributes of the various WiMAX deployment and spectrum alternatives that are available for consideration. Base station requirements are used as a means of comparing the relative merits of the various alternatives. This metric is generally agreed to be the major component of the total end-to-end network investment and hence a good indicator for assessing the relative viability of the business case when comparing two or more deployment alternatives. Some of the conclusions drawn are: • Deployments in the UHF (700 MHz) band are well-suited to lower density rural areas regardless of how much usable spectrum is available • For capacity-constrained environments requiring data densities in excess of 20 Mbps per sq-km it is desirable to have access to at least 30 MHz of usable spectrum, regardless of frequency band for a cost-effective deployment. • When one is faced with the option of FDD or TDD deployment, factors that must be considered include: DL to UL traffic asymmetry, need for interoperator synchronization, and the potential for interference from closely located operations in adjacent bands. • Although a more conservative reuse factor of 3 will result in lower interference and therefore, higher channel capacity the overall spectral efficiency will be reduced compared to reuse 1. • Base station solutions with adaptive beamforming offer the potential for both signal enhancement and interference mitigation to provide higher channel capacity and improved range. • Alternative usage models can greatly impact the base stations required to meet coverage requirements but lower initial deployment costs must be traded off against a reduced addressable market and the risk of adversely affecting performance for some customers. • Base stations with advanced antenna systems offer benefits that are highly likely to pay off in high density urban areas but may not always be the most cost-effective alternative in lower density environments that do not have high capacity requirements.
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Business Case Scenarios in the Deployment of a WiMAX™ Network
References 1. “A Comparative Analysis of Spectrum Alternatives for WiMAX Networks with Deployment Scenarios Based on the U.S. 700 MHz Band”, WiMAX Forum website, June 2008. 2. “Forum® Position Paper for WiMAX™ Technology in the 700 MHz Band”, WiMAX Forum website, March 2008. 3. “A Review of Spectrum Requirements for Mobile WiMAX™ Equipment to Support Wireless Personal Broadband Services”, WiMAX Forum website, September 2007. 4. “Mobile WiMAX - A Comparative Analysis of Mobile WiMAX™ Deployment Alternatives in the Access Network”, WiMAX Forum website, May 2007. 5. “Mobile WiMAX-Part II: A Comparative Analysis”, WiMAX Forum website, May 2006 6. “Mobile WiMAX-Part I: A Performance and Comparative Summary”, WiMAX Forum website, September 2006 7. P.H. Lehne, O. Rostbakken, and M. Petersen, “Estimating Smart Antenna Performance from th Directional Radio Channel Measurements”, Proceedings 50 IEEE Vehicle Tech. Conf. Sept 1999, pp 57-61. 8. “Introduction to MIMO Systems, Rohde & Schwarz Application Note 1MA102”, June 2006, Available on WiMAX Forum website
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