Mobile Wimax Link Budget

Chapter 4 Coverage of mobile WiMAX Mobile WiMAX Group

4.1 Introduction In order to investigate the feasibility of a WiMAX rollout, one has to be able to assess the number of base stations that will be needed in a specific area, dependent on the offered services and the number of active users. This is possible with the developed planning tool based on an accurate technical model. The tool takes into account the major technical characteristics of Mobile WiMAX together with the desired service specifications. It also has a certain degree of flexibility to introduce adaptations like e.g. new hardware. To start, we discuss the calculation of the link budget, which indicates to what extent the signal may weaken (Figure 4.1). Then, a propagation model is proposed to determine the range, by taking into account the link budget. Based on this range, we illustrate the calculation of the cell coverage area. In a next step, we calculate the bit rate per cell sector and finally, the cell areas and bit rates are combined to estimate the required number of base stations.

Figure 4.1 Link-budget for downlink

4.2 Link Budget You are planning a vacation. You estimate that you will need $1000 dollars to pay for the hotels, restaurants, food etc... You start your vacation and watch the money get spent at each stop. When you get home, you pat yourself on the back for a job well done because you still have $50 left in your wallet.

We do something similar with communication links, called creating a link budget. The traveler is the signal and instead of dollars it starts out with power. It spends its power (or attenuates, in engineering terminology) as it travels, is it wired or wireless. Just as you can use a credit card along the way for extra money infusion, the signal can get extra power infusion along the way from intermediate amplifiers such as microwave repeaters for telephone links or from satellite transponders for satellite links. The designer hopes that the signal will complete its trip with just enough power to be decoded at the receiver with the desired signal quality. In our example, we started our trip with $1000 because we wanted a budget vacation. But what if our goal was a first-class vacation with stays at five-star hotels, best shows and travel by QE2? A $1000 budget would not be enough and possibly we will need instead $5000. The quality of the trip desired determines how much money we need to take along. With signals, the quality is measured by the Bit Error Rate (BER). If we want our signal to have a low BER, we would start it out with higher power and then make sure that along the way it has enough power available at every stop to maintain this BER. A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system (Figure 4.2). It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feed line and miscellaneous losses. Randomly varying channel gains such as fading are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping.

Figure 4.2 illustration of link budget

The link budget depends on several parameters, which are discussed in this paragraph. Different parameter values can be chosen in the planning tool, and for both the downlink and the uplink a separate link budget is calculated. We also indicate which values are selected for the business modeling study.

4.2.1 What is a link? A link consists of three parts: 1. Transmitter (is BS in FL communication and is CPE in RL communication). 2. Receiver (is BS in RL communication and is CPE in FL communication). 3. Media. The very simplest form of a link equation is written as: =

+

+

−

This equation of course only talks about the signal power. We have not accounted for noise yet.

4.2.2 Base Station We consider a base station (BS) with three sectors, and there is a choice from three BS profiles: · · ·

Standard BS. BS with 2 x 2 MIMO. BS with 2 x 2 MIMO and 2 element AAS.

Most base stations which are now entering the market belong to the category “BS with 2 x 2 MIMO” (which is considered in our study). Every profile contains the values for six different parameters required for the link budget calculation. Table 4.1 gives an overview of them, where DL and UL stands for downlink and uplink respectively, and Tx and Rx for transmitter and receiver. Note that additionally a BS feeder loss of 0.5 dB is taken into account.

DL Tx power DL Tx antenna gain Other DL Tx gain UL Rx antenna gain Other UL Rx gain UL Rx noise figure

Standard BS

BS with 2 x 2 MIMO

35 dBm 16 dBi 0 dB 16 dBi 0 dB 5 dB

35 dBm 16 dBi 9 dB 16 dBi 3 dB 5 dB

Table 4.1 Base station parameters

BS with 2 x 2 MIMO and 2 element AAS 35 dBm 16 dBi 15 dB 16 dBi 6 dB 5 dB

4.2.3 Customer Premises Equipment (CPE) With regard to the CPE, we can choose from two profiles: · ·

Portable CPE. Mobile CPE.

The first type is comparable with e.g. as usual cable modem: they are installed indoors, have their own power supply and are usually connected via an Ethernet cable to the computer. They do not guarantee any form of mobility. Solutions with PCMCIA cards and receivers integrated in e.g. a laptop belong then to the second type. Every profile contains again six parameters (Table 4.2). UL Tx power UL Tx antenna gain Other UL Tx gain DL Rx antenna gain Other DL Rx gain DL Rx noise figure

Portable CPE 27 dBm 6 dBi 0 dB 6 dBi 0 dB 6 dB

Mobile CPE 27 dBm 2 dBi 0 dB 2 dBi 0 dB 6 dB

Table 4.2 CPE parameters

From the above six parameters for BS and CPE, we can calculate Equivalent isotropically radiated power (EIRP).

4.2.4 What is EIRP? In radio communication systems, Equivalent isotropically radiated power (EIRP) or, alternatively, Effective isotropically radiated power is the amount of power that a theoretical isotropic antenna (that evenly distributes power in all directions) would emit to produce the peak power density observed in the direction of maximum antenna gain. EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power emitted by an isotropic radiator with equivalent signal strength. The EIRP allows comparisons between different emitters regardless of type, size or form. From the EIRP, and with knowledge of a real antenna's gain, it is possible to calculate real power and field strength values.

Figure 4.3 illustration of EIRP

[

]=

[

Then,

]−

[

]+

[

Ø For FL communication ,BS is the transmitter and its EIRP can be calculated from Table 4.1 as: = (

+ (

)– (

+ +

+ )

)

Ø For RL communication ,CPE is the transmitter and its EIRP can be calculated from Table 4.2 as: = (

4.2.5 Receiver Sensitivity

)– (

+

/

)+ (

)

The receiver sensitivity is defined by: · · · ·

The thermal noise. The receiver SNR. The noise figure (Table 4.1 and Table 4.2). The implementation loss.

Now, the receiver sensitivity can be calculated as: =

(

)

+

+

+

]

4.2.5.1 The thermal noise The thermal noise is dependent on the channel bandwidth and can be estimated as (in dBm): = −

(

+

)

where Δf is the bandwidth in hertz over which the noise is measured. As physical bandwidth (BW), there is a choice from 1.25 MHz, 5 MHz, 10 MHz and 20 MHz, where today 10 MHz is the most standard value. For the calculation of the thermal noise, the bandwidth Δf has to be scaled to the effectively used bandwidth. So the value of BW has to be multiplied by the ratio between the numbers of used subcarriers (NUsed) and the total number of OFDM subcarriers or FFT size (NFFT), and the sampling factor (n). For each bandwidth, the model contains different values for NFFT and NUsed (Table 4.3). Note that NUsed is equal to the sum of the number of data subcarriers (NData) and pilot subcarriers (NPilot), together with the DC carrier. Table 4.3 also shows NData (used to determine the bit rates, Section 4.5) and the number of subchannels (NSubCh, used to calculate the subchanneling gain, Section 4.2.6). NFFT 128 512 1024 2084

1.25 MHz 5 MHz 10 MHz 20 MHz

NUsed 85 421 841 1681

NDataDL 72 360 720 1440

NDataUL 56 280 560 1120

NSubChDL 3 15 30 60

NSubChUL 4 17 35 70

Table 4.3 Parameters per channel bandwidth

The sampling factor n determines the subcarrier spacing (in conjunction with the bandwidth and used data subcarriers), and the useful symbol time. This value is set to 28/25 for channel bandwidths that are a multiple of any of 1.25, 1.5, 2 or 2.75 MHz (which is applicable in our case). The thermal noise is then determined by:

∶ ℎ

(

)=

= −

+

∗ 1000 ∗

∗

∗

≈ −147

Where, K is Boltzmann constant in joule/Kelvin and T is the room temperature in Kelvin 4.2.5.2 The receiver SNR The receiver SNR depends on the modulation scheme and the corresponding values are shown in Table 4.4, for two different forward error correction (FEC) methods (convolution code (CC) and convolution turbo code (CTC)) in an additive white Gaussian noise (AWGN) channel at a bit error rate (BER) of 10−6. As WiMAX adaptively selects the modulation scheme per user, the appropriate SNR value used in the link budget calculation is dynamically adapted. The

modulation scheme also defines the number of data bits per symbol, but this parameter only influences the bit rate per sector which will be discussed further in this chapter (Section 4.5). Modulation scheme QPSK 1/2 QPSK 3/4 16-QAM 1/2 16-QAM 3/4 64-QAM 1/2 64-QAM 2/3 64-QAM 3/4

SNR CC (AWGN, BER 10-6) 5 dB 8 dB 10.5 dB 14 dB 16 dB 18 dB 20 dB

SNR CTC (AWGN, BER 10-6) 2.5 dB 6.3 dB 8.6 dB 12.7 dB 13.8 dB 16.9 dB 18 dB

Data bit per symbol 1 1.5 2 3 3 4 4.5

Table 4.4 Parameters per modulation scheme

4.2.5.3 The noise figure Noise figure (NF) is a measure of degradation of the signal to noise ratio (SNR), caused by components in the RF signal chain. The noise figure is the ratio of the output NOISE POWER of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature T0 (usually 290 K). The noise figure is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise. It is a number by which the performance of a radio receiver can be specified. =

Note: In our calculations, the assumed values of NF in Table 4.1 and Table 4.2. 4.2.5.4 the implementation loss The implementation loss includes non-ideal receiver effects such as channel estimation errors, tracking errors, quantization errors, and phase noise. The assumed value is 2 dB.

4.2.6 Uplink Subchanneling Gain In the uplink direction, it will hardly occur that data is sent over all subcarriers simultaneously. To set off this effect, an uplink subchanneling gain is taken into account, based on the number of used subchannels per user and defined by: ℎ

= −10

NSubChUL is already given in Table 4.3 and NUsedSubChUL is based on the number of subchannels required for the offered uplink data rate per user, and will also depend on the modulation scheme (Section 4.5).

4.2.7 Margins Is defined as the amount by which a received signal level may be reduced without causing system performance to fall below a specified threshold value. To calculate the link budget, we have to consider several margins, such as the fade margin, the interference margin and a Building penetration loss (BPL) factor. 1. Fading margin: Fading covers the effect of the variation of the signal strength during the time on a fixed location. In contrast to shadowing which takes into account the variation of the signal strength between different locations on the same distance from the transmitter, fading is not incorporated in the propagation model. Ø Slow fading margin (lognormal fading margin): Slow fading or log-normal fading is the variation of the local mean signal level over a wider area, and has been observed by Young. The local mean is the mean value of the Rayleigh or Rician fading signal amplitude. This log-normal fading is caused by the obstacles (buildings, trees, etc.) that changes the average received signal level

and thus bring about shadowing. The variation of the signal amplitude local mean value over the wider area is log-normally distributed and thus it is called lognormal fading. =

(

Ø Fast fading margin:

,

,

)

The mobile station or base station receives in one moment the same signal arriving via different radio paths as mentioned in the previous section. The total received signal is a contribution of all the arrived signal multipath components based on the superposition principle. The different signal components, arriving via different radio paths, have different amplitude and phase due to the different lengths of the radio path and different reflection and diffraction properties. Thus, the sum of the received signals

can be constructive or a destructive depending on the phases of the multipath components. The assumed value for fast fading margin is 3 dB. 2. Interference margin: Due to co-channel interference (CCI) in frequency reuse deployments, users at the cell edge or the sector boundaries may suffer degradation in connection quality. The assumed interference margin is 2 dB for DL and 3 dB for UL respectively.

3. Building penetration loss (BPL): Buildings obstruct the transmitted electromagnetic signals. Since the used propagation model does not sufficiently take into account this effect, an extra correction on the link budget is added. The different possibilities are summarized in Table 4.5. Urban type Rural Suburban Urban Dense urban

Correction 12 dB 15 dB 18 dB 20 dB

Table 4.5 Urban corrections

4.2.8 Link Budget Calculation (Maximum Allowable Path Loss) With the data discussed in the previous sections, it is possible to calculate the link budget. §

For the downlink communication MAPL is specified as: =

§

− −

+

−

−

+

−

−

/

For uplink communication MAPL is specified as: =

− − +

+

−

+ −

−

4.2.9 Effect of Adverse Weather Conditions It is important to research any unusual weather conditions that are common to the site location. These conditions can include excessive amounts of rain or fog, wind velocity, or extreme temperature ranges. If extreme conditions exist that may affect the integrity of the radio link, it is recommended that these conditions be taken into consideration early in the planning process. ·

Rain and Fog: Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required. In most cases, the effects of fog are considered to be much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as

·

·

·

·

temperature inversion, or very still air accompanied by stratification. Temperature inversion can negate clearances, and still air along with stratification can cause severe refractive or reflective conditions, with unpredictable results. Temperature inversions and stratification can also cause ducting, which may increase the potential for interference between systems that do not normally interfere with each other. Where these conditions exist, it is recommended to have shorter paths and adequate clearances. Atmospheric Absorption: A relatively small effect on the link is from oxygen and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 14 GHz frequency range, which is approximately 0.01 dB/mile, is not very significant. Wind: Any system components mounted outdoors will be subject to the effect of wind. It is important to know the direction and velocity of the wind common to the site. Antennas and their supporting structures must be able to prevent these forces from affecting the antenna or causing damage to the building or tower on which the components are mounted. Antenna designs react differently to wind forces, depending on the area presented to the wind. This is known as wind loading. Most antenna manufacturers will specify wind loading for each type of antenna manufactured. Lightning: The potential for lightning damage to radio equipment should always be considered when planning a wireless link. A variety of lightning protection and grounding devices are available for use on buildings, towers, antennas, cables, and equipment, whether located inside or outside the site that could be damaged by a lightning strike. Lightning protection requirements are based on the exposure at the site, the cost of link down-time, and local building and electrical codes. If the link is critical, and the site is in an active lightning area, attention to thorough lightning protection and grounding is critical. Lightning Protection: To provide effective lightning protection, install antennas in locations that are unlikely to receive direct lightning strikes, or install lightning rods to protect antennas from direct strikes. Make sure that cables and equipment are properly grounded to provide low-impedance paths for lightning currents. Install surge suppressors on telephone lines and power lines. Lightning protection is recommended for both coaxial and control cables leading to the wireless device. The lightning protection should be placed at points close to where the cable passes through the bulkhead into the building, as well as near the wireless device. All smart Bridges products include grounding wires, so please make sure that the antenna is properly grounded.

4.2.10 Improving Coverage and Throughput · ·

Select data rate according to the actual utilization; with lower data rate allows longer distances to be achieved. Selection of Autofall back mode.

· · · · ·

Keeping the transmit power of equipment low and using a higher gain antenna will improve the data rate and coverage. Selecting equipment with RF interference mitigation capabilities Selecting equipment that takes care of multi-path issues Using MAC based authentication only for security and disabling a WEP/WPA/AES if higher end security standards are not required. This will improve the throughput. Selection of sector can improve the range of coverage in a particular direction

4.3 Propagation Model Median pathloss in a radio channel is generally estimated using analytical models based on either the fundamental physics behind radio propagation or statistical curve fitting of data collected via field measurements. For most of the practical deployment scenarios, particularly nonline- of-sight scenarios, statistical models based on empirical data are more useful. Although most of the statistical models for pathloss have been traditionally developed and tuned for a mobile environment, many of them can also be used for an NLOS fixed network with some modification of parameters. In the case of a line-of-sightbased fixed network, the free-space radio propagation model can be used to predict the median pathloss. Since WiMAX as a technology has been developed to operate efficiently even in an NLOS environment, we focus extensively on this usage model for the remainder of the appendix. We describe a few of the pathloss models that are relevant to NLOS WiMAX deployments.

4.3.1 Hata Model The Hata model is an analytical formulation based on the pathloss measurement data collected by Okumura in 1968 in Japan. The Hata model is one of the most widely used models for estimating median pathloss in macrocellular systems. The model provides an expression for median pathloss as a function of carrier frequency, BS and mobile station antenna heights, and the distance between the BS and the MS. The Hata model is valid only for the following range of parameters: · · · ·

150MHz ≤ f ≤ 1500MHz 30m ≤ hb ≤ 200m 1m ≤ hm ≤ 10m 1km ≤ d ≤ 20km

In these parameters, f is the carrier frequency in MHz, hb is the BS antenna height in meters, hm is the MS antenna height in meters, and d is the distance between the BS and the MS in km. According to the Hata model, the median pathloss in an urban environment is given by: = 69.55 + 26.16

− 13.82

ℎ + (44.9 − 6.55ℎ )

− (ℎ )

Where is expressed in the dB scale, and (ℎ ) is the MS antenna-correction factor. For a large city with dense building clutter and narrow streets, the MS antenna-correction factor is given by: (ℎ ) = 8.29[

(1.54 ∗ )] − 0.8

≤ 300

(ℎ ) = 3.20[

(11.75 ∗ )] − 4.97

≥ 300

For a small- or medium-size city, where the building-clutter density is smaller, the MS antennacorrection factor is given by: (ℎ ) = 1.11

− 0.7 ℎ − (1.56

− 0.8)

For a suburban area, the same MS antenna-correction factor as used for small cities is applicable, but the median pathloss is modified to be: =

− 2

28

− 5.4

For a rural area, the same MS antenna-correction factor as used for small cities is applicable, but the median pathloss is modified to be: =

− 4.78[

] − 18.33

− 40.98

The model may also be generalized to any clutter environment, such that the median pathloss is modified from that of a small urban city as:

4.3.2 COST-231 Hata Model

=

+

The Hata model is widely used for cellular networks in the 800MHz/900MHz band. As PCS deployments begin in the 1,800MHz/1,900MHz band, the Hata model was modified by the European COST (Cooperation in the field of Scientific and Research) group, and the extended pathloss model is often referred to as the COST-231 Hata model. This model is valid for the following range of parameters: · · · ·

150MHz ≤ f ≤ 2000MHz 30m ≤ hb ≤ 200m 1m ≤ hm ≤ 10m 1km ≤ d ≤ 20km

The median pathloss for the COST-231 Hata model is given by: = 46.3 + 33.9

− 13.82

ℎ + ( 44.9 − 6.55

The MS antenna-correction factor, a(hm), is given by: (ℎ ) = ( 1.11

− 0.7 )ℎ − (1.56

ℎ ) − 0.8)

− (ℎ ) +

For urban and suburban areas, the correction factor CF is 3dB and 0dB, respectively. The WiMAX Forum recommends using this COST-231 Hata model for system simulations and network planning of macrocellular systems in both urban and suburban areas for mobility applications. The WiMAX Forum also recommends adding a 10dB fade margin to the median pathloss to account for shadowing.

4.3.3 Erceg Model The Erceg model is based on extensive experimental data collected at 1.9GHz in 95 macrocells across the United States. The measurements were made mostly in suburban areas of New Jersey, Seattle, Chicago, Atlanta, and Dallas. The Erceg model is applicable mostly for fixed wireless deployment, with the MS installed under the eave/window or on the rooftop. The model, adopted by the IEEE 802.16 group as the recommended model for fixed broadband applications, has three variants, based on terrain type. 1. Erceg A is applicable to hilly terrain with moderate to heavy tree density. 2. Erceg B is applicable to hilly terrain with light tree density or flat terrain with moderate to heavy tree density. 3. Erceg C is applicable to flat terrain with light tree density. The Erceg model is a slope-intercept model given by: =

+

=

+ 10 ∝

+

Where is the median pathloss, PL is the instantaneous attenuation, and X is the shadow fades, A is the intercept and is given by free-space pathloss at the desired frequency over a distance of d0 = 100 m: 4

= 20

and α is the pathloss exponent and is modeled as a random variable with a Gaussian distribution around a mean value of − ℎ + ℎ . The instantaneous value of the pathloss exponent is given by: =

−

ℎ +

ℎ

+

Where x is a Gaussian random variable with zero mean and unit variance, and σα is the standard deviation of the pathloss exponent distribution. The parameters of the Erceg model, A, B, C, and σα for the various terrain categories, are given in Table 4.6. Parameters

Erceg Model A

Erceg Model B

Erceg Model C

a b c sa μS σS

4.6 0.0075 12.6 0.57 10.6 2.3

4 0.0065 17.1 0.75 9.6 3

3.6 0.005 20 0.59 8.2 1.6

Table 4.6 Parameters of Erceg Model

Unlike the Hata model, which predicts only the median pathloss, the Erceg model has both a median pathloss and a shadow-fading component, χ, a zero-mean Gaussian random variable expressed as y σ,

where y is a zero-mean Gaussian random variable with unit variance, and σ is the standard deviation of χ. The standard deviation σ is, in fact, another Gaussian variable with a mean of μS and a standard deviation of σS, such that σ = μS + z σS, z being a zero-mean unit variance Gaussian random variable. Strictly speaking, this base model is valid only for frequencies close to 1,900MHz, for an MS with omnidirectional antennas at a height of 2 meters and BS antenna heights between 10 meters and 80 meters. The base model has been expanded with correction factors to cover higher frequencies, variable MS antenna heights, and directivity. The extended versions of the Erceg models are valid for the following range of parameters: · · · ·

1900MHz ≤ f ≤ 3500MHz 10m ≤ hb ≤ 80m 2m ≤ hm ≤ 10m 0.1km ≤ d ≤ 8km

The median pathloss formula for the extended version of the Erceg model is expressed as: =

+ 10 log

+ ∆

+ ∆

+ ∆

The various correction factors in previous Equation corresponding to frequency, MS height, and MS antenna directivity are given by: ∆

∆

= 6 log

1900

= −10.8 log

∆

= −20 log

∆

= 0.64 ln

ℎ 2

ℎ 2

360

+ 0.54 ln

360

Where ΔPLMS is often referred to as the antenna-gain reduction factor and accounts for the fact that the angular scattering is reduced owing to the directivity of the antenna. The antenna-gain reduction factor can be quite significant; for example, using a 20° antenna can contribute to a ΔPLMS of 7 dB.

4.3.4 Walfish-Ikegami Model The Hata model and its COST-231 extension are suitable for macrocellular environments, but not for smaller cells that have a radius less than 1 km. The Walfish-Ikegami model applies to these smaller cells and is recommended by the WiMAX Forum for modeling microcellular environments. The model assumes an urban environment with a series of buildings as depicted in Figure 4.4, with the building heights, interbuilding distance, street width, and so on, as parameters. In this model, diffraction is

assumed to be the main mode of propagation, and the model is valid for the following ranges of parameters: · · · ·

800MHz ≤ f ≤ 2000MHz 4m ≤ hb ≤ 50m 1m ≤ hm ≤ 3m 0.2km ≤ d ≤ 5km

Figure 4.4 the Walfish-Ikegami model

The model is made up of three terms: =

+

+

where, Lfs is the free-space loss, Lrts is the rooftop-to-street diffraction loss, and Lmsd is the multiscreen loss. The model provides analytical expressions for each of the terms for a variety of scenarios and parameter settings. For the standard NLOS case, with BS antenna height 12.5m, building height 12m, building-to-building distance 50m, width 25m, MS antenna height 1.5m, orientation 30° for all paths, and in a metropolitan center, the equation simplifies to: 1.5 925 This equation is recommended by the WiMAX Forum to be used for system modeling. The use of an additional 10dB for fading margin is also recommended with this model. = −65.9 + 38

+ 24.5 +

The Walfish-Ikegami model also provides an expression for the urban canyon case, which has a LOS component between the BS and the MS. For the LOS case, the median pathloss expression is:

4.4 Cell Area

= −31.4 + 26

+ 20

Mobile WiMAX uses a cellular network structure and we consider a hexagonal cell area, defined as: 3 X d2 X sin(π/3), with d the coverage range as indicated in Figure 4.5.

Figure 4.5 Illustration of cell area calculation

4.5 Bit Rate per Sector Both the channel bandwidth and the modulation scheme have an important influence on the bit rate, and these two parameters were already discussed above (Table 4.3 and Table 4.4). Besides, the bit rate is also determined by the guard time, the overhead and the TDD down/up ratio. The guard time is intended to overcome multi path effects and in the planning tool the user can select a fraction from a particular set, specified in the standard (1/8 is used in our study). The overhead is defined as the percentage of time that no data is sent and is the time used for e.g. initialization and synchronization, and it also covers the headers (we assume an overhead of 20%). Finally, the ratio between the downlink and uplink time is defined by a TDD down/up ratio parameter (fixed at 3:1 in our model). The downlink bit rate is then given by: =

∗

∗

∗

∗

1− 1+

ℎ

∗

/

4.6 Required Number of Sites and Sectors The final goal of the planning tool is to deliver the number of sites and sectors required to cover a particular region, and this information will then be used to formulate different business scenarios. The area of the region, the user density and the desired downlink and uplink bit rate per user are additional input parameters. Operators also take into account that not every user simultaneously uses his connection, and for this purpose a parameter for simultaneous usage (overbooking) is introduced, which defines the percentage of the users that effectively use the service (we assume 5%). As already mentioned, WiMAX dynamically selects the best possible modulation scheme per user, which is illustrated in Figure 4.6.

Figure 4.6 Range of the different modulation schemes, indicated by different colors. The lighter the color, the less data bits per symbol (cf. Table 4.4)

4.7 Planning Tool: Graphical User Interface (GUI)

4.8 Link budget sample Table 4.7 shows a sample link budget for a WiMAX system for two deployment scenarios. In the first scenario, the mobile WiMAX case, service is provided to a portable mobile handset located outdoors; in the second case, service is provided to a fixed desktop subscriber station placed indoors. The fixed desktop subscriber is assumed to have a switched directional antenna that provides 6 dBi gain.

Table 4.7 Link budget sample