Link Budget-rf Planning

CDMA RF System Design Procedure

Chapter

2 2.1

Link Budgets and Associated NetPlan Inputs

Overview

A system design is usually based on trying to maximize coverage but still ensuring sufficient signal strength for calls. The coverage design has to be balanced with the requirements for quality and capacity within the system. Although all three factors are important in an analog design, the RF coverage is not dependent upon the number of traffic channels. In a CDMA system, however, the coverage and capacity are interrelated. With higher capacity, the cell coverage is reduced. All three principles (coverage, capacity, and quality) must be factored into a CDMA system design. The first step in a system design is setting up the link budget to model the path between the mobile and the base station. This link budget accounts for all of the gains and losses along the path. There are two main purposes for establishing a link budget for CDMA designs. The first main purpose is to establish system design assumptions (such as vehicle loss, building loss, ambient noise margin, maximum subscriber transmit power, etc.) which are used as inputs to NetPlan in the design process. The second main purpose of a link budget is to establish an estimate for maximum allowable path loss. This maximum allowable path loss number is used in conjunction with the propagation model in NetPlan to estimate cell site coverage. Analyzing the coverage based on a maximum allowable path loss is an important step since it can help determine major issues such as cell site placement problems (sites spaced too close or too far apart), terrain obstruction issues, and sites which may present interference problems (sites on mountain tops or near large bodies of water). By identifying these issues early in the design process, some of these issues can be resolved before going through the time and effort of simulations. This allows the simulator process to be used to concentrate on issues that can only be analyzed with the simulator rather than issues that can be addressed by coverage plots based on path loss only. A detailed discussion of the link budget and all its parameters can be found in Section 5 of the CDMA RF Planning Guide (version 1.7, February 16, 1996). It is recommended that this section of the RF Planning guide be read before generating a link budget or producing a propagation study. Once the parameters are understood, the following information can be used to determine initial link budget values and to show how these values are used to determine the inputs into NetPlan. Note:

This document assumes a detailed design process is followed using the NetPlan maximum allowable path loss step as well as the NetPlan CDMA simulator. Budgetary designs are not addressed.

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 2.2

RF Link Budget Parameters

As mentioned in the CDMA RF Planning Guide, the system designer will need to determine the specific link budget parameters to be used when designing the system. The following lists some of these parameters: •

Propagation Related Parameters: Building Loss Vehicle Loss Body Loss Noise Margin RF Feeder Losses Antenna Gain

•

CDMA Specific Parameters: Interference Margin Eb/No

•

Product Specific Parameters: Product Transmit Power Product Receiver Sensitivity

•

Reliability Parameter (Shadow Fade Margin)

The values within the link budget provide the designer with input parameters to be used within both the NetPlan maximum allowable path loss step as well as the NetPlan CDMA simulator.

2.3

Link Budget Assumptions

Due to the variability of the forward link, the CDMA link budget models only the reverse link. In addition, CDMA link budgets make simplifying assumptions regarding noise rise and Eb/No requirements. For instance, in the link budget Eb/No is considered a constant, in actuality, Eb/No is not a constant value but varies with respect to speed. A link budget must be determined for each different site configuration. The link budget for each site must incorporate any specific parameters that have been supplied by the customer (such as building penetrations, antenna heights, antenna gains, cable losses, coverage criteria, coverage reliability, etc.). If these parameters change from site to site, then the link budgets will need to be calculated for each site. Since a high percentage of subscriber units sold are portables and these units are often used in vehicles, it is recommended that a vehicle loss factor be included in all designs. If the customer does not specifically state a value to be used for vehicle loss, use an estimate of 6 dB.

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 2.4

Example Link Budgets Table 2-1: Example CDMA RF Link Budget - Uplink for 1.9 GHz Parameter

Unit

Portable Tx power

dBm

a

Portable Ant Gain

dBd

b

Body Loss

dB

c

Vehicle Loss

dB

d

Building Loss

dB

e

Base Ant Gain

dBd

f

Line Loss

dB

g

Eb/No

dB

h

Noise Figure

dB

k

Base Rx Sensitivity

Example 13 kB Link Budget

Notes

dBm m

* * * * * * *

Note 1

Example 8 kB Link Budget

23

23

-2.14

-2.14

2

2

6

6

0

0

Note 2

14.5

14.5

Note 4

2

2

7.3

7.0

6

6

Note 5

-119.1

-121.2

Note 1&2 Note 3

*

Interference Margin

dB

n

Note 6

3

3

Ambient Noise Rise

dB

p

* Note 7

0

0

Shadow Fade Margin

dB

r

Note 8

5.6

5.6

Max. Allowable Path Loss = a+b-c-d-e+f-g-m-n-p-r

dB

135.86

137.96

* These values are all used as input into the CDMA simulator. NOTES: 1. One has to be careful when specifying the portable transmit power. In the IS95 specifications (for 800 MHz), the minimum effective radiated power (ERP) for a Class III personal station is listed as 23 dBm. However, in the J-STD-008 specifications (for 1.9 GHz), the minimum isotropic effective radiated power (EIRP) for a Class II personal station is listed as 23 dBm. The ERP is calculated with respect to a dipole (antenna gains given in dBd) while the EIRP is calculated with respect to an isotropic antenna (antenna gains given in dBi). (dBd = dBi - 2.14) To illustrate this further, consider the following ERP and EIRP calculations: At 800 MHz

Portable Tx Power Portable Antenna Gain

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with respect to an isotropic antenna

with respect to a dipole

23 dBm 2.14 dBi 25.14 dBm EIRP

23 dBm 0 dBd 23 dBm ERP

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 At 1.9 GHz

Portable Tx Power Portable Antenna Gain

with respect to an isotropic antenna

with respect to a dipole

23 dBm 0 dBi 23 dBm EIRP

23 dBm -2.14 dBd 20.86 dBm ERP

As these tables show, if you are not careful to specify all of your values with respect to either an isotropic antenna or a dipole, the calculations can easily be off by roughly 2 dB. 2. The antenna gain values are given in terms of dBd since the NetPlan tool requires calculations to be done using antenna values in dBd. (dBd = dBi - 2.14) 3. Recommended value if no vehicular loss value is specified by the customer. 4. The value listed is an example line loss value. A 1-5/8” heliax cable @ 1.9 GHz has ~ 1.25 dB loss per 100 ft. Also, another 0.75 dB was assumed for jumpers and connectors. NOTE: An analysis should be done for each particular sector to determine the estimated line loss. This calculation should include all losses between the antenna and the base station such as the top jumper, the main transmission line, a surge protector, the bottom jumpers, and all connectors. Also, keep in mind that the line loss is frequency dependent. The following chart shows an example of this. Example main transmission line losses: 7/8” Foam Dielectric Coaxial Cable 1 5/8” Foam Dielectric Coaxial Cable

850 MHz 1.23 dB/100’ 0.767 dB/100’

1900 MHz 1.97 dB/100’ 1.25 dB/100’

5. Base Rx Sensitivity = kTB + Eb/No + NF - PG where the processing gain PG = B/R. The remaining parameters are detailed below. vocoder rate Eb/No NF kT B R 2-4

8 kbps 7.0 dB 6 dB -174 dBm/Hz 60.9 dB-Hz (1.228 MHz) 39.8 dB (9.6 kbps)

13 kbps 7.3 dB 6 dB -174 dBm/Hz 60.9 dB-Hz (1.228 MHz) 41.6 dB (14.4 kbps)

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 - These values assume flat fading and worse case speed (30 kmph @ 1% FER), with diversity and perfect decorrelation. - These values also assume a CSM chipset (MCC8s). When using a BSM chipset (MCC4s), the Eb/No values are approximately 2 dB greater which results in sensitivity numbers that are degraded by approximately 2 dB. 6. 50% loading = 3 dB, based on -10*log(1-% loading). NOTE: The 50% is an approximation used in this example link budget to try and maximize coverage at the expense of reducing capacity. This approximation will vary depending on the specific design requirements (capacity and coverage). Expected noise rise is better estimated from simulation studies. 7. If a value is known for the ambient noise of the system, it should be added to the link budget. 8. The shadow fade margin assumes the effects of soft handoff and multiple cells. The value shown here is an example of the fade margin required based on a multi-cell system simulation to achieve an area reliability of 97%. (An 8 dB standard deviation for lognormal shadowing and a propagation slope of 40 dB per decade were used in the simulation.) This value will vary depending on the actual area reliability that the system is designed to achieve.

2.5

Determining NetPlan Inputs to Estimate Coverage

The information obtained in the link budget is used in conjunction with a propagation model to estimate the coverage of each cell. As mentioned before, NetPlan is used along with the reverse link information in a CDMA system design to estimate the system coverage. This design is followed by a more detailed design using the CDMA simulator to analyze both the forward and reverse links. The link budget information is used to determine the specific values used as input in the NetPlan tool. Specifically, the information is used to calculate the system cutoff level (used when viewing or plotting coverage) and cell “ERP” levels used in generating coverage studies. Since the NetPlan tool allows for only one value to be used as a cutoff value for the entire system, all site specific variables need to be accounted for in the site’s “ERP” term. These variations include such parameters as percent loading, building loss, vehicle loss, line losses, and antenna gains. “ERP” is referred to here in quotes because if all of the variables of a site such as vehicle losses or building losses are accounted for in this term, then it is really no longer a true ERP value. A true ERP (effective radiated power) refers to the power that is being radiated out from an antenna.

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 All of the terms in a link budget must be accounted for within NetPlan to get an accurate coverage prediction. Therefore, the link budget terms must either be included in the cutoff value or in the site’s “ERP”. In order to make it easier to determine where each link budget parameter should be accounted for (cutoff level or “ERP” value), it is recommended to use the BTS receiver sensitivity value as the cutoff value and account for all other link budget terms in the site’s “ERP”. 2.5.1

Example Calculations of NetPlan Values

Using the above link budget, the following shows the values that would be used for the NetPlan maximum allowable path loss coverage studies. Table 2-2: NetPlan Cutoff Level (Min. Signal Strength) Reverse Link Parameter

Unit

Example 13 kB Link Budget

Example 8 kB Link Budget

Base Rx Sensitivity

dBm

-119.1

-121.2

Cutoff (Min. Signal Strength)

dBm

-119.1

-121.2

Table 2-3: NetPlan ERP (Receive Voice) Reverse Link Parameter

Unit

Portable Tx power

dBm

a

Note 1

Portable Ant Gain

dBd

b

Note 1&2

Body Loss

dB

Vehicle Loss

Notes

Example 13 kB Link Budget

Example 8 kB Link Budget

23

23

-2.14

-2.14

c

2

2

dB

d

6

6

Building Loss

dB

e

0

0

Base Ant Gain

dBd

f

14.5

14.5

Line Loss

dB

g

2

2

Interference Margin

dB

h

3

3

Ambient Noise Rise

dB

k

0

0

Shadow Fade Margin

dB

m

5.6

5.6

NetPlan Rv “ERP” = a+b-c-d-e+f-g-h-k-m

dB

16.76

16.76

Note 2

NOTES: 1. See Note 1 in section 2.4 “Example Link Budgets”. 2. The antenna gain values are in dBd since NetPlan requires all calculations to be done using antenna values in dBd. (dBd = dBi - 2.14) 2-6

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 As a double check to make sure that the proper values have been calculated for NetPlan, the “ERP” and the cutoff values when combined should equal the maximum allowable path loss calculated in the link budget. Table 2-4: Double Check of NetPlan Values Parameter NetPlan Rv “ERP” Cutoff (Min. Signal Strength)

2.5.2

Example 13 kB Link Budget

Unit

Example 8 kB Link Budget

dB

a

16.76

16.76

dBm

b

-119.1

-121.2

Calculated Path Loss from NetPlan Values = a-b

dB

135.86

137.96

Max. Allowable Path Loss from Link Budget

dB

135.86

137.96

Entering NetPlan Values

The NetPlan Rv “ERP” values may be entered via the “Edit Site” cell site editor window. Access to the “Edit Site” menu is gained through a pull down menu (see Figure 2-1: Edit Site - Pull Down Menu). Figure 2-1: Edit Site - Pull Down Menu

This action opens up the cell site editor window as seen in Figure 2-2: Edit Site. Two areas have been circled in this figure. The area marked “Area - A Common Input” denotes inputs which must be defined whether the cell site is analog or CDMA. These include name, location, propagation boundaries, etc. The area marked “Area - B Antenna Input” denotes inputs which are specific to each sector “antenna”. It is in area B that the NetPlan Rv “ERP” values will be entred.The Rv ERP values calculated for each sector may then be entered into the Rv ERP boxes of the edit site window. See Figure 2-3: Edit Site - Rv ERP below: Version 2.0 4/7/97

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 Figure 2-2: Edit Site

Area - A Common Input

Area - B Antenna Input

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 Figure 2-3: Edit Site - Rv ERP

Rv ERP (dBm) 2.5.3

NetPlan Image Parameters and Multiple Level Plots

In order to generate coverage for a system within NetPlan, the site specific values (such as ERP, antenna parameters, etc.) need to be set as well as the system “Image Parameters”. The image parameters (found via pull-down menus... Configure > Combined Image Parameters > Image Parameters) include the output resolution and the minimum signal strength. See Figure 2-4: Image Parameters for details. Figure 2-4: Image Parameters

1

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 NOTES: 1. A resolution of 100 m is required for simulations to get a more accurate output. 2. The minimum signal strength image parameter is an important parameter since it determines the threshold below which signal strength data will not be available in best server/sector images. In other words, regardless of whether the site propagation data was obtained for lower minimum signal strengths, the best server/sector image will contain no data for signal strengths below the defined minimum. Therefore, if you are designing a system to include in-vehicle loss but you think you may also want to look at a best server/sector plot to show on-street mobile coverage, then you want to make sure that you set your minimum signal strength low enough so that the on-street values are included. (For example, if you are designing a system with 6 dB in-vehicle loss and would normally look at a signal strength level of -119 dBm to show in-vehicle coverage, and you want to look at a best server plot showing “on-street” levels, your minimum signal strength would need to be at least -125 dBm.)

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 By setting different levels when viewing or plotting an image, you can show different coverage regions such as areas of in-building coverage, in-vehicle coverage, or on-street mobile coverage. These levels are accessed by assigning different cutoff points when assigning colors and values to the image sliders. The sliders are accessed via the “L” button to open the Layers window. With the image displayed, click the ellipsis [...] for “image” to open the “coloring” window. From there select the “intervals” button which will open the window with the coloring sliders. From here, differing colors and cutoff settings can be assigned for the image. See Figure 2-5: Setting Image Cutoff Levels for details. Figure 2-5: Setting Image Cutoff Levels

4 3 2 1

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CDMA RF System Design Procedure Chapter 2: Link Budgets and Associated NetPlan Inputs

2 As an example, assume the loss factors that you are using in your system design are as follow: inbuilding loss factor in the dense urban region is 20 dB, in-building loss factor for a residential region is 10 dB, and in-vehicle loss factor of 6 dB. Assume further that for your system the level that you are using for a cutoff to show in-vehicle coverage is -119 dBm. (This assumes that the “ERP” calculations include the in-vehicle loss but not the 20 dB and 10 dB building loss values for dense urban and residential regions.) Then if you wish to show a plot that highlights the areas where you expect coverage to be good based on the different loss factors (in-building dense urban, in-building residential, in-vehicle, on-street mobile), you would set up the different levels as depicted by the noted values in Figure 2-5: Setting Image Cutoff Levels: Note 1 2 3 4

2.6

area type in-building dense urban in-building residential in-vehicle on-street mobile

image cutoff level -105 dBm -115 dBm -119 dBm -125 dBm

Utilizing Link Budget Assumptions in Simulations

As mentioned before, the system design assumptions within the link budget are also used during the simulation portion of the design process. These values are listed with an asterisk in Table 2.1. For more details on how these values are used within the simulator, please refer to chapter 6 and X.

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