Lucent Technologies Bell Labs Innovations

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Network Wireless Systems

Subject: CDMA Link Budget Tutorial date: April 1, 2000

from: S. Vasudevan JW45C6000

Room 4c-23967, Whippany RoadWhippany, NJ 07981Ph:973-581-6862sv13@lucent.com

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Objective

The objective of this document is to provide a road map for populating a CDMA link budget for specifieddesign objectives, using Lucent equipment.

Additionally, explanations are provided for the entries in the CDMA reverse link budget.

Introduction

The objective of a link budget is to catalog all the losses and gains between the two ends of acommunication link, thus yielding the maximum loss in signal strength that can be tolerated betweentransmitter and receiver. This maximum allowable loss in signal strength is also known as available pathloss. It is specified in logarithmic units (decibels) and can in turn be translated into the greatest spatialdistance between transmitter and receiver at which reliable communication of the desired quality can stilltake place. In the context of wireless mobile communication systems, link budgets are a pre-requisite todetermining the locations of, as well as spacing between, cell sites in order to ensure reliable anduninterrupted communication as mobiles move through an area of intended radio coverage.

Since communication here between the mobiles and base stations is bi-directional, the quality of the linkfrom mobile to base station (uplink or reverse link) as well as from the base station to the mobile (downlinkor forward link) must be considered in system design.

This document however, focuses mainly on the CDMA reverse link budget and a discussion of the forwardlink budget is restricted to the checks that need to be made on forward link parameters after obtaining thereverse link available path loss.

Definitions of Link Budget Entries

The following set of definitions is to be read in conjunction with the appended reverse link budgetspreadsheet.

Mobile EIRP

This refers to the effective isotropically radiated power from the mobile at the antenna connector. For thePCS CDMA mobile, this value is 21 dBm while it is 23 dBm at cellular frequencies. These values are takenfrom minimum performance standards for a 200 milliwatt mobile.

Antenna Gain

This is the gain of the mobile antenna. At both cellular and PCS frequencies, this is a dipole whose gain canbe taken to be 2 dB.

For new applications such as the proposed CDMA system in the NMT band at 450 MHz, a dipole is notfeasible and the antenna gain is expected to be much lower (of the order of -1.5 dB for the helical antenna).

Head/Body Loss

This refers to the attenuation of the radio signal during both transmission and reception as the mobileantenna is held to the ear of the user. At PCS and cellular frequencies, this attenuation is mainly due to thehead of the user while at lower frequencies (and larger wavelengths) the entire human body could distort theradiation pattern of the mobile antenna.

Typical head/body loss values range from 2 to 5 dB. Values to be used in the link budget are typicallyprovided by the wireless network operator based on field measurements or prior experience. It is importantto obtain these values from the operator since any design loss raises design cell count.

The field measurement technique consists of first setting up a CW transmitter at a typical cell site location.Next the signal strength seen at a mobile antenna held to the ear of a user who walks a test route is logged.The difference between this signal strength and that seen by an isolated reference antenna held at the sameheight is logged and averaged. The result is an estimate of the head loss to be used in the link budget.

Receiver Antenna Gain

This refers to the gain of the Rx antenna at the base station. While the actual antennas used in the networkmay vary from site to site, a nominal, representative value is provided in the link budget based on thefrequency of operation and sectorization.

The following table provides nominal antenna gain values for PCS and cellular frequencies. The gain unitsare dBi or gain with respect to an isotropic radiator.

Omni Three sectorPCS 13 17.5

Cellular 10 14

In selecting an antenna of higher (or lower gain) for a specific application, the horizontal and verticalbeamwidths should not be compromised. Typical values for vertical and horizontal beamwidths are 60degrees and 6 degrees. This ensures adequate overlap between cells for soft handoff and also ensures thatthere is minimal gain variation across the cell.

For highway coverage, narrower horizontal beamwidth antennas of 30 degrees may be used.

More guidance on antenna selection can be found in the presentation “Antenna Usage: Recommendationsand Restrictions” available in the RF Engineering Library on the AMPS Systems Engineering web site.

Receiver Cable and Connector Losses

The receiver cable and connector losses are nominally taken to be 3 dB. When the cable length anddiameter (and hence attenuation/ft) are known, the actual cable losses may be substituted in the link budgetalong with an additional margin of 0.5 dB for connector (and duplexer) losses.

Typical cable diameters used are 7/8" and 1 5/8" and corresponding attenuations are 6.15 dB/100m and3.84 dB/100m.

Duplex configuration

In a duplex configuration, the transmit and one receive signal use the same cable. Hence one must accountfor losses in the duplexer where the mixing of the incoming and outgoing signals takes place.

Receiver Noise Figure

The receiver noise figure is a measure of SNR degradation caused by the base station receiver front end. Itis seen as an additional attenuation of the signal before it enters the demodulator past the base station front-end amplifier and filter.

There are two cases to consider here. The first is one where the base station receiver input is at the end ofthe base station cable leading down from the antenna. The second case is one where a low noise amplifierconnects to the antenna at the tower top and a modified base station receiver receives the (now amplified)signal at the other end of the tower cabling. In the first case, the base station noise figure and cable lossesare additive. In the latter case, a composite base station noise figure calculated on the basis of the cablelosses, low noise amplifier and modified base station receiver characteristics replaces the additivecombination of cable loss and receiver noise figure.

The following table lists the noise figures for various Lucent base station products.

Cellular PCSSeries II 5 N/AMiniCell 5 5Enhanced MiniCell N/A 4Modular Cell 4 4Microcell 4.5 4Micro-mini 4 4

Composite Noise Figure calculation with Tower-top Low Noise Amplifier (TTLNA)

Use the formula for a cascade (in series) of two ports. In this case, the devices are the TTLNA followed bythe cable, and modified base station receiver.

LNAc

Rc

LNA

cLNAc

GL

NF

G

LNFNF

)1

(

11 −+−+=

where all the quantities are expressed in linear units. Lc is the cable loss (equal to the noise figure of thecable) and the gain and noise figures of the TTLNA and the modified base station receiver are indicated bythe subscripts 'LNA' and 'Rc'. The expression can be further simplified to

LNA

RcLNAc G

NFLNFNF

1. −+= .

For the PCS minicell and a cable loss of 3 dB, the composite receiver noise figure is 5.6 dB.

For the Enhanced PCS minicell and a cable loss of 3 dB, the composite receiver noise figure is 5.1 dB

Currently, the modular cell, the microcell, and micro-mini do not support the use of TTLNA

Receiver Noise Density

This simply refers to the thermal noise floor at 290 K which is -174 dBm/Hz.

Interference Margin

Since all the mobiles in a CDMA system transmit over the same frequency carrier, the signals of thesemobiles appear as mutual interference at the base station receiver. The extra margin required at the basestation in order to ensure adequate performance for a mobile at cell edge is termed the interference marginin the reverse link budget. This margin can be shown to correspond to the rise in the noise floor that is seenat the serving cell site as a result of the operation of mobiles in its vicinity.

The interference margin can be simply calculated from the fractional loading of the system (typically 55%):

IMf

=−

101

110* lo g ( )

where f is the fraction loading of pole (0.55 in most cases).

However, if the system is being designed to specific capacity objectives, knowledge of the pole point(which is the maximum achievable capacity per sector) is necessary.

The following table shows the pole capacities

Omni Three Sector13 kbit vocoder 27 248 kbit vocoder (EVRC) 40 358 kbit vocoder with Orange 1.1 ASIC 50 44

Assumptions:

Note that voice activity is assumed to be 0.4. If the customer changes the voice activity factor, polecapacity, loading, and interference margin may all have to be changed.

The Eb/No requirement is chosen to be 7 dB for the Orange 1.0 ASIC and 6 dB for the Orange 1.1 ASIC.This is the appropriate value for achieving a field FER target of 2% (note the power control target should berun at 1%).

Given the required number of channels per sector, one obtains the fractional loading of pole using the tableabove and hence the interference margin. At 55% loading, the number of usable channels is:

Omni Three Sector13 kbit vocoder 15 138 kbit vocoder (EVRC) 22 20

8 kbit vocoder with Orange 1.1 ASIC 27 24

Raising system loading above 55% is generally not recommended. More aggressive loadings may bepossible in specialized configurations (e.g., EVRC fixed wireless with fixed, directional antennas) thatallow significantly higher pole points.

Erlang Capacity

If the required Erlang capacity per sector is specified as the design objective, the first step is to convert thisnumber into a required number of voice channels from the Erlang B tables at 2% blocking. (Note that werecommend operation of the system at 2% blocking.) Next, the fractional loading of pole can be determinedwith the aid of the table above and hence the required interference margin in the link budget.

The Erlangs corresponding to the previous table of voice channels are:

Omni Three Sector13 kbit vocoder 9 7.48 kbit vocoder (EVRC) 14.85 13.28 kbit vocoder with Orange 1.1 ASIC 19.25 16.6

Primary Erlangs

It should be noted that the above numbers are primary Erlangs, i.e. the traffic due to users in soft handoff isaccretive to these numbers.

Total Effective Noise plus Interference Density

This number may be seen as measuring the rise in the noise floor as a result of the noise contributions due tointerference from other users and the noise generated in the base station receiver.

Information rate

This refers to the maximum rate at which data is sent over the channel, which is either 14.4 kb/s or 9.6 kb/s.

Required Eb/No

The Energy per bit/noise spectral density ratio is an alternative means of referring to the required Signal-to-noise ratio or SNR. In the next section, we will relate Eb/No to SNR and explain the sequence ofcalculations leading up to the determination of receiver sensitivity.

Receiver Sensitivity

This is the minimum acceptable signal level at the base station receiver (before allowances are made forfading, penetration losses and soft handoff gains) for the desired Frame error rate (FER) performance.

Soft Handoff gain

The reverse link budget specifies how far out a mobile can go from a cell site and yet achieve the desiredSignal-to-noise ratio. In the case of CDMA a mobile at cell edge is in simultaneous communication withtwo or more cell sites resulting in overall link performance that is better than in the case when only a singlecell site is available for communication. This implies that the edge of each cell that abuts other cells can bemoved further out than if the cell were isolated. This improvement in coverage is captured by the softhandoff gain entry in the reverse link budget.

The soft handoff gain can essentially be viewed as a credit against the single-link fade margin. The creditcan be applied because of the presence of two links that are unlikely to fade simultaneously.

The actual value of the soft handoff gain depends on the edge coverage probability. This is because theimprovement due to the fact that the mobile has two links to communicate on, depends on the likelihood ofthe signal quality on at least one of the two being acceptable. When a system is being designed for a loweredge coverage probability, the improvement due to soft handoff is smaller since the likelihood ofunacceptable signal quality on both links is increased.

For the typical edge coverage probability targets of 90% and 75% and a fading standard deviation of 8 dB,the soft handoff gain values are 4 dB and 3 dB respectively.

Explicit Diversity Gain

This value is typically zero since the target Eb/No requirement of 7 or 6 dB presumes the availability ofreceive diversity.

In cases where there is no spatial diversity (due to space constraints at the site, for example) one mustincrease Eb/No by 5 dB.

Log-Normal Fade Margin

This is the margin that needs to be provided in the link budget to compensate for the fact that the averagepath loss at any fixed distance from the base station is a random variable that can exceed the average asignificant percentage of the time. The margin required is computed on the basis of the assumption that thisrandom variable is log-normally distributed with a mean equal to the average path loss at cell edge and thatit has a standard deviation of 8 dB. For a specified value of standard deviation (assumed or measured), thelognormal assumption allows computation of the fade margin for a specified probability of edge coveragefrom a standard Gaussian table.

The fade margin is calculated on the basis of the specified coverage objective which may be based on eitheredge or area coverage. The margin is based on a single-link computation. Adjustment for the presence ofmultiple links is contained in the soft handoff gain (see above).

For typical edge coverage objectives of 90% and 75%, the required fade margins are 10.3 and 5.6 dBrespectively. The margins correspond to percentiles of the lognormal distribution. For example, 90% ofthe fades are 10.3 dB or less (presuming 8 dB standard deviation) in a lognormal distribution.Accordingly, a design margin of 10.3 dB allows the mobile to combat 90% of fades at cell edge, resulting ina 90% probability of cell edge coverage.

Building/Vehicle Penetration Loss

This refers to the attenuation of the signal as it passes through one or more walls of buildings in the desiredcoverage area. The number is usually supplied by the network operator. Customer input on this point iscritical, since these values often raise design cell count significantly, and since no single value will penetrateall locations within all buildings. Operator requirements customarily derive from their insight into localbuilding structure, as well as their ability to purchase additional cells and their projections regarding thefraction of subscribers that are likely to be within buildings. Typical values used for penetration loss are 22,18, 15, and 10 dB for dense urban, urban, suburban, and rural environments.

The penetration loss may be specified as either the maximum value expected or as an average within theactual losses distributed about this value. In the latter case, a log-normal distribution with a standard

deviation of 8 dB is assumed for the random building penetration loss. A composite fade margin based on acombination of outdoor and indoor loss distributions is then used for the fade margin.

For example, the composite indoor plus outdoor fading distribution, when each has a standard deviation of

8 dB, has a standard deviation of 22 88 + or 11.3 dB. For an edge coverage probability of 90%, therequired fade margin is then 14.6 dB.

Design Objectives

A typical design objective would be specified as a combination of coverage and capacity objectives.

Coverage Objective

Coverage objective is typically specified as a target coverage probability at cell edge. Typical numbers are90% and 75% edge coverage. Achieving 90% edge coverage implies that at 90% of the locations at edge, acall can be initiated and kept up. In order to achieve this probability of coverage at cell edge it is necessaryto build in a margin into the reverse link budget. This number is specified as the log-normal fade marginand is 10.3 dB for 90% edge coverage and 5.6 dB for 75% edge coverage.

Due to difficulties associated with measuring coverage at cell edge (since the boundary is not clearlydefined), the coverage objective is often specified as an area coverage requirement. Using path loss models,one can relate area coverage to edge coverage and hence to fade margin requirement. We currently map95% area coverage to 90% edge coverage and 90% area coverage to 75% edge coverage. These mappingsdiffer somewhat from those found in the literature. These values presume a completely noise-limitedreceiver, whereas in CDMA cochannel interference at the receiver cannot be ignored.

Capacity Objective

The capacity objective may be specified as a required number of Erlangs or channels per sector, or as anErlang density for each morphology. Keeping the constraints on loading per carrier in mind, appropriateequipment choices can be made, interference margin in the reverse link budget can be calculated, andcarrier requirements at each site can be determined.

Trading capacity for coverage

By using the correct interference margin in the reverse link budget based on capacity inputs, any extra dBsavailable for coverage are automatically indicated in the available path loss.

Reverse Link Performance Objective

The Eb/No requirements are predicated on a 1% FER design target. Experience has indicated that theErlangs associated with the 1% target can be achieved while maintaining an average (across coverage area)measured FER of 2%. RIGHT HERE

Overall Reverse Link Budget Calculations

In this section, we work out the calculation of receiver sensitivity and make the correspondence betweenthis and the computations in the reverse link budget.

The required Signal to noise ratio at the base station receiver may be related to the required Eb/No (orEnergy per bit to Noise power spectral density) as follows:

WN

RESNR b

0

=

where R and W are the system information rate and carrier bandwidth respectively. The noise in the SNRconsists of the thermal noise, interference from other users as well as noise injected by the base stationamplifier. The above expression may be simplified and rewritten as a minimum signal requirement:

IMNFNRN

ES Rc

breq ++++= 0

0

where all terms are expressed in logarithmic units and the dependence on W, the carrier bandwidth, hasdisappeared. This equation is represented by the computations leading up the calculation of receiversensitivity in the reverse link budget.

Forward Link Check

The forward link check consists of making sure that, given the forward power constraints, all mobiles atcell edge can meet the required forward link Eb/No targets at a distance allowed by the reverse link budgetavailable path loss.

The forward link check is critical. A system design employing a reverse link budget will not be viable if theavailable forward link power cannot meet forward link Eb/No targets within the footprint dictated by thereverse link.

Forward Power output and Eb/No requirement

The following table lists the forward power output at the antenna connector (J4) to be used in the forwardlink spreadsheets to verify that acceptable FER targets are being met at cell edge on the downlink.

Cellular PCSSeries II 9 W – 33 W N/AMiniCell 4.5 W 8/16 wCompact MiniCell 20 W N/AEnhanced MiniCell N/A 8/16 wModular Cell 20 W 16 wMicrocell 10 w 8 wMicro-mini 16 w 16 w

For full mobility systems using the 8 kb vocoder, the Eb/No requirements are 6.8 dB at PCS and 7.3 dB atcellular frequencies.

Reverse Link Amplifier Backoff

In cases where the forward Eb/No requirement is not met, the available reverse link path loss may bereduced until this requirement is met. The approach is simply to decrement the available path in the reverselink until the Eb/No target is met in the forward link spreadsheet. If too large a reduction in path loss isnecessitated, it may be preferable to take a capacity penalty and reduce the number of users on both forwardand reverse links since forward Eb/No-s are much more sensitive to capacity changes than to reductions inpath loss. This penalty, if applied, is sometimes referred to as the “design tradeoff adjustment factor”, sinceit essentially entails adjustments that trade off capacity against coverage.

Reverse Link Budget Spreadsheet

An Example of Reverse Link Budget for PCS 8 kbps CDMA MiniCells for various morphologies using customer specificationsDU U SU Rural

WithoutTTLNA

WithoutTTLNA

WithTTLNA

WithTTLNA

Item Units Value Value Value Value Comments(a) Maximum Transmitted power per trafficchannel

dBm 23 23 23 23

(b) Transmit Cable, connector, combiner,and body losses

dB 2 2 2 2

(c) Transmitter Antenna Gain dBi 0 0 0 0 Antenna Gain is assumed as 2dBi, and it is includedin item (a)

(d) Transmitter EIRP per traffic channel (a-b+c)

dBm 21 21 21 21

(e) Receiver Antenna Gain dBi 17.5 17.5 17.5 17.5(f) Receiver Cable and Connector Losses dB 3 3 3 3 For the suburban and rural configurations, the cable

loss is included in item (g), the receiver noise figure

(g) Receiver Noise Figure dB 5 5 5.6 5.6(h) Receiver Noise Density dBm/Hz -174 -174 -174 -174(i) Receiver Interference Margin dB 3.01 3.01 3.01 3.01 =10log(1/(1-x/28)) where x is the number of channels

per carrier (14 for 50% pole loading), 28 is thesectorized pole capacity derived by assuming 0.5 ofvoice activity factor, 7 dB Eb/No requirement and0.85 of other sector to serving sector interferenceratio.

(j) Total Effective Noise plus InterferenceDensity=(g+h+i)

dBm/Hz -165.99 -165.99 -165.4 -165.4

(k1) Information Rate (10log(Rb)) forDigital or (k2) NAMPS traffic channel 3dBbandwidth

dB 39.8 39.8 39.8 39.8 41.6 for 13 kbps systems and 39.8 for 8 kbpssystems

(l1) Required Eb/No for Digital systems or(i2) Required C/I for NAMPS

dB 7 7 7 7

(m) Receiver sensitivity (j+k+l) dBm -119.17 -119.17 -118.53 -118.53(n) Hand-off Gain dB 4 4 3 3(o) Explicit Diversity Gain dB 0 0 0 0

(p) Log-normal Fade Margin dB 13.9 13.9 7.9 7.9 The fade margin is a composite number includingmargin for both outdoor (8 dB std. dev.) andindoor/in-vehicle fading (8/4 dB std. dev.). The valuescorrespond to 90% edge coverage for dense urbanand urban, and 75% edge coverage for suburban andrural.

(p') Building/Vehicle Penetration Loss dB 22 18 14.0 10.0(q) Maximum Path loss {d-m+(e-f)+o+n-p-p'}

dB 122.8 126.8 138.1 142.1

(r) Max Path Loss w/o penetration margin dB 144.8 144.8 152.1 152.1