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In an m-voice channel cell, one of the m traffic channels is the desired channel and the remaining m− 1 traffic channels are the interference channels.. BASIC SYSTEM DESIGN PHILOSOPHY 27

CDMA network design

9.1 BASIC SYSTEM DESIGN PHILOSOPHY

In Code Division Multiple Access (CDMA) systems, the capacity increase is based onhow much interference the desired signal can tolerate Prior to despreading, the signallevel of a desired signal is always below the interference level All the users have to sharethe same radio channel If one user takes more power than needed, then the others willsuffer and the system capacity will be reduced

In analog and TDMA systems, the most important key element is the carrier to

inter-ference ratio (C/I ) There are two different kinds of C/I One is the measured (C/I ),

which is used to indicate the voice quality in the system The higher the measured value,

the better it is The other is called the specified (C/I )s, which is the required value for

a specified performance of the cellular system For example, the (C/I )s in the American

mobile phone system (AMPS) is 18 dB Since in analog and TDMA systems, owing to

the spectral and geographical separations, the interference (I ) is much lower than the received signal (C), sometimes we can utilize field strength meter to measure C to deter-

mine the coverage of each cell The field strength meter therefore becomes a useful tool

in designing the TDMA system

In CDMA all the traffic channels are served solely by a single radio channel in every

cell In an m-voice channel cell, one of the m traffic channels is the desired channel and the remaining m− 1 traffic channels are the interference channels In this case, at thereceiver front end (prior to despreading) the interference is much stronger than the desired

channel C/I is hard to obtain by using the signal strength meter that will receive more

interference than the desired signal The key elements in designing a CDMA system aredifferent from the key element in designing a TDMA system We can design the CDMA

system based on the specified Eb/I0

C

I =

Eb I0



·

Rb B



· η ⇔ Eb I0 = G · C

Adaptive WCDMA: Theory And Practice.

Savo G Glisic Copyright ¶ 2003 John Wiley & Sons, Ltd.

ISBN: 0-470-84825-1

The left-hand side of equation (9.1) is derived from the right-hand side specifying that the

signal-to-noise ratio (SNR) after despreading is G times higher than the input SNR Values

of Eb/I0 for the forward-link channels and for the reverse-link channels are differentbecause of the different modulation schemes In general, there will be two different

requirements for C/I One (C /I )Ffor the forward-link channels and the other (C /I )Rfor

the reverse-link channels In Chapter 8 we used (Eb/I0)R = 7 dB and (Eb/I0)F= 5 dB So,

for a given Eb/I0 the network design should make sure that the required C/I is guaranteed

in each spot of the coverage area In the first step, we start with a simple, very muchapproximative approach to the problem in order to get the very first initial insight into thesystem parameters In the next iteration we will come up with a more detailed analysis

9.1.1 Uniform cell-size scenario

For the forward link a worst-case scenario is used to find the relation among the transmittedpowers of cell sites The position of the mobile for this case is shown in Figure 9.1

If we assume that the signal propagation losses can be approximated as R−4(shadowing

ignored at this stage) and if R is the cell parameter, then C/I at the mobile front end can

I (a) = I (2 adjacent cells) = (α2m2+ α3m3)R−4

I (i) = I (3 intermediate cells) = β(2R)−4

BASIC SYSTEM DESIGN PHILOSOPHY 273

α i(1, 2, 3) is the transmitted power of each voice channel in the cell

m i is the number of channels per cell

β and γ are transmitted powers of the combined adjacent cells at a distance 2R and 2.633R, respectively.

By solving the equation we get m1 as follows:

For C/I = −17 dB, we have m1 = 51

If there is no interference other than from the two close-in interfering cells then

If the total transmitted power P in each cell site is P1 = α1m1, P2 = α1m2, P3 = α3m3,

when m1, m2, m3 are given, then P1, P2 and P3 are the maximum transmitted powers of

P1 m1 = P2

m2 = P3

m3

Deduced from the equation, a design criterion that will be used in general for a CDMA

system of N cells can be expressed as

P i

m i = P j

m j = constantFor the reverse link the received signal from a desired mobile unit at the home cell site is

C Each signal of other m1 channels received at the home site is also C (owing to power control) The interference power of certain mobile units, say r · m1, from the two adjacentcells comes from the cell boundary (see the worst case scenario in Figure 9.2) Because

of the power control in each adjacent cell, the interference coming from the adjacent cell

for each voice channel would roughly be C at the home cell site So we have

BASIC SYSTEM DESIGN PHILOSOPHY 275

r depends on the size of the overlapped region in the adjacent cell and can be reasonably

assumed to be 1/6 (which is 0.166) if the system is properly designed If C/I = −17 dB,which is 50−1 and r12= r13= 0.166, then

m1 + 0.166 · (m2+ m3)≤ 51 ( 9.11) This is a relationship among the number of voice channels in each cell, m1, m2 and m3.From the reverse-link scenario, we can check to see whether all the conditions expressed

in the equations can be met The unknowns in these conditions come from the demanded

voice channels, m1, m2 and m3 Then, on the basis of the forward-link equations, we candetermine the maximum transmitted power of each cell

9.1.2 Nonuniform cell scenario

We may first assign the number of voice channels m in each cell owing to requirements

from demographical data Then we may calculate the total transmit power on the link channel in each cell from the worst-case scenario as shown in Figure 9.3

forward-The (C/I )F received at vehicle 1 is

C1 I1

F

(C2 /I )F received at vehicles 2 and 3 can be expressed as

C2 I2

F

F

F

=

C3 I3

F

=

C I

Fand Ia1 = Ia2 = Ia3= 0 ( 9.15)

−4

+ α2m2+ α3m3

R3 R2

−4

= α2· G α1 m1

R1R3

−4

+ α2m2

R2R3

P1 = m1α1

P2 = m2α2

BASIC SYSTEM DESIGN PHILOSOPHY 277

Figure 9.4 The worst-case scenario for reverse link.

The worst-case scenario for reverse link is depicted in Figure 9.4 On the basis of thepower control algorithm, all the signals will be the same on reaching the cell site

C1

I1

R

(m1 − 1)α

1R1−4+ r12m2α2R1−4+ r13m3α3R−41 + ˙Ia1

( 9.20)

where α1, α2 and α3 are the power of individual channels transmitted back to their

cor-responding cell sites r12and r13 are the portion of the total number of voice channels inthe adjacent cell that will interfere with the desired signal at cell 1 ˙Ia1is the interferencecoming from other users in other cells that are not cell 2 and cell 3, which is a relativelysmall value and can be neglected Similarly we have

C2

I2

R

r31m1 α1R3−4+ r32m2α2R3−4+ (m3− 1)α3· R3−4

(9.21)

where r is the percentage of total channels from the interfering cell received by the home

site Simplifying the equations gives

I C

R

≥ (m1− 1) + r12m2α

 2

α + r13m3α

 3

α

I C

R

≥ r21m1α

 1

α2 + (m2− 1) + r23m3α

 3

α2

I C

R

≥ r31m1α

 1

α3 + r32m2α

 2

α3 + (m3− 1) (9.22)

C I

R

=

C1I1

R

=

C2I2

R

=

C3I3

R

where R1, R2 and R3 are the radii of the three cells and k is a constant gain related to

the antenna heights at the cell sites Now equation (9.23) becomes

I C

R

R

≥ r21m1

R1R2

4

+ (m2− 1) + r23m3

R3R2

4

I C

R

≥ r31m1

R1R3

4

+ r32m2

R2R3

9.2 CDMA NETWORK PLANNING

In this section we provide more details on network planning and dimensioning Theapproach is based on References [1–5] WCDMA radio network dimensioning is theprocess through which the possible configurations and the amount of network equipment

is estimated, on the basis of the operator’s requirements related to the following:

CDMA NETWORK PLANNING 279

Coverage, which includes coverage regions, area type information, propagation conditions Capacity, which includes spectrum available, subscriber growth forecast, traffic density

information

Quality of Service, which includes area location probability (coverage probability),

block-ing probability, end user throughput

Dimensioning activities include radio link budget and coverage analysis, capacity mation, estimations on the amount of sites and base station hardware, radio networkcontrollers (RNCs), equipment at different interfaces and core network elements (i.e.circuit-switched domain and packet-switched domain core networks)

esti-9.2.1 Radio link budgets and coverage efficiency

The interference margin is needed in the link budget because of the loading of the

cell by other users The load factor, which will be later related to (Eb/N0)R defined

in equation (8.2) of Chapter 8, affects the coverage The more loading is allowed inthe system, the larger is the interference margin needed in the uplink, and the smaller

is the coverage area For coverage-limited cases a smaller interference margin is gested, while in capacity-limited cases a larger interference margin should be used Inthe coverage-limited cases the cell size is limited by the maximum allowed path loss inthe link budget, and the maximum air interference capacity of the base station site is notused Typical values for the interference margin in the coverage-limited cases are 1.0 to3.0 dB, corresponding to 20 to 50% loading Some headroom is needed in the mobilestation transmission power for maintaining adequate closed-loop fast power control Thisapplies especially to slow-moving pedestrian mobiles in which fast power control is able

sug-to effectively compensate the fast fading Typical values for fast fading margin are 2.0 sug-to5.0 dB for slow-moving mobiles

Handovers – soft or hard – give a gain against slow fading (lognormal fading) byreducing the required lognormal fading margin This is because the slow fading is partlyuncorrelated between the base stations, and by making handover the mobile can select

a better base station Soft handover gives an additional macro diversity gain against fast

fading by reducing the required Eb/N0relative to a single radio link, owing to the effect ofmacro diversity combining, as explained in Chapter 8, Section 8.7 The total soft handovergain is assumed to be between 2.0 and 3.0 dB in the examples given below, including thegain against slow and fast fading The following system assumptions given in Tables 9.1and 9.2 will be used in this section [1–5]

On the basis of this assumption, the link budget for three different services is shown

in Tables 9.3 to 9.5

Table 9.1 Assumptions for the mobile station

Speech terminal Data terminal

Table 9.2 Assumption for the base station

Antenna gain 18 dBi (three-sector base station)

Eb/N0 requirement Speech: 5.0 dB

144-kbps real-time data: 1.5 dB 384-kbps non-real-time data: 1.0 dB

Table 9.3 Reference link budget of adaptive multirate (AMR) 12.2-kbps voice service

(120 km h−1, in-car users, vehicular A type channel, with soft handover)

12.2-kbps voice service (120 km h−1, in-car)

Receiver (base station)

Thermal noise density (dBm Hz−1) −174 d

Base station receiver noise figure

(dB)

Receiver noise density (dBm Hz−1) −169 f = d + e

Receiver noise power (dBm) −103,2 g = f + 10∗log(3840000)

Receiver interference power (dBm) −103,2 i= 10 ∗log(10∗∗[(g + h)/10 − 10∗∗(g/ 10)]

Total effective noise + interference

Cable loss in the base station (dB) 2 o

Allowed propagation loss for cell

range (dB)

141,9 u = q − r + s − t

CDMA NETWORK PLANNING 281

Table 9.4 Reference link budget of 144-kbps real-time data service (3 km h−1, indoor user covered by outdoor base station, vehicular A type channel, with soft handover)

144-kbps voice service (120 km h−1, in-car)

Transmitter (mobile)

Receiver (base station)

Allowed propagation loss for cell range (dB) 133,8 v = r − s + t − u

It was assumed in Table 9.3 that mobile antenna gain is omnidirectional

The coverage efficiency of WCDMA is defined by the average coverage area per site,

in square kilometers per site, for a predefined reference propagation environment and

supported traffic density From the link budgets above, the cell range R can be readily

calculated for a known propagation model, like those defined in Chapter 8

The propagation model describes the average signal propagation in that environment,and it converts the maximum allowed propagation loss in decibels to the maximum cellrange in kilometers

Table 9.5 Reference link budget of non-real-time 384-kbps real-time data service (3 km h−1,

outdoor user, vehicular A type channel, no soft handover) 384-kbps non-real-time data, no soft handover

Transmitter (mobile)

Receiver (base station)

Allowed propagation loss for cell range (dB) 139,9 v = r − s + t − u

Example – with the fine tuning of (8.45) propagation model for an urban macrocell

with base station antenna height of 30 m, mobile antenna height of 1.5 m and carrierfrequency of 1950 MHz [1–5],

L = 137.4 + 35.2 log10(R) ( 9.27)

L is the path loss in dB and R is the range in kilometer In this case propagation coefficient

n = 3.52 is used For suburban areas an additional area correction factor of 8 dB is used.

L = 129.4 + 35.2 log (R) ( 9.28)

CDMA NETWORK PLANNING 283

So, the cell range of 12.2-kbps speech service with 141.9-dB path loss in Table 9.3

in suburban area would be 2.3 km The range of 144 kbps indoors with parameters from

Table 9.4 would be 1.4 km Once the cell range R is determined, the site area, which is

also a function of the base station sectorization configuration, can then be derived For acell of hexagonal shape covered by an omnidirectional antenna, the coverage area can be

approximated as 2.6 R2

9.2.2 Load factors and spectral efficiency

The second phase consists of estimating the amount of supported traffic per base stationsite When the frequency reuse is 1, the system is typically interference-limited For thispurpose modification of equation (8.12) gives

EbN0

In equation (9.30) the following notation is used W is the chip rate, P j is the receiver

signal power from user j , α j is the activity factor of user j, R j is the bit rate of user j and Itotal is the total receiver wideband power including thermal noise power in the basestation From equation (9.30) we have

The noise rise is defined as

Noise rise= Itotal

and the overall uplink load factor as

cells must be taken into account by the ratio of other-cell to own-cell interference, i:

i= other cell interferenceown cell interference ( 9.36)The uplink load factor now becomes

Eb /N0can be derived from link level simulations and from measurements It includes theeffect of the closed-loop power control and soft handover The effect of soft handover is

measured as the macro diversity combining gain relative to the single-link Eb/N0 result

The other-cell to own (serving)-cell interference ratio i is a function of cell environment or

cell isolation (e.g macro/micro, urban/suburban) and antenna pattern (e.g omni, 3-sector

or 6-sector) The parameters are further explained in Table 9.6

The load equation is commonly used to make a semianalytical prediction of the average

capacity of a WCDMA cell, without going into system-level capacity simulations Thisload equation can be used for the purpose of predicting cell capacity and planning noise

rise in the dimensioning process For a classical all-voice-service network, where all N users in the cell have a low bit rate R, equation (8.15) of Chapter 8 is valid and we have

W

So, the uplink load equation can be approximated and simplified to

ηUL= Eb/N0W/R · N · α · (1 + i) ( 9.39)

By using equation (9.39) in equation (9.34), an example for uplink noise rise is shown

in Figure 9.5 for data service, assuming an Eb/N0 requirement of 1.5 dB and i = 0.65.

The noise rise of 3.0 dB corresponds to a 50% load factor and the noise rise of 6.0 dB