Capacity & Interference in 3G

© Tallal Elshabrawy 2

SINR in CDMA Systems: Base Station-to-Mobile (Forward Link)

MT1

MT2

MTj

MTK

Single cell with K active Mobile Terminals

d2

dj

dK

d1

n

0 0 j

j K n

0 0 ji 1i j

P d dSINR

P d d Noise

Effective SINR because of Codes Cross-Correlation (Pseudo-Random Codes)

n n

0 0 j 0 0 j

j eff nn

0 0 j0 0 j

P d d GP d dSINR

1 1 K 1 P d d NoiseK 1 P d d NoiseG G

G is called the processing gain (Tsymbol/Tchip), reflects the decline in perceived interference due to spreading and de-spreadingThe term Noise reflects noise power in the spreaded bandwidthEffective SINR because of Codes Cross-Correlation (Orothgonal Codes)

n n

0 0 j 0 0 j

j eff n

0 0 j

P d d GP d dSINR

1 Noise0 K 1 P d d NoiseG

© Tallal Elshabrawy 3

SINR in CDMA Systems: Mobile-to-Base Station (Reverse Link)

MT1

MT2

MTj

MTK

Single cell with K active Mobile Terminals

d2

dj

dK

d1

n

0 0 j

j Kn

0 0 ii 1i j

P d dSINR

P d d Noise

Effective SINR because of Codes Cross-Correlation (Pseudo-Random Codes)

n n

0 0 j 0 0 j

j eff K Kn n

0 0 i 0 0 ii 1 i 1i j i j

P d d GP d dSINR

1 1P d d Noise P d d Noise

G G

G is called the processing gain (Tsymbol/Tchip), reflects the decline in perceived interference due to spreading and de-spreadingThe term Noise reflects noise power in the spreaded bandwidthEffective SINR because of Codes Cross-Correlation (Orothgonal Codes)

n n

0 0 j 0 0 j

j eff Kn

0 0 ii 1i j

P d d GP d dSI NR

1 Noise0 P d d Noise

G

© Tallal Elshabrawy 4

The Near-Far Effect

n

0 0 11 eff n

0 0 2

GP d dSI NR

P d d Noise

MT1

MT2

d1

d2

n

0 0 22 eff n

0 0 1

GP d dSINR

P d d Noise

The Near-Far EffectSINR2|eff is much worse than SINR1|eff

The strongest received mobile signal may capture the demodulator at the base stationThe Near-Far effect is the resultant of multiple users using the same transmit power level to communicate with the base station over the same frequency and in the same time within the same cell.

Mobile-to-Base Station Communication (Uplink or Reverse Link)

© Tallal Elshabrawy 5

Strength-Based Power ControlTo address the Near-Far Effect

Control the transmission power of each mobile terminal such that it is received at the base station at an equal level S

j eff

GSSINR

K 1 S Noise

Mobile-to-Base Station Communication (Uplink or Reverse Link)

nn n

0 0 1 0 0 2 0 0 jP d d P d d P d d S

Strength-Based Power Control

MT1

MT2

MTj

MTK

d2

dj

dK

d1

© Tallal Elshabrawy 6

Capacity of CDMA Cellular Systems

Assume interference from first tier (ring) of co-channel interferers

XRD

DD

D

DD

Di: interfering distance from ith co-channel interferenceNB No. of co-channel interfering sites

n

0 0n n

0 0 0 0

n

thn

n

th

thn

th

2

GP d RSI R

K 1 P d R 6KP d D

GSI R

K 1 6K R D

GSI R

K 1 6K Q

GK 1 6K Q

SI R

G1

SI RK

6 Q 1

G1

SI RFor n=4,Q= 3N K

2N 1

3

K: Number of Channels (i.e., Codes) per Cell

N: Cluster Size

Assume Forward Link and Ignore Noise

© Tallal Elshabrawy 7

Diversity in 2G/3G CDMA Systems

Multi-Path resistant RAKE Receiver can collect energy spread by the small-scale channel Suitable for bursty applications No need for frequency planning (frequency reuse of one) Soft blocking and soft handoff

TSymbol=X

Frequency- Selective Channel

τ2

∑

TChip=X/G

τ1 τ2 τNp

τNp

m(t)

c(t)

s(t)

r(t-tpd-τ2)

r(t-tpd-τNp)

w1

w2

wNp

r(t-tpd-τ1)

τ1