Capacity & Interference in 3G
SINR in CDMA Systems: Base Station-to-Mobile
(Forward Link)
SINR j 

P0 d0 dj
K
P d
i1
i j
0
0
dj

n

n
dj
 Noise
Effective SINR because of Codes CrossCorrelation (Pseudo-Random Codes)
SINR j
eff


P0 d0 dj
1
K  1 P0 d0 dj
G



n
n
1
 Noise
G


GP0 d0 dj
K  1 P0  d0
dj

n
dK

MT2


0  K  1 P  d

d
eff
MT1
0
© Tallal Elshabrawy
0
n
n
j
1
 Noise
G
MTK
 Noise
Effective SINR because of Codes CrossCorrelation (Orothgonal Codes)
P0 d0 dj
d1
n
G is called the processing gain (Tsymbol/Tchip), reflects the decline in
perceived interference due to spreading and de-spreading
The term Noise reflects noise power in the spreaded bandwidth
SINR j
MTj
d2


GP0 d0 dj

Single cell with K active
Mobile Terminals
n
Noise
2
SINR in CDMA Systems: Mobile-to-Base Station
(Reverse Link)
SINR j 

P0 d0 dj
K
 P d
0
i1
i j
0

n
di   Noise
dj
n
Effective SINR because of Codes CrossCorrelation (Pseudo-Random Codes)
SINR j
eff


P0 d0 dj


n
n
1 K
1
P0  d0 di   Noise

G i1
G
i j

GP0 d0 dj
K
 P d
i1
i j
0
dK

n
di   Noise
0
Effective SINR because of Codes CrossCorrelation (Orothgonal Codes)
eff


P0 d0 dj
K

n
0   P0  d0 di  
i1
i j
© Tallal Elshabrawy
n
1
Noise
G

MT2
d1
MTK
n
G is called the processing gain (Tsymbol/Tchip), reflects the decline in
perceived interference due to spreading and de-spreading
The term Noise reflects noise power in the spreaded bandwidth
SINR j
MTj
d2

GP0 d0 dj

MT1
Single cell with K active
Mobile Terminals
n
Noise
3
The Near-Far Effect
Mobile-to-Base Station Communication
(Uplink or Reverse Link)
d1
GP0  d0 d1 
n
SINR1
eff

P0  d0 d2   Noise
MT1
n
d2
GP0  d0 d2 
n
SINR2
eff

P0  d0 d1   Noise
n
MT2
The Near-Far Effect
SINR2|eff is much worse than SINR1|eff
The strongest received mobile signal may capture the demodulator at
the base station
The 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.
© Tallal Elshabrawy
4
Strength-Based Power Control
To 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
dj
dK
Mobile-to-Base Station Communication
(Uplink or Reverse Link)
Strength-Based Power Control

P0  d0 d1   P0  d0 d2   P0 d0 dj
n
SINR j
© Tallal Elshabrawy
eff
n

MTj
d2

n
MT2
d1
MTK
MT1
S
GS
K  1 S  Noise
5
Capacity of CDMA Cellular Systems
Assume Forward Link and Ignore Noise
 SIR 
 SIR 

GP0  d 0 R 
K  1 P0  d 0
n
R   6KP0  d 0 D 
n
n
G
K  1  6K R D 
G
K  1  6K  Q 
n
D
n
 SIR th
G
n
 K  1  6K  Q 

SIR th
G


 SIR  1 
th

K
n
 6  Q  1 





 G
 SIR  1 
th

For n=4,Q= 3N  K  
2
 N2  1 

 3


© Tallal Elshabrawy
Assume interference from first tier (ring)
of co-channel interferers
D
X
D
R
D
D
D
Di: interfering distance from ith
co-channel interference
NB No. of co-channel interfering
sites
K: Number of Channels (i.e., Codes) per Cell
N: Cluster Size
6
Diversity in 2G/3G CDMA Systems
w1
r(t-tpd-τ1)
τ1
TSymbol=X
m(t)
s(t)
TChip=X/G





∑
τ2
τ1 τ2
c(t)
w2
r(t-tpd-τ2)
τNp
FrequencySelective
Channel
wNp
r(t-tpd-τNp)
τNp
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
© Tallal Elshabrawy
7