WCDMA Capacity
Sarfraz Alam
WCDMA Overview
• Based on wideband Direct Sequence Code
Division Multiple Access (DS-CDMA)
• Uses 3.84Mcps chip rate that leads to a carrier
bandwidth of approximately 5MHz
• Supports highly variable user data rate
• Supports two basic modes of operation:
frequency division Duplex (FDD) and Time
Division Duplex (TDD)
• Designed to deploy in conjunction with GSM
Factors Affecting the Capacity
• Desired coverage
• Orthogonal variable spreading factor (OVSF)
code limitation
• Soft handover impact
• Adjacent Cell interference
• Target Signal to Noise Ratio (Eb/Nt)
• Traffic to total Power ratio Ec,DPCH / Ior
• etc
Selected Papers
1. ESG Quallcom, “Air Interface Cell Capacity of WCDMA
System”
[http://www.qualcomm.com/common/documents/w
hite_papers/Air_Interface_Cell_Capacity__RevB_new.
pdf]
2. T. Griparis, T. M. Lee, “The Capacity of WCDMA
Network: A CASE STUDY”
[http://www.utdallas.edu/~cpb021000/shared/pdfs/0
000001.pdf]
Mobility Channel & User Traffic for
Capacity Estimation
• Channels
- static channel, Case 1 (3km/hr), Case 2
(3km/hr with random mean power 0dB), Case
3 (120km/hr)
• User Traffic
- Speech: radio bearer 12.2kbps UL/DL
- Packet Data: 64 kbps UL/64, 144kbps DL
Release-99 Cell Capacity
• Assigned dedicated physical channel (DPCH)
for each call to carry traffic
• Cell-capacity: maximum simultaneous DPCHs
assigned within WCDMA cell
Uplink Cell Capacity
• Upper bound of the uplink capacity referred
as pole capacity
• Pole capacity can be estimated using the
standard uplink capacity equation.
W
Rb

Npole 
eq1
Eb * v * 1   
Nt
Uplink Cell Capacity
• From pole capacity, practical
cell capacity can be calculated
using uplink loading factor (η).

Eb
UL
 W
No

* N * v * 1

Rb
 Nuser  Npole * 
Signal of user J
 Eb / No  j  proces sin g gain of user j* total
received power
 Eb / No  j 
Solving for Pj
Pj 
1
1 w
.Itotal
Lj 
1
1 w

 Eb / No  j . Rj .Vj
N
N
j 1
j 1
 Pj  Lj *Itotal
Pj 
Noise Rise 
.... eq 2
Itotal
PN
u sin g eq2
Noise Rise 
Itotal
PN

1
N
1  Lj

1
1UL
j 1
considering int erference ratio  
by putting value ofNpole and η
.... eq 1
 Eb / No  j . Rj .Vj
Pj Lj * Itotal
Itotal
eq 2
w
Pj
.
vj . Rj Itotal  Pj
UL  1 .
N
N
j 1
j 1 1
 Lj  1 .
other cell int erference
own cell int erference
1
w
 Eb / No . Rj .vj
for voice service network wh ere all N user in the same cell have a low bit rate of R
 Nuser  N
W
 Eb / No . R .v
1
Simplified version UL 
 Eb / No * N *v*1 
W
R
Uplink Cell Capacity for Voice Service
• Example:
- Assuming uplink loading factor 75%, and
target Eb/No 5.1dB calculate pole capacity.
using eq2:
Eb
W
No  5.1dB 3.3
R

3.84 _*10 6
314
12.2*10 3
v0.6
1 1 0.6 1.6
u sin g eq 2
0.75 
N 76
3.3*0.6*1.6*N
314
Uplink Cell Capacity for Voice Service
Npole 
W
Eb
Nt
Rb
* v * 1   
eq1
AWGN
Case 1
Case 2
Case 3
Target Eb/Nt
5.1dB
11.9dB
9dB
7.2dB
Pole capacity
101
21
41
62
75% loading capacity
76
16
31
47
60% loading capacity
60
12
24
37
Uplink Cell Capacity and Throughput
for PS 64kbps
W
Rb

Npole is
 measured
eq1
• Cell capacity for PS data
in
terms
Eb * v * 1   
Nt
of the aggregated cell data throughput

TP Nuser * dataRate * 1 BLER

AWGN
Case 1
Case 2
Case 3
Target Eb/Nt
1.5dB
6.2dB
4.3dB
3.4dB
Pole capacity
27
9
14
17
75% loading capacity
20
7
10
13
60% loading capacity
16
5
8
10
Pole Throughput
1.7Mbps
0.6Mbps
0.9Mbps
1.1Mbps
75% loading throughput
1.3Mbps
0.4Mbps
0.7Mbps
0.8Mbps
60% loading throughput
1.0Mbps
0.3Mbps
0.5Mbps
0.7Mbps
Downlink Cell Capacity
• Downlink cell capacity can be estimated using
downlink pole capacity equation.
Npole 
1  oh *W Rb
Eb

* v *   Ioc 
Nt
I or

Downlink Cell Capacity for Voice service
AWGN
Case 1
Case 2
Case 3
Target Eb/Nt
5.1dB
13.4dB
10.8dB
7.8dB
Pole capacity
299
30
32
94
75% loading capacity
239
24
26
76
- Interference factor is assumed 0.3
- voice activity factor is 0.6
- overhead channel power percentage is 25%
- orthogonality factor varies from 0.1(AWGN)
to 0.7(Case2)
Downlink Cell Capacity using Ec,DPCH
Nuser , downlink 
1  oh 
v * Ec , DPCH
Ior
• Expected DPCH power depends on the target
Eb/Nt and power control margin
Downlink Cell Capacity using Ec,DPCH for
Voice Service

 Eb  W Ec, DPCH I or
*
*
 
Ior
Ioc
 Nt  Rb
AWGN
Case 1
Case 2
Case 3
Maximum Ec,DPCH / Ior
-16.6dB
-15.0dB
-7.7dB
-11.8dB
Adjusted Ec,DPCH / Ior
-22.6dB
-11.0dB
-15.7dB
-19.8dB
226
16
46
119
Capacity
Nuser , downlink 
1  oh 
v * Ec , DPCH
Ior
Downlink Cell Capacity using Ec,DPCH for
PS Data Rate 64kbps
AWGN
Case 1
Case 2
Case 3
Maximum Ec,DPCH / Ior
-13.6dB
-13.9dB
-6.4dB
-8.1dB
Adjusted Ec,DPCH / Ior
-19.1dB
-9.9dB
-14.4dB
-16.1dB
61
7
21
31
3.9Mbps
0.5Mbps
1.3Mbps
2.0Mbps
Capacity
Throughput
Downlink Cell Capacity using Ec,DPCH for
PS Data Rate 144kbps
AWGN
Case 1
Case 2
Case 3
Maximum Ec,DPCH / Ior
-9.9dB
-10.6dB
-8.1dB
-9.0dB
Adjusted Ec,DPCH / Ior
-15.9dB
-6.6dB
-10.1dB
-11.0dB
29
3
8
9
4.2Mbps
0.5Mbps
1.1Mbps
1.4Mbps
Capacity
Throughput
• The maximum cell throughput decrease as
DPCH data rates increase foe case2 and case 3
Conclusion
• Eb/Nt plays a vital role in determining the
WCDMA Cell Capacity
• Lower the Eb/Nt, higher the cell capacity
• Loading factor becomes 1, noise rise
approaches to infinity
• Increased in cell geometry by X dB requires
decrease in Ec,DPCH / Ior by X dB
• Capacity is very sensitive to mobility channel