Introduction System Model Metric Numerical Results Conclusion
CDMA vs. OFDM for Wideband Cellular Systems
Yong Peng
Ph.D. candidate, Wireless Networking LaboratoryDept. of Electrical EngineeringSouthern Methodist University
October, 2008
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
1. Motivation
I CDMA and OFDM are both proposed as candidates of thenext generation wireless cellular networks.
I Each of which has its own advantages and disadvantages.I Which is better? Answer should depend on, e.g.
I Metric used for comparisonI Channel conditionsI System complexity, etc..
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
2. The Metric
I Transport Spectral Efficiency (TSE): the distance weightedsum rate of all users in the cell per unit frequency.
I Unit: bit-meter per second per Hertz (bps ·m/Hz).I Motivated by the concepts of spectral efficiency (bps/Hz) and
transport capacity (bps ·m).
I For example: There are two users in one cell, they share1000 Hz bandwidth,
I user 1: rate=100 bps, distance=10 m;I user 2: rate=20 bps, distance=100 m;
TSE =100× 10 + 20× 100
1000= 3 bps ·m/Hz.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
3. Cellular Network Structure
Figure: Structure of the cellular system.Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
4. OFDM System Model
Figure: Cyclic prefix OFDM (CP-OFDM) and zero padding OFDM(ZP-OFDM) system model.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
5. CDMA System Model
Figure: CDMA system model with RAKE (CDMA-RAKE) and LMMSE(CDMA-LMMSE) receivers.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
6. Transport Spectral Efficiency
The TSE of CDMA is calculated as
The TSE of OFDM can be calculated in a similar manner.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
7. CDMA vs. OFDMVariation with number of users
0 10 20 30 40 50 60 7010
0
101
102
103
Number of Users/Cell0 10 20 30 40 50 60
100
101
102
103
TS
ECP−OFDM
ZP−OFDMCDMA−LMMSE
CDMA−RAKE
Figure: Comparison of the TSE with respect to different number of usersper cell.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
8. CDMA vs. OFDMVariation with SNR
60 70 80 90 100 110 120 13010
−1
100
101
102
103
SNR (dB)
TS
E
Flat Fading (CDMA, OFDM)
Multipath Fading (CDMA)
Multipath Fading (OFDM)
Figure: Comparison of the TSE of CP−OFDM and CDMA−RAKE withrespect to different number of multipaths.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
9. CDMA vs. OFDMVariation with SNR (cont.)
70 80 90 100 110 12010
0
101
102
103
SNR (dB)
TS
ECP−OFDM
CDMA−LMMSEZP−OFDM
CDMA−RAKE
Figure: Comparison of the TSE with respect to the input SNR.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
10. CDMA vs. OFDMVariation with cell radius
50 100 150 2000
50
100
150
200
250
Cell Radius
TS
ECP−OFDM
ZP−OFDM
CDMA−LMMSE
CDMA−RAKE
Figure: Comparison of the TSE with respect to the cell radius.
Asilomar ’08 Southern Methodist University
Introduction System Model Metric Numerical Results Conclusion
11. Conclusion
I Transport Spectral Efficiency:I (Flat fading) The TSE of all four schemes are approximately
the same;I (Multipath fading)
CP-OFDM>CDMA-LMMSE≈ZP-OFDMÀCDMA-RAKE;
I Complexity:CDMA-LMMSE≈ZP-OFDMÀCP-OFDM≈CDMA-RAKE;
I Base station density needed to achieve optimal TSE:CDMA-RAKE¿ZP-OFDM≈CDMA-LMMSE<CP-OFDM.
Asilomar ’08 Southern Methodist University
