1. Co-Channel Inteference in 802.11n Networks
S. Aon Mujtaba, Xiaowen Wang
Agere Systems

2. “Moore’s Law” for 802.11 Throughput Enhancement
Max PHY Layer
Point-to-Point
Throughput
250Mbps?
54Mbps
~5x
11Mbps
4.9x
2Mbps
5.5x 1G 2G 3G 4G

3. Options for Throughput Enchancement
assume that n is based on an evolution of a
Technique
Example
Throughput Scaling Factor
Bandwidth Expansion
20MHz  40MHz
Greater than 2x with tone filling in the guard band
Spatial Multiplexing
2,3, or 4 transmit antennas
2x, 3x, or 4x
Increase Puncturing Rate
¾  7/8
1.167x
Increase constellation size
64QAM  256QAM
1.33x
Reduce Guard Interval Overhead
(0.8us+3.2us)  (0.4us+6.4us)
1.17x

4. Let’s consider two System Architectures
System B
Target a max PHY throughput greater than 200Mbps
Constrain BW to 20MHz
introduce 4 transmit antennas for spatial multiplexing
Assume 4 receive antennas
64QAM, R=3/4, GI=0.8us, 64-point FFT
Achieve: 216Mbps
System A: “4x4x20”
Target a max PHY throughput greater than 200Mbps
Enhance BW to 40MHz
introduce 2 transmit antennas for spatial multiplexing
Assume 2 receive antennas
64-QAM, R=3/4, GI=0.8us, 64-point FFT
Achieve: 243Mbps
System B: “2x2x40”

5. Link Level Simulation Setup
Channel Model = exponentially decaying power delay profile
RMS delays spreads of 50ns, 100ns, 150ns
Uncorrelated antennas
Constellation size = 64-QAM
R=3/4 (.11a encoding, puncturing, and interleaving)
Same transmit power for System A and System B
Signal PSD for 40MHz is ½ that of 20MHz
Frequency offset = 100kHz
No phase noise impairment
1000 Byte packets
Spatial Multiplexing (no transmitter pre-coding)
Soft-output Zero-Forcing Receiver

6. Simulation Results (50ns RMS delay spread)26 28 30 32 34 36 38 1
0.1
4x4x20MHz
PER
2x2x40MHz
5dB
0.01
0.001
SNR (dB)

7. Simulation Results (100ns RMS delay spread)26 28 30 32 34 36 38 1
4x4x20MHz
PER
0.1
2x2x40MHz
0.01
SNR
4dB

8. Simulation Results (150ns RMS Delay Spread)28 30 32 34 36 38 40 42 1
4x4x20MHz
PER
0.1
2x2x40MHz
0.01
SNR (dB)

9. Observations
The 4x4 system has more self-interference than the 2x2 system
Hence, the need for higher SNR for decoding purposes
Increasing the BW from 20 to 40MHz does not degrade Eb/No:
Both data rate and BW increase by roughly 2x
However, SNR does reduce by 3dB for same transmit power
4x4 systems is very sensitive to ICI  hence, the 4x4 system fails at 150ns RMS delay spread
As the RMS delay spread increases from 50ns to 100ns, the benefit of 40MHz BW reduces by 1dB (at 1% PER)

10. Comments
In an isolated cell, a 2x2x40 system requires less decoding SNR than a 4x4x20 system
However, increasing the bandwidth reduces the number of channels available for frequency re-use
possibility for higher co-channel interference

11. Reuse=1/7 in 5GHz (BW=20MHz)
With 20MHz channelization, there are sufficient number of channels available in 5GHz to achieve the classical 1/7 reuse pattern, as shown: 2 2 7 3
Number of interferers in the 1st ring = 6 7 3 1 1 6 4 6 4 5 5 d

12. Reuse=1/4 in 5GHz (BW=40MHz)
With 40MHz channelization, let’s assume that the number of usable channels in 5GHz drops to 4. 4 4
Number of interferers in the 1st ring = 8 2 2 1 1 3 3 d

13. Modeling assumptions for calculating CCI
While co-channel interference (CCI) affects the entire cell, for sake of simplicity, we only consider the interference from Access Point to Access Point.
We assume that all Access Points transmit the same power
In going from 20MHz to 40MHz operation, we assume that the radius of the cells does not have to change since the SNR requried for 6Mbps reception is the same for System A and B.
I(r) r
CCI AP AP
STA

14. Translating distance to CCI
Power Level
20dB/decade
16dBm
Free
Space
33dB/decade
Break
Point r
I(r)
CCI AP AP

15. Translating CCI to SNR degradation

16. SNR degradation as a function of Re-Use
Cell Radius
SINRdB(1/7) – SINRdB(1/4)
20m
5.7 dB
30m
5.66 dB
40m
5.59 dB
50m
5.47 dB

17. Conclusions
In an isolated cell, bandwidth expansion coupled with spatial multiplexing provides a more robust path to throughput enhancement
2x2x40MHz is ~5dB more robust than 4x4x20MHz at 1% PER
In a multi-cellular deployment:
BW expansion increases Co-Channel interference
SNR degradation due to CCI increase is balanced out by increase in link robustness
“comparable” capacity of 2x2x40 and 4x4x20 systems