1. Multiple Antennas: Performance Gains with Channel Measurements
May 2003
Multiple Antennas: Performance Gains with Channel Measurements
Minnie Ho, Qinghua Li, Sumeet Sandhu
Cliff Prettie, David Cheung
Minnie Ho, Intel Corporation

2. Performance Metrics of Interest
May 2003
doc.: IEEE /279r0
May 2003
Performance Metrics of Interest
Capacity Gain
Definition: bits per second per Hz
Implication:
Efficiency in use of bandwidth
Increase in data rate for given bandwidth
Diversity Order
Definition: slope of BER curve vs. SNR
Increase in SNR at a fixed BER
Can translate to increase in range
Alamouti code (diversity order = 2)
Slope is 2
In this talk, we consider two performance metrics of interest. There are many others relevant to multiple antenna systems, such as degrees of freedom, coding gain, array gain, etc. Here, we focus on capacity and diversity order.
The plot shows an increase in diversity order using the Alamouti code.
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

3. Theoretical Gains are Well-Known
May 2003
Theoretical Gains are Well-Known
Shannon Capacity [Telatar]
Mr receive antennas
Mt transmit antennas
Capacity is linear with min(Mr, Mt)
Diversity Order
Maximum diversity order = Mr x Mt
ZF receiver [Winters]
Mr + Mt receive antennas
Mr-1 (users) interferers can be nulled out
Mt+1 diversity order
Questions:
How can we apply these results to real systems?
What are the gains with measured channels?
Minnie Ho, Intel Corporation

4. Techniques for Increasing Throughput
May 2003
Techniques for Increasing Throughput
Point-to-point MIMO
Increases maximum data rate between a user and an AP (PHY rate)
Improves spectral efficiency (bits/sec/Hz)
Point-to-multipoint SDMA
Simultaneous transmission from AP to multiple users
Increases throughput at the MAC SAP at an AP
SDMA
MIMO
Minnie Ho, Intel Corporation

5. Techniques for Increasing Diversity
May 2003
Techniques for Increasing Diversity
Switched diversity (selection between multiple antennas)
MRC
Alamouti
MMSE (minimum mean-squared error)
ZF (zero-forcing)
ML (maximum likelihood)
Minnie Ho, Intel Corporation

6. Measurement environment: Enterprise (office cubicles)
May 2003
Measurement environment:
Enterprise (office cubicles)
Stationary (night)
Automated positions
WLAN “AP”
WLAN “STA”
Minnie Ho, Intel Corporation

7. May 2003
Measurement Setup
At a given tone, the complex channel response from 1 transmit location to 1 receive location is measured
RF component responses compensated from measurements
Other frequency bands measured (UWB, full UNII, ISM)
750,000+ measurements in searchable database
Minnie Ho, Intel Corporation

8. Propagation Snapshot at 3.5 m
May 2003
doc.: IEEE /279r0
May 2003
Propagation Snapshot at 3.5 m
The second is the same movie but cut to 17 to 22 ns in time (takes 5 seconds to watch in real-time).
These are probably the most visually appealing movies we have – nice distinctive wavefront arrivals from a few different directions.  Some things you can note as you show the movie: the movies were generated from data collected over 4 STA (desktop) positions and 37x37 AP (ceiling) positions in the 2-8 GHz band at a distance of about 3.5m, in SC12, 3rd floor (cubicle environment).  The 4 STA positions are at the corners of a 6” square.  The 37x37 AP positions are spaced over a grid with 0.5” spacing (total travel of 1.5’x1.5’).  The movie consists of 5476 actual vector network analyzer measurements (4*37*37) taken using an automated system (robotic positioners, etc.).  To generate the movie we took the inverse Fourier transform of each measurement (resulting in the impulse response at each point).  Each frame of the movie is a slice in time of all the impulse responses, the colors representing the intensity of the energy (in volts) at that point on the array. 
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

9. Theoretical Capacity Results
May 2003
Theoretical Capacity Results
SDMA
4 antennas at the AP (Mr = 4)
1 antenna at each STA (Mt = 1)
Four “degrees of freedom”
AP should be able to support 4 simultaneous users
MIMO
2 antennas at the AP (Mr = 2)
2 antennas at the STA (Mt = 2)
Link should be able to support 2x the data rate
Minnie Ho, Intel Corporation

10. Measurement Locations and Grid
May 2003
Measurement Locations and Grid
12m 5m 7m 9m 3m
25m
14m
18m
13m
Minnie Ho, Intel Corporation

11. Measured Capacity: Downlink
May 2003
doc.: IEEE /279r0
May 2003
Measured Capacity: Downlink
SDMA increases the capacity of SISO between 3 and 4 times
The capacity of zero-forcing is close the maximum (coordination)
The capacity of zero-forcing is greater than 275 Mbps with a 90% probability.
Three 54 Mbps links with 62% MAC efficiency delivers 100 Mbps network throughput at AP.
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

12. Measured Capacity: Uplink
May 2003
doc.: IEEE /279r0
May 2003
Measured Capacity: Uplink
SDMA increases the capacity of SISO between 3 and 4 times.
MMSE technique reduces capacity and suffers from noise amplification and MAI.
The capacity of zero-forcing is greater than 275 Mbps with probability 90%.
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

13. Theoretical Diversity Gain
May 2003
Theoretical Diversity Gain
Theoretical
Maximum diversity order = Mr x Mt
Diversity order for ZF receiver = Mr – Mt + 1 [Winters et al]
SDMA
4 antennas at the AP (Mr = 4)
1 antenna at each of 3 STAs (Mt = 3)
ZF Diversity order should be 2 for each link
MIMO
2 antennas at the AP (Mr = 2)
2 antennas at the STA (Mt = 2)
MMSE Diversity order should be about 1
Minnie Ho, Intel Corporation

14. Measured Diversity: Downlink
May 2003
doc.: IEEE /279r0
May 2003
Measured Diversity: Downlink
4-antenna diversity gain compared to SISO is 1.6
5-antenna diversity gain compared to SISO is 2.6
The diversity gain for SDMA with 4 antennas vs SISO is 7.2/4.5 in downlink. 
2)       The diversity gain for SDMA with 5 antennas vs SISO is 7.2/2.8 in downlink. 
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

15. Measured Diversity: Uplink
May 2003
doc.: IEEE /279r0
May 2003
Measured Diversity: Uplink
4-antenna diversity gain compared to SISO is 1.5
5-antenna diversity gain compared to SISO is 2.0
3)       The diversity gain for SDMA with 4 antennas vs SISO is 7.2/4.8 in uplink. 
4)       The diversity gain for SDMA with 5 antennas vs SISO is 7.2/3.5 in uplink. 
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

16. Theoretical Capacity Results
May 2003
Theoretical Capacity Results
SDMA
4 antennas at the AP (Mr = 4)
1 antenna at each STA (Mt = 1)
AP should be able to support 4 simultaneous users
MIMO
2 antennas at the AP (Mr = 2)
2 antennas at the STA (Mt = 2)
Link should be able to support 2x the data rate
Minnie Ho, Intel Corporation

17. Measurement Locations and Grid
May 2003
Measurement Locations and Grid
12m 5m 7m 9m 3m
25m
14m
18m
13m
Minnie Ho, Intel Corporation

18. May 2003
Measured Capacity
Minnie Ho, Intel Corporation

19. May 2003
Minnie Ho, Intel Corporation

20. Theoretical Diversity Gain
May 2003
Theoretical Diversity Gain
Theoretical
Maximum diversity order = Mr x Mt
ZF Diversity order = Mr – Mt + 1 [Winters et al]
SDMA
4 antennas at the AP (Mr = 4)
1 antenna at each of 3 STAs (Mt = 3)
ZF Diversity order should be 2 for each link
MIMO
2 antennas at the AP (Mr = 2)
2 antennas at the STA (Mt = 2)
MMSE Diversity order should be about 1
Minnie Ho, Intel Corporation

21. Measured Diversity Diversity is of order 1 for uncoded MMSE
May 2003
doc.: IEEE /279r0
May 2003
Measured Diversity
Diversity is of order 1 for uncoded MMSE
Diversity is the same order as SISO
Coded gives more diversity (frequency diversity). The code is the standard a code with interleaving.
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

22. Conclusions Measured capacity less than (close to) theoretical
May 2003
doc.: IEEE /279r0
May 2003
Conclusions
Technique
Theoretical
Capacity
Measured
Diversity
Measured Diversity
Point-to-Point MIMO
(w/o channel knowledge)
Linear with number of antennas
Linear
(2x with 2x2 system)
Mr-Mt+1
Mr-Mt+1 (order 1 with 2x2 system with MMSE)
Point-to-Multipoint SDMA
Sub-linear
(3 users with 4 antennas)
(2 for 4 antennas, 3 users)
Down: 1.6
Up: 1.5
Measured capacity less than (close to) theoretical
Measured diversity less than (close to) theoretical
Practical system design is the challenge
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

23. For further information
May 2003
doc.: IEEE /279r0
May 2003
For further information
Send to Minnie Ho
To:
Subject: SmantIntel
For further information, send to Minnie Ho.
In the subject line, please include the following word: SmantIntel
Minnie Ho, Intel Corporation
Minnie Ho, Intel Corporation

24. May 2003
Backup Slides
Minnie Ho, Intel Corporation

25. BER Comparison for SDMA
May 2003
Downlink and BER 10-5
SDMA with 4 antennas requires 1ower SNR than SISO and SISO with selection in 86% and 74% of the time respectively.
SDMA with 5 antennas requires 1ower SNR than SISO and SISO with selection in 97% and 92% of the time respectively.
Uplink and BER 10-5
SDMA with 4 antennas requires 1ower SNR than SISO and SISO with selection in 65% and 50% of the time respectively.
SDMA with 5 antennas requires 1ower SNR than SISO and SISO with selection in 81% and 71% of the time respectively.
Minnie Ho, Intel Corporation