1. Multiple Antenna OFDM solutions for enhanced PHY
March 2004
doc.: IEEE /239r0
March 2004
Multiple Antenna OFDM solutions for enhanced PHY
Presented by: Alexandre Ribeiro Dias
Contributors: A. Ribeiro Dias, Stéphanie Rouquette-Léveil, Sébastien Simoens
Motorola Labs Paris
+33 (1)
Alexandre Ribeiro Dias - Motorola Labs
Colin Lanzl, Aware

2. Presentation Outline Introduction Multiple antenna techniques
March 2004
Presentation Outline
Introduction
Multiple antenna techniques
Multiple antenna PHY example
Alternative OFDM modulators
PHY simulation results
MAC simulation results
Possible MAC improvements
Conclusion
Alexandre Ribeiro Dias - Motorola Labs

3. Introduction (1/2) PAR requirements: Increase PHY performance (1/2):
March 2004
Introduction (1/2)
PAR requirements:
Define modifications to both PHY and MAC so that a maximum throughput of at least 100Mbps at the MAC SAP is enabled
Increase PHY performance (1/2):
If same bandwidth (proposed functional requirement) multiple antenna techniques are required to increase the peak data rate with good coverage (add advanced coding schemes?)
Which multiple antennas techniques should be used?
How many antennas can be considered?
Alexandre Ribeiro Dias - Motorola Labs

4. Introduction (2/2) Increase PHY data rate (2/2):
March 2004
Introduction (2/2)
Increase PHY data rate (2/2):
How does « 100Mbps at the MAC SAP » translates in terms of PHY data rate requirements ( depends on MAC efficiency  depends on MAC amendment)?
Increase MAC SAP goodput:
How high can the throughput be with an enhanced PHY and or e MACs?
How much can this efficiency be increased with backward compatibility constraints ?
Alexandre Ribeiro Dias - Motorola Labs

5. Multiple antenna techniques (1/5)
March 2004
Multiple antenna techniques (1/5)
How can multiple antennas at the AP can be used to improve performance of legacy STAs?
Partial channel state information, Closed-Loop (CL) technique UL
Maximum
Ratio
Combining
(MRC)
Transmit
Selection
(TS, per
subcarrier,
per antenna) DL
Alexandre Ribeiro Dias - Motorola Labs

6. Multiple antenna techniques (2/5)
March 2004
Multiple antenna techniques (2/5)
Space Time Block Codes (STBC) to benefit from spatial diversity (with MRC)
Increase communication reliability/coverage
Not optimal for high data rates
Alexandre Ribeiro Dias - Motorola Labs

7. Multiple antenna techniques (3/5)
March 2004
Multiple antenna techniques (3/5)
Spatial Division Multiplexing (SDM):
Data rate multiplied by number of transmit antennas
Transmit diversity not exploited
In general (depending on decoder) Nr  Nt
ML, ZF,
MMSE,
SIC…
based
receivers …
S/P
Alexandre Ribeiro Dias - Motorola Labs

8. Multiple antenna techniques (4/5)
March 2004
Multiple antenna techniques (4/5)
Hybrid schemes: increase data rate and exploit transmit diversity for higher robustness/range
SDM combined with STBCs (Open Loop OL)
S/P
SDM combined with TS (Closed-Loop CL)
Transmit
Selection
(TS, per
subcarrier,
per antenna)
S/P
N<Nt data streams
Alexandre Ribeiro Dias - Motorola Labs

9. Multiple antenna techniques (5/5)
March 2004
Multiple antenna techniques (5/5)
Open-Loop vs Closed-Loop?
CL techniques known to provide substantial gain but how CSI can be obtained at the TX?
Reciprocity assumption in TDD systems?
Are RF impairments identical on UL and DL?
Delay between UL and DL needs to be controlled
Feedback link?
Needs to be implemented and additional overhead…
Alexandre Ribeiro Dias - Motorola Labs

10. Multiple antenna PHY example
March 2004
Multiple antenna PHY example
How many antennas should be used?
Antenna configuration should depend on STA size (laptop, mobile handset)
Ex: fixed number of antennas at the AP, several number of antennas (hence multiple antenna techniques) at the STA depending on dimensions/legacy
AP 4 antennas, STA 1 antenna (legacy)
AP 4 antennas, STA 2 antennas (mobile handset)
AP 4 antennas, STA 4 antennas (laptop)
Techniques
MRC, TS?
STBC, SDM, Hybrid?
Modes
xQAM+code rate?
Alexandre Ribeiro Dias - Motorola Labs

11. Alternative OFDM modulators
March 2004
Alternative OFDM modulators
Cyclic Prefix OFDM (CP-OFDM) is an interesting option with simple equalisation scheme:
BUT:
Zero Padded OFDM (ZP-OFDM, CP replaced by zeros) solves the sensibility to channel zeros locations
Pseudo-Random Postfix OFDM (PRP-OFDM, CP replaced by pseudo-randomly weighted known postfix):
Keeps all advantages of ZP-OFDM
Low complexity channel estimation and tracking possible
Support for increased mobility (longer packets?)
Alternative OFDM modulators worth being considered
Alexandre Ribeiro Dias - Motorola Labs

12. PHY simulation results (1/3)
March 2004
PHY simulation results (1/3)
Simulation parameters:
256QAM investigated to increase spectral efficiency
NLOS TGn D channels (no spatial correlation)
Packet size: 512Bytes, 23dBm transmit power
ZF receivers when SDM-based techniques
All IEEE802.11a functional blocks are kept:
Multiple
Antenna
System
Processing
mapping
interleaver
coder
scrambler
OFDM modulation
IEEE802.11a block
New block
Legend:
Alexandre Ribeiro Dias - Motorola Labs

13. PHY simulation results (2/3)
March 2004
PHY simulation results (2/3)
Use of STBCs for increased reliability:
Important range increase, even with legacy STAs
Alexandre Ribeiro Dias - Motorola Labs

14. PHY simulation results (3/3)
March 2004
PHY simulation results (3/3)
Hybrid schemes for reliability and high data rate?
Hybrid schemes: range increase not very important, however reduces constraints on SNR requirements
Alexandre Ribeiro Dias - Motorola Labs

15. MAC simulation results
March 2004
MAC simulation results
Simulation Assumptions:
.11e MAC with Group Acknowledgement
1500 byte payload
16 MPDU per Group
Simulation Results:
Only 100 Mbit/s effective vs 162 Mbit/s theoretical with 3 streams and 64 QAM
MAC Efficiency is of only 54% for the 120 Mbit/s mode
.11e with Group Ack is too inefficient !
Alexandre Ribeiro Dias - Motorola Labs

16. Possible MAC improvements
March 2004
Possible MAC improvements
Need to increase the PHY burst size by aggregating numerous packets (without SIFS between each packet)
Keep individual FCS per packet for SR-ARQ
Possibility to rely on the Contention Free Period of .11e to introduce more efficient TDD/TDMA MAC
Alexandre Ribeiro Dias - Motorola Labs

17. Conclusion .11e MAC not efficient with high data rate PHY
March 2004
Conclusion
.11e MAC not efficient with high data rate PHY
With enhanced MAC, PHY data rate of 162Mbps might be sufficient (which is the best solution?)
Antenna configurations should depend on STA size
Number or antennas/techniques used should be chosen to reduce constraints on SNR requirements and keep reasonable range
Alexandre Ribeiro Dias - Motorola Labs