1. Optimal Combining of STBC and Spatial Multiplexing for MIMO-OFDM
July 2003
Optimal Combining of STBC and Spatial Multiplexing for MIMO-OFDM
Taehyun Jeon, Heejung Yu, and Sok-kyu Lee
Wireless LAN Modem Research Team, ETRI
Jihoon Choi and Yong H. Lee
KAIST
Taehyun Jeon, ETRI

2. Contents Spatial Multiplexing for MIMO Systems
July 2003
Contents
Spatial Multiplexing for MIMO Systems
Detection Methods
Transmit Diversity for MIMO Systems
Transmit Diversity using STBC
Combining of STBC and Spatial Multiplexing
Simulation results
Conclusion
Taehyun Jeon, ETRI

3. Spatial Multiplexing for MIMO Systems
July 2003
Spatial Multiplexing for MIMO Systems
Transmit independent parallel data streams through multiple antennas
Increase the data rate T times faster than SISO
Taehyun Jeon, ETRI

4. Detection of Spatial Multiplexed Signal
July 2003
Detection of Spatial Multiplexed Signal
Maximum Likelihood (ML) Detection
Complexity: ~ LT operation required (L: constellation size)
Linear Detection
V-BLAST Detection
Nulling & Canceling : Successive detection of data streams (layer by layer)
Complexity: ~ O(29T3/3)
Taehyun Jeon, ETRI

5. Spatial Multiplexing for MIMO-OFDM
July 2003
Spatial Multiplexing for MIMO-OFDM
OFDM sub-divides the wideband channel into multiple flat fading subcarriers and can be implemented simpler and more efficient demodulation processing (IFFT/FFT and FEQ) over single carrier systems
MIMO-OFDM with T=R=2: Complexity ~ O(29NT3/3) when V-BLAST detection used
Taehyun Jeon, ETRI

6. Transmit Diversity for MIMO Systems
July 2003
Transmit Diversity for MIMO Systems
Techniques compatible to IEEE a
Delay Diversity and Random Phase Diversity
Simple to implement but not significant performance gain with number of antenna increase
Space-Time Block Coding (STBC)
Simpler implementation than Trellis Coding
Alamouti code provides diversity order 2 with T=2 and R=1
Taehyun Jeon, ETRI

7. July 2003
STBC for MIMO-OFDM
Taehyun Jeon, ETRI

8. N x N MIMO Channels In terms of Spatial Multiplexing
July 2003
N x N MIMO Channels
In terms of Spatial Multiplexing
Data rate increases as number of antenna increases
No significant diversity advantages
In terms of Diversity
Maximum diversity gain can be achieved
Higher order modulation needed to increase the data rate  SNR loss
For best performance for a target data rate, optimal combining of above should be considered
Taehyun Jeon, ETRI

9. Combining of 2-layer Spatial Multiplexing and Alamouti Code
July 2003
Combining of 2-layer Spatial Multiplexing and Alamouti Code
S/P
STBC n x
IFFT M
FFT
STBC Based V -
BLAST
Detection
P/S a b y ^
Tx 1
Tx 2
Tx 3
Tx 4
t=2n
a2n
a2n+1
b2n
b2n+1
t=2n+1
-a*2n+1
-a*2n
-b*2n+1
b*2n
Taehyun Jeon, ETRI

10. Simulation Parameters
July 2003
Simulation Parameters
IEEE a PHY Based Frame
Number of Subcarriers (data subcarriers): 64 (48)
Number of Cyclic Prefix: 16
Sampling Rate: 20MHz
Modulation: QPSK, 16QAM, 256QAM
Number of Tx and Rx Antenna: 2, 4
Channel Coding: None
Channel Model: Independent MIMO Channel
ETSI/BRAN Channel Model B (RMS Delay Spread = 100ns)
Quasi Static Channel (no change within one frame)
Taehyun Jeon, ETRI

11. Simulation Results (T=2,R=2)
July 2003
Simulation Results (T=2,R=2)
Mode 2 (16QAM and Alamouti) performs better than Mode 1 (QPSK and 2-layered)
Diversity gain of order 4 in Mode 2 overcomes the ~4dB degradation of higher order modulation
Taehyun Jeon, ETRI

12. Simulation Results (T=4,R=2)
July 2003
Simulation Results (T=4,R=2)
Mode 2 (16QAM and 4x4 STBC) performs better than Mode 1 (QPSK and 2-layered Alamouti) beyond Eb/No=8dB
Diversity order 4 over 3
Taehyun Jeon, ETRI

13. Simulation Results (T=4,R=4)
July 2003
Simulation Results (T=4,R=4)
Three different combinations are tested
Mode 2 (16QAM and 2-layered Alamouti) performs best beyond 5dB
Mode 1 (QPSK and 4-layered) : Average diversity gain ~2.5
Mode 2 and Mode 3 : diversity gain about the same but 256QAM performs ~8dB worse than 16QAM
Taehyun Jeon, ETRI

14. July 2003
Conclusions
Optimal MIMO-OFDM system should be selected by trading-off the spatial multiplexing and diversity gain for a given antenna arrangement
Candidate combined systems of STBC and spatial multiplexing proposed and their performances compared for extended IEEE a systems
Performance evaluations with realistic MIMO channel model for further work items
Taehyun Jeon, ETRI