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

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

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

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

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

Transmit Diversity for MIMO Systems July 2003 Transmit Diversity for MIMO Systems Techniques compatible to IEEE 802.11a 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

July 2003 STBC for MIMO-OFDM Taehyun Jeon, ETRI

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

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

Simulation Parameters July 2003 Simulation Parameters IEEE 802.11a 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

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

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

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

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 802.11a systems Performance evaluations with realistic MIMO channel model for further work items Taehyun Jeon, ETRI