Performance Evaluation of Multi-Layered Space Frequency Time Codes for MIMO-OFDM Systems Dr. Samir Al-Ghadhban Assistant Prof. EE Dept, KFUPM, Saudi Arabia http://faculty.kfupm.edu.sa/ee/samir NCTT-MCP08 Aug, 2008
Outline • Background and motivation • IQ-Space Frequency Time codes • Multi-Layered STBC vs VBLAST • Multi-Layered SFT Codes NCTT-MCP08 Alghadhban 2
Introduction: Multiple Input Multiple Output (MIMO) Channels 1 1 • A MIMO channel is a wireless link between M T 2 2 transmit and M R receive antennas. Scatterers Tx Rx • MIMO channels boost the information capacity M M T R of wireless systems by order of magnitude [Telater95][Foschini98]. h t ( ) … h ( ) t 11 1 M T = H ( ) t � � � h ( ) t � h ( ) t M 1 M M R R T NCTT-MCP08 Alghadhban 3
Introduction: Open Loop MIMO Communication Systems Open Loop MIMO Communication Systems Transmit Spatial Diversity Multiplexing Differential Block Coding Block Coding Trellis Coding [Tar98] D-BLAST [Fos96] V-BLAST [Wal99] [Tar00] [Ala98][Tar99a] B 1 � B 1 B 2 � STBC B STBC � 2 B B 1 Combiner and 1 G V-BLAST 1 Detector Detector � B − K 1 B K-1 � B MIMO Fading K Channel NCTT-MCP08 Alghadhban 4 MIMO Fading B Channel K
Multi-layered STBC is a single user system that consists of K parallel STBC • It combines spatial STBC B 1 G 1 multiplexing with � B 1 transmit diversity. SGINC • It is a V-BLAST Detector system with STBC on � B K each layer. STBC B K G K MIMO Fading Channel NCTT-MCP08 Alghadhban 5
How does MLSTBC compare to V-BLAST and STBC? STBC V-BLAST B 1 � B B 1 2 � B 2 STBC STBC � B B 1 Combiner and 1 V-BLAST G 1 Detector Detector � B − K 1 MIMO Fading B K-1 � MLSTBC B Channel K MIMO Fading B Channel K STBC B 1 G 1 � B 1 SGINC Detector � B K STBC B K G K MIMO Fading Channel NCTT-MCP08 Alghadhban 6
Comparison of MLSTBC and V-BLAST over 4x4 MIMO-OFDM, N c =64 and L =4 0 0 0 10 10 10 V-BLAST-OFDM BPSK V-BLAST-OFDM QPSK V-BLAST-OFDM 16QAM 1/2 rate STBC-OFDM 256QAM MLSTBC-OFDM 16QAM MLSTBC-OFDM 256QAM MLSTBC-OFDM QPSK -1 10 -1 -1 10 10 -2 10 -2 -2 10 10 OFDM SER OFDM SER OFDM SER -3 10 -3 -3 10 10 -4 10 -4 -4 10 10 -5 10 -5 -5 -6 10 10 10 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 E s /N 0 E s /N 0 E s /N 0 NCTT-MCP08 Alghadhban 7
Motivation • Pervious work on MLSTBC over MIMO- OFDM systems didn’t take advantage of the available frequency diversity. • Our Goal is to design MLSTBC system that takes full frequency diversity advantage over MIMO-OFDM channels. • The solution is to add space frequency time (SFT) codes at each layer. NCTT-MCP08 Alghadhban 8
Design criteria of SFT codes • The maximum diversity available in MIMO-OFDM systems is M T LM R [Ben Lu 2000] . • The design criterion is to maximize the minimum effective length and break up channel correlation in frequency domain by interleaving. • To achieve this diversity, the minimum effective length of the SFT code should be equal to at least M T L , which needs large number of states for practical values. • For example, at M T =2 and L=3, we need 1024 states. And at L=4, we need 16384 states NCTT-MCP08 Alghadhban 9
Design criteria of SFT codes • Our goal is to simplify the design and reduce the number of states required to achieve the full spatial and frequency diversity. • Our approach is based on concatenating trellis coded modulation (TCM) and space time block codes (STBC). [Lateif 2003] • Spatial diversity is guaranteed by STBC and frequency diversity is provided by TCM. • We further reduce the number of states of TCM by using IQ-TCM [AlSemari 97]. NCTT-MCP08 Alghadhban 10
IQ-TCM [AlSemari97] • The minimum effective length of • Thus, when k is reduced by TCM is upper bounded by: a half, l min at most doubles and this is the reason behind the diversity increase of IQ- ≤ + l v k / 1 min TCM. Where v is the number of memory elements and k is the number of inputs. 8-states 4AM-TCM 3 -1 -1 3 In: Interleaver -3 -1 1 3 De: De-interleaver [ ] s ∈ − -3 1 1 3 1 -3 1 bps/Hz I I-TCM rate h* 1/2 � -3 1 s 4 -A M S ig n a l S e t { } h s ∈ I 1 6 - Q A M I-Dec -1 3 + In × De � s Q 3 -1 1 bps/Hz Q-Dec Q-TCM rate j 1/2 -3 1 [ ] s ∈ − -3 1 1 3 2 bps/Hz IQ-16QAM-TCM Q 1 -3 NCTT-MCP08 Alghadhban 11
2 bps/Hz Comparison • 8-states 8PSK-TCM: v =3, k =2 � l min =2 ≤ + l v k / 1 min • 8-states IQ-16QAM-TCM: v =3, k =1 � l min =4 8-states 4AM-TCM 3 -1 -1 3 In: Interleaver -3 -1 1 3 De: De-interleaver [ ] s ∈ − -3 1 1 3 1 -3 1 bps/Hz I I-TCM rate h* 1/2 � -3 1 s 4 -A M S ig n a l S e t { } h s ∈ I 1 6 - Q A M I-Dec -1 3 + In × De � s Q 3 -1 1 bps/Hz Q-Dec Q-TCM rate j 1/2 -3 1 [ ] s ∈ − -3 1 1 3 2 bps/Hz IQ-16QAM-TCM Q 1 -3 NCTT-MCP08 Alghadhban 12
IQ-SFT Alamouti STBC Code In= Interleaver P/S= Parallel to Time Time CP= Cyclic Prefix Serial converter b 2 1 s 1, I I-TCM 1, I 1 − * s s s 2 2,1 1,1 − s * 1 s + In − = 1,2 s * = IFFT P/S 2,2 s CP s b 1 � 2 � 1, Q 1, Q Q-TCM j − s s * 1, N N c c 2, Nc Encoder STBC b s 1 s * 2, I I-TCM s 2, I 2 2,1 1,1 s * s s = 2,2 IFFT P/S s * = s CP 2 1,2 + In 2 � 1 � b s s * 2, Q 2, Q s Q-TCM j 2, N N c 1, Nc c I-Dec (R e) Tim e 2 Tim e 1 1 s � 1 Q -Dec (Im ) 2 t STBC y 1 t y Decoder 2 1,1 1,1 Rem Com biner at t y S/P FFT t 1 De y 2 t = 1,2 I-Dec Y CP each = 1,2 1 (R e) t Y 2 1 � 1 � subcarrier � s t y 2 N t 1 Q -Dec y 2 (Im ) 1, L c 1, L NCTT-MCP08 Alghadhban 13
Advantages of concatenated IQ-TCM-STBC at 2bps/Hz FCS Minimum number of states to achieve full diversity Length ( M T LM R ) L Tarokh STTC 8PSK-STBC IQ-16QAM-STBC QPSK 2 64 4 2 3 1024 16 4 4 16384 64 8 5 262144 256 16 6 4194304 1024 32 7 67108864 4096 64 NCTT-MCP08 Alghadhban 14
The discrete received signal over T time slots at the i th subcarrier is M R : total number of receive antennas N G : number of transmit antennas per group M T : total number of transmit antennas = + Y H S V i i i i S 1, i S IQ -SFT 2, i = + H H � H V O FD M G 1, i 2, i K i , i D em odulator � 1 M ulti-Layered S K i , IQ -SFT D ecoder S k,i is the k th STBC at the i th IQ -SFT O FD M D em odulator layer. G K H k,i is the M R × N G MIMO matrix from group k to the receiver at the i th subcarrier. NCTT-MCP08 Alghadhban 15
Due to the short code length of STBC, the received signals over T slots are rearranged into a vector ˆ = + STBC y Hx η B 1 G 1 x � B 1 1 x ˆ ˆ ˆ = 2 + H H � H η SGINC 1 2 K � Detector x K � B K STBC x k is the symbols of the k th layer. B K G K MIMO Fading Channel ˆ H ⋅ × is the MIMO M T N k G matrix from group k to the receiver. NCTT-MCP08 Alghadhban 16
Serial Group Interference Nulling and Cancellation (SGINC) • Group interference nulling : Based on an ordering criterion, assume that the first detected group is the k th group. Then, the algorithm calculates the orthonormal bases of the null space of: ˆ ˆ ˆ ˆ = � � H H H H H − + k 1 k 1 k 1 K H N • Denote the orthonormal bases of the null space of by , k k then the received signal for the i th group after nulling is: � = = + � � y N y H x η k k k k k � H Where is the post-processing channel matrix. k NCTT-MCP08 Alghadhban 17
SGINC � = � H � x H y • STBC Combiner: k k k • IQ-SFT Decoder • Group interference cancellation : After Decoding the k th Layer, its contribution is subtracted from the received signal and the processing is repeated serially for each group. • Ordering: – MaxMin FN – MaxAverage FN – Blind power allocation • Number of receive antennas should be greater than or equal to number of layers. NCTT-MCP08 Alghadhban 18
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