V- BLAST : Speed and Ordering Madhup Khatiwada IEEE New Zealand Wireless Workshop 2004 (M.E. Student) 2 nd September, 2004 University of Canterbury Alan MacDiarmid Centre, Industrial Research Ltd. Supervisor – Dr. P. J. Smith Lower Hutt, Wellington

MIMO Wireless System Multiple Input Multiple Output (MIMO) –Multiple antennas at source and destination. Motivation : Current wireless systems –Capacity constrained networks. –Issues related to quality and coverage MIMO increases capacity –MIMO uses independent channel fading due to multipath propagation to increase capacity. –No extra expensive bandwidth required !! –Multiple independent samples of the same signal at the receiver gives rise to “diversity”. Coding Modulation Demodulation Decoding N TXersM RXers Channel H

System Model R : received vector H : quasi-static channel matrix s : transmitted vector n : white Gaussian noise vector Diversity exhibited : - Spatial diversity - Transmit diversity - Receive diversity

Spatial Multiplexing Detection Maximum Likelihood (ML): optimum and most time consuming detection method Linear detection –Zero-Forcing (ZF): pseudo inverse of the channel, simple –Minimum mean-squared error (MMSE) : simple detection with intermediate performance Simulation Parameters Number of Antennas – 4 x 4 Modulation Scheme – BPSK Channel Model – Flat Rayleigh Fading

V-BLAST –extracts data streams by ZF or MMSE filter with ordered successive interference cancellation (SIC) –Steps for V-BLAST detection 1.Ordering: choosing the “best” channel 2.Interference Nulling/ Reduction : using ZF or MMSE 3.Slicing: making a symbol decision 4.Canceling: subtracting the detected symbol 5.Iteration: going to the first step to detect the next symbol Simulation Parameters Number of Antennas – 4 x 4 Modulation Scheme – BPSK Channel Model – Flat Rayleigh Fading

Key references in the literature on the subject matter Foschini, G. J. and Gans, M. J., “ On limits of wireless communications in a fading environment when using multiple antennas,” Wireless Personal Communications, Vol. 6, pp , 1998 Hassell, C.Z.W., Thompson, J., Mulgrew, B. and Grant, P.M., “A comparison of detection algorithms including BLAST for wireless communication using multiple antennas” The 11th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Vol. 1, pp. 698 – 703, Sept Golden, G. D., Foschini, G. J., Valenzuela, R. A., and Wolniansky, P. W., “ Detection algorithm and initial laboratory results using V-BLAST space-time communication architecture,” IEE Lett., Vol. 35, No. 1, pp , January 1999 Benjebbour, A., Murata, H. and Yoshida, S., “Comparison of ordered successive receivers for space-time transmission,” Vehicular Technology Conference, VTC 2001 Fall. IEEE VTS 54th, Vol. 4, pp , 7-11 Oct. 2001

Issues of Ordering and Speed Ordering Optimal ordering plays a significant role in achieving high performance  In V-BLAST, the best channel is picked based on highest SINR, but picking up channels based on highest SINR value doesn’t seem to be the optimal way of ordering. Switching Switching based on SINR or Eigenvalues of the channel helps in reducing the computational time by adding very little complexity to the system.  Switching is a scheme in which the receiver switches its detection technique from V-BLAST MMSE to simple MMSE depending upon the channel condition (either SINR values or eigenvalues of the channel).

Significance of Optimal Ordering Picking the first best channel can effect significantly in the whole system performance Simulation Parameters Number of Antennas – 4 x 4 Modulation Scheme – BPSK Channel Model – Flat Rayleigh Fading

Significance of Switching Switching allows us to trade off performance with time Simulation Parameters Number of Antennas – 4 x 4 Modulation Scheme – BPSK Channel Model – Flat Rayleigh Fading Simulation time reduced by nearly 60% at 10 db SNR case

Significance of Ordering and Switching in Overloaded system For systems with less receive antennas, optimal ordering can be a crucial issue for system performance Simulation Parameters Number of Antennas – 4 x 2 Modulation Scheme – BPSK Channel Model – Flat Rayleigh Fading

Conclusion Optimal ordering plays a vital role in increasing the performance of V-BLAST systems with MMSE. Switching techniques may be useful in reducing processing time while adding very little complexity to the system.