V- BLAST : Speed and Ordering

1
V- BLAST Speed and Ordering

• Madhup Khatiwada IEEE New Zealand
  Wireless Workshop 2004
• (M.E. Student) 2nd September,
  2004
• University of Canterbury Alan MacDiarmid
  Centre, Industrial Research Ltd.
• Supervisor Dr. P. J. Smith Lower
  Hutt, Wellington

2
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.

Channel H
N TXers
M RXers
3
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
4
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

5
V-BLAST

• V-BLAST
• extracts data streams by ZF or MMSE filter with
  ordered successive interference cancellation
  (SIC)
• Steps for V-BLAST detection
• Ordering choosing the best channel
• Interference Nulling/ Reduction using ZF or
  MMSE
• Slicing making a symbol decision
• Canceling subtracting the detected symbol
• 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

6
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. 311-335, 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, 18-21 Sept. 2000
• 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. 14-16, January 1999
• Benjebbour, A., Murata, H. and Yoshida, S.,
  Comparison of ordered successive receivers for
  space-time transmission, Vehicular Technology
  Conference, 2001. VTC 2001 Fall. IEEE VTS 54th
  , Vol. 4 , pp. 2053 - 2057 , 7-11 Oct. 2001

7
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 doesnt 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).

8
Significance of Optimal Ordering

• Simulation Parameters
• Number of Antennas 4 x 4
• Modulation Scheme BPSK
• Channel Model Flat Rayleigh Fading

• Picking the first best channel can effect
  significantly in the whole system performance

9
Significance of Switching
Simulation time reduced by nearly 60 at 10 db
SNR case

• Simulation Parameters
• Number of Antennas 4 x 4
• Modulation Scheme BPSK
• Channel Model Flat Rayleigh Fading

• Switching allows us to trade off performance
  with time

10
Significance of Ordering and Switching in
Overloaded system

• Simulation Parameters
• Number of Antennas 4 x 2
• Modulation Scheme BPSK
• Channel Model Flat Rayleigh
  Fading

• For systems with less receive antennas, optimal
  ordering can be a crucial issue for system
  performance

11
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.