Semi-Blind Equalization for OFDM using Space-Time Block Coding and Channel Shortening

Alvin LeungYang You

EE381K-12May 1, 2008

General Background/Motivation Multiple antenna communication systems

Higher capacityLeveraging multiple antennas

Spatial Diversity Space-Time Coding

Frequency selective channels Modeled as FIR filter Orthogonal Frequency Division Multiplexing (OFDM)

Channel Shortening

Needed in order to preserve cyclic convolution in OFDM Designed as FIR filter – compresses channel energy

Alamouti Coding

Orthogonalizes transmitted symbol

Objective

Evaluate a combination of blind channel shortening and semi-blind channel estimation in a multi-antenna ST-OFDM system over a realistic channel model (3GPP TR 25.996 spatial channel model).

System Model – TX

Θ1 and Θ2 - Linear precoders [JxK] M(.) - Alamouti space-time encoding OFDM

IFFT +CP

IFFT +CP

System Model – Channel and RX

wChannel

ShortenerFFT-CP

D1 and D2 - effective frequency domain channelsw(n) is AWGNM(.) removes space-time codingΓ equalizes z(n) to obtain symbol estimates

Channel Shortening for Equal Channels

Bit Error Rate = 5.3191e-005 Bit Error Rate = 9.5745e-005

Bit Error Rate = 0.0089 Bit Error Rate = 4.5745e-004

Constellation Comparison – Two Channels

Channel Estimate Comparison

Conclusions

Computation of SVD in estimation very complex – O(N3)

Iterative channel estimation assumes very slowly changing channel

Blind channel shortener based on ergodic statistics – requires large sample size

We are exploring training based approaches