1. Semi-Blind Equalization for OFDM using Space-Time Block Coding and Channel Shortening Alvin Leung Yang You EE381K-12 March 04, 2008

2. General Background/Motivation Multiple transmit/receive antennas for communication systems  Higher capacity than single transmit/receive systems  Leveraging multiple antennas Space-time block coding  Frequency selective channels Multicarrier modulation Divide channel into many flat fading bins  Channel estimation

3. System Model 2 TX Antennas x 1 RX Antenna (MISO) R = (h0 * s0) + (h1 * s1) + n Multipath channel model with additive Gaussian noise

4. Orthogonal Frequency Division Multiplexing subchannel magnitude carrier channel frequency http://www.ece.utexas.edu/~rheath/research

5. Training vs Blind vs Semi-Blind Channel Estimation Must estimate the channel to recover the signal Training-based  Use pilot symbols/tones  Consume bandwidth, capacity loss Blind  Use inherent structure of signal  Constrained to special cases, less accurate Semi-blind  Use minimal amount of known symbols w/ inherent structure of signal

6. Low Complexity MIMO Blind, Adaptive Channel Shortening [R. K. Martin et al. 2005] OFDM requires cyclic prefix (CP) be longer than channel Can shorten the channel to ensure this condition Blind technique induces channel shortening by restoring the redundancy of CP Low complexity; shorten multiple channels simultaneously Unclear how to leverage multiple transmit antennas or estimate channel N samplesv samples CP: Cyclic Prefix CP s y m b o l i s y m b o l ( i+1) copy http://www.ece.utexas.edu/~rheath/research

7. Alamouti Coding [S. Alamouti 1998] Rate-1 code using simple coding matrix at transmitter and combiner at receiver Can be generalized to 2 transmitters and M receivers Adds structure and redundancy to received signal for channel estimation and robustness

8. Semi-Blind Channel Estimation [S. Zhou et al. 2002] Scalar ambiguity exists if channel estimated blindly from Alamouti structure Only two known symbols necessary to resolve true channel Less wasted symbols than training based

9. Proposed Project We leverage Martin’s blind channel shortening algorithm to reduce the channel length, allowing a shorter cyclic prefix Channel is estimated with semi-blind technique based on Alamouti with two known symbols Channel estimate used for frequency domain equalization, recover transmitted signal