Georgialnstitute of Technology CSIP Center for Sigmal Image pnocessing Space-Time and Space-Frequency Coded Orthogonal Frequency Division Multiplexing Transmitter Diversity Techniques King f. Lee
Space-Time and Space-Frequency Coded Orthogonal Frequency Division Multiplexing Transmitter Diversity Techniques King F. Lee
Introduction Frequency-selective fading is a dominant impairment in mobile communications Fading reduces receive signal-to-noise ratio and degrades the bit-error-rate(BER) Frequency selectivity of the channel, i.e., delay spread, induces inter-symbol interference(ISi) To combat frequency-selective fading, diversity techniques must be resilient to ISI Transmitter diversity techniques are attractive especially for portable receivers where current drain and physical size are important constraints Georgia Institute of Technology Center for Signal and Image Processing
Georgia Institute of Technology Center for Signal and Image Processing Introduction • Frequency-selective fading is a dominant impairment in mobile communications. – Fading reduces receive signal-to-noise ratio and degrades the bit-error-rate (BER). – Frequency selectivity of the channel, i.e., delay spread, induces inter-symbol interference (ISI). • To combat frequency-selective fading, diversity techniques must be resilient to ISI. • Transmitter diversity techniques are attractive, especially for portable receivers where current drain and physical size are important constraints
Background Space-time block coding has emerged as an efficient means of achieving near optimal transmitter diversity gain [Alamouti 98, Tarokh 99 Existing implementations are sensitive to delay spreads and, therefore, are limited to flat fading environments such as indoor wireless networks Orthogonal frequency division multiplexing (OF DM) With a sufficiently long cyclic prefix can convert frequency-selective fading channels into multiple flat fading subchannels Combine space-time block code and OFDM Georgia Institute of Technology Center for Signal and Image Processing
Georgia Institute of Technology Center for Signal and Image Processing Background • Space-time block coding has emerged as an efficient means of achieving near optimal transmitter diversity gain [Alamouti 98,Tarokh 99]. • Existing implementations are sensitive to delay spreads and, therefore, are limited to flat fading environments, such as indoor wireless networks. • Orthogonal frequency division multiplexing (OFDM) with a sufficiently long cyclic prefix can convert frequency-selective fading channels into multiple flat fading subchannels. Combine space-time block code and OFDM
Space-Time Block Code -I EXample Assume two transmit antennas and one receive antenna The space-time block code transmission matrix is For each pair of symbols transmit Antenna ffT. x1-x2 Antenna#2:yt Georgia Institute of Technology Center for Signal and Image Processing
Georgia Institute of Technology Center for Signal and Image Processing Example • Assume two transmit antennas and one receive antenna. • The space-time block code transmission matrix is • For each pair of symbols transmit Antenna #1: Antenna #2: Space-Time Block Code - I * 1 2 x x − * 2 1 x x 1 2 2 * * 2 1 x x x x = − G
Space-Time Block Code -l The received signals are y=a,x+a2* 2+n y2==01X2+c2x1+n2 Calculate the decision variables as 米=a+2=(1P+1)x++n Vi-aly (a2+1)-h+an Similar to that of a two-branch maximal ratio combining receiver diversity system Unfortunately, the technique is sensitive to delays Georgia Institute of Technology Center for Signal and Image Processing
Georgia Institute of Technology Center for Signal and Image Processing • The received signals are • Calculate the decision variables as • Similar to that of a two-branch maximal ratio combining receiver diversity system! • Unfortunately, the technique is sensitive to delays. Space-Time Block Code - II ( ) ( ) 2 2 * * * 1 1 1 2 2 1 2 1 1 1 2 2 2 2 * * * 2 2 1 1 2 1 2 2 1 2 2 1 ˆ . ˆ x y y x x y y x = + = + + + = − = + − + 1 1 1 2 2 1 * * 2 1 2 2 1 2 . y x x y x x = + + = − + +