BROADBAND BEAMFORMING

Presented by:Kalpana Seshadrinathan

INTRODUCTIONEqualization Distortion caused by the channel results in

Inter-Symbol Interference (ISI) which has to be compensated for to reduce the probability of error

Such a compensator is called an equalizerProblem in Broadband Communications High Data Rates are transmitted and

multipath distortion is more severe Low complexity equalizers required without

sacrificing too much in ISI mitigation

TYPES OF EQUALIZERS

Maximum Likelihood Sequence Estimation (MLSE) Optimal in terms of probability of error Disadvantage: Exponential increase in computation

requirements with increase in channel memory

Linear equalizers Sub-optimal approach Used when computational complexity of MLSE is

prohibitive Commonly used optimality criteria for choosing

filter coefficients include mean-squared error and peak distortion

BROADBAND BEAMFORMERS

MLSE is not implementable in the broadband case due to computational complexityA broadband beamformer reduces the ISI to narrowband levels in the receiverThe space-time receiver is made up of an antenna array followed by an FIR filter bankOptimal MAP equalization is then performed on the beamformer output

Koca and Levy, 2002

POWER COMPLEMENTARITY

Noise at the filter output is colored and cannot be applied to a trellis-based equalizer

Filter coefficients are hence chosen to satisfy the power complementarity property to ensure white noise at receiver output [Koca and Levy, 2002]

Power Complementarity property of an N-channel filterbank is defined by [Vaidyanathan, 1993]

1 ) ( ) (~

1

z Wz Wi

N

ii

EFFECT OF OVERSAMPLING

Oversampling in the receive filter colors the noise

BEAMFORMER DESIGN

The channel is modeled by an L-ray complex baseband impulse response with Additive White Gaussian Noise

Filter coefficients are designed using Lagrangian optimizationObjective function is the mean-squared error at the beamformer outputThe power complementarity constraint is imposed with suitable modifications to account for oversampling

)()(1

l

L

ll tatg

RESULTS

CONCLUSIONS AND FUTURE WORK

The impulse response of the channel is seen to be shortened effectively by the beamformer ISI is reduced to about 3 baud intervals and MAP equalization can be performed on the beamformer outputFuture work may incorporate co-channel interference suppression in the beamformer design