Anand G. Dabak

Performance analysis of closed-loop transmit diversity in the presence of feedback delay

In this paper, a bit-error-rate (BER) analysis for closed-loop transmit diversity in a time-selective Rayleigh fading channel containing feedback delay is presented. In the absence of feedback delay, closed-loop transmit diversity always outperforms open-loop transmit for a given transmitted signal energy. This is no longer true in the presence of feedback delay. We derive closed-form expressions of the average BER for this case assuming QPSK and BPSK signaling. The results of the analysis are instrumental for comparing closed-loop with open-loop schemes under given operating conditions. In particular, we demonstrate that, for a given transmitted energy and number of transmit antennas, open-loop outperforms closed-loop at sufficiently fast channel fading. We also show that, for a given transmitted signal energy and fading rate, closed-loop outperforms open-loop for sufficiently large numbers of transmit antennas while the total average transmitted signal energy is kept constant. For some special cases, closed-form expressions for the fading rate at which the performance of open-loop is equal to closed-loop are obtained.

Function-based modeling of binaural processing: Interaural level

The function-based modeling approach applies optimal estimation theory to sensory phenomena for determining how relevant sensory parameters are extracted from stimuli and how the characteristics of the resulting optimal processing system compare with those of the sensory system. This approach is applied to the neural system involved in the binaural localization of sustained high-frequency sound sources: the lateral superior olive (LSO) of the cat. The sufficient statistic produced by the optimal processor is shown to be related to the interaural level difference. This level difference is processed optimally when the inputs are excitatory from one ear and inhibitory from the opposite ear. Response characteristics of LSO single units are remarkably similar, thereby strongly supporting the notion that LSO units are intimately involved in high-frequency binaural hearing. Optimal processor theory is also used to assess lateralization performance when the hearing thresholds of the two ears differ.

Decision feedback equalization for Bluetooth systems

Gaussian frequency shift keying modulation has been chosen as the modulation technique for the physical layer of Bluetooth. Bluetooth is a standard for low cost and low power wireless communications between various mobile devices. The optimal demodulation of a GFSK signal involves an extremely complex Viterbi decoder. Therefore designers have opted for the noncoherent detection of GFSK which uses a frequency discriminator, followed by symbol by symbol detection. We describe a decision feedback equalizer to be added after the discriminator. The DFE receiver gives gains in excess of 2 dB. We also describe how to increase the current data rate of a Bluetooth system by increasing the symbol rate and not the alphabet size.

Signal constellations for non-Gaussian communication problems

On the basis of a geometric theory of detection, the authors extend the notion of a signal constellation, a concept deeply rooted in Gaussian problems, to the non-Gaussian case. Significant differences between optimal designs for Gaussian and non-Gaussian situations are shown. In particular, square-wave signals are much more important in heavy-tailed, non-Gaussian noise situations than in Gaussian ones. Furthermore, design guidelines for non-Gaussian problems can vary with the number of signal set members and can depend on SNR. The extent to which suboptimal designs affect performance (using Gaussian-based designs in non-Gaussian situations, for example) can be predicted from calculations of the Kullback information, but only in the sense of determining how the logarithmic error probability rates differ.<<ETX>>