Limited feedback precoding for MIMO-OFDM and non-coherent demodulation for orthogonal space time coded continuous phase modulation

Tarkesh Pande, Purdue University

Abstract

In the first part of this research we investigate various techniques to reduce the feedback requirement in a MIMO-OFDM system with precoding. Transmitter precoding for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is an effectiving way of leveraging the diversity gains afforded by a multiple transmit-multiple receive antenna system in a frequency selective environment. In the limited feedback scenario, optimal precoder representation for narrowband MIMO systems using moderately sized codebooks designed on the Grassmann manifold has been shown to perform remarkably well. In MIMO-OFDM systems precoder matrices have to be designed for all subcarriers and the amount of feedback can get prohibitively large. This is especially true for next generation wireless local area networks and wireless metropolitan area networks which have a large number of subcarriers. We will present, three different techniques to reduce this feedback requirement and the performance of these algorithms are numerically shown to provide improvement over existing schemes. In the second part of this work, we study the relationship between the size of a packing or code on the Grassmann manifold and its minimum distance. A coding theoretic approach typically involves deriving Gilbert-Varshamov and Hamming bounds which relate the size of the packing to its minimum distance. A key computation in the derivation is that of finding the volume of a metric ball for a given radius and distance metric. We present exact closed form analytical expressions of the metric ball volume for line packings in the Grassmann manifold with respect to different distance metrics. Simplified approximations to the metric ball volume are then derived and are shown to result in better approximations to the Gilbert-Varshamov and Hamming bounds than those present in literature. For the general subspace packing case furnished with the projection two-norm distance metric, a lower bound on the metric ball volume is provided based on a recently derived marginal distribution [1]. In the third part of this research we present non-coherent receivers for a nonlinear modulation scheme called orthogonal space-time continuous phase modulation (OST-CPM). Typically, the performance of non-coherent detectors for linear modulations is 3-dB worse than those of the corresponding coherent detectors. We show that for a reasonable increase in receiver complexity non-coherent receivers for OST-CPM perform less than 1-dB away from the corresponding coherent detectors.

Degree

Ph.D.

Advisors

Love, Purdue University.

Subject Area

Electrical engineering

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