16 research outputs found

    Transmitter Diversity and Coding Schemes

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    Masters ThesisWe analyze wireless communication systems employing multiple-transmit multiple-receive antennas in this thesis. Space-time codes have been proposed in the literature as an efficient means of coding over multiple transmit antennas. In particular, the rank and the determinant of code difference matrices have been shown to be important in the design of space time codes in quasi-static fading channels. In this thesis, we investigate the problem of maximizing the coding gain given by the minimum of the determinants of the code difference matrices. We show that equal singular values of the code difference matrices is a necessary and sufficient condition for obtaining optimal coding gain. We also show that equal singular values lead to robust codes. Finally, we present the construction of enhanced dimensional trellis coded modulation (EDTCM) codes based on equal singular values

    Transmitter diversity and coding schemes

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    We analyze wireless communication systems employing multiple-transmit multiple-receive antennas in this thesis. Space-time codes have been proposed in the literature as an efficient means of coding over multiple transmit antennas. In particular, the rank and the determinant of code difference matrices have been shown to be important in the design of space time codes in quasi-static fading channels. In this thesis, we investigate the problem of maximizing the coding gain given by the minimum of the determinants of the code difference matrices. We show that equal singular values of the code difference matrices is a necessary and sufficient condition for obtaining optimal coding gain. We also show that equal singular values lead to robust codes. Finally, we present the construction of enhanced dimensional trellis coded modulation (EDTCM) codes based on equal singular values

    Feedback in multiple antenna systems: Bounds, design criterion and construction

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    In this thesis, we study wireless communication systems equipped with multiple antennas at the transmitter and the receiver. In particular, we investigate the design and performance analysis of transmission schemes when partial channel state information is available at the transmitter via a feedback channel. Assuming that the feedback information is used for beamforming, we study the design of the transmission schemes and the feedback channel with the finite rate feedback constraint. The important contribution of this thesis is a geometrical framework for studying beamforming schemes for multiple antenna systems in the presence of finite rate channel feedback. We present intuitive geometrical interpretations to the beam forming problem and demonstrate the utility of the geometrical framework through the derivation of a fundamental lower bound on the error performance of beam-forming schemes. Using the tight lower bound, we present a distortion measure for quantization based on the resulting outage probability which bears interesting similarities with the mean squared distortion metric commonly used in source coding. In particular, we show that finite rate feedback systems approach the perfect channel information case as t-12-B t-1 for a single receive antenna where B is the number of feedback bits and t is the number of transmit antennas. Our geometrical framework also leads to a design criterion for good beamformers. Our design criterion for beamformer construction minimizes the maximum inner product between any two beamforming vectors in the beamformer codebook. We show the equivalence of beamformer construction with two other interesting problems in literature, viz., unitary space time code design for non-coherent communications and the subspace packing problem. We show that good beamformers are good packings of 2-dimensional subspaces in a 2t-dimensional real Grassmannian manifold with chordal distance as the distance metric. Finally, we extend our analysis and construction methods to multiple receive antenna systems. We introduce the notion of partial rank beamforming and derive a rate region where rank-m beamforming can be expected to perform better than the full rank space time codes. Rate threshold Rth,m, which is the maximum of the rates in the optimal rate region of rank-m beamforming, is shown to vary as kmk-mlog 2&parl0;tm&parr0; , where k = min(r, t), r being the number of receive antennas. (Abstract shortened by UMI.

    Generalized Beamforming for MIMO Systems with Limited Transmitter Information

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    Conference PaperIn this work, we investigate the outage performance of beamforming schemes in wireless systems equipped with multiple transmit and receive antennas. In particular, we analyze the outage performance of unit rank beamforming in the presence of finite rate channel feedback at the transmitter. Further, we present finite size beamformer codebook constructions which result in near-optimal outage performance for unit rank beamforming. The constructions obtained for the unit rank beamforming scheme are then extended to higher rank beamforming schemes with quantized channel information. We show that significant performance improvements as well as reduction in decoding complexity can result from a small number of feedback bits

    Finite Rate Feedback Design for Multiple Transmit Antennas

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    Conference PaperMany practical systems employ a feedback channel to convey the channel state information to the transmitter.The capacity of the feedback channels is usually limited to a few bits per frame. We design feedback based transmission schemes for multiple transmit antenna systems in this work. The important contribution of this work is a design criterion for the construction of optimal beamformers with a constraint on the rate of the feedback channel. Beamforming schemes also lead to reduced complexity compared to space time coding schemes at the cost of a few bits of feedback.Texas Advanced Technology ProgramNokiaNational Science Foundatio

    Design of Multiple Antenna Coding Schemes with Channel Feedback

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    Conference PaperIn this paper, we derive multiple antenna transmission strategies in the presence of limited channel information at the transmitter and the receiver. In particular, we look at the cases of complete channel information, channel phase information and channel amplitude information at the transmitter. We highlight that transmission along the eigenvector of the channel corresponding to the maximum eigenvalue minimizes error probability, when complete channel information is available at the transmitter. In the case where only the channel phase information is available at the transmitter, we derive the beamformer which minimizes the error probability. We also show that, in the presence of channel amplitude information at the transmitter without any phase information, selection diversity at the transmitter is the best beamforming strategy. We evaluate the penalty in SNR incurred by the transmission schemes in the case of limited channel information compared to the case of complete channel information at the transmitter.Noki

    Design of space-time codes with optimal coding gain

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    Conference PaperSpace time codes have been proposed in the literature as an efficient means for improving the data rates over fading channels with multiple transmit antennas. In particular, the rank and the determinant of code difference matrices have been shown to be important in the design of space time codes for fading channels. In the present work, we investigate the problem of maximizing the coding gain of space-time codes, given by the minimum of the determinants of all the code difference matrices. We rely on equality of the singular values of the code difference matrices as a necessary and sufficient condition for obtaining the optimal coding gain. Finally, we discuss the construction of trellis codes and present simulation results.Noki

    On Beamforming with Finite Rate Feedback in Multiple Antenna Systems

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    Journal PaperIn this paper, we study a multiple antenna system where the transmitter is equipped with quantized information about instantaneous channel realizations. Assuming that the transmitter uses the quantized information for beamforming, we derive a universal lower bound on the outage probability for any finite set of beamformers. The universal lower bound provides a concise characterization of the gain with each additional bit of feedback information regarding the channel. Using the bound, it is shown that finite information systems approach the perfect information case as (t-1)2<sup>-B/(t-1)</sup>, where <i>B</i> is the number of feedback bits and <i>t</i> is the number of transmit antennas. The geometrical bounding technique, used in the proof of the lower bound, also leads to a design criterion for good beamformers, whose outage performance approaches the lower bound. The design criterion minimizes the maximum inner product between any two beamforming vectors in the beamformer codebook, and is equivalent to the problem of designing unitary space time codes under certain conditions. Finally, we show that good beamformers are good packings of 2-dimensional subspaces in a 2<i>t</i>-dimensional real Grassmannian manifold with chordal distance as the metric.Noki

    On the outage proabability of a class of signaling schemes for multiple antennas

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    Conference PaperIn this work, we investigate large SNR behavior of the outage probability for a class of signaling schemes designed for flat fading Rayleigh multiple antennas systems. Outage probability is important in a practical system since it reflects the frame error rates achievable for a given rate of transmission. With an assumption of short-term power constraint, which diminishes the effectiveness of time power control, our analysis characterizes the diversity advantage of different schemes. Our main result is the characterization of a large class of signaling schemes, all of which are guaranteed to have the identical rate of decay of outage probability with SNR given by the maximum spatial diversity available in the system. We show that several important multiple antenna signaling schemes like space time coding and various beamforming techniques, designed with different amount of channel state information at the transmitter are included in the proposed class of signaling schemes. Such a classification of schemes, based on similar error performance, helps in addressing practical issues like tradeoffs in feedback resources and decoding complexity required by each scheme. Finally, the techniques developed in this paper can also be used to derive the diversity gain of an arbitrary signaling scheme.Noki
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