Browsing by Author "Memarzadeh, Mahsa"

Now showing 1 - 7 of 7
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    Broadcast Space-Time Coding
    (2001-08-20) Memarzadeh, Mahsa; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we study transmission techniques for broadcast channels with a single transmitter and multiple receivers. The transmitter is assumed to be equipped with multiple antennas. Further, each receiver is interested only in part of the transmitted information. Using Gaussian code based information theoretic bounds, we analyze two transmit beamforming techniques. The first technique, zero-forcing beamformer, uses the channel information for all users to send spatially orthogonal signals to different users, i.e., no user receives interference from the other user signals. The second method uses the single-user optimal beamformer with no effort to reduce interference at the receivers. It is shown that the above transmit beamforming techniques are similar to multiuser receivers used in non-orthogonal CDMA systems. In particular, the zero-forcing beamformer is similar to a decorrelating detector and the single-user beamformer is identical to a matched filter. The comparison with CDMA multiuser receivers is strengthened by results on spectral efficiency of proposed beamformers, which follow the behavior of decorrelating and matched-filter receiver.
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    Code Design for Multiple-Antenna Systems
    (2001-04-20) Memarzadeh, Mahsa; Center for Multimedia Communications (http://cmc.rice.edu/)
    We propose a systematic method for the design of space-time codes for AWGN slowly and fast Rayleigh fading channels. This can be accomplished by adopting a concatenated space-time code structure, where an orthogonal transmit diversity system constitutes the inner encoder. We will show that this will cause in decoupling of the problems of spatial and temporal diversity gains maximization, involved in the design of space-time codes. This decoupling significantly simplifies the code design procedure and presents a systematic code construction technique. In the case of slowly fading channels, where no temporal diversity gain is available, the concatenated structure of the space-time code, will help to decouple the problems of spatial diversity and coding gains maximization. However in a fast fading channel, the proposed system will decouple the problems of spatial and temporal diversity gains maximization. At the end, some issues involved in designing codes for downlink broadcast channels will be discussed.
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    Code design for multiple-antenna systems
    (2001) Memarzadeh, Mahsa; Aazhang, Behnaam
    We propose a systematic method for the design of space-time codes for AWGN slowly and fast Rayleigh fading channels. This can be accomplished by adopting a concatenated space-time code structure, where an orthogonal transmit diversity system constitutes the inner encoder. We will show that this will cause in decoupling of the problems of spatial and temporal diversity gains maximization, involved in the design of space-time codes. This decoupling significantly simplifies the code design procedure and presents a systematic code construction technique. In the case of slowly fading channels, where no temporal diversity gain is available, the concatenated structure of the space-time code, will help to decouple the problems of spatial diversity and coding gains maximization. However in a fast fading channel, the proposed system will decouple the problems of spatial and temporal diversity gains maximization. At the end, some issues involved in designing codes for downlink broadcast channels will be discussed.
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    Design of Coded Modulation Schemes for Orthogonal Transmit Diversity
    (2001-06-20) Borran, Mohammad Jaber; Memarzadeh, Mahsa; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we propose a technique to decouple the problems of spatial and temporal diversity gain maximization involved in the design of space-time codes in fast Rayleigh fading channels by using orthogonal transmit diversity systems. We will also introduce the idea of constellation expansion (in dimension or size) to design coded modulation schemes with maximum temporal diversity gain for orthogonal transmit systems. The Chernoff upper bound for the error probability is used to show that the code design criterion reduces to the maximization of Hamming and product distances in the expanded constellation. The proposed technique is demonstrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to the coding schemes designed for single transmit and receive antennas.
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    Design of Coded Modulation Schemes for Orthogonal Transmit Diversity
    (2001-05-20) Borran, Mohammad Jaber; Memarzadeh, Mahsa; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we propose a technique to decouple the problems of maximizing spatial diversity gain, temporal diversity gain, and coding gain, involved in the design of space-time codes in Rayleigh fading channels. For this purpose, we will use a set of concatenated codes with orthogonal transmit diversity system as their inner code. In the case of slowly fading channels, no temporal diversity is available, and the proposed technique decouples the problems of maximizing spatial diversity gain and coding gain. The design criterion for the outer codewill be shown to be based on the maximization of the free Euclidean distance. For fast fading channels, by using the new idea of constellation expansion (in dimension or size), we decouple the code design problem into two simpler problems, namely maximizing the spatial diversity gain, and maximizing the temporal diversity and coding gain. In this case, the code design criteria for the outer encoder will be shown to reduce to the maximization of Hamming and product distances in the expanded constellation. The proposed techniques are illustrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to some existing space-time trellis codes with the same complexities.
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    Optimal borehole communication using multicarrier modulation
    (2007) Memarzadeh, Mahsa; Johnson, Don H.
    Transmission of down-hole measurements to the surface while the oil well is being drilled (logging while drilling-LWD-telemetry) is the key to the efficient and successful recovery of hydrocarbon resources. The most promising method of telemetry uses compressional acoustic waves to transmit data along the drillstring. In this technique, telemetry signals are encoded as acoustic wave pulses that propagate through the drillstring and are subsequently received and converted back to electric signals for data recovery at the surface receiver. Based on the analysis of the wave propagation properties in the drillstring, I propose and design an acoustic orthogonal frequency division multiplexing (OFDM) transmission scheme to efficiently communicate through the channel. To prove the feasibility of the proposed communication technique, numerous field tests were successfully designed and implemented in various drilling oil rigs. Then I characterize the performance of the oil well telemetry system and propose a joint source-channel coding approach wherein the end-to-end distortion in reconstructing telemetry signals at the surface receiver is directly minimized. I show that the acoustic OFDM technique enables simple implementation of the joint source-channel coding design criterion and present the methodology for optimizing different transmission scheme components. Realistic LWD acoustic telemetry design examples demonstrate that the optimal design results in significant gains in system performance and throughput as compared to standard design approaches.
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    Optimal digital communication of analog signals
    (2005-03-01) Memarzadeh, Mahsa; Johnson, Don; Digital Signal Processing (http://dsp.rice.edu/)
    In this paper, the problem of optimally communicating analog sources using a bandwidth and power limited digital system is considered. We propose and analyze optimal combined source-channel coding schemes that jointly optimize the compression of the source while controlling the individual bit error probabilities to minimize the mean-squared distortion in reconstructing the signal. For a spectrally shaped channel, the proposed schemes are most efficiently implemented using multicarrier modulation. We optimize the power allocation across subchannels and the constellation within each subchannel. The results indicate that the optimal power allocation is not achieved by a water-filling solution and significant gains (several dB) can be obtained by our optimal design.