Browsing by Author "Ahmed, Nadeem"

Now showing 1 - 10 of 10
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    Asymptotic Performance of Transmit Diversity via OFDM for Multipath Channels
    (2002-11-01) Ahmed, Nadeem; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
    Many wireless systems exploit transmit diversity for more reliable detection of signals at the receiver. To accomplish this, coding is spread across multiple transmit antennas. An example of this is the well known "Alamouti transmit diversity", where a very simple coding scheme across multiple transmit antennas allows systems to attain performance similar to systems with multiple receive antennas. The major drawback is that this system only works when a "flat-fading" model for the channel is assumed; when used in a multipath environment, the system breaks down. Here we show that when the Alamouti code is placed within an OFDM structure, using adjacent frequency bands rather than consecutive symbol intervals, it can asymptotically achieve the same performance in multipath fading as the Alamouti code in flat-fading.
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    Blind Crosstalk Cancellation for DMT Systems
    (2002-09-01) Ahmed, Nadeem; Warke, Nirmal; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
    As the deployment of broadband communication systems such as DSL continues to grow, system performance in terms of capacity and error rates is severely limited by crosstalk interference. In order to continue deploying high speed DSL service, it becomes necessary to mitigate this crosstalk interference. All current crosstalk mitigation techniques require precise knowledge of the crosstalk coupling functions between the twisted pair wires carrying DSL service within a cable binder. In practice, this is near impossible to obtain, without coordination between the different DSL services. In this paper, we present a crosstalk cancellation technique that is blind to the coupling functions between the wire pairs. Our technique uses the statistical properties of the received crosstalk signal (that can be easily estimated), rather than the exact coupling functions themselves, making it very attractive for practical implementation. We show with the help of simulations for a realistic ADSL system with HDSL and T1 crosstalk interference using actual measured crosstalk coupling functions that the proposed blind crosstalk cancellation technique can achieve significant gains in terms of rate and reach improvement for the ADSL system. We also show that the performance of the proposed blind crosstalk cancellation technique is robust to a jitter in the crosstalk symbol timing estimate that is required to construct the cancellation signal.
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    Delay -limited throughput maximization in fading channels
    (2005) Ahmed, Nadeem; Baraniuk, Richard G.
    Fading channels, seen in many wireless systems, provide a hostile environment for reliable communication. Conventional analysis of fading channels has been performed from the single-attempt paradigm . That is, the amount of information that can be reliably communicated with a single codeword transmission attempt is quantified. This works well for idealized, delay-unconstrained systems that always transmit a single, infinite-length codeword. However, practical systems are delay-limited since they must use finite-length codewords. Therefore, the conventional performance metrics based on the single-attempt paradigm have drawbacks for delay-limited systems: &epsis;- capacity does not provide a measure of error-free performance, while delay-limited capacity underestimates performance. For delay-limited systems, transmitters need not restrict themselves to a single transmission attempt per codeword. In fact, practical communication protocols, such as TCP or ARQ, retransmit data when errors occur. Clearly, there is a disconnect in the design of delay-limited systems (multi-attempt) and the conventional measures used to quantify their performance (single-attempt). In this thesis we provide a new analysis framework for delay-limited systems based on the multi-attempt paradigm . We maximize the average communications throughput by optimizing system parameters and use the maximum throughput as a measure of delay-limited communication performance. We consider two common scenarios, the first being only when the receive has channel state information (CSI-R), while in the second both transmitter and receiver it (CSI-RT). With CSI-R, the average transmit power is held constant and throughput is maximized by performing optimal rate selection . With CSI-RT, the transmitter knows the condition of the channel at the time of transmission and can vary the power accordingly. Our analysis is done for an average power constraint on the transmitted signal. We also consider the scenario if an additional peak power constraint on the transmitted signal is added. Therefore throughput is maximized by performing optimal rate selection and power control . As a pre-requisite for throughput maximization, we also solved the outage minimization problem for signals with both peak and average power constraints. We propose maximum &epsis;-throughput (M&epsis;T) and maximum zero-outage throughput (MZT) as measures of best-case communications performance when there is, and is not, a restriction on the maximum number of transmission attempts per codeword, respectively. We show that a far greater throughput is achieved with the multi-attempt approach than the single-attempt approach. The increased throughput comes at the cost of queueing delays that are not present when transmitters are limited to a single transmission attempt. Therefore, we also consider the important situation in which throughput is maximized with a constraint on the queueing delay. In this thesis we provide the procedure to maximize communications throughput for systems and give some non-intuitive design guidelines for delay-limited communication systems in fading channels. Our novel analysis shows that some conventionally held wisdom for delay-unconstrained systems does not hold for delay-limited systems.
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    Delay-Limited Throughput Maximization for Fading Channels using Rate and Power Control
    (2004-12-01) Ahmed, Nadeem; Khojestapour, Mohammad; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
    The fading channels seen in many wireless systems provide a particularly hostile environment for reliable communication. Current metrics for evaluating the performance limits of fading channels have shortcomings. Ergodic capacity, representing the ultimate error-free communications limit, only applies to systems with infinite coding delay. Practical systems are delay-limited and must use finite-length codes. For delay-limited systems /spl epsi/-capacity and delay-limited capacity are typically used to quantify the communications performance. However, /spl epsi/-capacity is not an estimate of error-free performance while delay-limited capacity tends to be an overly conservative measure. We model practical systems as a single server queue and quantify the communications performance as the average throughput through the queue. Throughput is maximized by optimally selecting the transmission rate and power control strategy. Using this approach we arrive at striking conclusions. First, we show that a throughput very close to ergodic capacity can be achieved with a small coding delay. Second, the optimal transmission rate for some systems can be higher than the ergodic capacity of the channel. Third, we demonstrate the notion that power adaptation does not improve communication performance does not hold for delay-limited systems.
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    Joint Detection Strategies for Orthogonal Frequency Division Multiplexing
    (2000-04-20) Ahmed, Nadeem; Center for Multimedia Communications (http://cmc.rice.edu/)
    Traditional OFDM implementations use conventional Fourier filters for data modulation, via the IFFT operation. Much research suggests wavelet based OFDM provides performance gains, due to superior spectral containment properties of wavelet filters. When compared in simulation, neither system was found fundamentally better than the other. Rather than focus on filter bank design, joint detection strategies provide a relatively simple method for improving system performance. The conventional matched filter used in OFDM systems, is only optimal when the channel introduces no carrier distoration. We propose an optimal detector, which maximizes the probability of making correct decisions, and a suboptimal method which decorrelates carriers based on knowledge of the channel. Both joint detection methods provide significant performance gains in OFDM systems over conventional matched filtering.
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    Joint detection strategies for orthogonal frequency division multiplexing
    (2000) Ahmed, Nadeem; Baraniuk, Richard G.
    Traditional OFDM implementations use conventional Fourier filters for data modulation, via the IFFT operation. Much research suggests wavelet based OFDM provides performance gains, due to superior spectral containment properties of wavelet filters. When compared in simulation, neither system was found fundamentally better than the other. Rather than focus on filter bank design, joint detection strategies provide a relatively simple method for improving system performance. The conventional matched filter used in OFDM systems, is only optimal when the channel introduces no carrier distoration. We propose an optimal detector, which maximizes the probability of making correct decisions, and a suboptimal method which decorrelates carriers based on knowledge of the channel. Both joint detection methods provide significant performance gains in OFDM systems over conventional matched filtering.
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    Optimal Transmit Spectra for Communication in the Presence of Crosstalk and Imperfect Echo Cancellation
    (2001-09-01) Ahmed, Nadeem; Baraniuk, Richard G.; Shaver, Donald; Digital Signal Processing (http://dsp.rice.edu/)
    In many communication systems, including Digital Subscriber Lines, performance is severely limited by crosstalk interference. Previous work has presented a general framework for designing optimal transmit spectra for crosstalk avoidance. The technique uses the channel, noise, and interference characteristics to setup and solve an optimization problem which maximizes the capacity of neighboring lines, while maintaining spectral compatibility with other services. This joint signaling and optimal power distribution technique yields significant performance gains over conventional fixed spectra in terms of bit-rates and performance margins. In general, the spectra that result from this scheme have both an echo cancelled and frequency division multiplexed region. To ease the analysis, this technique assumed that the echo canceller has perfect echo rejection capability, which in practice is not true. In this paper, we propose an extension to these techniques, in which we factor the performance of practical echo cancellers into the optimization procedure. When echo rejection is not perfect, as is generally the case, our technique shows significant performance gains over previous techniques. As the performance of the echo canceller increases, our technique converges to the same solution.
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    Optimal Transmit Spectra for Communication in the Presence of Crosstalk and Imperfect Echo Cancellation
    (2001-11-01) Ahmed, Nadeem; Baraniuk, Richard G.; Shaver, Donald; Digital Signal Processing (http://dsp.rice.edu/)
    In many communication systems, including Digital Subscriber Lines, performance is severely limited by crosstalk interference. Previous work has presented a general framework for designing optimal transmit spectra for crosstalk avoidance . The technique uses the channel, noise, and interference characteristics to setup and solve an optimization problem which maximizes the capacity of neighboring lines, while maintaining spectral compatibility with other services. This joint signaling and optimal power distribution technique yields significant performance gains over conventional fixed spectra in terms of bit-rates and performance margins. In general, the spectra that result from this scheme have both an echo cancelled and frequency division multiplexed region. To ease the analysis, this technique assumed that the echo canceller has perfect echo rejection capability, which in practice is not true. In this paper, we propose an extension to these techniques, in which we factor the performance of practical echo cancellers into the optimization procedure. When echo rejection is not perfect, as is generally the case, our technique shows significant performance gains over previous techniques. As the performance of the echo canceller increases, our technique converges to the same solution.
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    Throughput Maximization for ARQ-like Systems in Fading Channels with Coding and Queuing Delay Constraints
    (2004-11-01) Ahmed, Nadeem; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
    Practical delay-limited communication systems often employ retransmission algorithms such as ARQ to ensure reliable communications in fading channels. Maximizing the communications throughput in such systems can cause excessive queueing delays due to the random number of retransmission attempts required for each codeword. In this paper we consider the problem of delay-limited throughput maximization with a constraint on the expected waiting-time, which incorporates both queueing and coding delays. We explore the trade-off between queueing and coding delays and propose a novel queue management technique for fading channels.
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    Throughput Measures for Delay-constrained Communications in Fading Channels
    (2003-10-01) Ahmed, Nadeem; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
    Fading channels, often seen in wireless systems, provide an unfavorable environment for reliable communications. Current methods for evaluating the performance of fading channels include ergodic capacity and epsilon-capacity. Ergodic capacity quantifies the ultimate reliable communication limit of the fading channel. It is only achievable with infinite coding delay, making it impossible to achieve in practice. epsilon-capacity, achievable with finite coding delay, does not provide a measure of error-free communications performance. Since practical communication systems are delay-constrained, it is possible to retransmit codewords when errors occur. We provide a new analysis framework that accounts for codeword retransmission in the analysis of fading channels. We introduce new measures, maximum zero-outage throughput and maximum epsilon-throughput, that predict the performance of practical systems and show that ergodic capacity and epsilon-capacity are special cases of our definitions. We also provide a measure that characterizes the performance of a system with more complex receiver design, using "incremental diversity" to improve throughput.