Browsing by Author "Ahmed, Nasir"

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    Cooperative Communications in the Fading Channel
    (2005-03-01) Ahmed, Nasir; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Cooperative coding is a communication paradigm that pools distributed resources of different nodes in a network, such that the nodes act like a collaborative system instead of greedy adversarial participants. Cooperation has shown promise in increasing throughput and providing better power efficiency in wireless networks. In this work, we consider a basic example of cooperative communication, relay coding, and consider methods to improve the power efficiency by employing feedback and using power control. We consider power control policies based on the degree of transmitter channel knowledge. First, when perfect feedback is available, we show results for the optimal power control policy for any network code. We show that by using the decode and forward relaying protocol, in some cases it is possible to approach the universal lower bound on the outage probability for the block fading relay channel. Second, when a finite rate of feedback is available, we see that only a few feedback bits are necessary to achieve most of the gains that the perfect feedback policy has over constant power transmission. Based on these results, it is evident that future network protocols should utilize feedback in order to fully exploit the potential gains of network coding.
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    Exploiting Feedback in Cooperative Relay Networks
    (2005-08-01) Ahmed, Nasir; Center for Multimedia Communications (http://cmc.rice.edu/)
    Recent results on multiple antenna transmission techniques have shown great po- tential in their ability to improve the overall performance in fading channels. Despite the promise shown by employing multiple antenna's, practical implementations may not be feasible due to size and hardware limitations of mobile nodes. Cooperative Cod- ing is a new transmission paradigm which pools together the resources of neighboring nodes in a network to improve performance without requiring multiple antennas at any of the mobile devices. The power of node collaboration can be seen by considering the relay channel, the simplest cooperative network. Recently, protocols have been developed for the wireless relay channel that allow the network to behave as a virtual multiple antenna system. In this thesis we show that in addition to e±cient network protocols, exploit- ing channel state information can yield even more performance in the relay setting by allowing for temporal power and rate control. When power control is used for a given transmission rate, minimizing the outage probability is the appropriate method to maximize performance in the block fading channel. In a relay setting, we derive the optimal power control strategy when the transmitters in the network have perfect knowledge of the network channel state. In practice having perfect channel state knowledge at the transmitters is not possible. In this direction, we derive a power control policy that minimizes the outage probability based on the rate of the feedback link. Interestingly, we observe that only a few bits of feedback are needed to extract much of the gains of the perfect feedback power control policy. For applications that can support a variable rate of transmission, such as data transfers, the feedback can be used to vary both the transmission rate and power. The appropriate performance metric in this case is throughput. We derive throughput maximizing policies for various cooperative transmission protocols. Once again, we show that with a limited rate of feedback, signi¯cant throughput gains are possible in relay networks. Interestingly, we show that simultaneous power and rate adaptation is usually not needed. For small average power constraints, power control is imperative, while for large average powers, rate control is su±cient to achieve a large throughput. Our results reveal that power and rate adaptation can lead to signi¯cant perfor- mance improvements. Even a few bits of feedback can lead to large power savings and throughput gains, and as a result, channel state feedback can be readily implemented with minimal communication overhead in next generation protocols.
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    Finite Backlog Effects on Downlink Scheduling
    (2003-11-20) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In a multi-user downlink system, the use of channel estimates in the scheduling process can significantly improve the spectral efficiency. Recent work has shown that from a sum rate capacity standpoint, and assuming infinite queue backlogs, the optimal transmission scheme is to select the user with the largest instantaneous channel gain. A more realistic scenario is considered in this work, in which the downlink scheduling problem with finite queue backlogs is analyzed. It is shown that although time division scheduling is optimal for infinite backlogs, this is not the case for finite buffer sizes. In fact, a simple broadcast scheduler is shown to be more power efficient than the maximum rate time division scheduler under such a scenario.
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    Multilevel coding of broadcast video over wirless channels
    (2002-05-20) Cakareski, Zeljko; Ahmed, Nasir; Dhar, Aditya; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    This paper addresses the problem of broadcasting video over wireless channels. In such a scenario, different end users perceive the video signal through channels with different quality. We propose a source-channel coding system, that partitions the video data in such a way that maintains a baseline quality of service even for the user with the worst channel. The system consists of a layered source representation that is robust to error propagation effects, combined with a multilevel channel coding scheme that provides an unequal error protection to the different layers of the encoded video. Experimental results show that our system is robust and guarantees a baseline quality of service to all users over a wide range of channel conditions.
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    On Power Control with Finite Rate Feedback for Cooperative Relay Networks
    (2004-03-01) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Power control strategies with finite rate feedback are studied for the cooperative channel. The main contribution of this paper is to show that quantized feedback information can lead to significant reduction in outage probability for the cooperative relay network. To obtain an increase in diversity order and significant reductions in outage probability over constant power cooperative signaling, we develop algorithms that exploit the channel states of all network links. With one feedback bit, the proposed power control algorithm is shown to double the diversity order of constant power transmission. To quantify the performance increase of using power control in the cooperative network, we derive a lower bound on the diversity order. Based on these results, it is evident that future network protocols should utilize feedback in order to exploit the potential gains of network coding.
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    Outage Minimization and Optimal Power Control for the Fading Relay Channel
    (2004-05-01) Ahmed, Nasir; Khojastepour, Mohammad; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this work, we show that in the wireless relay network, a tremendous savings in energy can be achieved by having side information at the transmitters and by employing power control. We present efficient protocols and the corresponding optimal power control policies that approach the universal lower bound on the outage probability of the block fading relay channel. Each of the proposed protocols have their own utility for specific channel conditions. However, a hybrid protocol between two known coding schemes is the best scheme for all channel conditions and is sufficient to approach the lower bound on outage probability. Unlike the single link channel, we show that exploiting the knowledge of the channel at the transmitters can significantly lower the outage even if the transmit powers at the source and relay have to be kept constant. In this case, it is also demonstrated that the lower bound on outage is closely followed by the outage probability of the hybrid protocol. Our results reveal that exploiting the right network protocol in conjunction with power control result in orders of magnitude savings in power over direct transmission for a target performance level.
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    Outage Minimization with Limited Feedback for the Fading Relay Channel
    (2006) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this work, we consider practical methods to approach the theoretical performance limits in the fading relay channel under different assumptions of transmitter channel knowledge. Specifically, we consider two degrees of transmitter channel knowledge: (i) perfect feedback is available and power control is employed, and (ii) no channel state knowledge is available at the transmitters and only spatial power allocation is possible. First, when perfect feedback is available, the optimal power control policy determines the ultimate limits of performance for constant rate transmission in the slow fading environment. However, in practice, perfect channel knowledge is not possible at the transmitters due to the finite capacity of the feedback links. We find practical methods to approach this performance limit through the use of power control with finite rate feedback. The finite rate feedback results are shown for the low complexity, full diversity amplify-and-forward(AF) protocol. Interestingly, we see that only a few feedback bits are needed to achieve most of the gains of the optimal perfect feedback power control algorithm. Second, we consider the performance limit when the transmitters have no channel state knowledge, and derive the optimal spatial power allocation between the source and relay for a given sum power constraint for the AF protocol. For most practical cases of interest, equal power allocation between the source and relay is shown to be nearly optimal. Our work suggests that there is minimal power savings from using spatial power allocation at the transmitters. To obtain large performance improvements over constant power transmission, it is imperative to have feedback for each realization of the channel state to allow for temporal power control.
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    Performance Improvements with Feedback in Cooperative Relay Networks
    (2005-08-01) Ahmed, Nasir; Center for Multimedia Communications (http://cmc.rice.edu/)
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    Performance improvements with feedback in cooperative relay networks
    (2006) Ahmed, Nasir; Aazhang, Behnaam
    Recent results on multiple antenna transmission techniques have shown great potential in their ability to improve the overall performance in fading channels. Despite the promise shown by employing multiple antenna's, practical implementations may not be feasible due to size and hardware limitations of mobile nodes. Cooperative Coding is a new transmission paradigm that overcomes these limitations by pooling together the resources of neighboring nodes in a network to create a distributed antenna array. The power of node collaboration can be seen by considering the relay channel, the simplest cooperative network. Recently, protocols have been developed for the wireless relay channel that allow the network to behave as a virtual multiple antenna system. In this thesis we show that in addition to efficient network protocols, exploiting channel state information can yield even more performance in the relay setting by allowing for temporal power and rate control. When power control is used for a given transmission rate, minimizing the outage probability is the appropriate method to maximize performance in the block fading channel. In a relay setting, we derive the optimal power control strategy when the transmitters in the network have perfect knowledge of the network channel state. In practice having perfect channel state knowledge at the transmitters is not possible. In this direction, we derive a power control policy that minimizes the outage probability based on the rate of the feedback link. Interestingly, we observe that only a few bits of feedback are needed to extract much of the gains of the perfect feedback power control policy. For applications that can support a variable rate of transmission, such as data transfers, the feedback can be used to vary both the transmission rate and power. The appropriate performance metric in this case is throughput. We derive throughput maximizing policies for various cooperative transmission protocols. Once again, we show that with a limited rate of feedback, significant throughput gains are possible in relay networks. Interestingly, we show that simultaneous power and rate adaptation is usually not needed. For small average power constraints, power control is imperative, while for large average powers, rate control is sufficient to achieve a large throughput. Our results reveal that power and rate adaptation can lead to significant performance improvements. Even a few bits of feedback can lead to large power savings and throughput gains, and as a result, channel state feedback can be readily implemented with minimal communication overhead in next generation protocols.
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    Power Issues in Communication Systems
    (2002-04-20) Ahmed, Nasir; Ahmed, Nasir; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this work, we are concerned with power efficient communication techniques for wireless mobile devices. In this direction the effects of nonlinear amplifiers in communications systems are first studied, as they tend to consume a large fraction of available battery power. Next, a new information theoretic upper bound is proposed that takes into account power efficiency, and the bound achieving input distributions are analyzed and shown to be strikingly different that those that achieve channel capacity. Next, in order to examine methods to achieve the power efficient input distributions, results on shaping gains are restated, with particular emphasis on non-equiprobable signalign in two dimensions. With slight changes in classical techniques we are able to achieve power efficient signaling schemes that reduce the wasted power from RF amplification. Finally, simulation results are shown to verify the savings in wasted energy obtained through the use of these power efficient signalign schemes.
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    Power issues in communication systems
    (2002) Ahmed, Nasir; Aazhang, Behnaam
    In this work, we are concerned with power efficient communication techniques for wireless mobile devices. In this direction the effects of nonlinear amplifiers in communications systems are first studied, as they tend to consume a large fraction of available battery power. Next, a new information theoretic upper bound is proposed that takes into account power efficiency, and the bound achieving input distributions are analyzed and shown to be strikingly different than those that achieve channel capacity. Next, in order to examine methods to achieve the power efficient input distributions, results on shaping gains are restated, with particular emphasis on non-equiprobable signaling in two dimensions. With slight changes in classical techniques we are able achieve power efficient signaling schemes that reduce the wasted power from RF amplification. Finally, simulation results are shown to verify the savings in wasted energy obtained through the use of these power efficient signaling schemes.
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    Throughput Gains with Limited Feedback in Cooperative Relay Networks
    (2005-06-01) Ahmed, Nasir; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this work, we consider delay-constrained systems in cooperative relay networks, and derive methods to maximize the throughput when a limited feedback link to the transmitter nodes exist. We derive the optimal rate control policy, and show that with just a few bits of feedback, most of the gains of the perfect feedback rate control policy are achievable. We maximize the throughput for popular methods of transmission in a network such as relaying, routing, and direct transmission. It is observed that for a finite rate of feedback, the throughput maximizing outage probability can be relatively high. This suggests using higher rate codes, and allowing some outages in an effort to increase the overall throughput. Interestingly, for large average power constraints, power control is unnecessary to maximize the throughput. On the other hand, for small average power constraints, we show that power control is more important than rate control.