Browsing by Author "Khoshnevis, Ahmad"

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    Achievable Diversity and Multiplexing in Multiple Antenna Systems with Quantized Power Control
    (2005-05-01) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    We consider a multiple antenna system with finite rate feedback, in which the quantized channel state information at the transmitter is used solely for temporal power control. We show that similar to systems without feedback, the tradeoff between diversity order and multiplexing gain exists. However, unlike the systems with feedback that apply both rate and power control, systems with only power control are unable of achieving non-zero diversity order at the maximum multiplexing gain. The analysis is based on asymptotic behavior of the distribution of order statistics of the eigenvalues of channel matrix, which is a key step in evaluating the diversity order.
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    Coding-Spreading Tradeoff for Lattice Codes
    (2001-04-20) Khoshnevis, Ahmad; Center for Multimedia Communications (http://cmc.rice.edu/)
    A fixed bandwidth expansion can be achieved either by coding or spreading, while each have different effect on the resultant signal space. Coding increases both Shannon and Fourier bandwidth whereas spreading only increases the Fourier bandwidth. In this document we are looking for the optimum combination of coding and spreading, in a Code Division Multiple Access (CDMA) system, that minimizes the average frame error rate under fading channel with multiple antennas at transmitter and receiver. Using the theory of lattice code, we show that in a system with K users, the optimum spreading factor N equals K. Simulation results support the analysis. In simulations we used Minimum Mean Square Error (MMSE), and Matched Filter (MF) as multi-user detector. We also assumed that receiver knows the Channel State Information (CSI). In case of multiple antennas Alamouti scheme at transmitter and Maximum Ratio Combining (MRC) at the receiver are applied.
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    Coding-spreading tradeoff for lattice codes
    (2001) Khoshnevis, Ahmad; Aazhang, Behnaam
    A fixed bandwidth expansion can be achieved either by coding or spreading, while each have different effect on the resultant signal space. Coding increases both Shannon and Fourier bandwidth whereas spreading only increases the Fourier bandwidth. In this document we are looking for the optimum combination of coding and spreading, in a code division multiple access (CDMA) system, that minimizes the average frame error rate under fading channel with multiple antenna at transmitter and receiver. Using the theory of lattice code, we show that in a system with K users, the optimum spreading factor N equals K. Simulation results support the analysis. In simulations we used Minimum Mean Square Error (MMSE), and Matched Filter (MF) as multi-user detector. We also assumed that receiver knows the channel state information (CSI). In case of multiple antennas Alamouti scheme [Ala98] at transmitter and maximum ratio combining (MRC) at the receiver are applied.
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    An Introduction to Wideband CDMA
    (2000-05-20) Khoshnevis, Ahmad; Center for Multimedia Communications (http://cmc.rice.edu/)
    The main goal of this project is to design a space-time code system for wideband code division multiple access (Wideband CDMA). In this work we tried to make the essential foundations for this goal. There are two key questions. First is the tradeoff between coding and spreading in a wideband channel. Since the bandwidth expansion is achievable both from coding and from spreading the signal in a code division multiple access (CDMA) system. And second is the type of spreading. Since in DS-SS system the signal power is spread in a wide bandwidth, in limit, the capacity approaches zero. We also looked at the tradeoffs between direct sequence and frequency hopped spread spectrum in this work.
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    Network Channel Estimation in Cooperative Wireless Networks
    (2003-05-20) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    In distributed wireless networks, where nodes actively participate in helping communication for other nodes, they are typically unaware of their neighourhood and hence have to "estimate" it before sending any useful data. In this paper, we formalize the concept of node neighbourhood by introducing the notion of network channel, in which all nodes become part of a large channel. The notion of network channel is then used to study routing in decode and forward networks. To do the same, we introduce the concept of network coherence time, which denotes the time for which the network topology remains approximately constant. The new concepts are used to study the tradeoffs between encoding rate of route discovery packets, number of discovered routes and accuracy of subsequent network channel estimation. Finally, we propose a simple adaptive algorithm for route selection using outage capacity as the metric for route selection, and show that our algorithm outperforms the existing route selection based on the minimum hop count.
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    On Diversity and Multiplexing Gain of Multiple Antenna Systems with Transmitter Channel Information
    (2004-10-01) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    We quantify the multiplexing-diversity tradeoff of a multiple-input multiple-output (MIMO) system, when the channel state information (CSI) is known perfectly at the receiver and partially at the transmitter. The partial knowledge of CSI at the transmitter consists of the quantized value of one of the eigenvalues and perfect knowledge of eigenvectors of the channel matrix. The key result is that while multiplexing gain cannot be increased beyond minimum number of transmit and receive antennas, diversity order for each multiplexing gain can be substantially increased by using only a few bits of feedback at the transmitter. For example, with 1 bit of feedback in a 2x3 system, for multiplexing gains of 0, 1, and 2, diversity gains of 42, 6, and 2 can be achieved, respectively. Thus, while the tradeoff between diversity advantage and multiplexing gain is still present, its behavior is significantly changed by channel knowledge at the transmitter. The major reason for this different tradeoff can be attributed to addition of long-term power control, which allows the transmitter to switch between modes for reducing outage and increasing throughput based on signal to noise ratio along different eigenvalues.
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    On the Asymptotic Performance of Multiple Antenna Channels with Fast Channel Feedback
    (2005-10-01) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we analyze the asymptotic performance of multiple antenna channels where the transmitter has either perfect or finite bit channel state information. Using the diversitymultiplexing tradeoff to characterize the system performance, we demonstrate that channel feedback can fundamentally change the system behavior. Even one-bit of information can increase the diversity order of the system compared to the system with no transmitter information. In addition, as the amount of channel information at the transmitter increases, the diversity order for each multiplexing gain increases and goes to infinity for perfect transmitter information. The major reason for diversity order gain is a â location-dependentâ temporal power control, which adapts the power control strategy based on the average channel conditions of the channel.
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    On the Overhead-Delay Tradeoff in Carrier Sense Collision Channel
    (2005-09-01) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we study the impact of delay constraints on the throughput of a queued multiple-access system. We model the channel as a collision channel with \emph{carrier sense} to capture the inherent information sharing due to broadcast nature of the wireless channels. Since the queue state information is unknown to other nodes and a delay-bounded communication is desired, we show that a fraction of throughput is lost as protocol overhead. More importantly, we show that there is a tradeoff between protocol overhead and queuing delay; larger delays allowing smaller overheads. In addition, we show that larger network loads can also be used to reduce protocol overhead, which is in direct contrast of the behavior exhibited by commonly used medium access protocols.
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    Performance of Quantized Power-Control in Multiple Antenna Systems
    (2004-06-01) Khoshnevis, Ahmad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we analyze the outage probability of a single user system with multiple antennas at the transmitter, single antenna at the receiver, and finite rate feedback power control. The optimum power control is complex and the analysis is not tractable. Hence we propose a sub-optimal power allocation scheme, which is asymptotically optimum, with very low computational complexity. Analyzing the proposed algorithm we show that the diversity order can potentially be increased unboundedly by increasing the feedback rate and without increasing number of transmit or receive antennas. We find a closed form approximation to this diversity-like gain at large SNRs, as a function of number of transmit antennas, number of quantization levels, and average available SNR. Simulation results confirm the validity of the analysis.
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    Physical layer algorithms with limited feedback: Power control and coding strategies
    (2007) Khoshnevis, Ahmad; Sabharwal, Ashutosh
    The uncertainty in the received signal is the main limiting factor for the throughput of any communication system with limited resources. In this thesis we analyze the potential gain in throughput by managing channel uncertainty, noise, and interference using algorithms such as power control and coding techniques applied at the transmitter. Our contributions are (i) design and analysis of algorithms based on limited information about the uncertainties, and (ii) taking into account the resources required for collecting these information. The thesis is divided into two parts. The first part considers the case of channel information at the transmitter. Since most receivers implicitly or explicitly estimate the channel, often a closed loop system is designed to provide the channel state information at the transmitter. We model a finite capacity feedback link by assuming that only few bits of information about the channel are available at the transmitter. We develop a power control algorithm that is asymptotically optimum. Using the designed power control scheme, we characterize the tradeoff between diversity order and multiplexing gain of the multiple antenna systems, and conclude that feedback can significantly reduce uncertainty due to channel fading. In the second part we consider a multiple access channel with two transmitters and a single receiver, and analyze the impact of cooperation between the senders on the system throughput. In two case studies we establish the importance of cooperation, and the tradeoff between throughput and access overhead. Further, we show that the cost associated with establishing cooperation may exceed the consequent gain. Thus, as the major contribution of the second part, we develop a model based on two-way channel. We are the first to find upper and lower bounds on achievable rates of multiple access two-way channels with noisy feedback. Since the feedback link uses the same channel resources, the cost of feedback is automatically considered. By constructing a coding scheme we show that the two-way channel model is essential in having throughput gain in multiple access channel with noisy feedback. We also demonstrate that feedback based cooperation can reduce uncertainty due to interference and additive noise.