R-3 Repository :: Browsing by Author "Erkip, Elza"

Browsing by Author "Erkip, Elza"

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Beamformer Design with Feedback Rate Constraints : Criteria and Constructions
(2003-07-20) Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this work, we provide a geometrical framework for the analysis and design of beamformer codebooks with finite number of beamformer vectors. We present a design criterion for good beamfomer codebooks and show the equivalence of the beamformer design problem to two other known problems. First, the beamformer design problem can be directly posed as a problem of packing 2 dimensional subspaces in a 2t dimensional Grassmannian manifold, t being the number of transmit antennas. And second, under certain conditions, the beamformer design problem is equivalent to the construction of unitary space time constellations.
A comparative study of multiple accessing schemes
(1997-11-20) Erkip, Elza; Aazhang, Behnaam; Fargues, M.P., Hippenstiel, R.D.; Center for Multimedia Communications (http://cmc.rice.edu/)
We compare the performance of different accessing schemes (frequency division, time division and code division) for the uplink in a wireless communication system. We assume an additive white Gaussian noise channel with multipath fading. We consider the case when the receiver can track the channel parameters, but the transmitters cannot. We look at three different measures of performance: the Shannon capacity, the delay limited capacity and probability of outage. Shannon capacity is better suited for systems where the delay requirements are not as stringent, whereas the delay limited capacity and probability of outage give the performance when there are strict delay requirements or when the channel is slowly fading. We observe that all three schemes have the same achievable rates of transmission in Shannon sense, but the code division multiple access scheme performs better than frequency and time division in terms of delay limited capacity and probability of outage.
Computationally Efficient Iterative Multiuser Detection and Decoding
(1998-11-20) Das, Suman; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We describe an iterative detection and decoding scheme for the uplink in a convolutionally coded direct sequence code division multiple access (DS-CDMA) system. We consider maximum-a-posteriori (MAP) decoding and reduce the complexity through an iterative interference cancelation scheme combined with a suboptimal channel decoding algorithm. The MAP priors are updated at every iteration step. We investigate the performance of this low complexity scheme and observe that it is close to optimal.
Distorted Channel Capacity: A Geometrical Approach
(2000-06-20) Das, Suman; Cavallaro, Joseph R.; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
Shannon's capacity result has often been used as a benchmark to compare the performance of practical codes. Shannon's theorem helps us to calculate the maximum rate at which data can be transferred without making any errors. Most of the practical codes never achieve this zero error bound. Moreover most of the digital data like audio and video can tolerate various amount of errors. In this paper we introduce the notion of distorted channel capacity - the maximum rate at which data can be transferred when we allow certain amount of error in the recovered bits. We also calculate the distorted channel capacity for a gaussian channel and a fading channel.
Increasing uplink capacity via user cooperation diversity
(IEEE, 1998-08-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
Mobile users' capacity is limited by the fact that within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. We are proposing a new form of diversity, whereby diversity gains are achieved via the cooperation of in-cell users. Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
Iterative Multiuser Detection and Decoding
(1998-11-20) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
An iterative detection and decoding algorithm is explored for a convolutionally coded DS-CDMA system. The optimum decoding strategy is to consider the trellises of all the users simultaneously, but this has exponential complexity in the number of users. We use an iterative +multistage detection scheme combined with the MAP algorithm to reduce the complexity of the receiver without significant performance degradation. The simulation results show that our algorithm consistently outperforms other existing low complexity algorithms. We also provide a framework where the similarities and differences between various algorithms become transparent. Based on this framework, we make a quantitative comparison of the various schemes under investigation.
Low Complexity Iterative Multiuser Detection and Decoding for Real-Time Applications
(IEEE, 2005-07-01) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
This paper presents a low-complexity multiuser decoding technique that can be implemented in real time for a convolutionally coded direct sequence code division multiple access (DS-CDMA) system. The main contribution, denoted here as the iterative prior update (IPU), consists of iterative interference cancellation and prior updates on sequences of coded bits combined with M-algorithm and list decoding. We illustrate performance gains over other low-complexity sequence detection and decoding strategies and argue that the algorithm converges within a few iterations and requires only a small size buffer for keeping track of the priors along iterations. The fact that the we can use existing available architectures for Viterbi decoding with slight modifications and can meet the real-time processing constraints makes the IPU algorithm an attractive alternative for cellular systems.
Maximum Weight Basis Decoding of Convolutional Codes
(2000-06-20) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
The effectiveness of the convolutional code increases with the constraint length of the code. Unfortunately the decoding complexity of Viterbi algorithm grows exponentially with the constraint length. In this paper we propose a suboptimal decoding method based on the calculation of the maximum weight basis of the convolutional code. We extend the algorithm to incorporate the ideas of list decoding method. The complexity of the algorithm grows only quadratically with the constraint length and the performance of the algorithm is comparable to the optimal Viterbi decoding method.
Maximum Weight Basis Decoding of Convolutional Codes
(IEEE, 2002-11-01) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
In this paper we describe a new suboptimal decoding technique for linear codes based on the calculation of maximum weight basis of the code. The idea is based on estimating the maximum number locations in a codeword which have least probability of estimation error without violating the codeword structure. In this paper we discuss the details of the algorithm for a convolutional code. The error correcting capability of the convolutional code increases with the constraint length of the code. Unfortunately the decoding complexity of Viterbi algorithm grows exponentially with the constraint length. We also augment the maximal weight basis algorithm by incorporating the ideas of list decoding technique. The complexity of the algorithm grows only quadratically with the constraint length and the performance of the algorithm is comparable to the optimal Viterbi decoding method.
Multiple access schemes over multipath fading channels
(1998-10-20) Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We compare the performance of different accessing schemes for communication over an additive white Gaussian noise multiple access channel with multipath fading. We assume that only the receiver can track the channel variations. We investigate time division, frequency division and code division multiple access schemes and consider the Shannon capacity as well as the probability of outage. Although orthogonal code division multiple access has the same total capacity with frequency and time division, it has better probability of outage. We also provide an extension of our comparison to a CDMA system employing non-orthogonal codes and multiuser detection.
New Spread Spectrum Techniques for Multiple Antenna Transmit Diversity
(1999-12-20) Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a new scheme for multiple antenna transmission in the context of spread spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple atennas. We show that this new scheme greatly improves the throughout over crrently known multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed feedback rate.
On Beamforming with Finite Rate Feedback in Multiple Antenna Systems
(2003-10-20) Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In 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^-B/(t-1), where B is the number of feedback bits and t 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 2t-dimensional real Grassmannian manifold with chordal distance as the metric.
On Channel State Information in Multiple Antenna Block Fading Channels
(2000-11-20) Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
Motivated by large capacity gains in multiple antenna systems when perfect channel state information is available at both receiver and transmitter, we examine the achievable rates of channel measurement based techniques. We assume that instantaneous channel state is not known a priori and measured at the receiver and transmitter using part of the total system resources. The receiver side information is obtained using a preamble. For transmitter side information, two classes of feedback channel are studied, a time-division duplex mode and a two-way channel mode. For each class, we study both low-rate quantized and reverse preamble feedback. Lower bounds for achievable rates are derived for the proposed methods, and these bounds are used for resource allocation among the preamble and the feedback. For slowly fading channels, significant gains over non-measurement schemes are possible even with a single bit of feedback.
Performance limits on beamforming with finite rate feedback for multiple antenna systems
(2002-11-20) Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this work, we address the problem of evaluating the outage performance of beamformers designed for multiple transmit and single receive antenna systems when quantized channel state information is available at the transmitter. We present a tight universal lower bound for the outage probability of an arbitrary beamformer codebook consisting of a finite number of beamforming vectors. We also characterize the loss in outage performance due to the quantized nature of the channel information available at the transmitter. In particular, we show that the outage probability of beamforming based on quantized channel information approaches the performance of perfect information beamforming as (t-1)2^-B/(t-1), where B is the number of bits used to quantize the vector channel and t is the number of transmit antennas.
Spreading and Power Allocation for Multiple Antenna Transmission using Decorrelating receivers
(2003-05-20) Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a new scheme for multiple antenna transmission in the context of spread spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple antennas. We show that this strategy of using Multiphase Spreading (MPS) on different antennas greatly improves the throughput over currently known spread-spectrum multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed rate of channel state information, which might be provided via a feedback link, at the transmitter. We demonstrate the differences in optimal power distribution for maximizing capacity and minimizing probability of outage. When the transmission from the two antennas uses orthogonal spreading, we find that optimizing the power does not give much gain over the equal power transmission. However, when the transmissions are not orthogonal as in the case of MPS, then allocating power to maximize throughput gives considerable gain over equal power transmission. We also consider the effect of imperfections in the feedback channel on the optimal power allocation and show that our power allocation scheme is robust to feedback errors.
User Cooperation Diversity--Part I: System Description
(2003-11-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
Mobile users' data rate and quality of service are limited by the fact that, within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. In this two-part paper, we propose a new form of spatial diversity, in which diversity gains are achieved via the cooperation of mobile users. Part I describes the user cooperation strategy while Part II focuses on implementation issues and performance analysis. Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity for both users but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
User Cooperation Diversity--Part II: Implementation Aspects and Performance Analysis
(2003-11-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
This is the second in a two-part series of papers on a new form of spatial diversity, where diversity gains are achieved through the cooperation of mobile users. Part I described the user cooperation concept and proposed a cooperation strategy for a conventional CDMA system. Part II investigates the cooperation concept further and considers practical issues related to its implementation. In particular, we investigate the optimal and suboptimal receiver design, and present performance analysis for the conventional CDMA implementation proposed in Part I. We also consider a high rate CDMA implementation and a cooperation strategy when assumptions about the channel state information at the transmitters are relaxed. We illustrate that, under all scenarios studied, cooperation is beneficial in terms of increasing system throughput and cell coverage, as well as decreasing sensitivity to channel variations.

Browsing by Author "Erkip, Elza"

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