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45
DMT optimality of LRaided linear decoders for a general class of channels, lattice designs, and system models
 IEEE TRANS. INFOM. THEORY
, 2010
"... The work identifies the first general, explicit, and nonrandom MIMO encoderdecoder structures that guarantee optimality with respect to the diversitymultiplexing tradeoff (DMT), without employing a computationally expensive maximumlikelihood (ML) receiver. Specifically, the work establishes the ..."
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Cited by 33 (4 self)
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The work identifies the first general, explicit, and nonrandom MIMO encoderdecoder structures that guarantee optimality with respect to the diversitymultiplexing tradeoff (DMT), without employing a computationally expensive maximumlikelihood (ML) receiver. Specifically, the work establishes the DMT optimality of a class of regularized lattice decoders, and more importantly the DMT optimality of their latticereduction (LR)aided linear counterparts. The results hold for all channel statistics, for all channel dimensions, and most interestingly, irrespective of the particular latticecode applied. As a special case, it is established that the LLLbased LRaided linear implementation of the MMSEGDFE lattice decoder facilitates DMT optimal decoding of any lattice code at a worstcase complexity that grows at most linearly in the data rate. This represents a fundamental reduction in the decoding complexity when compared to ML decoding whose complexity is generally exponential in rate. The results’ generality lends them applicable to a plethora of pertinent communication scenarios such as quasistatic MIMO, MIMOOFDM, ISI, cooperativerelaying, and MIMOARQ channels, in all of which the DMT optimality of the LRaided linear decoder is guaranteed. The adopted approach yields insight, and motivates further study, into joint transceiver designs with an improved SNR gap to ML decoding.
FiniteSNR DiversityMultiplexing Tradeoff via Asymptotic Analysis of Large MIMO Systems
, 2010
"... Diversitymultiplexing tradeoff (DMT) was characterized asymptotically (SNR> infinity) for i.i.d. Rayleigh fading channel by Zheng and Tse [1]. The SNRasymptotic DMT overestimates the finiteSNR one [2]. This paper outlines a number of additional limitations and difficulties of the DMT framewo ..."
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Cited by 15 (9 self)
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Diversitymultiplexing tradeoff (DMT) was characterized asymptotically (SNR> infinity) for i.i.d. Rayleigh fading channel by Zheng and Tse [1]. The SNRasymptotic DMT overestimates the finiteSNR one [2]. This paper outlines a number of additional limitations and difficulties of the DMT framework and discusses their implications. Using the recent results on the sizeasymptotic (in the number of antennas) outage capacity distribution, the finiteSNR, sizeasymptotic DMT is derived for a broad class of fading distributions. The SNR range over which the finiteSNR DMT is accurately approximated by the SNRasymptotic one is characterized. The multiplexing gain definition is shown to affect critically this range and thus should be carefully selected, so that the SNRasymptotic DMT is an accurate approximation at realistic SNR values and thus has operational significance to be used as a design criterion. The finiteSNR diversity gain is shown to decrease with correlation and power imbalance in a broad class of fading channels, and such an effect is described in a compact, closed form. Complete characterization of the outage probability (or outage capacity) requires not only the finiteSNR DMT, but also the SNR offset, which is introduced and investigated as well. This offset, which is not accounted for in the DMT framework, is shown to have a significant impact on the outage probability for a broad class of fading channels, especially when the multiplexing gain is small. The analytical results and conclusions are validated via extensive Monte Carlo simulations. Overall, the sizeasymptotic DMT represents a valuable alternative to the SNRasymptotic one.
Bit Allocation Laws for MultiAntenna Channel Feedback Quantization: MultiUser Case
"... This paper addresses the optimal design of limitedfeedback downlink multiuser spatial multiplexing systems. A multipleantenna basestation is assumed to serve multiple singleantenna users, who quantize and feed back their channel state information (CSI) through a shared ratelimited feedback cha ..."
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Cited by 15 (5 self)
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This paper addresses the optimal design of limitedfeedback downlink multiuser spatial multiplexing systems. A multipleantenna basestation is assumed to serve multiple singleantenna users, who quantize and feed back their channel state information (CSI) through a shared ratelimited feedback channel. The optimization problem is cast in the form of minimizing the average transmission power at the basestation subject to users’ target signaltointerferenceplusnoise ratios (SINR) and outage probability constraints. The goal is to derive the feedback bit allocations among the users and the corresponding channel magnitude and direction quantization codebooks in a highresolution quantization regime. Toward this end, this paper develops an optimization framework using approximate analytical closedform solutions, the accuracy of which is then verified by numerical results. The results show that, for channels in the real space, the number of channel direction quantization bits should be (M−1) times the number of channel magnitude quantization bits, where M is the number of basestation antennas. Moreover, users with higher requested qualityofservice (QoS), i.e. lower target outage probabilities, and higher requested downlink rates, i.e. higher target SINR’s, should use larger shares of the feedback rate. It is also shown that, for the target QoS parameters to be feasible, the total feedback bandwidth should scale logarithmically with the geometric mean of the target SINR values and the geometric mean of the inverse target outage probabilities. In particular, the minimum required feedback rate is shown to increase if the users ’ target parameters deviate from the corresponding geometric means. Finally, the paper shows that, as the total number of feedback bits B increases, the performance of the limitedfeedback system approaches the perfectCSI system as 2 −B/M2
Optimal throughputdiversitydelay tradeoff in MIMO ARQ blockfading channels
 IEEE Trans. Inf., Theory
, 2008
"... In this paper, we consider an automaticrepeatrequest (ARQ) retransmission protocol signaling over a blockfading multipleinput, multipleoutput (MIMO) channel. Unlike previous work, we allow for multiple fading blocks within each transmission (ARQ round), and we constrain the transmitter to fixed ..."
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Cited by 10 (0 self)
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In this paper, we consider an automaticrepeatrequest (ARQ) retransmission protocol signaling over a blockfading multipleinput, multipleoutput (MIMO) channel. Unlike previous work, we allow for multiple fading blocks within each transmission (ARQ round), and we constrain the transmitter to fixed rate codes constructed over complex signal constellations. In particular, we examine the general case of average inputpowerconstrained constellations with a fixed signalling alphabet of finite cardinality. This scenario is a suitable model for practical wireless communications systems employing orthogonal frequency division multiplexing techniques over a MIMO ARQ channel. Two cases of fading dynamics are considered, namely shortterm static fading where channel fading gains change randomly for each ARQ round, and longterm static fading where channel fading gains remain constant over all ARQ rounds pertaining to a given message. As our main result, we prove that for the blockfading MIMO ARQ channel with a fixed signalling alphabet satisfying a shortterm power constraint, the optimal signaltonoise ratio (SNR) exponent is given by a modified Singleton bound, relating all the system parameters. To demonstrate the practical significance of the theoretical analysis, we present numerical results showing that practical
Joint power control and beamforming codebook design for MISO channels with limited feedback
 Global Telecommun. Conf. (Globecom
"... Abstract — This paper investigates the joint design and optimization of the power control and beamforming codebooks for the singleuser multipleinput singleoutput (MISO) wireless systems with a ratelimited feedback link. The problem is cast in the form of minimizing the outage probability subject ..."
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Cited by 8 (6 self)
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Abstract — This paper investigates the joint design and optimization of the power control and beamforming codebooks for the singleuser multipleinput singleoutput (MISO) wireless systems with a ratelimited feedback link. The problem is cast in the form of minimizing the outage probability subject to the transmit power constraint and cardinality constraints on the beamforming and power codebooks. We show that by appropriately choosing and fixing the beamforming codebook and optimizing the power codebook for that beamforming codebook, it is possible to achieve a performance very close to the optimal joint optimization. Further, this paper investigates the optimal tradeoffs between beamforming and power codebook sizes for different number of feedback bits and transmit antennas. Given a target outage probability, our results provide the optimal codebook sizes independent of the target rate. As the outage probability decreases, we show that the optimal joint design should use fewer feedback bits for beamforming and more feedback bits for power control. The jointly optimized beamforming and power control modules combine the power gain of beamforming and diversity gain of power control, which enable it to approach the performance of the system with perfect channel state information as the feedback link capacity increases to infinity — something that is not possible with beamforming or power control alone. I.
Bits About the Channel: Multiround Protocols for Twoway Fading Channels
 In Preparation. May 19, 2009 DRAFT
"... Most communication systems use some form of feedback, often related to channel state information. In this paper, we study diversity multiplexing tradeoff for both FDD and TDD systems, when both receiver and transmitter knowledge about the channel is noisy and potentially mismatched. For FDD systems, ..."
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Cited by 6 (3 self)
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Most communication systems use some form of feedback, often related to channel state information. In this paper, we study diversity multiplexing tradeoff for both FDD and TDD systems, when both receiver and transmitter knowledge about the channel is noisy and potentially mismatched. For FDD systems, we first extend the achievable tradeoff region for 1.5 rounds of message passing to get higher diversity compared to the best known scheme, in the regime of higher multiplexing gains. We then break the mold of all current channel state based protocols by using multiple rounds of conferencing to extract more bits about the actual channel. This iterative refinement of the channel increases the diversity order with every round of communication. The protocols are ondemand in nature, using high powers for training and feedback only when the channel is in poor states. The key result is that the diversity multiplexing tradeoff with perfect training and K levels of perfect feedback can be achieved, even when there are errors in training the receiver and errors in the feedback link, with a multiround protocol which has K rounds of training and K − 1 rounds of binary feedback. The above result can be viewed as a generalization of Zheng and Tse, and Aggarwal and Sabharwal, where the result was shown to hold for K = 1 and K = 2 respectively. For TDD systems, we also develop new achievable strategies with multiple rounds of communication between the transmitter and the receiver, which use the reciprocity of the forward and the feedback channel. The multiround TDD protocol achieves a diversitymultiplexing tradeoff which uniformly dominates its FDD counterparts, where no channel reciprocity is available.
Hybrid ARQ in MultipleAntenna Slow Fading Channels: Performance Limits and Optimal Linear Dispersion Code Design
, 2009
"... This paper focuses on studying the fundamental performance limits and linear dispersion code design for the MIMOARQ slow fading channel. Optimal average rate of wellknown HARQ protocols is analyzed. The optimal design of spacetime coding for the MIMOARQ channel is discussed. Informationtheoreti ..."
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Cited by 6 (0 self)
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This paper focuses on studying the fundamental performance limits and linear dispersion code design for the MIMOARQ slow fading channel. Optimal average rate of wellknown HARQ protocols is analyzed. The optimal design of spacetime coding for the MIMOARQ channel is discussed. Informationtheoretic measures are used to optimize the rate assignment and derive the optimum design criterion, which is then used to evaluate the optimality of existing spacetime codes. A different design criterion, which is obtained from the error probability analysis of spacetime coded MIMOHARQ, is presented. Examples are studied to reveal the gain of ARQ feedback in spacetime coded MIMO systems.
Throughput maximization over slowly fading channels using erroneous quantized feedback
 in IEEE Transactions on Communications
, 2009
"... We study rate and power adaptation of data codewords over slowly fading channels, when quantized and erroneous channel state information (CSI) is available to the transmitter. The goal is to maximize the expected data rate using superposition (multilayer) coding and power control at the transmitter ..."
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Cited by 6 (1 self)
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We study rate and power adaptation of data codewords over slowly fading channels, when quantized and erroneous channel state information (CSI) is available to the transmitter. The goal is to maximize the expected data rate using superposition (multilayer) coding and power control at the transmitter. The proposed CSI quantizer structure resembles channel optimized scalar quantizers (COSQs) with a newly introduced quasigrey bitmapping scheme. Our results show that superposition coding provides significant gains when feedback is more erroneous or channel uncertainty at the transmitter is high, whereas power control is more effective with more reliable feedback. I.
Diversity order gain with noisy feedback in multiple access channels
 in Proc. IEEE International Symposium on Information Theory
, 2008
"... Abstract—In this paper, we study the effect of feedback channel noise on the diversitymultiplexing tradeoff in multiuser MIMO systems using quantized feedback, where each user has m transmit antennas and the basestation receiver has n antennas. We derive an achievable tradeoff and use it to show t ..."
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Cited by 5 (4 self)
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Abstract—In this paper, we study the effect of feedback channel noise on the diversitymultiplexing tradeoff in multiuser MIMO systems using quantized feedback, where each user has m transmit antennas and the basestation receiver has n antennas. We derive an achievable tradeoff and use it to show that in SNRsymmetric channels, a single bit of imperfect feedback is sufficient to double the maximum diversity order to 2mn compared to when there is no feedback (maximum is mn at multiplexing gain of zero). Further, additional feedback bits do not increase this maximum diversity order beyond 2mn. Finally, the above diversity order gain of mn over nonfeedback systems can also be achieved for higher multiplexing gains, albeit requiring more than one bit of feedback. I.
Decentralized power control with twoway training for multiple access
 in Proc. ISIT
, 2008
"... Abstract—In this work, we analyze the diversitymultiplexing performance of a MIMO multiple access wireless system with noncooperating transmitters. Each of the transmitters and receiver use noisy and mismatched versions of the channel estimate to implement decentralized power control. While accou ..."
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Cited by 5 (5 self)
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Abstract—In this work, we analyze the diversitymultiplexing performance of a MIMO multiple access wireless system with noncooperating transmitters. Each of the transmitters and receiver use noisy and mismatched versions of the channel estimate to implement decentralized power control. While accounting for the resources consumed in training, we show that with relatively simple power control, regardless of the number of transmitters, we can achieve double the maximum diversity order of a system with no instantaneous channel state information at the transmitters. Intuitively, the gain can be attributed to using temporal degrees of freedom enabled by power control without coding over multiple coherence intervals. I.