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48
Bounding Performance and Suppressing Intercarrier Interference in Wireless Mobile OFDM
 IEEE Trans. Commun
, 2003
"... While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequencydivision multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve the performance. In this paper, we first ..."
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Cited by 44 (1 self)
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While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequencydivision multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve the performance. In this paper, we first derive a matchedfilter bound (MFB) for OFDM transmissions over doubly selective Rayleigh fading channels, which benchmarks the best possible performance if ICI is completely canceled without noise enhancement. We then derive universal performance bounds which show that the timevarying channel causes most of the symbol energy to be distributed over a few subcarriers, and that the ICI power on a subcarrier mainly comes from several neighboring subcarriers. Based on this fact, we develop lowcomplexity minimum meansquare error and decisionfeedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICIimpaired OFDM with affordable complexity. In the relatively low Doppler frequency region, biterror rate of the DFE receiver is close to the MFB.
Downlink capacity of interferencelimited MIMO systems with joint detection
 IEEE Trans. Wireless Commun
, 2004
"... The capacity of downlink cellular multipleinput multipleoutput (MIMO) cellular systems, where cochannel interference is the dominant channel impairment, is investigated in this paper, mainly from a signalprocessing perspective. Turbo spacetime multiuser detection (ST MUD) is employed for intrac ..."
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Cited by 32 (7 self)
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The capacity of downlink cellular multipleinput multipleoutput (MIMO) cellular systems, where cochannel interference is the dominant channel impairment, is investigated in this paper, mainly from a signalprocessing perspective. Turbo spacetime multiuser detection (ST MUD) is employed for intracell communications, and is shown to closely approach the ultimate capacity limits in Gaussian ambient noise for an isolated cell. Then it is combined with various multiuser detection methods for combating intercell interference. Among various multiuser detection techniques examined, linear minimummeansquareerror (MMSE) MUD and successive interference cancellation are shown to be feasible and effective. Based on these two multiuser detection schemes, one of which may outperform the other for different settings, an adaptive detection scheme is developed, which together with a Turbo ST MUD structure offers substantial performance gain over the well known VBLAST techniques with coding in this interferencelimited cellular environment. The obtained multiuser capacity is excellent in high to medium signaltointerference ratio scenario. Nonetheless, numerical results also indicate that a further increase in system complexity, using basestation cooperation, could lead to further significant increases of the system capacity. The asymptotic multicell MIMO capacity with linear MMSE MUD preprocessing is also derived, and this analysis agrees well with the simulation results.
BEqualdiagonal QR decomposition and its application to precoder design for Vol
 Proceedings of the IEEE 2389 and Choi: NearInstantaneously Adaptive HSDPAStyle OFDM Versus MCCDMA Transceivers
, 2007
"... Abstract—In multipleinput and multipleoutput (MIMO) multiuser detection theory, the QR decomposition of the channel matrix H can be used to form the backcancellation detector. In this paper we propose an optimal QR decomposition, which we call the equaldiagonal QR decomposition, or shortly the Q ..."
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Cited by 23 (4 self)
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Abstract—In multipleinput and multipleoutput (MIMO) multiuser detection theory, the QR decomposition of the channel matrix H can be used to form the backcancellation detector. In this paper we propose an optimal QR decomposition, which we call the equaldiagonal QR decomposition, or shortly the QRS decomposition. We apply the decomposition to precoded successivecancellation detection, where we assume that both the transmitter and the receiver have perfect channel knowledge. We show that, for any channel matrix H, there exists a unitary precoder matrix S, such that HS = QR, where the nonzero diagonal entries of the upper triangular matrix R in the QR decomposition of HS are all equal to each other. The precoder and the resulting successivecancellation detector have the following properties. a) The minimum Euclidean distance between two signal points at the channel output is equal to the minimum Euclidean distance between two
Analysis of Decision Feedback Detection for MIMO RayleighFading Channels and the Optimization of Power and Rate Allocations
, 2004
"... For an uncoded, Ktransmit, Nreceive antenna coherent narrowband communication system employing a decorrelating decision feedback detector (DDFD), the exact average (over channel realizations) joint error probability (JEP) as well as the average persymbol error probabilities (SEPs) are derived w ..."
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Cited by 22 (2 self)
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For an uncoded, Ktransmit, Nreceive antenna coherent narrowband communication system employing a decorrelating decision feedback detector (DDFD), the exact average (over channel realizations) joint error probability (JEP) as well as the average persymbol error probabilities (SEPs) are derived without making any simplifying assumptions on error propagation. It is proved that the diversity orders of the JEP and the SEP (of every symbol) is limited by error propagation to CI. Based on our exact error probability analysis, however, we suggest an optimization of JEP over nonnegative quadrature amplitude modulation (QAM) constellation sizes (rates) and average powers across transmitters which yield significant improvements over the usual equal power and equal rate assignment. In fact, the JEP of such an optimized design has the much improved diversity order of (which is also the diversity order obtained through the optimum
Optimal sequences for CDMA with decisionfeedback receivers
 IEEE Trans. Inform. Theory
, 2003
"... Abstract—We consider a symbolsynchronous codedivision multipleaccess (CDMA) system that is equipped with a multiuser decisionfeedback receiver and for which power control is available. The users are each assigned a qualityofservice (QoS) threshold to be guaranteed by the system, and to cover s ..."
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Cited by 14 (0 self)
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Abstract—We consider a symbolsynchronous codedivision multipleaccess (CDMA) system that is equipped with a multiuser decisionfeedback receiver and for which power control is available. The users are each assigned a qualityofservice (QoS) threshold to be guaranteed by the system, and to cover scenarios for which there are multiple classes of users, these are not required to be equal to each other. For an ideal decisionfeedback receiver, it is known that with enough power the system can always meet the users ’ QoS thresholds, so we instead minimize the sum of the users ’ received powers over system designs (i.e., signature sequences, powercontrol policy, and decisionfeedback receiver) which guarantee the QoS requirements. It is found that the optimal design produces two classes of users, those whose sequences and powers satisfy with equality the generalized Welch bound inequality and those oversized users that are mutually orthogonal to each other and the rest of the users. In terms of power and bandwidth savings, the optimal sequences for the decisionfeedback receiver are found to compare very favorably to optimal designs for linear receivers and to random sequences for the decisionfeedback receiver. Index Terms—Codedivision multiple access (CDMA), decisionfeedback receiver, linear minimum meansquareerror (MMSE) receiver, power control, qualityofservice (QoS), signature sequences. I.
An improved complex sphere decoder for VBLAST systems
 IEEE Signal Processing Letters, Vol.11, no.9
, 2004
"... Abstract—A new complex sphere decoding algorithm is presented for signal detection in VBLAST systems, which has a computational cost that is significantly lower than that of the original complex sphere decoder (SD) for a wide range of SNRs. Simulation results on a 64QAM system with 23 transmit and ..."
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Cited by 13 (1 self)
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Abstract—A new complex sphere decoding algorithm is presented for signal detection in VBLAST systems, which has a computational cost that is significantly lower than that of the original complex sphere decoder (SD) for a wide range of SNRs. Simulation results on a 64QAM system with 23 transmit and 23 receive antennas at an SNR per bit of 24 dB show that the new sphere decoding algorithm obtains the ML solution with an average cost that is at least 6 times lower than that of the original complex SD. Further, the new algorithm also shows robustness with respect to the initial choice of sphere radius. Index Terms—Branch and bound, complex sphere decoder, ML detection, VBLAST. I.
Multiuser Detection for Overloaded CDMA Systems
 IEEE TRANS. INFORM. THEORY
, 2002
"... Multiuser detection for overloaded CDMA systems, in which the number of users is larger than the dimension of the signal space, is of particular interest when bandwidth is at a premium. In this paper, certain fundamental questions are answered regarding the asymptotic forms and performance of subopt ..."
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Cited by 12 (2 self)
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Multiuser detection for overloaded CDMA systems, in which the number of users is larger than the dimension of the signal space, is of particular interest when bandwidth is at a premium. In this paper, certain fundamental questions are answered regarding the asymptotic forms and performance of suboptimum multiuser detectors for cases where the desired and/or interfering signal subspaces are of reduced rank and/or have a nontrivial intersection. In the process, two new suboptimum detectors are proposed that are especially well suited to overloaded systems, namely the group pseudodecorrelator and the group MMSE detector, and the former is seen to be the correct extension of the group decorrelator in the sense that it is the limiting form (in the lownoise regime) of the group MMSE detector. Pseudodecorrelation is also used as a feedforward filter in a new decision feedback scheme. For the particular case of realvalued modulation, it is shown that the recent proposals of the socalled "improved" linear (aka "linearconjugate" or "widely linear") detectors were more simply derived earlier using the idea of minimal sufficiency which we also apply to the new detectors of this paper.
Geometric Derivation of ExpectationMaximization and Generalized Successive Interference Cancellation Algorithms with Applications to CDMA Channel Estimation
, 2002
"... The expectationmaximization (EM) algorithm is well established as a computationally efficient method for separable signal parameter estimation. Here, a new geometric derivation and interpretation of the EM algorithm is given that facilitates the understanding of EM convergence properties. Geometri ..."
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Cited by 11 (8 self)
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The expectationmaximization (EM) algorithm is well established as a computationally efficient method for separable signal parameter estimation. Here, a new geometric derivation and interpretation of the EM algorithm is given that facilitates the understanding of EM convergence properties. Geometric consider ations lead to an alternative separable signal parameter estimator based on successive cancellation. The new Generalized Successive Interference Cancellation (GSIC) algorithm is then applied to multiuser delay and channel estimation for codedivision multiple access (CDMA) radiolocation and communication. The radiolocation application uses a handshaking protocol, in which multiple reference nodes transmit ac knowledge (ACK) CDMA packets to a master node which then computes roundtrip travel times (RTTs). Simulation results comparing EM and GSIC are presented for the CDMA channel estimation problem.
Bandwidth efficient multiple access (BEMA): A new strategy based on signal design with qualityofservice constraints for successivedecodingtype multiuser receivers
 IEEE Trans. Commun
, 2001
"... Abstract—This paper considers the design of signature waveforms for successivedecodingtype multiuser receivers [including the optimum successive decoder (OSD)] in a correlatedwaveform multipleaccess channel. The problem is to obtain signature waveforms that require as little bandwidth as possibl ..."
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Cited by 11 (4 self)
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Abstract—This paper considers the design of signature waveforms for successivedecodingtype multiuser receivers [including the optimum successive decoder (OSD)] in a correlatedwaveform multipleaccess channel. The problem is to obtain signature waveforms that require as little bandwidth as possible while allowing the receiver to meet a given set of qualityofservice (QoS) objectives. The QoS objectives are specified for each user in terms of capacity, or equivalently, the signaltointerference ratio. A (generally unachievable) lower bound is obtained on the minimum bandwidth required to achieve these QoS constraints. Moreover, a simple algorithm is proposed for obtaining signal sets that meet the QoS constraints when used with the OSD, and which, while not optimal, require a bandwidth that can be very close to the minimum required bandwidth. It is also shown that such signal sets allow for a significantly more efficient use of bandwidth than do orthogonal signals used in time or frequencydivision multiple access (TDMA/FDMA). Based on our signal design approach, we propose a new multipleaccess strategy that we refer to as bandwidthefficient multiple access (BEMA). While BEMA is more bandwidth efficient than TDMA or FDMA, it retains their desirable feature of needing only singleuser coding (and decoding) for each user. Index Terms—Bandwidth, multiaccess communication, multiuser channel, qualityofservice, signal design, successivedecoding.
VBLAST without Optimal Ordering: Analytical Performance Evaluation for Rayleigh Fading Channels
 IEEE Trans. Comm
, 2004
"... is simple, and hence, a popular choice for a multipleinput multipleoutput (MIMO) receiver. Its biterror rate (BER) performance has been studied mainly using numerical (Monte Carlo) techniques, since exact analytical evaluation presents serious difficulties. Close examination of the problem of BLA ..."
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Cited by 11 (9 self)
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is simple, and hence, a popular choice for a multipleinput multipleoutput (MIMO) receiver. Its biterror rate (BER) performance has been studied mainly using numerical (Monte Carlo) techniques, since exact analytical evaluation presents serious difficulties. Close examination of the problem of BLAST BER performance analysis reveals that the major difficulty for analytical evaluation is due to the optimal ordering procedure. Hence, we analyze the algorithm performance without optimal ordering. While this is a disadvantage of the analysis, there are certain advantages as well. Exact closedform analytical evaluation is possible for arbitrary number of transmit and receive antennas in an independent, identically distributed Rayleigh fading channel, which provides deep insight and understanding that cannot be gained using the Monte Carlo approach alone. A result on the maximum ratio combining weights, which is used at each detection step, is derived to obtain a number of results: independence of noise, distribution of signaltonoise ratio (SNR), and block or biterror rates. We present a detailed analysis and expressions for uncoded error rates at each detection step, which hold true for any modulation format and take simple closed form in some cases. Asymptotic form of these expressions for large SNRs is particularly simple. Extensive Monte Carlo simulations validate the analytical results and conclusions. Index Terms—Biterror rate (BER), Bell Labs layered space–time (BLAST), error propagation, multiantenna systems, multipleinput multipleoutput (MIMO), performance analysis. I.