While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequency-division 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 matched-filter 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 time-varying 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 low-complexity minimum mean-square error and decision-feedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICI-impaired OFDM with affordable complexity. In the relatively low Doppler frequency region, bit-error rate of the DFE receiver is close to the MFB.

For an uncoded, K-transmit, N-receive antenna coherent narrow-band communication system employing a decorrelating decision feedback detector (D-DFD), the exact average (over channel realizations) joint error probability (JEP) as well as the average per-symbol 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

Abstract—This paper considers the design of signature waveforms for successive-decoding-type multiuser receivers [including the optimum successive decoder (OSD)] in a correlated-waveform multiple-access 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 quality-of-service (QoS) objectives. The QoS objectives are specified for each user in terms of capacity, or equivalently, the signal-to-interference 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 frequency-division multiple access (TDMA/FDMA). Based on our signal design approach, we propose a new multiple-access strategy that we refer to as bandwidth-efficient multiple access (BEMA). While BEMA is more bandwidth efficient than TDMA or FDMA, it retains their desirable feature of needing only single-user coding (and decoding) for each user. Index Terms—Bandwidth, multiaccess communication, multiuser channel, quality-of-service, signal design, successive-decoding.

This paper considers joint delay (timeof -arrival (TOA)) and channel estimation for DS- CDMA communication systems, with an emphasis on the radiolocation problem. A (]eneralized Successive Interference Cancellation ((]SIC) algorithm is developed for multiuser delay/channel estimation in the presence of frequency-selective fading. A ray-tracing propagation model is employed to obtain realistic channel impulse reponses for the (]SIC simulation. Simulation results are presented showing the performance of radiolocation using the ray-tracing propagation model, (]SIC algorithm, and a linearized positioning solution.

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 non-trivial intersection. In the process, two new suboptimum detectors are proposed that are especially well suited to overloaded systems, namely the group pseudo-decorrelator 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 low-noise regime) of the group MMSE detector. Pseudo-decorrelation is also used as a feedforward filter in a new decision feedback scheme. For the particular case of real-valued modulation, it is shown that the recent proposals of the so-called "improved" linear (aka "linear-conjugate" 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.

strategy where transmitter pulses are continually designed at the base station and are dynamically allocated to the transmitters via a feedback channel. Such pulses (or “signature waveforms”) are designed to conserve bandwidth while simultaneously enabling the receiver at the base station to meet a Quality-of-Service (QoS) specification for each transmitter. The key technical problem in BEMA communication is therefore the design of the transmitter pulses for the base-station receiver. In an earlier paper [1], we presented solutions to this problem that were shown to be superior (in terms of strict bandwidth) to common signaling schemes such as Time-, Frequency-, and Code-Division Multiple Access (TDMA, FDMA, and CDMA). This paper uses the framework developed in [1], but considers strictly time-limited transmitter pulses and the root-mean squared (rms) bandwidth measure. As in [1], significant bandwidth savings over the traditional multiple-access strategies are obtained. However, in contrast to the rank-conserving approach of [1], the bandwidth gains of this paper are realized by tailoring the signature waveform design to conserve rms bandwidth via eigenvalue-optimization problems. Index Terms—Bandwidth, decision feedback, multiaccess communication, multiuser channel, Quality-of-Service, signal design. I.

Abstract—Uplink communication in a cellular radio network is considered where the base station in each cell employs linear or nonlinear (decision feedback) multiuser receivers. For any such receiver, the problem of interest is that of minimizing the total transmit power under the constraint that all the users of the network achieve their quality-of-service objective in terms of signal-to-interference ratio (SIR). When the solution is feasible for the desired SIR requirements, the optimum powers are computed with a distributed iterative power control strategy suitable for implementation at each base station. While the deterministic algorithm requires both in-cell and out-of-cell user information, the stochastic algorithm proposed in this paper can be implemented at the base stations in a truly distributed manner requiring knowledge of only in-cell parameters. Such an algorithm was proposed recently for the case where base stations use linear (single user) matched filter (MF) receivers. However, the feasibility region in terms of attainable SIRs for a well-designed multiuser receiver, particularly for a nonlinear receiver that employs decision feedback, is generally much larger than it is for the linear MF receiver. The stochastic power control algorithm in this paper, for linear or nonlinear multiuser receivers, converges in the mean-square sense to the minimal powers when the target SIRs are feasible. The second major focus of this paper is to improve the convergence properties of the conventional stochastic approximation based power control strategy by using the more recent results on averaging. Convergence issues of both the “nonaveraged ” and “averaged ” algorithms are investigated, and numerical examples are presented to demonstrate the performance improvement due to averaging. Index Terms—Code-division multiple access, decision-feedback receivers, multiuser detection, power control, stochastic approximation. I.

We propose a class of layered space-time block codes (STBCs) for the quasi-static multi-input multi-output (MIMO) Rayleigh fading channel. The proposed codes use singleinput single-output (SISO) component codes based on algebraic number theory in the so called diagonal BLAST (D-BLAST) architecture. Based on a pairwise error probability analysis, we show that the diversity order of the frame error probability (FEP) of the proposed codes is Æà   à à   � in a Æ receive and à transmit antenna (Æ � Ã) system with à layers. Further, this diversity order is achieved with an average complexity, at moderate to high signal to noise ratio (SNR), of about Ç Ã per symbol interval. The error probability analysis also yields design criteria for the component SISO codes. Through simulation results we show that even with a much lower complexity the FEP performance of the D-BLAST lattice codes for moderate to high spectral efficiencies and a wide range of SNR is generally close to the performance offered by the universal lattice codes of [1] and the linear dispersion codes [2]. 1

Previous results on blind multiuser detection apply in situations where the signal parameters of the users of interest are known, and those of the interferers are unknown. In this paper, we consider the new paradigm of an-user system, in which users are active, and the problem is to detect users of interest out of those active users when the signal parameters (codes, amplitudes) of the users of interest are known, as are the codes of all users. What is not known at the receiver, however, is , the number of active interferers, and the identity of these interferers. A solution to such a problem could be to ignore the knowledge of the remaining codes, and apply known blind multiuser detectors based on stochastic approximation or subspace tracking techniques. However, it is shown here that the additional knowledge of those codes can be used to obtain an interference-identificationbased blind multiuser receiver that has much faster convergence properties. We illustrate the underlying principle in the context of blind group detection in synchronous direct-sequence/code-division multiple-access (DS/CDMA) systems operating in channels that exhibit frequency-selective fading.

Abstract—This paper presents a method for jointly designing the transmitter–receiver pair in a block-by-block communication system that employs (intrablock) decision feedback detection. We provide closed-form expressions for transmitter–receiver pairs that simultaneously minimize the arithmetic mean squared error (MSE) at the decision point (assuming perfect feedback), the geometric MSE, and the bit error rate of a uniformly bit-loaded system at moderate-to-high signal-to-noise ratios. Separate expressions apply for the “zero-forcing ” and “minimum MSE” (MMSE) decision feedback structures. In the MMSE case, the proposed design also maximizes the Gaussian mutual information and suggests that one can approach the capacity of the block transmission system using (independent instances of) the same (Gaussian) code for each element of the block. Our simulation studies indicate that the proposed transceivers perform significantly better than standard transceivers and that they retain their performance advantages in the presence of error propagation. Index Terms—Bit error rate, block precoding, channel capacity, decision feedback detection, minimum mean-square error, mutual information, zero-forcing. I.