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Communications, IEEE Transactions on

Issue 1 • Date Jan 2000

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Displaying Results 1 - 22 of 22
  • Code-spread CDMA using maximum free distance low-rate convolutional codes

    Page(s): 135 - 144
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (392 KB)  

    In code division multiple-access (CDMA) systems, maximum total throughput assuming a matched filter receiver can be obtained by spreading with low-rate error control codes. Previously, orthogonal, bi-orthogonal and super-orthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible low-rate convolutional codes with maximum free distance. The performance of these codes for spectrum spreading in a CDMA system is evaluated and shown to outperform that of orthogonal and super-orthogonal codes as well as conventionally coded and spread systems. We also show that the proposed low rate codes will give simple encoder and decoder implementations. With these codes, any rate 1/n, n⩽512, are obtained for constraint lengths up to 11, resulting in a more flexible and powerful scheme than those previously proposed View full abstract»

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  • Time-selective signaling and reception for communication over multipath fading channels

    Page(s): 83 - 94
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    The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code-division multiple-access systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel, which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the intersymbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the intersymbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudorandom codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels View full abstract»

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  • A matrix-algebraic approach to successive interference cancellation in CDMA

    Page(s): 145 - 151
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (200 KB)  

    In this paper, we describe linear successive interference cancellation (SIC) based on matrix-algebra. We show that linear SIC schemes (single stage and multistage) correspond to linear matrix filtering that can be performed directly on the received chip-matched filtered signal vector without explicitly performing the interference cancellation. This leads to an analytical expression for calculating the resulting bit-error rate which is of particular use for short code systems. Convergence issues are discussed, and the concept of ε-convergence is introduced to determine the number of stages required for practical convergence for both short and long codes View full abstract»

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  • Worst case equalizer for noncoherent HIPERLAN receivers

    Page(s): 28 - 36
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB)  

    Coherent detection of HIPERLAN Gaussian minimum-shift keying signals calls for complex and expensive receivers. However, when the channel delay spread is limited to at the most 50% of the symbol time and a reliable line-of-sight component of the radiated signal exists (Rician fading model), noncoherent detectors are capable of achieving a good performance. Based on the above motivations, we compare four different demodulation techniques, namely the following: (1) one-bit differential detector; (2) discriminator detector; (3) limiter discriminator detector; and (4) limiter discriminator integrator detector (LDID). The intersymbol interference introduced by these demodulators is nonlinear (with respect to the data symbols) and a decision-feedback equalizer (DFE) based on a mean square-error criterion may not be appropriate. Moreover, at this high speed, a DFE may be very complex to implement. Hence, we propose a new DFE design method that increases the eye-diagram aperture by removing the worst case interference. Performance of the above demodulators in the presence of a simple nonlinear DFE (with feedback part only) is computed in terms of the bit-error rate (BER) by means of the saddle-point approximation. This procedure, for static channels, turns out to be a very general tool with a simple and robust implementation. The same method can be applied, for multipath fading channels, to the BER evaluation as part of a semianalytic approach. The main conclusion from this work is that for LDID demodulators and in the presence of Rician fading channels with an average normalized root mean square delay spread of 0.3 and dual antenna diversity, the new equalizer lowers the outage probability from 60% to 10% at a BER of 10-4 View full abstract»

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  • Joint source-channel decoding for variable-length encoded data by exact and approximate MAP sequence estimation

    Page(s): 1 - 6
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    Joint source-channel decoding based on residual source redundancy is an effective paradigm for error-resilient data compression. While previous work only considered fixed-rate systems, the extension of these techniques for variable-length encoded data was independently proposed by the authors and by Demir and Sayood (see Proc. Data Comp. Conf., Snowbird, UT, p.139-48, 1998). We describe and compare the performance of a computationally complex exact maximum a posteriori (MAP) decoder, its efficient approximation, an alternative approximate decoder, and an improved version of this decoder are suggested. Moreover, we evaluate several source and channel coding configurations. The results show that our approximate MAP technique outperforms other approximate methods and provides substantial error protection to variable-length encoded data View full abstract»

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  • Joint source-channel and multiuser decoding for Rayleigh fading CDMA channels

    Page(s): 13 - 16
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (132 KB)  

    We consider joint source-channel and multiuser decoding for frequency selective Rayleigh fading code-division multiple-access channels. The block source-channel encoder is defined by a vector quantizer. We investigate optimal (minimum mean-square error) decoding and “user-separated” decoding of lower complexity. The studied decoders are soft in the sense that they utilize all soft information available at the receiver. Simulations indicate significant performance gains of the introduced decoders compared with a tandem approach that uses maximum-likelihood multiuser detection plus table-lookup decoding View full abstract»

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  • Asynchronous multicarrier DS-CDMA using mutually orthogonal complementary sets of sequences

    Page(s): 53 - 59
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (176 KB)  

    The use of sets of multiple spreading sequences per user in multicarrier code-division multiple-access (CDMA) is investigated. Each user is assumed to have a distinct set of spreading sequences, with a different spreading sequence for each carrier in each user's set. We show that when these sets of sequences are chosen to be the mutually orthogonal (MO) complementary sets of sequences, multiple-access interference is minimal on a nonfading channel. As a result of the autocorrelation sidelobe cancellation properties of the MO complementary sequences, it is possible to pack symbols more closely together on the nonfading channel, resulting in a higher data rate than in multicarrier CDMA using the same spreading sequence for each carrier. The resulting communication system scheme results in an easily parallelized receiver architecture that may be useful in nonfading coherent channels, such as the optical fiber channel or the Rician channel with a strong line-of-sight component. On the Rayleigh fading channel, the performance of the system is identical to that of multicarrier CDMA employing a single spreading sequence per user, with only a minimal increase in receiver complexity View full abstract»

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  • On the PDF of the sum of random vectors

    Page(s): 7 - 12
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (136 KB)  

    There are various cases in physics and engineering sciences (especially communications) where one requires the envelope probability density function (PDF) of the sum of several random sinusoidal signals. According to the correspondence between a random sinusoidal signal and a random vector, the sum of random vectors can be considered as an abstract mathematical model for the above sum. Now it is desired to obtain the PDF of the length of the resulting vector. Considering the common and reasonable assumption of uniform distributions for the angles of the vectors, many researchers have obtained the PDF of the length of the resulting vector only for special cases. However in this paper, the PDF is obtained for the most general case in which the lengths of vectors are arbitrary dependent random variables. This PDF is in the form of a definite integral, which may be inappropriate for analytic manipulations and numerical computations. So an appropriate infinite Laguerre expansion is also derived. Finally, the results are applied to solve a typical example in computing the scattering cross section of random scatterers View full abstract»

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  • Link activation protocols for a mobile communication network with directive/adaptive antennas

    Page(s): 60 - 74
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (540 KB)  

    A noncentralized fully distributed mobile communication network with directive/adaptive antennas is proposed. The spatial and temporal synchronization of transmitting and receiving antenna beams is accomplished by employing the global positioning system information and a synchronized spatial search algorithm. The link setup between a communication transmitter (CT) and a communication receiver (CR) depends on the success of both forward and reverse links. A two-way communication link protocol is defined as a basic agreement for the link activation in the network. Two link search algorithms, for additive white Gaussian noise channels, are proposed for use in hostile communication environments as follows: (1) a sequential ascending search power algorithm using an optimal set of link power levels and (2) a sequential Bayesian search algorithm (SBSA) using Bayes rule to update the CR location information. The optimal SBSA can be obtained by using dynamic programming, but the computational complexity is unfeasibly high. The experimental results in which the CT uses a noncoherent matched filter receiver for spread-spectrum code acquisition and the unfriendly interceptor uses a wide-band energy detector for both link search algorithms are presented View full abstract»

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  • Theory of the performance of a DECT receiver in a multipath channel

    Page(s): 75 - 82
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (204 KB)  

    The performance of a digital enhanced cordless telecommunications (DECT) receiver in a multipath channel is studied. The following issues were investigated: irreducible bit-error rate (BER) performance of the differentially coherent detector in Rayleigh and Rician channels; BER performance in the presence of both the noise in the receiver and a multipath interference. The theoretical estimates of the irreducible error rate developed in the paper showed reasonable agreement with the data obtained from computer-based simulation of the DECT link through a Rayleigh multipath channel. The combined effect from receiver noise and multipath interference has been studied, and asymptotic formulas for the total error probability have been developed. The results of calculating the error probabilities using these asymptotics demonstrate good agreement with measured data View full abstract»

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  • Design of channel-optimized vector quantizers in the presence of channel mismatch

    Page(s): 118 - 124
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (144 KB)  

    We study the design of channel-optimized vector quantizers in the presence of channel mismatch. We show that when the statistics of the channel bit-error rate (BER) are not known, a minimax solution is the one obtained by designing for the worst possible channel. Then, we consider the case when the probability density function of channel BER is known and propose an algorithm that provides a minimum average distortion. Also, by using an estimate of the channel BER at the decoder, we develop a decoder-adaptive scheme that further improves the performance. In all cases, we have limited ourselves to table-lookup decoders, which amount to very small computational complexities. Finally, the utilization of lookup tables at the encoder and the effects of imperfect estimation of channel BERs are considered View full abstract»

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  • Zero-forcing equalizability of FIR and IIR multichannel systems with and without perfect measurements

    Page(s): 17 - 22
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (128 KB)  

    We study the linear zero-forcing equalizability of a communication channel. Necessary and sufficient conditions are given in terms of the zeros of the transfer function. Detailed procedures of equalizer design for minimum sensitivity with respect to modeling errors and white noise are also presented. It is found that the worst case effect of channel mismatch on the performance may be modeled as equivalent losses in signal-to-noise ratio View full abstract»

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  • Combined rate and power adaptation in DS/CDMA communications over Nakagami fading channels

    Page(s): 162 - 168
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (212 KB)  

    We consider combined rate and power adaptations in direct-sequence code-division multiple-access communications, where the transmission power and the data rate are adapted relative to channel variations. We discuss the power gain that the combined adaptations provide over power adaptation. Then, we consider an integrated voice and data transmission system that offers a constant bit rate voice service, using power adaptation and a variable bit rate data service with rate adaptation. We present an expression for the required average transmission power of each traffic type having different quality-of-service specifications and discuss the capacity gain over power adaptation for voice and data View full abstract»

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  • Subspace blind adaptive detection for multiuser CDMA

    Page(s): 169 - 175
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    Direct adaptive realizations of the linear minimum mean-square error (MMSE) receiver for direct-sequence code-division multiple access possess the attractive feature of not requiring any explicit information of interference parameters such as timing, amplitudes, or spreading sequences; however, they need a training sequence for the desired user. Previously, a new blind adaptive receiver was proposed based on an anchored least mean-squared (LMS) algorithm that requires only the spreading code and symbol timing of the desired user but obviates the need for a training sequence. In this work, it is analytically demonstrated that the blind LMS algorithm always provides (nominally) faster convergence than the training driven LMS-MMSE receiver of but at the cost of increased tap-weight fluctuations or misadjustment. Second, the property that the optimal MMSE or minimum-output energy filter coefficients lies in the signal subspace is exploited to propose a new efficient blind adaptive receiver requiring fewer adaptive coefficients. Improved detector characteristics (superior convergence rates and steady-state signal-to-interference-plus-noise ratios) is indicated by analysis and supported by simulation View full abstract»

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  • Averaged stochastic gradient algorithms for adaptive blind multiuser detection in DS/CDMA systems

    Page(s): 125 - 134
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (248 KB)  

    In this paper, we present a blind adaptive gradient (BAG) algorithm for code-aided suppression of multiple-access interference (MAI) and narrow-band interference (NBI) in direct-sequence/code-division multiple-access (DS/CDMA) systems. This BAG algorithm is based on the concept of accelerating the convergence of a stochastic gradient algorithm by averaging. This ingenious concept of averaging was invented by Polyak and Juditsky (1992)-this paper examines its application to blind multiuser detection and NBI suppression in DS/CDMA systems. We prove that BAG has identical convergence and tracking properties to recursive least squares (LMS) but has a computational cost similar to the least mean squares (LMS) algorithm-i.e., an order of magnitude lower computational cost than RLS. Simulations are used to compare our averaged gradient algorithm with the blind LMS and LMS schemes View full abstract»

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  • Efficient identification of non-Gaussian mixtures

    Page(s): 106 - 117
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB)  

    Non-Gaussian mixtures are commonly used as parametric models for impulsive interference superimposed on a Gaussian background. In this study, the problem of efficient batch and recursive estimation of the parameters of such mixtures via minimization of the Kullback-Leibler distance is considered. The maximum-likelihood estimator (MLE) provides the starting point for the development of these estimators. First, it is shown that the MLE yields consistent estimates of the parameters, despite the existence of multiple roots in the Kullback-Leibler distance function. Since direct implementation of the MLE is difficult, an alternative estimator, designed with the objective of maximizing the likelihood function, is proposed. A simulation study of the proposed estimator reveals that it performs very well in terms of attaining the Cramer-Rao lower bound. Stochastic approximation is also considered in this study. A “complete data” modification to the standard recursion is presented, which greatly facilitates its implementation. Three initiating estimators are developed for these recursions: an iterative likelihood-based scheme, a batch estimator that uses a histogram of the data, and an iterative scheme that integrates the concept of complete data into the method-of-moments estimator. Upon initiating the modified stochastic approximation recursion with these estimators, it is seen that excellent estimates of the parameters can be obtained View full abstract»

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  • A matrix-algebraic approach to linear parallel interference cancellation in CDMA

    Page(s): 152 - 161
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (244 KB)  

    Linear parallel interference cancellation (PIC) schemes are described and analyzed using matrix algebra. It is shown that the linear PIC, whether conventional or weighted, can be seen as a linear matrix filter applied directly to the chip-matched filtered received signal vector. An expression for the exact bit-error rate (BER) is obtained, and conditions on the eigenvalues of the code correlation matrix and the weighting factors to ensure convergence are derived. The close relationship between the linear multistage PIC and the steepest descent method (SDM) for minimizing the mean squared error (MSE) is demonstrated. A modified weighted PIC structure that resembles the SDM is suggested which approaches the minimum MSE (MMSE) detector rather than the decorrelator. It is shown that for a K-user system, only K PIC stages are required for the equivalent matrix filter to be identical to the the MMSE filter. For fewer stages, techniques are devised for optimizing the choice of weights with respect to the MSE. One unique optimal choice of weights is found, which will lead to the minimum achievable MSE at the final stage. Simulation results show that a few stages are sufficient for near-MMSE performance View full abstract»

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  • Optimally regularized channel tracking techniques for sequence estimation based on cross-validated subspace signal processing

    Page(s): 95 - 105
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB)  

    New methods for maximum-likelihood sequence estimation based on the Viterbi algorithm (VA) are presented. In the proposed scheme, the channel estimator and the Viterbi processor operate concurrently. At any given time-step, the sequence provided to the channel estimator comes from the survivor with the best metric value. These already known modifications of the traditional decision-directed VA cause large variance in the estimated channel coefficients. In fact, sequences with a high error rate may be used to perform estimation, and also the adjustment term of the channel tracking algorithm may exhibit abrupt changes caused by a “survivor swap”, (that is by the event in which a different survivor has the best metric at step n, with respect to the step n-1). The proposed regularization procedure forces the channel vector to lie in the appropriate a priori known subspace: while the variance decreases, a certain amount of bias is introduced. The variance-bias tradeoff is then automatically adjusted by means of a cross-validation “shrinkage” estimator, which is at the same time optimal in a “small sample” predictive sum of squares sense and asymptotically model mean squared-error optimal. The method is shown by means of hardware experiments on a wide-band base station to be extremely more effective than per survivor processing, minimum survivor processing, and traditional decision directed approaches View full abstract»

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  • Optimal diversity allocation in multiuser communication systems. II. Optimization

    Page(s): 45 - 52
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (188 KB)  

    For pt.I see ibid., vol.47, no.1828-36 (1999). In Part I, a class of multicarrier systems was proposed to study the effect of the method of diversity allocation on the performance of coherent multiuser communication systems operating over fading channels. In this paper, optimization over the proposed class of systems is considered for a fixed number of users per unit bandwidth. The first case studied is a system where the only noise not attributable to users in the system is additive white Gaussian noise. It is observed that either a system employing exclusive allocation, where users are allocated time-bandwidth resources that are not simultaneously shared with other users, or a system employing maximum resource sharing, where all users simultaneously share time-bandwidth resources, is optimal. Next, the preferable of these two extreme forms of resource allocation is determined. For any reasonable signal-to-noise ratio (SNR) and user density, it is shown that the system employing exclusive resource allocation is optimal in a single-cell environment with perfect subchannel separation at the receiver. Finally, the optimization is repeated in the presence of partial-band interference (PBI). Once again, either a system employing exclusive resource allocation or a system employing a maximum resource sharing scheme is observed to be optimal. The presence of the PBI increases the range of user densities and SNRs where a system employing a maximum resource sharing scheme is optimal, particularly when the probability of a particular time-bandwidth slot experiencing interference is high View full abstract»

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  • On the computation and reduction of the peak-to-average power ratio in multicarrier communications

    Page(s): 37 - 44
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    For any code C defined over an equal energy constellation, it is first shown that at any time instance, the problem of determining codewords of C with high peak-to-average power ratios (PAPR) in a multicarrier communication system is intimately related to the problem of minimum-distance decoding of C. Subsequently, a method is proposed for computing the PAPR by minimum-distance decoding of C at many points of time. Moreover an upper bound on the error between this computed value and the true one is derived. Analogous results are established for codes defined over arbitrary signal constellations. As an application of this computational method, an approach for reducing the PAPR of C proposed by Jones and Wilkinson (1996) is revisited. This approach is based on introducing a specific phase shift to each coordinate of all the codewords where phase shifts are independent of the codewords and known both to the transmitter and the receiver. We optimize the phase shifts offline by applying our method for computing the PAPR for the coding scenario proposed by the ETSI BRAN Standardization Committee. Reductions of order 4.5 dB can be freely obtained using the computed phase shifts. Examples are provided showing that most of the gain is preserved when the computed optimal phase shifts are rounded to quantenary phase-shift keying (PSK), 8-PSK, and 16-PSK type phase shifts View full abstract»

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  • Computationally efficient optimal power allocation algorithms for multicarrier communication systems

    Page(s): 23 - 27
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (108 KB)  

    We present an optimal, computationally efficient, integer-bit power allocation algorithm for discrete multitone modulation. Using efficient lookup table searches and a Lagrange-multiplier bisection search, our algorithm converges faster to the optimal solution than existing techniques and can replace the use of suboptimal methods because of its low computational complexity. Fast algorithms are developed for the data rate and performance margin maximization problems View full abstract»

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Aims & Scope

IEEE Transactions on Communications focuses on all telecommunications including telephone, telegraphy, facsimile, and point-to-point television by electromagnetic propagation.

 

 

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Editor-in-Chief
Robert Schober
University of British Columbia