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Communications, IEEE Transactions on

Issue 9 • Date Sept. 2000

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Displaying Results 1 - 24 of 24
  • Abstracts of forthcoming manuscripts

    Page(s): 1601 - 1602
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    Freely Available from IEEE
  • Noncoherent iterative (turbo) decoding

    Page(s): 1488 - 1498
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    Previously, noncoherent sequence detection schemes for coded linear and continuous phase modulations have been proposed, which deliver hard decisions by means of a Viterbi algorithm. The current trend in digital transmission systems toward iterative decoding algorithms motivates an extension of these schemes. In this paper, we propose two noncoherent soft-output decoding algorithms. The first solution has a structure similar to that of the well-known algorithm by Bahl et al. (1974), whereas the second is based on noncoherent sequence detection and a reduced-state soft-output Viterbi algorithm. Applications to the combined detection and decoding of differential or convolutional codes are considered. Further applications to noncoherent iterative decoding of turbo codes and serially concatenated interleaved codes are also considered. The proposed noncoherent detection schemes exhibit moderate performance loss with respect to corresponding coherent schemes and are very robust to phase and frequency instabilities View full abstract»

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  • Soft-decision demodulation design for COVQ over white, colored, and ISI Gaussian channels

    Page(s): 1499 - 1506
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    In this work, the design of a q-bit (scalar and vector) soft-decision demodulator for Gaussian channels with binary phase-shift keying modulation is investigated. The demodulator is used in conjunction with a soft-decision channel-optimized vector quantization (COVQ) system. The COVQ is constructed for an expanded (q>1) discrete channel consisting of the concatenation of the modulator, the Gaussian channel, and the demodulator. It is found that as the demodulator resolution q increases, the capacity of the expanded channel increases, resulting in an improvement of the COVQ performance. Consequently, the soft-decision demodulator is designed to maximize the capacity of the expanded channel. Three Gaussian channel models are considered as follows: (1) additive white Gaussian noise channels; (2) additive colored Gaussian noise channels; and (3) Gaussian channels with intersymbol interference. Comparisons are made with (a) hard-decision COVQ systems, (b) COVQ systems which utilize interleaving, and (c) an unquantized (q=∞) soft-decision decoder proposed by Skoglund and Hedelin (1999). It is shown that substantial improvements can be achieved over COVQ systems which utilize hard decision demodulation and/or channel interleaving. The performance of the proposed COVQ system is comparable with the system by Skoglund and Hedelin-though its computational complexity is substantially less View full abstract»

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  • Performance of truncated type-II hybrid ARQ schemes with noisy feedback over block fading channels

    Page(s): 1477 - 1487
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    This paper considers truncated type-II hybrid automatic repeat-request (ARQ) schemes with noisy feedback over block fading channels. With these ARQ techniques, the number of retransmissions is limited, and, similar to forward error correction (FEC), error-free delivery of data packets cannot be guaranteed. Bounds on the average number of transmissions, the average coding rate as well as the reliability of the schemes are derived using random coding techniques, and the performance is compared with FEC. The random coding bounds reveal the achievable performance with block codes and maximum-likelihood soft-decision decoding. Union upper bounds and simulation results show that over block fading channels, these bounds can be closely approached with simple terminated convolutional codes and soft-decision Viterbi decoding. Truncated type-II hybrid ARQ and the corresponding FEC schemes have the same probability of packet erasure; however, the truncated ARQ schemes offer a trade-off between the average coding rate and the probability of undetected error. Truncated ARQ schemes have significantly higher average coding rates than FEC at high and medium signal-to-noise ratio even with noisy feedback. Truncated ARQ can be viewed as adaptive FEC that adapts to the instantaneous channel conditions View full abstract»

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  • Another application for trellis shaping: PAR reduction for DMT (OFDM)

    Page(s): 1471 - 1476
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    A bound for the possible reduction of the peak-to-average ratio (PAR) dependent on the rate as well as possible practical procedures are presented. The idea of trellis shaping, originally used to minimize average transmit power in single-carrier systems, is applied to the problem of PAR reduction in multicarrier transmission. Its impact, as a function of code rate, as well as design practicability is considered using metrics in both the time and frequency domains View full abstract»

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  • Successive interference cancellation in multicarrier DS/CDMA

    Page(s): 1530 - 1540
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    In this paper, we present a successive interference cancellation (SIC) scheme for a multicarrier (MC) direct-sequence code-division multiple-access (CDMA) system, using band-limited spreading waveforms to prevent self-interference. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, a minimum mean-squared error (MMSE) receiver, and matched filter (MF) detection with maximal-ratio combining (MRC). We also consider suboptimal combining using pilot symbol-assisted modulation (PSAM) to make the system more realistic. Analytic bit-error probabilities for SIC, MMSE, and MF in correlated Rayleigh fading channels are derived. The theoretical results for SIC, MMSE, and MF are shown to agree well with simulations. In particular, SIC and MMSE are shown to achieve better performance than MF with MRC; when the number of users is small, SIC provides better performance than does MMSE. Further, the correlation among different subcarriers is studied, and only large subcarrier correlation coefficients result in an obvious worsening of performance. Finally, we derive results for the performance degradation that an SIC scheme experiences in MC CDMA due to phase and timing tracking errors. It is shown that SIC can still retain a performance advantage over MF-MRC, although the advantage of SIC decreases with increasing tracking errors, especially when subcarrier correlation coefficients are small View full abstract»

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  • Innovations-based MAP detection for time-varying frequency-selective channels

    Page(s): 1507 - 1519
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    By using the innovations process, this paper provides a unification and extension of the existing maximum a posteriori (MAP) detectors (MAPDs). The practically important topics of linear modulations, time-varying frequency-selective channels, differential phase detection, and fractional sampling are accounted for. The MAPDs are derived under different conditions of optimality and a priori knowledge as follows: when the MAP criterion is applied to the constellation mapper's input bits or output symbols, when all observations or only a fixed number of future observations (i.e. fixed-lag MAPDs) from a transmission are available, when the time-varying channel impulse response is perfectly known, and when only the Gaussian-distributed channel's mean and autocovariance and the noise variance are known. As these quantities are actually unknown, their estimation in the context of MAP detection is also discussed. The MAPDs are characterized through simulation and a novel, unified analysis. Although MAPDs are less suited to hardware implementation than the traditional maximum-likelihood sequence detectors, the MAPDs can accept nonuniform a priori bit or symbol probabilities and provide soft outputs. In this way, the MAPDs are well suited to iterative decoding, and so they will become increasingly integral to high-performance receiver designs View full abstract»

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  • Decomposition of fading dispersive channels-effects of mismatch on the performance of MLSE

    Page(s): 1467 - 1470
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    Decompositions of fading dispersive channels find utility in channel modeling and in the estimation of channel state information in a receiver. This letter considers decompositions based on the f-power series and Karhunen-Loeve expansions and investigates their relative ability to model selected channels. Development of such models should enable the design of reduced complexity receiver structures View full abstract»

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  • A reduced-state soft-statistics-based MAP/DF equalizer for data transmission over long ISI channels

    Page(s): 1441 - 1446
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    In this contribution, a novel version of the reduced-state maximum a posteriori probability/decision-feedback (MAP/DF) equalizer for intersymbol interference channels with long impulse responses is presented. The main feature of the proposed equalizer is that the soft statistics generated by the MAP receiver are employed to recursively compute a suitable index of the actual reliability of the (soft) decisions feeding the feedback filter. Therefore, in the presented equalizer, the usual (over-optimistic) assumption of error-free decisions at the input of the feedback filter is relaxed and improved performances are experienced, especially on channels with long impulse responses View full abstract»

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  • Adaptive multiuser CDMA detector for asynchronous AWGN channels-steady state and transient analysis

    Page(s): 1541 - 1549
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    A two-stage adaptive multiuser detector in an additive white Gaussian noise code-division multiple-access channel is proposed and analyzed. Its first stage is an asynchronous one-shot decorrelator which in terms of computational complexity only requires inversion of K symmetric K×K matrices for all K users. In addition, the K inversions can be done in parallel, and the computed results for one user can be reused by all other users as well, resulting in a latency of only one bit, same as its synchronous counterpart. The decorrelated tentative decisions are utilized to estimate and subtract multiple-access interference in the second stage. Another novel feature of the detector is the adaptive manner in which the multiple-access interference estimates are formed, which renders prior estimation of the received signal amplitudes and the use of training sequences unnecessary. Adaptation algorithms considered include steepest descent (as well as its stochastic version), and a recursive least squares-type algorithm that offers a faster transient response and better error performance. Sufficient conditions for the receiver to achieve convergence are derived. The detector is near-far resistant, and is shown to provide substantial steady-state error performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals View full abstract»

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  • Blind joint soft-detection assisted slow frequency-hopping multicarrier DS-CDMA

    Page(s): 1520 - 1529
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    A novel multiple-access scheme based on slow frequency-hopping multicarrier direct-sequence, code-division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrow-band CDMA and third-generation wide-band CDMA systems. Blind joint soft-detection of the SFH/MC DS-CDMA signals is investigated, assuming that the receiver has no knowledge of the associated frequency-hopping (FH) patterns invoked. The system's performance is evaluated over the range of Nakagami multipath fading channels. The results show that blind joint soft-detection achieves the required bit-error rate performance, while blindly acquiring the FH patterns employed. This is advantageous during the commencement of communications or during soft handover View full abstract»

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  • Theoretical and practical limits of next-generation high-speed digital subscriber loops

    Page(s): 1447 - 1450
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    For the single-pair HDSL (HDSL2), the concept of having different upstream and downstream power spectral densities is highlighted. The signal-to-noise ratio (SNR) at the input of the central office HDSL terminal unit (HTU-C) can be enhanced by boosting the power output of the remote user HDSL terminal unit (HTU-R). Further improvement in performance can be achieved by reducing the HTU-C NEXT interference. Two techniques are proposed to reach this goal. The first approach is to reduce the HTU-C transmitter power in the downstream direction for shorter reach cables. The second approach is borrowed from the analog frequency-modulation preemphasis/deemphasis frequency tilting in the upstream direction. Combining the three techniques leads to at least 6-dB SNR margin improvement for the full reach carrier serving area loops View full abstract»

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  • Performance of combined DDLL and AGC loop for direct-sequence spread-spectrum systems

    Page(s): 1455 - 1458
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    The automatic gain control (AGC) loops are usually used in communication systems to stabilize the performance. In this paper, the combined performance of digital delay-locked loop (DDLL) and AGC loop for direct-sequence spread-spectrum systems is analyzed. Especially, the inherent coupling effects between DDLL and AGC loop are developed. The numerical results show that the DDLL with AGC loop can offer a reliable and satisfactory performance View full abstract»

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  • Distance properties of finite-impulse response channels

    Page(s): 1429 - 1431
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    High-speed data communication over channels with limited bandwidth gives rise to intersymbol interference at the receiver usually resulting in a poor receiver error performance. An important parameter that determines the receiver error performance is the minimum Euclidean distance between two received sequences. Channel properties that maximize this distance are investigated. It is shown that maximum distance channels exist for any length of interference View full abstract»

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  • Multiuser decision-feedback receivers for asynchronous CDMA systems over a mismatched flat Rayleigh fading channel

    Page(s): 1563 - 1570
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    In this paper, the design of decision-feedback multiuser (DFM) receivers for asynchronous code-division multiple access (CDMA) systems over a mismatched flat Rayleigh fading channel is studied, and tight upper and lower bounds on the pair-wise symbol-error probability, which has been averaged over the channel fading, time delays, and phase angles, are derived for some of the DFM detection schemes. The performance bounds, which are computed by efficient analytic techniques, can serve as reliable references for CDMA system design. It is shown that the proposed system, which takes into account the effect of mismatched fading on the system design, has some gain over the system where the estimation of fading is assumed to be exact View full abstract»

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  • Carrier-frequency estimation for transmissions over selective channels

    Page(s): 1580 - 1589
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    This paper deals with carrier-frequency estimation for burst transmissions over frequency-selective channels. Three estimation schemes are proposed, all based on the use of known training sequences. The first scheme employs an arbitrary sequence and provides joint maximum-likelihood (ML) estimates of the carrier frequency and the channel response. Its implementation complexity is relatively high but its accuracy achieves the Cramer-Rao bound. The second scheme is still based on the ML criterion, but the training sequence is periodic, which helps to reduce the computational load. The third scheme also employs periodic sequences, but its structure comes from heuristic reasoning. Theoretical analysis and simulations are employed to assess the performance of the three schemes View full abstract»

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  • Iterative equalization with adaptive soft feedback

    Page(s): 1462 - 1466
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    In this letter, a novel equalization algorithm applying soft-decision feedback and designed for binary transmission is introduced. In contrast to conventional decision-feedback equalization (DFE), iterations are necessary, because a simple matched filter serves as feedforward filter, which collects signal energy, but creates noncausal intersymbol interference. The rule for generating soft decisions is adapted continuously to the current state of the algorithm. In most cases, standard DFE methods are clearly outperformed. For a class of certain channel impulse responses, performance of maximum-likelihood sequence estimation is attained, in principle. The high performance of the scheme is explained using results from neural network theory View full abstract»

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  • Turbo decoding for wireless systems with imperfect channel estimates

    Page(s): 1437 - 1440
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    A new metric for turbo decoding on Rayleigh fading channels with noisy channel estimates is derived. By including the error variance of the channel estimate in the decoder metric derivation, we calculate the correct channel reliability factor for this case. Simulations show that the signal-to-noise ratio gain of using this new metric, compared to ignoring the uncertainty of the channel estimate, may be as large as 1 dB at a bit error probability of 10-3. This gain is obtained without any complexity increase View full abstract»

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  • Multistage multiuser detection for FHMA

    Page(s): 1550 - 1562
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    A multistage multiuser detector (MMD) is presented for frequency-hopping/code-division multiple access (FH/CDMA.) The MMD reduces the bit-error rate (BER) over the conventional detector by exploiting prior knowledge of the addresses and energies of the user signals. This detector is a conservative multiuser detector which is robust to unknown users and has a complexity that is only linear in the number of users. The performance analysis of the synchronous MMD includes both theoretical and simulation BERs for the noiseless case as well as simulation results that include the presence of noise. The MMD is then extended to the fully asynchronous case which makes this work the first to propose an FH/CDMA multiuser detector for asynchronous communications. This asynchronous MMD is compared via simulation to the conventional detector View full abstract»

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  • Blind adaptive detection of DS/CDMA signals on time-varying multipath channels with antenna arrays using high-order statistics

    Page(s): 1590 - 1600
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    A new approach based on multiscale decomposition and higher-order statistics is presented for the simultaneous solution of multiuser interference and time-varying multipath propagation in the uplink of a cellular direct-sequence spread-spectrum code-division multiple-access (DS/CDMA) system. Each channel between the mobile transmitter and the base-station receiver is unknown and arbitrarily varying with time. The optimum filter achieving separation and multipath compensation is time-variant. The typical approach in many multiuser detectors previously proposed is to assume that the channel is almost static (time invariant) and attempt detection according to this model. Slow variations of the channels are then compensated using adaptive algorithms that force the estimates (of the channels or of the separating filters) to be constantly in search of a convergence point. If the channel coefficients variations in time are fast with respect to the convergence time of the adaptive algorithm, significant degradation may result. In this work, we depart from this traditional approach and we investigate new kernels that more accurately can characterize the time-varying nature of the detection problem. As a first step, we show that the super-exponential framework can still be applied in a time-variant environment. Then, we describe a multiresolution decomposition of the filter components, essentially constraining its variations in time to remain within the solution subspace. The resulting algorithm overcomes some of the drawbacks associated with slow convergence and insufficient tracking capability typical of many blind approaches and several nonblind methods based on the slow fading assumption View full abstract»

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  • Low-complexity carrier-phase estimator suited to on-board implementation

    Page(s): 1451 - 1454
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    In this paper, we describe and analyze a new feedforward carrier-phase algorithm suited to on-board digital implementation. The algorithm studied here (for phase recovery of M-ary phase-shift keying signals) has the particularity to perform all calculations in polar coordinates in order to reduce the implementation complexity. The defined phase estimator is operating at very low signal-to-noise ratio with small frequency offsets. Its performance is close to those of the Viterbi and Viterbi algorithm View full abstract»

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  • A fast linear programming algorithm for blind equalization

    Page(s): 1432 - 1436
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    A fast implementation of a special non-MSE cost function for blind equalization is presented here. This baud-rate equalization algorithm is based on a convex cost function coupled with a simple linear constraint on the equalizer parameters. For a generic class of channels with persistently exciting quadrature amplitude modulation input signals, this new algorithm allows the convergence of equalizer parameters to a unique global minimum achieving intersymbol interference suppression and carrier phase recovery View full abstract»

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  • Improved lightpath (wavelength) routing in large WDM networks

    Page(s): 1571 - 1579
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    We address the problem of efficient circuit switching in wide area networks. The solution provided is based on finding optimal routes for lightpaths and semilightpaths. A lightpath is a fully optical transmission path, while a semilightpath is a transmission path constructed by chaining several lightpaths together, using wavelength conversion at their junctions. The problem thus is to find an optimal lightpath/semilightpath in the network in terms of the cost of wavelength conversion and the cost of using the wavelengths on links. In this paper, we first present an efficient algorithm for the problem which runs in time O(k2n+km+kn log(kn)), where n and m are the number of nodes and links in the network, and k is the number of wavelengths. We then analyze that the proposed algorithm requires O(d 2nk02+mk0 log n) time for a restricted version of the problem in which the number of available wavelengths for each link is bounded by k0 and k0=o(n), where d is the maximum in-degree or out-degree of the network. It is surprising to have found that the time complexity for this case is independent of k. It must be mentioned that our algorithm can be implemented efficiently in the distributed computing environment. The distributed version requires O(kn) time and O(km) messages. Compared with a previous O(k2n+kn2) time algorithm, our algorithm has the following advantages. (1) We take into account the physical topology of the network which makes our algorithm outperform the previous algorithm. In particular, when k is small [e.g., k=O(log n)] and m=O(n), our algorithm runs in time O(n log2 n), while the previous algorithm runs in time O(n log n). (2) Since our algorithm has high locality, it can be implemented on the network distributively View full abstract»

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  • OFDM blind carrier offset estimation: ESPRIT

    Page(s): 1459 - 1461
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    In orthogonal frequency-division multiplex (OFDM) communications, the loss of orthogonality due to the carrier-frequency offset must be compensated before discrete Fourier transform-based demodulation can be performed. This paper proposes a new carrier offset estimation technique for OFDM communications over a frequency-selective fading channel. We exploit the intrinsic structure information of OFDM signals to derive a carrier offset estimator that offers the accuracy of a super resolution subspace method, ESPRIT 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