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Communications, IEEE Transactions on

Issue 10 • Date Oct. 2002

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Displaying Results 1 - 21 of 21
  • Abstracts of forthcoming manuscripts

    Page(s): 1706
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    Freely Available from IEEE
  • Vector channel modeling and prediction for downlink beamforming in frequency-division duplex systems

    Page(s): 1571 - 1576
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (395 KB) |  | HTML iconHTML  

    Researchers have studied the behavior of the vector channel in order to study the quality of beamforming methods and to increase downlink signal power and signal-to-interference ratio. Jeng (see IEEE Trans. Antennas Propagat., vol.46, p.749-57, 1998) showed that knowledge beyond simply directions of arrival of the vector channel at one frequency may not be useful at another. We propose that if the ratio between two frequencies, f1 and f2, is close to one, then the autoregressive (AR) coefficients which model the behavior of the vector channel at f1 are essentially the same AR coefficients as those which model the vector channel at f2. This implies that accurate predictions of the vector channel at f2 can be performed knowing only the channel at f1, as well as enough knowledge of the channel at f2 in order to provide initial conditions. Ray-tracing simulations of downtown Austin, TX, show that this hypothesis holds for f1 = 1.85 GHz and f2 = 1.93 GHz, which correspond to uplink and downlink frequencies, respectively, in US personal communications systems. View full abstract»

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  • Some analytical tools for the design of space-time convolutional codes

    Page(s): 1593 - 1599
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (516 KB) |  | HTML iconHTML  

    Space-time convolutional codes have shown considerable promise for providing improved performance for wireless communication through combined diversity and coding gain. An efficient design procedure is presented for optimizing the coding and diversity gain measures proposed in the first papers on space-time codes. The procedure is based on some simple lower and upper bounds on coding gain. The same calculations needed to compute these bounds can be used to check either necessary or sufficient conditions on space-time codes which achieve maximum diversity gain. A new simple, but useful, measure of code performance is also suggested which augments existing measures. The use of the design procedure is illustrated and new codes are provided. These codes are shown to outperform the space-time convolutional codes provided in the initial papers introducing space-time codes. View full abstract»

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  • Variable bit allocation for FH-CDMA wireless communication systems

    Page(s): 1637 - 1644
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (356 KB) |  | HTML iconHTML  

    In the wireless indoor environment, the channel may vary slowly as users and the interferers may move at slow speeds. A frequency-hopped code-division multiple-access system can adapt to the different interference levels in hopping patterns and assign the slots different bit rates to increase the system capacity. We show that the maximum throughput bit-rate/channel-assignment problem is NP-hard. Several practical ad hoc bit-allocation algorithms are designed based on the insights derived from exhaustive searches. The algorithms that achieve the most system capacity perform interference avoidance. Users concentrate their throughput in a small fraction of the slots with low interference by transmitting large signal constellations, while avoiding the channels with large interference. Simulations show that the flexibility of users to adjust their bit rates to the interference environment can significantly increase the system capacity. View full abstract»

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  • Exact probability of error of BPSK communication links subjected to asynchronous interference in Rayleigh fading environment

    Page(s): 1577 - 1579
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (298 KB) |  | HTML iconHTML  

    We derive a new expression for the exact bit-error probability for the detection of a coherent binary phase-shift keying signal experiencing a number of asynchronous interferers having unequal power levels in Rayleigh fading channels. The new expression is readily computed in terms of the coefficients of a Hermite polynomial. View full abstract»

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  • Performance of parallel and serial concatenated codes on fading channels

    Page(s): 1600 - 1608
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (701 KB) |  | HTML iconHTML  

    The performance of parallel and serial concatenated codes on frequency-nonselective fading channels is considered. The analytical average upper bounds of the code performance over Rician channels with independent fading are derived. Furthermore, the log-likelihood ratios and extrinsic information for maximum a posteriori (MAP) probability and soft-output Viterbi algorithm (SOVA) decoding methods on fading channels are developed. The derived upper bounds are evaluated and compared to the simulated bit-error rates over independent fading channels. The performance of parallel and serial codes with MAP and SOVA iterative decoding methods, with and without channel state information, is evaluated by simulation over independent and correlated fading channels. It is shown that, on correlated fading channels, the serial concatenated codes perform better than parallel concatenated codes. Furthermore, it has been demonstrated that the SOVA decoder has almost the same performance as the MAP decoder if ideal channel state information is used on correlated Rayleigh fading channels. View full abstract»

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  • Efficiency and reliability of call admission control schemes using call-level behavior

    Page(s): 1645 - 1651
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (489 KB) |  | HTML iconHTML  

    While the efficiency of call admission control (CAC) schemes can be improved by taking advantage of call-level behavior, it is also important to make sure that the reliability of the CAC schemes is maintained. This paper studies the reliability aspect of CAC schemes for the reference model that consists of the M/M/c loss system at the call level, and the bufferless link with single-class exponential on-off fluid traffic at the burst level. As a measure of reliability, the probability that an admitted call violates a loss-ratio target is used, and its analytic expression is derived for the reference model by using performability analysis and the nearly completely decomposability approximation. It is shown that the CAC schemes previously proposed to take advantage of call-level behavior are not acceptably reliable. In addition, it is shown that, for most cases in practice, if the reliability of a CAC scheme taking advantage of call-level behavior is to be maintained at a reasonable level, the improvement in efficiency relative to the static scheme will be small. View full abstract»

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  • Adaptive cochannel interference cancellation in space-time coded communication systems

    Page(s): 1580 - 1583
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (269 KB) |  | HTML iconHTML  

    Space-time coding is a powerful scheme that combines channel coding, modulation, and multiple transmit antennas to achieve higher data rates and combat fading in wireless systems. In this letter, we propose a multiple-input multiple-output minimum mean-square error spatial-filtering-based adaptive antenna arrays method to suppress cochannel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time structure, thereby significantly improving the system's interference suppression ability without significant bit error rate performance degradation. View full abstract»

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  • Rotated TCM systems with dual transmit and multiple receive antennas on Nakagami fading channels

    Page(s): 1609 - 1616
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (563 KB) |  | HTML iconHTML  

    This paper analyzes the bit-error rate performance of a dual transmission-multiple reception antenna system in conjunction with a rotated trellis-coded modulation scheme over Nakagami fading channels. A simple way to combine the exact calculation of the pairwise error event probability with the transfer function bounding techniques is described. This approach allows us to evaluate an extremely tight theoretical upper bound on the average bit-error probability. The parameters which dominate the performance of these transmission schemes on fading channels are extracted from this evaluation. Optimum adaptive and fixed-phase rotations have been calculated, showing that most of the power efficiency obtained by using the optimum adaptive-phase rotation can be retained by using the optimum fixed-phase rotation. View full abstract»

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  • Analysis of phase noise effects in OFDM modems

    Page(s): 1696 - 1705
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (595 KB) |  | HTML iconHTML  

    The effects of the phase noise (PN) on orthogonal frequency-division multiplex modems are evaluated. Three receivers are studied: a coherent receiver, a common phase error correction receiver (which is a receiver specially designed to combat PN) and a differential receiver. The impact of the PN on the decision signal-to-noise ratio (SNR) of each of these receivers is computed as a function of the PN spectrum. The resulting formulas are extremely simple. The theory is applicable to a wide range of PN models, and unifies and extends previous results on the topic. The conditions under which the decision SNR yields correct symbol error rate predictions are discussed. Simulations are reported that confirm the results. View full abstract»

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  • Cochannel interference reduction in dynamic-TDD fixed wireless applications, using time slot allocation algorithms

    Page(s): 1627 - 1636
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (563 KB) |  | HTML iconHTML  

    In this paper, we consider a fixed wireless cellular network that uses dynamic time division duplex (D-TDD). We analyze the signal-to-interference ratio (SIR) outage performance of a D-TDD fixed cellular system, and propose a scheme to improve the outage probability performance. First, outage probability is evaluated using an analytical model, when omnidirectional antennas are deployed at a base-station site and a subscriber site. Our model is verified, using Monte Carlo simulations. According to our investigation, the outage performance of the D-TDD system is severely limited by a strong interference from the cochannel cell on the downlink, while the reference cell is in the uplink cycle. To improve the outage performance during uplink receptions, we introduce two time-slot allocation methods, combined with sector antennas: max min{SIR} and max{SIR}. Max min{SIR} is an exhaustive search algorithm for assigning subscribers to a few extra uplink time slots, so as to maximize the minimum SIR expectation value over the extra uplink time-slots region. It is used as a performance benchmark in our analysis. Meanwhile, the max{SIR} is a simpler and efficient algorithm for improving the outage performance. It is established that the performance difference between the two algorithms is not noticeable. Especially, the difference is negligible, when the dynamic range of the traffic pattern between uplink and downlink is small. Also, the outage performance of a system that employs the max{SIR} algorithm combined with sectored antennas is compared to that of a system employing adaptive-array antennas. The proposed system shows promise, and offers a compromise between system complexity and network guaranteed availability. View full abstract»

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  • A fast-acquiring blind predictive DFE

    Page(s): 1557 - 1560
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (293 KB) |  | HTML iconHTML  

    This article presents a blind predictive decision-feedback equalization (DFE) scheme motivated by the work of Labat et al. (see idid., vol.46, p.921-30, July 1998). The proposed scheme outperforms another previously proposed blind predictive DFE scheme, and also eliminates the filter interchange required in the scheme of Labat et al. View full abstract»

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  • Performance of the decorrelator receiver for DS-CDMA mobile radio system employing RAKE and diversity through Nakagami fading channel

    Page(s): 1566 - 1570
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (454 KB) |  | HTML iconHTML  

    Multiple-access interference (MAI) and multipath fading are two of the most significant factors limiting the capacity and performance of direct-sequence code-division multiple-access (DS-CDMA) systems. In this paper, synchronous multiuser receivers that combine antenna diversity, RAKE reception, and a multipath decorrelator for MAI cancellation are analyzed in a Nakagami faded environment using a maximal ratio combiner or a selection combiner. A coherent binary phase-shift keying employing DS-CDMA is considered. Arbitrary branch correlation is also considered for any diversity order in the case of identical severity fading on the branches. View full abstract»

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  • Smart antenna arrays with oscillating beam patterns: characterization of transmit diversity in semi-elliptic coverage

    Page(s): 1549 - 1556
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (626 KB) |  | HTML iconHTML  

    By applying carefully selected time-varying delays to the array elements of a smart antenna located at the base station (BS), small oscillations are generated in the beam pattern. These oscillations create a time-varying channel demonstrating intra-symbol time variation and characterized by coherence time TC. At a single-antenna mobile station (MS), the time-varying channel (with coherence time TC) creates a time diversity which is exploited to enhance the mobile's performance (by introducing oversampling to the mobile receiver). We present a channel model which characterizes the time-varying channel that results from beam pattern oscillation. We then use our channel model to evaluate the coherence time, TC, at the mobile station (MS). The channel model presented corresponds to the so-called geometric-based stochastic channel model (GSCM), with a semi-elliptic coverage area. This geometric approach allows us to stochastically model the parameters of the time-varying channel impulse response. Simulations based on the GSCM show that 7-fold time diversity can be exploited at the MS (when beam pattern movement is small), which significantly improves the MS receiver probability-of-error performance. View full abstract»

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  • Temporal/spatial fiber-optic CDMA systems with post- and pre-optical amplification

    Page(s): 1688 - 1695
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (528 KB) |  | HTML iconHTML  

    We present a thorough analysis based on photon-counting techniques on temporal/spatial fiber-optic code-division multiple-access systems incorporating both post- and pre-optical amplifiers. In this analysis, we consider the effect of shot noise, thermal noise, and source extinction on system bit-error rate. Our results can be used to estimate the maximum tolerable amount of quantum fluctuations in the received signal. View full abstract»

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  • Detection of linear modulations in the presence of strong phase and frequency instabilities

    Page(s): 1617 - 1626
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (594 KB) |  | HTML iconHTML  

    Noncoherent sequence detection algorithms, previously proposed by the authors, have a performance which approaches that of coherent detectors and are robust to phase and frequency instabilities. These schemes exhibit a negligible performance loss in the presence of a frequency offset, provided this offset does not exceed an order of 1% of the signaling frequency. For higher values, the performance rapidly degrades. In this paper, detection schemes are proposed, characterized by high robustness to frequency offsets and capable of tolerating offset values up to 10% of the signaling frequency. More generally, these detection schemes are very robust to rapidly varying phase and frequency instabilities. The general case of coded linear modulations is addressed, with explicit reference to M-ary phase shift keying and quadrature amplitude modulation. View full abstract»

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  • Performance of diversity schemes for OFDM systems with frequency offset, phase noise, and channel estimation errors

    Page(s): 1561 - 1565
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (562 KB) |  | HTML iconHTML  

    We provide expressions for the bit error rate of various transmit and receive diversity schemes for orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency offset, phase noise, and channel estimation errors. The derivations are also applicable for a general multiplicative distortion of the received signal. Our results show that with perfect channel estimates, practical values of the phase noise do not significantly degrade the performance of the various diversity methods for binary phase-shift keying modulation. In contrast, the transmit diversity schemes for OFDM are much more sensitive to channel estimation errors than maximal ratio combining receive diversity. View full abstract»

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  • Timing error recovery in turbo-coded systems on AWGN channels

    Page(s): 1584 - 1592
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (614 KB) |  | HTML iconHTML  

    The classical method for decoding turbo codes is the Bahl-Cocke-Jelinek-Raviv (1974) algorithm which, in the majority of existing papers, is analyzed under the assumption of perfect channel knowledge and synchronization. In reality, however, its properties change significantly when synchronization is not ideal. We show that the influence of timing synchronization offset can be modeled as a decreased effective signal-to-noise ratio, which provides results consistent with simulations. Furthermore, we present a general model for the soft bit output of the BCJR algorithm and apply it to the situation when the received signal is sampled with a timing offset. Finally, we use the derived properties of the algorithm in a simple timing synchronizer which is shown to outperform the classical maximum-likelihood non-data-aided symbol synchronizer without a need of using additional pilot symbols. View full abstract»

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  • Reductions of multiple-access interference in fiber-grating-based optical CDMA network

    Page(s): 1680 - 1687
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (479 KB) |  | HTML iconHTML  

    A fiber Bragg grating (FBG) encoder/decoder scheme based on correlation subtractions of nearly orthogonal M-sequence codes is presented. With proper coder design, a receiver can reject interfering users and obtain quasi-orthogonality between optical code-division multiple-access (CDMA) users in the network. However, optical CDMA networks may be degraded by multiple-access interference (MAI) due to nonflattened incoherent sources and nonideal FBG coders. A compensating module is therefore proposed to compensate for such MAI effects. As a result, the MAI effects induced by nonideal FBG coders can be perfectly eliminated by the compensating module. With spectral width reduction on the incoherent source, the scheme can partly compensate the MAI effects induced by nonflattened sources and further reduce the average error probability in the system performance. View full abstract»

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  • Performance and testbed study of topology reconfiguration in IP over optical networks

    Page(s): 1662 - 1679
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3790 KB) |  | HTML iconHTML  

    With the widespread deployment of Internet protocol/wavelength division multiplexing (IP/WDM) networks, it becomes necessary to develop traffic engineering (TE) solutions that can effectively exploit WDM reconfigurability. More importantly, experimental work on reconfiguring lightpath topology over testbed IP/WDM networks is needed urgently to push the technology forward to operational networks. This paper presents a performance and testbed study of topology reconfiguration for IP/WDM networks. IP/WDM TE can be fulfilled in two fashions, overlay vs. integrated, which drives the network control software, e.g., routing and signaling protocols, and selects the corresponding network architecture model, e.g., overlay or peer-to-peer. We present a traffic management framework for IP over reconfigurable WDM networks. Three "one-hop traffic maximization"-oriented heuristic algorithms for lightpath topology design are introduced. A reconfiguration migration algorithm to minimize network impact is presented. To verify the performance of the topology design algorithms, we have conducted extensive simulation study. The simulation results show that the topologies designed by the reconfiguration algorithms outperform the fixed topology with throughput gain as well as average hop-distance reduction. We describe the testbed network and software architecture developed in the Defense Advanced Research Projects Agency (DARPA) Next Generation Internet (NGI) SuperNet Network Control and Management project and report the TE experiments conducted over the testbed. View full abstract»

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  • Analysis of synchronization algorithms with time-out control over networks with exponentially symmetric delays

    Page(s): 1652 - 1661
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (864 KB) |  | HTML iconHTML  

    We study the performance of a class of time-offset estimation algorithms for synchronization of master-slave nodes based on asynchronous transfer of timing cells when GPS is not used. We implement a synchronization control mechanism based on cell acknowledgment time-out (TO) with wait or no wait options. We analyze the mechanism reliability and performance parameters over symmetric links using an exponential cell delay variation model. We show that the maximum-likelihood offset estimator does not exist for the exponential likelihood function. We analytically provide RMS error result comparisons for five ad hoc offset estimation algorithms: the median round delay, the minimum round delay, the minimum link delay (MnLD), the median phase, and the average phase. We show that the MnLD algorithm achieves the best accuracy over symmetric links without having to impose a strict TO control, which substantially speeds up the algorithm. We also discuss an open-loop estimation updating mechanism based on standard clock models. 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