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Communications, IEEE Transactions on

Issue 12 • Date Dec. 2002

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Displaying Results 1 - 25 of 37
  • Space-time block-coded OFDM systems with RF beamformers for high-speed indoor wireless communications

    Page(s): 1899 - 1901
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (248 KB) |  | HTML iconHTML  

    A beamforming network is considered for a space-time block-coded orthogonal frequency-division multiplexing system in a high-speed indoor wireless network. We find that choosing the most powerful beams for transmission provide the best performance in the absence of interference. In the presence of interference, an iterative two-metric beam-selection method is proposed. View full abstract»

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  • Abstracts of forthcoming manuscripts

    Page(s): 2149
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    Freely Available from IEEE
  • Reviewers list 2002 IEEE communications society IEEE Transactions on communications editorial staff 2002

    Page(s): 2150 - 2154
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    Freely Available from IEEE
  • Author index

    Page(s): 2155 - 2163
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    Freely Available from IEEE
  • Subject index

    Page(s): 2163 - 2192
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    Freely Available from IEEE
  • A new family of soft-output adaptive receivers exploiting nonlinear MMSE estimates for TDMA-based wireless links

    Page(s): 1935 - 1945
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (713 KB) |  | HTML iconHTML  

    Decision-directed adaptive receivers suffer performance degradation on time varying and intersymbol interference-impaired links because of two major problems: the use of predicted channel estimates due to the unavoidable decision delay of any detector, and the unreliability of hard decisions used for channel estimation and tracking. It is shown here that combining a recursive nonlinear symbol estimator with a channel estimator with a low prediction order may alleviate this performance degradation. In particular, it is here proposed to employ the nonlinear minimum mean-square error (NL-MMSE) filtered and fixed-lag smoothed estimates of the transmitted symbols in place of the usual hard decisions for channel estimation and tracking. It is also shown that these NL-MMSE estimates can be recursively computed on the basis of a linear transformation of the vector of the a posteriori probabilities (APPs) of the states of the channel. This approach allows the prediction order of the channel estimates to be limited and, at the same time, limits the performance degradation due to erroneous hard decisions. Another result presented here is that the use of NL-MMSE estimates in place of hard decisions is not based on mere intuition only, but is a straightforward consequence of the statement of the problem of MMSE channel estimation when the overly optimistic assumption of correct decisions is dropped. On the basis of this novel approach, a new family of soft-output adaptive receivers is presented for time-division multiple-access-based radio communications. The proposed family of adaptive receivers is based on an APP-computer and exploits the APPs for both channel estimation and detection. The versatility of the APPs ensures that the architecture of the proposed receiver is flexible, so that several estimators and detectors can be embedded in it. View full abstract»

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  • Performance of hybrid ARQ schemes for the fading channel

    Page(s): 1882 - 1885
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (294 KB) |  | HTML iconHTML  

    We define and adopt a theoretical method, based on the sphere-packing bound, for evaluating and comparing the achievable performance of different hybrid automatic repeat request schemes for the fading channel. The channel state evolution is assumed to be represented by a finite-state Markov chain. To assess the correctness and the accuracy of the analysis, some simulation results, based on the use of turbo codes, are also presented. View full abstract»

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  • Performance enhancement of multiuser MIMO wireless communication systems

    Page(s): 1960 - 1970
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (733 KB) |  | HTML iconHTML  

    This paper describes a new approach to the problem of enhancing the performance of a multiuser multiple-input-multiple-output (MIMO) system for communication from one base station to many mobile stations in both frequency-flat and frequency-selective fading channels. This problem arises in space-division multiplexing systems with multiple users where many independent signal streams can be transmitted in the same frequency and time slot through the exploitation of multiple antennas at both the base and mobile stations. Our new approach is based on maximizing a lower bound for the product of signal-to-interference plus noise ratio (SINR) of a multiuser MIMO system. This provides a closed-form (noniterative) solution for the antenna weights for all the users, under the constraint of fixed transmit power. Our solution is shown by simulation to have better performance than previously proposed iterative or noniterative solutions. In addition, our solution requires significantly reduced complexity over a gradient search-based method that directly optimizes the product SINRs while still maintaining similar performance. Our solution assumes channel state information is present at the base station or transmitter. View full abstract»

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  • Fast initialization of Nyquist echo cancelers using circular convolution technique

    Page(s): 1865 - 1870
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (431 KB) |  | HTML iconHTML  

    For full-duplex high-speed data transmission over the two-wire line using the same frequency band, it is required to sufficiently suppress the echo. The use of a conventional adaptation method may take a long time to train the echo canceler. Fast training can be achieved by initializing the coefficients of the echo canceler with an estimate of the impulse response of the echo path. We propose a method for fast initialization of the echo canceler by using a circular convolution technique. The proposed method enables the use of real-valued training signals instead of complex-valued ones, resulting in significant reduction of the initialization time as well as the implementation complexity. Finally, the performance of the proposed method is analyzed and verified by computer simulation. View full abstract»

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  • Soft-input decoder for decoding of internally channel coded fiber-optic CDMA communication systems

    Page(s): 1994 - 2002
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (486 KB) |  | HTML iconHTML  

    We propose using a soft-input decoder for the decoding of internally convolutional coded Poisson noise-dominated fiber-optic code-division multiple-access (CDMA) communication systems using optical orthogonal codes. We first compute the coded symbol log-likelihoods at the output of the Poisson noise-dominated channel, which is then used by a soft-input maximum-likelihood decoder, for a fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. Furthermore, we develop a discrete soft-output channel model for a Poisson noise-dominated channel, with which we evaluate the upper bound on the bit error probability of the internally coded Poisson noise-dominated fiber-optic CDMA system using a soft-input decoder. It is shown that the soft-input decoder significantly outperforms the hard-input decoder. Furthermore, the performance of the soft-input decoder is also evaluated in the presence of different values of dark current. View full abstract»

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  • Nonblocking WDM switching networks with full and limited wavelength conversion

    Page(s): 2032 - 2041
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (540 KB) |  | HTML iconHTML  

    In previous years, with the rapid exhaustion of the capacity in wide area networks led by Internet and multimedia applications, demand for high bandwidth has been growing at a very fast pace. Wavelength-division multiplexing (WDM) is a promising technique for utilizing the huge available bandwidth in optical fibers. We consider efficient designs of nonblocking WDM permutation switching networks. Such designs require nontrivial extensions from the existing designs of electronic switching networks. We first propose several permutation models in WDM switching networks ranging from no wavelength conversion, to limited wavelength conversion, to full wavelength conversion, and analyze the network performance in terms of the permutation capacity and network cost, such as the number of optical cross-connect elements and the number of wavelength converters required for each model. We then give two methods for constructing nonblocking multistage WDM switching networks to reduce the network cost. View full abstract»

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  • A differential coding method for the symmetrically differential polarization shift-keying system

    Page(s): 2042 - 2051
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (744 KB) |  | HTML iconHTML  

    This paper presents a novel differential coding method for the symmetrically differential polarization shift-keying system. The proposed differential coding method, which can overcome the slow polarization variation in the fiber, constructs the reference frame by setting the sum, the difference, and the cross product of previous two symbols as the first, second, and the last axes of the signal space. Obviously, each axis is determined by the previous two symbols together. Thus, at the receiver, the new scheme constructs a noisy reference frame more accurately than other schemes and yields better performance. The optimal constellation for the system is symmetric and easy to find. The analytical integral form for the bit error rate (BER) of the proposed system is derived, and the saddle point method is applied to obtain the analytical results. It is found that the performance in terms of BER is much better than that of other differential polarization shift-keying systems. The analytical results agree with the simulation very well. View full abstract»

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  • Modeling and compensation of asymmetry in optical recording

    Page(s): 2052 - 2063
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (624 KB) |  | HTML iconHTML  

    Asymmetry, also known as domain bloom, is a systematic imperfection caused by the writing process in optical or magneto-optical recording. At the reading end of the system, asymmetry causes shifts of adjacent signal transitions in opposite directions. We present a simple nonlinear model for the replay signal in the presence of asymmetry. The model is specified by a single parameter, which is proportional to asymmetry, and its accuracy is demonstrated by application to replay signals from digital video disk drives. Based on the proposed model, a maximum-likelihood sequence detector is designed for replay signals with asymmetry. Simple modifications of the proposed detector lead to significant reductions in complexity, while the attainable performance, evaluated both analytically and through error-rate simulations, is superior to that of conventional techniques. View full abstract»

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  • On the performance of a novel quasi-synchronous trellis-coded CDMA system

    Page(s): 1984 - 1993
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (677 KB) |  | HTML iconHTML  

    We introduce a novel signal set defined over a signal space that consists of L (L≥2) orthogonal planes, and a quasi-synchronous trellis-coded code-division multiple-access (TC-CDMA) system based on it. The proposed scheme makes efficient use of the available processing gain to improve power and/or bandwidth efficiency for practical multiuser interference environments. Having a multiplanar signal constellation structure, the proposed scheme provides several options for a given required data rate, which makes it better adapted to dynamic channel conditions. Analytical bounds and simulation results indicate that at practical error rates and 2 b/s/Hz the proposed scheme is approximately 1.2 dB better than a TC-CDMA system based on 8-PSK, and 3 dB better at 3 b/s/Hz compared to TC-CDMA using 16-QAM. Additionally, the proposed system is approximately 1 dB better than a multicoded system using two signature sequences per user. View full abstract»

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  • Outdoor MIMO wireless channels: models and performance prediction

    Page(s): 1926 - 1934
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (497 KB) |  | HTML iconHTML  

    We present a new model for multiple-input-multiple-output (MIMO) outdoor wireless fading channels and their capacity performance. The proposed model is more general and realistic than the usual independent and identically distributed (i.i.d.) model, and allows us to investigate the behavior of channel capacity as a function of the scattering radii at transmitter and receiver, distance between the transmit and receive arrays, and antenna beamwidths and spacing. We show how the MIMO capacity is governed by spatial fading correlation and the condition number of the channel matrix through specific sets of propagation parameters. The proposed model explains the existence of "pinhole" channels which exhibit low spatial fading correlation at both ends of the link but still have poor rank properties, and hence, low ergodic capacity. In fact, the model suggests the existence of a more general family of channels spanning continuously from full rank i.i.d. to low-rank pinhole cases. We suggest guidelines for predicting high rank (and hence, high ergodic capacity) in MIMO channels, and show that even at long ranges, high channel rank can easily be sustained under mild scattering conditions. Finally, we validate our results by simulations using ray tracing techniques. Connections with basic antenna theory are made. View full abstract»

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  • On the accuracy of Gaussian approximations in the error analysis of DS-CDMA with OQPSK modulation

    Page(s): 2064 - 2071
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (693 KB) |  | HTML iconHTML  

    This paper considers the bit error probability analysis of direct-sequence code-division multiple access with offset quadrature phase-shift keying (OQPSK) modulation and random signature sequences. A statistical characterization of the decision variable at the output of the correlation receiver is obtained, and several techniques for evaluating the error performance, previously derived for binary phase-shift keying (BPSK)-modulated systems, are then extended to the case of OQPSK. This includes exact bit error evaluation, as well as various approximation methods based on Gaussian modeling of the multiple-access interference terms. These techniques, known as the standard and improved Gaussian approximations, have increasing complexity depending on the level of accuracy required. For BPSK, the standard approximation is typically found to be inaccurate, unless the number of users becomes large, and only the improved approximation yields sufficient accuracy in general. But for the case of OQPSK, the relative performance comparison developed in this paper shows that, for all practical purposes, the simple standard approximation is surprisingly very accurate even with a small number of users. View full abstract»

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  • Optimal training and redundant precoding for block transmissions with application to wireless OFDM

    Page(s): 2113 - 2123
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (870 KB) |  | HTML iconHTML  

    The adoption of orthogonal frequency-division multiplexing by wireless local area networks and audio/video broadcasting standards testifies to the importance of recovering block precoded transmissions propagating through frequency-selective finite-impulse response (FIR) channels. Existing block transmission standards invoke bandwidth-consuming error control codes to mitigate channel fades, and training sequences to identify the FIR channels. To enable block-by-block receiver processing, we design redundant precoders with cyclic prefix and superimposed training sequences for optimal channel estimation and guaranteed symbol detectability, regardless of the underlying frequency-selective FIR channels. Numerical results are presented to access the performance of the designed training and precoding schemes. View full abstract»

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  • Adaptive interference suppression for DS-CDMA

    Page(s): 1902 - 1905
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (336 KB) |  | HTML iconHTML  

    We propose an adaptive scheme to reject multiuser interference using as little as 20-30 taps in direct-sequence code-division multiple-access systems employing arbitrary processing gains (31-400 considered here). We demonstrate that our receiver using this scheme does not require training signals, either initially when the desired user powers on in the presence of several strong interferers, or during transmission, as other users (possibly near-far interferers) gain access to the system. Moreover, knowledge of interfering users spreading codes, timing, or phase information is not required with our adaptive receiver. View full abstract»

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  • Cyclic prefixing or zero padding for wireless multicarrier transmissions?

    Page(s): 2136 - 2148
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (841 KB) |  | HTML iconHTML  

    Zero padding (ZP) of multicarrier transmissions has been proposed as an appealing alternative to the traditional cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM) to ensure symbol recovery regardless of the channel zero locations. In this paper, both systems are studied to delineate their relative merits in wireless systems where channel knowledge is not available at the transmitter. Two novel equalizers are developed for ZP-OFDM to tradeoff performance with implementation complexity. Both CP-OFDM and ZP-OFDM are then compared in terms of transmitter nonlinearities and required power backoff. Next, both systems are tested in terms of channel estimation and tracking capabilities. Simulations tailored to the realistic context of the standard for wireless local area network HIPERLAN/2 illustrate the pertinent tradeoffs. View full abstract»

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  • Adaptive antennas for space-time codes in outdoor channels

    Page(s): 1918 - 1925
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (621 KB) |  | HTML iconHTML  

    Space-time codes have been introduced to improve mobile system performance in a multipath fading environment. We consider a multiple-input multiple-output (MIMO) system with m mobile antennas and n base station antennas, in which there are L multipaths at the base station at distinct angles of arrival. We show that when the channel has no intersymbol interference (ISI), then adaptive antennas in the form of beamforming, can be combined with space-time coding, to achieve a diversity gain of mL and a large signal-to-noise ratio (SNR) gain whenever n≥L. When the channel has ISI, beamforming can be used by the MIMO systems to achieve an SNR gain over a single-input multiple-output system, although both systems have the same diversity gain. View full abstract»

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  • Low-complexity FFT structures for OFDM transceivers

    Page(s): 1878 - 1881
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (287 KB) |  | HTML iconHTML  

    Orthogonal frequency-division multiplexing is a multiple-access technique with modulation and demodulation implemented by an inverse discrete Fourier transform (DFT) and a DFT, respectively. In a downlink (uplink) environment, an individual receiver (transmitter) may only use a small number of subchannels at any given time, in which case it does not make sense to require full DFT demodulation (inverse DFT modulation). Several existing low-complexity techniques for computing a partial DFT or inverse DFT with power-of-two size are examined. Low-complexity fast Fourier transform structures for full, few input, and few output nonpower-of-two transforms are derived. View full abstract»

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  • Analysis of a partial decorrelator in a multicell DS-CDMA system

    Page(s): 1895 - 1898
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (355 KB) |  | HTML iconHTML  

    For a multicell code-division multiple-access (CDMA) system, we propose a partial decorrelator that decodes a user by suppressing only the in-cell interferers. As a result, each user suffers only from other-cell interference and enhanced receiver noise. By analysis, we show that in random CDMA systems, the partial decorrelator outperforms the conventional receiver, within the operating regime of the conventional receiver. In simulation, we observe that when users have equal received powers at their respective receivers, a multicell system with partial decorrelator receivers yields roughly 1.5 times the capacity of the conventional system. View full abstract»

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  • Complexities, error probabilities, and capacities of optical OOK-CDMA communication systems

    Page(s): 2009 - 2017
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (512 KB) |  | HTML iconHTML  

    The complexity in the hardware implementation of traditional optical code-division multiple-access correlation receivers with double optical hardlimiters is discussed. A comparison with the implementation of chip-level receivers is presented as well. In addition, the bit error probabilities and the throughput capacities for both chip-level and correlation systems (without hardlimiters) are derived and evaluated under code-correlation constraints equal to one and two. Our results reveal that chip-level receivers are much simpler and their performances are competitive with that of traditional correlation receivers with double optical hardlimiters. Further, the throughput capacity of chip-level systems can be increased by almost a factor of 3.4 when increasing the code-correlation constraint from one to two. View full abstract»

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  • Near-explicit state probabilities of multiservices loss systems

    Page(s): 2091 - 2103
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1346 KB) |  | HTML iconHTML  

    The Kaufman-Roberts (1981) multirate recurrence equation is converted to a functional equation subject to a single step parameter d, which defines the state-dependent recurrence depth of the multirate model. This parameter, d, is verified to be the joint solution of two moment-generating functions. In contrast to the multirate transform domain solution, the characteristic function of the functional model may be inverted explicitly. But this inverse is scaled by d and yields a new state-dependent probability density based on two types of scaled gamma functions. They uniformly cover wide areas of continuous states, capacities C, and demands. A two-moments series expansion of d in the transform domain signifies that it models the effective bitrate of the multirate connections under progress within a complete sharing mode of C. Selected case studies show very good agreements to the exact Kaufman-Roberts' state probabilities and to the high accurate Mitra-Morrison's (1994) tail probabilities. Further partial sharing mode investigations include an efficient approximation for connection admission controls by trunk reservation covering partial and full fairness. The resulting state and tail probabilities deviate only slightly from Roberts' approximations of the past. All solutions achieve time complexities of O(1) per state, which amount to O(SC) with iterative multirate recurrence solutions for S service classes. View full abstract»

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  • Capture probability in Rician fading channels with power control in the transmitters

    Page(s): 1889 - 1891
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (288 KB) |  | HTML iconHTML  

    The capture probability for signals over Rician fading channels (with equal mean power) is analyzed. Expressions for probability density function and cumulative distribution function for power of n interferers are derived, validating with a known expression for Rayleigh fading. These equations are applied to slotted ALOHA, showing an increase in throughput. View full abstract»

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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