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Communications, IEEE Transactions on

Issue 7 • Date July 1997

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Displaying Results 1 - 20 of 20
  • Author's reply to "A correction to Kurt Metzger's algorithm in the paper 'On the probability density of intersymbol interference' "

    Publication Year: 1997
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (40 KB)  

    Pokrajac and Nikoli (see ibid., vol.43, no.12, p.2868, 1995) have correctly identified a mistake by Metzger (see ibid., vol.COM-35, no.4, p.396-402). The examples presented by Metzger were calculated correctly, but the carelessness in the formulation of the equation is regretted. It was intended to show the feasibility of the basic transformation, and not to give an exhaustive investigation of the numerical possibilities for executing the equation. Some more sophisticated ways for performing the evaluation are included. View full abstract»

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  • A new key generation method for frequency-domain speech scramblers

    Publication Year: 1997 , Page(s): 749 - 752
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (124 KB)  

    This paper describes a method which uses “derangement” permutations as scrambling keys in a frequency-domain speech scrambling system. Subjective tests based on the number test confirm that such keys produce scrambled speech with virtually no residual intelligibility. A new derangement generation algorithm is introduced for key generation. This algorithm maps each integer g, in the range of 0⩽g<(n-1)!, to a distinct derangement of 2 elements, View full abstract»

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  • Combination of BCM and TCM with 90°, 180°, and 270° phase rotational invariance

    Publication Year: 1997 , Page(s): 800 - 808
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (360 KB)  

    Two systematic schemes to combine block-coded modulation (BCM) and trellis-coded modulation (TCM) are proposed. In the first scheme, the signal points which are used in TCM should be partitioned by the proper BCM systems constructed by a multilevel construction method. The asymptotic coding gain will be 6.02-3.01/n dB, where n is the length of the BCM systems. Since the average transmission power can be reduced by using a denser signal lattice, the main idea of the second scheme is to construct the TCM system based on the signal points of a denser lattice formed by a proper BCM system. Hence, we can get a higher coding gain which is a combination of both the distance gain of TCM and the density gain of BCM. Theoretically, the net coding gain of this proposed scheme can reach 5.27 dB in the case of an 8-D constellation and 7.15 dB in the case of a 16-D constellation. For practical implementation, the phase ambiguity problems for both schemes are considered and solved by a differential encoder/decoder View full abstract»

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  • Gradient estimation for sensitivity analysis and adaptive multiuser interference rejection in code-division multiple-access systems

    Publication Year: 1997 , Page(s): 848 - 858
    Cited by:  Papers (19)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (392 KB)  

    In this paper, we consider a direct-sequence code-division multiple-access (DS-CDMA) system in the framework of a discrete-event dynamic system (DEDS) in order to optimize the system performance. Based on this formulation, we develop infinitesimal perturbation analysis (IPA) for estimating the sensitivity of the average probability of bit error to factors ranging from near-far effects to imperfections in power control. The above estimates are shown to be unbiased, and this technique is then further incorporated into a stochastic gradient algorithm for achieving adaptive multiuser interference rejection for such systems, which is also subject to frequency nonselective slow fading. We use an IPA-based stochastic training algorithm for developing an adaptive linear detector with the average probability of error being the minimization criterion. We also develop a practical implementation of such an adaptive detector where we use a joint estimation-detection algorithm for minimizing the average probability of bit error. A sequential implementation that does not require a stochastic training sequence or a preamble is also developed View full abstract»

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  • Design of optimal interpolation filter for symbol timing recovery

    Publication Year: 1997 , Page(s): 877 - 884
    Cited by:  Papers (15)  |  Patents (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (316 KB)  

    We propose an optimal interpolation filter for symbol timing recovery in a digital receiver where the input analog-to-digital (A-D) conversion sampling clock is not synchronized to the transmitter symbol clock. The optimal filter is designed by minimizing the mean-square error (MSE) at the output of the receiver, assuming that the correct timing difference between the nonsynchronized input samples and the correct strobe synchronized to the symbol is given. The MSE minimization procedure results in a system of linear equations which can easily be solved to yield the optimal filter coefficients. We also analyze the symbol tracking performance of a symbol synchronizer that employs the proposed optimal interpolation filter. Although the proposed optimal filter is designed assuming the availability of the correct timing difference, we show that it minimizes the timing estimation error variance even when the computed timing difference values deviate from the ideal, we also show that the timing estimation is virtually unbiased if the length of the interpolation filter is greater than or equal to 4. Simulation results are included to show the performance improvement realizable by employing the optimal interpolation filter View full abstract»

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  • Joint phase and timing recovery with CPM signals

    Publication Year: 1997 , Page(s): 867 - 876
    Cited by:  Papers (22)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (356 KB)  

    We describe joint carrier phase and timing recovery algorithms for CPM signaling. They may be employed with any CPM format, and with either full or reduced state detectors. Their implementation is fully digital, and involves a limited computational complexity. Simulation results show that these algorithms have excellent tracking performance. When operated in their simplest form, however, they may exhibit false locks. In particular, this occurs with multilevel and partial response formats. A simple solution to the false lock problem is proposed View full abstract»

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  • Very low variable-rate convolutional codes for unequal error protection in DS-CDMA systems

    Publication Year: 1997 , Page(s): 753 - 755
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (76 KB)  

    Convolutional codes based on Hadamard sequences can achieve both orthogonality and variable rate. In contrast with other convolutional coding techniques, these codes incur no processing gain penalty in direct-sequence code-division multiple-access (DS-CDMA) systems. We describe a class of such codes called “telescopic protection codes”. These codes enable variable-rate error protection in the manner of rate-compatible punctured-convolutional (RCPC) codes, but preserve the Hadamard orthogonality properties necessary to maintain processing gain that RCPC codes lack View full abstract»

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  • Efficient estimation of continuous phase modulation with unknown carrier phase

    Publication Year: 1997 , Page(s): 765 - 767
    Cited by:  Papers (1)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (144 KB)  

    An efficient algorithm is presented for the estimation of continuous phase modulation (CPM) sequences over the Gaussian channel with unknown carrier phase. It operates on the excess phase trellis like the Viterbi algorithm, except that it is not a maximum-likelihood (ML) estimator. Its decision metric for survivor selection at each node is chosen so that it achieves the node error event probability of coherent ML estimation in the limit as the carrier phase remains constant over a long interval, so that the observation interval for forming the metric can be made large. It is shown for the case of minimum-shift-keying (MSK) that the bit-error probability (BEP) of perfectly coherent ML estimation is attained View full abstract»

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  • Bhattacharyya bound, cutoff rate, and constellation design for the companding channel

    Publication Year: 1997 , Page(s): 779 - 787
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (360 KB)  

    In modern voiceband data communication, the received signal is subject to nonlinear quantization noise due to companding. Under certain conditions, this quantization noise may become dominant and cause serious degradation in performance. In this paper, we first calculate the Bhattacharyya bound on the error probability between two signal points, which we then use to obtain some insight into the effects of the companding channel. Next, we compute the cutoff rate, and then use these results to design new “optimal” signal constellations for the companding channel. The bound, cutoff rate, and new constellations are computed assuming a signal-dependent noncircular Gaussian channel transition density. In addition, we obtain “optimal” a priori probabilities for this channel View full abstract»

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  • A spread spectrum communications channel sounder

    Publication Year: 1997 , Page(s): 840 - 847
    Cited by:  Papers (8)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (216 KB)  

    The design of a microwave channel probing analyzer is presented and discussed. Special attention is paid to the characteristics of the transmission path to be measured; the choice of spread spectrum probing signal; the signal processing methods used; modulation, demodulation, and synchronization at the transmitter and receiver sites; the path model to be fitted to the measured data and finally, the impact of all of these issues upon the necessary hardware and software. Following a formulation of the channel-probing problem, transfer function regression techniques are reviewed to illustrate the advantages of periodic averaging over established correlation methods. The construction of an experimental sounder based on these methods is described. The equipment has been used to measure the time-variant frequency-selective channel dynamics of a short (158 km) troposcatter transhorizon link at a frequency of 11.647 GHz over a 31.25-MHz bandwidth. Off-line, nonlinear spectrum analysis, based on Prony's method and singular value decomposition, is used to estimate parameters of a high-resolution delay model that accurately reflects the underlying ray structure of the transmission path View full abstract»

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  • A TDM-based multibus packet switch

    Publication Year: 1997 , Page(s): 859 - 866
    Cited by:  Papers (4)  |  Patents (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (216 KB)  

    A new packet switch architecture using two sets of time-division multiplexed buses is proposed. The horizontal buses collect packets from the input links, while the vertical buses distribute the packets to the output links. The two sets of buses are connected by a set of switching elements which coordinate the connections between the horizontal buses and the vertical buses so that each vertical bus is connected to only one horizontal bus at a time. The switch has the advantages of: (1) adding input and output links without increasing the bus and I/O adaptor speed; (2) being internally unbuffered; (3) having a very simple control circuit; and (4) having 100% throughput under uniform traffic. A combined analytical-simulation method is used to obtain the packet delay and packet loss probability. Numerical results show that for satisfactory performance, the buses need to run about 30% faster than the input line rate. With this speedup, even at a utilization factor of 0.9, each input adaptor requires only 31 buffers for a packet loss rate of 10-6. The output queue behaves essentially as an M/D/1 queue View full abstract»

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  • Introducing erasures in decision-feedback equalization to reduce error propagation

    Publication Year: 1997 , Page(s): 757 - 760
    Cited by:  Papers (42)  |  Patents (23)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (120 KB)  

    A simple modification of the decision feedback equalizer (DFE) slicer is proposed to reduce the effect of error propagation. A comparison of the performance of the modified DFE and conventional DFE is made for specific channels. On these channels, the modified DFE performs only marginally better than the conventional DFE in terms of average error probability, but may offer some advantages in terms of error probability conditioned on specific input sequences and in terms of the distribution of error burst lengths. Some examples are given, concerning binary PAM and multilevel quadrature amplitude modulation (M-QAM) systems View full abstract»

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  • A fundamental lower bound to the performance of phase estimators over Rician-fading channels

    Publication Year: 1997 , Page(s): 775 - 778
    Cited by:  Papers (13)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (144 KB)  

    We present a fundamental lower bound to the mean-square error of estimators of the phase of a sine wave passed through a Rician-fading channel. Explicit expressions for the bound with Gaussian and Rayleigh channels are provided. The bound is derived by analyzing the performance of the Bayes minimum mean-square error phase estimator, and therefore represents the best performance attainable by physically realizable phase estimators over a Rician channel View full abstract»

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  • Digital television on ATM networks: optimum chain for coding and transmission

    Publication Year: 1997 , Page(s): 829 - 839
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (252 KB)  

    This paper presents a global optimization of the transmission chain for video on asynchronous transfer mode (ATM) channels. This optimum control includes compression, transmission, and decoding. First, the gain in statistical multiplexing is put forward to demonstrate that transmitting at variable rates on asynchronous multiplexing links is more efficient than exploiting the constant rate on synchronous links. The joint optimization relies on both the entropy rate and the end-to-end image quality. This requires the characterization of the video sources as entropy generators, and the development of the entropy rate-distortion functions in the coder and the transmission channel. Quantizers and variable-length entropy coders in coding, traffic, and queues in transmission multiplexing each lead to performance functions expressing quality in terms of entropy rate. The objective measures of quality are, respectively, the PSNR in terms of the output data rate and the cell loss in terms of the network loads. The main advantage of transmitting on variable bit-rate channels is to permit the generation of image sequences at constant subjective quality on the coding side, and the saving of transmission bandwidth through a gain in statistical multiplexing on the network side. Mirrored control actions are described for coding and multiplexing; they lead to a unique global optimum of the transmission chain. Since the decoders are generally slaved to the coding and transmission performances, they are restricted to perform independent optimum signal reconstruction View full abstract»

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  • Further results on differential detection of GMSK signals

    Publication Year: 1997 , Page(s): 761 - 764
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (204 KB)  

    We evaluate the performance, in terms of bit-error rate, of 1- and 2-bit differential detectors for Gaussian minimum-shift keying (GMSK) signals by using a very fast numerical technique which makes use of the saddle point approximation. Moreover, the performance of each detector is optimized by using a closed-form expression of the optimum threshold in the slicer View full abstract»

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  • Symbol-by-symbol MAP demodulation of CPM and PSK signals on Rayleigh flat-fading channels

    Publication Year: 1997 , Page(s): 788 - 799
    Cited by:  Papers (54)  |  Patents (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (336 KB)  

    Demodulation using the symbol-by-symbol maximum a posteriori probability (MAP) algorithm is presented. The algorithm is derived for the case of continuous phase modulation (CPM) signals transmitted over Rayleigh flat-fading channels, and a corresponding receiver structure is specified. It is shown that the MAP algorithm requires computing, for each trellis branch, the sum of the products of the weights of all paths through the trellis which pass through that branch, and that this generic computational problem can be solved efficiently by an approach that uses a forward and backward recursion through the trellis. Simulation results are presented which show both the hard and soft decision performance of the MAP receiver to be robust, even in the presence of fade rates of up to 30% of the symbol rate. The application of the receiver concept to phase-shift keying (PSK) signals is also discussed, and then evaluated via simulation. The concept of joint demodulation and decoding using iterative processing techniques is introduced. It is shown that the MAP receiver is well suited for iterative processing applications due to its use of a priori symbol probabilities and its production of optimal soft decisions. Simulation results for the reception of quaternary PSK (QPSK) show that the bit error rate (BER) performance of the iterative MAP receiver can approach that of a receiver operating with perfect knowledge of the fading process View full abstract»

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  • A convex projections method for improved narrow-band interference rejection in direct-sequence spread-spectrum systems

    Publication Year: 1997 , Page(s): 772 - 774
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (84 KB)  

    A method is presented for enhancing the narrow-band interference rejection capability of direct-sequence spread-spectrum systems employing an adaptive notch filter. The method, based on projection onto convex sets, restores that part of the spread-spectrum signal distorted by the filter. Simulation results are presented which show the output bit-error rate (BER) improvement gained by using the signal restoration scheme View full abstract»

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  • Performance analysis of slotted fiber-optic code-division multiple-access (CDMA) packet networks

    Publication Year: 1997 , Page(s): 819 - 828
    Cited by:  Papers (26)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (332 KB)  

    This paper examines code-division multiple-access (CDMA) techniques used in slotted fiber-optic packet networks. Since the inherent properties and signal processing of the conventional communication channels are different from those of the fiber-optic channels, new code sequences must be constructed for fiber-optic applications. The goal of our research is to analyze the performance of fiber-optic CDMA packet networks using code sequences with given orthogonality properties View full abstract»

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  • Neural networks for modeling nonlinear memoryless communication channels

    Publication Year: 1997 , Page(s): 768 - 771
    Cited by:  Papers (32)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (84 KB)  

    This paper presents a neural network approach for modeling nonlinear memoryless communication channels. In particular, the paper studies the approximation of the nonlinear characteristics of traveling-wave tube (TWT) amplifiers used in satellite communications. The modeling is based upon multilayer neural networks, trained by the odd and even backpropagation (BP) algorithms. Simulation results demonstrate that neural network models fit the experimental data better than classical analytical TWT models, View full abstract»

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  • Impact of diversity reception on fading channels with coded modulation. III. Co-channel interference

    Publication Year: 1997 , Page(s): 809 - 818
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (476 KB)  

    For pt.II see ibid., vol.45, no.6, p.55-67, 1997. In previous work, we have studied the impact of diversity on coded digital communication systems operating over fading channels. In particular, we have shown that diversity may be thought of as a way of making the channel more similar to a Gaussian one. The present paper extends this analysis to fading channels affected by co-channel interference (CCI). Three receiver models are examined, namely, with coherent detection and perfect channel-state information (CSI), with differential; and with pilot-tone detection. We study the effect of diversity on the irreducible error floor caused by CCI and fading, and the asymptotic behavior of the channel as the diversity order increases. Our results show that, when perfect CSI is available, diversity is able to turn asymptotically the channel into a CCI-free additive white Gaussian noise (AWGN) channel with the same signal-to-noise ratio (SNR). On the other hand, differential and pilot-tone detection do not remove interference in the limit. Nevertheless, also with these detection schemes, diversity achieves significant gains when the SNR is large enough. Calculation of the channel cutoff rate provides guidelines for the design of coded systems with CCI in fading environments. A wide range of examples, validated by computer simulation, illustrates our conclusions 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