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Selected Areas in Communications, IEEE Journal on

Issue 6 • Date Jun 2001

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Displaying Results 1 - 20 of 20
  • Adaptive narrowband interference rejection in DS-CDMA systems: a scheme of parallel interference cancellers

    Page(s): 1103 - 1114
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (272 KB) |  | HTML iconHTML  

    In the presence of narrowband interference (NBI), the performance of the direct-sequence code-division multiple access communication system can be improved by using various interference cancellation schemes. In this paper, a scheme of parallel interference cancellers (PICs), which includes a set of NBI cancellation filters and a set of multiple access interference (MAI) cancellation filters, is developed to improve the signal-to-interference ratio (SIR) and handle the MAI effect. In order to avoid performance deterioration due to unreliable initial detection, a robust coefficient γ is introduced in our proposed design criterion. A recursive least-square algorithm is employed to update the tap coefficients of the PICs. In other words, no training or retraining period is needed in our proposed PICs. Finally, the SIR performance of the proposed detector is analyzed and compared to that of the conventional linear detector View full abstract»

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  • Load analysis of the soft handoff scheme in a CDMA cellular system

    Page(s): 1147 - 1152
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (124 KB) |  | HTML iconHTML  

    In a code-division multiple-access cellular system, the soft handoff with macrodiversity is known to increase the number of available channels in each cell from the interference-reduced radio environment. This capability, however, also incurs extra load on the system due to the increase in the number of channels occupied and reserved for macrodiversity. The objective is to evaluate the associated loads, thereby enabling effective channel management. We first differentiate two kinds of loads attributable to new and handoff calls, respectively. Further dividing handoff calls into two kinds, we derive a load balance equation from which three different loads at a cell are all exactly obtained in association with the size of the soft handoff region. Comparative analysis with the case of no soft handoff region, i.e., of hard handoff, shows the appropriate proportion of channels reserved for the macrodiversity in a given soft handoff region View full abstract»

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  • Optimal adaptive multiuser detection in unknown multipath channels

    Page(s): 1115 - 1127
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (348 KB) |  | HTML iconHTML  

    An optimal multiuser detector in the weighted least squares (WLS) sense is derived. This detector, which includes the maximum likelihood multiuser detector as a special case, consists of two parts: a bank of linear fractionally chip spaced minimum mean squared error (MMSE) filters, and a nonlinear WLS metric minimizer. It is shown that the symbol spaced samples at the output of the MMSE filter bank provide a set of sufficient statistics for WLS detection. The relationship between the taps of a centralized decision feedback detector and the MMSE filter bank is derived. It is proven that all the necessary parameters for implementing the WLS detector can be realized by adaptively training a centralized decision feedback detector. Therefore, the WLS detector achieves optimal joint synchronization and data detection even in the presence of colored noise, such as narrowband interference, without any a priori knowledge of the users' signatures, multipath channel taps or statistics of the colored noise. Significant features of the WLS detector are that: (1) the WLS detector is a generalization of the maximum likelihood multiuser detector that employs a bank of matched filters; (2) it is implemented adaptively; and (3) it has structural flexibility in terms of implementation complexity View full abstract»

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  • CRDA: a collision resolution and dynamic allocation MAC protocol to integrate data and voice in wireless networks

    Page(s): 1153 - 1163
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (208 KB) |  | HTML iconHTML  

    One key requirement for radio access in advanced, third generation (3G) mobile communication systems is the ability to accommodate a variety of services via a flexible and efficient medium access control (MAC) protocol. The paper presents and evaluates a new multiple access protocol termed CRDA (collision resolution and dynamic allocation), which has the potential to meet the above requirement. CRDA is basically a slotted packet-reservation multiple access technique with dedicated reservation slots, which allows the main shortcoming of previous PRMA schemes, i.e., contention, to be overcome through the integration in the MAC protocol of a code-division multiple-access (CDMA) transmission mode used to access the reservation slots. This prevents collisions during the reservation phase and enhances channel throughput, notably in the case of mixed voice/data traffic. Our simulations of the CRDA MAC integrate voice channels with data sources, generating what we call advanced data traffic, which has a very similar shape to the actual traffic generated by World Wide Web (WWW) applications. The standard assumption of Poissonian data traffic is also considered. Our conclusion is that the CRDA MAC protocol satisfactorily accommodates both types of traffic View full abstract»

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  • Orthogonal frequency division multiplex synchronization techniques for frequency-selective fading channels

    Page(s): 999 - 1008
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (224 KB) |  | HTML iconHTML  

    The effect of time-domain and frequency-domain synchronization errors is quantified in the context of various coherently and noncoherently detected 1, 2, and 4 bits/symbol OFDM constellations, in order to demonstrate the wide applicability of the techniques proposed for mitigating the bit error rate (BER) performance degradations inflicted. A reference symbol is proposed and a range of correlation techniques are suggested for coarse and fine synchronization. Their performance is studied over time-dispersive Rayleigh fading channels, with the conclusion that the proposed synchronization techniques result in virtually unimpaired BERs over the range of wideband channels investigated in comparison to a perfectly synchronized system View full abstract»

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  • A multicarrier GMSK modulator

    Page(s): 1070 - 1079
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (232 KB) |  | HTML iconHTML  

    A multicarrier Gaussian minimum shift keying (GMSK) modulator has been developed and implemented. The design contains four GMSK modulators, which generate GMSK modulated carriers at the specified center frequencies. Utilization of the redundancy in the stored waveforms reduces the size of the GMSK trajectory look-up table to less than one-quarter of the original size in the modulator. Conventionally, the power ramping and output power level controlling are performed in the analog domain. A novel digital ramp generator and output power level controller perform both the burst ramping and the dynamic power control in the digital domain. The power control is realized by scaling the ramp curve, which follows a raised cosine/sine curve. The four GMSK modulated signals are combined together in the digital domain. The digital multicarrier GMSK modulator is designed to fulfil the spectrum and phase error specifications of the GSM 900 and DCS 1800 base stations View full abstract»

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  • An effective multiuser receiver for DS/CDMA systems

    Page(s): 1019 - 1028
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (284 KB) |  | HTML iconHTML  

    Multiuser detection (MUD) is a key technique for combating multiple access interference (MAI) in CDMA systems. We propose a simple and effective multiuser receiver, which we refer to as the objective function based multiuser detector (OFMUD). This is a successive multistage joint detector based on the statistical optimization of an objective function. Two objective functions are considered in this paper. The first one is based on a maximum likelihood solution, and the second one is based on the least squared error solution. A key feature of the proposed detector is that among all the K user bits, we update the estimate of one and only one bit (the one that is most likely to be wrong) in every stage. Analytical and numerical results show that significant performance improvement can be achieved compared with the conventional DS/CDMA receiver. At the same time, the near-far problem is alleviated. In addition, it is shown that the proposed receiver can outperform the popular multiuser parallel interference cancellation receiver in most cases with a smaller computational cost View full abstract»

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  • Rapid acquisition of PN signals for DS/SS systems using a phase estimator

    Page(s): 1128 - 1137
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (240 KB) |  | HTML iconHTML  

    We propose a new scheme for rapid acquisition of PN signals in direct-sequence spread spectrum (DS/SS) systems by estimating the phase of the received PN signal with the use of an auxiliary signal. The auxiliary signal can be generated by a sum of the phase shifted PN signals. The phase of the incoming PN signal is estimated using the properties of cross correlation between the PN signal and the auxiliary signal. True phase alignment is detected using a conventional serial search scheme, where the initial phase of the local PN generators is set to a value obtained by the phase estimator. The performance of the proposed acquisition scheme is analytically evaluated in terms of the mean acquisition time. Numerical results show that the proposed scheme can achieve acquisition at least two times faster than the conventional scheme in the nominal operating condition View full abstract»

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  • A multistep linear prediction approach to blind asynchronous CDMA channel estimation and equalization

    Page(s): 1090 - 1102
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (304 KB) |  | HTML iconHTML  

    A multistep linear prediction approach is presented for blind channel estimation, multiuser interference (MUI) suppression, and detection of asynchronous short-code direct sequence code division multiple access signals in multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. We exploit the previously proposed multistep linear prediction approach for blind multiple-input multiple-output channel estimation in conjunction with the structure imposed by the desired user's spreading code sequence. With the knowledge of the desired user's code sequence, only the second-order statistics of the data are needed under certain sufficient conditions on the underlying multiuser MIMO transfer function. Based on the desired user's channel estimate, a linear minimum mean square error filter is designed for simultaneous equalization and MUI suppression. Three illustrative simulation examples are presented View full abstract»

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  • Adaptive cell sectorization for CDMA systems

    Page(s): 1041 - 1051
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (232 KB) |  | HTML iconHTML  

    Given the user distribution in a cell, we investigate the two problems of how to appropriately sectorize the cell such that we minimize the total received power and the total transmit power of all the users, while giving each user acceptable quality of service in both cases. For the received power optimization problem, we show that the optimum arrangement equalizes the number of users in each sector. The transmit power optimization is formulated as a graph partitioning problem that is polynomially solvable. We provide an algorithm that finds the best sectorization assignment as well as the optimal transmit powers for all the users. The computational complexity of the algorithm is polynomial in the number of users and sectors. For both the received power optimization and the transmit power optimization, under nonuniform traffic conditions, we show that the optimum arrangement can be quite different from uniform cell sectorization (equal width sectors). We also formulate and solve the transmit power optimization and cell sectorization problem in a multicell scenario that would improve the capacity of a hot spot in the network. We observe that, with adaptive sectorization, where the sector boundaries are determined in response to users' locations, received and transmit power savings are achieved, and the number of users served by the system (system capacity) is increased compared to uniform sectorization of the cell View full abstract»

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  • Variations on optimal and suboptimal handoff control for wireless communication systems

    Page(s): 1173 - 1185
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (272 KB) |  | HTML iconHTML  

    The design of handoff algorithms for cellular communication systems based on signal-strength measurements is addressed. The system is modeled using a hybrid framework: a mixture of continuous state and discrete event systems. The handoff problem is formulated as an optimization problem to control the switchings within the discrete event system. Performance is evaluated as a function of the expected number of handoffs, the expected handoff delay, and the expected number of signal degradations. A signal degradation occurs when the signal level falls below a threshold. The cost of handoff delay is explicitly specified, in contrast to prior work. Various optimization problems are posed to trade off between these quantities. Based on the optimal solutions which are obtained through dynamic programming, suboptimal versions are proposed for ease of implementation. The performance of the suboptimal algorithm which trades off between the expected number of handoffs and the expected number of signal degradations is improved through the use of signal averaging; however, this algorithm suffers from excessive handoff delay. Therefore, the tradeoff between handoff delay and number of handoffs is considered. The corresponding suboptimal algorithm provides nearly one handoff and almost no delay, which is ideal if call quality is also good. Finally, an algorithm which is a combination of the two previous algorithms is explored View full abstract»

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  • Performance of an array receiver with a Kalman channel predictor for fast Rayleigh flat fading environments

    Page(s): 1164 - 1172
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (232 KB) |  | HTML iconHTML  

    We develop an approach for using an antenna array for tracking fast Rayleigh flat fading channels and suppressing cochannel interference. The fast flat fading process is assumed to be a general autoregressive (AR) process in order to characterize temporal variation of channels and evaluate its effect on the receiver structure and performance. The optimal array receiver structure that minimizes the probability of error for BPSK signals is derived, which includes a Kalman filter to predict the fading channels. A simple integral expression for the probability of error is also derived for the optimal receiver. In particular, we analyze the case with identical shaping filters. An irreducible probability of error is shown to exist due to the prediction error of multiple channels. Another interesting observation from the study is that the diversity gain with m antenna elements in the presence of k interferences is usually greater than (m-k), even in the presence of channel prediction error. Simulations are carried out to verify the theoretical analysis View full abstract»

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  • Optimum spreading bandwidth for selective RAKE reception over Rayleigh fading channels

    Page(s): 1080 - 1089
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (244 KB) |  | HTML iconHTML  

    Building on the developments in the performance analysis of generalized selection combining (GSC), this paper examines the optimum spreading bandwidth for a fixed-complexity GSC diversity receiver operating over independent identically distributed Rayleigh paths. For this purpose, the study considers three performance criteria: (1) average combined signal-to-noise ratio (SNR) at the GSC output; (2) average bit error probability (BEP); and (3) outage probability of the instantaneous combined SNR at the GSC output. For the average BEP criterion, results are presented for both coherent and noncoherent combining. For the average combined SNR and some instances of the average BEP optimization problem, an accurate approximate estimate of this optimum bandwidth in the form of a solution of a transcendental equation is provided. In other cases, where the optimization is not easily tractable in an analytic fashion, a numeric-search procedure is used to find this optimum bandwidth for different performance criteria and system parameters of interest. Finally, simplified rule-of-thumb-type formulas are also presented as a good reference for picking the optimum spreading bandwidth given a set of system parameters and a particular performance criterion of interest View full abstract»

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  • Performance of ultrawideband SSMA using time hopping and M-ary PPM

    Page(s): 1186 - 1196
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (356 KB) |  | HTML iconHTML  

    Wireless spread spectrum multiple access (SSMA) using time hopping and block waveform encoded (M-ary) pulse position modulated (PPM) signals is analyzed. For different M-ary PPM signal designs, the multiple-access performance in free-space propagation renditions is analyzed in terms of the number of users supported by the system for a given bit error rate, signal-to-noise ratio, bit transmission rate, and number of signals in the M-ary set. The processing gain and number of simultaneous users are described in terms of system parameters. Tradeoffs between performance and receiver complexity are discussed. Upper bounds on both the maximum number of users and the total combined bit transmission rate are investigated. This analysis is applied to ultrawideband impulse radio modulation. In this modulation, the communications waveforms are practically realized using subnanosecond impulse technology. A numerical example is given that shows that impulse radio modulation is theoretically able to provide multiple-access communications with a combined transmission capacity of hundreds of megabits per second at bit error rates in the range 10-4 to 10-7 using receivers of moderate complexity View full abstract»

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  • Exploiting data and code interactions to reduce the power variance for CDMA sequences

    Page(s): 1061 - 1069
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (196 KB) |  | HTML iconHTML  

    This paper investigates the effect of interactions between the Walsh codes and data sequences on the statistical moments of the forward-link CDMA signal. Of primary interest is the normalized fourth-order moment, which is referred to as the “power variance”. Several techniques for reducing the power variance of the CDMA signal are discussed that are based on Walsh code selection and data encoding. Certain groupings of Walsh codes, referred to as “active quadruples”, are shown to be useful in predicting the potential of each technique. Results demonstrate the reduction in the power variance afforded by two novel approaches referred to as “channel hopping” and “data reversal” View full abstract»

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  • GQR models for multipath Rayleigh fading channels

    Page(s): 1009 - 1018
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB) |  | HTML iconHTML  

    In this paper, reduced complexity statistical models for the representation of wide sense stationary-uncorrelated scattering doubly selective fading channels are developed. Their derivation is based on the evaluation of Fourier integrals by means of Gaussian quadrature rules. The accuracy and the complexity of the proposed models is assessed, and is then compared to that provided by other modeling techniques available in the literature View full abstract»

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  • On the architecture and performance of a hybrid image rejection receiver

    Page(s): 1029 - 1040
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (216 KB) |  | HTML iconHTML  

    This paper describes a hybrid image rejection receiver. The hybrid image rejection receiver contains a modified Hartley (1928) image rejection mixer and a digital image rejection processor. The modified Hartley image rejection mixer performs similarly to an original Hartley image rejection receiver but provides two digital outputs. In one output it enhances the desired signal and in the other output it enhances the image signal. The digital image rejection processor first measures the mismatching effect in the analog devices and then suppresses the image signal by compensating for the mismatching effect. We also propose a simplified implementation method for the hybrid image rejection receiver to reduce its computation complexity. Computer simulation was used to evaluate the performance of this simplified implementation method to include the quantization effect introduced by the A/D converters. Simulation results show that the proposed hybrid image rejection receiver achieves much better performance than the original Hartley image rejection receiver. This architecture greatly relaxes the matching requirements of the analog devices and has a low complexity for an IC implementation View full abstract»

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  • Genetic algorithm assisted joint multiuser symbol detection and fading channel estimation for synchronous CDMA systems

    Page(s): 985 - 998
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (280 KB) |  | HTML iconHTML  

    A novel multiuser code division multiple access (CDMA) receiver based on genetic algorithms is considered, which jointly estimates the transmitted symbols and fading channel coefficients of all the users. Using exhaustive search, the maximum likelihood (ML) receiver in synchronous CDMA systems has a computational complexity that is exponentially increasing with the number of users and, hence, is not a viable detection solution. Genetic algorithms (GAs) are well known for their robustness in solving complex optimization problems. Based on the ML rule, GAs are developed in order to jointly estimate the users' channel impulse response coefficients as well as the differentially encoded transmitted bit sequences on the basis of the statistics provided by a bank of matched filters at the receiver. Using computer simulations, we showed that the proposed receiver can achieve a near-optimum bit-error-rate (BER) performance upon assuming perfect channel estimation at a significantly lower computational complexity than that required by the ML optimum multiuser detector. Furthermore, channel estimation can be performed jointly with symbol detection without incurring any additional computational complexity and without requiring training symbols. Hence, our proposed joint channel estimator and symbol detector is capable of offering a higher throughput and a shorter detection delay than that of explicitly trained CDMA multiuser detectors View full abstract»

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  • Implementation and performance evaluation for mobility management of a wireless PBX network

    Page(s): 1138 - 1146
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (248 KB) |  | HTML iconHTML  

    As wireless technology advances, wireless products are integrated with enterprise networking to offer cordless terminal mobility. Most corporations have deployed wireless PBXs at the departmental level. However, the mobility management mechanism that integrates these facilities at the corporation level may not be available. This paper describes a mobility management mechanism for an enterprise wireless telephone network. We show how, to modify the call model of the private branch exchange (PBX) to accommodate mobility management for an enterprise network. Our design was implemented on a commercial PBX product called Jupiter. An analytical model is proposed to evaluate the performance of the implemented system. Our study shows that with a large number of WPBXs and long Internet message delays, the misrouting probability can be limited to within 1%. This performance result is considered satisfactory View full abstract»

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  • Performance enhancement of CDMA cellular systems with augmented antenna arrays

    Page(s): 1052 - 1060
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (176 KB) |  | HTML iconHTML  

    The use of base station adaptive antenna arrays is an attractive way to increase the capacity of code division multiple access cellular systems. In this paper, a system with an adaptive minimum redundancy array (MRA) at the base stations is proposed. This system uses the high-resolution signal angle of the arrival estimation algorithm, ESPRIT, in conjunction with array augmentation techniques, and linear least squares adaptation. For the MRA, we propose to use virtual array elements at the locations where there is no sensor element. All real and virtual sensor outputs are weighted and combined to extract the desired signal components and suppress interference. To simplify the evaluation of the interference from adjacent cells, the concentric circle cell geometry is employed in place of the common hexagonal cell geometry. The performance of the proposed system is quantified by comparison with the omnidirectional antenna, and the adaptive uniform linear array (ULA) with the same number of elements and same array aperture, using realistic simulations. It is shown that, for a four-element array, there is about a 4- and a 1-dB improvement in the despread output signal-to-interference-plus-noise ratio of the proposed system over the omnidirectional antenna and the conventional ULA, respectively View full abstract»

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Aims & Scope

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

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Editor-in-Chief
Muriel Médard
MIT