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Wireless Communications, IEEE Transactions on

Issue 3 • Date May 2003

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Displaying Results 1 - 22 of 22
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  • An iterative maximum SINR receiver for multicarrier CDMA systems over a multipath fading channel with frequency offset

    Page(s): 560 - 569
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (651 KB) |  | HTML iconHTML  

    A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver. View full abstract»

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  • Efficient decision feedback equalization for sparse wireless channels

    Page(s): 570 - 581
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (681 KB) |  | HTML iconHTML  

    A new efficient decision feedback equalizer (DFE) appropriate for channels with long and sparse impulse response (IR) is proposed. Such channels are encountered in many high-speed wireless communications applications. It is shown that, in cases of sparse channels, the feedforward and feedback (FB) filters of the DFE have a particular structure, which can be exploited to derive efficient implementations of the DFE, provided that the time delays of the channel IR multipath components are known. This latter task is accomplished by a novel technique, which estimates the time delays based on the form of the channel input-output cross-correlation sequence in the frequency domain. A distinct feature of the resulting DFE is that the involved FB filter consists of a reduced number of active taps. As a result, it exhibits considerable computational savings, faster convergence, and improved tracking capabilities as compared with the conventional DFE. Note that faster convergence implies that a shorter training sequence is required. Moreover, the new algorithm has a simple form and its steady-state performance is almost identical to that of the conventional DFE. View full abstract»

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  • Decentralized dynamic power control for cellular CDMA systems

    Page(s): 549 - 559
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (772 KB) |  | HTML iconHTML  

    The control of transmit power has been recognized as an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. Indeed, power control allows for mobile users to share radio resources equitably and efficiently in a multicell environment. Much of the work on power control for CDMA systems found in the literature assumes a quasi-static channel model, i.e., the channel gains of the users are assumed to be constant over a sufficiently long period of time for the control algorithm to converge. In this paper, the design of dynamic power control algorithms for CDMA systems is considered without the quasi-static channel restriction. The design problem is posed as a tradeoff between the desire for users to maximize their individual quality of service and the need to minimize interference to other users. The dynamic nature of the wireless channel for mobile users is incorporated in the problem definition. Based on a cost minimization framework, an optimal multiuser solution is derived. The multiuser solution is shown to decouple, and effectively converge, to a single-user solution in the large system asymptote, where the number of users and the spreading factor both go to infinity with their ratio kept constant. In a numerical study, the performance of a simple threshold policy is shown to be near that of the optimal single-user policy. This offers support to the threshold decision rules that are employed in current cellular CDMA systems. View full abstract»

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  • Optimizing an array of antennas for cellular coverage from a high altitude platform

    Page(s): 484 - 492
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1037 KB) |  | HTML iconHTML  

    In a wireless communications network served by a high altitude platform (HAP) the cochannel interference is a function of the antenna beamwidth, angular separation and sidelobe level. At the millimeter wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells. We present a method for predicting cochannel interference based on curve-fit approximations for radiation patterns of elliptic beams which illuminate cell edges with optimum power, and a means of estimating optimum beamwidths for each cell of a regular hexagonal layout. The method is then applied to a 121 cell architecture. Where sidelobes are modeled as a flat floor at 40-dB below peak directivity, a cell cluster size of four yields carrier-to-interference ratios (CIRs), which vary from 15 dB at cell edges to 27 dB at cell centers. On adopting a cluster size of seven, these figures increase, respectively, to 19 and 30 dB. On reducing the sidelobe level, the improvement in CIR can be quantified. The method also readily allows for regions of overlapping channel coverage to be shown. View full abstract»

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  • Spreading and power allocation for multiple antenna transmission using decorrelating receivers

    Page(s): 436 - 445
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (576 KB) |  | HTML iconHTML  

    We propose a new scheme for multiple antenna transmission in the context of spread-spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple antennas. We show that this strategy of using multiphase spreading (MPS) on different antennas greatly improves the throughput over currently known spread-spectrum multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed rate of channel state information, which might be provided via a feedback link, at the transmitter. We demonstrate the differences in optimal power distribution for maximizing capacity and minimizing probability of outage. When the transmission from the two antennas uses orthogonal spreading, we find that optimizing the power does not give much gain over the equal power transmission. However, when the transmissions are not orthogonal as in the case of MPS, then allocating power to maximize throughput gives considerable gain over equal power transmission. We also consider the effect of imperfections in the feedback channel on the optimal power allocation and show that our power allocation scheme is robust to feedback errors. View full abstract»

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  • Use of multiple antennas for DS/CDMA code acquisition

    Page(s): 424 - 430
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (448 KB) |  | HTML iconHTML  

    A generalized acquisition scheme is proposed for direct sequence code-division multiple-access systems with multiple antennas. The proposed scheme employs grouping of multiple antennas as a means of a tradeoff between two important factors determining the mean acquisition time, combining gain and search time. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh-fading channels with consideration of spatial correlations. Numerical results show that the use of the largest number of antenna groups is preferable to reducing the mean acquisition time at low signal-to-interference ratio (SIR) values. At high SIR values, on the contrary, the mean acquisition time is found to increase in proportion to the number of antenna groups. In a typical environment, the presence of spatial correlation is shown to increase or decrease the mean acquisition time within 50% compared with the uncorrelated fading case. View full abstract»

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  • Performance of CDMA random access systems with packet combining in fading channels

    Page(s): 413 - 419
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (618 KB) |  | HTML iconHTML  

    We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance. View full abstract»

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  • Coded differential space-time modulation for flat fading channels

    Page(s): 582 - 590
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (793 KB) |  | HTML iconHTML  

    In this paper, powerful coding techniques for differential space-time modulation (DSTM) over Rayleigh flat fading channels and noncoherent detection without channel state information at the receiver are investigated. In particular, multilevel coding, bit-interleaved coded modulation, and so-called hybrid coded modulation (HCM) are devised and compared. For improved noncoherent reception multiple-symbol differential detection (MSDD) is adapted to DSTM. In order to reduce the computational effort required for MSDD, a low-complexity version of MSDD is applied. Evaluating the ergodic channel capacity for the different schemes as appropriate performance measure, HCM with simplified MSDD is shown to offer a favorable tradeoff between complexity and achievable power efficiency. Simulation results employing turbo codes in properly designed HCM schemes confirm the predictions from information theory. View full abstract»

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  • Spread-time CDMA resistance in fading channels

    Page(s): 446 - 458
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1083 KB) |  | HTML iconHTML  

    We study and examine the effect of fading channels on the performance of spread-time/code division multiple access (ST/CDMA). We consider two different models for fading channel and obtain the average signal to interference plus noise ratio (SIR) for ST/CDMA in both cases. We also obtain the SIR for spread-spectrum/CDMA (SS/CDMA) and compare the results with that of ST/CDMA. Depending upon the transmitted chip pulse shape, and in the absence of thermal noise, spread-time (ST) performance is equal to or greater than spread spectrum (SS). For example ST/CDMA has 0.62 dB higher SIR value compared to SS/CDMA with root raised cosine pulse shape with rolloff factor 0.22 used in W-CDMA. The spread-time technique was first proposed by Crespo et al. (1991, 1995), and US Patents no. 5177768, 5175744, 5175743, 5173923, and is considered as the time-frequency dual of spread spectrum technique. View full abstract»

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  • Hermitian codes for frequency-hop spread-spectrum packet radio networks

    Page(s): 529 - 536
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (650 KB) |  | HTML iconHTML  

    Hermitian codes are an attractive alternative to Reed-Solomon codes for use in frequency-hop spread-spectrum packet radio networks. For a given alphabet size, a Hermitian code has a much longer block length than a Reed-Solomon code. This and other considerations suggest that Hermitian codes may be superior for certain applications. Analytical results are developed for the evaluation of the packet error probability for frequency-hop transmissions using Hermitian coding. We find there are several situations for which Hermitian codes provide much lower packet error probabilities than can be obtained with Reed-Solomon codes. In general, as the code rate decreases or the symbol alphabet size increases, the relative performance of Hermitian codes improves with respect to Reed-Solomon codes. Performance evaluations are presented for an additive white Gaussian noise channel and for certain partial-band interference channels, and the packet error probability is evaluated for both errors-only and errors-and-erasures decoding. View full abstract»

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  • Analysis of fade margins for soft and hard handoffs in cellular CDMA systems

    Page(s): 431 - 435
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (339 KB) |  | HTML iconHTML  

    The effect of handoff techniques on fade margin is investigated. We provide an accurate comparison for soft and hard handoffs in a two-cell system. An upper bound on the fade margin for hard-handoff systems is derived and an approximation formula is obtained empirically. Simulation results show that this approximation is very accurate over reasonable ranges of system parameters. Extension of our analysis to multiple-cell systems are illustrated using a three-cell system as an example. View full abstract»

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  • A channelized digital ultrawideband receiver

    Page(s): 502 - 510
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (591 KB) |  | HTML iconHTML  

    A channelized digital ultrawideband (UWB) receiver that efficiently samples the UWB signal at a fraction of the chip frequency is proposed. The received signal is channelized in the frequency domain by employing a bank of mixers and low-pass filters. After sampling at a much reduced frequency, digital synthesis filters optimally estimate the transmitted signals. The signal-to-noise ratio (SNR) of the proposed receiver has been solved and compared against an ideal conventional receiver, which is defined as a receiver that samples at the signal Nyquist rate. When finite resolution analog-to-digital converters (ADC) are employed in the presence of a large narrowband interferer, the proposed receiver significantly outperforms the ideal conventional receiver. For example, the SNR of the proposed receiver is as much as 20 dB higher than the ideal conventional receiver when a 4-bit ADC is used in the presence of a 50 dB (relative to the noise floor) brickwall narrowband interferer with a bandwidth of 15% of the chip frequency. View full abstract»

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  • Block differentially encoded OFDM with maximum multipath diversity

    Page(s): 420 - 423
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (336 KB) |  | HTML iconHTML  

    This paper proposes a novel block differentially encoded orthogonal frequency-division multiplexing for multicarrier transmissions over frequency-selective fading channels. Choosing appropriate system parameters, we divide the set of correlated subchannels into subsets of independent subchannels. Within each subset, differential unitary space-time modulation is performed by treating each subchannel as a transmit antenna. In addition to low complexity, the proposed system enjoys maximum multipath diversity and high coding advantages. Analytic evaluation and corroborating simulations reveal its performance merits. View full abstract»

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  • Large system performance of second-order linear multistage CDMA receivers

    Page(s): 591 - 600
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (850 KB) |  | HTML iconHTML  

    We analyze the performance of a second-order linear multistage multiuser code-division multiple-access receiver. The receiver's filtered output is designed to converge to that of the linear minimum mean-squared error solution as the number of stages increase. Our analysis is based on a related second-order stationary iterative solution method. We derive the large system output signal to interference-plus-noise ratio for each stage. We use this result to perform a numerical optimization with respect to the two second-order parameters of our receiver. Within this iterative framework, we can achieve performance extremely close to the optimal linear multistage multiuser receiver. View full abstract»

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  • Wireless access to the World Wide Web in an integrated CDMA system

    Page(s): 472 - 483
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1324 KB) |  | HTML iconHTML  

    We propose a wireless access mechanism for web traffic packets in an integrated wireless code-division multiple-access system that has both voice and Web traffic. The proposed scheme is a medium-access control layer/link layer (MAC/LL) scheduling algorithm that consists of a two level control: admission control and packet scheduling. The admission control restricts the number of users in the system such that quality-of-service requirements [target signal-to-interference ratio (SIR) and delay] for both voice and Web traffic can be met. The packet scheduling balances the system interference on a slot-by-slot basis such that the target SIRs can be achieved for all users (voice and Web browsing sessions) with a higher scheduling priority for voice. Designing admission control for Web users based on the average offered rate per session is difficult due to the high variations in the offered load generated by heavy tailed distributions for Web traffic. To overcome this problem, we propose an admission control algorithm that adaptively estimates the aggregate average load based on load measurements using a sliding observation window. View full abstract»

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  • A new simple model for land mobile satellite channels: first- and second-order statistics

    Page(s): 519 - 528
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (732 KB) |  | HTML iconHTML  

    We propose a new shadowed Rice (1948) model for land mobile satellite channels. In this model, the amplitude of the line-of-sight is characterized by the Nakagami distribution. The major advantage of the model is that it leads to closed-form and mathematically-tractable expressions for the fundamental channel statistics such as the envelope probability density function, moment generating function of the instantaneous power, and the level crossing rate. The model is very convenient for analytical and numerical performance prediction of complicated narrowband and wideband land mobile satellite systems, with different types of uncoded/coded modulations, with or without diversity. Comparison of the first- and the second-order statistics of the proposed model with different sets of published channel data demonstrates the flexibility of the new model in characterizing a variety of channel conditions and propagation mechanisms over satellite links. Interestingly, the proposed model provides a similar fit to the experimental data as the well-accepted Loo's (1985) model but with significantly less computational burden. View full abstract»

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  • Forward-link soft-handoff in CDMA with multiple-antenna selection and fast joint power control

    Page(s): 459 - 471
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (900 KB) |  | HTML iconHTML  

    We consider forward-link soft-handoff with multiple antenna selection and fast joint power control at high data rates in a cellular code-division multiple-access network, where signals are directed to a mobile station (MS) from antennas located at the same or different base stations. The total power transmitted to any mobile is divided among the active antennas selected according to the momentary channel conditions so as to maximize the signal-interference ratio at each MS. Multiple-antenna selection is used to mitigate the effects of both short- and long-term fading, and achieve the best soft-handoff with respect to system capacity and complexity. To achieve capacity gains with soft-handoff, we derive optimum handoff thresholds corresponding to the optimum handoff region in different cell environments. Numerical results demonstrate that under high Doppler spread and large handoff-delay conditions, the proposed soft-handoff employing two transmit antennas and the optimum handoff threshold achieves a significant gain in microcell environments, but not in macrocell environments. View full abstract»

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  • Noncoherent space-time equalization

    Page(s): 537 - 548
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (971 KB) |  | HTML iconHTML  

    In this paper, noncoherent equalization is combined with multiple receive antennas. The resulting noncoherent space-time equalization (NSTE) schemes are analyzed and compared with the corresponding coherent receivers. In particular, noncoherent linear equalization (NLE), noncoherent decision-feedback equalization (NDFE), and noncoherent sequence estimation (NSE) are considered. For NLE and NDFE novel approximations for the signal-to-distortion ratio (SDR) are derived and verified by simulations. It is shown that NSTE can suppress interfering users and exploit diversity as efficiently as coherent STE. However, NSTE is more robust against channel phase variations than the combination of coherent STE and synchronization. Robust noncoherent recursive least squares (NC-RLS) algorithms, which compare favorably with the conventional RLS algorithm with additional carrier synchronization loop, are proposed for fast filter adaptation. View full abstract»

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  • Joint signal precoding in the downlink of spread-spectrum systems

    Page(s): 511 - 518
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (430 KB) |  | HTML iconHTML  

    The downlink capacity tends to be the limiting factor in future communications systems. We propose two schemes for the downlink of code-division multiple-access systems that help reduce the multiuser interference by jointly precoding the transmitted signal based on knowledge of the downlink channel. Methods that operate on blocks of bits are first developed and a bitwise simplification of these for systems with large processing gains are derived. The use of multiple transmit antennas are also considered. These methods provide a substantial capacity increase without requiring complex multiuser detectors at the mobile terminals, and can be applied for instance to time-division duplex systems, which have reciprocal channels. View full abstract»

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  • Reducing authentication signaling traffic in third-generation mobile network

    Page(s): 493 - 501
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (594 KB) |  | HTML iconHTML  

    In the Universal Mobile Telecommunication System (UMTS), authentication functions are utilized to identify and authenticate a mobile station (MS) and validate the service request type to ensure that the user is authorized to use the particular network services. The authenticating parties are the authentication center (AuC) in the home network and the MS. In the UMTS, the serving general packet radio service support node (SGSN) accesses the AuC to obtain the authentication data, and delegates the AuC to perform mutual authentication with the MS. Since the cost for accessing AuC is expensive, the SGSN may obtain an array of authentication vectors (AVs) at a time so that the number of accesses can be reduced. On the other hand, if the size K of the AV array is large, the AV array transmission from the AuC to the SGSN may be expensive. Thus, it is desirable to select an appropriate K value to minimize the authentication network signaling cost. We propose an analytic model to investigate the impact of K on the network signaling traffic, which is validated by simulation experiments. Then, we propose an automatic K-selection mechanism that dynamically selects the size of the AV array to reduce the network signaling cost. Our study indicates that the automatic K-selection mechanism effectively identifies appropriate size of the authentication vector array. View full abstract»

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  • A BEM-based detector for CPM signals transmitted over frequency-flat fading channels

    Page(s): 409 - 412
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (321 KB) |  | HTML iconHTML  

    In this paper, the expectation-maximization (EM) algorithm for maximum a posteriori (MAP) estimation of a random vector is applied to the problem of symbol detection for continuous phase modulation signals transmitted over time-selective Rayleigh-fading channels. This results in a soft-in-soft-out detection algorithm suitable for iterative detection/decoding schemes. Simulation results show that the error performance provided by the proposed solution is very close to that of a MAP detector endowed with an ideal knowledge of the channel state both in uncoded and coded transmissions. View full abstract»

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The IEEE Transactions on Wireless Communications publishes high-quality manuscripts on advances in the state-of-the-art of wireless communications.

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Editor-in-Chief
Jeff Andrews
Cullen Trust for Higher Education Endowed Professor of Engineering