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Selected Areas in Communications, IEEE Journal on

Issue 7 • Date Jul 2001

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Displaying Results 1 - 19 of 19
  • Constructing multirate CDMA signals with controlled peak-to-average power ratio using balanced incomplete block designs

    Page(s): 1255 - 1263
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (196 KB) |  | HTML iconHTML  

    A multirate modulation scheme with controlled peak-to-average power (PAV) ratio and suitable for code division multiple access techniques is introduced. The scheme utilizes codes drawn from a fixed set of Walsh-Hadamard codes but other code families can also be used. The strategy employs balanced incomplete block designs to define the multirate data symbols, with the objective being to reduce depletion of the code pool while controlling the PAV ratio. Existing multirate schemes are reviewed. Code depletion and PAV ratio characteristics of the proposed scheme are compared to existing schemes. Symbol error performance of the proposed scheme in an additive white Gaussian noise channel is examined. It is concluded that the proposed scheme, unlike the two other existing schemes that are examined, strikes a good balance between PAV ratio, code depletion, and error performance View full abstract»

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  • Unified spatial diversity combining and power allocation for CDMA systems in multiple time-scale fading channels

    Page(s): 1276 - 1288
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (344 KB) |  | HTML iconHTML  

    In a mobile wireless system, fading effects can be classified into large-scale (long-term) effects and small-scale (short-term) effects. We use transmission power control to compensate for large-scale fading and exploit receiver antenna (space) diversity to combat small-scale fading. We show that the interferences across the antennas are jointly Gaussian in a large system, and then characterize the signal-to-interference ratio for both independent and correlated (across the antennas) small-scale fading cases. Our results show that when each user's small-scale fading effects are independent across the antennas, there is a clear separation between the gains of transmission power control and diversity combining, and the two gains are additive (in decibels). When each user's small-scale fading effects are correlated across the antennas, we observe that, in general, the gains of transmission power control and diversity combining are coupled. However, when the noise level diminishes to zero, using maximum ratio combining “decouples” the gains and achieves the same diversity gain as in the independent case. We then characterize the Pareto-optimal (minimum) transmission power allocation for the cases of perfect and noisy knowledge of the desired user's large-scale fading effects. We find that using antenna diversity leads to significant gains for the transmission power View full abstract»

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  • Robust vector quantization for wireless channels

    Page(s): 1365 - 1373
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (168 KB) |  | HTML iconHTML  

    This study focuses on two issues: parametric modeling of the channel and index assignment of codevectors, to design a vector quantizer that achieves high robustness against channel errors. We first formulate the design of a robust zero-redundancy vector quantizer as a combinatorial optimization problem leading to a genetic search for a minimum-distortion index assignment. The performance is further enhanced by the use of the Fritchman (1967) channel model that more closely characterizes the statistical dependencies between error sequences. This study also presents an index assignment algorithm based on the Fritchman model with parameter values estimated using a real-coded genetic algorithm. Simulation results indicate that the global explorative properties of genetic algorithms make them very effective in estimating Fritchman model parameters, and use of this model can match index assignment to expected channel conditions View full abstract»

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  • Slow frequency-hopping multicarrier DS-CDMA for transmission over Nakagami multipath fading channels

    Page(s): 1211 - 1221
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (260 KB) |  | HTML iconHTML  

    A novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrowband CDMA and third-generation wideband CDMA systems. The frequency hopping patterns are controlled by a set of constant-weight codes. Consequently, multirate communications can be implemented by selecting the corresponding sets of constant-weight codes having the required weights controlling the SFH patterns invoked. Two FH schemes, namely random and uniform FH, are considered and their advantages as well as disadvantages are investigated. We assume that the system operates in a multipath fading environment and a RAKE receiver structure with maximum ratio combining (MRC) is used for demodulation. The system's performance is evaluated over the range of multipath Nakagami (1960) fading channels, under the assumption that the receiver has all explicit knowledge of the associated frequency-hopping (FH) patterns invoked. Furthermore, the performance of the SFH/MC DS-CDMA system is compared to that of the conventional single-carrier (SC) DS-CDMA system and that of the conventional MC DS-CDMA system, under the assumptions of constant system bandwidth and of constant transmitted signal power View full abstract»

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  • On the distribution of zeros of mobile channels with application to GSM/EDGE

    Page(s): 1289 - 1299
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (332 KB) |  | HTML iconHTML  

    The distribution of zeros of mobile channels is investigated and the results obtained are applied to the GSM/EDGE system. The taps of the discrete-time overall impulse response can be modeled as correlated complex Gaussian random variables, where the correlations depend on the transmit filter, the power delay profile of the channel, and the receiver input filter. For calculation of the density of zeros of the overall transfer function, a result from the mathematical literature is used. From this density, two cumulative distributions which are relevant for the design of suboptimum receivers are derived. Our investigations show that for the power delay profiles specified for GSM/EDGE, an allpass prefilter which transforms the impulse response in its minimum phase equivalent should be employed if decision-feedback equalization (DFE) or reduced-state sequence estimation (RSSE) are used at the receiver. On the other hand, if impulse response truncation using a linear prefilter is applied, the truncated impulse response should have a length of three as shown View full abstract»

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  • Optimizing time and space MIMO antenna system for frequency selective fading channels

    Page(s): 1395 - 1407
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (348 KB) |  | HTML iconHTML  

    Smart or adaptive antennas promise to provide significant increases in system capacity and performance in wireless communication systems. In this paper, we investigate the use of adaptive antennas at the base and mobile stations, operating jointly, to maximize the average signal-to-interference and noise ratio (SINR) of each packet in the system for frequency selective channels with prior knowledge of the channel at the transmitter. Our approach is based on deriving an analytic formula for the average packet SINR and using the Lagrange multiplier method to determine an optimum. We derive necessary conditions for an optimum solution and propose an analytical expression for the optimum. Our analytical expression is not guaranteed to be the global optimum but it does satisfy the derived necessary conditions and, in addition for frequency flat channels, our results reduce to expressions for optimal weights previously published. To demonstrate the potential of the proposed system, we provide Monte Carlo simulation results of the system bit-error rates and make comparisons with other adaptive antenna systems. These show that significant improvements in performance are possible in a wireless communications context View full abstract»

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  • Open-loop power control error in cellular CDMA overlay systems

    Page(s): 1246 - 1254
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (288 KB) |  | HTML iconHTML  

    Adaptive power control has widely been used in DS/CDMA systems to overcome the so-called “near-far” problem. This paper studies the adaptive open-loop power control of a cellular CDMA system, which is overlaid in the downlink by a narrowband signal. The effects of downlink power allocation schemes to power control error in the presence of narrowband interference are analyzed. In order to get a minimum power control error in the CDMA overlay situations, an optimum downlink power allocation scheme is used, which works well for a wide range of signal to narrowband interference ratio View full abstract»

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  • ATCP: TCP for mobile ad hoc networks

    Page(s): 1300 - 1315
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (196 KB) |  | HTML iconHTML  

    Transport connections set up in wireless ad hoc networks are plagued by problems such as high bit error rates, frequent route changes, and partitions. If we run the transmission control protocol (TCP) over such connections, the throughput of the connection is observed to be extremely poor because TCP treats lost or delayed acknowledgments as congestion. We present an approach where we implement a thin layer between Internet protocol and standard TCP that corrects these problems and maintains high end-to-end TCP throughput. We have implemented our protocol in FreeBSD, and we present results from extensive experimentation done in an ad hoc network. We show that our solution improves the TCP's throughput by a factor of 2-3 View full abstract»

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  • Improving battery performance by using traffic shaping techniques

    Page(s): 1385 - 1394
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (264 KB) |  | HTML iconHTML  

    We present a new approach to minimizing energy consumption by addressing battery management techniques that exploit the charge recovery effect inherent to many secondary storage batteries. We review results that pertain to the capacity of a battery and its dependence on the intensity of the discharge current. The phenomenon of charge recovery that takes place under bursty or pulsed discharge conditions is identified as a mechanism that can be exploited to enhance the capacity of a battery. The bursty nature of many data traffic sources suggests that data transmissions may provide natural opportunities for charge recovery. We explore stochastic models to track charge recovery in conjunction with bursty discharge processes. Using the postulated model, we identify the improvement to battery capacity that results from a pulsed discharge driven by bursty stochastic discharge demand. The insight from this analysis leads us to propose discharge shaping techniques that trade-off energy efficiency with delay View full abstract»

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  • Two suboptimal algorithms for downlink beamforming in FDD DS-CDMA mobile radio

    Page(s): 1264 - 1275
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (352 KB) |  | HTML iconHTML  

    Two suboptimal algorithms are proposed for downlink beamforming in FDD DS-CDMA mobile radio by using uplink beamforming weights. One is a null-constrained method, which maintains the same null positions for both uplink and downlink beam patterns; the other is a frequency-calibrated method which constrains the same main beam positions for both patterns. We also evaluate the multicell downlink capacity of DS-CDMA systems using a per-user-per-weight beamforming scheme. Outer cell interference is modeled as an AWGN process whose variance is proportional to the average intracell total transmitted power. Computer simulations are given to compare the single cell and multicell capacities using different downlink beamforming weight generation algorithms View full abstract»

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  • Transmit antennae space-time block coding for generalized OFDM in the presence of unknown multipath

    Page(s): 1352 - 1364
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (484 KB) |  | HTML iconHTML  

    Transmit antenna diversity has been exploited to develop high-performance space-time coders and simple maximum-likelihood decoders for transmissions over flat fading channels. Relying on block precoding, this paper develops generalized space-time coded multicarrier transceivers appropriate for wireless propagation over frequency-selective multipath channels. Multicarrier precoding maps the frequency-selective channel into a set of flat fading subchannels, whereas space-time encoding/decoding facilitates equalization and achieves performance gains by exploiting the diversity available with multiple transmit antennas. When channel state information is unknown at the receiver, it is acquired blindly based on a deterministic variant of the constant-modulus algorithm that exploits the structure of space-time block codes. To benchmark performance, the Cramer-Rao bound of the channel estimates is also derived. System performance is evaluated both analytically and with simulations View full abstract»

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  • Satellite diversity in mobile satellite CDMA systems

    Page(s): 1324 - 1333
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (228 KB) |  | HTML iconHTML  

    The paper addresses the exploitation of satellite diversity in a satellite mobile network. In particular, we focus on the impact of diversity on service availability and on system capacity, considering the forward link of a CDMA system with a multisatellite and multibeam architecture. The analysis includes the effects of path blockage, intrabeam and interbeam interference, imperfect power control, and fading correlation in the time-domain due to nonideal interleaving. A closed-form solution is given for the estimation of system capacity. We show that satellite diversity is essential in providing service availability in urban and suburban areas, while the impact of satellite diversity on system capacity may be positive or negative, mainly depending on the fading channel characteristics. In particular, diversity becomes more and more beneficial for increasing fading time-domain correlation (i.e., for low mobile speed and/or limited interleaving depth). The analytical results have been validated by means of Monte Carlo simulation View full abstract»

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  • A access-based clustering protocol for multihop wireless ad hoc networks

    Page(s): 1201 - 1210
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (220 KB) |  | HTML iconHTML  

    The ad hoc network is a temporary wireless system without a fixed (wired or wireless) infrastructure. Many clustering algorithms have been proposed to partition mobile users into clusters to support routing and network management. Most previous studies, however, focus on algorithm design, lacking an overall evaluation of clustering overheads. We design a multiple access scheme for the broadcast of control messages, and propose a new access-based clustering protocol (ABCP) whose cluster formation is heavily influenced by the outcome of the multiple access. By taking into account many delicate aspects in the clustering process, the ABCP provides a generic, flexible, rapidly deployed and stable cluster architecture for the upper layer protocols. Simulation is used to compare ABCP with the other clustering strategy in terms of cluster stability and overheads. Since ABCP makes clustering decision directly based on the result of channel access, it requires fewer control overheads and has shorter convergence time than the other clustering criteria. We also demonstrate that the resulting cluster structure by ABCP behaves stable in face of topology changes View full abstract»

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  • Data-aided frequency estimation for PSK signaling in frequency-selective fading

    Page(s): 1408 - 1419
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (348 KB) |  | HTML iconHTML  

    A new data-aided frequency estimator is introduced for phase-shift keying signals transmitted over frequency-selective fading channels. This estimator is developed based on a maximum likelihood criterion. It assumes the use of a special class of pilots, called near-i.i.d. (independent identically distributed) sequences, with impulsive fourth-order moments. With the help of such pilots, the proposed method can estimate frequency offsets without the need for channel information. The pilots of GSM and IS-136 mobile communication systems have been observed as being near-i.i.d., and statistical analysis indicates that the proposed estimate is almost unbiased if the pilot is near-i.i.d. The advantage of the proposed estimator over conventional methods is demonstrated via computer simulation View full abstract»

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  • A beamspace-time interference cancelling CDMA receiver for sectored communications in a multipath environment

    Page(s): 1374 - 1384
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (268 KB) |  | HTML iconHTML  

    A beamspace-time (BT) receiver is proposed for interference suppression and multipath diversity reception in sectored wireless code division multiple access communications. The scheme involves two stages. First, a set of adaptive space-time diversity processors, in the form of beamformer-correlator pairs, is constructed which provides effective suppression of unwanted interference and reception of signals from a prescribed space-time region. Second, the output data obtained by these processors are maximum ratio combined to capture the signal multipath components coherently. The proposed BT receiver is blind in that no training signal is required. The only information required is the signature sequence, timing and a rough estimate of the angle of arrival of the signal for selecting the sector of interest View full abstract»

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  • Energy efficient battery management

    Page(s): 1235 - 1245
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (272 KB) |  | HTML iconHTML  

    A challenging aspect of mobile communications consists in exploring ways in which the available run time of terminals can be maximized. We present a detailed electrochemical battery model and a simple stochastic model that captures the fundamental behavior of the battery. The stochastic model is then matched to the electrochemical model and used to investigate battery management techniques that may improve the energy efficiency of radio communication devices. We consider an array of electrochemical cells. Through simple scheduling algorithms, the discharge from each cell is properly shaped to optimize the charge recovery mechanism, without introducing any additional delay in supplying the required power. Then, a battery management scheme, which exploits knowledge of the cells' state of charge, is implemented to achieve a further improvement in the battery performance. In this case, the discharge demand may be delayed. Results indicate that the proposed battery management techniques improve system performance no matter which parameters values are chosen to characterize the cells' behavior View full abstract»

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  • Soft iterative multisensor multiuser detection in coded dispersive CDMA wireless channels

    Page(s): 1334 - 1351
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (464 KB) |  | HTML iconHTML  

    A soft iterative multisensor array receiver for coded multiuser wideband code-division multiple-access wireless uplink channels is proposed, such channels are typically both frequency- and time-selective. A new equivalent discrete-time synchronous representation is used to model asynchronous multiuser dispersive channels that employ, in general, random spreading. The proposed scheme suppresses multiuser interference over a wide range of user signal powers, by iteratively exchanging soft information between a minimum mean-square-error (MMSE) multiuser demodulator and a bank of single-user decoders, feeding back the outputs of the latter to aid in soft multiuser multisensor MMSE-RAKE demodulation and subtractive interference cancellation, in the second and subsequent iterations. It displays near-far resistance since it behaves like a successive interference subtracter across iterations. The array responses are obtained via a multipass estimation scheme that uses both (temporal) pilot symbols and soft estimates fed back from the decoders to effectively provide multisymbol pilot signals and thence successively refined estimates with increasing iterations, while seeking neither to rely on the array geometry nor to estimate the directions from which users' signals arrive at the receiver. Simulation studies indicate that this scheme performs close to the single-user case with a two-sensor receiver array, and perfect channel state information, after four iterations; alternatively, it allows significantly increased user capacity compared with conventional receivers, and suffers only a modest loss with estimated array responses View full abstract»

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  • Network-assisted resource management for wireless data networks

    Page(s): 1222 - 1234
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (196 KB) |  | HTML iconHTML  

    We propose a framework for network-assisted radio resource management in wireless data networks. This type of radio resource management techniques offer implementation and capacity benefits compared to conventional, interference-measurement based, dynamic channel assignment (DCA) algorithms. The basic idea is to use interbase signaling to shift most of the burden of the resource allocation from the air interface to the backbone infrastructure. By exchanging channel assignment as well as other relevant information in real time through the backbone network, each base can calculate the impact of a resource assignment on the system. As a result, rapid interference measurements, which are typically needed to implement DCA schemes, are replaced by a limited amount of path loss measurements and the computation of interference conditions by the base stations. This significantly reduces the measurement and over-the-air signaling requirements, and can also provide an opportunity for a better optimization of the system performance. We focus on two specific algorithms: network-assisted least-interference-based dynamic packet assignment (NA-LI-DPA) and network-assisted dynamic packet assignment with throughput optimization (NA-DPA). NA-LI-DPA closely resembles a least-interference-based dynamic channel assignment algorithm, and NA-DPA attempts to further improve the overall system throughput. The algorithms, as defined, are appropriate for a best-effort data service, where the primary goal is to provide a higher throughput. However, it will be clear from the discussion that it is also feasible to alter the algorithms to optimize performance metrics other than throughput, e.g., to ensure a certain quality of service. We show through simulation that, for a system like enhanced general packet radio service (EGPRS) system, NA-DPA can provide a throughput that is 50% higher than random packet assignment, and 25% higher than that obtained by conventional DCA algorithms View full abstract»

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  • A multimode CDMA with reduced intercell interference for broadband wireless networks

    Page(s): 1316 - 1323
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (124 KB) |  | HTML iconHTML  

    This paper investigates the application of CDMA to broadband wireless access systems commonly known as LMDS networks. After a review of the intercell interference in the emerging LMDS networks based on TDMA, we analyze the interference in CDMA-based networks and show that, while conventional CDMA is superior to TDMA in terms of the worst-case interference on the upstream channel, it turns out to be inferior to TDMA in terms of interference on the downstream channel. Next, based on the observation that strong interference occurs only for a small range of user locations, we introduce a multimode CDMA concept that efficiently handles this interference by assigning orthogonal spreading sequences whose number and length are a function of the user position with respect to the base station. We show that in terms of worst-case SIR, the proposed technique leads to a significant improvement with respect to conventional CDMA and TDMA 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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Meet Our Editors

Editor-in-Chief
Muriel Médard
MIT