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Wireless Communications, IEEE Transactions on

Issue 2 • Date Mar 2003

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Displaying Results 1 - 20 of 20
  • Adaptive multirate CDMA for uplink throughput maximization

    Page(s): 218 - 228
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (841 KB) |  | HTML iconHTML  

    We determine the optimal adaptive rate and power control strategies to maximize the total throughput in a multirate code-division multiple-access system. The total throughput of the system provides a meaningful baseline in the form of an upper bound to the throughput achievable with additional restrictions imposed on the system to guarantee fairness. Peak power and instantaneous bit energy-to-noise spectral density constraints are assumed at the transmitter with matched filter detection at the receiver. Our results apply to frequency selective fading in so far as the bit energy-to-equivalent noise power spectral density ratio definition can be used as the quality-of-service metric. The bit energy-to-equivalent noise power spectral density ratio metric coincides with the bit-error rate metric under the assumption that the processing gains and the number of users are high enough so that self-interference can be neglected. We first obtain results for the case where the rates available to each user are unrestricted, and we then consider the more practical scenario where each user has a finite discrete set of rates. An upper bound to the maximum average throughput is obtained and evaluated for Rayleigh fading. Suboptimal low-complexity schemes are considered to illustrate the performance tradeoffs between optimality and complexity. We also show that the optimum rate and power adaptation scheme with unconstrained rates is in fact just a rate adaptation scheme with fixed transmit powers, and it performs significantly better than a scheme that uses power adaptation alone. View full abstract»

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  • Comprehensive performance analysis of a TCP session over a wireless fading link with queueing

    Page(s): 344 - 356
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (690 KB) |  | HTML iconHTML  

    A link model-driven approach toward transmission control protocol (TCP) performance over a wireless link is presented. TCP packet loss behavior is derived from an underlying two-state continuous time Markov model. The approach presented here is (to our knowledge) the first that simultaneously considers (1) variability of the round-trip delay due to buffer queueing; (2) independent and nonindependent (bursty) link errors; (3) TCP packet loss due to both buffer overflow and channel errors; and (4) the two modes of TCP packet loss detection (duplicate acknowledgments and timeouts). The analytical results are validated against simulations using the ns-2 simulator for a wide range of parameters; slow and fast fading links; small and large link bandwidth-delay products. For channels with memory, an empirical rule is presented for categorizing the impact of channel dynamics (fading rate) on TCP performance. View full abstract»

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  • Smart-antenna operation for indoor wireless local-area networks using OFDM

    Page(s): 392 - 399
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (556 KB) |  | HTML iconHTML  

    This paper reports link-level Monte Carlo simulations for a system that is compatible with the physical layer of the 5-GHz IEEE 802.11a wireless-local-area network and utilizes an adaptive antenna array at the access point for single-user smart-antenna operation, as well as for space-division multiple access (SDMA). For the spatial indoor radio propagation channel, complex impulse-response recordings are used. These are obtained in wideband channel-sounder measurements in three different buildings at 5.3 GHz. Thus, no unrealistic assumptions about channel conditions are involved. The paper studies how the packet-error-rate performance for the downlink (DL) is affected by time evolution of the radio channel that takes place after the uplink operation in which channel estimation is performed, and before DL operation in which the estimated channel information is utilized. Based on simulations two-user SDMA is possible with four-antenna elements under indoor propagation conditions and with six antennas three users can simultaneously be served. Delay spreads, coherence bandwidths, and correlation properties (in space, frequency, and polarization) of the radio channels obtained in the measurements are also discussed. The results suggest that indoor time-division-duplex systems with access-point-controlled scheduling are desirable communication systems which can benefit from SDMA. View full abstract»

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  • Hierarchical cache design for enhancing TCP over heterogeneous networks with wired and wireless links

    Page(s): 205 - 217
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (558 KB) |  | HTML iconHTML  

    TCP is a reliable transport protocol tuned to perform well in traditional networks made up of links with low bit-error rates. Networks with higher bit-error rates, such as those with wireless links and mobile hosts, violate many of the assumptions made by the transmission control protocol (TCP), causing degraded end-to-end performance. We propose a two-layer hierarchical cache architecture for enhancing TCP performance over heterogeneous networks with both wired and wireless links. A new network-layer protocol, called new snoop (NS), is designed. The main idea is to cache the unacknowledged packets at both the mobile switch center (MSC) and base station (BS), to form a two-layer cache hierarchy. If a packet is lost due to transmission errors in the wireless link, the BS takes the responsibility to recover the loss. When a handoff occurs, the packets cached at the MSC can help to minimize the latency of retransmissions due to temporal disconnection. NS can preserve the end-to-end TCP semantics and is compatible with existing TCP applications. Its implementation only requires code modification at the BS and MSC. Simulation results show that NS is significantly more robust in dealing with unreliable wireless links and handoffs as compared with the original snoop scheme, as well as some other existing TCP enhancements. View full abstract»

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  • Error performance analysis of an energy sequence estimation receiver for binary FSK on frequency-selective fading channels

    Page(s): 260 - 269
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (721 KB) |  | HTML iconHTML  

    Energy detection of frequency-shift keying (FSK) signals is the optimum noncoherent detection technique yielding a minimum bit-error rate (BER) on the additive white Gaussian noise channel. It is usually used when carrier phase estimation is difficult. When FSK signals are passed through a frequency-selective multipath fading channel, the multipath delay in the channel results in a multitone waveform being received during each signaling interval. An effective practical sequence estimation receiver for this situation is proposed. It makes use of energy detectors followed by a Viterbi (1979) or sequence estimator that uses an energy difference metric. Statistical properties of the detection variable are derived. Analytical upper and lower bounds on the BER are derived and the results compared with computer simulations to show the effectiveness of the technique. View full abstract»

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  • Outage probability of dual-branch diversity systems in presence of co-channel interference

    Page(s): 310 - 319
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (880 KB) |  | HTML iconHTML  

    Outage probability of two low-complexity dual-branch diversity schemes with the consideration of both minimum carrier-to-interference ratio and the carrier-to-noise ratio constraints is investigated. More specifically, closed-form expressions for this outage probability for selection combining and switch and stay combining in single-interferer Rayleigh fading environment are derived, first under the assumption of perfectly estimated independent diversity paths, then by taking into account the effect of branch correlation and outdated/imperfect channel estimates. When applicable, the results obtained as special cases of the offered generic closed-form expressions are compared with those previously reported in the literature. In addition, some numerical examples are provided and discussed. View full abstract»

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  • Uplink user capacity in a CDMA macrocell with a hotspot microcell: exact and approximate analyses

    Page(s): 364 - 374
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1082 KB) |  | HTML iconHTML  

    This paper studies the number of voice users (user capacity) supported on the uplink of a single-macrocell/single-microcell code-division multiple-access system. A "hotspot" microcell is embedded within a larger macrocell and operates over the same bandwidth as the larger cell. Analytic methods are presented for computing user capacity which account for propagation loss, multiple-access interference, power control, and random locations of user terminals, as well as two distinct methods by which users select base stations (tiers). Along with the exact user capacity, a technique for making accurate approximations is also presented. Simulation results verify both the exact and approximate analytical methods. This simulation is also employed to study the capacity gains of a third, more optimal, tier-selection scheme. These results point to differences in capacity performance based on the tier-selection method, as well as on the traffic density within the hotspot region. View full abstract»

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  • Space-time diversity systems based on linear constellation precoding

    Page(s): 294 - 309
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1330 KB) |  | HTML iconHTML  

    We present a unified approach to designing space-time (ST) block codes using linear constellation precoding (LCP). Our designs are based either on parameterizations of unitary matrices, or on algebraic number-theoretic constructions. With an arbitrary number of Nt transmit- and Nr receive-antennas, ST-LCP achieves rate 1 symbol/s/Hz and enjoys diversity gain as high as NtNr over (possibly correlated) quasi-static and fast fading channels. As figures of merit, we use diversity and coding gains, as well as mutual information of the underlying multiple-input-multiple-output system. We show that over quadrature-amplitude modulation and pulse-amplitude modulation, our LCP achieves the upper bound on the coding gain of all linear precoders for certain values of Nt and comes close to this upper bound for other values of Nt, in both correlated and independent fading channels. Compared with existing ST block codes adhering to an orthogonal design (ST-OD), ST-LCP offers not only better performance, but also higher mutual information for Nt>2. For decoding ST-LCP, we adopt the near-optimum sphere-decoding algorithm, as well as reduced-complexity suboptimum alternatives. Although ST-OD codes afford simpler decoding, the tradeoff between performance and rate versus complexity favors the ST-LCP codes when Nt, Nr, or the spectral efficiency of the system increase. Simulations corroborate our theoretical findings. View full abstract»

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  • A self-encryption mechanism for authentication of roaming and teleconference services

    Page(s): 400 - 407
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (384 KB) |  | HTML iconHTML  

    A simple authentication technique for use in the global mobility network (GLOMONET) is proposed. This technique is based on the concept of distributed security management, i.e., the original security manager administrates the original authentication key (long-term secret key) acquired when a user makes a contract with his home network, while a temporary security manager is generated for a roaming user in the visited network that provides roaming services. The temporary security manager will take the place of the original security manager when the roaming user stays in the service area of the visited network. In the proposed authentication protocol for the regular communication phase, the procedures of the original security manager and the temporary security manager are the same except for introducing different parameters. Furthermore, the proposed technique not only reduces the number of transmissions during the authentication phase, but it also can decrease the complexity of mobile equipment. The idea behind the proposed technique is to introduce a simple mechanism which is called "self-encryption". We also suggest that this mechanism can be easily adopted as the authentication function for the secure teleconference service. View full abstract»

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  • Experimental characterization of the MIMO wireless channel: data acquisition and analysis

    Page(s): 335 - 343
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (471 KB) |  | HTML iconHTML  

    Detailed performance assessment of space-time coding algorithms in realistic channels is critically dependent upon accurate knowledge of the wireless channel spatial characteristics. This paper presents an experimental measurement platform capable of providing the narrowband channel transfer matrix for wireless communications scenarios. The system is used to directly measure key multiple-input-multiple-output parameters in an indoor environment at 2.45 GHz. Linear antenna arrays of different sizes and construction with up to ten elements at transmit and receive are utilized in the measurement campaign. This data is analyzed to reveal channel properties such as transfer matrix element statistical distributions and temporal and spatial correlation. Additionally, the impact of parameters such as antenna element polarization, directivity, and array size on channel capacity are highlighted. The paper concludes with a discussion of the relationship between multipath richness and path loss, as well as their joint role in determining channel capacity. View full abstract»

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  • Ping-pong effects in linear parallel interference cancellation for CDMA

    Page(s): 357 - 363
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (504 KB) |  | HTML iconHTML  

    The convergence behavior of linear parallel interference cancellation is investigated. Especially the so-called ping-pong effect, where the bit-error rate performance is found to oscillate between two different convergence patterns is studied in detail. This effect is shown to be a direct consequence of the extreme eigenvalues of the correlation matrix, allowing for an analytical approach. Intervals for the dominating eigenvalues within which ping-pong effects can occur are specified and illustrated by examples. It is shown that the decision statistic for traditional parallel cancellation will always exhibit oscillating behavior with either short or long codes. Relaxation factors, leading to weighted cancellation, are shown to be effective for alleviating oscillations and ping-pong effects at the expense of convergence rate. Asymptotic analysis for large systems is applied to uncover the convergence behavior for long code systems. View full abstract»

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  • Training-based channel estimation for multiple-antenna broadband transmissions

    Page(s): 384 - 391
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (515 KB) |  | HTML iconHTML  

    This paper addresses the problem of training sequence design for multiple-antenna transmissions over quasi-static frequency-selective channels. To achieve the channel estimation minimum mean square error, the training sequences transmitted from the multiple antennas must have impulse-like auto correlation and zero cross correlation. We reduce the problem of designing multiple training sequences to the much easier and well-understood problem of designing a single training sequence with impulse-like auto correlation. To this end, we propose to encode the training symbols with a space-time code, that may be the same or different from the space-time code that encodes the information symbols. Optimal sequences do not exist for all training sequence lengths and constellation alphabets. We also propose a method to easily identify training sequences that belong to a standard 2m-PSK constellation for an arbitrary training sequence length and an arbitrary number of unknown channel taps. Performance bounds derived indicate that these sequences achieve near-optimum performance. View full abstract»

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  • A mixed neural-genetic algorithm for the broadcast scheduling problem

    Page(s): 277 - 283
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (427 KB) |  | HTML iconHTML  

    The broadcast scheduling problem (BSP) arises in frame design for packet radio networks (PRNs). The frame structure determines the main communication parameters: communication delay and throughput. The BSP is a combinatorial optimization problem which is known to be NP-hard. To solve it, we propose an algorithm with two main steps which naturally arise from the problem structure: the first one tackles the hardest contraints and the second one carries out the throughput optimization. This algorithm combines a Hopfield neural network for the constraints satisfaction and a genetic algorithm for achieving a maximal throughput. The algorithm performance is compared with that of existing algorithms in several benchmark cases; in all of them, our algorithm finds the optimum frame length and outperforms previous algorithms in the resulting throughput. View full abstract»

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  • Theoretical analysis and performance of OFDM signals in nonlinear fading channels

    Page(s): 284 - 293
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (912 KB) |  | HTML iconHTML  

    This paper presents an analytical framework to calculate the average symbol-error rate (SER) of uncoded orthogonal frequency-division multiplexing (OFDM) systems in realistic scenarios impaired by transmitter nonlinearity and frequency-selective fading channels. The results are applicable to cyclically extended OFDM signals characterized by a high number of carriers, which can be modeled as complex Gaussian processes. To avoid intercarrier interference, we also assume that the symbol duration is shorter than the channel coherence time. We derive analytical SER results in Rayleigh and Rice frequency-selective fading channels, for both the nonlinear amplification and the ideal predistortion case. Simulations results demonstrate the validity of the analytical results. View full abstract»

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  • Performance comparison of transmit diversity and beamforming for the downlink of DS-CDMA system

    Page(s): 320 - 334
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1027 KB) |  | HTML iconHTML  

    A theoretical analysis has been made to compare the capacity performances of transmit diversity and beamforming in the downlink of direct-sequence code-division multiple-access (DS-CDMA) system. The performances of selection transmit diversity (STD), beamforming (BF), and beam selection transmit diversity (BSTD), which is a hybrid scheme of STD and BF, have been studied at system level with capacity as the performance index. Both fading and interference are jointly considered in multipath propagation environment. It was found that the comparison result strongly relates to the availability of path diversity. The system capacity is maximized by the BF in the presence of path diversity, otherwise it is the BSTD that achieves the best overall capacity performance. View full abstract»

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  • Optimum power control for successive interference cancellation with imperfect channel estimation

    Page(s): 375 - 383
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (488 KB) |  | HTML iconHTML  

    Successive interference cancellation, in conjunction with orthogonal convolutional codes, has been shown to approach the Shannon capacity for an additive white Gaussian noise channel. However, this requires highly accurate estimates for the amplitude and phase of each user's signal. We derive an optimal power control strategy specifically designed to maximize the overall capacity under the constraint of a high degree of estimation error. This power control strategy presents a general formula of which other power control algorithms are special cases. Even with estimation error as high as 50%, capacity can be approximately doubled relative to not using interference cancellation. In addition, when properly applied to multicell mobile networks, this power control scheme can reduce the handset transmit power, and therefore other-cell interference, by more than an order of magnitude. View full abstract»

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  • A class of nonorthogonal rate-one space-time block codes with controlled interference

    Page(s): 270 - 276
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (504 KB) |  | HTML iconHTML  

    Complex, rate one, orthogonal block space-time codes (STCs) exist only for two transmit antennas and were proposed by Alamouti (1998). Designing rate-one block STCs for more than two transmit antennas implies giving up orthogonality. For more than two transmit antennas a class of rate one block STCs with a controlled number of interference terms per detected symbol is presented. Conditions that link the matrix formulation of the receive equation to the matrix formulation of a multiuser detection scheme are discussed. The block STCs design problem is closely related to the class of linear real Hadamard codes. View full abstract»

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  • Estimation of Nakagami-m fading channel parameters with application to optimized transmitter diversity systems

    Page(s): 250 - 259
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (795 KB) |  | HTML iconHTML  

    An optimum power loading algorithm for transmitter diversity systems over correlated and unbalanced Nakagami (1960) paths and its performance evaluation under perfect channel estimate conditions are derived. In addition, various online estimators of the required Nakagami channel parameters for optimized power loading and the comparison of their mean square error via Monte Carlo simulations are presented. Some of these estimators are used to obtain the performance of optimized transmitter diversity systems under imperfect channel estimation (ICE). These numerical results show that the diversity gain of these optimized systems compared with equipower systems increases as the severity of fading decreases and as the degree of branch imbalance increases even under ICE. On the other hand, in weakly correlated and (or) unbalanced branches, optimized transmitter diversity systems offer negligible gain or even losses compared with unoptimized systems because of the ICE. View full abstract»

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  • Performance of adaptive MC-CDMA detectors in rapidly fading Rayleigh channels

    Page(s): 229 - 239
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (816 KB) |  | HTML iconHTML  

    Multicarrier code-division multiple access (MC-CDMA) combines multicarrier transmission with direct sequence spread spectrum. Different approaches have been adopted which do not assume a perfectly known channel. We examine the forward-link performance of decision-directed adaptive detection schemes, with and without explicit channel estimation, for MC-CDMA systems operating in fast fading channels. We analyze theoretically the impact of channel estimation errors by first considering a simpler system employing a threshold orthogonality restoring combining (TORC) detector with a Kalman channel estimator. We show that the performance deteriorates significantly as the channel fading rate increases and that the fading rate affects the selection of system parameters. We examine the performance of more realistic schemes based on the minimum mean square error (MMSE) criterion using least mean square (LMS) and recursive least square (RLS) adaptation. We present a discussion which compares the decision-directed and pilot-aided approaches and explores the tradeoffs between channel estimation overhead and performance. We find that there is a fading rate range where each method provides a good tradeoff between performance and overhead. We conclude that the MMSE per carrier decision-directed detector with RLS estimation combines good performance in low to moderate fading rates, robustness in parameter variations, and relatively low complexity and overhead. For higher fading rates, however, only pilot-symbol-aided detectors are appropriate. View full abstract»

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  • MIMO CDMA antenna system for SINR enhancement

    Page(s): 240 - 249
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (639 KB) |  | HTML iconHTML  

    We present a system to enhance signal-to-interference plus noise ratio (SINR) for multiple-input-multiple-output (MIMO) direct-sequence code-division multiple-access (DS/CDMA) communications in the downlink for frequency-selective fading environments. The proposed system utilizes a transmit antenna array at the base station and a receive antenna array at the mobile station with finite-impulse response filters at both the transmitter and receiver. We arrive at our system by attempting to find the optimal solution to a general MIMO antenna system. A single user joint optimum scenario and a multiuser SINR enhancement scenario are derived. In addition, a simplified one-finger receiver structure is introduced. Numerical results reveal that significant system performance and capacity improvement over conventional approaches are possible. We also investigate the sensitivity of the proposed system to channel estimation errors. 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