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Wireless Communications, IEEE Transactions on

Issue 4 • Date April 2007

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Displaying Results 1 - 25 of 49
  • IEEE Transactions on Wireless Communications

    Page(s): c1 - c3
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    Freely Available from IEEE
  • On the Spectral Efficiency of VSG CDMA in Nakagami and Rician Fading

    Page(s): 1153 - 1158
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (289 KB) |  | HTML iconHTML  

    This paper presents new analytical expressions for the spectral efficiency of variable spreading gain code division multiple access (VSG CDMA) networks in a Nakagami and Rician fading environments. These are used to gain insight into the impact of fading and background noise on the optimal transmit powers. Numerical results for a dual class and 5-class system, reveal that the spectral efficiency is optimized at higher signal-to-noise ratio levels when each user is retaining the same bit energy. However, at lower levels of the signal-to-noise-ratio, the spectral efficiency would be optimized when a larger amount of energy is allocated to the classes of higher data rate, as compared to the lower rate classes View full abstract»

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  • Maximum Asymptotic Stable Throughput of Opportunistic Slotted ALOHA and Applications to CDMA Networks

    Page(s): 1159 - 1163
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    In this paper we study the maximum asymptotic stable throughput of an opportunistic slotted ALOHA protocol. We provide a characterization of the maximum stable throughput as the number of users in the system goes to infinity. We then apply our findings to CDMA networks with the signal-to-interference-ratio (SIR) threshold model. It is shown that the slotted ALOHA protocol with the power/transmission control rule that equalizes the reception powers achieves 1 - O(logN/radicN) channel utilization, which is defined as the throughput divided by the optimal throughput N achieved by scheduling. This implies that the slotted ALOHA is asymptotically optimal in the sense that its channel utilization converges to 1 as the spreading gain goes to infinity View full abstract»

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  • Power Management in MIMO Ad Hoc Networks: A Game-Theoretic Approach

    Page(s): 1164 - 1170
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (218 KB) |  | HTML iconHTML  

    This paper considers interference characterization and management in wireless ad hoc networks using MIMO techniques. The power allocation in each link is built into a non-cooperative game where a utility function is identified and maximized. Due to poor channel conditions, some links have very low data transmission rates even though their transmit powers are high. Therefore, a mechanism for shutting down links is proposed in order to reduce cochannel interference and improve energy efficiency. The multiuser water-filling and the gradient projection methods are compared with the proposed game theoretic approach in terms of system capacity and energy efficiency. It is shown that using the proposed method with the link shut-down mechanism allows the MIMO ad hoc network to achieve the highest energy efficiency and the highest system capacity View full abstract»

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  • Decision Directed Channel Estimation Aided OFDM Employing Sample-Spaced and Fractionally-Spaced CIR Estimators

    Page(s): 1171 - 1175
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    In this letter we characterize the substantial difference between two channel estimation approaches, namely the sample-spaced (SS) and the fractionally-spaced (FS) channel impulse response (CIR) estimators. The achievable performance of decision-directed channel estimation (DDCE) methods employing both the SS- and the FS-CIR estimators is analyzed in the context of an OFDM system. The performance of the two estimation methods is compared and it is shown that the DDCE scheme employing the projection approximation subspace tracking (PAST)-aided FS-CIR estimator outperforms its SS-CIR estimator-based counterpart View full abstract»

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  • Time and Frequency Synchronization for OFDMA Uplink System using the SAGE Algorithm

    Page(s): 1176 - 1181
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (385 KB) |  | HTML iconHTML  

    In this letter, we propose an iterative time and frequency synchronization scheme for orthogonal frequency-division multiple access (OFDMA) uplink system using the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step of the SAGE algorithm is intended to remove the multiple-access interference (MAI) caused by the other asynchronous users. Then, the maximization step is utilized in the timing and frequency offset estimation of each asynchronous user using the MAI-canceled signals. Simulation results present that the proposed scheme shows performance enhancement in the presence of multiple asynchronous users View full abstract»

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  • On Some Near Optimal Low Complexity Detectors for MIMO Fading Channels

    Page(s): 1182 - 1186
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (221 KB) |  | HTML iconHTML  

    We introduce several low complexity sub-optimal MIMO detection schemes based on the List-BLAST algorithm, which exhausts the constellation points in the first layer of a BLAST scheme to generate multiple candidate solutions from which the maximum likelihood solution is determined. The candidates can also be used as initial points for the space alternating generalized expectation-maximization (SAGE) algorithm to further improve performance. The proposed schemes can achieve close to optimal performance for both hard and soft output detection with lower complexity than that of the sphere detection in our simulation settings View full abstract»

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  • A Low Complexity RLS-PASTd Algorithm for Blind Multiuser Detection in Dispersive CDMA Channels

    Page(s): 1187 - 1192
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (412 KB) |  | HTML iconHTML  

    For joint blind channel estimation and multiuser detection in direct-sequence code division multiple access (DS-CDMA) systems, a combined scheme is proposed, which couples the recursive least-squares (RLS) adaptations and the PASTd subspace tracking algorithm. A major advantage of the RLS-PASTd algorithm is that it admits an adaptive implementation with low computational complexity instead of singular value decomposition. Simulation results demonstrate the fast tracking capability and high steady-state estimation accuracy of the proposed algorithm in both the time-invariant and time-variant environments View full abstract»

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  • Joint Frequency Offset and Channel Estimation for OFDM Systems Using Pilot Symbols and Virtual Carriers

    Page(s): 1193 - 1202
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (517 KB) |  | HTML iconHTML  

    We consider joint estimation of carrier frequency offset and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) systems with pilot symbols and virtual subcarriers (VCs). We derive the receive-signal correlation structure due to the pilots and VCs, give the evidence of joint multivariate Gaussian distribution of the received samples, and derive an approximate maximum likelihood (ML) frequency offset estimator. We also derive the asymptotic mean-square error (MSE) and an approximate Cramer-Rao bound (CRB) and establish the asymptotic unbiasedness. Without pilots, in high signal-to-noise ratio, our estimator is equivalent to Liu and Tureli's estimator with Nv virtual carriers. When the pilot number (Np) is greater than the channel length L, our estimator acts as a subspace-based estimator with Nv + Np - L virtual carriers. A decision-directed joint ML estimator is derived to iteratively update the estimates of frequency offset, data symbols and CIR. The optimal pilot and virtual carrier placement strategies are also discussed. The resulting decision-directed joint estimator performs within 0.8 dB of the ideal case even when the frequency offset is as large as 20% View full abstract»

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  • New Design for Linear Precoding over STBC in the Presence of Channel Correlation

    Page(s): 1203 - 1207
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (215 KB) |  | HTML iconHTML  

    In order to account for spatial correlation in a multiple input multiple output (MIMO) channel using precoding over space-time block codes (STBC), an approach was proposed for the design of a linear precoder (H. Sampath and A. Paulraj, 2002). This approach works well for orthogonal STBC but fails in the case of non-orthogonal STBC. Based on a min-max problem formulation, a new design of the linear precoder is proposed in this paper. Simulation results in the case of quasi-orthogonal STBC (ABBA) are presented in order to show the gain over the conventional design approach and the non-precoded system View full abstract»

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  • Burst-Based Orthogonal ST Block Coding for CPM

    Page(s): 1208 - 1212
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB) |  | HTML iconHTML  

    In this paper, we propose a simple orthogonal space-time block coding (OSTBC) technique for continuous-phase modulation (CPM). Although the straightforward combination of orthogonal designs (ODs) and CPM was deemed impossible in X. Zhang and M.P. Fitz (2003) G. Wang and X-G Xia (2004) we show that this is easily accomplished with a burst-based approach. In fact, using the proposed technique ODs can be combined with any CPM scheme. After an appropriate ST combining at the receiver, the same detection techniques as in case of single-antenna transmission can be applied. This is a significant advantage over previously proposed ST coding schemes for CPM. We also derive accurate approximations for the bit error rate and the frame error rate of CPM with OSTBC. Both analysis and simulations show in good agreement the excellent performance of the proposed scheme View full abstract»

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  • Average Orthogonality Factor in WCDMA Downlinks for Independent Rayleigh Fading Multipath Channels

    Page(s): 1213 - 1217
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    In wideband code division multiple access (WCDMA) system capacity analysis and simulations, downlink interference due to loss of orthogonality between spreading codes is modeled by means of the orthogonality factor (OF). This study derives closed-form expressions for the time-averaged OF over independent Rayleigh fading multipath channels, in which the average power gains of each path can be distinct or identical. Two approximation approaches proposed in related literature are tested against the analytical results presented here View full abstract»

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  • Switching Rate and Dwell Time in M-of-N Selection Diversity

    Page(s): 1218 - 1223
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (291 KB) |  | HTML iconHTML  

    Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a selection diversity receiver where M out of a total of N independent branches are selected for combining. We show that the switching rate can be many times the Doppler frequency, while the average dwell time can be a small fraction of the reciprocal Doppler frequency. The brevity of the dwell times suggests difficulty in obtaining channel state information, which in turn calls into question performance analyses of idealized HS/MRC structures. Our results also suggest that HS/MRC should be frame-based, rather than continuously acting in time View full abstract»

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  • Iterative Channel Estimation Based on B-splines for Fast Flat Fading Channels

    Page(s): 1224 - 1229
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (155 KB) |  | HTML iconHTML  

    We propose novel low-complexity iterative channel estimators based on B-splines. Local splines are adopted for computational simplicity. Minimum mean square error (MMSE) local splines with integral sampling are derived. The MSE of the proposed estimators depends on signal-to-noise ratio, fading rate, sampling interval, spline order and the number of weighting coefficients; these dependencies are investigated. The linear and cubic local splines with as few as seven weighting coefficients are capable of achieving MSE and BER performance comparable to those of the Wiener filter and the spheroidal basis expansion. However, a significantly lower complexity is achieved using B-splines View full abstract»

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  • Soft-Information Assisted Near-Optimum Nonlinear Detection for BLAST-type Space Division Multiplexing OFDM Systems

    Page(s): 1230 - 1234
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (288 KB) |  | HTML iconHTML  

    In this contribution, a nonlinear hybrid detection scheme based on a novel soft-information assisted genetic algorithm (GA) is proposed for a turbo convolutional (TC) coded space division multiplexing (SDM) aided orthogonal frequency division multiplexing (OFDM) system. Our numerical results show that the performance of the currently known GA-assisted system can be improved by about 2 dB with the aid of the GA's population-based soft solution, approaching the optimum performance of the soft-information assisted maximum likelihood (ML) detection, while exhibiting a lower complexity, especially in high-throughput scenarios. Furthermore, the proposed scheme is capable of achieving a good performance even in the so-called overloaded systems, where the number of transmit antennas is higher than the number of receiver antennas View full abstract»

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  • A case for considering hyper-Rayleigh fading channels

    Page(s): 1235 - 1239
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    This work is motivated by the problem of characterizing small-scale radio propagation environments for wireless sensor networks. If sensors are statically deployed near the ground or within structures, temporal fading may not exist but the channel may nevertheless experience severe frequency-selective behavior. The work presents real-world, frequency-selective fading data measured for in-vehicle wireless sensor applications. This fading data often exhibit statistics more severe than predicted by the Rayleigh fading model; a scenario referred herein as to as being hyper-Rayleigh. A two-ray, small-scale model is proposed as a new worst-case for this application space. View full abstract»

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  • Coverage-preserving routing protocols for randomly distributed wireless sensor networks

    Page(s): 1240 - 1245
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (315 KB) |  | HTML iconHTML  

    Sensing coverage is an important issue for sensor networks, since it is viewed as one of the critical measures of performance offered by a sensor network. The design of a routing protocol is generally independent of the sensing coverage issue. Since some nodes may become invalid after they have used up their own energy resource, the achievable sensing coverage will gradually degrade as time passes. Different routing protocols may motivate different distributions of energy dissipation among nodes, and thus induce different changes in the network topology after some nodes have died out. This implies that different routing protocols will lead to different sensing coverage when some nodes are no longer available. Considering the impact on the sensing coverage of a network, we have proposed coverage-preserving routing protocols which are modified from the LEACH and virtual grid routing protocols. These proposed protocols can substantially improve the performance of sensing coverage. According to the simulation results, the sensing coverage degradation of the coverage-preserving protocols is slower than that of the other baseline protocols. For the time duration maintaining the network coverage over 50%, a gain of 20% in overall sensing coverage can be obtained by using the coverage-preserving protocols. View full abstract»

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  • Blind Recursive Tracking of Carrier Frequency Offset (CFO) Vector in MC-CDMA Systems

    Page(s): 1246 - 1255
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    A recursive algorithm for estimating and updating the effective carrier frequency offset (CFO) vector in a multicarrier code-division multiple-access (MC-CDMA) system is proposed in this work. The recursive relation is derived based on the expectation maximization (EM) algorithm with a quadratic constraint. This new approach enables the use of linear estimation theory to tackle the CFO estimation problem with or without training data, which leads to an analytic CFO estimate in closed form. Furthermore, the multiple access interference (MAI) is mitigated using the second order statistics of the interference-plus-noise vector, which is updated in a recursive manner under the EM formulation, too. When reaching a converged estimate, a fixed-norm quadratic constraint is imposed so that the final CFO estimate is robust to an imprecise covariance matrix estimate caused by insufficient data samples. It is demonstrated by computer simulation that the performance of an MC-CDMA system without the CFO information can be restored by the proposed scheme in the sense that its bit error probability (BEP) performance is close to that with perfect CFO knowledge View full abstract»

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  • On the Geometrical Characteristic of Wireless Ad-Hoc Networks and its Application in Network Performance Analysis

    Page(s): 1256 - 1265
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (541 KB) |  | HTML iconHTML  

    A wireless ad-hoc network can be roughly considered as one consisting of a collection of mobile nodes distributed in a finite region, which adopts a non-centralized and self-organized structure. In such networks, messages are transmitted, received and forwarded in a finite geometrical region. In addition, the transmission of messages is highly dependent on the locations of the mobile nodes. As a result, the geometrical relationships between the nodes, and especially the distance between them are of fundamental importance. In this paper, we propose a space decomposition method to analyze the probability distribution of the distance between nodes in an ad-hoc network. In particular, we derive two theoretical expressions for the probability distribution of the distance between nodes under the assumption that the nodes are independently and uniformly distributed in either a rectangular region or hexagonal region. Further results on the node degree distribution and max-flow capacity of the network are then presented based upon these expressions View full abstract»

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  • Low Complexity Rake Receivers in Ultra-Wideband Channels

    Page(s): 1265 - 1275
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (602 KB) |  | HTML iconHTML  

    One of the major issues for the design of ultra-wideband (UWB) receivers is the need to recover the signal energy dispersed over many multipath components, while keeping the receiver complexity low. To this aim we consider two schemes for reduced-complexity UWB Rake receivers, both of which combine a subset of the available resolved multipath components. The first method, called partial Rake (PRake), combines theirs/ arriving multipath components. The second is known as selective Rake (SRake) and combines the instantaneously strongest multipath components. We evaluate and compare the link performance of these Rake receivers in different UWB channels, whose models are based on extensive propagation measurements. We quantify the effect of the channel characteristics on the receiver performance, analyzing in particular the influence of small-scale fading statistics. We find that for dense channels the performance of the simpler PRake receiver is almost as good as that of the SRake receiver, even for a small number of fingers. In sparse channels, however, the SRake outperforms the PRake significantly. We also show that for a fixed transmitted energy there is an optimum transmission bandwidth View full abstract»

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  • A Markovian Framework for Performance Evaluation of IEEE 802.11

    Page(s): 1276 - 1265
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (499 KB) |  | HTML iconHTML  

    A new approach for modeling and performance analysis of the IEEE 802.11 medium access control (MAC) protocol is presented. The approach is based on the so-called system approximation technique, where the protocol service time distribution of the IEEE 802.11 MAC protocol is studied and approximated by an appropriate phase-type distribution, leading to the construction of a versatile queueing model which is amenable to analysis and, at the same time, general enough to allow for bursty arrival process as well as key statistical characteristics of the protocol operations. The versatility of the model is demonstrated by considering Markov modulated and on/off arrival processes as well as various data frame size distributions. The accuracy of the analytical results is verified by simulation. View full abstract»

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  • Queuing Analysis of Power Management in the IEEE 802.11 Based Wireless LANs

    Page(s): 1286 - 1294
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    Energy efficiency is an important aspect in wireless networks and power management is a technique that can save energy. We investigate the power management in the IEEE 802.11 based infrastructure mode wireless LANs and seek the optimal power management parameters that can achieve good energy efficiency without degrading other performances. We propose two models for the power management scheme: the M/G/1 queue with bulk service model and the D/G/1 queue model. From the models, we obtain the analytical results for the energy efficiency and the response time performance metrics, which are controlled by a listen interval. Our simulation results are in good agreement with the analysis. Based on the analytical models and simulation, we propose to select the largest listen interval with the satisfaction of response time requirement. The analytical models can be used to predict system performance if system configurations are given. This can help system deployment and system optimization. View full abstract»

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  • A quasi-orthogonal group space-time architecture to achieve a better diversity-multiplexing tradeoff

    Page(s): 1295 - 1307
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (487 KB) |  | HTML iconHTML  

    Most existing MIMO (multiput-input multiput-output) schemes optimize only either the diversity gain or the multiplexing gain. To obtain a good tradeoff between these two, the quasi-orthogonal group space-time (QoGST) architecture is proposed, wherein the transmit stream is subgrouped but encoded via an inter-group space-time block encoder, with group interference suppression at the receiver. This paper also considers another combined space-time coding and layered space-time architecture, which we refer to as group layered space-time (GLST), where space-time block coding is employed within each group. Under the assumption of Rayleigh fading and a prior perfect channel state information at the receiver, a performance analysis will demonstrate that both QoGST and GLST can achieve a good diversity-multiplexing tradeoff. QoGST is even superior to GLST. Simulation results will validate our analysis and further show that compared to the existent layered space-time block code (LSTBC) scheme, both QoGST and GLST can achieve a significant performance gain. View full abstract»

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  • Battery Power Efficiency of PPM and FSK in Wireless Sensor Networks

    Page(s): 1308 - 1319
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (551 KB) |  | HTML iconHTML  

    As sensor nodes are typically powered by nonrenewable batteries, energy efficiency is a critical factor in wireless sensor networks (WSNs). Orthogonal modulations appropriate for the energy-limited WSN setup have been investigated under the assumption that batteries are linear and ideal, but their effectiveness is not guaranteed when more realistic nonlinear battery models are considered. In this paper, based on a general model that integrates typical WSN transmission and reception modules with realistic battery models, we derive two battery power-conserving schemes for two M-ary orthogonal modulations, namely pulse position modulation (PPM) and frequency shift keying (FSK), both tailored for WSNs. Then we analyze and compare the battery power efficiency of PPM and FSK over various wireless channel models. Our results reveal that FSK is more power-efficient than PPM in sparse WSNs, while PPM may outperform FSK in dense WSNs. We also show that in sparse WSNs, the power advantage of FSK over PPM is no more than 3 dB; whereas in very dense WSNs, the power advantage of PPM over FSK can be much more significant as the constellation size M increases. View full abstract»

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  • An Approximate Analysis of Buffered S-ALOHA in Fading Channels Using Tagged User Analysis

    Page(s): 1320 - 1326
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (229 KB) |  | HTML iconHTML  

    Tagged user analysis (TUA) is a generic approximate method of analyzing random access protocols for finite-user finite-buffer systems. This technique decouples the channel contention behavior from the user queuing behavior and allows the use of classical queuing theory results to be directly applicable to the analysis of finite-user finite-buffer random access methods. In this paper, we extend TUA to analyze finite buffer S-ALOHA operating over flat fading radio channels and derive expressions for system performance indices like throughput, average packet delay, blocking probability and queue length. It is shown that for a moderate number of active users, the simulation and analytical results fit closely View full abstract»

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Aims & Scope

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