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Communications Letters, IEEE

Issue 3 • Date March 2007

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Displaying Results 1 - 22 of 22
  • IEEE Communications Letters - Table of contents

    Publication Year: 2007 , Page(s): C1 - C2
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  • Hierarchical Constellation Based Adaptive Relay Scheme in Multi-Hop Networks

    Publication Year: 2007 , Page(s): 225 - 227
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (170 KB) |  | HTML iconHTML  

    In order to improve cell capacity in multihop networks, we propose a novel adaptive relay scheme based on hierarchical constellations in which a symbol consists of multiple bits with varying degrees of robustness against channel errors. These bits are transferred to multiple users, such as relays and destinations, adaptively according to the users channel conditions and this adaptive transfer enables the capacity of multi-hop networks to be unproved View full abstract»

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  • Multi-user Fair Scheduling in the Downlink of CDMA Packet Systems

    Publication Year: 2007 , Page(s): 228 - 230
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (116 KB) |  | HTML iconHTML  

    A fair scheduling algorithm is proposed to improve the system throughput while maintaining the fairness in the downlink of a code-division multiple access (CDMA) system employing AMC and multicodes. R. Kwan and C. Leung suggested an optimal scheduling to maximize the total throughput based on simultaneous transmissions strategy. In this letter, we extend the optimal scheduling so that some degree of fairness can be maintained. We formulate a mixed-integer nonlinear programming problem to assign the radio resources such as the transmit power and codes to several users effectively. The result shows that the proposed algorithm provides a significant throughput gain over PF scheduling with one-by-one transmission at the same level of fairness View full abstract»

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  • A Simple Accurate Method for Generating Autocorrelated Nakagami-m Envelope Sequences

    Publication Year: 2007 , Page(s): 231 - 233
    Cited by:  Papers (19)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (201 KB) |  | HTML iconHTML  

    We propose a simple accurate method for generating autocorrelated Nakagami-m envelope sequences. The method allows for arbitrary values of fading parameter and nonisotropic fading scenarios. In essence, Nakagami-m samples are first drawn and then rearranged to match the Nakagami-m autocorrelation. The rearrangement is made in accordance with the rank statistics of an underlying Rayleigh reference sequence with the desired autocorrelation. Examples illustrate the excellent performance of the new method View full abstract»

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  • Performance Analysis of LDPC-Coded Space-Time Modulation over MIMO Fading Channels

    Publication Year: 2007 , Page(s): 234 - 236
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (156 KB) |  | HTML iconHTML  

    A closed-form upper bound on the error performance is proposed for LDPC-coded space-time modulation over MIMO block/slow fading channels based on the analysis framework developed for the fast fading case. This follows from the observation that the pairwise error probability (PEP) in all these fading cases is determined by a certain metric of codewords, with respect to which we can enumerate all distinct PEPs and thus concisely formulate the union bound. Simulation results indicate that the bound is useful to benchmark the performance of iterative decoding and detection algorithms View full abstract»

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  • Pilot Design for Inter-Cell Interference Mitigation in MIMO OFDM Systems

    Publication Year: 2007 , Page(s): 237 - 239
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (134 KB) |  | HTML iconHTML  

    In this letter, the design of pilots aiming at mitigating the inter-cell interference in MIMO OFDM systems is addressed. It is shown that if an ideal channel between interferer and receiver is assumed, pilots resulting in a variation coefficient of zero, and therefore best performance on system level, can be designed. The properties of these pilots are described, and numerical calculations as well as simulation results are presented to verify the theoretical analysis. View full abstract»

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  • Feedback of Downlink Channel State Information Based on Superimposed Coding

    Publication Year: 2007 , Page(s): 240 - 242
    Cited by:  Papers (3)  |  Patents (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (184 KB) |  | HTML iconHTML  

    In closed-loop FDD MIMO system, downlink channel state information (DL-CSI) is usually feedback to base station in forms of codebook or CQI, both of which aim at lowering the feedback quantity at the cost of limited feedback precision and heavy processing complexity at mobile side. Meanwhile, the recently proposed direct channel feedback method incurs great system overhead due to its exclusive occupation of uplink bandwidth resources. We propose a low-cost feedback method for DL-CSI, which spreads unquantized and uncoded DL-CSI and superimposes it onto uplink user data sequences (UL-US). Exclusive occupation of system resources by DL-CSI can thus be avoided. Due to spreading, DL-CSI can be estimated accurately with little power allocation at the cost of some UL-US's SER performance View full abstract»

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  • Greedy Geographic Routing using Dynamic Potential Field for Wireless Ad Hoc Networks

    Publication Year: 2007 , Page(s): 243 - 245
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (134 KB) |  | HTML iconHTML  

    Greedy geographic routing has been considered as an attractive approach due to its simplicity and scalability. However, greedy routing is insufficient in terms of routing convergence because of the local maximum problem. In this letter, we propose a novel greedy routing scheme based on a potential field method that dynamically eliminates the local maximum condition View full abstract»

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  • Level Crossing Rates in Transmit Beamforming Systems

    Publication Year: 2007 , Page(s): 246 - 248
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (122 KB) |  | HTML iconHTML  

    In this paper, the second-order statistics of the signal to noise ratio (SNR) at the output of a multiple-input-multiple-output (MIMO) system employing maximal ratio combining (MRC) are analyzed. Exact expressions for the level crossing rate (LCR) and the average fade duration (AFD) are derived for the Rayleigh propagation and arbitrary channel matrix dimensions. The analytical results are validated by simulation View full abstract»

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  • An Optimum Subcarrier and Bit Allocation Period Based on Mobile Speed for OFDM Systems

    Publication Year: 2007 , Page(s): 249 - 251
    Cited by:  Papers (1)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (156 KB) |  | HTML iconHTML  

    A subcarrier and bit allocation period (SBAP) is proposed based on the mobile speed for OFDM systems. It is assumed that users have different SBAPs according to their mobile speeds. The overhead is considered while transmitting the subcarrier and bit assignment information (SBAI). The optimum SBAP of each user is determined such that the overall throughput including SBAI can be maximized. Simulation results show that the proposed scheme outperforms other schemes. It is also demonstrated that the proposed scheme is insensitive to an error in the estimation of mobile speeds View full abstract»

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  • A Relay-Assisted Handover Technique with Network Coding over Multihop Cellular Networks

    Publication Year: 2007 , Page(s): 252 - 254
    Cited by:  Papers (10)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (132 KB) |  | HTML iconHTML  

    A relay-assisted handover technique exploiting network coding for multihop cellular networks is described. In the scheme, each MS (mobile station) in the outer region receives data related to the home cell from the home BS (base station) as well as a coded version of data related to a neighboring BS from a relay node located in the cell boundary via the use of network coding. Thus, MSs can achieve more reliable macro diversity with the assistance of the relay-based handover technique. In a simulation, it is demonstrated that this technique is very effective in maintaining QoS (quality of service) in terms of outage probability over a multi-cell environment View full abstract»

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  • Average Error Performance of M-ary Modulation Schemes in Nakagami-q (Hoyt) Fading Channels

    Publication Year: 2007 , Page(s): 255 - 257
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (141 KB) |  | HTML iconHTML  

    Presented are exact-form expressions for the average error performance of various coherent, differentially coherent, and noncoherent modulation schemes in Nakagami-q (Hoyt) fading channels. The expressions are given in terms of the Lauricella hypergeometric function, FD (n); for nges1, which can be evaluated numerically using its integral or converging series representation. It is shown that the derived expressions reduce to some existing results for Rayleigh fading as special cases View full abstract»

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  • Accurate Upper Bound of SINR-based Call Admission Threshold in CDMA Systems with Imperfect Power Control

    Publication Year: 2007 , Page(s): 258 - 260
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (181 KB) |  | HTML iconHTML  

    Since the capacity of CDMA networks is interference-limited, it is vital to have a call admission control (CAC) mechanism to preserve the signal quality in terms of the signal-to-interference-and-noise ratio (SINR). SINR-based CAC schemes compare the SINR of the incoming call with a threshold value (SINRth). The call is accepted if the SINR is greater than SINRth, otherwise it is rejected. The choice of the SINRth value is restricted by two opposing factors: the signal quality and the network utilization. Setting SINR th at high value is desirable to increase the signal quality. However, a high SINRth value increases the blocking probability (Pb) and reduces the network utilization. An upper bound of SINRth (SINRth_ub) in CDMA systems with imperfect power control has been determined. However, the accuracy of that upper bound is questionable since the possibility of power control (PC) infeasibility has not been taken into consideration. Also, the analysis used the normal distribution to model SINR instead of the widely-accepted lognormal distribution. Moreover, the noise and the inter-cell interference were ignored. In this letter, we derive a more accurate upper bound by taking the possibility of PC infeasibility into consideration and by using the lognormal distribution of SINR for imperfect PC. In addition, our analysis takes the noise and the inter-cell interference into account View full abstract»

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  • Outage optimality of opportunistic amplify-and-forward relaying

    Publication Year: 2007 , Page(s): 261 - 263
    Cited by:  Papers (67)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (101 KB) |  | HTML iconHTML  

    In this paper, we show that optimal selection and transmission of a single relay among a set of multiple amplify-and-forward (AF) candidates minimize the outage probability (i.e., outage-optimal) and outperform any other strategy that involves simultaneous transmissions from more than one AF relay under an aggregate power constraint. This outage optimality demonstrates that cooperation benefits are maximized with intelligent scheduling among AF relays. View full abstract»

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  • Timing Synchronization in Ultra-Wideband Systems with Delay Line Combination Receivers

    Publication Year: 2007 , Page(s): 264 - 266
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (206 KB) |  | HTML iconHTML  

    This paper proposes a novel non-data aided timing synchronization algorithm designated as "timing with delay line combination" (TDLC), to improve the synchronization speed of differential impulse radio ultra-wideband systems. Continuously integrating the combinative output signal provided by the proposed "frame-differential delay line" (FDL), it is found that the integration output exhibits a sharp rising or falling transition at the moment that the first signal per symbol arrives. By accurately detecting the time of the rising or falling edge, the symbol boundary can be simply derived. The proposed scheme achieves timing synchronization within a single symbol duration View full abstract»

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  • The Control-Plane Stability Constraint in Optical Burst Switching Networks

    Publication Year: 2007 , Page(s): 267 - 269
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (111 KB) |  | HTML iconHTML  

    In this letter, we consider the performance of optical burst switching networks with electronic header processing. We find that the finite processing capacity of the electronic core-node header processors can have a significant impact on the maximum throughput and minimum burst length that can be supported. We derive necessary conditions for control-plane stability in OBS networks in terms of the physical design parameters of the system. Based on this analysis, we present a general design procedure and expressions for determining the minimum average burst length to ensure that the control-plane does not become the throughput bottleneck of the OBS system View full abstract»

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  • Stability of Networked Control Systems with Time-Varying Delays

    Publication Year: 2007 , Page(s): 270 - 272
    Cited by:  Papers (18)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (205 KB) |  | HTML iconHTML  

    A robust Smith predictor is proposed to compensate for the round trip delay of networked control systems (NCSs); An analytical TCP model is adopted. We show that stability of the closed-loop system is guaranteed up to some maximum upper bound of the round trip delay View full abstract»

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  • Improving TCP Vegas Fairness in Presence of Backward Traffic

    Publication Year: 2007 , Page(s): 273 - 275
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (86 KB) |  | HTML iconHTML  

    TCP Vegas provides better performance compared to traditional TCP Reno schemes. However, backward traffic significantly degrades Vegas efficiency causing underutilization of the available bandwidth and unfairness. In this letter, we present an enhanced congestion control mechanism for TCP Vegas able to correctly remove the undesired impact of reverse traffic on bandwidth sharing. Furthermore, our proposal is easily deployable because it does not require neither clock synchronization nor any special support at the intermediate routers View full abstract»

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  • Optimization of TFRC Loss History Initialization

    Publication Year: 2007 , Page(s): 276 - 278
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (130 KB) |  | HTML iconHTML  

    This letter deals with the initialization of the loss history structure in the TFRC (TCP-friendly rate control) mechanism. This initialization occurs after the detection of the first loss event after every slowstart phase. The loss history is crucial for the algorithm since it returns the packet loss rate estimation. This estimation is used in the TFRC equation to compute the sending rate. In this letter, we propose a new method to compute the packet loss rate which is more computationally efficient and remains as accurate as the classical commonly used method. The motivation of this work is to reduce the computation time and formulate a unified computation scheme. This method is based on the Newton's algorithm issued from numerical analysis of the TCP throughput equation. This proposal is evaluated analytically and the results show a significant improvement in terms of the computation time View full abstract»

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  • Gradient Projection Decoding of LDPC Codes

    Publication Year: 2007 , Page(s): 279 - 281
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (479 KB) |  | HTML iconHTML  

    A new practical method for decoding low-density parity check (LDPC) codes is presented. The followed approach involves reformulating the parity check equations using nonlinear functions of a specific form, defined over Rrho, where rho denotes the check node degree. By constraining the inputs to these functions in the closed convex subset [0,1]rho ("box" set) of Rrho, and also by exploiting their form, a multimodal objective function that entails the code constraints is formulated. The gradient projection algorithm is then used for searching for a valid codeword that lies in the vicinity of the channel observation. The computational complexity of the new decoding technique is practically sub-linearly dependent on the code's length, while processing on each variable node can be performed in parallel allowing very low decoding latencies. Simulation results show that convergence is achieved within 10 iterations, although some performance degradations relative to the belief propagation (BP) algorithm are observed View full abstract»

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  • "Small-world" effects of shadowing in pulse-coupled distributed time synchronization

    Publication Year: 2007 , Page(s): 282 - 284
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (152 KB) |  | HTML iconHTML  

    Performance of pulse-coupled distributed synchronization in wireless ad hoc networks is known to depend critically on the topology of the corresponding connectivity graph. This paper evaluates the impact of log-normal shadowing on the convergence of distributed discrete-time phase locked loops (PLLs). It is argued that the beneficial effect of shadowing on distributed synchronization can be seen as an instance of the "small-world" phenomenon. Evidence of this is further provided by evaluating average path length and clustering coefficient of the wireless network through simulations View full abstract»

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  • An Efficient Transmission Scheme for Minimizing User Waiting Time in Video-On-Demand Systems

    Publication Year: 2007 , Page(s): 285 - 287
    Cited by:  Papers (8)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (96 KB) |  | HTML iconHTML  

    To take the advantage of skewed popularity of videos, efficient video-on-demand (VOD) systems are more likely to deliver the most popular videos through periodic broadcasting and the least popular videos through on-demand multicasting. While videos delivered through multicasting usually share a pool of server channels, broadcasting of each video demands one or more channels dedicated to it. Given a total number of available channels, distributing them for individual broadcasting and the multicasting pool to achieve the optimal average user waiting time is a nonlinear optimization problem. This letter addresses this problem by proposing a hybrid transmission scheme, which uses dynamic programming approach to ensure optimally for any given number of channels and request arrival rate View full abstract»

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IEEE Communications Letters  is aimed at satisfying the need for rapid dissemination of information, offering a vehicle for authors to expose their results to be used in future developments. This ensures early recognition of work. 

 

 

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George K. Karagiannidis
Aristotle University of Thessaloniki
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