ISSN:1124-318X    

DOI:10.1002/ett.1

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractWe consider a wireless sensor network whereKsensors must deliver their messages within a frame ofNtime slots by spending a given finite energy. If the messages are not transmitted within the required delay they become useless and the residual energy is wasted. The channel is block‐fading, with independent fades for each sensor and each slot. Sensors know the fading levels up to the current slot, but do not know the future fading levels. The receiver collects the signal on all the slots of the frame and performs joint decoding of all the messages. We characterize the region of long‐term average achievable rates and we show that the optimal policy tends to waterfilling in time (optimal policy without delay constraints) asNincreases. In this setting, we also characterize the long‐term average capacity region per unit energy by showing that the optimal policy is ‘one‐shot’, totally decentralized and extremely simple, i.e., every user spends all its available energy on the first slot of the frame, the fading gain of which is larger than a pre‐computed time‐varying threshold. Furthermore, the ‘one‐shot’ policy not only makes the most efficient use of the energy, but also reduces to the minimum the interference to other users as it makes all the users transmit with the minimum energy per bit required for reliable communications. These characteristics make the ‘one‐shot’ policy appealing for systems with severe energy limitation as wireless sensor ne

ISSN:1124-318X    

DOI:10.1002/ett.2

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractIn orthogonal frequency division multiplexing (OFDM) systems, a co‐channel interference (CCI) canceller has been proposed to reduce CCI from other cells. When the received power of the desired and the interference signals is nearly the same, the probability of detection error becomes high. Therefore, even though a convolutional code is employed, performance is not much improved. In this paper, to solve this problem, we propose to apply frequency spread coding (FSC) and/or frequency interleaving (FI) to the CCI canceller for OFDM systems. Performance of the system using FSC or FI is evaluated by computer simulation. Furthermore, the system using FSC to the adjacent subcarriers, which have high correlation, and the system using FSC to the separated subcarriers, which have low correlation, are also compared. Copyright © 2003

ISSN:1124-318X    

DOI:10.1002/ett.3

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractA joint scheme combining error‐control coding (ECC) with complex‐field coding (CFC) is proposed for space–time communications through flat or frequency‐selective fading multiple‐input multiple‐output (MIMO) channels. It is shown that the diversity gain of this joint coding scheme is the product of the free distance of the ECC, the complex‐field block encoder size, and the number of receiving antennas, which is computationally prohibitive to achieve using ECC alone. The decoding complexity with the proposed iterative decoder is comparable to a system without CFC. Numerical simulations for perfectly interleaved and HIPERLAN channels with both single and multiple antennas show great potential of the proposed joint coding scheme. Copyri

ISSN:1124-318X    

DOI:10.1002/ett.4

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractIn this paper we present a new set of blind equalization algorithms for multicarrier CDMA systems with receiver diversity. We first re‐examine the cost function in the well‐known subspace method and interpret it in terms of the noise projector, which can be effectively approximated by a special weighted spectral decomposition of the data autocorrelation matrix. By adding a user‐specific cost term to the common cost function, we show that all user channel responses can be estimated in parallel. A block algorithm with lower complexity is derived first, followed by a recursive algorithm using RLS‐type matrix updating. Our new approach treats additional antennas in the same way as a higher processing gain and provides a programmable structure to compromise between spectral and spatial diversity. Simulations show our method is near–far resistant, and our adaptive algorithm has fast convergence. Copyright

ISSN:1124-318X    

DOI:10.1002/ett.5

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractIn the past decade, the market for wireless mobile communications has been growing at a rapid pace. A future wireless mobile network is desired to support integrated services, including voice, data, video, etc. A well‐designed multiple access control (MAC) protocol is crucial in supplying a satisfactory level of performance, such as delay or throughput. A dynamic code assignment in hybrid multi‐code CDMA/TDMA is proposed in this paper. The code assignment scheme is designed to follow the time‐varying traffic characteristics of mobile users. The base station (BS) assigns more codes to mobiles during congestion, but releases them when congestion subsides. Congestion is estimated from the queue lengths of mobiles. A queueing model is established for performance analysis, in which the traffic source is taken to be a Markov‐modulated Poisson process (MMPP). Copyright ©

ISSN:1124-318X    

DOI:10.1002/ett.6

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractRay‐optical propagation models are often utilized for the prediction of the field strength (and delay spread) in mobile radio networks. However, the practical usage of these deterministic models is limited by their high computational demands. A new method for the acceleration of ray‐optical models is presented in this paper. It is based on a single preprocessing of the database in which the mutual visibility relations between the walls and the edges of the buildings are determined. The propagation model is implemented for urban and indoor scenarios, and comparisons with measurements show the gain in computation efficiency as well as in achieved prediction accuracy. Copyright © 200

ISSN:1124-318X    

DOI:10.1002/ett.7

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractIn multiple‐access communication systems several users share the system resources, thus creating co‐channel interference to each other. Multiuser detection, applied at the receiver side, is a powerful technique to combat co‐channel interference, in contrast to single‐user approaches that treat the interference as additive noise. In this paper, we investigate the feasibility of achieving increased spectral efficiency in GSM‐like TDMA networks by applying multiuser detection. We focus on downlink operation and consider two mobile users that share the same physical channel within the same cell or in two adjacent cells. No synchronization between the base stations is assumed. Mobile terminals exploit multiple receiving antennas. We propose a simple iterative receiver algorithm that performs joint channel and data estimation. Results of simulations that evaluate the receiver's performance are presented. Copyright

ISSN:1124-318X    

DOI:10.1002/ett.8

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractThis paper proposes a new diversity technique for broadband wireless access systems that uses orthogonal frequency division multiplexing (OFDM), and describes its performance. This technique consists of two components. One is optimal antenna selection in the reception phase and the other is signal transmission over each OFDM subchannel. Simulations and experiments are used to verify that the proposed technique offers excellent performance. In addition, to offset the quantization noise caused by large differences in the received power levels on the different antennas, a new automatic gain‐controlling (AGC) method is proposed and its performance is described. Copyright © 2003

ISSN:1124-318X    

DOI:10.1002/ett.9

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY

摘要:

AbstractThe problem of estimating propagation delays of orthogonally modulated signals in asynchronous DS‐CDMA systems over time‐varing Rayleigh‐fading channels is treated in this paper. The maximum likelihood (ML) estimator and its unaffordable complexity for implementation are discussed. Some suboptimal solutions, e.g., whitened sliding correlator, MMSE estimator, subspace‐based estimator, approximate ML estimator, are proposed to combat the multiple access interference in the fading channels. The performance of these estimators are evaluated with the computer simulations and shown to have better acquisition performance than the standard sliding correlator. They also achieve reduced computational complexity compared with the ML estimator, while maintaining an acceptable performance degradation. Copyright ©

ISSN:1124-318X    

DOI:10.1002/ett.10

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2003

数据来源: WILEY