Showing papers on "Fading published in 1995"

Information capacity and power control in single-cell multiuser communications

Abstract: We consider a power control scheme for maximizing the information capacity of the uplink in single-cell multiuser communications with frequency-flat fading, under the assumption that the users attenuations are measured perfectly. Its main characteristics are that only one user transmits over the entire bandwidth at any particular time instant and that the users are allocated more power when their channels are good, and less when they are bad. Moreover, these features are independent of the statistics of the fading. Numerical results are presented for the case of single-path Rayleigh fading. We show that an increase in capacity over a perfectly-power controlled (Gaussian) channel can be achieved, especially if the number of users is large. By examining the bit error-rate with antipodal signalling, we show the inherent diversity in multiuser communications over fading channels.

Finite-state Markov channel-a useful model for radio communication channels

Abstract: The authors first study the behavior of a finite-state channel where a binary symmetric channel is associated with each state and Markov transitions between states are assumed. Such a channel is referred to as a finite-state Markov channel (FSMC). By partitioning the range of the received signal-to-noise ratio into a finite number of intervals, FSMC models can be constructed for Rayleigh fading channels. A theoretical approach is conducted to show the usefulness of FSMCs compared to that of two-state Gilbert-Elliott channels. The crossover probabilities of the binary symmetric channels associated with its states are calculated. The authors use the second-order statistics of the received SNR to approximate the Markov transition probabilities. The validity and accuracy of the model are confirmed by the state equilibrium equations and computer simulation. >

On channel estimation in OFDM systems

Abstract: The use of multi-amplitude signaling schemes in wireless OFDM systems requires the tracking of the fading radio channel. The paper addresses channel estimation based on time-domain channel statistics. Using a general model for a slowly fading channel, the authors present the MMSE and LS estimators and a method for modifications compromising between complexity and performance. The symbol error rate for a 18-QAM system is presented by means of simulation results. Depending upon estimator complexity, up to 4 dB in SNR can be gained over the LS estimator.

Wireless Information Networks

Abstract: Preface. PART I INTRODUCTION TO WIRELESS NETWORKS. 1 Overview of Wireless Networks. 1.1 Introduction. 1.2 Network Architecture and Design Issues. 1.3 Key Trends in Wireless Networking. 1.4 Outline of the Book. Questions. 2 Evolution of the Wireless Industry. 2.1 Introduction. 2.2 Three Views of the Wireless Industry. 2.3 Three Generations of Cellular Networks. 2.4 Trends in Wireless Technologies. Questions. PART II CHARACTERISTICS OF RADIO PROPAGATION. 3 Characterization of Radio Propagation. 3.1 Introduction. 3.2 Multipath Fading and the Distance-Power Relationship. 3.3 Local Movements and Doppler Shift. 3.4 Multipath for Wideband Signals. 3.5 Classical Uncorrelated Scattering Model. 3.6 Indoor and Urban Radio Propagation Modeling. Questions. Problems. Projects. 4 Modeling and Simulation of Narrowband Signal Characteristics. 4.1 Introduction. 4.2 Modeling Path Loss and Slow Shadow Fading. 4.3 Doppler Spectrum of Fast Envelope Fading. 4.4 Statistical Behavior of Fast Envelope Fading. 4.5 Simulation of Fast Envelope Fading. Questions. Problems. Projects. 5 Measurement of Wideband and UWB Channel Characteristics. 5.1 Introduction. 5.2 Time-Domain Measurement Techniques. 5.3 Frequency-Domain Measurement Techniques. 5.4 Advances in Frequency-Domain Channel Measurement. Questions. Problems. Project. 6 Modeling of Wideband Radio Channel Characteristics. 6.1 Introduction. 6.2 Wideband Time-Domain Statistical Modeling. 6.3 Wideband Frequency-Domain Channel Modeling. 6.4 Comparison Between Statistical Models. 6.5 Ray-Tracing Algorithms. 6.6 Direct Solution of Radio Propagation Equations. 6.7 Comparison of Deterministic and Statistical Modeling. 6.8 Site-Specific Statistical Model. Appendix 6A: GSM-Recommended Multipath Propagation Models. Appendix 6B: Wideband Multipath Propagation Models. Questions. Problems. Projects. PART III MODEM DESIGN. 7 Narrowband Modem Technology. 7.1 Introduction. 7.2 Basic Modulation Techniques. 7.3 Theoretical Limits and Practical Impairments. 7.4 Traditional Modems for Wide-Area Wireless Networks. 7.5 Other Aspects of Modem Implementation. Questions. Problems. Projects. 8 Fading, Diversity, and Coding. 8.1 Introduction. 8.2 Radio Communication on Flat Rayleigh Fading Channels. 8.3 Diversity Combining. 8.4 Error-Control Coding for Wireless Channels. 8.5 Space-Time Coding. 8.6 MIMO and STC. Questions. Problems. Projects. 9 Broadband Modem Technologies. 9.1 Introduction. 9.2 Effects of Frequency-Selective Multipath Fading. 9.3 Discrete Multipath Fading Channel Model. 9.4 Adaptive Discrete Matched Filter. 9.5 Adaptive Equalization. 9.6 Sectored Antennas. 9.7 Multicarrier, OFDM, and Frequency Diversity. 9.8 Comparison of Traditional Broadband Modems. 9.9 MIMO in Frequency-Selective Fading. Appendix 9A: Analysis of the Equalizers. Questions. Problems. Projects. 10 Spread-Spectrum and CDMA Technology. 10.1 Introduction. 10.2 Principles of Frequency-Hopping Spread Spectrum. 10.3 Principles of Direct-Sequence Spread Spectrum. 10.4 Interference in Spread-Spectrum Systems. 10.5 Performance of CDMA Systems. Questions. Problems. PART IV SYSTEMS ASPECTS. 11 Topology, Medium Access, and Performance. 11.1 Introduction. 11.2 Topologies for Local Networks. 11.3 Cellular Topology for Wide-Area Networks. 11.4 Centrally Controlled Assigned Access Methods. 11.5 Distributed Contention-Based Access Control. Questions. Problems. Project. 12 Ultrawideband Communications. 12.1 Introduction. 12.2 UWB Channel Characteristics. 12.3 Impulse Radio and Time-Hopping Access. 12.4 Direct-Sequence UWB. 12.5 Multiband OFDM. Questions. Problems. 13 RF Location Sensing. 13.1 Introduction. 13.2 RF Location-Sensing Techniques. 13.3 Modeling The Behavior of RF Sensors. 13.4 Wireless Positioning Algorithms. Questions. Problems. 14 Wireless Optical Networks. 14.1 Introduction. 14.2 Implementation. 14.3 Eye Safety. 14.4 IR Channel Characterization and Data-Rate Limitations. 14.5 Modulation Techniques for Optical Communications. 14.6 Multiple Access and Data Rate. Questions. 15 Systems and Standards. 15.1 Introduction. 15.2 GSM, GPRS, and EDGE. 15.3 CDMA and HDR. 15.4 Other Historical Systems. 15.5 Wireless LANs. 15.6 Speech Coding in Wireless Systems. Questions. References. Index. About the Authors.

Per-Survivor Processing: a general approach to MLSE in uncertain environments

Abstract: Per-survivor processing (PSP) provides a general framework for the approximation of maximum likelihood sequence estimation (MLSE) algorithms whenever the presence of unknown quantities prevents the precise use of the classical Viterbi algorithm. This principle stems from the idea that data-aided estimation of unknown parameters may be embedded into the structure of the Viterbi algorithm itself. Among the numerous possible applications, the authors concentrate on (a) adaptive MLSE, (b) simultaneous trellis coded modulation (TCM) decoding and phase synchronization, (c) adaptive reduced state sequence estimation (RSSE). As a matter of fact, PSP is interpretable as a generalization of decision feedback techniques of RSSE to decoding in the presence of unknown parameters. A number of algorithms for the simultaneous estimation of data sequence and unknown channel parameters are presented and compared with "conventional" techniques based on the use of tentative decisions. Results for uncoded modulations over interSymbol interference (ISI) fading channels and joint TCM decoding and carrier synchronization are presented. In all cases, it is found that PSP algorithms are clearly more robust than conventional techniques both in tracking a time-varying channel and acquiring its characteristics without training. >

Interchannel interference analysis of OFDM in a mobile environment

Abstract: Orthogonal frequency division multiplexing with a suitably chosen guard interval is an effective means of eliminating intersymbol interference for high-rate transmission over fading dispersive channels. Time variations of the channel, however, lead to a loss of subchannel orthogonality, resulting in interchannel interference (ICI). The article examines the effects of ICI through analysis and simulation, in the context of a system design for HDTV digital video broadcasting to mobile receivers. It is shown that ICI can be modeled as an additive Gaussian random process that leads to an error floor which can be determined analytically as a function of the Doppler frequency. Antenna diversity and trellis coding are then examined as methods for reducing this error floor.

Variable rate QAM for mobile radio

Abstract: Quadrature amplitude modulation (QAM) schemes which vary the number of modulation levels in accordance with the mobile radio fading channel variations are investigated. Important parameters considered are the fading rate and the block size used. We describe how the adaptive QAM modems can be employed and consider their use in a DECT-like TDD packet structure. System performance in the presence of cochannel interference is also considered. Simulations show that the variable rate system has about 5 dB improvement in channel SNR over a fixed 16-level QAM system for BER's between 10/sup -2/ and 10/sup -5/ and channel SNR's between 25 and 40 dB. >

Performance of maximal-ratio diversity systems in a correlated Nakagami-fading environment

Abstract: The bit error rate (BER) performance of an M-branch maximal-ratio combiner (MRC) for the detection of signals in a correlated Nakagami-fading channel is analyzed. Coherent and incoherent detection of frequency shift-keying (FSK) and phase-shift keying (PSK) signals are considered. It is assumed that the fading parameters in each diversity branch are identical. The effect of correlation is studied by assuming two types of correlation among the quadrature components of the signals in each diversity branch. Outage probabilities are also calculated for the digital radio communication systems via the correlated fading channel. >

On the accuracy of a first-order Markov model for data transmission on fading channels

Abstract: Transmission of data blocks on a fading mobile radio channel is considered. The binary process of the success and failure of the data blocks is investigated, and the accuracy of a Markov approximation is studied. It is shown through analysis and simulation that a first-order Markov process is a good approximation. Data-link performance of ARQ protocols is considered as an example of application.

Coherent DS-CDMA performance in Nakagami multipath fading

Abstract: The coherent reception of direct sequence-code division multiple access (DS-CDMA) signals in a multipath fading channel is considered. The channel model assumes independent paths with Nakagami fading statistics; this model includes the Rayleigh channel os a special case and is a reasonable model for a frequency-selective fading channel. The bit error rate (BER) performance of a RAKE receiver under various multipath fading conditions is derived and evaluated. The results indicate that over 50 users may access the channel (assuming 127 chips per bit) simultaneously under typical fading conditions, and can achieve an uncoded BER of 0.01 with an average received signal-to-noise ratio per bit of 10 dB. >

Low-complex frame synchronization in OFDM systems

Abstract: Orthogonal frequency-division multiplexing (OFDM) systems have gained an increased interest due to their use in wireless applications such as mobile communication systems. A novel data-based frame synchronization method for OFDM-systems is presented. OFDM frames are shown to contain sufficient information to synchronize a system without the use of pilots. The cyclic extension, preceding OFDM frames, is of decisive importance for this method. Based on only the sign bits of the in-phase and the quadrature components of the received OFDM signal, the maximum likelihood solution is derived. This solution basically consists of a correlator, a moving sum and a peak detector. The stability of the generated frame-clock is improved significantly by averaging over a few number of frames. Simulations show that this low-complex, averaging method can be used to synchronize an OFDM system on twisted pair copper wires and in slowly fading radio channels.

CDMA overlay situations for microcellular mobile communications

Abstract: Direct sequence code division multiple access communications is a promising approach to cellular mobile communications, which operates in an environment characterized by multipath Rician fading. In this paper, the CDMA network is assumed to share common spectrum with a narrowband microwave user. Because of the presence of the narrowband waveform, an interference suppression filter at each CDMA receiver is employed to reject the narrowband interference. The problem of interference from adjacent cells is also considered. Average power control is assumed to combat the near/far problem, and multipath diversity, in conjunction with simple interleaved channel coding, is considered for improving the performance of the CDMA system. >

Multitone spread spectrum multiple access communications system in a multipath Rician fading channel

Abstract: The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated. >

Time division multiple access methods for wireless personal communications

Abstract: Time division multiple access (TDMA) is a classic approach to multiple access in digital cellular wireless communications systems. The authors summarize a number of frequency and time slot allocation techniques for enhancing the capacity and flexibility of TDMA-based systems. They also describe how the problems of fading, delay spread, time variability and interference affect TDMA systems, and how they may he countered and even exploited by appropriate techniques of detection, diversity, coding, adaptive equalization and slow frequency hopping (FH). It is worth emphasizing that the use of one of these techniques, slow random FH, results in a system that is in effect a hybrid of TDMA and code division multiple access (CDMA). >

Synchronization requirements for multi-user OFDM on satellite mobile and two-path Rayleigh fading channels

Abstract: The performance of multi-user orthogonal frequency division multiplexing (OFDM) with raised cosine time domain pulse shaping is studied on the satellite mobile channel (SMC) and the two-path Rayleigh fading channel. A multi-user time and carrier frequency synchronization scheme is discussed. The relationship between the synchronization requirement and the roll-off rate (/spl beta/) of the pulse shaping is explored. In Gaussian and typical Rician (K=10) channels, multi-user OFDM with /spl beta/=0.2 requires timing and carrier frequency synchronization accuracies of 4% and 2%, respectively, in order to avoid severe degradation due to multi-user adjacent channel interference, while on the two-path Rayleigh channel these values must be better than 2% for error floors less than 10/sup -3/. Timing and carrier frequency accuracies are normalized to T and 1/T respectively, where T is the symbol duration. Multi-user OFDM is less affected by the multipath time delay and slightly more severely affected by the Doppler frequency shift than traditional FDM with root Nyquist filtering (FDM/RNF) for the same value of /spl beta/. >

Wireless telephone distribution system with time and space diversity transmission

Abstract: A wireless communication system combines time and space diversity to reduce fading and simplify receiver design. In particular, a data packet which carries digital telephone traffic, is transmitted at three different times from three different antennas (A-C). The mobile subscriber receiver (10) thus receives the same data packet at three different times from three different antennas, and uses the best data packet or combination of the data packets to reduce the effects of fading. A transfer station (54, 56, 58) receives a time division multiplex multiple access (TDMA) signal from a base station carrying telephone data packet traffic to form three data packet repeats at spatially diverse antennas locations. The transfer station further modulates a code division multiple access (CDMA) system using a TDMA signal which links the mobile subscriber receiver to the transfer station. Each data packet received at the transfer station is thus retransmitted at three different times to the mobile subscriber station on a CDMA link.

Adaptive averaging methodology for handoffs in cellular systems

Abstract: The purpose of this paper is to show how information in signal strength measurements can be exploited to improve the quality of handoff decisions, for both large and small cells. Averaging of signal strength fluctuations is required. This leads to the following tradeoff problem for the averaging interval for the signal strength measurements. If the interval is too short, the fading fluctuations are not sufficiently smoothed out. If the interval is too long, delay in handoff increases. With this tradeoff in mind, we present a method to adaptively change the averaging interval. The method is based on estimating the maximum Doppler frequency, f D , as a means to obtain mobile velocity, the key to the tradeoff. A method used for estimating f D from the squared deviations of the signal envelope is outlined. Exact analysis for the f D estimate as a function of squared deviations of the logarithmically compressed signal envelope in Rayleigh fading is presented. An extension of the algorithm for robustness in a Rician fading environment is given. Sensitivity issues of the estimates are considered. An adaptive scheme for optimal averaging is outlined.

Method and apparatus for coherent communication reception in a spread-spectrum communication system

Abstract: A method and apparatus is provided for facilitating coherent communication reception. A received reference symbol coded spread-spectrum communication signal is despread with a spreading code to derive a stream of reference samples (152) and a stream of data samples (158). The channel response is estimated by utilizing the stream of reference samples (152). An offset frequency detector (443) determines an offset to be applied to the received signal via a frequency locked loop (456), while a timing control (176) compensates for slow timing drift and fast fading based on power estimates derived from the stream of reference samples (152) and/or the stream of data samples (158). A rate estimator determines the rate at which the information was encoded, and the rate information is used to optimize the timing control (176), frequency offset detector (443) and channel estimator (154). Thus an improved detection of estimated data symbols from the stream of received data samples (158) is provided.

Analysis of LEO, MEO, and GEO global mobile satellite systems in the presence of interference and fading

Abstract: Several multisatellite and multispot systems have been recently proposed for provision of mobile and personal services with global coverage, adopting GEO or non-GEO (i.e., MEO, LEO) satellite constellations. The paper addresses an in-depth analysis of these constellations, evaluating both geometrical performance measures and cochannel interference levels caused by extensive frequency reuse. The geometrical analysis yields the statistics for coverage, frequency of satellite hand-overs, and link absence periods. The interference analysis is based on a general model valid for all access techniques, which is here applied to the case of FDMA. The outage probability as a function of the specification on carrier-to-interference power ratio is evaluated for four selected constellations. Several techniques are introduced for interference reduction in non-GEO systems, in which the satellites coverage areas may intersect: spot turnoff, intraorbital plane frequency division, and interorbital plane frequency division. The effects of Rice fading have also been analyzed by means of an analytic approximated method. The overall analysis allows a fair comparison between LEO, MEO, and GEO constellations. >

Multi-rate schemes in DS/CDMA systems

Abstract: Different modulation schemes supporting multiple data rates in a direct sequence code division multiple access (DS/CDMA) system are studied, focusing on how to support personal communication services. Both AWGN and multipath Rayleigh fading channels are considered. It is shown that the multi processing-gain scheme and the multi-channel scheme have almost the same performance. However, the multi-channel scheme has some advantages due to near-far resistance, easier code design and easier multi-user receiver construction. The drawback though, is the need for linear amplifiers. A multi-modulation scheme is also possible, but the performance for the users with the high data rates is significantly worse than for the other schemes.

System for wireless transmission and receiving of information and method of operation thereof

Abstract: A system and method is disclosed for wireless transmission of information which is subject to fading by using a RF carrier modulated with a subcarrier modulated with the information. The system has a bus interface which communicates with a digital signal processor which controls the transmitting and receiving circuitry functions. The bus interface is for connection to a computer bus which is connected to a computer which originates information to be transmitted by transmitting circuitry and which receives information from receiving circuitry. The digital signal processor provides first and second encoded information streams each comprising the information to be transmitted with the second stream being delayed by a time delay interval with respect to the first stream which is equal to or greater than the fading interval. The first and second encoded information streams modulate cycles of the subcarrier to produce first and second parallel information streams which are time offset by the time delay interval. The receiving circuitry has a detector for detecting transmitted first and second parallel information streams with the second parallel information stream being delayed from the first parallel information stream by the time delay interval. The digital signal processor determines if faded information is present in the frames of the detected first and second parallel information streams by processing the error correction code therein to determine if a number of bit errors are present which exceed the bit error correction capacity of the error correction code. The digital signal processor places an error marker within the detected first and second parallel information streams to mark each faded information unit and controls replacement of each error marker within at least one of the first and second parallel information streams with replacement data bits within a frame in one of the first and second parallel information streams which were time offset at transmission by the time delay interval to produce error free transmitted information.

Innovations-based MLSE for Rayleigh fading channels

Abstract: This paper deals with the use of the innovations approach for developing a general and practical MLSE algorithms for signals transmitted over Rayleigh fading channels. The proposed MLSE receiver can be implemented by a bank of FIR time-invariant filters followed by a Viterbi processor and is applicable to any practically modulated signal over either frequency-nonselective or selective, fast or slowly fading channels. Simulation results are presented that demonstrate the superiority of the innovations-based receiver over differential detection in the presence of fast fading. To further improve the error performance, the derived MLSE receiver is combined with diversity reception. The simulation also shows that the diversity technique results in a significant improvement of the error performance. A unified interpretation of MLSE of signals transmitted over both Gaussian noise channels in the presence of ISI and Rayleigh fading channels is obtained by using the innovations approach. >

Error rates for Rayleigh fading multichannel reception of MPSK signals

Abstract: This paper provides a new closed form expression for calculating the probability that an MPSK signal will lie in a particular decision region when received over N independent and identically distributed Rayleigh fading channels corrupted by additive white Gaussian noise This expression is applied to provide a unified method to derive the exact symbol error rate and bit error rate for MPSK signals considering N channel diversity reception The N channel diversity reception techniques considered are maximal ratio combining (MRC) and selection combining MRC with identical channels and dissimilar channels is considered The results for MRC can be extended to provide an approximation for the error rates of MPSK under equal gain combining >

Co-channel interference cancelling receiver for TDMA mobile systems

Abstract: A method for co-channel interference cancellation in TDMA mobile systems exploiting the characteristics of mobile channels is proposed. The independently fading multipath channels provide a distinct waveform coding on each of the co-channel signals, which is a basis for signal separation in receivers. This enables the use of joint detection methods in receivers provided that accurate channel estimates can be obtained for all the cochannels. A joint detection and channel estimation algorithm for multiple co-channel signals are derived. The performance of the receiver is verified by simulations with the GSM system assuming that the strongest interferer is cancelled.

On the performance of different detection techniques for OFDM-CDMA in fading channels

Abstract: The combination of orthogonal frequency division multiplexing (OFDM) with code division multiple access (CDMA) systems has attained increasing significance in mobile radio communications. In this paper basic OFDM-CDMA detection techniques based on simple equalization algorithms, and improved detection techniques, such as the newly proposed iterative detection applying the minimum mean square error (MMSE) criterion, are analyzed in the mobile radio environment. This analysis enables one a comparison between known and new OFDM-CDMA detection techniques in terms of spectral efficiency and receiver complexity.

Adaptive arrays and MLSE equalization

Abstract: In digital cellular communication systems, receivers are designed to combat the problems of fading, time dispersion, and interference. Separately, these problems can be solved using antenna diversity, equalization, and adaptive array processing, respectively. Joint solutions have been proposed which combine adaptive array processing with either linear equalization (LE) or decision feedback equalization (DFE). However, LE is sensitive to spectral nulls and DFE has the problem of decision error feedback. In this paper, an algorithm is proposed which combines adaptive array processing with MLSE equalization to mitigate fading, time dispersion, and interference. The optimal metric for the MLSE structure is derived. The receiver reduces to well known forms for special cases. D-AMPS simulation results illustrate the potential performance of the proposed algorithm, including the case when the fading on the different antennas is correlated.

A multiuser receiver for code division multiple access communications over multipath channels

Abstract: A multiuser communication system is considered where K users share a channel with multipath propagation by using code division for multiple access. Data modulation is carried out by binary phase shift keying and direct sequence spread spectrum signaling. The micro-cellular communication media is modeled as a frequency selective fading channel with multipath propagation. The multipath diversity of the received signals from the K users is exploited by a bank of K RAKE correlators. Algorithms based on the maximum likelihood rule have been developed for estimating the complex channel coefficients as well as for detection of the desired data packets from the sufficient statistics provided by the RAKE correlators. The performance of the resulting multiuser detector is evaluated analytically and via Monte Carlo simulations. The results indicate that the estimator of the channel coefficients has a variance close to the Cramer-Rao lower bound, and that the proposed multiuser detector is capable of eliminating the near-far effect as well as processing the signals propagated through multiple paths. >

Optimum transmission ranges in multihop packet radio networks in the presence of fading

Abstract: The evaluation of optimum transmission ranges in a packet radio network in a fading and shadowing environment is considered. It is shown that the optimal probability of transmission of each user is independent of the system model and is p/sub o//spl sime/0.271. The optimum range should be chosen so that on the average there are /spl chi/(G/b)/sup 2//spl eta// terminals closer to the transmitter than the receiver, where G is the spread spectrum processing gain, b is the outage signal-to-noise ratio threshold, /spl eta/ is the power loss factor and /spl chi/ depends on the system parameters and the propagation model. The performance index is given in terms of the optimal normalized expected progress per slot, given by /spl thetav/(G/b)/sup 1//spl eta// where /spl thetav/ is proportional to the square root of /spl chi/. A comparison with the results obtained by using deterministic propagation models shows, for typical values of fading and shadowing parameters, a reduction up to 40% of the performance index. >

A selection combining scheme for RAKE receivers

Abstract: A selection combining scheme for a RAKE receiver operating over a multipath fading channel is introduced, by which the m largest channel outputs are selected instead of only the largest one, as in the conventional selection combining receiver. Expressions for the error probability of this scheme for an exponential multipath intensity profile (MIP) with arbitrary decay constant are found by first deriving the joint density function of the m ordered channel outputs, then averaging the conditional error probability over the joint density function. The performance is compared with that of maximal ratio combining in both an interference-limited and a noise-limited environment. The interference-limited environment chosen is a multicell CDMA system. Numerical results show that the performance of the selection combining scheme is superior to that of conventional selection combining, and can be very close to that of maximal ratio combining, depending upon the value of m and the rate of decay of the MIP.

A frequency and timing period acquisition technique for OFDM systems

Abstract: Symbol Pattern Abstract This paper introduces a frequency and timing period acquisition technique for orthogonal frequency division multiplexing (OFDM) systems. The proposed technique estimates both the frequency and timing pesiod offsets at the time by using only one pilot symbol with its suitable frequency assignment. Pseudo noise (PN) sequences are introduced to assign these frequencies of the pilot symbol so that the acquisition range can be widened. Numerical examples are given to show the estimate variances for both additive white Gaussian noise (AWGN) and multipath fading channels.