Showing papers on "Multipath propagation published in 2004"

Cooperative diversity in wireless networks: Efficient protocols and outage behavior

Abstract: We develop and analyze low-complexity cooperative diversity protocols that combat fading induced by multipath propagation in wireless networks. The underlying techniques exploit space diversity available through cooperating terminals' relaying signals for one another. We outline several strategies employed by the cooperating radios, including fixed relaying schemes such as amplify-and-forward and decode-and-forward, selection relaying schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal. We develop performance characterizations in terms of outage events and associated outage probabilities, which measure robustness of the transmissions to fading, focusing on the high signal-to-noise ratio (SNR) regime. Except for fixed decode-and-forward, all of our cooperative diversity protocols are efficient in the sense that they achieve full diversity (i.e., second-order diversity in the case of two terminals), and, moreover, are close to optimum (within 1.5 dB) in certain regimes. Thus, using distributed antennas, we can provide the powerful benefits of space diversity without need for physical arrays, though at a loss of spectral efficiency due to half-duplex operation and possibly at the cost of additional receive hardware. Applicable to any wireless setting, including cellular or ad hoc networks-wherever space constraints preclude the use of physical arrays-the performance characterizations reveal that large power or energy savings result from the use of these protocols.

A review of antennas and propagation for MIMO wireless communications

Abstract: Multiple-input-multiple-output (MIMO) wireless systems use multiple antenna elements at transmit and receive to offer improved capacity over single antenna topologies in multipath channels In such systems, the antenna properties as well as the multipath channel characteristics play a key role in determining communication performance This paper reviews recent research findings concerning antennas and propagation in MIMO systems Issues considered include channel capacity computation, channel measurement and modeling approaches, and the impact of antenna element properties and array configuration on system performance Throughout the discussion, outstanding research questions in these areas are highlighted

Design of a multiband OFDM system for realistic UWB channel environments

Abstract: In February 2002, the Federal Communications Commission allocated 7500 MHz of spectrum for unlicensed use of commercial ultra-wideband (UWB) communication devices. This spectral allocation has initiated an extremely productive activity for industry and academia. Wireless communications experts now consider UWB as available spectrum to be utilized with a variety of techniques, and not specifically related to the generation and detection of short RF pulses as in the past. There are many differences between real-world behavior of narrow-band and UWB systems. All wireless systems must be able to deal with the challenges of operating over a multipath propagation channel, where objects in the environment can cause multiple reflections to arrive at the receiver (RX). For narrow-band systems, these reflections will not be resolvable by the RX when the narrow-band system bandwidth is less than the coherence bandwidth of the channel. The large bandwidth of UWB waveforms, instead, significantly increases the ability of the RX to resolve the different reflections in the channel. The UWB channel model developed by the IEEE 802.15.3a standard body is described in this paper. For highly dispersive channels, an orthogonal frequency-division multiplexing (OFDM) RX is more efficient at capturing multipath energy than an equivalent single-carrier system using the same total bandwidth. OFDM systems possess additional desirable properties, such as high spectral efficiency, inherent resilience to narrow-band RF interference, and spectral flexibility, which is important because the regulatory rules for UWB devices have not been finalized throughout the entire world. This paper describes the design of a UWB system optimized for very high bit-rate, low-cost, and low-power wireless networks for personal computing (PC), consumer electronics (CE), and mobile applications. The system combines OFDM modulation technique with a multibanding approach, which divides the spectrum into several sub-bands, whose bandwidth is approximately 500 MHz. The system described in this paper has been selected by several key industry organizations [Mulitband OFDM Alliance, WiMedia, Wireless Universal Serial Bus (USB)] because of its very good technical characteristics for the diverse set of high performance short-range applications that are eagerly anticipated for CE, PC, and mobile applications.

Ultra-wideband radio technology

Abstract: Dedication.Preface.Acknowledgements.1. History.2. The Regulatory Climate.3. UWB in Standards.4. Generating and Transmitting UWB Signals.5. Radiation of UWB Signals.6. Propagation of UWB Signals.7. Receiving UWB Signals.8. UWB System Limits and Capacity.9. Applications and Future Directions.Appendix A: Excerpts from the FCC First Report and Order.Appendix B: Summary of Multipath Model for IEEE P802.15.3a.Appendix C: Free-space Transmission of Pulses.Appendix D: Glossary.Index.

UltraWideBand indoor positioning systems and their use in emergencies

Abstract: Reliable and accurate indoor positioning for moving users requires a local replacement for satellite navigation. An UltraWideBand (UWB) system is particularly suitable for such local systems, including temporary installations supporting emergency services inside large buildings. The requirements for emergencies will be very variable, but will generally include: good radio penetration through structures, the rapid set-up of a stand-alone system, tolerance of high levels of reflection, and high accuracy. The accuracy should be better than 1 m, as sometimes it matters which side of a door you are, and locations should be in 3 dimensions. Support for robots as well as people would call for still better accuracy. Rapid set-up implies very little surveying of the fixed terminals, and positioning relative to the mobile and fixed terminals. A radio system that measures ranges between fixed and mobile terminals matches these requirements, but the requirements for accuracy and for dealing with multipath need a bandwidth of more than 1 GHz. Thus UWB is the preferred solution, as it has the specific advantage of high accuracy, even in the presence of severe multipath. This paper presents the features and system design options for UWB positioning systems, and shows how they match the indoor location demands of emergency services. The main features that are covered are: the deployment of terminals (how many, and where), the minimum requirements for fixed terminal surveying, integration (hybridisation) with GNSS, and solving for position inside the network. The main system design options are: whether the mobile terminals are transceivers or solely receivers, the UWB signal design and frequency span, and the use of the same signal for communications. The paper includes results from a demonstration UWB indoor positioning system being built at TRT (UK).

Performance of differential chaos-shift-keying digital communication systems over a multipath fading channel with delay spread

Abstract: The performance of the noncoherent differential chaos-shift-keying (DCSK) communication system over a multipath fading channel with delay spread is evaluated. Analytical expressions of the bit error rates are derived under the assumption of an independent Rayleigh fading two-ray channel model. Analytical and simulated results are presented and compared. The multipath performance of the DCSK system is also compared with that of the coherent CSK system as well as conventional generic waveform communication schemes.

Optimal pilot waveform assisted modulation for ultrawideband communications

Abstract: Ultrawideband (UWB) transmissions induce pronounced frequency-selective fading effects in their multipath propagation. Multipath diversity gains can be collected to enhance performance, provided that the underlying channel can be estimated at the receiver. To this end, we develop a novel pilot waveform assisted modulation (PWAM) scheme that is tailored for UWB communications. We select our PWAM parameters by jointly optimizing channel estimation performance and information rate. The resulting transmitter design maximizes the average capacity, which is shown to be equivalent to minimizing the mean-square channel estimation error, and thereby achieves the Crame/spl acute/r-Rao lower bound. Application of PWAM to practical UWB systems is promising because it entails simple integrate-and-dump operations at the frame rate. Equally important, it offers a flexible UWB channel estimator, capable of striking desirable rate-performance tradeoffs depending on the channel coherence time.

Bit error rate analysis of DS-CDMA with joint frequency-domain equalization and antenna diversity combining

Abstract: To improve the DS-CDMA signal transmission performance in a frequency-selective fading channel, the frequency-domain equalization (FDE) can be applied, in which simple one-tap equalization is carried out on each subcarrier component obtained by fast Fourier transform (FFT). Equalization weights for joint FDE and antenna diversity combining based on maximal ratio combining (MRC), zero-forcing (ZF), and minimum mean square error (MMSE) are derived. The conditional bit error rate (BER) is derived for the given set of channel gains in a frequency-selective multipath fading channel. The theoretical average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation. Performance comparison between DS- and multi-carrier (MC)-CDMA both using FDE is also presented.

Analog space-time coding for multiantenna ultra-wideband transmissions

Abstract: Ultra-wideband (UWB) transmissions have well-documented advantages for low-power, peer-to-peer, and multiple-access communications. Space-time coding (STC), on the other hand, has gained popularity as an effective means of boosting rates and performance. Existing UWB transmitters rely on a single antenna, while ST coders have mostly focused on digital linearly modulated transmissions. In this paper, we develop ST codes for analog (and possibly nonlinearly) modulated multiantenna UWB systems. We show that the resulting analog system is able to collect not only the spatial diversity, but also the multipath diversity inherited by the dense multipath channel, with either coherent or noncoherent reception. Simulations confirm a considerable increase in both bit-error rate performance and immunity against timing jitter, when wedding STC with UWB transmissions.

Source localization using vector sensor array in a multipath environment

Abstract: Coherent signals from distinct directions is a natural characterization of the multipath propagation effect. This paper addresses the problem of coherent/fully correlated source localization using vector sensor arrays. The maximum likelihood (ML) and minimum-variance distortionless response (MVDR) estimators for source direction-of-arrival (DOA) and signal polarization parameters are derived. These estimators require no search over the polarization parameters. In addition, a novel method for "decorrelating" the incident signals is presented. This method is based on the polarization smoothing algorithm (PSA) and enables the use of eigenstructure-based techniques, which assume uncorrelated or partially correlated signals. The method is implemented as a preprocessing stage before applying eigenstructure-based techniques, such as MUSIC. Unlike other existing preprocessing techniques, such as spatial smoothing and forward-backward (FB) averaging, this method is not limited to any specific array geometry. The performance of the proposed PSA preprocessing combined with MUSIC is evaluated and compared to the Crame/spl acute/r-Rao Bound (CRB) and the ML and MVDR estimators. Simulation results show that the MVDR and PSA-MUSIC asymptotically achieve the CRB for a scenario with two coherent sources with and without an uncorrelated interference source. A sensitivity study of PSA-MUSIC to source polarization was also conducted via simulations.

High-speed satellite mobile communications: technologies and challenges

Abstract: Central features of future 4G mobile communication systems are high-speed data transmission (up to 1 Gb/s) and interactive multimedia services. For effective delivery of these services, the network must satisfy some stringent QoS metrics, defined typically in terms of maximum delay and/or minimum throughput. Mobile satellite systems will be fully integrated with the terrestrial cellular systems to provide ubiquitous global coverage to diverse users. The challenges for future broadband satellite systems, therefore, lie in the proper deployment of state-of-the-art satellite technologies to ensure seamless integration of the satellite networks into the cellular systems and its QoS frameworks, while achieving, as far as possible, efficient use of satellite link resources. The paper presents an overview of future high-speed satellite mobile communication systems, the technologies deployed or planned for deployment, and the challenges. Focusing in particular on nonlinear downlink channel behavior, shadowing and multipath fading, various physical channel models for characterizing the mobile satellite systems are presented. The most prominent technologies used in the physical layer, such as coding and modulation schemes, multiple-access techniques, diversity combining, etc., are then discussed in the context of satellite systems. High-speed and QoS-specific technologies, such as onboard processing and switching, mobility and resource management, IP routing and cross-layer designs, employed in the satellite systems are also discussed.

Multipath-enabled super-resolution for rf and microwave communication using phase-conjugate arrays.

Abstract: We demonstrate experimentally that phase-conjugate techniques can be used to achieve super-resolution focusing of electromagnetic waves in a multipath indoor environment at 2.45 GHz. The focusing phenomena was used to direct independent signals to two locations separated by approximately one-half wavelength, thereby creating two simultaneous channels at the same frequency. An increase in channel capacity is shown to be achievable by an experimental transmission of a 1 Mbps signal over two channels created using a four element phase-conjugate array.

A semi-blind channel estimation method for multiuser multiantenna OFDM systems

Abstract: A subspace-based blind method is proposed for estimating the channel responses of a multiuser and multiantenna orthogonal frequency division multiplexing (OFDM) uplink system. It gives estimations to all channel responses subject to a scalar matrix ambiguity and does not need precise channel order information (only an upper bound for the orders is required). Furthermore, the scalar ambiguity matrix can be easily resolved by using only one pilot OFDM block, given that the number of users is smaller than the number of symbols in the pilot symbol block. Equalization methods are discussed based on the estimated channels. By using partial knowledge of the channels, a multipath subspace method is proposed that reduces the computational complexity. Simulations show that the methods are effective and robust.

Statistical analysis of the UWB channel in an industrial environment

Abstract: In this paper, we present a statistical model for the ultra-wideband (UWB) channel in an industrial environment. Based on a set of measurements in a factory hall, we find that the abundance of metallic scatterers causes dense multipath scattering. This can be seen to produce mostly a Rayleigh distributed small-scale fading signal, with only a few paths exhibiting Nakagami distributions. For the power delay profile, we suggest a generalization of the Saleh-Valenzuela model where clusters with different excess delays have different ray power decay constants; the decay constants follow a linear dependence on the delay. This model provides an excellent fit to the measured data. We also note that for non-line-of-sight scenarios at larger distances, several hundred multipath components need to be collected to capture 50% of the available energy.

Link error prediction methods for multicarrier systems

Abstract: Multicarrier modulations such as OFDM with adaptive modulation and coding (AMC) are well suited for high data rate broadband systems that operate in multipath environments and are considered as promising candidates for future generation cellular systems (e.g., 4G). Cellular system performance is normally investigated with system level simulations that are computationally complex. For broadband multicarrier systems, incorporating a detailed physical layer emulator into the system simulator becomes impractical, so there is a need for simplified link performance predictors. However, due to the large variability of the channel in the frequency domain, two links with the same average SNR can experience drastically different performance, thus making it difficult to accurately predict the instantaneous link performance such as the frame error rate. In this paper, the accuracy of two FER prediction methods is studied: Packet error rate indicator (PER-indicator) and exponential effective SIR mapping (Exp-ESM). Both methods are shown to have accuracy within a few tenths of a dB under a wide range of modulation schemes, coding rates and channel types. These methods are then extended to handle more advanced link enhancements such as hybrid ARQ and Alamouti encoding. The Exp-ESM method has slightly better accuracy than the PER-indicator, and is the preferred link error predictor for a system simulator.

Blind multiuser detection: from MOE to subspace methods

Abstract: The minimum output energy (MOE) multiuser receiver has been shown to approach the minimum mean-square-error (MMSE) receiver at high signal-to-noise ratio (SNR). However, performance degradation is incurred by noise induced channel estimation error. In this paper, we propose a Power of R (POR) technique to significantly improve the performance of the MOE receiver. It is shown that the new receiver asymptotically converges to the MMSE receiver without performance penalty. The convergence is established either under high SNR, with large exponent raised in the power of the covariance matrix, or with sufficiently large number of data samples. Connection between our POR method and a widely studied subspace method is investigated from the respective optimization criteria. Asymptotic equivalence between these two methods is also established. Extensive simulations based on finite data samples show that the proposed method significantly outperforms the subspace method in systems with medium to heavy loading, severe multipath distortion, or smaller processing gain. Moreover, adaptive implementation of the proposed method exhibits very robust performance in a dynamic loading environment.

Orthogonal time-frequency signaling over doubly dispersive channels

Abstract: This paper develops a general framework for communication over doubly dispersive fading channels via an orthogonal short-time Fourier (STF) basis. The STF basis is generated from a prototype pulse via time-frequency shifts. In general, the orthogonality between basis functions is destroyed at the receiver due to channel dispersion. The starting point of this work is a pulse scale adaptation rule first proposed by Kozek to minimize the interference between the basis functions. We show that the average signal-to-interference-and-noise (SINR) ratio associated with different basis functions is identical and is maximized by the scale adaptation rule. The results in this paper highlight the critical impact of the channel spread factor, the product of multipath and Doppler spreads, on system performance. Smaller spread factors result in lesser interference such that a scale-adapted STF basis serves as an approximate eigenbasis for the channel. A highly effective iterative interference cancellation technique is proposed for mitigating the residual interference for larger spread factors. The approximate eigendecomposition leads to an intuitively appealing block-fading interpretation of the channel in terms of time-frequency coherence subspaces: the channel is highly correlated within each coherence subspace whereas it is approximately independent across different subspaces. The block-fading model also yields an approximate expression for the coherent channel capacity in terms of parallel flat-fading channels. The deviation of the capacity of doubly dispersive channels from that of flat-fading channels is quantified by studying the moments of the channel eigenvalue distribution. In particular, the difference between the moments of doubly dispersive and flat-fading channels is proportional to channel spread factor. The results in this paper indicate that the proposed STF signaling framework is applicable for spread factors as large as 0.01.

Group-orthogonal multicarrier CDMA

Abstract: In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.

Measured joint Doppler-delay power profiles for vehicle-to-vehicle communications at 2.4 GHz

Abstract: Measured per-tap Doppler spectra are presented for a frequency selective vehicle-to-vehicle or mobile-to-mobile wireless communications link in various multipath environments in Atlanta, Georgia. The measurements were taken using the direct sequence spread spectrum (DSSS) technique at 2.45 GHz. The environments, chosen for their exceptionally long delay spreads, include an expressway, an urban T-intersection, and an exit ramp. The different environments produced quite different spectra. Also, for a given channel, the spectra corresponding to different delays were different, implying a non-separable channel model.

Underwater acoustic channel characterisation for medium-range shallow water communications

Abstract: The ability to effectively communicate underwater has numerous applications for researchers, marine commercial operators and defence organizations. As electromagnetic waves cannot propagate over long distances in seawater, acoustics provides the most obvious choice of channel. Although acoustics has been used effectively for point-to-point communications in deep-water channels, acoustics has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and nonGaussian noise are two of the major factors that limit acoustic communication performance in shallow water. Although it is known that medium-range shallow water propagation is dominated by time-varying multipath arrivals, very few measurements of the variability of the multipath structure are available. In this paper, we present channel measurements made in a shallow water channel (depth 15-20 m) up to a range of 1 km. An analysis of the temporal variability of the arrival structure is presented. Most communication systems make the assumption that the noise is additive and Gaussian. Snapping shrimp dominate the ambient noise spectrum above a few kHz in warm shallow waters. It is known that snapping shrimp noise is impulsive and highly nonGaussian. These noise characteristics need to be taken into account when designing communication systems if robust and near-optimal performance is desired. An analysis of the ambient noise characteristics from some warm shallow water channels is also presented.

Theory Of Code Division Multiple Access Communication

Abstract: Preface. 1 Introduction to Cellular Mobile Radio Communication. 1.1 CellularMobile Radio Systems. 1.2 Frequency Division and Time Division Multiple Access. 1.3 Direct Sequence CDMA. 1.4 Frequency-Hopped CDMA. 1.5 Pulse Position-Hopped CDMA. 1.6 Organization of the Text. 1.7 Comments. Problems. 2 Introduction to Spread Spectrum Communication Systems. 2.1 Modulation Formats for SS Communication. 2.2 Correlation and Spectral Properties of Modulated Signals. 2.3 Generation of DS SS Signals. 2.4 Frequency-Hopped SS Signals. 2.5 Pulse Position-Hopped SS Signals. 2.6 Orthogonal and Quasi-Orthogonal Expansions of SS Signals. 2.7 Comments. Problems. 3 Reception of Spread Spectrum Signals in AWGN Channels. 3.1 Problem Formulation. 3.2 Neyman-Pearson Hypothesis Testing Concept. 3.3 Coherent Reception of DS CDMA Signals (Uplink Transmission). 3.4 Coherent Reception of DS CDMA Signals (Downlink Transmission). 3.5 Reception of DS DPSK SS Signals. 3.6 Reception of FH SS Signals. 3.7 Reception of PPH SS Signals. 3.8 Comments. Problems. 4 Forward Error Control Coding in Spread Spectrum Systems. 4.1 Introduction to Block Coding. 4.2 First-Order Reed-Muller Code. 4.3 Noncoherent Reception of Encoded DS CDMA Signals. 4.4 Introduction to Convolutional Coding. 4.5 Convolutional Coding in DS CDMA Systems. 4.6 Orthogonal Convolutional Codes. 4.7 Coding in FH and PPH CDMA Systems. 4.8 Concatenated Codes in CDMA Systems. 4.9 Comments. Problems. 5 CDMA Communication on Fading Channels. 5.1 Statistical Models of Multipath Fading. 5.2 Coherent Reception of Faded Signals. 5.3 Forward Transmission over a Multipath Faded Channel in a DS CDMA System. 5.4 Reverse Transmission over a Multipath Faded Channel in a DS CDMA System. 5.5 Interleaving for a Rayleigh Channel. 5.6 FH SS Communication over Rayleigh Faded Channels. 5.7 Comments. Problems. 6 Pseudorandom Signal Generation. 6.1 Pseudorandom Sequences and Signals. 6.2 Finite-Field Arithmetic. 6.3 Maximum-Length Linear Shift Registers. 6.4 Randomness Properties of Maximal-Length Sequences. 6.5 Generating Pseudorandom Signals (Pseudonoise) from Pseudorandom Sequences. 6.6 Other Sets of Spreading Sequences. 6.7 Comments. Problems. 7 Synchronization of Pseudorandom Signals. 7.1 Hypothesis Testing in the Acquisition Process. 7.2 Performance of the Hypothesis Testing Device. 7.3 The Acquisition Procedure. 7.4 Modifications of the Acquisition Procedure. 7.5 Time Tracking of SS Signals. 7.6 Coherent Reception of Uplink Transmitted Signals in the DS CDMA System. 7.7 Comments. Problems. 8 Information-Theoretical Aspects of CDMA Communications. 8.1 Shannon Capacity of DS CDMA Systems. 8.2 Reliability Functions. 8.3 Capacity of FH CDMA Systems. 8.4 Uplink Multiple-Access Channels. 8.5 Downlink Multiple-Access Channels. 8.6 Multiuser Communication in the Rayleigh Fading Channels. 8.7 Comments. Problems. 9 CDMA Cellular Networks. 9.1 General Aspects of CDMA Cellular Networks. 9.2 Other-Cell Relative Interference Factors. 9.3 Handoff Strategies. 9.4 Power Control. 9.5 Erlang Capacity of CDMA System. 9.6 Interference Cancellation in the Reverse Link of the DS CDMA System. 9.7 User Coordination in the Forward Link of the DS CDMA System. 9.8 Third-Generation Wireless Cellular Networks. 9.9 Comments. Problems. Appendix A: Analysis of the Moments of the Decision Statistics for the FH CDMA Communication System. Bibliography. Index.

Analysis of electromagnetic field polarizations in multiantenna systems

Abstract: This paper provides an analytical framework useful for assessing the use of all six electric and magnetic electromagnetic field polarizations for multiantenna communications systems. The approach uses a mapping between the induced signal currents and the received electromagnetic field in order to formulate a diversity interpretation of the six-polarization problem. Application of the framework to a simple, yet representative, channel model demonstrates that for full multipath elevation and azimuthal angle spread, six communication modes are theoretically possible. However, to implement the system requires more antenna design work since a straightforward implementation is found to reduce the potential number of modes to three.

Likelihood function for QRM-MLD suitable for soft-decision turbo decoding and its performance for OFCDM MIMO multiplexing in multipath fading channel

Abstract: This paper proposes likelihood function generation of complexity-reduced maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD) suitable for soft-decision turbo decoding and investigates the throughput performance using QRM-MLD with the proposed likelihood function in multipath Rayleigh fading channels for orthogonal frequency and code division multiplexing (OFCDM) with multiple-input multiple-output (MIMO) multiplexing Simulation results show that by using the proposed likelihood function generation scheme for soft-decision turbo decoding following QRM-MLD in 4-by-4 MIMO multiplexing, the required average received signal energy per bit-to-noise power spectrum density ratio (E/sub b//N/sub o/) at the average block error rate (BLER) of 10/sup -2/ at a 1-Gbps data rate is significantly reduced compared to that using hard-decision decoding in OFCDM access with 16 QAM modulation, the coding rate of 8/9, and 8-code multiplexing with a spreading factor of 8 assuming a 100-MHz bandwidth Furthermore, we show that by employing QRM-MLD associated with soft-decision turbo decoding for 4-by-4 MIMO multiplexing, the throughput values of 500 Mbps and 1 Gbps are achieved at the average received E/sub b//N/sub o/ of approximately 45 and 93 dB by QPSK with the coding rate of R = 8/9 and 16 QAM with R = 8/9, respectively, for OFCDM access assuming a 100-MHz bandwidth in a twelve-path Rayleigh fading channel

Antenna Isolation, Wideband Multipath Propagation Measurements, and Interference Mitigation for On-frequency Repeaters

Abstract: On-frequency repeaters provide network operators with a cost-effective means of extending coverage to areas of low signal strength. Their ability to transmit and receive on the same channel essentially renders their operation transparent to the rest of the network. The maximum gain of an on-frequency repeater is, however, limited by the amount of isolation that exists between the transmitter and receiver antennas. Adaptive cancellation techniques may be used to mitigate a lack of isolation, but their performance is limited by their ability to track the time-varying multipath in the channel which exists between the two antennas. This paper presents antenna isolation and wideband power delay profile measurement results for typical repeater installations. In addition, simulation results are presented from a novel adaptive filtering approach that allows repeaters to operate with gains equal to or greater than the existing isolation.

A dual frequency microstrip patch antenna for high-precision GPS applications

Abstract: Since the Global Positioning System (GPS) was launched, significant progress has been made in GPS receiver technology but the multipath error remains an unsolved problem. As solutions based on signal processing are not adequate, the most effective approach to discriminate between direct and multipath waves is to specify new and more restrictive criteria in the design of the receiving antenna. An innovative low profile, lightweight dual band (L1+L2) GPS radiator with a high multipath-rejection capability is presented. The proposed solution has been realized by two stacked shorted annular elliptical patch antennas. In what follows, a detailed account of the design process and antenna performances is given, presenting both simulated and experimental results.

The effect of NBI on UWB time-hopping systems

Abstract: This letter presents an analysis of the effect of narrowband interference (NBI) on ultrawideband (UWB) time-hopping (TH) systems in the presence of multipath fading using both analytical derivations and simulations. Our analysis demonstrates that NBI may be an issue in some instances. In addition, we suggest three NBI suppression schemes for combating NBI in UWB TH systems. Single-link performance of these schemes in conjunction with a Rake-type receiver structure is estimated for both the ideal all-Rake receiver and the simpler partial-Rake receiver in an indoor environment. Two UWB pulse shapes that meet the Federal Communications Commission rules for UWB communications are considered in the investigation.

Performance of multicarrier CDMA with successive interference cancellation in a multipath fading channel

Abstract: A high capacity, low complexity, and robust system design for a successive interference cancellation (SIC) system is developed and analyzed. Multicarrier code-division multiple access (MC-CDMA) is used to suppress multipath and to overcome the multipath channel estimation problem in single-carrier SIC systems. In addition, an optimal power control algorithm for MC-CDMA with SIC is derived, allowing analytical bit-error rate expressions to be found for an uncoded system. Low-rate forward error-correcting codes are then added to the system to achieve robustness. It is found that the capacity of the coded system approaches the additive white Gaussian noise capacity for SIC, even in a fading multipath channel with channel estimation error. This indicates that MC-CDMA is very attractive for systems employing SIC.

Power savings analysis of peak-to-average power ratio in OFDM

Abstract: Orthogonal frequency division multiplexing (OFDM) has become a popular modulation method in high-speed wireless networks because of its robustness against multipath fading, its simple equalizer structure, and its high bandwidth efficiency. However, in the time domain, the OFDM signal has a large peak-to-average power ratio (PAR), which translates to low power amplifier efficiencies. In this paper we examine and quantify the power savings from PAR reductions. We also relate any power savings to the computational power costs of selected mapping, which is a prominent PAR reduction scheme that results in negligible performance degradation.

Analysis of Non Ambiguous BOC Signal Acquisition Performance

Abstract: The Binary Offset Carrier planned for future GNSS signals, including several GALILEO signals as well as GPS M-code, presents a high degree of spectral separation from conventional signals. It also greatly improves positioning accuracy and enhances multipath rejection. However, with such a modulation, the acquisition process is made more complex. Specific techniques must be employed in order to avoid unacceptable errors. This paper assesses the performance of three methods allowing to acquire and track a BOC signal unambiguously : the Bump-Jumping technique, the ”BPSK-like” technique, and the Sub Carrier Phase Cancellation (SCPC) technique. A detailed study of these methods is developed taking into accounts the filtering effects. In order to validate the theoretical results, intensive simulations have been carried out and performance of BOC signal acquisition for the different methods have been assessed and compared with initial theoretical analysis.

Analysis of diffuse optical wireless channels employing spot-diffusing techniques, diversity receivers, and combining schemes

Abstract: In this letter, the performance of an indoor optical wireless spot-diffusing system using various multibeam transmitter configurations, in association with direction diversity and combining techniques, is assessed and compared under the impact of multipath dispersion and ambient light noise through theoretical analysis and computer simulation. Computer simulation for three different multibeam transmitter configurations and a conventional diffuse transmitter is carried out. Diversity receiver and wide field-of-view (FOV) receiver configurations are evaluated in conjunction with the proposed configurations. For the diversity-detection case, a receiver comprising an array of narrow FOV detectors (three and seven segments) oriented in different directions is used to maximize the collected signals and minimize noise. A novel line-strip multibeam system (LSMS) is investigated with single and diversity receiver configurations, and is compared with other spot-diffusing methods. Combining schemes, including selection combining, maximum ratio combining, and equal gain combining are employed for the presented configurations. Our results indicate that the performance improvement obtained through the use of LSMS with a three-direction diversity receiver is about 20 dB signal-to-noise ratio enhancement over the conventional diffuse system, and 26 dB when combining techniques are used. Root mean square delay-spread performance for the proposed configurations, at different positions on the communication floor, are also evaluated and compared.