Showing papers on "Multipath propagation published in 1996"

Recent advances in high-speed underwater acoustic communications

Abstract: In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

A rate-splitting approach to the Gaussian multiple-access channel

Abstract: It is shown that any point in the capacity region of a Gaussian multiple-access channel is achievable by single-user coding without requiring synchronization among users, provided that each user "splits" data and signal into two parts. Based on this result, a new multiple-access technique called rate-splitting multiple accessing (RSMA) is proposed. RSMA is a code-division multiple-access scheme for the M-user Gaussian multiple-access channel for which the effort of finding the codes for the M users, of encoding, and of decoding is that of at most 2M-1 independent point-to-point Gaussian channels. The effects of bursty sources, multipath fading, and inter-cell interference are discussed and directions for further research are indicated.

A geometrically based model for line-of-sight multipath radio channels

Abstract: This paper provides a geometrically based model for single bounce multipath components in line-of-sight (LOS) microcell radiowave propagation channels. The model characterizes the excess delay, direction-of-arrival, and received power of multipath components and is particular useful for microcell and PCS systems with low transmitter and receiver antenna heights. The model includes analytical expressions for the probability density functions for all critical channel parameters and is thus useful for analytical purposes. The model is also useful for simulating both wideband and narrowband systems and in particular the model has been used extensively in a study of code division multiple access (CDMA) cellular radio systems employing adaptive antennas and switched beam systems at the base station.

A comparison of theoretical and empirical reflection coefficients for typical exterior wall surfaces in a mobile radio environment

Abstract: This paper presents microwave reflection coefficient measurements at 1.9 GHz and 4.0 GHz for a variety of typical smooth and rough exterior building surfaces. The measured test surfaces include walls composed of limestone blocks, glass, and brick. Reflection coefficients were measured by resolving individual reflected signal components temporally and spatially, using a spread-spectrum sliding correlation system with directional antennas. Measured reflection coefficients are compared to theoretical Fresnel reflection coefficients, applying Gaussian rough surface scattering corrections where applicable. Comparisons of theoretical calculations and measured test cases reveal that Fresnel reflection coefficients adequately predict the reflective properties of the glass and brick wall surfaces. The rough limestone block wall reflection measurements are shown to be bounded by the predictions using the Fresnel reflection coefficients for a smooth surface and the modified reflection coefficients using the Gaussian rough surface correction factors. A simple, but effective, reflection model for rough surfaces is proposed, which is in good agreement with propagation measurements at 1.9 GHz and 4 GHz for both vertical and horizontal antenna polarizations. These reflection coefficient models can be directly applied to the estimation of multipath signal strength in ray tracing algorithms for propagation prediction.

OFDM codes for peak-to-average power reduction and error correction

Abstract: Orthogonal frequency division multiplexing (OFDM) is a promising way to provide large data rates at reasonable complexity in wireless fading channels. However, a major disadvantage of OFDM is its large peak-to-average power ratio, which significantly decreases the efficiency of the transmitter power amplifier and hence forms a major obstacle to implementing OFDM in portable communication systems. This paper shows the possibility of using complementary codes for both decreasing the peak-to-average power (PAP) ratio and error correction. Set sizes and minimum distance properties of these codes are derived. It is shown that specific subsets of complementary codes have a minimum distance of up to half the code length, while their PAP ratio is only 3 dB. Simulation results demonstrate the viability of using these codes in multipath fading channels. It is currently planned to implement OFDM with complementary codes in the Wireless ATM Network Demonstrator (WAND), a joint European ACTS program.

A subspace method for signature waveform estimation in synchronous CDMA systems

Abstract: Synchronous code-division multiple-access (CDMA) techniques possess intrinsic protection against co-channel interference due to orthogonal codes employed and thus, offers higher capacity than existing frequency-division multiple-access (FDMA) or time-division multiple-access (TDMA) systems. In the presence of multipath, however, each signal is subject to frequency-selective fading and the orthogonality condition does not necessarily hold leading to increased cross correlation. In these scenarios, multiuser detection need to be performed to suppress interference and recover the message symbols. To implement such a technique, explicit knowledge of the (nonorthogonal) signature waveforms of all users is required. We propose a blind estimation scheme that provides closed-form estimates of the signature waveforms by exploiting the structure information of the data output. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of the signature waveforms. Based on this observation, a multiple signal classification (MUSIC)-like algorithm is derived. Performance analysis of the new approach is also presented.

Modelling and equalization of rapidly fading channels

Abstract: Basis expansion ideas are employed in order to equalize frequency-selective, rapidly fading channels. For certain multipath fading channels (e.g. the mobile radio channel), the time variations of the coefficients can be modelled as a combination of a small number of complex exponentials, under the assumption of linearly changing path delays. Based on this observation, novel adaptive and decision feedback algorithms are derived which exploit such an explicit modelling of the channel's variations. The problem of estimating the frequencies of the exponentials is also addressed using second- and higher-order cyclic statistics. By integrating ideas from cyclostationary signal analysis, both batch and recursive methods are developed. Finally, some illustrative simulations are presented.

Effects of antenna directivity and polarization on indoor multipath propagation characteristics at 60 GHz

Abstract: In millimeter-wave indoor communications systems, the radiation patterns and polarizations of the antennas at base stations and remote terminals have a significant influence on channel characteristics. The work reported in this paper investigated the effects of the radiation patterns of the antennas at remote terminals on multipath propagation characteristics. These effects were investigated by indoor propagation measurements at 60 GHz conducted in a modern office room and by ray-tracing simulations based on geometrical optics. Multipath channel characteristics are compared in terms of impulse responses and their root-mean-square (rms) delay spreads for an omnidirectional antenna and for three directive antennas with different beam widths. From the results of measurements and ray-tracing simulations, the use of a directive antenna at the remote terminal is demonstrated to be an effective method of reducing the effects of multipath propagation. Further reduction in the multipath effects is found to be achieved by the use of circular polarization instead of linear polarization with the directive antennas.

Multistage linear receivers for DS-CDMA systems

Abstract: A new family of multistage low-complexity linear receivers for direct sequence code division multiple access (DS-CDMA) communications is introduced. The objective of the proposed design is to mitigate the effect of multiple access interference (MAI), the most significant limiting factor of user capacity in the conventional DS-CDMA channel. The receivers presented here employ joint detection of multiple users and therefore require knowledge of all the signature codes and their timing. In addition, for a multipath environment, reliable estimates of the received powers and phases are assumed available for maximal ratio RAKE combining. Each stage of the underlying design recreates the overall modulation, noiseless channel, and demodulation process. The outputs of these stages are then linearly combined. The combining weights can be chosen to implement different linear detectors, including the decorrelating and minimum mean square error (MMSE) detectors. In this paper, we focus on implementing the MMSE detector. Simulation results illustrate that significant performance gains can be achieved in both synchronous and asynchronous systems.

Cochannel signal processing system

Abstract: A method and apparatus for processing cochannel signals received at a sensor array (110) in a cumulant-based signal processing and separation engine to obtain a desired set of output signals (38) or parameters. For use in a signal recovery system, the output signals are recovered and separated versions of the originally transmitted cochannel signals. An important feature that distinguishes the cumulant-based system from other signal separation and recovery systems is that it generates an estimated generalized steering vector associated with each signal source, and representative of all received coherent signal components attributable to the source. This feature enables the invention to perform well in multipath conditions, by combining all coherent multipath components from the same source. In a receiver/transmitter system (316), the estimated generalized steering vectors associated with each source are used to generate transmit beamformer weight vectors that permit cochannel transmission to multiple user stations (310). The basic cumulant-based processing and separation engine can also be used in a variety of applications, such as high density recording, complex phase angle equalization, receiving systems with enhanced effective dynamic range, and signal separation in the presence of strong interference. Various embodiments and extensions of the basic cumulant-based system are disclosed.

Spatio-temporal coding for wireless communications

Abstract: A compact model is derived for the multiple-input multiple output (MIMO), dispersive, spatially selective wireless communication channel. The multivariate information capacity behavior is then analyzed for various input and output antenna configurations. For high SNR conditions, the MIMO channel can exhibit a capacity slope in bits per dB of power increase that is proportional to the minimum of the number multipath components, the number of input antennas, or the number of output antennas. The discrete matrix multitone (DMMT) is proposed as an asymptotically optimal, practical spatio-temporal coding structure. Experimental examples that support the theoretical results are presented.

Emerging applications of multirate signal processing and wavelets in digital communications

Abstract: Multirate systems and filter banks have traditionally played an important role in source coding and compression for contemporary communication applications, and many of the key design issues in such applications have been extensively explored. We review developments on the comparatively less explored role of multirate filter banks and wavelets in channel coding and modulation for some important classes of channels. Some representative examples of emerging potential applications are described. One involves the use of highly dispersive, broadband multirate systems for wireless multiuser communication (spread spectrum CDMA) in the presence of fading due to time-varying multipath. Another is the wavelet-based diversity strategy referred to as "fractal modulation" for use with unpredictable communication links and in broadcast applications with user-selectable quality of service. A final example involves multitone (multicarrier) modulation systems based on multirate filter banks and fast lapped transforms for use on channels subject to severe intersymbol and narrowband interference. Collectively, these constitute intriguing, interrelated paradigms within an increasingly broad and active area of research.

Strobe & Edge Correlator Multipath Mitigation for Code

Abstract: GPS multipath effects occur when reflected satellite signals (from surfaces around or under the antenna) interfere with the direct satellite signal. Multipath causes receivers to make inaccurate measurements, or even lose lock on the signal. Multipath conditions are always present. For high precision Diiferential GPS, multipath is usually the single largest source of error. Ashtech has developed three new correlator schemes which are now implemented in new receivers to satisfy the multipath mitigation objectives for all applications. Edge CorrelatorTM is an enhancement of AshtechB’s proven Super CIA technique for reducing multipath on code. Strobe CorrelatorTM is a totally new and revolutionary idea that uses a very simple hardware structure and onty one extra correlator for a better code tracking. Enhanced Strobe CorrelationTM also uses a very simple hardware structure and three extra correlators, and provides multipath mitigation for both code and carrier. The Strobe Correlation techniques (patent pending) provide the best muhipath mitigation available. Strobe Correlation consumes less than 1 watt of power, totally removes the effect of far multipath and substantially reduces the effect of near multipath. The paper focuses on Edge and Strobe Correlator techniques comparing them to the One-Chip Wide and Narrow Correlator technologies in a multipath mitigation perspective. Results will be shown for simulations and real-life applications.

Performance of wireless CDMA with M-ary orthogonal modulation and cell site antenna arrays

Abstract: An antenna array-based base station receiver structure for wireless direct-sequence code-division multiple-access (DS/CDMA) with M-ary orthogonal modulation is proposed. The base station uses an antenna array beamformer-RAKE structure with noncoherent equal gain combining. The receiver consists of a "front end" beamsteering processor feeding a conventional noncoherent RAKE combiner. The performance of the proposed receiver with closed loop power control in multipath fading channels is evaluated. Expressions for the system uncoded bit-error probability (BEP) as a function of the number of users, number of antennas, and the angle spread are derived for different power control scenarios. The system capacity in terms of number of users that can be supported for a given uncoded BEP is also evaluated. Analysis results show a performance improvement in terms of the system capacity due to the use of antenna arrays and the associated signal processing at the base station. In particular, analysis results show an increase in system capacity that is proportional to the number of antennas. They also show an additional performance improvement due to space diversity gain provided by the array for nonzero angle spreads.

Mitigation of multipath effects in global positioning system receivers

Abstract: A technique for minimizing or eliminating the effect of multipath signals in a receiver processing pseudorandom (PRN) code signals, such as in a global positioning system (EPS) receiver. The presence of multipath signals adversely affects both code measurements and carrier phase measurements of received PRN signals. One aspect of the invention provides for improved code tracking in the presence of multipath signals, by sampling the received code with a multipath mitigation window (MMW) (FIG. 25D) that results in a code error function (FIG. 25F) that reduces or eliminates the multipaht effects. The MMW, which may by any of a number of preferred waveforms (FIGS. 35b-35E), provides a code error function that varies in opposite directions from zero at a desired tracking point (402), but assumes a nearly zero value when the MMW is advanced from the tracking or synchronization point by more than a small fraction of a code chip. Because of this nearly zero code error value on the early side of the desired tracking point, delayed multipath signals will have a corresponding code error function that is nearly zero (FIG. 25F) at the desired tracking point of the directly received signals, and the multipath signals will, therefore, have little or no effect on the desired tracking point and on code synchronization. The effects of multipath signals on carrier phase measurements are minimized by sampling the received signals, together with their possible multipath components, before and immediately after code transitions (vectors A and B, respecitvely). The vector rellationship of the directly received (D) and multipath (M) signals is such that performing a vector average of the two types of samples (A and B) produces the directly received signal (D) and its correct phase, with many, if not all, of the multipath components (M) eliminated.

SNR-based multipath error correction for GPS differential phase

Abstract: Carrier phase multipath is currently the limiting error source for high precision Global Positioning System (GPS) applications such as attitude determination and short baseline surveying. Multipath is the corruption of the direct GPS signal by one or more signals reflected from the local surroundings. Multipath reflections affect both the carrier phase measured by the receiver and signal-to-noise ratio (SNR). A technique is described which uses the SNR information to correct multipath errors in differential phase observations. The potential of the technique to reduce multipath to almost the level of receiver noise was demonstrated in simulations. The effectiveness on real data was demonstrated with controlled static experiments. Small errors remained, predominantly from high frequency multipath. The low frequency multipath was virtually eliminated. The remaining high frequency receiver noise can be easily removed by smoothing or Kalman filtering.

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.

Signal correlation technique for a receiver of a spread spectrum signal including a pseudo-random noise code that reduces errors when a multipath signal is present

Abstract: A receiver of a radio frequency signal having a pseudo-random noise (PRN) code modulated on a carrier, and techniques of processing such a signal that are especially adapted for ranging applications. Such an application is in a global positioning system (GPS or GLONASS) receiver. Both of the receiver DLL code and PLL carrier loops include a loop component that senses an error in its main loop caused by the presence of a multipath signal. The main loop is continuously adjusted by this sensed error, thereby causing the loop to track more precisely and minimize the effect of the multipath signal. The result is a more accurate range measurement.

Modeling of nondirected wireless infrared channels

Abstract: We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and r.m.s. delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion.

Multicarrier CDMA with adaptive frequency hopping for mobile radio systems

Abstract: A modified multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) system has been proposed for use over slow multipath fading channels with frequency selectivity in the reverse link transmission of a cellular network. Instead of transmitting data substreams uniformly through subchannels, data substreams hop over subchannels with the hopping patterns adaptively adjusted to the channel fading characteristics. The problem of determining the optimal hopping pattern is formulated as a multiobjective optimization problem, for which an efficient algorithm, based on the water-filling (WF) principle, is designed to solve the problem practically. Simulation results show that the performance in terms of the average bit-error probability (BEP) (over all users) is better than that of single carrier RAKE receiver systems, conventional MC CDMA systems applying moderate error protection, or diversity systems with different combining techniques.

Geometrically based statistical channel model for macrocellular mobile environments

Abstract: We develop a statistical geometric propagation model for a macrocell mobile environment that provides the statistics of angle-of-arrival of the multipath components. This channel model assumes that each multipath component of the propagating signal undergoes only one bounce traveling from the transmitter to the receiver and that scattering objects are located uniformly within a circle around the mobile. This geometrically based single bounce macrocell (GBSBM) channel model provides three important parameters that characterize a channel: the power of the multipath components, the time-of-arrival (TOA) of the components, and the angle-of-arrival (AOA) of the components. Doppler spectra and fading envelopes obtained using the GBSBM model is compared with Clarke's (1968) model. The results show that the rate of fading at the base station is lower than at the mobile and the reduction in the Doppler spread at the station is dependent on the direction of motion of the mobile with respect to the base station and the radius of the scattering circle.

An Adaptive SNR-Based Carrier Phase Multipath Mitigation Technique

Abstract: This paper describes an improved technique that mit- igates specular multipath in GPS differential carrier phase measurements. It adaptively estimates the spec- tral parameters (frequency, amplitude, phase offset) of multipath in the associated SNR., and then constructs a profile of the multipath error in the carrier phase. A multipath correction is subsequently made by sub- tracting the profile from the actual phase measurement data. The technique is demonstrated on ground based ex- perimental data, as well as flight data from the atmo- spheric research satellite CRISTA-SPAS. Ground ex- periments were conducted on static platforms in severe multipath environments. Multipath was deliberately introduced by either strategic placement of reflectors or electronic injection. This allowed for some control over the strength and frequency of the multipath. The experimental methods are described in detail. Aver- aging the results from 43 ground and 18 flight data sets, the differential carrier phase multipath was re- duced by 47%. The complete results for both ground and flight tests are presented and are accompanied by discussions of individual cases.

Wireless infrared communication links using multi-beam transmitters and imaging receivers

Abstract: We discuss two modifications to the design of wireless infrared links that can yield dramatic performance improvements. In nondirected, non-line-of-sight (non-LOS) links, replacement of the diffuse transmitter by one that projects multiple narrow beams can reduce the path loss, typically increasing the link SNR by about 20 dB. In both non-directed, LOS and non-directed, non-LOS links, replacement of the non-imaging receiver by one employing an imaging light concentrator and a segmented photodetector can reduce the received ambient light noise and multipath distortion. This can yield SNR improvements of tens of decibels.

Optimal decorrelating receivers for DS-CDMA systems: a signal processing framework

Abstract: Code division multiple access (CDMA) schemes allow a number of asynchronous users to share a transmission medium with minimum cooperation among them. However, sophisticated signal processing algorithms are needed at the receiver to combat interference from other users and multipath effects. A discrete-time multirate formulation is introduced for asynchronous CDMA systems, which can incorporate multipath effects. This formulation reveals interesting links between CDMA receivers and array processing problems. In this framework, linear receivers are derived that can completely suppress multiuser interference (decorrelating receivers). A criterion is introduced, which guarantees the decorrelating property, while providing optimal solutions in the presence of noise. Parametric FIR designs as well as nonparametric solutions are delineated, and their performance is analyzed. The proposed receivers are resistant to near-far effects and do not require the estimation of the users' and noise powers.

A low-power CMOS chipset for spread spectrum communications

Abstract: In future personal communication services, high-bandwidth multimedia data will be delivered over wireless pico-cellular networks to high densities of users. The system described here is designed to simultaneously supply 1 Mb/s to up to 50 users in a single cell, requiring a system bandwidth in excess of 50 Mb/s. The use of spread-spectrum techniques, in particular direct sequence code-division multiple access (CDMA), provides a multiple-access strategy to maintain parallel, separate streams of real-time data to all users, and to reduce sensitivity to multipath, narrow band fades, and interference present in the radio environment. Three chips in standard digital 0.8 /spl mu/m CMOS technology implement the critical parts of a spread spectrum transceiver: the digital modulator; the analog receiver front-end and ADC; and the digital receiver baseband processor. The transmit system transmitter is based loosely on the US IS-95 digital cellular CDMA standard, but at more than an order of magnitude higher date rate.

GPS Network Design and Error Mitigation for Real-Time Continuous Array Monitoring Systems

Abstract: In this paper a near real-time (typically a baseline update once per hour, or once per day) continuous array system, with at least three dual-frequency GPS receivers connected to the IGS network, and many single frequency receivers, is proposed for monitoring applications covering a region of 50km radius, A weighted differential GPS method, appropriate for the suggested array design, can be implemented in order to eliminate or mitigate the orbit bias, including the effects of SA. A local area epoch-by-epoch and satellite-by-satellite ionospheric delay model, determined using dual-frequency observations, is used to correct single frequency observations. Tropospheric delay is taken into account by modelling it as a first order Gauss-Markov (or random walk) process with temporal correlations. Multipath elimination is achieved by using an Finite Impulse Response (FIR) lowpass filter for the local ionospheric delay model and multipath extraction using an FIR bandpass filter to account for the multipath in real-time. The biases dependent on the GPS receiver and antenna are also discussed and associated optimaI network designs for data processing are suggested. In addition, efficient and reliable ambiguity resolution, and automatic cycle slip detection and repair procedures will be incorporated. This system would be capable of millimeter horizontal accuracy and centimeter vertical accuracy, and is intended to address real-time applications for earthquake studies, and for volcano and engineering deformation monitoring.

Concatenated orthogonal/PN spreading sequences and their application to cellular DS-CDMA systems with integrated traffic

Abstract: In this paper, we investigate the application of the concatenated orthogonal/PN spreading scheme for a cellular direct sequence code-division multiple access (DS-CDMA) system with integrated traffic. The performance of the system is evaluated in terms of user capacity. In order to incorporate traffic with a wide range of source rates, line rates (adjusted data rates before spreading) have to be selected for transmission. For traffic with source rates higher than the line rate of concern, we propose the use of concatenated orthogonal/PN spreading sequences to subdivide a high rate stream into parallel line rate streams. Therefore, in this paper, we first analyze the properties of the concatenated orthogonal/PN spreading sequences. The results are used to evaluate their performance for homogenous voice traffic in various cellular mobile environments with multipath fading, lognormal shadowing, and path loss. Our results show that the proposed spreading scheme offers a significant improvement in the forward link capacity as compared to using the conventional nonconcatenated long PN sequence, especially if the multipath fading is Rician (e.g., microcellular and indoor picocellular systems). Incorporating the notion of line rate, we then evaluate the performance of a system with integrated voice and video traffic. Special emphasis is placed on the effect of line rate selection on the overall capacity which leads to the optimal selection of line rates.

Performance evaluation of experimental 50-Mb/s diffuse infrared wireless link using on-off keying with decision-feedback equalization

Abstract: We report an experimental nondirected optical link for short-range, indoor data transmission at 50 Mb/s. The system uses on-off keying (OOK) and achieves low bit-error rates (BERs) in the presence of intersymbol interference, background light noise, and shadowing, with a range of 2.9 m in a skylit room. The transmitter produces an eye-safe Lambertian pattern at 806 nm with an average power of 474 mW. The receiver utilizes a hemispherical concentrator with a hemispherical bandpass optical filter, a 1-cm/sup 2/ silicon p-i-n photodiode, and a high-impedance hybrid preamplifier to achieve a high signal-to-noise ratio (SNR). A high-pass filter is used to mitigate fluorescent light noise, with quantized feedback removing the resulting baseline wander. A decision-feedback equalizer provides resistance to intersymbol interference due to multipath. The system and its components are characterized, and compared to theory. We observe that decision-feedback equalization yields a reduction of multipath power penalties that is in good agreement with theory.

Combined multiuser detection and diversity reception for wireless CDMA systems

Abstract: Code division multiple-access (CDMA) techniques using interference cancellation are being explored for the capacity increase in third-generation universal mobile telecommunications systems. However, multipath fading is a major constraint on the performance of wireless CDMA systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near far problem. Multiuser detection, exploiting the knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelating detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference.