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To achieve space-air-ground-sea integrated communication networks for future sixth generation (6G) communications, unmanned aerial vehicle (UAV) communications applying to maritime scenarios serving as mobile base stations have recently attracted more attentions. The UAV-to-ship channel modeling is the fundamental for the system design, testing, and performance evaluation of UAV communication systems in maritime scenarios. In this paper, a novel non-stationary multi-mobility UAV-to-ship channel model is proposed, consisting of three kinds of components, i.e., the line-of-sight (LoS) component, the single-bounce (SB) components resulting from the fluctuation of sea water, and multi-bounce (MB) components introduced by the waveguide effect over the sea surface. In the proposed model, the UAV as the transmitter (Tx), the ship as the receiver (Rx), and the clusters between the Tx and Rx, can be seen as moving with arbitrary velocities and arbitrary directions. Then, some typical statistical properties of the proposed UAV-to-ship channel model, including the temporal autocorrelation function (ACF), spatial cross-correlation function (CCF), Doppler power spectrum density (PSD), delay PSD, angular PSD, stationary interval, and root mean square (RMS) delay spread, are derived and investigated. Finally, by comparing with the available measurement data, the accuracy of proposed channel model is validated.

Most millimeter wave (mmWave) channel measurements are conducted with different configurations, which may have large impacts on propagation channel characteristics. In addition, the comparison of different mmWave bands is scarce. Moreover, mmWave massive multiple-input multiple-output (MIMO) channel measurements are absent, and new propagation properties caused by large antenna arrays have rarely been studied yet. In this paper, we carry out mmWave massive MIMO channel measurements at 11-, 16-, 28-, and 38-GHz bands in indoor environments. The space-alternating generalized expectation-maximization algorithm is applied to process the measurement data. Important statistical properties, such as average power delay profile, power azimuth profile, power elevation profile, root mean square delay spread, azimuth angular spread, elevation angular spread, and their cumulative distribution functions and correlation properties, are obtained and compared for different bands. New massive MIMO propagation properties, such as spherical wavefront, cluster birth-death, and non-stationarity over the antenna array, are validated for the four mmWave bands by investigating the variations of channel parameters. Two channel models are used to verify the measurements. The results indicate that massive MIMO effects should be fully characterized for mmWave massive MIMO systems.

The millimeter wave (mmWave) communications and massive multiple-input multiple-output (MIMO) are both widely considered to be the candidate technologies for the fifth generation mobile communication system. It is thus a good idea to combine these two technologies to achieve a better performance for large capacity and high data-rate transmission. However, one of the fundamental challenges is the characterization of mmWave massive MIMO channel. Most of the previous investigations in mmWave channel only focus on single-input single-output links or MIMO links, whereas the research of massive MIMO channels mainly focus on a frequency band below 6 GHz. This paper investigates the channel behaviors of massive MIMO at a mmWave frequency band around 26 GHz. An indoor mmWave massive MIMO channel measurement campaign with 64 and 128 array elements is conducted, based on which, path loss, shadow fading, root-mean-square (RMS) delay spread, and coherence bandwidth are extracted. Then, by using our developed ray-tracing simulator calibrated by the measurement data, we make the extensive ray-tracing simulations with 1024 antenna elements in the same indoor scenario, and get insights into the variation tendency of mean delay and the RMS delay with different array elements. It is observed that the measurement and the ray-tracing-based simulation results have reached a good agreement.

This paper presents a novel ultrawideband wireless spread spectrum millimeter-wave (mmWave) channel sounder that supports both a wideband sliding correlator mode and a realtime spread spectrum mode, also known as wideband correlation or direct correlation. Both channel sounder modes are capable of absolute propagation delay (time of flight) measurements with up to 1 GHz of radio frequency null-to-null bandwidth, and can measure multipath with a 2-ns time resolution. The sliding correlator configuration facilitates long-distance measurements with angular spread and delay spread for up to 185 dB of maximum measurable path loss. The real-time spread spectrum mode is shown to support short-range, small-scale temporal, and Doppler measurements (minimum snapshot sampling interval of 32.753 μs) with a substantial dynamic fading range of 40 dB for human blockage and dynamic urban scenarios. The channel sounder uses field programmable gate arrays, analog-to-digital converters, digital-to-analog converters, and low-phase-noise rubidium standard references for frequency/time synchronization and absolute time delay measurements. Using propagation theory, several methods are presented here to calibrate and verify the accuracy of the channel sounder, and an improved diffraction model for human blockage, based on the METIS model but now including directional antenna gains, is developed from measurements using the channel sounder. The mmWave channel sounder described here may be used for accurate spatial and temporal raytracing calibration, to identify individual multipath components, to measure antenna patterns, for constructing spatial profiles of mmWave channels, and for developing statistical channel impulse response models in time and space.

Time domain synchronous OFDM (TDS-OFDM) has higher spectral efficiency and faster synchronization than standard cyclic prefix OFDM (CP-OFDM), but suffers from the difficulty of supporting 256QAM in low-speed vehicular channels with long delay spread and the performance loss over fast time-varying vehicular channels. This paper addresses how to efficiently use the compressive sensing (CS) theory to solve those problems. First, we break through the conventional concept of cancelling the interferences if present, and propose the idea of using the inter-block-interference (IBI)-free region of small size to reconstruct the high-dimensional sparse multipath channel, whereby no interference cancellation is required any more. In this way, without changing the current signal structure of TDS-OFDM at the transmitter, the mutually conditional time-domain channel estimation and frequency-domain data detection in conventional TDS-OFDM receivers can be decoupled. Second, we propose the parameterized channel estimation method based on priori aided compressive sampling matching pursuit (PA-CoSaMP) algorithm to achieve reliable performance over vehicular channels, whereby partial channel priori available in TDS-OFDM is used to improve the performance and reduce the complexity of the classical CoSaMP signal recovery algorithm. Simulation results demonstrate that the proposed scheme can support the 256QAM and gain improved performance over fast fading channels.

This paper presents a novel and practical study on the position-based radio propagation channel for High-Speed Railway by performing extensive measurements at 2.35 GHz in China. The specification on the path loss model is developed. In particular, small scale fading properties such as K-factor, Doppler frequency feature and time delay spread are parameterized, which show dynamic variances depending on the train location and the transceiver separation. Finally, the statistical position-based channel models are firstly established to characterize the High-Speed Railway channel, which significantly promotes the evaluation and verification of wireless communications in relative scenarios.

In order to realize a vehicle area network for multimedia entertainment systems, which demand capacities of up to 1 Gbps, and vehicle control links between many parts of the car, which demand high reliability and low latency, this paper proposes a millimeter-wave system that offers high capacity and low latency transmission. Propagation measurement results show the maximum delay spread of 6 - 7 ns can be achieved with omni antennas without complicated equalizers. Feasible advances in beam forming antenna performance will reduce the maximum delay spread inside the car to only 1 - 2 ns. This paper proves that millimeter-wave vehicle area networks can support multimedia communications as well as control links by employing a basic single carrier modem with simple rake receivers; no equalization is needed.

In this paper we propose a novel method for communication over underwater acoustic channels that exhibit simultaneously large delay spread and Doppler spread, such as those found in the surf zone. In particular, we propose a coded pulse-shaped multicarrier scheme that converts the doubly dispersive channel into an inter-carrier interference (ICI) channel with small ICI spread. The resulting ICI is mitigated using a soft noncoherent equalizer that leverages sparsity in the delay-power profile to generate near-optimal bit estimates with low complexity. The noncoherent equalizer uses a delay-power-profile estimate (rather than a channel estimate) which is obtained from pilots. Numerical simulations with surf-zone-like channels demonstrate performance close to genie-aided bounds.

Traditional synchronization techniques applied to impulse-radio ultra-wideband (UWB) result in prohibitively long acquisition times, due to the extremely large search space. Additionally, in dense multipath environments, there exist a larger number of cells within the uncertainty region that can lead to acquisition lock. Locking to an arbitrary multipath component may result in unacceptable performance for many applications (range error in positioning systems for example). In this paper, we present a modified framework for the analysis of UWB acquisition which accommodates multiple lock cells. The framework divides the acquisition process into two distinct phases. The two phases are termed "coarse" and "fine" acquisition. The coarse acquisition phase is a fast implementation of traditional serial search which takes advantage of the large number of cells which can terminate the search process. Fine acquisition exploits statistics derived from the first phase and the clustered nature of multipath arrivals to determine the earliest arriving path, even when it is severely attenuated. We show that the first phase provides a substantial improvement in mean acquisition time compared with traditional serial search and that the second phase provides robust estimation of the first arriving path in ranging applications.

The autocorrelation receiver (AcR) is a suboptimum, low-complexity receiver architecture particularly suited to ultra-wideband (UWB) communication systems. As the bit rate increases, interference among pulses due to multipath propagation causes serious impairments of the AcR's performance. To mitigate this effect, we propose an appropriate design of the chip code and of the delay hopping (DH) code. We provide conditions to be satisfied by the DH code in order to reduce the nonlinear intersymbol interference (ISI) and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. By extending the length of the chip code, we show that N/sub p/ transmitted pulses per symbol can be employed to suppress the average linear ISI of N/sub p/-1 previous symbols. Simulated results confirm the performance improvement in terms of bit-error rate. However, in previous work it has been shown that the noise power linearly increases with N/sub p/. Although a large number of pulses per symbol is favorable for ISI mitigation, we show that the transmission of a single pulse minimizes the probability of error, for bit rates lower than an upper bound depending on the channel root mean-square delay spread and on the noise power.

This paper aims to lay a solid foundation for the application of ultra-wideband (UWB) radio in vehicle environments by exploring the characteristics of UWB channels within a vehicle. A comprehensive measurement campaign was conducted to gather a set of channel impulse responses of UWB links within and outside an armored vehicle. Based on the experimental data, the channel's temporal characteristics, path loss, and capacities are analyzed. The distributions of delay spread and amplitudes of multipath signals are also studied, showing correspondence with the IEEE 802.15.3a UWB channel model. The UWB channels within the vehicle are then modeled by extracting relevant cluster and ray arrival rates and cluster and ray decay factors.

Ultra-wideband indoor line-of-sight frequency-domain channel measurements have been performed in the 2-6 GHz frequency band using three different transmit/receive antenna combination pairs. The effects of antenna directivity on path loss and multipath propagation in the channel are analyzed extensively for various omnidirectional and directional antenna combinations. A statistical model of the path loss in the channel is presented, in which all path loss exponents result in better than free space loss. The shadow fading experienced in the channel is shown to highly agree with the normal probability assumption. There is a significant reduction in root mean square delay spread when directional antennas are used at the transmitter and receiver or solely at the receiver with respect to an omnidirectional/omnidirectional antenna pair. Finally, a distance-dependent statistical impulse response model of the channel is presented along with simulation results that significantly concur with the measured data.

Multicarrier code-division multiple access (MC-CDMA) is a promising multiplexing technique for future mobile radio systems, which require multirate transmission capability to efficiently support various services with wide range of data rates. In this paper, a generic performance analysis for the multicode and variable spreading factor multirate schemes in MC-CDMA is presented. Results comparing and pointing out notable differences in the error rate performance of these multirate schemes in conjunction with six different combining techniques are presented for the synchronous downlink and both the synchronous and asynchronous uplink. Furthermore, the analysis takes into account also the intersymbol interference caused by the multipath delay components exceeding the guard interval, which is commonly omitted in the literature.

Fourth-generation (4G) systems are expected to support data rates of the order of 100 Mb/s in the outdoor environment and 1 Gb/s in the indoor/stationary environment. In order to support such large payloads, the radio physical layer must employ receiver algorithms that provide a significant increase in spectrum efficiency (and, hence, capacity) over current wireless systems. Recently, an explosion of multiple-input-multiple-output (MIMO) studies have appeared with many journals presenting special issues on this subject. This has occurred due to the potential of MIMO to provide a linear increase in capacity with antenna numbers. Environmental considerations and tower loads will often restrict the placing of large antenna spans on base stations (BSs). Similarly, customer device form factors also place a limit on the antenna numbers that can be placed with a mutual spacing of 0.5 wavelength. The use of cross-polarized antennas is widely used in modern cellular installations as it reduces spacing needs and tower loads on BSs. Hence, this approach is also receiving considerable attention in MIMO systems. In order to study and compare various receiver architectures that are based on MIMO techniques, one needs to have an accurate knowledge of the MIMO channel. However, very few studies have appeared that characterize the cross-polarized MIMO channel. Recently, the third-generation partnership standards bodies (3GPP/3GPP2) have defined a cross-polarized channel model for MIMO systems but this model neglects the elevation spectrum. In this paper, we provide a deeper understanding of the channel model for cross-polarized systems for different environments and propose a composite channel impulse model for the cross-polarized channel that takes into account both azimuth and elevation spectrum. We use the resulting channel impulse response to derive closed-form expressions for the spatial correlation. We also present models to describe the dependence of cross-polarization discrimination (XPD) on distance, azimuth and elevation and delay spread. In addition, we study the impact of array width, signal-to-noise ratio, and antenna slant angle on the mutual information (MI) of the system. In particular, we present an analytical model for large system mean mutual information values and consider the impact of elevation spectrum on MI. Finally, the impact of multipath delays on XPD and MI is also explored. © 2006 IEEE.

A novel stochastic channel model for the indoor propagation channel is presented. It is especially for, but not limited to future communication systems with multiple antennas like space division multiple access (SDMA), spatial filtering for interference reduction (SFIR), or multiple-input/multiple-output (MIMO). The model is designed for indoor scenarios, straight forward extendable to urban environments. It is based on physical wave propagation. The new approach describes the channel by multipath components, each characterized by its transfer matrix (including loss), delay, direction of arrival, and departure. The appearance and disappearance of multipath components over time is modeled as a birth and death process, a marked Poisson process. This enables first-time the correct modeling of spatial and temporal correlations. In each modeling step, path properties change according to the motion of transmitter and receiver. The changing delay times of propagation paths yield a realistic Doppler behavior of the channel. Deterministic ray tracing results are used to produce the huge data sets required for the statistical evaluation of the parameters of the proposed model. This method enables an automated parameter extraction for new environments or frequencies. The ray tracing tool has been verified by narrowband, wideband, and directional channel measurements. The novel stochastic spatial channel model allows the simulation of third-generation broadband radio systems including arbitrary antenna configurations and patterns. System simulations for the bit-error rate of radio links can be performed including intelligent antenna configurations like SDMA, SFIR, or MIMO. Furthermore, the capacity of complete systems can be investigated.

A significant number of propagation and channel modeling papers have reported channel parameters like RMS delay spread, correlation bandwidth, and the Rician K factor, derived by various new methods from instantaneous, or snapshot measurements. This conflicts with the original definitions of these parameters, which formally should be derived from time averages, under an assumption of ergodicity and applied for the assessment of time-averaged link performance. It appears that the origins of fading channel characterization parameters, and the conditions under which they can be estimated and applied are now often ignored, leaving interpretation of new results subject to skepticism. This paper, therefore, provides a critical review of the estimation of frequency correlation and one of these parameters, correlation bandwidth, using channel impulse response estimates derived from propagation measurements. The practical application for knowledge of frequency correlation is summarized. Then, the derivation of the equation relating a channel's average power delay profile to its frequency correlation function via a Fourier transform is reviewed, with emphasis on conditions needed for validity. An alternate method, free from most of these conditions is also reported. Examples are given and comparisons are made of results from analyses using the two methods to estimate frequency correlation on Rayleigh and Rician mobile radio channels, which are shown to have significantly different frequency correlation characteristics. Finally, a measure of frequency correlation that is free from ambiguity concerning the value (e.g., 0.5, 0.75, or 1/e) at the correlation band edges is recommended.

This article presents measurement results and models for 60-GHz channels. Multipath components were resolved in time by using a sliding correlator with 10-ns resolution and in space by sweeping a directional antenna with 7/spl deg/ half power beamwidth in the azimuthal direction. Power delay profiles (PDPs) and power angle profiles (PAPs) were measured in various indoor and short-range outdoor environments. Detailed multipath structure was retrieved from PDPs and PAPs and was related to site-specific environments. Results show an excellent correlation between the propagation environments and the multipath channel structures. The measurement results confirm that the majority of the multipath components can be determined from image based ray tracing techniques for line-of-sight (LOS) applications. For non-LOS (NLOS) propagation through walls, the metallic structure of composite walls must be considered. From the recorded PDPs and PAPs, received signal power and statistical parameters of angle-of-arrival and time-of-arrival were also calculated. These parameters accurately describe the spatial and temporal properties of millimeter-wave channels and can be used as empirical values for broadband wireless system design for 60-GHz short-range channels.

In this paper, empirical channel models and parameters are derived from the wideband measured data at 5.3 GHz in outdoor mobile communications. The path loss exponents and intercepts are obtained by using the least square method. The mean excess delay and mean root-mean-square (rms) delay spread are within 29-102 ns and 22-88 ns, respectively. The correlation distances and bandwidths are within 1-11 /spl lambda/ and 1.2-11.5 MHz, respectively, when the envelope correlation coefficients equal 0.7 in line-of-sight cases. These correlation values depend strongly on the base station antenna heights. The window length for averaging out the fast fading components is about 1-2 m for microcells and picocells. The multipath number distributions follow both Poisson's and Gao's distributions, but Gao's distribution is better in the high probability region. Large excess delays up to 1.2 /spl mu/s and rms delay spread about 0.42 /spl mu/s are found in the urban rotation measurements, where the receiver is close to a large open square.

This paper provides the results of a complete study about the indoor radio propagation channel at 17 GHz. Wideband parameters, as coherence bandwidth or rms delay spread, and coverage are analyzed for the design of an OFDM-based broadband WLAN. A discussion of some adequate system parameters is also included. The report presents the characterization results as well as a previously developed indoor channel model; both of them have been checked and compared to bibliography results in order to verify their reliability This work was supported in part by the European Commission and in part by the Spanish National Plan projects WIND-FLEX (IST-1999-10025) and CICYT-2FD97-1066-C02-02 Publicado

Empirical results characterizing the joint statistical properties of the local azimuth spread (AS), the local delay spread (DS), and the shadow (slow) fading component are presented. Measurement data from typical urban, bad urban, and suburban (SU) environments have been analyzed. It is found that a log-normal distribution accurately fits the distribution function of all the investigated parameters. The spatial autocorrelation function of both AS, DS, and shadow fading can be modeled with an exponential decay function. However, for SU environments the spatial autocorrelation function is better characterized by a composite of two exponential decaying functions. A positive cross correlation is found between the AS and the DS, while both parameters are negatively correlated with shadow fading. All essential parameters required for the implementation of a simulation model considering the joint statistical properties of the AS, DS, and shadow fading are provided.

The design of advanced radio systems requires knowledge of higher order channel statistics, such as the time delay spread and the angle of arrival spread. Time delay has been measured in a number of cities, while angle of arrival has been measured in only a few. Since the link geometry is not the same for all measurements, it is not clear how to compare the measured values, or if they are applicable to other building environments, antenna height, etc. To clarify these issues, we have used a three-dimensional ray tracing code, called the vertical plane launch (VPL) method, to simulate time delay spread and angle of arrival spread in different building environments. Results for elevated base station antennas show that the distribution of root mean square (RMS) delay spreads is not sensitive to the statistical properties of the buildings but increases with distance to the mobiles and increases as the base antenna is lowered below the rooftops. In contrast, the distribution of RMS angle spread is sensitive to the distribution of building heights and to the height of the base station antenna but not to the distance to the mobile. The influence of street geometry on delay and angle spread for low base station antennas is also discussed.

This article presents results of a wide-band measurement campaign conducted at 38 GHz. The objective of the research was to determine multipath and time varying channel behavior of short-hop millimeter-wave point-to-multipoint radio links during various weather events. 73963 power delay profiles (PDPs) were captured on three links, each comparable to proposed local multipoint distribution systems (LMDS) in a campus environment. Multipath was observed in unobstructed LOS links during rain but not during clear weather. Short-term variation of the received signal over 1-2 min observation periods is described by a Rician distribution with a K factor which varies as a function of rain rate. Measured rain attenuation exceeds Crane's (1996) model predictions by several decibels. A novel prediction technique is presented that applies canonical antenna patterns and site specific information to estimate worst case multipath channel characteristics including relative power, time of arrival (TOA), and angle of arrival (AOA) of each multipath component. New metrics, the excess delay zone and relative power zone, are defined and contour plots are developed to determine potential reflectors from an area site map. These results and models provide useful guidelines for the design of millimeter-wave wireless communication systems.

Due to the site specific nature of microcellular operational environments, propagation models are required to take into account the exact position, orientation and electrical properties of individual buildings, and hence, ray-tracing techniques have emerged as the dominant methods to predict propagation in such environments. A novel hybrid three-dimensional (3-D) ray-tracing algorithm which can evaluate scenarios incorporating many thousands of objects by utilising the concept of “illumination zones,” is presented. In order to evaluate the accuracy of the presented model, comparisons of narrow-band and wide-band predictions with measurements are performed for a variety of scenarios. First, power comparisons show that very accurate predictions can be achieved (RMS errors less than 3.7 dB). Then, wide-band analysis shows that since the RMS delay spread for systems with finite bandwidth is a function of the multipath phase, only average measured and predicted RMS delay spread values can be compared and as a result, limited averaging can produce large RMS errors. With sufficient averaging the achieved wide-band accuracy in terms of the predicted RMS delay spread, is adequate for most planning purposes.

We investigate the effect of time delay spread on trellis coded modulation (TCM) in a wireless radio environment where equalization is not employed to mitigate the effects of frequency selective fading when the time delay spread is small. Using a random variable decomposition technique and a Gaussian approximation of the intersymbol interference terms, we obtain explicit bounds for the pairwise error probability of TCM over multipath Rayleigh fading channels characterized by various power delay profiles. A method to calculate an upper bound of the bit error rate (BER) based on Jamali and LeNgoc (1995) bound is also presented. These bounds are used to evaluate TCM performance as well as investigate the delay spread tolerance limit of TCM, including I-Q TCM, over frequency selective fading channels.

Noncoherent demodulators are very attractive for high performance radio LAN (HIPERLAN) systems because of their low implementation costs and their inherent robustness against frequency and carrier phase offsets. However, when the channel is time dispersive, the nonlinear intersymbol interference (ISI) introduced by these demodulators precludes the use of conventional linear equalization strategies. We present an alternative noncoherent receiver structure followed by a nonlinear equalizer, which includes a RAM and a Viterbi detector, capable of equalizing nonlinear multipath fading channels. In addition, we also present a new algorithm specifically for noncoherent demodulators, which allows estimation of all useful signal values at the input of the equalizer to be stored in the RAM. By means of computer simulations, we report the performance and computational complexity tradeoffs of the receiver/equalizer structure, including antenna diversity. We show that demodulators which consist of a complex receiver and a Viterbi detector are much more robust against multipath fading channels than traditional real noncoherent demodulators. The results suggest that in a typical HIPERLAN scenario, where the channel delay spread is less than 50 ns and a reliable line of sight component exists, it is feasible to combat multipath effects using noncoherent demodulation.

Spread spectrum (SS) multiple access techniques have been proposed for third generation broadband wireless access. We develop an analytical framework to quantify the effects of spreading bandwidth on SS systems operating in dense multipath environments in terms of the receiver performance, receiver complexity, and multipath channel parameters. In particular, we consider wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. The focus of the paper is to characterize the combined signal of the RAKE receiver fingers tracking the strongest multipath components. Closed form expressions for the mean and the variance of the total RAKE receiver output signal-to-noise ratio (SNR) are derived in terms of the number of RAKE fingers, spreading bandwidth, and multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain. A representative result indicates that for SS systems with 5 MHz signal bandwidth operating in a channel with constant power delay profile having 5 /spl mu/s spread, the average SNR gain from increasing the number of RAKE fingers from one to three is 3.8 dB and from three to five is 1.5 dB. Furthermore, the reduction in the variation of SNR is 1.1 dB and 0.4 dB for the same increments in the number of fingers.

Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimize the total transmit power. This is done by assigning each user a set of subcarriers and by determining the number of bits and the transmit power level for each subcarrier. We obtain the performance of our proposed algorithm in a multiuser frequency selective fading environment for various time delay spread values and various numbers of users. The results show that our proposed algorithm outperforms multiuser OFDM systems with static time-division multiple access (TDMA) or frequency-division multiple access (FDMA) techniques which employ fixed and predetermined time-slot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the bit-error rate (BER), or the area of coverage for a given outage probability.

This paper reports on experimental results and their interpretations for an indoor extremely high frequency (EHF) multipath channel. It is intended to help in establishing design guidelines for indoor wireless communications systems using millimeter waves. It deals with measurements obtained for the narrow and wideband indoor radio channels at 37.2 GHz within a typical concrete building at Laval University. Two kinds of transmission antennas, omnidirectional and directional, are used to investigate the propagation characteristics for the indoor channel. Under line-of-sight (LOS) conditions, the distance-power law exponent is found to be lower than the free space condition with walls playing a major role on the sustaining of high level signals. Large- and smallscale variations extracted from the original data are shown to follow log-normal and Rice distributions, respectively. The observed wideband impulse response has delay spread extending over a range up to 40 ns and a maximum root mean square (rms) value of about 16 ns. Both amplitude and phase behaviors of the signals are available for a better understanding of the various effects.

This paper presents the throughput and delay analysis of a packet-switched code division multiple access (CDMA) network based on the hybrid direct sequence (DS)/slow frequency hopping (SFH) spread-spectrum multiple access (SS MA) technique with Q-, B-, and D-PSK modulation using analytical and measured pico cellular channels. The performance of the hybrid DS/SFH, DS, and SFH multiple access techniques have been compared in a pico cellular personal communications network (PCN) environment. Multipath and multiple access interference are considered. The performance is evaluated for a given delay spread and a fixed bandwidth. The effects of forward error correction (FEC) coding and diversity techniques, such as selection diversity and maximal ratio combining on the performance, are also investigated.

The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading. >

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 /spl mu/s or less. >

The authors investigated experimentally, analytically, and by simulation the irreducible errors such as error floor for high signal-to-noise ratio that are introduced by time dispersion in a slowly fading Rayleigh channel. A multipath indoor propagation environment with controlled delay spread was created to validate this approach. The system is close to the Digital European Cordless Telecommunications (DECT) standard with 1 Mb/s GMSK-modulated signals being transmitted in FDM/TDMA/TDD format and received by a differential direct-conversion receiver. The measured and simulated error probability floor is Pe approximately=5*10/sup -3/ as a consequence of delay spread that is 10% of the bit length. This value is approximated by a closed-form analytical description of phase jitter due to large group delay bursts. Simulation results obtained previously by others for comparable cases are partially corroborated and partially disproved. >

An accurate software/hardware bit-by-bit error simulator for mobile radio communications is described. Simulation results in indoor and outdoor channels are compared with theoretical results. Bit error rate (BER) results in simulated frequency-selective fading channels generated by several channel models such as two-ray, constant amplitude, and simulated indoor radio channel impulse models (SIRCIMs) are presented. It is shown that BER is not only dependent on the RMS delay spread, but also on the distribution of temporal and spatial multipath components in local areas. An important result is that a two-ray Rayleigh fading model is a poor fit for indoor wireless channels and, if used, can underestimate the BER by orders of magnitude. A real-time bit error simulation of video transmission using the bit-by-bit error simulator is described. The simulator, called BERSIM, is shown to be a useful tool for evaluating emerging data transmission products for digital mobile communications. >

The authors report experimental and modeling studies that investigate the dependence of indoor radio channel multipath characteristics on a transmit/receive range. Specifically, a simple model for estimation of the complex baseband equivalent impulse response for indoor channels is explained. Using this model, the relationship between RMS delay spread and range on static indoor channels is estimated. Results show that this relationship is nonmonotonic and has a maximum at a range that depends on the building dimensions and the electrical properties of reflecting surfaces. The model is used in infinite, as well as finite, resolution modes to supplement the measurements of impulse response characteristics in different buildings using a limited resolution channel sounder. Experimental and modeling results are combined to derive conclusions that confirm the anticipated nonmonotonic relationship exists in empty buildings. Measurement results demonstrate that furniture has the effect of destroying this relationship, making RMS delay spread almost independent of range. >

If indoor radio propagation channels are modeled as linear filters, they can be characterized by reporting the parameters of their equivalent impulse response functions. The measurement and modeling of estimates for such functions in two different office buildings are reported. The resulting data base consists of 12000 impulse response estimates of the channel that were obtained by inverse Fourier transforming of the channel's transfer functions. It is shown that the number of multipath components in each impulse response estimate is a normally-distributed random variable with a mean value that increases with increasing antenna separations; a modified Poisson distribution shows a good fit to the arrival time of the multipath components; amplitudes are lognormally distributed over both local and global areas, with a log-mean value that decreases almost linearly with increasing excess delay; for small displacements of the receiving antenna, the amplitude of the multipath components are correlated; the amplitudes of adjacent multipath components of the same impulse response function show negligible correlations; and the RMS delay spread over large areas is normally distributed with mean values that increase with increasing antenna separation. >

Performance predictions for troposcatter channels with multipath are developed using the eigenvalues in the Karhunen-Loeve representation of the tropo signal. Flat Rayleigh fading predictions, in the presence of jamming, are easily extended to multipath channels with this technique. >

Describes a low-complexity equalization technique for improving the reliability of portable radio links in the presence of multipath time delay spread. A technique that operates on hard-limited received signals, with only the phase information available, is presented. Suboptimum receivers based on a maximum likelihood estimation criterion are discussed, including a sequence estimator and a decision feedback phase detector. A low-complexity adaptive phase equalizer structure using decision-directed phase tracking is proposed. The equalizer does not require the multiplication operations required in most conventional equalization algorithms. The author also proposes a receiver that includes a time-reversal structure and a joint estimator for optimum timing recovery and equalizer training. The time-reversal structure plays a crucial role in maximizing the compensation capability of the phase equalizer. The combined use of phase equalization and diversity reception is also considered. >

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10-4outage probability at a 10-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.

This paper discusses cross-pol cancellation for microwave radio system applications. A model for cross-pol interference coupling during multipath fades is postulated and some preliminary performance goals are presented. A design for an IF canceller consisting of cross-coupled adaptive tapped delay lines and a digital controller is presented and discussed. Predicted canceller performance as a function of tap spacing and number is discussed and compared to measured data. The design of a two-channel, dual-polarized lab fade simulator is presented, and methods to characterize canceller performance and fade parameters are also briefly discussed.

Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.

Time delay jitter in digital personal communications systems, caused by moving the antenna by a few wavelengths, was studied at two office buildings and two residences at a frequency of 850 MHz. A wide-band pseudonoice code technique was used. The results indicate that peak-to-peak time delay, variations on the order of 375 ns may be encountered. The worst case jitter was not necessarily found when there was a large time delay spread. It was also found that the excess mean delay and the root mean-square (rms) time delay spread of the delay distribution profiles were approximately the same.

Frequency reuse is an attractive way for increasing transmission capacity of digital radio systems. Propagation impairments such as multipath fading or antenna imperfections require the introduction of new techniques to fight against the degradation of crosspolarization discrimination. This paper presents the performance of new kinds of cross-polarization interference cancellers for a dual 16 QAM system over dispersive radio channels. The structure of the studied cancellers is a decision feedback one. Topics include a multipath propagation model, canceller description, and the theoretical and dynamic simulated performance of the receiver. For asynchronous clocks for the horizontally and vertically polarized channels, the results show that the receiver performance is critically sensitive to the timing delay between the main and the interfering signals.

To accurately predict the performance of broadcast teletext systems, one must know the characteristics of typical over-the-air broadcast television channels. Consequently, ensembles of typical complex impulse responses of broadcast teletext channels in the VHF and UHF frequency ranges are required for computer performance simulations. To circumvent the lack of comprehensive measured channel data, a theoretical multipath propagation model for suburban and urban environments was developed to gain insight into the nature of typical television multipath channels. In this model, both the direct and scatter paths consist of a line-of-sight and ground-reflected component. The basis of the propagation model is an expression for the ratio of the scattered path to direct path field strength caused by vertically oriented dielectric slabs of specified thickness, height, and width. The reflector location, orientation, width, height, and thickness are randomized for each Monte Carlo simulation cycle. A scatter table of relative amplitude versus multipath delay is maintained for subsequent analysis and formation of typical multipath channel impulse responses. A perceived desired-to-undesired ratio (PDUR), which is essentially a signal-to-clutter ratio, is computed to eliminate channels of unacceptable video quality, as these would be of no interest to the teletext application. A modification to account for near-field scattering is described. Finally, mobile channel applications are discussed.

Multipath propagation is a severe problem in conventional FM broadcasting since this phenomenon causes nonlinear distortions after demodulation. Several proposals for the correction of multipath corrupted FM signals were given in some recent papers, but up to now there exist no satisfactory solutions with sufficiently low hardware complexity for commercial application. The main idea of the present paper is to take advantage of some a priori knowledge of the multipath transfer function with the goal to develop a nonrecursive equalizer structure with minimum hardware expense. For adaptive adjustment of the equalizer, we use a recently introduced constant modulus algorithm (CMA) in a modified form. Instead of direct coefficient adjustment, we regard the parameters of the transmission channel as unknowns which are updated adaptively. The equalizer coefficients are uniquely defined by the channel parameters and can be determined in one step. The advantage of this method is a large improvement of the convergence behavior in comparison to existing solutions. The methods presented here are tested by means of an FM hardware system including FM transmitter, multipath simulation, FM demodulator, and an FIR multipath equalizer, which allows real-time investigations of the equalizer performance under well-defined multipath conditions.

The results of indoor multipath propagation measurements using 10 ns, 1.5 GHz, radarlike pulses are presented for a medium-size office building. The observed channel was very slowly time varying, with the delay spread extending over a range up to about 200 ns and rms values of up to about 50 ns. The attenuation varied over a 60 dB dynamic range. A simple statistical multipath model of the indoor radio channel is also presented, which fits our measurements well, and more importantly, appears to be extendable to other buildings. With this model, the received signal rays arrive in clusters. The rays have independent uniform phases, and independent Rayleigh amplitudes with variances that decay exponentially with cluster and ray delays. The clusters, and the rays within the cluster, form Poisson arrival processes with different, but fixed, rates. The clusters are formed by the building superstructure, while the individual rays are formed by objects in the vicinities of the transmitter and the receiver.