Showing papers on "Fading published in 1987"

Antenna diversity in mobile communications

Abstract: The conditions for antenna diversity action are investigated. In terms of the fields, a condition is shown to be that the incident field and the far field of the diversity antenna should obey (or nearly obey) an orthogonality relationship. The role of mutual coupling is central, and it is different from that in a conventional array antenna. In terms of antenna parameters, a sufficient condition for diversity action for a certain class of high gain antennas at the mobile, which approximates most practical mobile antennas, is shown to be zero (or low) mutual resistance between elements. This is not the case at the base station, where the condition is necessary only. The mutual resistance condition offers a powerful design tool, and examples of new mobile diversity antennas are discussed along with some existing designs.

Capacity of Slotted ALOHA in Rayleigh-Fading Channels

Abstract: Fading channels may improve the capacity of slottedALOHA networks. Rayleigh fading and spatially distributed packet transmitters diminish mutual packet interference and thus reduce the average blocking probability of a shared receiver. A transmission model, combining Poisson statistics of the offered data traffic with Rayleigh statistics for the fading channel, is developed. Analytical and numerical results are presented, indicating significant improvements to signaling in cellular mobile networks and in packet radio systems with fading radio channels.

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Abstract: 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.

Fading Channel Issues in System Engineering

Abstract: The character of multipath-induced propagational fading is reviewed, along with the interpretations underlying use of the Rayleigh fading model to describe the process statistics. The relationship between this model and contemporary laboratory fading simulators is also outlined. The effects of the fading upon data communications are described, along with the techniques of modulation, diversity, coding, and adaptive equalization used in modern modem designs for operation over such channels. Finally, the system engineering problems are discussed of attempting to provide quantitative estimates of long-term link or network performance that takes into account the longer term channel variabilities.

Trellis Coded Modulation for 4800-9600 bits/s Transmission Over a Fading Mobile Satellite Channel

Abstract: The combination of trellis coding and MPSK signaling with the addition of asymmetry to the signal set is discussed with regard to its suitabllity as a modulation/coding scheme for the fading mobile satellite channel. For MPSK, introducing nonuniformity (asymmetry) into the spacing between signal points in the constellation buys a further improvement in performance over that achievable with trellis coded symmetric MPSK, all this without increasing the average or peak power, or changing the bandwidth constraints imposed on the system. Whereas previous contributions have considered the performance of trellis coded modulation transmitted over an additive white Gaussian noise (AWGN) channel, the emphasis in this paper is on the performance of trellis coded MPSK in the fading environment. The results will be obtained by using a combination of analysis and simulation. It will be assumed that the effect of the fading on the phase of the received signal is fully compensated for either by tracking it with some form of phase-locked loop or with pilot tone calibration techniques. Thus, our results will only reflect the degradation due to the effect of the fading on the amplitude of the received signal. Also, we shall consider only the case where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. Finally, the impact of the availability of channel state information on average bit error probability performance is assessed.

Highly Efficient Digital Mobile Communications with a Linear Modulation Method

Abstract: Although linear modulation methods can achieve high spectrum efficiency, very little attention has been directed to their use in mobile radio systems. This is mainly due to the fact that the nonlinearity of the transmitter power amplifier tends to spread the spectrum and thus eliminate any spectrum efficiency advantage gained through the use of linear modulation methods. In this paper, a linear modulation system is proposed, which solves the above difficulty and which gives higher spectrum efficiency than conventional digital FM. The modulation/demodulation method is \pi /4 shift QPSK and phase-shift detection with a limiter-discriminator and an integrate-and-dump filter. By introducing a cartesian coordinate negative feedback control, 35 percent power efficiency at 10 W output power and - 60 dB relative out-of-band radiation are simultaneously achieved with a class "AB" amplifier, owing to the 29 dB feedback gain. The receiver configuration is easy to realize and gives immunity against fast fading through the use of noncoherent detection with limiter-discriminator. By using a novel decision method, bit error rate performances under both nonfading and fading condition are comparable to those obtained by digital FM. These results make it possible for linear modulation methods to achieve higher spectrum efficiency than is possible with conventional digital FM methods in mobile radio communications.

Macroscopic Diversity in Frequency Reuse Radio Systems

Abstract: Macroscopic diversity is a technique that can facilitate high quality and ubiquitous communications between low-power portable radiotelephones and data terminals, and radio base stations (ports) that are connected to the local network. It uses radio signals from several base stations to mitigate the effect of shadow fading, a variation of signal strength over space created by the presence of buildings, foliage, and terrain variations. With a path loss exponent of four and a shadow fading standard deviation of 10 dB, four-branch macroscopic diversity results in a 13 dB improvement in signal strength and a 15 dB improvement in signal to cochannel interference ratio for high user capacity interference-limited operation. (Both figures are for 99 percent statistical coverge of the service area.) The improvement in signal to cochannel interference ratio is equivalent to a factor-of-five savings of spectrum.

Apparatus and method for transmitting digital data over a radio communications channel

Abstract: A digital radio RF signalling protocol communicates either digitized voice or digital data signals and indicates the type of information being communicated to the receiver. Multiple repeats of control signals are provided for face protection and to ensure correct receipt even under adverse communications channel conditions. When the destination transceiver receives a transmitted digital data burst from the transmitter, the destination transceiver transmits a responsive acknowledge message and may specify in the acknowledge message which portions of the received message were received incorrectly and should be retransmitted by the originating transceiver. The originating transceiver determines the contents of subsequently-transmitted data burst based upon the information it receives from the destination transceiver by way of the acknowledge message. As deleterious phenomena (e.g., noise and/or fading) degrade communcations channel quality, the acknowledge message handshaking causes the originating transceiver to repeat incorrectly received data packets until the destination transceiver finally receives those data packets correctly.

Forward Error Correction Coding for Fading Compensation in Mobile Satellite Channels

Abstract: Fading in mobile satellite communications severely degrades the performance of data transmission. The channel is modeled with nonfrequency selective Rice and Rayleigh fading. Also, stored channel simulation is used for hardware data transmission. FEC coding with Viterbi decoding of convolutional codes, and Berlekamp-Massey decoding of Reed-Solomon codes, are used to compensate for the fading. In addition to interleaving, channel state and erasure information improve the performance of the decoder. The BER after decoding is calculated for specific codes on several channels and for different transmission schemes. Using very simple channel state and erasure information gives 2-7 dB additional coding gain. These gains have been verified by hardware data transmission on synthetic fading channels and stored mobile satellite channels.

Mobile digital communications via tone calibration

Abstract: A new digital signaling technique that is particularly suited for channels impaired by multipath fading is presented. The proposed modulation scheme employs a continuous-wave (CW) tone to calibrate the mobile channel against the multipath-induced phase uncertainties. This technique is applicable to quaternary phase shift keying as well as to more complicated signal constellations such as M-ary phase shift keyed schemes. The advantages of tone calibration are: 1) robustness of the receiver and 2) elimination of the link dependent error floor. Furthermore, since the CW tone can also be used for coherent signal demodulation, carrier phase acquisition can be achieved within a bit time. This property is particularly attractive when a burst of data with a short burst length has to be detected. This radio technique is useful for both the terrestrial mobile and the newer satellite-aided mobile communication (SAMC) services.

Error Probability Evaluation for Systems Employing Differential Detection in a Rician Fast Fading Environment and Gaussian Noise

Abstract: A method is presented for determining the error probability of a receiver using differential detection in the presence of Gaussian noise and fast Rician fading. Equations for the covariances of the fading component are derived, which include the effect of IF filter distortion. It is shown that these equations may be readily evaluated numerically. A simple formula for the error probability is derived for systems using BPSK and a matched filter receiver. An example of the error probability is given using this receiver. Also given is an example of a system using MSK with a practical IF filter. Different spectral shapes and bandwidths for the fading process are investigated for this example and their effect on the error probability is determined.

Channel Modeling and Threshold Signal Processing in Underwater Acoustics: An Analytical Overview

Abstract: An overview of underwater acoustic channel modeling and threshold signal processing is presented, which emphasizes the inhomogeneous, random, and non-Ganssian nature of the generalized channel, combined with appropriate weak-signal detection and estimation. Principal attention is given to the formal structuring of the scattered and ambient acoustic noise fields, as well as that of the desired signal, including both fading and Doppler "smear" phenomena. The role of general receiving arrays is noted, as well as their impact on spatial and temporal signal processing and beam forming, as indicated by various performance measures in detection and estimation. The emphasis here is on limiting optimum threshold systems, with some attention to suboptimum cases. Specific first-order probability density functions (pdf's) for the non-Ganssian components of typical underwater acoustic noise environments are included along with their field covariances. Several examples incorporating these pdf's are given, to illustrate the applications and general methods involved. The fundamental role of the detector structure in determining the associated optimum estimators is noted: the estimators arc specific linear or nonlinear functionals of the original optimum detector algorithm, depending on the criterion (i.e., minimization of the chosen error or cost function) selected. Results for both coherent and incoherent modes of reception are presented, reflecting the fact that frequently signal epoch is not known initially at the receiver. To supplement the general discussion, a selected list of references is included, to provide direct access to specific detailed problems, techniques, and results, for which the present paper is only a guide.

Error Probablity for Optimal and Suboptimal Quadratic Receivers in Rapid Rayleigh Fading Channels

Abstract: The exact performance of optimal and suboptimal quadratic receivers in a binary hypothesis test between jointly distributed zero mean complex Gaussian variates is derived. The probability of error is given as a function of the characteristic values of a generalized eigenvalue problem set up in terms of the covariance matrix of the received signal-plus-noise and in the matrix of the quadratic form of the receiver. The results include the exact performance of various types of suboptimal receivers including those previously derived for the envelope matched filter for MFSK and the noncoherent DBPSK receiver in rapid Rayleigh fading, nonfrequency-selective channels. Also, the performance of near-optimal stationary process-long observation time [SPLOT] receivers, "energy detectors," and other approximately optimal receivers may be calculated for noncoherent signaling in the same channel.

Adaptive Equalization Techniques for HF Channels

Abstract: Data transmission at rates of 1.2 kbits/s or higher through voiceband ionospheric channels is subject to impairment from severe linear distortion, fast channel time variations, and severe fading. In this paper, we have focused on the performance of DFE (decision feedback equalization) receivers for communication over 3 kHz bandwidth HF channels. We describe the results of simulations for a wide range of fading rates on simulated and real recorded HF channels, using fractionally spaced DFE receivers. Both LMS (least mean square) and FRLS (fast recursive least squares) adaptation algorithms with periodic restart were evaluated, and both ideal-reference and decision-directed operation was observed. The results indicate that FRLS adaptation yields superior performance to LMS in rapid fading conditions, but that this performance advantage diminishes at low signal-to-noise ratios. Also, fade rates greater than about 1 Hz produced relatively high error rates, irrespective of which adaptation method was employed. Finally, a novel modification of the simple LMS algorithm which improves its tracking ability was evaluated. This involved preceding the LMS DFE receiver with an adaptive lattice whitening filter.

The fading rates of some natural dyes

Abstract: The fading rate curves of selected natural dyes were constructed from color difference measurements made using a tristimulus colorimeter. Examination of the curves showed that most natural dyes fade rapidly initially followed by a slower rate of fading. Only the most lightfast natural dyes fade at a constant rate over time. The implications of these findings for museum textiles are discussed.

Diversity Combining for Channels with Fading and Partial-Band Interference

Abstract: Diversity combining techniques that employ a ratiothreshold test (RTT) are suggested for communications with fading and partial-band interference. We consider a system with binary orthogonal signaling and noncoherent demodulation. The fading channel is modeled as a group of independent narrow-band channels, each with nonselective Rician fading. We assume that the partial-band interference is Gaussian, and we include additive white Gaussian quiescent noise in the analysis to account for wide-band noise sources. The performance measures we use to evaluate the diversity combining schemes are the narrow-band interference rejection capability and the signal-to-noise ratio requirement over the entire range of partial-band interference duty factors. The performances of the RTT with square-law combining and the RTT with majority logic decoding are compared to each other and to the performance of the optimum diversity combining technique.

Direct-Sequence Spread-Spectrum Receiver for Communication on Frequency-Selective Fading Channels

Abstract: The reception of direct-sequence spread-spectrum signals on frequency-selective fading communication channels is considered. The fading statistics are described using the wide-sense-stationary uncorrelated-scattering (WSSUS) channel model. It is shown that, under certain assumptions about this channel such as time-invariance over the duration of a data symbol, an orthogonal representation for the received distorted signal can be found. The optimum incoherent receiver can then be realized with reasonable complexity. The analysis shows that exploiting the inherent diversity of a frequency-selective channel can reduce the receiver error probability by several orders of magnitude. The optimum selective channel and the jamming susceptibility of the receiver are discussed.

Measurements and models of a land mobile satellite channel and their applications to MSK signals

Abstract: The fading and shadowing effects observed on land mobile satellite signals are characterized in statistical terms. Models are developed in terms of probability distribution of the signal's envelope and phase as well as its rate of change with time. The multipath fading and shadowing effects modelled are typical of those encountered in a mobile-satellite link in rural and suburban areas. A comparison of the models with experimental data at 870 MHz and 1542 MHz is given. Applications of the models to predict performance of a minimum shift keying (MSK) signal at 2400 bits/s show that large margins are required to compensate for the effects of fading and shadowing. In addition, the results show that conventional coherent demodulation of a MSK signal may not be feasible due to phase variation caused by fading and shadowing. On the other hand, the results show that random FM has negligible effect on the probability of error of the MSK signal at 2400 bits/s when frequency demodulation is used.

Roadside tree attenuation measurements at UHF for land mobile satellite systems

Abstract: Tree attenuation results at 870 MHz are described for experiments conducted in October 1985 and March 1986 in Central Maryland. These experiments employed a helicopter as a source platform and a van with receiver and data acquisition instrumentation. Tree attenuation results were obtained for the cases in which the van was stationary and in motion. The experiments were performed for the purpose of providing the designers of planned land mobile satellite systems with important elements in the determination of link parameter requirements; namely, the expected fading statistics due to roadside trees for both mobile and stationary vehicles. Single tree attenuation results gave worst case median fades as high as 15 dB although roadside tree values were noted to produce fades in excess of 20 dB for small percentages of time. The cumulative fade distributions and their relative contributions as a function of path elevation angle, right side versus left side driving, and different road types are derived from the field measurements. Upon comparing the attenuations from bare deciduous trees (March 1986) with those due to trees in full foliage (October 1985), the increase in dB attenuations were, in general, less than 25 percent for the dynamic cases, and less than 40 percent for the worst case static configuration. This result demonstrates thai the dominant fading is caused by the wooded tree branches as opposed to the leaves on these branches. The tail end of the observed fade distributions was observed to follow lognormal distributions with respect to dB attenuation.

Channel Simulation to Facilitate Mobile-Satellite Communications Research

Abstract: In designing a mobile satellite network, engineers and technologists are faced with wide-ranging issues for which there is no prior database. System engineers must address such issues as adequate margin to combat multipath fading, the level of adjacent channel protection required to allow transmission in narrow-band channels, the cochannel protection required to allow for frequency reuse in a multiplebeam system, and the level of intermodulation distortion tolerable for single-channel-per-carrier operation. Technologists, on the other hand, must determine the performance of various system components. For example, in the ground segment, modem and speech codec performance must be evaluated in the presence of thermal noise, fading, and other impairments. To enable system designers and technologists to optimize their design and pursue their research and development, the Jet Propulsion Laboratory has developed a mobile satellite channel simulator in hardware. This simulator has sufficient flexibility to facilitate many forms of tests and experiments.

A Wireless Network for Wide-Band Indoor Communications

Abstract: 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.

Logarithmic PCM weighted QAM transmission over Gaussian and Rayleigh fading channels

Abstract: The transmission of 8-bit μ-law PCM signals using M-level weighted QAM (WQAM), M =16, 64 and 256, over Gaussian and Rayleigh fading channels is examined. The weighting process modifies the positions of the QAM constellation points so that the overall distortion in the recovered information-bearing source signal is reduced. The PCM bits are mapped to the WQAM points such that the most significant bits have a lower probability of being in error than the least significant bits. Gray coding of the constellation points is also used. The WQAM systems have been optimised for the same average signal energy per transmitted symbol as for unweighted QAM. Optimised systems have also been derived for the same peak signal energy per symbol. The theoretical and simulation results using speech indicate that 16-level and 256-level WQAM have a gain of up to 3 and 5dB, respectively, over unweighted QAM for the Gaussian channel. The gains due to weighting for 16-level QAM operating over an ideal slow Rayleigh fading channel are up to 5 dB.

Analysis for spectrum efficiency in single cell trunked and cellular mobile radio

Abstract: Spectrum efficiencies for single-cell trunked and cellular mobile radio systems are analyzed, taking into consideration fading circumstances and call blocking probability. Two kinds of transmit power control methods are considered. One retains area mean power at the receiver constant and the other retains local mean power constant. For single-cell trunked systems, it has been shown how spectrum efficiency is limited by the required traffic quality or permitable adjacent channel interference. A narrower channel spaced system shows superiority to wide-band systems, if an adjacent channel interference protection ratio is lower than about -40 dB and a few percent interference probability is permitted. On comparing cellular systems, superiority in spectrum efficiency is determined by the product of protection ratio and channel spacing. Call blocking probability and interference have little effect on spectrum efficiency comparison. For both systems, the two kinds of transmit power control and the two interchannel interference criteria show no significant difference between their effects on spectrum efficiency.

Engineering Calculation of Overflow Probabilities in Buffers with Markov-Interrupted Service

Abstract: Interrupted service, which may occur in fading radio channels, in low-priority channels which can be preempted or in systems with failures may make severe demands on buffer size if overflow is to be avoided. This paper analyzes a buffer with a Markov-interrupted timeslotted server, generalizing earlier work on independent random interruptions. An equivalent service distribution is defined for use in an approximate M/G/1 model, which in turn gives buffer probabilities and overflow probabilities. For very small overflow probabilities, the necessary buffer size is found from a further analytic approximation to the tail of the buffer size distribution. The accuracy of the two approximations together is good, shown by an example of a fading radio channel.

An LMS Adaptive Array for Multipath Fading Reduction

Abstract: Multipath fading often poses a serious hindrance in radiocommunication. The application of a least-mean-square (LMS)adaptive array to the problem of multipath fading reduction is discussed. However, it is known that multipath components are in general correlated with one another. We examine the effect of the correlation on the performance of the LMS adaptive array. When the correlation coefficient does not equal or approximate 1, the LMS adaptive array suppresses the multipath signals significantly by nulling. On the other hand, when the correlation coefficient nearly equals 1, the LMS adaptive array prevents the output signalpower from decreasing. Therefore, the LMS adaptive array mayreduce the multipath fading effectively for any correlation coefficient value. A reference signal in the LMS adaptive array is also discussed. It is shown that synchronization in the referencesignal generation must be extremely accurate. Moreover, aprocessor configuration is proposed which may generate thereference signal with the required accuracy.

The Maritime Satellite Communication Channel--Channel Model, Performance of Modulation and Coding

Abstract: Towards the year 2000, maritime satellite communications using the INMARSAT system will employ a second and third generation of satellites and new ship earth stations (SES). The new SES standards will use very small antennas with gains between 0 and 15 dBi. At the lower end of SES there will be no antenna stabilization. The communication channel for such small stations is described by a model including multipath fading, Doppler shift, and noise. The results of an extensive measurement program were used to determine the parameters of the channel model, which depend on antenna type and elevation angle. Analytical calculations as well as synthetic and stored channel hardware simulations have been used to determine the performance of several modulation schemes. A complete data link using PSK modems with AFC/Costas loop, interleaving, and FEC codecs at 1.2 kbits/s was built up around a hardware maritime channel simulator, to study the performance of data transmission on the small SES maritime channel. Theoretical and measured results are given for interleaved Viterbi decoding with channel state information and Reed-Solomon codes. The measurements show that with interleaved FEC schemes, the required E_{b}/N_{o} for a BER 10-5is in the range of 9-15 dB and the effects of multipath fading are almost compensated for.

Correction to "Design and Experimental Results for a Direct-Sequence Spread-Spectrum Radio Using Differential Phase-Shift Keying Modulation for Indoor, Wireless Communications"

Abstract: We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.

The effects of multipath delay spread on timing recovery

Abstract: Equations for the recovered timing for a squaring timing recovery circuit under multipath radio propagation are derived. Both coherent and differential detections are studied. If delay spread is much smaller than the symbol duration, the recovered timing can be approximated by the centroid of the power delay profile, p(t). Two cases of timing loop bandwidth are considered. If the fading frequency is much lower than the bandwidth of the timing loop, the instantaneous sample of p(t) is used to generate the timing clock. If the fading frequency is much higher than the loop bandwidth, the ensemble average of p(t) over fading samples is used to recover the timing. A computer simulation is performed for a system operating in a frequency-selective, slowly fading environment. It is found that for root mean square (rms) delay spread less than or equal to 0.1 of the symbol duration, a squaring timing loop with either narrow or wide bandwidth can properly determine the timing detection. The main mechanism of the "irreducible bit error rate" in this case is the closure of the eye-pattern instead of timing error.

Coded FH/SS Communications in the Presence of Combined Partial-Band Noise Jamming, Rician Nonselective Fading, and Multiuser Interference

Abstract: In this paper we address the problem of combatting combined interference in spread-spectrum communication links. We consider frequency-hopped spread-spectrum systems with M -ary FSK modulation and noncoherent demodulation which employ forward-error-control coding. The interference consists of partial-band noise jamming, nonselective Rician fading, other-user interference, and thermal noise. The coding schemes which we analyze include: ReedSolomon codes (with or without diversity and error-only, erasure-only, or parallel erasure/error decoding), binary, nonbinary, and dual- k convolutional codes with and without side information (information about the state of the channel), and concatenated schemes (Reed-Solomon outer codes with either inner detection-only block codes or inner convolutional codes). In all cases we derive 1) the minimum signal-to-jammer energy ratio required to guarantee a desirable bit error rate as a function of ρ, the fraction of the band which is jammed, when the number of interfering users is fixed; and 2) the maximum number of users that can be supported by the system as a function of ρ, when the signal-to-jammer energy ratio is fixed.

System applications for wireless indoor communications

Abstract: Buildings present a hostile environment for radio communications, with in-building radio propagation difficult to predict and continuously changing. W ireless indoor communications can be used in a bride range of applications to provide for user mobility, easy setup, and reconfiguration of terminals, telephones, and so forth. Such applications can be divided $#to two classes: (1) Narrowband applications, such as paging or wireless PBX systems for voice and low rate data and (2) Wideband applications, such as a wireless local area network capable of handling data rates u p to several Megabits per second. Buildings, however, present a hostile environment for radio communications, with in-building radio propagation difficult to predict and continuously changing. Problems include multipath and shadow fading, which reduce the coverage region, and dispersion due to delay spread, which limits the maximum data rate. Furthermore, in buildings with multiple users, interference among users can be a serious problem, and the available spectrum might be much less than that required to handle all potential users. In this article we describe various antenna diversity techniques, including selection, maximal ratio and optimum combining, and show how diversity, in combination with other techniques, can be used to overcome these problems. One such technique is resource sharing, which recognizes that, at any moment, only ;I small fraction of all users are experiencing transmission difficulties, and assigns extra system resources (for example, bandwidth or time slots) as needed to protect those users.\" We first briefly describe the indoor radio environment and show why diversity can be effective. We then describe techniques for narrowband applications , such as wireless PBX's, that can be used to increase the coverage region, eliminate interference problems, and permit frequency reuse within the same coverage region, thereby dramatically increasing system capacity. Finally, we describe techniques for wide-band applications, such as wireless local area networks , that can be used to increase both the maximum data rate and the coverage region. With indoor radio communication, there is rarely a line of sight between the transmitter and receiver, and multiple signal propagation paths exist. The signals from these paths combine both constructively and destructively at the receiver to produce multipath fading. For narrowband transmission, where the propagation delays associated with the various paths are extremely small compared to the inverse of the signal bandwidth, the channel can be considered as a Rayleigh fading channel with frequency flat fading [ 141. This Rayleigh fading …