Dispersive Fading Research Articles

An adaptive receiver for the time- and frequency-selective fading channel

An adaptive receiver is presented in this paper for the reception of linearly modulated signals transmitted over a time- and frequency-selective fading channel. The channel is modeled as a truncated power series which represents the dispersive fading channel as a sum of three elementary flat-fading channels. The proposed receiver consists of a sequence estimator with a parallel channel estimator. The channel estimator recovers the instantaneous fading processes associated with each elementary channel and filters them to generate one-step predictions of each fading process. Some implementation difficulties and solutions are also discussed. Computer simulations using quadrature phase-shift keying (QPSK) and channels with moderate delay spreads and fade rates have been used to evaluate the performance of the receiver. The results show that our technique has potential in channels with delay spread of about 20%, signal-to-noise ratio (SNR) greater than 15 dB, and applications requiring bit-error rates (BER's) less than 10/sup -2/.

Fraction spaced Multi-Carrier modulation

This paper presents a digital implementation of a Multi-Carrier Modulation scheme without spectral overlapping between sub-carriers. This scheme uses aQAM modulation and a square root Nyquist pulse shaping function. It guarantees orthogonality between sub-carrier signals even after transmission over a dispersive fading channel. The modulator and the demodulator use aK-pointDFT and a set of polyphase filters. Therefore all single-carrier equalization and detection techniques can be used in this Multi-Carrier Modulation system.

Bidirectional decoding of convolutional codes for indoor cellular radio

Bidirectional suboptimal breadth-first decoding of convolutional codes is an attractive technique for slowly varying and quasi-static fading channels as it restricts the extent of decoding errors due to correct path loss to very heavy noise or interference regions. This paper compares the performance of such a decoding scheme to the Viterbi algorithm over wideband TDMA indoor radio links where equalization and space diversity are also used to combat dispersive fading and cochannel interference. On the basis of equal computational complexity and equal decoding delay, suboptimal, breadth-first, bidirectional decoding of a long constraint length convolutional code is shown to be superior to Viterbi decoding of a shorter constraint length code. Furthermore, this advantage increases as the outage criterion (in terms of bit error rate) becomes more stringent which makes bidirectional decoding particularly attractive for data applications and makes channel coding a more attractive alternative to increasing the space diversity order at the receiver.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of unequalized frequency-hopped TDMA with convolutional coding on dispersive fading channels

Increasing demand for wireless personal communications has stimulated research on new digital radio technologies that are optimized for various service applications and environments. This paper discusses the performance of a slow-frequency-hopped time-division multiple-access (SFH-TDMA) technique, which has been proposed as a “high-tier” extension of a low-complexity TDMA architecture optimized for low-power pedestrian applications. The SFH-TDMA technique considered uses QPSK modulation and rate-1/2 convolutional coding. Numerical results for a wide range of fading rates are obtained through analytical calculation of the effective signal-to-noise ratio combined with a simulation approach which incorporates measured multipath channels and actual frequency correlation among contiguous hopping channels. The results indicate that the SFH-TDMA technique can tolerate root-mean-square (rms) delay spread up to several bit periods without a need for adaptive equalization, but also point to the need for fast power control when the fading is slow and the rms delay spread is much smaller than the bit period. This work is targeted toward understanding the implications to local exchange networks of wireless technology alternatives that could provide access to those networks.

Frequency hopped spread spectrum multiple access over cellular dispersive fading channels

The authors present a general method based on the concept of circularly symmetric signals, for the evaluation of the average probability of error of uncoded asynchronous noncoherent frequency hopped spread spectrum multiple access (FH-SSMA) communications over cellular dispersive fading channels. This method provides an accurate approximation and a tight upper bound to the symbol error probability of orthogonal MFSK over nonselective and selective fading channels, respectively. Nonselective fading is considered for the two extreme cases: a system with no interleaving and a channel with fading which is slow relative to the hopping rate; and a system which is fully interleaved. The authors consider a general class of Gaussian wide-sense-stationary uncorrelated scattering (GWSSUS) fading channels. Several conclusions are drawn from the application of this method, namely the influence of power control on nonselective fading. This is compared with the selective fading case, for which a different situation occurs. The Gaussian approximation method is also presented and compared.

Dispersive fading effects in CDMA radio systems

The effect of multipath delay spread in a code division multiple access (CDMA) radio communications system where multiple PN receivers are used is reported. Ten multipath power delay profiles obtained from measurements in New York City are examined. Results show that the increase of interference due to diffuse uncaptured energy is about 1 to 1.4 dB on the average. This increase in interference results in at least 20% capacity reduction in CDMA radio systems. >

Compound strategies of coding, equalization, and space diversity for wide-band TDMA indoor wireless channels

Compound strategies of equalization and space diversity in the form of an optimum baseband combiner are attractive for wideband time division multiple access (TDMA) portable communication radio links in order to combat dispersive fading and cochannel interference. The authors investigate the performance of such a scheme in conjunction with convolutional coding and soft-decision Viterbi decoding via a semianalytical technique based on the method of moments. Such an approach avoids a Gaussian characterization of interference and yields results for both ideal interleaving and no interleaving. With dual space diversity, three taps per forward filter, and a data rate of 10 Mb/s, it is shown that, although a third space diversity branch remains preferable in terms of performance, channel coding can be a viable alternative, particularly in terms of outage rate, to increasing the space diversity order, even in the absence of interleaving, provided the signal-to-interference ratio is sufficiently high. >

Multipath propagation effects on digital radio equipped with a plane reflector repeater

Effects of multipath propagation on a digital radio hop employing a plane reflector as a passive repeater were analysed by using a ray tracing method. The calculations of raypath parameters on the hop show that a large-aperture plane reflector increases attenuation fading due to the main ray's angle-of-arrival variation by its sharp radiation pattern, and it can reduce dispersive fading. A digital transmission experiment on a reflector-equipped hop showed that plane reflectors could be used for small-capacity digital microwave systems.

Performance of antenna diversity and decision feedback equalisers in narrowband digital cellular systems

Performance results are given for the combined use of a decision feeback equaliser with antenna diversity in time dispersive Rayleigh fading channels such as those typically encountered in narrowband digital cellular mobile systems. The results show that the use of dual antenna diversity provides significant gains over nondiversity receivers with decision feedback equalisers, and that this performance is robust in the presence of uncompensated paths and correlation between the diversity ports.

Coding and optimum baseband combining for wide-band TDMA indoor wireless channels

Optimum baseband combining with coherent BPSK/QPSK is an attractive technique for wide-band TDMA portable communication radio links in view of its ability not only to combat dispersive fading but also to cancel cochannel interference. The performance of such a scheme for data rates ranging from 1 to 20 Mbits/s is investigated. It is shown that convolutional coding with soft-decision Viterbi decoding can be a viable alternative to increasing the space diversity order even in the absence of interleaving if the effective diversity order is sufficiently large. A number of performance curves for various combinations of transceiver and channel parameters are presented.

Mathematical analysis of the three-ray dispersive fading channel model

The paper presents a rigorous mathematical analysis of the three-ray multipath dispersive fading channel model. The channel transfer function in this model consists of three terms representing a direct ray, a ray refracted from the atmosphere and a ray reflected from the ground. The channel transfer function, conditioned on the amplitude of the direct ray, is proven to be a complex Gaussian process whose probability distribution is independent of the statistical moments of the rays' delays. Explicit formulas are presented for several pertinent single-variable probability density functions, covariances and joint probability density functions of the channel transfer function and of the channel power gain function.

Equaliser techniques for QAM transmissions over dispersive mobile radio channels

Channel equalisation of 32 Mbit/s data transmissed via 16-level QAM at 1.9 GHz over dispersive fading channels is addressed using computer simulations. The multipath propagation channels used had three independent Rayleigh fading paths. The conventional equaliser systems were found to be unsatisfactory and an alternative system was conceived based on a RAKE-type equaliser controlling an iterative decision feedback equaliser. Although complex, simulations showed this equaliser system to yield an order of magnitude reduction in BER compared to that for a linear equaliser at SNRs in excess of 15 dB. Advantages are presented of using error correction coding in the conventional way and in an iterative manner to aid convergence of the equaliser. Gains of 2 to 3 dB were achieved by iterative decoding but at the expense of considerable increase in complexity. When second order spatial diversity was used we achieved improvements of about 5 to 7 dB when both systems were uncoded despite the inherent diversity that can be gained from equalising a wideband channel. By operating at SNRs greater than 25 dB, we were able to simulate transmission at 32 Mbit/s with a BER below 10−3.

Space diversity improvement factors for overwater path in New Zealand

Space diversity improvement factors for an overwater path in New Zealand are derived from field trial data and compared with predicted results. Improvement factors of about 50 were measured for single frequency fading, and those of about 30 were achieved for dispersive fading.

Dispersive microwave fading and lower atmospheric structure: An observational study

Simultaneous measurements of microwave fading and lower atmospheric conditions were carried out in Georgia in 1986 as a part of the continuing effort to study the line‐of‐sight digital radio performance. Acoustic sounding was a principal means of exploring the mechanism that causes dispersive fading. It was found that dispersive multipath fading occurs during nights with strong radiation inversion accompanied by a subsiding air mass. Subsidence inversion causes dispersion, and the origin of the air mass affects the mean received power level under multipath propagation conditions. When the height of the nocturnal boundary layer coincides with the height of the ground‐based radiation inversion layer and is at 100–150 m, fading is heavy, active, and dispersive for a 57‐km path with both transmitting and receiving antennae at 60 m above grade. Case studies of the significant episodes are presented, and the statistics of the different types of fading associated with the types of meteorological conditions are summarized for the 5‐month study period.

Diversity protections for digital radio-summary of ten-year experiments and studies

A summary is presented of a series of critical experiments that led to recent discoveries of large improvement factors for digital radio performance by antenna pattern diversity, antenna angle diversity, frequency diversity, and space diversity. The measured diversity improvement factors for digital radio against multipath dispersive fading are larger than those predicted by the existing analog radio model by at least one order of magnitude. Applications of these findings will lead to substantial savings in the cost for diversity protections for digital radio routes. These discoveries stimulated the development of new models of diversity improvement factors for digital radio and the development of the DRDIV computer program for engineering digital radio routes. Background information is given on multipath fading and diversity concepts, and a glossary of terms is included.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pseudoerror Monitor for 16 QAM 140 Mbit/s Digital Radio

This paper presents the performance of a pseudoerror monitoring technique for a 16 QAM 140 Mbit/s digital radio in presence of multipath dispersive fading. The so-called pseudoerrors, generated by means of a threshold modification of two, namely, secondary receivers are entered into an extrapolating function to obtain a fast bit error ratio (BER) calculation. A counting time of 10 ms was retained in order to follow fading depth changes up to 100 dB/s and fading notch speeds up to 300 MHz/s approximately. We have considered three structures for the receiver: without equalization, with IF amplitude equalization, and decision feedback equalization (DFE). The results obtained show the estimated and real BER within a margin that includes the two recommended CCIR values: 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> . In particular, the estimated signature obtained in the case of using IF equalization reveals that this fast BER calculation could be an effective choice to control a frequency diversity switch, even in the presence of a fading activity with rapid variations.

Statistical Model for Amplitude and Delay of Selective Fading

The transmission performance of digital radio systems is controlled by spectral distortion caused by multipath fading. To evaluate this performance for digital systems with high-order modulation schemes, a statistical model for frequency-selective fading is needed. New propagation data obtained in Gainesville, Florida, were used to generalize Rummler's model to include group delay response. The introduction of the delay response data into the model of the fading channel enabled the classification of the fades as minimum phase and nonminimum phase. We found that 24 percent of all fades have significant delay distortion, and can be characterized as being minimum phase or non-minimum phase. In the range of practical interest, there are as many minimum phase as nonminimum phase fades. The results of this work will facilitate a better understanding of the fading channel, which will be beneficial in the engineering of radio routes and digital radio design. The results also demonstrate the need for a description of the geographical occurrence of dispersion, which will differ from that for multipath fading at a single frequency. This is based on the observation, presented in this paper, that the relative amount of dispersive fading is significantly greater in Gainesville, Florida, than in Palmetto, Georgia. The availability of a dispersive fading map will facilitate the accurate engineering of digital radio routes.

The performance of digital radio relay systems

This paper outlines some of the different types of digital radio relay systems that are now in worldwide service, identifies the factors which contribute to their performance, and illustrates the performance standards that are currently being achieved. The CCIR performance standards for high capacity digital radio systems are discussed, together with some of the system planning guidelines in current use, and these are compared with worldwide results from a number of system field evaluations. The comparison shows that the CCIR standards can be achieved on established microwave networks, using advanced equipment designs which incorporate diversity and adaptive equalizers to overcome the effects of dispersive fading. The performance and availability of medium and lowcapacity systems are also briefly reviewed, and predictions of non-availability due to rainfall fading are given for some typical systems. The paper concludes that the performance standards for digital radio systems will be in accordance with the highest quality grades required for transmission media in an Integrated Services Digital Network.

Adaptive Phase and Amplitude Modulation on a Frequency Dispersive Fading Channel

Differentially coherent phase shift keying (DPSK) is commonly used to provide data communication on frequency dispersive channels. To obtain higher data rates one can increase the number of phases or employ combined amplitude-phase modulation. In this paper we compare the performance of multilevel DPSK with the performance of Type I amplitude-phase modulation on a frequency dispersive channel with additive noise. For the Type I system differential modulation is used to amplitude encode a single bit, and ordinary DPSK is used to encode up to four phase bits. Curves of bit error probability versus the effective signal-signal-to-noise ratio of the channel are presented for both the amplitude and phase encoded bits. From these curves the appropriate amount of amplitude modulation to be employed in a high rate Type I system can be obtained. Performance comparisons between multilevel DPSK and Type I modulation are then presented under conditions of both equal average and peak transmitter power.

Reception of pulse-amplitude-modulated signals transmitted over a dispersive fading channel with nonstationary statistics

The optimum linear receiver is described for the reception of PAM signals over a dispersive, randomly time-varying channel which causes intersymbol interference. For a wide-sense stationary channel the receiver is relatively simple. The results also apply to the reception of multiplexed signals where the modulation scheme is linear.