Multipath Fading Effects on Digital Radio

An experimental study of multipath fading effects on an 8-PSK 6 GHz digital radio system was conducted on a 26.4 mi test link near Atlanta, Georgia. Results were obtained for a non-diversity arrangement and a space-diversity arrangement employing a cophasing combiner operating at 70 MHz and fed by two antennas separated by 30 ft. Multipath fading was found to have a significant impact on bit error rate performance, i.e., the digital radio system was fragile by comparison to conventional analog FM radio. Modest in-band linear amplitude dispersion, 0.2 dB/MHz for example, was sufficient to cause aBERgeq10^{-3}. Such dispersion occurred at (average power) fade depths as shallow as 20 dB. However, average fade depth was a poor indicator of BER performance. The measured hop missed short haul outage objectives by an order of magnitude without space-diversity, and was close to acceptable when diversity was activated.     

Diversity protections for digital radio-summary of ten-yearexperiments 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     

Digital audio broadcasting in the FM band based on continuous phase modulation

A method for broadcasting digital audio signals simultaneously with existing analog frequency modulation radio (88-108 MHz) in adjacent channels is presented. The digital transmission is based on continuous phase modulation (CPM) and a proper reduced-state sequence estimator. With the proposed method, the power level and the symbol rate of the transmitter signal is determined in a manner that the interference the CPM signal poses for the analog FM signal in adjacent channels remains below a level according to the radio frequency emission mask defined by international rules. Due to the multipath propagation of the transmitted signal, the transmission behavior of the radio channel is determined by high dispersion up to 85 /spl mu/s. With the selected bit rate, the receiver has to cope with a channel memory of up to 17 bits. Since Viterbi detection is not feasible due to the number of channel states, detection is performed by a reduced-state sequence estimator that is able to eliminate the complete channel interference by decision feedback. Simulation results show that the detector almost achieves the detection quality of the optimum receiver. CPM achieves data rates of up to 200 kb/s inside a 200 kHz FM channel, which is sufficient for transmission of digital compressed audio signals at compact disc quality. The encouraging results of field tests will be published in another paper.     

Decision Feedback Equalization for Multipath Induced Interference in Digital Microwave LOS Links

The performance of a 49-QPRS, 90 Mbit/s digital radio receiver equipped with a decision feedback equalizer (DFE) to counter multipath fading is investigated via computer simulation. The simulation includes the transmitted data, multipath fade model, receiver model, and DFE. The results indicate that a DFE equipped with five forward and five feedback taps can adequately compensate a 40 dB minimum-phase fade anywhere in the receiver passband. The study is extended to other receiver configurations including the use of space diversity and/or slope equalizers and the use of a transversal equalizer (TE) with the same delay-span in place of the DFE. The results indicate that the DFE equipped receiver outperforms the TE receiver and that still better performance may be achieved using a combination of space diversity and DFE.     

Application of coding and diversity to UHF satellite soundbroadcasting systems

Results are presented of a study of alternative means by which to reduce the required link margins in a satellite sound broadcasting system, thus improving the economic feasibility of the service and increasing the possibility of sharing a radio-frequency allocation with terrestrial services. Propagation models are presented that account for the effects of shadowing and multipath in the operating environments. Comparisons are made of the performance that results using time, frequency, and space diversity to moderate the effects of shadowing and multipath for vehicular and portable receivers. It is shown that space diversity has the broadest applicability in restoring the channel performance that would otherwise be degraded by Rayleigh fading channels and by quasistationary AWGN (additive white Gaussian noise) channels. Example link budgets are given for candidate satellite sound broadcasting system designs that use analog FM and digital modulation techniques     

Mesure des effets de la propagation sur un faisceau hertzien numérique à 140 Mbit/s utilisant une modulation à huit états de phase

This paper presents the main field test results on a 140 Mbit/s digital radio system using eight phase shift keying. System performance, in terms of outage time and net fade margin, has been measured for various receiving configurations (space diversity, intermediate frequency slope equalizer).     

Dispersion Signatures, a Statistically Based, Dynamic, Digital Microwave Radio System Measurement Technique

Outage in digital microwave radio systems during multipath fading is highly correlated with the channel amplitude dispersion. This characteristic is exploited in a new digital microwave radio system measurement technique which yields dispersion signatures. These dispersion signatures can be readily applied to assess the effects of time varying channels on digital radio equipment. Thus, dispersion signatures can be used for in-service monitoring of equipment operation during actual fading. Routine maintenance assessment and laboratory evaluation of digital microwave radio equipment can also be readily carried out. The laboratory evaluation results can be used for system design both with and without diversity.     

Digital audio broadcasting in the FM band by means of contiguous band insertion and precanceling techniques

A low-complexity method of linearly combining an analog frequency-modulation (FM) signal with a low-level multitone signal for simultaneous broadcasting of digital data and analog FM is proposed and evaluated. The analog FM signal can be recovered with a standard FM receiver. The digital multitone signal is added symmetrically at the band edges of the 200-kHz FM signal in contiguous bands with variable width. To maximize the data rate, the digital bandwidth is maximized subject to a maximum allowed distortion of the demodulated analog FM signal. The impact of the analog FM signal on, digital reception is cancelled by means of transmitter precanceling techniques. Conditions for precanceling techniques for multipath channels are formulated. The main advantage of the contiguous band insertion method is that the rate of the control channel is considerably lower compared to previously proposed methods of carrier insertion without significant reduction in data rate. We present a number of numerical results for acoustical signals.     

Cofrequency Cross-Polarized Operation of a 91 Mbit/s Digital Radio

Current digital radio systems have spectral efficiencies of only half that attained by FDM-FM analog systems. This paper describes a system, that exploits the inherent ruggedness of digital modulation and the concept of frequency reuse to achieve spectral efficiencies of 4.5 bits/s/Hz which is comparable to that achieved by analog systems. The major technical challenge in implementing such a system was devising a means to maintain adequate cross-polarization discrimination during multipath fading. Details are given on extensive digital and analog field experiments that were conducted to verify that cofrequency cross-polarized operation during multipath fading was feasible. The key to achieving satisfactory performance is shown to be the use of space diversity with in-phase combining.     

Outage Performance of QAM Digital Radio Using Adaptive Equalization and Switched Space Diversity Reception

Recently, the ability of anti-fading measures to reduce the outage which occurs on digital microwave radio links has been the subject of considerable study. Ideal and nonideal adaptive equalization in both the time and frequency domain have been evaluated for nondiversity reception using various performance criteria. Ideal adaptive equalization and space diversity reception have been considered using a recently published model of multipath fading on space diversity radio channels. In this paper, we determine the outage of 16-QAM and 64-QAM digital radio systems using adaptive slope equalization, finite-tap decision feedback equalization, and switched space diversity reception. The outage is evaluated by computing the probability of occurrence of those channel realizations which cause the bit error rate to exceed a critical value. The dependence of the outage prediction on the equalization method and the outage defining criterion is investigated by also considering ideal adaptive equalization and a signal-to-distortion ratio, respectively.     

MMSE Equalization of Interference on Fading Diversity Channels

Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channel to channel, the equalizer can also reduce its effect on the quality of information transfer, important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem. In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymhol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce ISI effects to levels small enough for satisfactory communication. The analysis also develops he concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the MMSE processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.     

Simultaneous broadcasting of analog FM and digital audio signals bymeans of adaptive precanceling techniques

A method for broadcasting digital audio simultaneously with existing analog frequency modulation (FM) radio is presented. The method is based on precomputing the response of the host analog FM signal at the digital receiver and precanceling it at the transmitter. As a result, the digital transmission is free from interference from analog FM. We select the rate and power level of the digital transmission in a manner that the interference the digital data incur on the analog FM signal remains at acceptably low levels. The digital transmission is based on adaptive orthogonal frequency-division multiplexing (OFDM) (adaptive multicarrier). The frequencies and number of carriers of the digital multicarrier modem are judiciously selected in a time-varying fashion so as to cause a negligible distortion in a standard receiver for analog FM. Simulations based on conservative nonoptimized signal design indicate that data rates up to about 130 kb/s inside the 200-kHz FM channel are achievable for acoustic test signals. We present a number of numerical examples where the average digital data use up to 50% of the 200-kHz power spectrum with digital signal power levels 25-35 dB below the analog signal. Due to the resulting variable-rate digital transmission, a control channel is required. A method of precanceling with multiple orthogonal direct-sequence spread-spectrum schemes is also presented     

An investigation of space-path hybrid diversity scheme for basestation reception in CDMA mobile radio

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 μs or less     

Characteristics of a High Capacity 16 QAM Digital Radio System in Multipath Fading

Multipath fading effects on a 200 Mbit/s 16 QAM digital radio system have been experimentally investigated. It was found that the outage probability can be accurately estimated using only the in-band frequency response without knowledge of the delay difference between two incident waves. New methods to evaluate the improvement factors for both space diversity reception and dynamic equalizing, which are required to combat multipath fading effects, were also proposed. In particular, the significance of noticing the delay dispersion sign was pointed out when attaining sufficient equalization effect. The validity of these evaluations was confirmed by simulation experiments and a field trial.     

Prediction methods for rain attenuation statistics at variable path angles and carrier frequencies between 13 and 100 Ghz

Fade depth and space diversity statistics of propagation along earth-satellite paths have been calculated from radar reflectivity data of rain using modeling procedures. The reflectivity data base was obtained during the summer of 1973 at Wallops Island, Va., using a high resolutionSband radar interfaced with a computer and digital processing system. Fade statistics have been calculated for various path angles at several frequencies between 13 and 100 GHz. Subsequent analysis has demonstrated the ability to predict the following: 1) fade statistics at other path elevation angles given similar type statistics at a particular path angle, 2) space diversity statistics at other frequencies, given similar type statistics at a particular frequency, and 3) fade statistics at a third frequency given similar type statistics at two other frequencies. Although a specific data base was used pertaining to the climatology at Wallops Island, the techniques developed are general and may be applied to existing or future "fade measurements" at other climatological locations.     

Diversity scheme employing inverse filtering for multipath radiochannels

A new diversity scheme, termed maximum path resolution (MPR) reception, and applicable to multipath radio channels. Is introduced. The technique exploits the frequency diversity of a spreading signal received via a multipath channel by applying aperiodic inverse filtering in such a way that the path with the greatest power is enhanced, while the others are effectively cancelled. The technique is analysed in the context of a CTDMA system and its performance is verified via simulation     

Some Experimental Results on In-Band Amplitude Dispersion and a Method for Estimating In-Band Linear Amplitude Dispersion

In-band amplitude measurement was conducted on a 63.4 km test path with an oversea section (normal reflection point was on the ground), using a frequency band from 4440 to 4500 MHz. Results on peak-to-peak amplitude dispersion and linear amplitude dispersion were obtained for both nondiversity and idealized space diversity receptions. Potential improvements by space diversity in terms of reductions of amplitude dispersion were remarkable in the large amplitude dispersion region. An analysis of the in-band amplitude pattern was made in order to identify which multiray is most responsible for amplitude dispersion during multipath fading. The main cause of amplitude dispersion was found to be waves irregularly reflected from the sea surface and waves reflected from the ground, with a long delay but small amplitude. A frequency correlation function for received power, which is necessary for estimating linear amplitude dispersion, was derived theoretically, for the three-path model. The theoretical frequency correlation agrees well with that measured on this path.     

Precoded Modulo-Precanceling Systems for Simulcasting Analog FM and Digital Data

We present techniques for simulcasting low-power digital data and analog FM over fading channels. Our methods have strong connections to what are referred to as dirty paper coding techniques in that a low-power digital data signal is modulo-added to the host analog FM signal. Due to the low power levels of the digital data and the suppression capability of analog FM, a standard analog FM receiver can be used to reliably recover the analog audio signal. We develop digital receivers that work reliably over fading channels given imperfect channel state information. Spreadresponse precoding is exploited to not only provide rate-1 temporal diversity benefits, but also to simplify the design of the modulo-precanceler and the receiver. Our theoretical and simulation-based performance analysis of the digital receivers and our MSE-based analog FM distortion analysis suggest that precoded modulo-precanceling schemes provide substantially higher data rates than existing precanceling strategies at the cost of transmission delay and transmitter and receiver complexity. The systems we present are also readily compatible with conventional channel coding techniques as well as with analog FM postcancelers, which can improve the digital receiver bit-error-rate performance at the expense of receiver complexity.     

A broad-band 11 GHz radio propagation experiment

The designer of a digital radio system (DRS) requires quantitative data on flat and frequency-selective fading in order to assess the need for diversity and equalization and also the feasibility of using the same repeater sites developed for systems in the lower bands. Preliminary results from an experiment which measured the statistical distribution of the complex channel response across a 1 GHz bandwidth at 11 GHz on a 36 km path are described. On this test path the cumulative distribution function of signal attenuation due to rain was found to be about an order of magnitude greater than that due to multipath fading. Incuded are typical records of the complex frequency response during frequency selective fading (showing large variations of group delay), joint cumulative distributions of amplitude, amplitude slope, and notch speed.     

基于OFDM技术的短波电台系统研究

OFDM（正交频分复用）技术是一种多载波数字调制技术,在无线多径衰落信道下具有较好的传输特性,文中设计短波电台系统采用了OFDM技术来对克服多径衰落带来的干扰,并且给出了系统的设计方案。利用Simulink建模,并在瑞利衰落信道和高斯信道下进行了仿真,结果显示出性能上有较大的改进。