Phase Fading Research Articles

Cooperative Strategies and Capacity Theorems for Relay Networks

Coding strategies that exploit node cooperation are developed for relay networks. Two basic schemes are studied: the relays decode-and-forward the source message to the destination, or they compress-and-forward their channel outputs to the destination. The decode-and-forward scheme is a variant of multihopping, but in addition to having the relays successively decode the message, the transmitters cooperate and each receiver uses several or all of its past channel output blocks to decode. For the compress-and-forward scheme, the relays take advantage of the statistical dependence between their channel outputs and the destination's channel output. The strategies are applied to wireless channels, and it is shown that decode-and-forward achieves the ergodic capacity with phase fading if phase information is available only locally, and if the relays are near the source node. The ergodic capacity coincides with the rate of a distributed antenna array with full cooperation even though the transmitting antennas are not colocated. The capacity results generalize broadly, including to multiantenna transmission with Rayleigh fading, single-bounce fading, certain quasi-static fading problems, cases where partial channel knowledge is available at the transmitters, and cases where local user cooperation is permitted. The results further extend to multisource and multidestination networks such as multiaccess and broadcast relay channels.

Phase Noise and Fading Effects on System Performance in MT-DS-SS

Multicarrier (MC) modulation and code division multiple access (CDMA) schemes have seen much recent attention for the high capacities and flexibility they can provide. A potential difficulty with these systems is their sensitivity to the effects of imperfect frequency up/down conversion due to local oscillator phase noise and frequency offset. In this paper, we provide a general method to upper bound and/or approximate system performance in multitone direct-sequence spread spectrum (MT-DS-SS) signaling in the presence of imperfect synchronization. We model phase noise as a slow random processes with small variance. In particular, we assume coherent detection with binary phase-shift keying (BPSK) modulation and use a phase noise model based upon one for a practical phase-locked loop (PLL). Comparisons between simulations and analysis show excellent agreement, and also show that system degradation is dominated by the common phase noise, and that the intersubcarrier-same-user interference (IS-SUI) contribution is very small. The approach we employ provides an effective analytical/numerical method for performance evaluation for low target error probability values, on the order of 10/sup -6/ or lower. In addition, our method can also be easily applied to any multicarrier system with other frequency offset/phase noise models.

PSK and DPSK trellis codes for fast fading, shadowed mobile satellite communication channels

The performance of 8-PSK and 8-DPSK trellis codes is presented for a class of fast fading, land mobile satellite communication channels. The fading model is Rician but, in addition, the line-of-sight path is subjected to a fast lognormal attenuation that represents tree shadowing. The fading parameters used in this study represent the degree of shadowing and are based on measured data. The primary application considered is for digital speech transmission and thus, bit error probabilities in the order of 10/sup -3/ are emphasized. Sensitivity of the bit error probability to amplitude fading, amplitude and phase fading, and decoding delay is presented. Performance is determined via digital computer simulation. Optimal four- and eight-state codes are determined and optimality is found to be dependent on the presence of lognormal shadowing. >

Decision-Directed Diversity Combiners--Principles and Simulation Results

Diversity combining methods for medium- and high-capacity digital microwave radio links employing QAM signaling are studied theoretically and by computer simulations. The selective fading diversity channels are represented by a two-path model with complex envelopes. Emphasis is put to find simple receiver structures where combining is controlled by detected symbols to minimize a given error criterion. Two criteria are discussed-minimum mean-square error (MMSE) and minimum projection (MP). The latter criterion is believed to be new and its behavior is analyzed in two extreme cases. It turns out that the MP combiners obey the maximal ratio rule in the absence of dispersion and the minimum distortion rule in the absence of noise. This versatility is achieved with minor circuit complexity. The results have been verified by computer simulations for a 4 PSK 70 Mbit/s system. The resulting diversity signatures are very narrow on the frequency axis, indicating good performance. Moreover, the MP combining tolerates extremely well nonminimum phase fading in one diversity branch, situations where most cophasing schemes have difficulties.

Short Range Skywave Field Strengths From Tall AM Broadcast Towers

Calculations of mediumwave skywave field strengths within 100 km of an AM broadcast station using a 186 degree tower are presented. The analysis is novel in that the high vertical elevation angle radiation from the tower is calculated using a method of moments technique. The results show that skywave field strengths are several times stronger than predicted when using the FCC tower radiation formula, and that within 100 km the 10 percent time skywave field strength can in some cases be of comparable amplitude to the groundwave signal amplitude. An analysis of the resulting signal envelope amplitude and phase fading statistics is also presented.

Comparison of simultaneous line-of-sight signals at 9.6 and 34.5 GHz

Signals at 9.6 and 34.52 GHz, propagated simultaneously over a slant line-of-sight overwater path, have been analyzed to compare the power spectra of phase-of-arrival variations and fading and to determine the coherence of these signals with regard to both phase variations and fading. The phase data at the two radio frequencies exhibited nearly identical power spectra from 0.01 to 5 Hz and very high coherence from 0.01 to 0.1 Hz. The coherence dropped rapidly above 0.1 Hz and was in most cases less than 0.4 above 0.5 Hz. The power spectra of fading were similar in shape at the two frequencies, but the fading spectral density was consistently higher at 34.52 GHz than at 9.6 GHz from 0.1 to 5 Hz. The shape of the coherence function for fading was similar to that of the corresponding phase coherence function, but the fading coherence was lower at the low spectral frequencies. The possible effect of the small spatial separation of the propagation path on the coherence analysis is discussed.

The variability of time shifts in the spaced fading records

The variations in phase path recorded at three closely spaced points can be used for measuring horizontal drifts at the E-region level. In this communication the variability of time shifts in spaced fading records have been investigated using simultaneous spaced phase path and fading records. The observed variability in time shifts in the spaced fading records can be interpreted as due to continuous changes in the direction of arrival, and interference between more than one signal. Portions of the spaced phase path records influenced by changes in the direction of arrival and interference effects can be easily identified, and reliable horizontal drift measurements can be made from the phase path records that are not contaminated by the above two effects. No such identification is possible in the spaced fading records.

Probability of decoding error for random phase and Rayleigh fading channels

In this paper, the probability of error of two time-varying channels with memory, 1) the random phase channel and 2) the Rayleigh fading channel, is discussed. The input is assumed to be one of M equiprobable waveforms. An upper bound to the probability of error is derived by convetting the channel into a memoryless one by means of scrambling. A lower bound to the probability of error is derived by assuming that except for an additive noise, all the channel parameters are completely known at the receiver and, therefore, are not considered to be random variables any more. Following this assumption, it is shown that the channel is converted into a memoryless channel. For each one of the two channels, there is a region of SNR where the upper and lower bounds are close together and therefore yield a good estimate to the actual probability of error. Furthermore, using random coding and scrambling, this probability of error may actually be achieved (for a certain region of rates).

A Study of Phase Fading of Ionospheric Reflections

Pulsed 2�28 Mc/s radio waves were transmitted imd measurements were made on the first-hop, vertically-incident reflections from the ionosphere. A method of continuously recording the changes of phase difference between the signals at two aerials is described. The general features of the records are described and it is shown that the quasi-periodic type of fading can be explained in terms of two component rays being reflected from two reflecting areas of the ionosphere and interfering at the receiving aerials. The horizontal separation of the two regions is about 50 km in the case analysed. The direction of arrival of the radio waves, the velocity of the amplitude pattern over the ground, and the statistical distribution of the phase difference are also discussed for this type of fading.