Showing papers on "Fading distribution published in 1995"

Information capacity and power control in single-cell multiuser communications

Abstract: We consider a power control scheme for maximizing the information capacity of the uplink in single-cell multiuser communications with frequency-flat fading, under the assumption that the users attenuations are measured perfectly. Its main characteristics are that only one user transmits over the entire bandwidth at any particular time instant and that the users are allocated more power when their channels are good, and less when they are bad. Moreover, these features are independent of the statistics of the fading. Numerical results are presented for the case of single-path Rayleigh fading. We show that an increase in capacity over a perfectly-power controlled (Gaussian) channel can be achieved, especially if the number of users is large. By examining the bit error-rate with antipodal signalling, we show the inherent diversity in multiuser communications over fading channels.

Coherent DS-CDMA performance in Nakagami multipath fading

Abstract: The coherent reception of direct sequence-code division multiple access (DS-CDMA) signals in a multipath fading channel is considered. The channel model assumes independent paths with Nakagami fading statistics; this model includes the Rayleigh channel os a special case and is a reasonable model for a frequency-selective fading channel. The bit error rate (BER) performance of a RAKE receiver under various multipath fading conditions is derived and evaluated. The results indicate that over 50 users may access the channel (assuming 127 chips per bit) simultaneously under typical fading conditions, and can achieve an uncoded BER of 0.01 with an average received signal-to-noise ratio per bit of 10 dB. >

Innovations-based MLSE for Rayleigh fading channels

Abstract: This paper deals with the use of the innovations approach for developing a general and practical MLSE algorithms for signals transmitted over Rayleigh fading channels. The proposed MLSE receiver can be implemented by a bank of FIR time-invariant filters followed by a Viterbi processor and is applicable to any practically modulated signal over either frequency-nonselective or selective, fast or slowly fading channels. Simulation results are presented that demonstrate the superiority of the innovations-based receiver over differential detection in the presence of fast fading. To further improve the error performance, the derived MLSE receiver is combined with diversity reception. The simulation also shows that the diversity technique results in a significant improvement of the error performance. A unified interpretation of MLSE of signals transmitted over both Gaussian noise channels in the presence of ISI and Rayleigh fading channels is obtained by using the innovations approach. >

Analysis of the impact of channel estimation errors on the performance of a decision-feedback equalizer in fading multipath channels

Abstract: A coherent receiver with a decision-feedback equalizer (DFE) operating on a Rayleigh fading channel under a suitable adaptive algorithm is considered. In the analysis of a DFE, a common assumption is that the receiver has perfect knowledge of the channel impulse response. However, this is not the case in practice, and for a rapidly fading channel, errors in channel tracking can become significant. We analyze theoretically the impact of these errors on the performance of a multichannel DFE. The expressions obtained for the achievable average MPSK bit error probabilities depend on the estimation error covariance. In order to specify this matrix, we focus on a special case when a Kalman filter is used as an optimal channel estimator. In this case, the probability of bit error can be assessed directly in terms of channel fading model parameters, the most interesting of which is the fading rate. Our results show the penalty imposed by imperfect channel estimation, as well as the fading-induced irreducible error rates. >

On the error probability of linearly modulated signals on frequency-flat Ricean, Rayleigh, and AWGN channels

Abstract: The method used in Aghamohammadi and Meyr (1990) for finding the error probability of linearly modulated signals on Rayleigh frequency-flat fading channels has been applied to the more general case of Ricean fading. A signal received on a fading channel is subject to a multiplicative distortion (MD) and to the usual additive noise. Following a compensation of the MD, the signal provided to the detector may be thought to include only a single additive distortion term ("final noise"), which comprises the effects of the original additive noise, the MD, and the error in MD compensation. An exact expression for the probability density function of the final noise is derived. This allows calculation of error probability for arbitrary types of linear modulations. Results for many cases of interest are presented. Furthermore, as special cases of Ricean fading, error probability for Rayleigh fading and non-fading channels are obtained which either match the results or complete the approximate derivations formerly known from the literature. >

Bandwidth efficient QPSK in cochannel interference and fading

Abstract: The performances of QPSK in the presence of cochannel interference in both nonfading and fading environments are analyzed. Three approaches for representing the cochannel interference are investigated. These are a precise error probability method, a sum of sinusoids (sinusoidal) model, and a Gaussian interference model. In addition to determining precise results for the performance of QPSK in cochannel interference, we examine the validity of these two interference models in both additive white Gaussian noise (AWGN) environments and in different flat fading environments; Rayleigh, Ricean, and Nakagami. Nyquist pulse shaping is considered and the effects of cross channel ISI produced by the cochannel interference are accounted for in the precise interference model. Also accounted for are the random symbol and carrier timing offsets of the interfering signals. Two performance criteria are considered. These are the average bit error rate and the interference penalty. The latter is defined as the increase in signal-to-noise power ratio (SNR) required by a system with cochannel interference in order to maintain the same BER as a system without interference. Attention is given, in particular, to the outdoor microcellular fading environment. In this environment, the fading experienced by the interfering signals may be represented by a Rayleigh-fading model while the fading experienced by the desired signal may be represented by a Ricean or a Nakagami-fading model. >

Matched filter-bound for time-discrete multipath Rayleigh fading channels

Abstract: The matched filter bound (MFB) for the two-path channel derived in Mazo (1991) is extended to Rayleigh fading channels with an arbitrary number of time-discrete paths. General results on antenna diversity reception and communication over correlated-path fading channels are presented. Examples of the MFB for several practically useful channels are given. >

Multiple-accessing over frequency-selective fading channels

Abstract: This work considers the transmission of information from many independent sources to a common receiver over a channel impaired by multipath propagation. In cellular radio communications this is the case of the uplink. We start by examining the achievable rate region of the multiuser frequency-selective fading channel without knowledge of the channel on the transmission end. It has been shown that SSMA (Spread Spectrum Multiple Access) is theoretically capable of higher data rates than FDMA (Frequency Division MuZfiple Access) or slow frequency-hopping[l]. When the average received power for all the users is equal, which corresponds to a perfect slow power control, we show that the maximum spectral efficiency of SSMA exceeds that of FDMA or slow frequency-hopping by 5772 nats/s/Hz for many users and Rayleigh fading. Also, we formulate the optimal multiple-access scheme when all the channels are known to the transmitters. In turns out that only one user should transmit at any given frequency. Morecver, the input power spectra for the transmitters are water-filling formulae both in frequency and time. It is shown that the spectral efficiency for the optimal scheme is significantly higher than both those of SSMA and FDMA.

Packet error rate in the non-interleaved Rayleigh channel

Abstract: Two analytical methods for determining the packet error rate in the non-interleaved Rayleigh fading channel are presented. The first method is an exact evaluation of the packet error rate, whereas the second method is based on a k-state digital channel model. The fading model is a nonfrequency selective Rayleigh fading with a first-order low pass spectrum for the quadrature fading components. The analytical methods are complemented by simulations. The influence of the fading bandwidth on the packet error rate and the packet throughput is discussed. Block codes as well as convolutional codes are considered. The results show that without forward error correction the throughput for slow fading is higher than for fast fading. With forward error correction the packet error rate is sensitive to the fading bandwidth. Furthermore it is shown that the convolutional code with intra packet interleaving is least sensitive to slow fading. >

On the distribution of cumulated interference power in Rayleigh fading channels

Abstract: This paper deals with the distribution of cumulated instantaneous interference power in a Rayleigh fading channel for an infinite number of interfering stations, where each station transmits with a certain probability, independently of all others. If all distances are known, a necessary and sufficient condition is given for the corresponding distribution to be nondefective. Explicit formulae of density and distribution functions are obtained in the interesting special case that interfering stations are located on a linear grid. Moreover, the Laplace transform of cumulated power is investigated when the positions of stations follow a one- or two-dimensional Poisson process. It turns out that the corresponding distribution is defective for the two-dimensional models.

Assessment of a pico-cellular system using propagation measurements at 1.9 GHz for indoor wireless communications

Abstract: Narrow-band propagation measurements were conducted at two floors of a multi-storied building at 1.9 GHz to study the behavior of the Rice factor and local mean power. To evaluate the influence of the Rice factor on a pico-cellular system, the outage probability is calculated for two different cases: (i) Rician fading for the desired and interfering signals with different Rice factors; (ii) desired signal with Rician fading and interfering signals with Rayleigh fading with a different local mean power for each interfering signal. Finally, a pico-cellular structure is determined as an illustration. >

Symbol-aided channel estimation with nonselective Rayleigh fading channels

Abstract: New algorithms are proposed to implement coherent detection of PSK signals transmitted over frequency-flat Rayleigh channels. Channel gain estimates are derived both from known symbols multiplexed with the data stream and from tentative data decisions. The performance of these algorithms is assessed by computer simulation with uncoded and trellis-coded modulation. Comparisons are also made with differentially coherent detection (DPSK) and with perfect coherent detection (corresponding to ideal knowledge of the fading process). Simulation results demonstrate that these algorithms can track adequately the severe fading fluctuations encountered on a Rayleigh channel and that their superiority over DPSK becomes impressive with fast fading.

Performance of RAKE receivers in Nakagami fading channel with arbitrary fading parameters

Abstract: A new expression for the bit error rate of RAKE receivers with either coherent or noncoherent/differentially coherent binary demodulation schemes in a Nakagami fading channel is derived. The analysis assumes an arbitrary number of diversity branches with arbitrary fading parameters.

Jitter diversity in multipath environments

Abstract: The lack of capacity in mobile communication systems is an increasing problem. A channel model for the use of antenna arrays at the base station is presented. A new diversity scheme, jitter diversity, is suggested with a simple algorithm. The technique is to use an antenna array and move the main beam slightly to avoid deep fades, giving substantial improvement in the fading distribution and the mean power. Linear antenna arrays with 1-19 elements are applied to propagation environments with different scatterer spread. The influence of the environments on the received antenna pattern, such as the effective gain, is investigated. Experiments support the main findings, namely that the environments reduce the directivity of the perceived antenna diagram by several dB. Jitter diversity is a very simple approach, compared to other combining methods, as it only includes the variation of a single parameter, the phase between elements. It should therefore be simple to implement in practice.

Fast Power Control to Counteract Rayleigh Fading in Cellular Radio Systems

Abstract: In urban areas, cellular radio signals are subject to Rayleigh fading, which results in rapid changes in the channel gain. For TDMA-systems without the possibility of frequency hopping, this can ca ...

Bit-error-rate performance for asynchronous DS-CDMA over multipath fading channels

Abstract: The authors analyse the performance of uncoded and coded asynchronous DS-CDMA systems over multipath fading channels. The fading-channel model adopted is a general one; by varying the parameters it is possible to model Rician or Rayleigh fading. Using these fading models, the authors developed the probability of bit error for the asynchronous DS-CDMA by modelling the multiple access interference (MAI) as a Gaussian process, as well as an improved Gaussian process. For coded systems, they investigated the effect of bit-to-bit dependence, due to the MAI, and fading within a codeword on the computation of the probability of bit error.

A two-state Rician model for predicting indoor wireless communication performance

Abstract: The received signal amplitude of an indoor wireless channel is generally modeled as Rayleigh or Rician fading depending on the situation. In addition to these, other fading models including the Gilbert, two-state analog, and log-normal are used for fading channels in general. Bultitude [1987] found that his measurements of several indoor channels best fit the Rician model. The present authors describe a two-state Rician fading model that better matches Bultitude's data. For DPSK, the bit-error-rate performance of the two-state Rician model is worse than the pure Rician model by about 8 dB.

Transformation of a Rayleigh fading channel into a set of parallel AWGN channels and its advantage for coded transmission

Abstract: The authors consider transmission over a memoryless Rayleigh fading channel. It is shown that an appropriate orthogonal transformation at the transmit side and block decision feedback equalisation at the receive side transform the Rayleigh fading channel into a set of AWGN channels. The advantage for coded transmission is demonstrated.

Optimum detection of fading signals in impulsive noise

Abstract: This paper deals with the synthesis and the analysis of optimum receivers to detect one out of M equally likely, equi-energy, fading signals in impulsive noise, modelled as a compound Gaussian, possibly correlated process. We show that the conventional coherent and incoherent detectors are still optimum, independent of the noise as well as the fading probability density functions. The performance analysis has been carried on with reference to the general case of arbitrarily distributed disturbance: in order to simplify the analysis, asymptotical expressions have been developed for high signal-to-noise ratios as well as high signal space dimensionality. Interestingly enough, this allows separating the effect of the noise spikyness from that of the fading law. Results indicate that, for deep fading, the noise marginal distribution does not dramatically affect the error probability, nor is it influential on the limit operating characteristics corresponding to infinite signal space dimensions. For non-fluctuating signals, instead, the noise distribution plays a primary role: spiky noise usually produces performance impairment; moreover, the limit performance in impulsive disturbance may exhibit marked deviations from the well-known stepwise shape which is typical of Gaussian channels. >

Performance of multistage multiuser detection in a fading multipath channel

Abstract: In this paper we study the performace of multiuser detection in cellular uplink reception. The main motivation in this paper is not to study the performance in a near-far scenario, but to demonstrate the achievable capacity gain. To this end we apply the joint estimation and multistage detection algorithm developed in [3] in a multipath fading environment. We show that multiuser detection can be used effectively to increase the system capacity and range. The assessment is carried out by extensive simulations with up to 60 users using length 63 Gold codes in a two path Rayleigh channel, both with and without U2-rate coding. II. SYSTEM DESCRIPTION Assume K asynchronous BPSK CDMA transmitters, each assignedaspreading waveform s ~ , ( t ) =E;”=, 5’f)II~,(t-jT,) where the j th chip of the kth user’s spreading sequence is given by Sf) E {-l7 l}, k = 1, ..., K . The kth user’s waveform is time-limited to [0, 2’). All users transmit simultaneously in a common frequency band by modulating the spreading waveforms with corresponding symbols b r ) E {-1, 1). The receiver demodulates the signal

Matched filter bound for OFDM on Rayleigh fading channels

Abstract: A matched filter bound (MFB) analysis is presented, in which the normalised Doppler rate is unrestricted, and thus applicable to OFDM signalling. In contrast to the static channel case, the optimal matched filter receiver is shown to be time varying and the probability of error is shown to depend on the transmitted pulse shape.

Symbol error rate analysis for MPSK under Rician fading channels with fading compensation based on time correlation

Abstract: In order to combat the effects of channel fading, a pilot symbol-aided modulation based on fading time correlation has been proposed. In this paper we present results of analytical studies on this technique which were not included in previous works. We derive a new formula to determine the symbol error rate of MPSK scheme under Rician fading channels. The results are compared with a perfect carrier-referenced system. Computer simulations are also carried out to verify the analytical results. >

1 and 2 stage decision feedback coherent detectors for DQPSK in fading channels

Abstract: Novel coherent detectors for DQPSK and /spl pi//4-DQPSK on fading channels are presented. The phase reference for the decision device is derived by mean square error estimation of the fading process. A finite impulse response filter is used for this purpose. The decision device is a traditional coherent detector. Both 1 and 2 stage detectors are considered. The proposed detectors show significantly better performance in fast fading compared to a differential detector. The detector may be used with FDMA, TDMA, CDMA and OFDM systems.

A coded 16 QAM scheme for fast fading mobile radio channels

Abstract: An adaptive Viterbi algorithm, derived from a dynamic estimate of the fading channel is used for the decoding of a convolutional coded 16 QAM system in a mobile environment. The estimates are obtained by a sequence of known pilot symbols embedded in the data stream, and perform compensation for Rayleigh fading. The likelihood criterion in the Viterbi decoder is also modified by these channel estimates through a metric weighting function. We demonstrate through computer simulations, that our new technique achieves a BER improvement of 7-10 dB at P/sub e/=10/sup -3/ in a fast flat Rayleigh fading environment compared to an uncoded system. The BER performance of our new technique in a co-channel interference (CCI) controlled environment is also studied, and the results show that it may achieve a 40% to 85% improvement in capacity over the standard modem scheme for the new US digital cellular system, /spl pi//4-QPSK. >

Multiple tone interference of frequency-hopped noncoherent MFSK signals transmitted over Ricean fading channels

Abstract: This paper investigates the performance degradation resulting from multitone interference of orthogonal, noncoherent frequency-hopped M-ary frequency-shift keyed receivers (FH/MFSK) where the effect of thermal and other wideband noise is not neglected. The multiple, equal power jamming tones are assumed to correspond to some or all of the possible FH M-ary orthogonal signaling tones. Furthermore, the channel is modeled as a Ricean fading channel, a possibility precluded when thermal noise is neglected; and both the signaling tones and the multiple interference tones are assumed to be affected by channel fading. Only band multitone interference is considered, and it is assumed that jammed hop bands contain a single interfering tone at one of the M possible signaling frequencies. When the information signal is not affected by fading but the multiple interference tones are, the effect on the overall performance of the FH/MFSK system is very small when the power of the individual interference tones is greater than or equal to the information signal power. When the information signal power exceeds the power of the individual interference tones, the counterintuitive result of poorer overall system performance when the multiple interfering tones experience fading as compared with when they do not, is obtained. This trend is accentuated as M increases. When the information signal experiences fading, the effect of fading multiple interference tones on overall system performance lessens, and for a Rayleigh faded information signal, fading of the multiple interference tones has no effect on overall system performance regardless of M.

Pre-diversity using coding, multi-carriers and multi-antennas

Abstract: In this paper, we describe a prediversity (diversity at transmitter) scheme using coding, multi-carriers and multi-antennas designed for fading channels. Burst errors occur frequently in the Rayleigh flat fading mobile channel. To solve this kind of problem, the time interleaving technique has been utilized. However it can not deal with slow fadings, and has a long processing delay. Our proposal requires no time delay and provides a fixed BER characteristic not only for a slow fading but also for a fast fading channel. We derive an upper-bound for the BER. Through computer simulations, we show that the proposed system with (7,4) block code, 7-carriers and 7-antennas achieves 15 dB improvement in a slow fading channel (Doppler frequency f/sub D/=10 Hz) compared with a time interleaving system. As another application, we apply the pre-diversity technique to normal MPSK post-diversity system using trellis code.

A hidden Markov model (HMM)-based MAP receiver for Nakagami fading channels

Abstract: Signalling over Rayleigh fading channels can be classed as a general Gaussian problem. Optimal linear filtering can then be applied to jointly estimate the channel and detect the information sequence. For fading channels with non-Gaussian distributions, optimal linear filtering does not necessarily yield the best channel estimates. To exploit the channel memory, a first order finite Markov chain model (HMM) that statistically characterizes the Nakagami-m fading process is used to aid the channel estimation. Based on this, a maximum a posteriori (MAP) receiver using coherent detection is presented for binary PAM signals.

Apparatus and method for data transmission to inhibit radio signal fading with sequential transmission of data groups based upon power levels

Abstract: Radio communication apparatus and method for inhibiting radio signal Rayleigh fading comprises apparatus for varying a nominal output power level in accordance with a sequence of power levels. The sequence may be pseudo-random. The nominal output power level may be one of the system power levels inhibiting normal fading and the sequence of power levels varies the output power in a range between adjacent nominal power levels.