Showing papers on "Fading published in 1991"

Correlation model for shadow fading in mobile radio systems

Abstract: A simple autocorrelation model for shadow fading in mobile radio channels is proposed. The model is fitted to both large cells and microcells. Results show that the model fit is good for large to m ...

An analysis of pilot symbol assisted modulation for Rayleigh fading channels (mobile radio)

Abstract: The author presents pilot-symbol-assisted modulation (PSAM) on a solid analytical basis, a feature missing from previous work. Closed-form expressions are presented for the bit error rate (BER) in binary-phase-shift-keying (BPSK) and in quadrature-phase-shift-keying (QPSK), for a tight upper bound on the symbol error rate in 16 quadrature-amplitude-modulation (16-QAM), and for the optimized receiver coefficients. The error rates obtained are lower than for differential detection for any combination of signal-to-noise ratio (SNR) and Doppler spread, and the performance is within 1 dB of a perfect reference system under slow-fading conditions and within 3 dB when the Doppler spread is 5% of the symbol rate. >

The land mobile satellite communication channel-recording, statistics, and channel model

Abstract: The communication channel between the MARECS satellite at 26 degrees W and a cruising van was measured and recorded in European areas exhibiting satellite elevations from 13 to 43 degrees . Different environments and mobile antennas were tested. The results of an extensive statistical evaluation include spectra of the fading amplitude; probability density, and distribution of the received signal power; and the percentage of time for fade and nonfade periods. Based on the physical phenomena of multipath fading and signal shadowing, an analog model of the land mobile satellite channel which can readily be used for software and hardware fading simulation is developed. The most important parameter of this model is the time-share of shadowing. The Rice factor which characterizes the channel during unshadowed periods, can vary from 3.9 to 18.1 dB. Block error probability density, error gap distribution, and block error probability are discussed. >

Statistical channel impulse response models for factory and open plan building radio communicate system design

Abstract: Statistical radio channel impulse response models are presented for the analysis and design of wireless factory and open plan office communication systems. The models incorporate first- and second-order statistics to characterize the discrete impulse responses of indoor radio channels for both line-of-sight (LOS) and obstructed (OBS) topographies. The effects of large-scale transmitter-receiver separation distance, small-scale receiver movement, and models for the correlation of multipath component amplitudes over 1 m local areas are developed from 1.3 GHz measurements. SIRCIM, a computer simulator based on the models presented, has recreated multipath power delay profiles and CW fading profiles that are representative of measured data. Large-scale models for path loss are implicitly included in this work. >

Fading in lightwave systems due to polarization-mode dispersion

Abstract: System fading caused by polarization-mode dispersion is investigated at 1.7 Gb/s using highly-birefringent, dispersion-shifted fiber at 1.55 mu m. The observed fading, which is manifested by random fluctuations of the bit error rate for a fixed receiver power, is observed to depend on the environmental conditions of the fiber, with the time constant for fading varying from minutes to hours depending on the rate of change of the ambient temperature. The mean dispersion penalty inferred from the observed fluctuations in the bit error rate is consistent with a square-law dependence on the polarization-mode dispersion for small penalties. >

Adaptive equalization for TDMA digital mobile radio

Abstract: Adaptive equalization for a TDMA (time-division multiple-access) digital cellular system is discussed. A survey of adaptive equalization techniques that includes their performance characteristics and limitations and their implementation complexity is presented. The design of adaptive equalization algorithms for a narrowband TDMA system is considered. It is concluded that, on the basis of implementation complexity and performance in the presence of multipath distortion and signal fading, MLSE (maximum-likelihood sequence estimation) and DFE (decision feedback equalization) are viable equalization methods for mobile radio. >

Analysis of equal gain diversity on Nakagami fading channels

Abstract: An infinite series for the complementary probability distribution function (CDF) of the signal-to-noise ratio (SNR) at the output of L-branch equal-gain (EG) diversity combiners in Nakagami (1960) fading channels is derived. The bit error rate for a matched filter receiver is analyzed for the L-branch EG combiner and different fading parameters. Both coherent phase shift keying (CPSK) and differential coherent phase shift keying (DCPSK) are considered. The effects of gain unbalance between branches on the probability distribution of the SNR and on the bit error rates are investigated. Bit error rate results are also obtained for coherent and noncoherent reception of frequency shift keying (FSK). The effects of gain unbalances on FSK modulations are also investigated. Bit error rates for EG combining on Rayleigh fading channels are obtained for L>2. These results are presented as a special case of the more generalized Nakagami fading model. >

Outage probabilities of cellular mobile radio systems with multiple Nakagami interferers

Abstract: Closed-form expressions for outage probabilities of mobile radio channels experiencing multiple, cochannel, independent Nakagami interferers are derived. This is done for the case of Nakagami fading alone with an arbitrary number of interferers. Analytical results for the case of Nakagami fading combined with log-normal shadowing are obtained for a single interferer. The case of multiple shadowed interferers is examined by simulation. The fading severity parameter in the Nakagami distribution may be varied to model different fading conditions. Interferers with similar and different Nakagami statistics are analyzed. The probability of cochannel interference is related to the reuse distance, which is one of the key parameters in the design of cellular mobile radio systems. In addition, the effects of specifying a minimum signal power requirement for satisfactory reception are investigated. A number of system examples that illustrate applications of the results are included. >

Signal and interference statistics of a CDMA system with feedback power control

Abstract: A feedback power control approach that allows power commands to be updated at a higher rate than the rate of multipath fading is investigated. The signal and interference statistics as received at the base stations after power control are obtained for a simulated direct-sequence code-division multiple-access (CDMA) system, which includes multiple base stations with diversity receivers and a large number of power-controlled users continuously moving at various speeds. It is shown that often-used analyses based on perfect average power control lead to optimistic capacity results (by 25% to 60%) because interference is underestimated by 1 to 2 dB. >

Bandwidth efficient QAM schemes for Rayleigh fading channels

Abstract: Techniques to improve the BER performance of 16-level QAM transmission over Rayleigh fading channels for PCM systems are presented, simulations were carried out with a carrier frequency of 1.9GHz, data rate of 64 kbit/s and mobile speed of 30mph. The residual BER was reduced by over an order of magnitutde, by introducing circular constellation coupled differential amplitude and phase encoding, compared with a square QAM constellation. When an oversampling and interpolation technique was combined with circular constellation with differential encoding another order of magnitude reduction in residual BER was obtained. By expanding the number of QAM levels to 64, and on using the two extra bits gained for block coding, the BER was reduced 10-6 for channel SNRs in excess of 35 dB. Decreasing the data throughput to 48 kbit/s using 3/4 rate RS(60, 44, 12) code, 16 level QAM, interleaved over 40ms, transmitted over a Rayleigh fading channel yielded a BER of 10-6 for channel SNRs above 25 dB.

BER performance of QDPSK with postdetection diversity reception in mobile radio channels

Abstract: The bit-error-rate performance of Nyquist raised-cosine-filtered quaternary differential phase shift keying (RC-QDPSK) and pi /4-shift RC-QDPSK with postdetection diversity reception is theoretically analyzed for Rayleigh fading channels. A selection combiner, equal-gain combiner, and maximal-ratio combiner are considered for postdetection diversity reception. Diversity improvements on the average BER due to additive white Gaussian noise, cochannel interference, random FM noise, and delay spread of the multipath channel are calculated. Also analyzed is the combined effect of diversity reception and error control. The required E/sub b//N/sub 0/ and allowable RMS delay spread for achieving a certain BER and spectrum efficiency in cellular mobile systems are calculated when short BCH forward error correcting codes are used. The performance of automatic repeat request combined with time diversity reception is investigated. >

Dual diversity combining and equalization in digital cellular mobile radio

Abstract: The performance of digital data transmission over frequency-selective fading channels is investigated. For statistically independent diversity paths, estimates of average attainable error rates and outage probabilities as functions of system parameters are provided. The dependences among the important system parameters are exhibited graphically for several examples, including quaternary phase-shift keying (QPSK). In the optimized uncoded QPSK with 1.5 b/s/Hz, two orders of magnitude in outage probability can be gained by diversity reception. When one compares the uncoded average probability of error for the optimized mean squared error (MSE) systems one finds at most an order-of-magnitude difference among the different equalizers investigated except for the zero-forcing equalizer, whose performance is drastically inferior to the others. Again, dual diversity can provide two orders of magnitude improvement in the average error rate or in outage probability for the uncoded optimized systems. >

Improved assessment of interference limits in cellular radio performance

Abstract: Reuse distance aid cluster size are evaluated for cellular mobile radio systems. Rayleigh fading, log-normal shadowing, and area mean power (which is inversely proportional to the fourth power of propagation distance) are considered, using the technique of Schwartz and Yeh (1982) for determining the probability density function of the sum of several log-normal variables in order to obtain the cochannel interference probabilities. These results are used for analyzing the radio spectrum efficiency, taking traffic intensity into consideration. Different modulation methods, namely, analog FM with 30 kHz and 12.5 kHz channel spacing, SSB (single sideband) with 5 kHz spacing, and digital modulation with 25 kHz channel spacing, with adequate protection ratios are compared. An example of the calculation of system bandwidth and cluster size using this procedure is presented. >

OFDM for data communication over mobile radio FM channels. I. Analysis and experimental results

Abstract: The performance of OFDM/FM modulation for digital communication over Rayleigh-fading mobile radio channels is described. The use of orthogonal frequency division multiplexing (OFDM) over mobile radio channels was proposed by Cimini (1985). OFDM transmits blocks of bits in parallel and reduces the bit error rate (BER) by averaging the effects of fading over the bits in the block. OFDM/FM is a modulation technique in which the OFDM baseband signal is used to modulate an FM transmitter. OFDM/FM can be implemented simply and inexpensively by retrofitting existing FM communication systems. Expressions are derived for the BER and word error rate (WER) within a block when each subchannel is QAM-modulated. Several numerical methods are developed to evaluate the overall BER and WER. An experimental OFDM/FM system was implemented and tested using unmodified VHF FM radio equipment and a fading channel simulator. The BER and WER results obtained from the hardware measurements agree closely with the numerical results. >

Computer models for fading channels with applications to digital transmission

Abstract: The authors describe computer models for Rayleigh, Rician, log-normal, and land mobile satellite fading channels. All computer models for the fading channels are based on the manipulation of a white Gaussian random process. This process is approximated by a sum of sinusoids with random phase angle. These models compare very well with analytical models in terms of their probability distribution of envelope and phase of the fading signal. For the land mobile satellite fading channel, results of level crossing rate and average fade duration are given. These results show that the computer models can provide a good coarse estimate of the time statistic of the faded signal. Also, for the land mobile satellite fading channel, the results show that a 3-pole Butterworth shaping filter should be used with the model. An example of the application of the land mobile satellite fading channel model to predict the performance of a differential phase-shift keying signal is described. >

Bit error probabilities MDPSK over the nonselective Rayleigh fading channel with diversity reception

Abstract: Data transmission using M-ary differential phase shift keying (MDPSK) over the nonselective Rayleigh fading channel with diversity reception is considered. While previous studies on error probability mostly assume no fading fluctuation, the author considers, exclusively, the case in which the fading process fluctuates from one symbol interval to the next. Exact bit error probability results for 2, 4, and 8 DPSK as well as tight upper bounds are derived. Some applications of the results are discussed. >

Error performance of multiple symbol differential detection of PSK signals transmitted over correlated Rayleigh fading channels

Abstract: A study is made of the error performance of multiple-symbol differential detection of uncoded PSK (phase shift keying) signals transmitted over correlated flat Rayleigh fading channels. It is found that the optimal detector uses a decoding metric which is a quadratic form of Gaussian variates. By using the characteristic function/residue theorem approach, the authors are able to derive an exact expression for the pairwise error event probability for the optimal detector. Subsequently, it is found that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor commonly associated with conventional differential detection. It is also found that the error performances of these detectors are not very sensitive to the mismatch between the decoding metric and the channel fading statistics. >

A digital DS spread-spectrum receiver with joint channel and Doppler shift estimation

Abstract: A digital spread-spectrum receiver design is presented for communication over multipath channels with severe Doppler shifts. The characteristics of the underwater channel relevant to spread-spectrum system design are discussed, and a channel model for short-range communications (less than 10 km) is defined. The receiver considered uses a digital coherent RAKE combiner, coupled with an extended Kalman filter (EKF)-based estimator for channel parameters and pseudonoise code delay. Receiver performance is evaluated by computing average bit-error rate (BER) versus iterations of the EKF joint estimator, using both fixed and time-varying channels. It is shown that the BER obtained using the EKF joint estimator closely tracks the optimum BER obtained when the channel, delay, and Doppler parameters are known exactly. Finally, the Cramer-Rao lower bound for time-invariant joint channel, delay, and Doppler estimation is derived, and compared with the ensemble averaged mean-squared error of the EKF estimator. >

Digital audio broadcasting system

Abstract: A digital audio broadcasting system that is capable of providing a large number of high quality stereophonic channels to mobile receivers in an environment with severe multipath delay and fading. Optimum combination of frequency and time diversity to guarantee robust performance in the mobile channel, with its multipath delay and frequency-selective fading effects. The system is based upon a dynamic single channel per carrier assignment of each stereo channel to many carriers. Intersymbol interference degradations caused by multipath delay are mitigated via an adaptive equalizer in the receiver. This dynamic single channel per carrier system preserves the simplicity inherent in the single channel per carrier assignment while it incorporates the ability to address frequency-selective fading by providing substantial frequency diversity. The frequency diversity is achieved via a slow frequency hop implementation in which the assignment of a number of stereo channels to an equal number of carrier frequencies is changed periodically. The system offers the simplicity of single channel per carrier assignment of stereo channels while it achieves the powerful performance benefits of frequency diversity and adaptive equalization for the mobile channel.

Broadband CDMA for personal communications systems

Abstract: Multipath propagation in a broadband CDMA environment is described. A propagation model for broadband spread-spectrum signals is presented. Experimental results relating to the sharing of the band by fixed service microwave users and mobile personal communications network (PCN) users are discussed. Field tests indicate that PCN systems can provide high-quality communications when sharing the spectrum with fixed-service microwave systems in suburban and urban areas. >

Burst coherent demodulation with combined symbol timing, frequency offset estimation, and diversity selection

Abstract: A low-overhead burst coherent demodulation method that jointly estimates symbol timing and carrier frequency offset and then performs diversity selection is studied. It coherently demodulates individual bursts of TDMA (time division multiple access) symbols by operating solely on random data within the burst without requiring training sequences. Its performance is robust against frequency offset between transmitter and receiver, thereby eliminating the need for a highly stable frequency reference. The performance of this demodulation method in a fading channel can be further improved by using a diversity selection technique based on a quality measure derived as part of the joint timing/frequency offset estimation process. Simulations and experiments have confirmed that two-branch diversity using this method can provide reliable speech communication using TDMA with a transmission rate of 450 kb/s for a portable radio channel with an RMS delay spread of 555 ns or less. >

MLSE equalization and decoding for multipath-fading channels

Abstract: The performance of a receiver using a combined MLSE (maximum likelihood sequence estimation) equalizer/decoder and D-diversity reception is analyzed for multipath Rayleigh fading channels. An upper bound on the (decoded) bit error probability is derived. Comparisons to simulation results show that this upper bound is quite tight when the system has a high signal-to-noise ratio or when diversity reception is used. The upper bound involves an infinite series that must be truncated at a point where the remainder can be safely assumed to be small. An algorithm based on a one-directional stack algorithm is proposed for this calculation because it makes efficient use of computer memory. >

Exact matched filter bound for two-beam Rayleigh fading

Abstract: The matched filter bound (MFB) is exactly evaluated for a channel modeled by the sum of two delayed and independently Rayleigh-fading beams. It is shown that, if the two beams have comparable average powers, and if the delay spread is moderate or large, then considerable gain is obtained from a diversity-like effect. That is, it is as if the two beams could be detected separately and their results combined. This is true even though significant interference would seem to be present between the beams. >

Autonomous adaptive frequency assignment for TDMA portable radio systems

Abstract: A simple autonomous procedure performed by each radio port (base station) to determine its own transmitting frequency is proposed. This procedure consists of signal strength measurements and an algorithm which selects the frequency with minimum interference from other ports. The algorithm converges rapidly while adapting to changes of operational conditions, such as installing new ports, which changes the system configuration, or adding new buildings, which causes different shadow fading. This method is significantly superior to a random assignment method both in resultant channel quality and in traffic-handling capability. For a regular-grid configuration, this method performs nearly as well as an a priori optimal frequency assignment method. It is also found that this assignment algorithm is robust against short-term signal fluctuations and it can be performed completely autonomously by each radio port. >

A new method for phase synchronization and automatic gain control of linearly modulated signals on frequency-flat fading channels

Abstract: An optimal phase synchronization and automatic gain control (AGC) scheme for coherent reception of linearly modulated signals on frequency-flat mobile fading channels is presented. The channel model and receiver performance are described. It is shown that using the technique allows the irreducible error floors (due to random FM) known from the noncoherent methods to be practically eliminated. Depending on the fastness of the fading, large power gains over the noncoherent methods are achieved. Unfavorable analog signal processing and/or the high bandwidth inefficiency of the FDM-pilot coherent methods are also avoided. >

Adaptive equalization and diversity combining for mobile radio using interpolated channel estimates

Abstract: The authors demonstrate the feasibility of a digital cellular radio (DCR) system which employs a jointly adaptive decision-feedback equalizer and diversity combiner In particular, the authors utilize the current estimates of the channel impulse response (CIR) at each diversity branch to compute the receiver parameters periodically The authors propose a novel block-adaptive strategy which computes the time-varying CIR by interpolating a set of CIR estimates obtained through periodic training Although incurring some inherent processing delay and throughput reduction, this interpolation strategy has the advantage of immunity to decision errors which would quite likely occur during a deep fade It is shown that the system performance is limited, in the form of an irreducible bit error rate at high signal-to-noise ratios (SNRs), by the CIR estimation of the rapidly fading channel >

Optimal detection of digital data over the nonselective Rayleigh fading channel with diversity reception

Abstract: Based on the criterion of minimum symbol error probability, an analysis is made of symbol-by-symbol detection of a sequence of digital data transmitted using linear suppressed-carrier modulation over L independent diversity channels with AWGN (additive white Gaussian noise) and slow nonselective Rayleigh fading. The optimal receiver is derived, but is found to be difficult to implement in practice because of its exponential growth in complexity as a function of sequence length. Suboptimal decision-feedback approximations are then suggested which are linear and readily implementable and can be integrated as generalized differentially coherent receivers. The exact bit error probabilities of these suboptimal receivers are obtained. Tight upper bounds on these error probabilities are also obtained which show simply how they behave as a function of signal-to-noise ratio and order of diversity. A main conclusion of this work is that optimal data detection on a fading channel should be performed using MMSE (minimum mean squared error) estimates of the quadrature amplitudes of the channel fading processes as a coherent reference. >

Burst coherent demodulation with combined symbol timing, frequency offset estimation, and diversity selection

Abstract: A low-overhead burst coherent demodulation method that jointly estimates symbol timing and carrier frequency offset and then performs diversity selection is studied. It coherently demodulates individual bursts of TDMA (time division multiple access) symbols by operating solely on random data within the burst without requiring training sequences. Its performance is robust against frequency offset between transmitter and receiver, thereby eliminating the need for a highly stable frequency reference. The performance of this demodulation method in a fading channel can be further improved by using a diversity selection technique based on a quality measure derived as part of the joint timing/frequency offset estimation process. Simulations and experiments have confirmed that two-branch diversity using this method can provide reliable speech communication using TDMA with a transmission rate of 450 kb/s for a portable radio channel with an RMS delay spread of 555 ns or less. >