Showing papers on "Fading published in 1996"

Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas

Abstract: This paper addresses digital communication in a Rayleigh fading environment when the channel characteristic is unknown at the transmitter but is known (tracked) at the receiver. Inventing a codec architecture that can realize a significant portion of the great capacity promised by information theory is essential to a standout long-term position in highly competitive arenas like fixed and indoor wireless. Use (n T , n R ) to express the number of antenna elements at the transmitter and receiver. An (n, n) analysis shows that despite the n received waves interfering randomly, capacity grows linearly with n and is enormous. With n = 8 at 1% outage and 21-dB average SNR at each receiving element, 42 b/s/Hz is achieved. The capacity is more than 40 times that of a (1, 1) system at the same total radiated transmitter power and bandwidth. Moreover, in some applications, n could be much larger than 8. In striving for significant fractions of such huge capacities, the question arises: Can one construct an (n, n) system whose capacity scales linearly with n, using as building blocks n separately coded one-dimensional (1-D) subsystems of equal capacity? With the aim of leveraging the already highly developed 1-D codec technology, this paper reports just such an invention. In this new architecture, signals are layered in space and time as suggested by a tight capacity bound.

Variable-rate variable-power MQAM for fading channels

Abstract: We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.

OFDM channel estimation by singular value decomposition

Abstract: We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels

Abstract: Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.

Performance of multicarrier DS CDMA systems

Abstract: In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system.

On verifying the first-order Markovian assumption for a Rayleigh fading channel model

Abstract: The use of received signal-to-noise ratio (SNR) as the side information in communication systems has been widely accepted especially when the channel quality is time varying. On many occasions, this side information is treated as the received SNR of the current channel symbol or that of previous symbols. In particular, the first-order Markov channel provides a mathematically tractable model for time-varying channels and uses only the received SNR of the symbol immediately preceding the current one. With the first-order Markovian assumption, given the information of the symbol immediately preceding the current one, any other previous symbol should be independent of the current one. Although the experimental measurements confirm the usefulness of the first-order Markovian assumption, one may argue that second or higher-order Markov processes should provide a more accurate model. We answer this question by showing that given the information corresponding to the previous symbol, the amount of uncertainty remaining in the current symbol should be negligible.

A rate-splitting approach to the Gaussian multiple-access channel

Abstract: It is shown that any point in the capacity region of a Gaussian multiple-access channel is achievable by single-user coding without requiring synchronization among users, provided that each user "splits" data and signal into two parts. Based on this result, a new multiple-access technique called rate-splitting multiple accessing (RSMA) is proposed. RSMA is a code-division multiple-access scheme for the M-user Gaussian multiple-access channel for which the effort of finding the codes for the M users, of encoding, and of decoding is that of at most 2M-1 independent point-to-point Gaussian channels. The effects of bursty sources, multipath fading, and inter-cell interference are discussed and directions for further research are indicated.

Comparison of diversity combining techniques for Rayleigh-fading channels

Abstract: Several methods of diversity combining for a Rayleigh-faded channel are evaluated and compared. The methods considered are, for coherent reception, maximal ratio combining (MRC), selection combining (SC), and a generalization of SC, whereby the two (three) signals with the two (three) largest amplitudes are coherently combined. We will call this method second (third) order SC, and denote it SC2 (SC3). Similar techniques are also investigated for noncoherent reception, with equal gain combining (EGC) replacing MRC, and noncoherent versions of SC2 and SC3. Numerical results indicate that SC2 and SC3 significantly enhances the bit-error rate (BER) performance relative to that achievable with SC, and under certain conditions approaches the performance achieved by MRC or EGG. The performance enhancement of SC2 and SC3 is especially noticable for noncoherent reception, where EGC is seen to provide the best performance only for low BER values. In fact, when the BER is 10/sup -3/ or greater, SC2 and SC3 performed comparably to EGG, and in some cases performed better than EGC.

Good lattice constellations for both Rayleigh fading and Gaussian channels

Abstract: Recent work on lattices matched to the Rayleigh fading channel has shown how to construct good signal constellations with high spectral efficiency. We present a new family of lattice constellations, based on complex algebraic number fields, which have good performance on Rayleigh fading channels. Some of these lattices also present a reasonable packing density and thus may be used at the same time over a Gaussian channel. Conversely, we show that particular versions of the best lattice packings (D/sub 4/, E/sub 6/, E/sub 8/, K/sub 12/, /spl Lambda//sub 16/, /spl Lambda//sub 24/), constructed from totally complex algebraic cyclotomic fields, present better performance over the Rayleigh fading channel. The practical interest in such signal constellations rises from the need to transmit information at high rates over both terrestrial and satellite links. Some further results in algebraic number theory related to ideals and their factorization are presented and the decoding algorithm used with these lattice constellations are illustrated together with practical results.

OFDM codes for peak-to-average power reduction and error correction

Abstract: Orthogonal frequency division multiplexing (OFDM) is a promising way to provide large data rates at reasonable complexity in wireless fading channels. However, a major disadvantage of OFDM is its large peak-to-average power ratio, which significantly decreases the efficiency of the transmitter power amplifier and hence forms a major obstacle to implementing OFDM in portable communication systems. This paper shows the possibility of using complementary codes for both decreasing the peak-to-average power (PAP) ratio and error correction. Set sizes and minimum distance properties of these codes are derived. It is shown that specific subsets of complementary codes have a minimum distance of up to half the code length, while their PAP ratio is only 3 dB. Simulation results demonstrate the viability of using these codes in multipath fading channels. It is currently planned to implement OFDM with complementary codes in the Wireless ATM Network Demonstrator (WAND), a joint European ACTS program.

A subspace method for signature waveform estimation in synchronous CDMA systems

Abstract: Synchronous code-division multiple-access (CDMA) techniques possess intrinsic protection against co-channel interference due to orthogonal codes employed and thus, offers higher capacity than existing frequency-division multiple-access (FDMA) or time-division multiple-access (TDMA) systems. In the presence of multipath, however, each signal is subject to frequency-selective fading and the orthogonality condition does not necessarily hold leading to increased cross correlation. In these scenarios, multiuser detection need to be performed to suppress interference and recover the message symbols. To implement such a technique, explicit knowledge of the (nonorthogonal) signature waveforms of all users is required. We propose a blind estimation scheme that provides closed-form estimates of the signature waveforms by exploiting the structure information of the data output. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of the signature waveforms. Based on this observation, a multiple signal classification (MUSIC)-like algorithm is derived. Performance analysis of the new approach is also presented.

Modelling and equalization of rapidly fading channels

Abstract: Basis expansion ideas are employed in order to equalize frequency-selective, rapidly fading channels. For certain multipath fading channels (e.g. the mobile radio channel), the time variations of the coefficients can be modelled as a combination of a small number of complex exponentials, under the assumption of linearly changing path delays. Based on this observation, novel adaptive and decision feedback algorithms are derived which exploit such an explicit modelling of the channel's variations. The problem of estimating the frequencies of the exponentials is also addressed using second- and higher-order cyclic statistics. By integrating ideas from cyclostationary signal analysis, both batch and recursive methods are developed. Finally, some illustrative simulations are presented.

Carrier frequency acquisition and tracking for OFDM systems

Abstract: We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

Emerging applications of multirate signal processing and wavelets in digital communications

Abstract: Multirate systems and filter banks have traditionally played an important role in source coding and compression for contemporary communication applications, and many of the key design issues in such applications have been extensively explored. We review developments on the comparatively less explored role of multirate filter banks and wavelets in channel coding and modulation for some important classes of channels. Some representative examples of emerging potential applications are described. One involves the use of highly dispersive, broadband multirate systems for wireless multiuser communication (spread spectrum CDMA) in the presence of fading due to time-varying multipath. Another is the wavelet-based diversity strategy referred to as "fractal modulation" for use with unpredictable communication links and in broadcast applications with user-selectable quality of service. A final example involves multitone (multicarrier) modulation systems based on multirate filter banks and fast lapped transforms for use on channels subject to severe intersymbol and narrowband interference. Collectively, these constitute intriguing, interrelated paradigms within an increasingly broad and active area of research.

Low bit-rate video transmission over fading channels for wireless microcellular systems

Abstract: We consider the transmission of QCIF resolution (176/spl times/144 pixels) video signals over wireless channels at transmission rates of 64 kb/s and below. The bursty nature of the errors on the wireless channel requires careful control of transmission performance without unduly increasing the overhead for error protection. A dual-rate source coder is presented that adaptively selects a coding rate according to the current channel conditions. An automatic repeat request (ARQ) error control technique is employed to retransmit erroneous data-frames. The source coding rate is selected based on the occupancy level of the ARQ transmission buffer. Error detection followed by retransmission results in less overhead than forward error correction for the same quality. Simulation results are provided for the statistics of the frame-error bursts of the proposed system over code division multiple access (CDMA) channels with average bit error rates of 10/sup -3/ to 10/sup -4/.

A perceptually optimized 3-D subband codec for video communication over wireless channels

Abstract: Visual communication over wireless channels is becoming important in multimedia. Because of the limited bandwidth and high error rates of the wireless channel, the video codec should be designed to have high coding efficiency in maintaining acceptable visual quality at low bit rates and robustness to suppress the distortion due to transmission errors. The coding efficiency of a 3D subband video codec is optimized by removing not only the redundancy due to spatial and temporal correlation but also perceptually insignificant components from video signals. Unequal error protection is applied to the source code bits of different perceptual importance. An error concealment method is employed to hide the distortion due to erroneous transmission of perceptually important signals. The evaluation of each signal's perceptual importance is made first from measuring the just-noticeable distortion (JND) profile as the perceptual redundancy inherent in video signals, and then from allocating JND energy to signals of different subbands according to the sensitivity of human visual responses to spatio-temporal frequencies. Simulation results show that acceptable visual quality can be maintained in transmitting video sequences with low bit rates (<64 kbps) over the wireless channel of high error rates (up to BER=10/sup -2/), and the distortion due to erroneous transmission of coded data can be effectively suppressed. In the simulation, the noisy channel is assumed to be corrupted by the random errors depending on the average strength of the received wave and the burst errors due to Rayleigh fading.

A deterministic digital simulation model for Suzuki processes with application to a shadowed Rayleigh land mobile radio channel

Abstract: We present a novel computer simulation model for a land mobile radio channel. The underlying channel model takes for granted non-frequency-selective fading but considers the effects caused by shadowing. For such a channel model we design a simulation model that is based on an efficient approximation of filtered white Gaussian noise processes by finite sums of properly weighted sinusoids with uniformly distributed phases. In all, four completely different methods for the computation of the coefficients of the simulation model are introduced. Furthermore, the performance of each procedure is investigated on the basis of two quality criteria. All the presented methods have in common the fact that the resulting simulation model has a completely determined fading behavior for all time. Therefore, the simulation model can be interpreted as a deterministic model that approximates stochastic processes such as Rayleigh, log-normal, and Suzuki (1977) processes.

Performance of wireless CDMA with M-ary orthogonal modulation and cell site antenna arrays

Abstract: An antenna array-based base station receiver structure for wireless direct-sequence code-division multiple-access (DS/CDMA) with M-ary orthogonal modulation is proposed. The base station uses an antenna array beamformer-RAKE structure with noncoherent equal gain combining. The receiver consists of a "front end" beamsteering processor feeding a conventional noncoherent RAKE combiner. The performance of the proposed receiver with closed loop power control in multipath fading channels is evaluated. Expressions for the system uncoded bit-error probability (BEP) as a function of the number of users, number of antennas, and the angle spread are derived for different power control scenarios. The system capacity in terms of number of users that can be supported for a given uncoded BEP is also evaluated. Analysis results show a performance improvement in terms of the system capacity due to the use of antenna arrays and the associated signal processing at the base station. In particular, analysis results show an increase in system capacity that is proportional to the number of antennas. They also show an additional performance improvement due to space diversity gain provided by the array for nonzero angle spreads.

Low-overhead, low-complexity [burst] synchronization for OFDM

Abstract: A rapid synchronization method is presented for an OFDM system using either a continuous transmission or a burst operation over a time-varying, fading channel. It will acquire the signal and provide channel estimation upon the receipt of just one training sequence of two symbols in the presence of unknown symbol and frame timing, large carrier and sampling frequency offsets, and very low SNRs, while maintaining low latency and low complexity. It can then track the signal with the same algorithms. Digital TV and wireless LAN are used as examples.

SBR image approach for radio wave propagation in tunnels with and without traffic

Abstract: We propose a deterministic approach to model the radio propagation channels in tunnels with and without traffic. This technique applies the modified shooting and bouncing ray (SBR) method to find equivalent sources (images) in each launched ray tube and sums the receiving complex amplitude contributed by all images coherently. In addition, the vector effective antenna height (VEH) is introduced to consider the polarization-coupling effect resulting from the shape of the tunnels. We verify this approach by comparing the numerical results in two canonical examples where closed-form solutions exist. The good agreement indicates that our method can provide a good approximation of high-frequency radio propagation inside tunnels where reflection is dominant. We show that the propagation loss in tunnels can vary considerably according to the tunnel shapes and the traffic inside them. From the results we also find a "focusing" effect, which makes the power received in an arched tunnel higher than that in a rectangular tunnel. Besides, the deep fading that appears in a rectangular tunnel is absent in an arched tunnel. The major effect of the traffic is observed to be the fast fading due to the reflection/obstruction of vehicles. Additional considerations, such as time delay, wall roughness, and wedge diffraction of radio wave propagation in tunnels are left for future studies.

Adaptive modulation system with variable coding rate concatenated code for high quality multi-media communication systems

Abstract: This paper proposes an adaptive modulation system with a concatenated code (AMS-CC) for land mobile communications to achieve high quality, high bit rate transmission in Rayleigh fading environments. The proposed system adaptively controls the coding rate of the inner convolutional code, symbol rate, and modulation level according to the instantaneous fading channel conditions. The performances in Rayleigh fading environments are evaluated theoretically and by computer simulation. The results show that the proposed system can realize higher quality transmission without the degradation in average bit rate compared to conventional adaptive modulation systems.

Design and analysis of turbo codes on Rayleigh fading channels

Abstract: The performance of turbo codes using coherent BSPK signaling on Rayleigh fading channels is considered. In regions of low signal-to-noise, performance analysis uses simulations of typical turbo coding systems. For higher signal-to-noise regions beyond simulation capabilities, an average upper bound is used, where the average is over all possible interleaving schemes. Fully-interleaved and exponentially-correlated Rayleigh channels are explored. Furthermore, the design issues relevant to turbo codes are explored for the correlated fading channel.

Multicarrier CDMA with adaptive frequency hopping for mobile radio systems

Abstract: A modified multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) system has been proposed for use over slow multipath fading channels with frequency selectivity in the reverse link transmission of a cellular network. Instead of transmitting data substreams uniformly through subchannels, data substreams hop over subchannels with the hopping patterns adaptively adjusted to the channel fading characteristics. The problem of determining the optimal hopping pattern is formulated as a multiobjective optimization problem, for which an efficient algorithm, based on the water-filling (WF) principle, is designed to solve the problem practically. Simulation results show that the performance in terms of the average bit-error probability (BEP) (over all users) is better than that of single carrier RAKE receiver systems, conventional MC CDMA systems applying moderate error protection, or diversity systems with different combining techniques.

An adaptive power control and coding scheme for mobile radio systems

Abstract: We propose a dynamic combined power control and forward error correction control (FEC) algorithm for mobile radio systems that can minimize the power consumed by wireless transmitters while increasing the number of simultaneous connections. This algorithm is distributed, where individual transmitter-receiver pairs determine the minimal power and FEC to satisfy specified quality-of-service (QOS) constraints. We present simulation results showing that this algorithm outperforms previous algorithms that use only power control.

Geometrically based statistical channel model for macrocellular mobile environments

Abstract: We develop a statistical geometric propagation model for a macrocell mobile environment that provides the statistics of angle-of-arrival of the multipath components. This channel model assumes that each multipath component of the propagating signal undergoes only one bounce traveling from the transmitter to the receiver and that scattering objects are located uniformly within a circle around the mobile. This geometrically based single bounce macrocell (GBSBM) channel model provides three important parameters that characterize a channel: the power of the multipath components, the time-of-arrival (TOA) of the components, and the angle-of-arrival (AOA) of the components. Doppler spectra and fading envelopes obtained using the GBSBM model is compared with Clarke's (1968) model. The results show that the rate of fading at the base station is lower than at the mobile and the reduction in the Doppler spread at the station is dependent on the direction of motion of the mobile with respect to the base station and the radius of the scattering circle.

Analysis of adaptive multistage interference cancellation for CDMA using an improved Gaussian approximation

Abstract: We consider a simple model for adaptive multistage interference cancellation within a CDMA system, and seek to develop an accurate analytical expression for the performance of this system. Previous work on interference cancellation has relied heavily on simulation techniques or a simple Gaussian approximation (GA). The standard GA ran lead to bit-error rate (BER) results which are optimistic for the conventional receiver, and this also occurs when the approximation is applied to the interference cancellation problem. Additionally, this approximation does not allow the second order effects of the multiple access interference (MAI) to be included in the performance estimates. Several improvements on the standard GA have been suggested which result in accurate performance results for a standard CDMA receiver. This paper presents an analytical expression for the probability of bit error for an adaptive multistage interference canceller, using an improved Gaussian approximation (IGA) for MAI. The BER at any stage of interference cancellation can be recursively computed from the signal-to-noise ratio (SNR), the statistics of the random powers of users, and the processing gain of the CDMA system. The performance of the resulting EER expression is compared with simulation results. Since the second order effects of MAI can be included, the analytical framework presented here can also be used to evaluate the performance of multistage interference cancellation in arbitrary fading environments, and we present results for the performance of interference cancellation in lognormal fading environments.

Comment on "Estimate of channel capacity in Rayleigh fading environment

Abstract: Lee (see ibid., vol.39, p.187, no.8, 1990) proposed a very simple and elegant model for estimating the capacity of slowly fading channels. A small error in his paper, however, has led to an inexact picture. We correct that inaccuracy. As a consequence, the loss in capacity of a Rayleigh fading channel with respect to the additive white Gaussian noise (AWGN) channel turns out to be negligible for all values of the signal-to-noise ratio that are of interest. Furthermore, we extend the analytical results to the situation with M-branch diversity and Ricean fading channels. The lower moments of the associated distributions, i.e., of the /spl chi//sup 2/-distribution (Nakagami with integer m-parameter) and Rice distribution agree when the parameters of the distribution are suitably chosen. With that choice of parameters, the capacities of the M-branch diversity and of the Ricean fading channel become almost identical.

Estimation and equalization of fading channels with random coefficients

Abstract: The time-varying tap coefficients of frequency selective fading channels are typically modeled as random processes with low-pass power spectra. However, traditional adaptive techniques usually make no assumption on the channel's time variations and hence do not exploit this information. Kalman filtering methods are derived to track the channel by employing a multichannel autoregressive description of the time-varying taps in a decision-feedback equalization framework. Fitting a model to the variations of the channel's taps is a challenging task because the tap coefficients are not observed directly. A novel linear method is proposed in order to estimate the model parameters from input/output data. The consistency of the proposed method is shown, and some illustrative simulations are presented.

Outage probability in cellular mobile radio due to Nakagami signal and interferers with arbitrary parameters

Abstract: Outage probability provides a fundamental performance measure of the grade of service for cellular mobile radio systems. Determination of the outage probability in a Nakagami (1960) mobile environment is particularly important since Nakagami fading is, as shown by various empirical measurements, the most appropriate model in many practical applications. Effective techniques have been developed recently to determine outage probability in the presence of multiple Nakagami interferers by assuming that the fading parameters of both signal and co-channel interferences are integer-valued. However, the general problem with arbitrary Nakagami signal and interferers remains unsolved. A new technique is presented, which allows the Nakagami distributions for both signal and interferers to be arbitrary. Exact formulas for probability of outage with and without a constraint on minimum signal power are derived, and computer results are also presented to illustrate the theory.