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Showing papers on "Fading published in 1998"


Journal ArticleDOI
Siavash Alamouti1
TL;DR: This paper presents a simple two-branch transmit diversity scheme that provides the same diversity order as maximal-ratio receiver combining (MRRC) with one transmit antenna, and two receive antennas.
Abstract: This paper presents a simple two-branch transmit diversity scheme. Using two transmit antennas and one receive antenna the scheme provides the same diversity order as maximal-ratio receiver combining (MRRC) with one transmit antenna, and two receive antennas. It is also shown that the scheme may easily be generalized to two transmit antennas and M receive antennas to provide a diversity order of 2M. The new scheme does not require any bandwidth expansion or any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC.

13,706 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined the performance of using multi-element array (MEA) technology to improve the bit-rate of digital wireless communications and showed that with high probability extraordinary capacity is available.
Abstract: This paper is motivated by the need for fundamental understanding of ultimate limits of bandwidth efficient delivery of higher bit-rates in digital wireless communications and to also begin to look into how these limits might be approached. We examine exploitation of multi-element array (MEA) technology, that is processing the spatial dimension (not just the time dimension) to improve wireless capacities in certain applications. Specifically, we present some basic information theory results that promise great advantages of using MEAs in wireless LANs and building to building wireless communication links. We explore the important case when the channel characteristic is not available at the transmitter but the receiver knows (tracks) the characteristic which is subject to Rayleigh fading. Fixing the overall transmitted power, we express the capacity offered by MEA technology and we see how the capacity scales with increasing SNR for a large but practical number, n, of antenna elements at both transmitter and receiver. We investigate the case of independent Rayleigh faded paths between antenna elements and find that with high probability extraordinary capacity is available. Compared to the baseline n = 1 case, which by Shannon‘s classical formula scales as one more bit/cycle for every 3 dB of signal-to-noise ratio (SNR) increase, remarkably with MEAs, the scaling is almost like n more bits/cycle for each 3 dB increase in SNR. To illustrate how great this capacity is, even for small n, take the cases n = 2, 4 and 16 at an average received SNR of 21 dB. For over 99% of the channels the capacity is about 7, 19 and 88 bits/cycle respectively, while if n = 1 there is only about 1.2 bit/cycle at the 99% level. For say a symbol rate equal to the channel bandwith, since it is the bits/symbol/dimension that is relevant for signal constellations, these higher capacities are not unreasonable. The 19 bits/cycle for n = 4 amounts to 4.75 bits/symbol/dimension while 88 bits/cycle for n = 16 amounts to 5.5 bits/symbol/dimension. Standard approaches such as selection and optimum combining are seen to be deficient when compared to what will ultimately be possible. New codecs need to be invented to realize a hefty portion of the great capacity promised.

10,526 citations


Journal ArticleDOI
TL;DR: This paper describes the statistical models of fading channels which are frequently used in the analysis and design of communication systems, and focuses on the information theory of fading channel, by emphasizing capacity as the most important performance measure.
Abstract: In this paper we review the most peculiar and interesting information-theoretic and communications features of fading channels. We first describe the statistical models of fading channels which are frequently used in the analysis and design of communication systems. Next, we focus on the information theory of fading channels, by emphasizing capacity as the most important performance measure. Both single-user and multiuser transmission are examined. Further, we describe how the structure of fading channels impacts code design, and finally overview equalization of fading multipath channels.

2,017 citations


Journal ArticleDOI
TL;DR: This work focuses on the multiaccess fading channel with Gaussian noise, and defines two notions of capacity depending on whether the traffic is delay-sensitive or not, and characterize the throughput capacity region which contains the long-term achievable rates through the time-varying channel.
Abstract: In multiaccess wireless systems, dynamic allocation of resources such as transmit power, bandwidths, and rates is an important means to deal with the time-varying nature of the environment. We consider the problem of optimal resource allocation from an information-theoretic point of view. We focus on the multiaccess fading channel with Gaussian noise, and define two notions of capacity depending on whether the traffic is delay-sensitive or not. We characterize the throughput capacity region which contains the long-term achievable rates through the time-varying channel. We show that each point on the boundary of the region can be achieved by successive decoding. Moreover, the optimal rate and power allocations in each fading state can be explicitly obtained in a greedy manner. The solution can be viewed as the generalization of the water-filling construction for single-user channels to multiaccess channels with arbitrary number of users, and exploits the underlying polymatroid structure of the capacity region.

1,216 citations


Journal ArticleDOI
TL;DR: A minimum mean-square-error (MMSE) channel estimator is derived, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels and can significantly improve the performance of OFDM systems in a rapid dispersion fading channel.
Abstract: Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

1,039 citations


Journal ArticleDOI
TL;DR: Very high diversity orders can be achieved and this results in an almost Gaussian performance over the fading channel, this multidimensional modulation scheme is essentially uncoded and enables one to trade diversity for system complexity, at no power or bandwidth expense.
Abstract: The increasing need for high data-rate transmissions over time- or frequency-selective fading channels has drawn attention to modulation schemes with high spectral efficiency such as QAM. With the aim of increasing the "diversity order" of the signal set we consider multidimensional rotated QAM constellations. Very high diversity orders can be achieved and this results in an almost Gaussian performance over the fading channel, This multidimensional modulation scheme is essentially uncoded and enables one to trade diversity for system complexity, at no power or bandwidth expense.

1,030 citations


Journal ArticleDOI
TL;DR: Trellis and lattice codes designed for additive white Gaussian noise channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains, to obtain trellis-coded adaptive MQAM.
Abstract: We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive M-ary quadrature amplitude modulation (MQAM) to obtain trellis-coded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple four-state trellis code, and an effective 3.6-dB coding gain for an eight-state trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trellis-coded adaptive MQAM to that of coded modulation with built-in time diversity and fixed-rate modulation. The adaptive method exhibits a power savings of up to 20 dB.

947 citations


Journal ArticleDOI
TL;DR: This tutorial reviews the key concepts in spatial channel modeling and presents emerging approaches, and reviews the research issues in developing and using spatial channel models for adaptive antennas.
Abstract: Spatial antenna diversity has been important in improving the radio link between wireless users. Historically, microscopic antenna diversity has been used to reduce the fading seen by a radio receiver, whereas macroscopic diversity provides multiple listening posts to ensure that mobile communication links remain intact over a wide geographic area. In later years, the concepts of spatial diversity have been expanded to build foundations for emerging technologies, such as smart (adaptive) antennas and position location systems. Smart antennas hold great promise for increasing the capacity of wireless communications because they radiate and receive energy only in the intended directions, thereby greatly reducing interference. To properly design, analyze, and implement smart antennas and to exploit spatial processing in emerging wireless systems, accurate radio channel models that incorporate spatial characteristics are necessary. In this tutorial, we review the key concepts in spatial channel modeling and present emerging approaches. We also review the research issues in developing and using spatial channel models for adaptive antennas.

917 citations


Journal ArticleDOI
01 Sep 1998
TL;DR: It is shown that in the largest majority of cases, these error-rate expressions can be put in the form of a single integral with finite limits and an integrand composed of elementary functions, thus readily enabling numerical evaluation.
Abstract: Presented here is a unified approach to evaluating the error-rate performance of digital communication systems operating over a generalized fading channel. What enables the unification is the recognition of the desirable form for alternate representations of the Gaussian and Marcum Q-functions that are characteristic of error-probability expressions for coherent, differentially coherent, and noncoherent forms of detection. It is shown that in the largest majority of cases, these error-rate expressions can be put in the form of a single integral with finite limits and an integrand composed of elementary functions, thus readily enabling numerical evaluation.

851 citations


Journal ArticleDOI
TL;DR: Performance limits for two closely related communication scenarios involving a wireless system with multiple-element transmitter antenna arrays are derived and it is shown that, when properly chosen, even a small amount of side information can be quite valuable.
Abstract: We derive performance limits for two closely related communication scenarios involving a wireless system with multiple-element transmitter antenna arrays: a point-to-point system with partial side information at the transmitter, and a broadcast system with multiple receivers. In both cases, ideal beamforming is impossible, leading to an inherently lower achievable performance as the quality of the side information degrades or as the number of receivers increases. Expected signal-to-noise ratio (SNR) and mutual information are both considered as performance measures. In the point-to-point case, we determine when the transmission strategy should use some form of beamforming and when it should not. We also show that, when properly chosen, even a small amount of side information can be quite valuable. For the broadcast scenario with an SNR criterion, we find the efficient frontier of operating points and show that even when the number of receivers is larger than the number of antenna array elements, significant performance improvements can be obtained by tailoring the transmission strategy to the realized channel.

819 citations


Proceedings ArticleDOI
07 Jun 1998
TL;DR: A minimum mean-square-error (MSE) channel estimator is derived, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels and can significantly improve the performance of OFDM systems in a rapid dispersion fading channel.
Abstract: Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

Journal ArticleDOI
01 Dec 1998
TL;DR: Novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers.
Abstract: The time-varying impulse response of rapidly fading mobile communication channels is expanded over a basis of complex exponentials that arise due to Doppler effects encountered with multipath propagation. Blind methods are reviewed for estimating the bases' parameters and the model orders. Existing second-order methods are critiqued and novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers. Illustrative simulations are also presented.

Proceedings ArticleDOI
16 Aug 1998
TL;DR: Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
Abstract: Mobile users' capacity is limited by the fact that within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. We are proposing a new form of diversity, whereby diversity gains are achieved via the cooperation of in-cell users. Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity but also to a more robust system, where users' achievable rates are less susceptible to channel variations.

Journal ArticleDOI
TL;DR: The results show that an ultra-wide bandwidth signal does not suffer multipath fading, and the robustness of the UWB signal in multipath is quantified through cumulative distribution functions of the signal quality in various locations of the building.
Abstract: The results of an ultra-wide bandwidth (UWB) signal propagation experiment, using bandwidth in excess of 1 GHz, performed in a typical modern office building are presented. The robustness of the UWB signal in multipath is quantified through cumulative distribution functions of the signal quality in various locations of the building. The results show that an UWB signal does not suffer multipath fading.

Journal ArticleDOI
TL;DR: A quasi-analytical experimental analysis is described in this paper to quantify the tradeoff between energy capture and diversity level in a RAKE receiver using measured received waveforms obtained from ultrawide bandwidth signal propagation experiments.
Abstract: A quasi-analytical experimental analysis is described in this paper to quantify the tradeoff between energy capture and diversity level in a RAKE receiver using measured received waveforms obtained from ultrawide bandwidth signal propagation experiments.

Journal ArticleDOI
16 Aug 1998
TL;DR: It is shown that variable-rate coding is not needed to achieve capacity and, even when the CSIT is not perfect, the capacity achieving power allocation is of the waferfilling type.
Abstract: We study the capacity of some channels whose conditional output probability distribution depends on a state process independent of the channel input and where channel state information (CSI) signals are available both at the transmitter (CSIT) and at the receiver (CSIR). When the channel state and the CSI signals are jointly independent and identically distributed (i.i.d.), the channel reduces to a case studied by Shannon (1958). In this case, we show that when the CSIT is a deterministic function of the CSIR, optimal coding is particularly simple. When the state process has memory, we provide a general capacity formula and we give some more restrictive conditions under which the capacity has still a simple single-letter characterization, allowing simple optimal coding. Finally, we turn to the additive white Gaussian noise (AWGN) channel with fading and we provide a generalization of some results about capacity with CSI for this channel. In particular, we show that variable-rate coding (or multiplexing of several codebooks) is not needed to achieve capacity and, even when the CSIT is not perfect, the capacity achieving power allocation is of the waferfilling type.

Proceedings ArticleDOI
05 Oct 1998
TL;DR: The effect of fading correlations in multi-element antenna (MEA) communication systems is investigated and it is shown that the MEA capacity can be expressed as the sum of the capacities of several subchannels, whose gains are affected by the fading correlation.
Abstract: We investigate the effect of fading correlations in multi-element antenna (MEA) communication systems. The enormous capacity of an MEA system can potentially be reduced by fading correlation. To model the narrowband Rayleigh-fading multipath environment, we propose an abstract model. Using this model, the fading statistics can be determined from the geometrical parameters of the MEA and the multipath environment. This, model allows us to directly observe how the choice of antenna geometry affects capacity. We show that the MEA capacity can be expressed as the sum of the capacities of several subchannels, whose gains are affected by the fading correlation. As the fading correlation becomes higher, the disparity between the gains of these subchannels becomes larger and as a result some of the subchannels do not convey information at any significant rate.

Journal ArticleDOI
TL;DR: It is shown that successive decoding is optimal, and the optimal decoding order and power allocation can be found explicitly as a function of the fading states; this is a consequence of an underlying polymatroid structure that is exploited.
Abstract: For pt.I see ibid., vol.44, no.7, p.2796-815 (1998). In multiaccess wireless systems, dynamic allocation of resources such as transmit power, bandwidths, and rates is an important means to deal with the time-varying nature of the environment. We consider the problem of optimal resource allocation from an information-theoretic point of view. We focus on the multiaccess fading channel with Gaussian noise, and define two notions of capacity depending on whether the traffic is delay-sensitive or not. In the present paper, we introduce a notion of delay-limited capacity which is the maximum rate achievable with delay independent of how slow the fading is. We characterize the delay-limited capacity region of the multiaccess fading channel and the associated optimal resource allocation schemes. We show that successive decoding is optimal, and the optimal decoding order and power allocation can be found explicitly as a function of the fading states; this is a consequence of an underlying polymatroid structure that we exploit.

Proceedings ArticleDOI
08 Nov 1998
TL;DR: A channel estimation technique is developed for an OFDM system with multiple transmit and multiple receive antennas and is shown to be optimal within the class of linear MMSE estimators.
Abstract: A channel estimation technique is developed for an OFDM system with multiple transmit and multiple receive antennas. The approach exploits the channel structure in the time domain and the frequency domain. The method is appropriate for OFDM systems operating in a time varying channel. A multi-input-multi-output OFDM (MOFDM) system model is defined, followed by a proposed excitation strategy for sufficient identification of the channel. The estimation strategy is shown to be optimal within the class of linear MMSE estimators. Experimental results of the algorithm are provided.

Journal ArticleDOI
TL;DR: The theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and its performance over a frequency-flat Rayleigh fading channel are presented and it is concluded that STCM can provide significant SNR improvement over simple delay diversity.
Abstract: This paper presents the theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach, data is encoded by a space-time (ST) channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem, as applied to the IS-136 cellular standard, are presented. We present the frame error rate (FER) performance results as a function of the signal-to-noise ratio (SNR) and the maximum Doppler frequency, in the presence of timing and frequency offset errors. Simulation results show that for a 10% FER, a 32-state eight-phase-shift keyed (8-PSK) ST code with two transmit and two receive antennas can support data rates up to 55.8 kb/s on a 30-kHz channel, at an SNR of 11.7 dB and a maximum Doppler frequency of 180 Hz. Simulation results for other codes and other channel conditions are also provided. We also compare the performance of the proposed STCM scheme with delay diversity schemes and conclude that STCM can provide significant SNR improvement over simple delay diversity.

Journal ArticleDOI
16 Aug 1998
TL;DR: The capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths is investigated and it is shown that if white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths L/spl tilde/ with energy spread out.
Abstract: We investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We show that the capacity of the channel in the wideband limit is the same as that of a wideband Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be "peaky" in time or frequency. In particular, we show that if white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths L/spl tilde/ with energy spread out, and in fact approaches 0 as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter L/spl tilde//sub crit/ is defined in terms of system parameters to delineate the threshold on L over which such overspreading phenomenon occurs.

Journal ArticleDOI
TL;DR: This work derives the expected intersymbol/interchannel interference of such a nonorthogonal FDM (NOFDM) system under the assumption of a wide-sense stationary uncorrelated scattering (WSSUS) channel and compares OFDM and NOFDM schemes with regard to robustness against delay/Doppler spread.
Abstract: A new approach to multicarrier digital communication over time-varying, frequency selective fading channels is presented. We propose a transmission signal set whose basic structure is similar to standard orthogonal frequency division multiple access (OFDM)-setups, i.e., a system of functions generated by time and frequency-shifted versions of a pulse-like prototype function known as a Weyl-Heisenberg (WH) system. Unlike previous OFDM studies, however, which are restricted to the case of orthonormal pulses, we consider nonorthogonal pulses that are adapted to realistically available a priori knowledge of the channel. Perfect transmultiplexing in the case of an ideal channel is incorporated as a mathematical side-constraint. We derive the expected intersymbol/interchannel interference of such a nonorthogonal FDM (NOFDM) system under the assumption of a wide-sense stationary uncorrelated scattering (WSSUS) channel. Based on this result, we compare OFDM and NOFDM schemes with regard to robustness against delay/Doppler spread.

Journal ArticleDOI
01 Feb 1998
TL;DR: In this paper, the angular domain is considered for flat fading and frequency-selective fading channels and a generic channel model is introduced incorporating directions of arrival and a set of parameters is given for its configuration.
Abstract: The definition of flat fading and frequency-selective fading channels is extended to low-rank and high-rank channels to include the angular domain. By physical reasoning a generic channel model is introduced incorporating directions of arrival and a set of parameters is given for its configuration. The time evolution of the scattering processes and directions of arrival is taken into account. Analytical expressions are derived for space-correlation coefficients and frequency-correlation coefficients. Spatial correlation decreases with increasing angular spread and decreasing angle of incidence, measured from array broadside.

Journal ArticleDOI
TL;DR: The performance and design of turbo codes using coherent BPSK signaling on the Rayleigh fading channel is considered and Turbo interleaver design criteria are developed and architectural modifications are proposed for improved performance.
Abstract: The performance and design of turbo codes using coherent BPSK signaling on the Rayleigh fading channel is considered. In low signal-to-noise regions, performance analysis uses simulations of typical turbo coding systems. For higher signal-to-noise regions beyond simulation capabilities, an average upper bound is used in which 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 examined for the correlated fading channel. Turbo interleaver design criteria are developed and architectural modifications are proposed for improved performance.

Journal ArticleDOI
TL;DR: An exact closed-form solution for the reliability of an ideal M-branch MMSE (minimum mean-squared error) diversity combiner operating in a Rayleigh-fading channel with N interferers, each having some specified average power is derived.
Abstract: We derive an exact closed-form solution for the reliability of an ideal M-branch MMSE (minimum mean-squared error) diversity combiner operating in a Rayleigh-fading channel with N interferers, each having some specified average power. The reliability is defined as the probability, taken over fading of the desired and interfering signals, that the combiner's output signal-to-interference ratio (SINR) is greater than some specified threshold. This kind of metric is important in evaluating the potential capacity improvements of using diversity combining and adaptive array processing in interference-limited wireless systems. Our result is remarkably simple, fast, straightforward to compute, and numerically stable. We show a set of special cases, which relate to standard results and reveal valuable insights into how this type of array processing operates in interference-limited environments. We also present a set of numerical examples, which show that our calculated reliabilities agree with estimates from Monte Carlo simulation.

Journal ArticleDOI
TL;DR: A synchronous multiuser system operating in an additive white Gaussian noise channel, with or without multipath fading, is considered and it is shown that both multiple access and intersymbol interference can be eliminated by means of a suitable transmitter precoding scheme.
Abstract: A synchronous multiuser system operating in an additive white Gaussian noise channel, with or without multipath fading, is considered. It is shown that when either a conventional single user receiver or the RAKE receiver is employed, both multiple access and intersymbol interference can be eliminated by means of a suitable transmitter precoding scheme. Transmitter precoding represents a linear transformation of transmitted signals, such that the mean squared errors at all receivers are minimized. Precoding, with both conventional single user receiver and with the RAKE receiver, results in near-far resistant performance and outperforms considerably the respective schemes without precoding. The crucial assumption, in the multipath case, is that the transmitter knows the multipath characteristics of all channels and that channel dynamics are sufficiently slow so that multipath profiles remain essentially constant over the block of precoded bits.

Journal ArticleDOI
TL;DR: The authors have accurately approximated the Rayleigh-lognormal distribution by the K distribution, which is simpler and thus more appropriate for analysis and design of wireless communication systems.
Abstract: The Rayleigh-lognormal distribution, which has proved useful for modelling fading-shadowing wireless channels, has a complicated integral form. The authors have accurately approximated it by the K distribution. This distribution is simpler and thus more appropriate for analysis and design of wireless communication systems.

Journal ArticleDOI
Jack Harriman Winters1
TL;DR: The results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas, and that the same diversity benefit can be obtained at the remotes and base stations using multiple base-station antennas only.
Abstract: In this paper, we study the ability of transmit diversity to provide diversity benefit to a receiver in a Rayleigh fading environment. With transmit diversity, multiple antennas transmit delayed versions of a signal to create frequency-selective fading at a single antenna at the receiver, which uses equalization to obtain diversity gain against fading. We use Monte Carlo simulation to study transmit diversity for the case of independent Rayleigh fading from each transmit antenna to the receive antenna and maximum likelihood sequence estimation for equalization at the receiver. Our results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas. Thus, we can obtain the same diversity benefit at the remotes and base stations using multiple base-station antennas only.

Journal ArticleDOI
TL;DR: A novel approach to stable noise modeling is introduced based on the LePage series representation and the results obtained are useful for the prediction of noise statistics in a wide range of environments with deterministic and stochastic power propagation laws.
Abstract: This paper addresses non-Gaussian statistical modeling of interference as a superposition of a large number of small effects from terminals/scatterers distributed in the plane/volume according to a Poisson point process. This problem is relevant to multiple access communication systems without power control and radar. Assuming that the signal strength is attenuated over distance r as 1/r/m, we show that the interference/clutter could be modeled as a spherically symmetric /spl alpha/-stable noise. A novel approach to stable noise modeling is introduced based on the LePage series representation. This establishes grounds to investigate practical constraints in the system model adopted, such as the finite number of interferers and nonhomogeneous Poisson fields of interferers. In addition, the formulas derived allow us to predict noise statistics in environments with lognormal shadowing and Rayleigh fading. The results obtained are useful for the prediction of noise statistics in a wide range of environments with deterministic and stochastic power propagation laws. Computer simulations are provided to demonstrate the efficiency of the /spl alpha/-stable noise model in multiuser communication systems. The analysis presented will be important in the performance evaluation of complex communication systems and in the design of efficient interference suppression techniques.

Journal ArticleDOI
TL;DR: The simulation model's amplitude and phase probability density function (PDF) is investigated, but also higher order statistics [e.g., level-crossing rate (LCR) and average duration of fades (ADFs] are investigated].
Abstract: Rice's (9144, 1945) sum of sinusoids can be used for an efficient approximation of colored Gaussian noise processes and is therefore of great importance to the software and hardware realization of mobile fading channel models. Although several methods can be found in the literature for the computation of the parameters characterizing a proper set of sinusoids, less is reported about the statistical properties of the resulting (deterministic) simulation model. In this paper, not only is the simulation model's amplitude and phase probability density function (PDF) investigated, but also higher order statistics [e.g., level-crossing rate (LCR) and average duration of fades (ADFs)]. It is shown that due to the deterministic nature of the simulation model, analytical expressions for the PDF of the amplitude and phase, autocorrelation function (ACF), crosscorrelation function (CCF), LCR, and ADFs can be derived. We also propose a new procedure for the determination of an optimal set of sinusoids, i.e., the method results for a given number of sinusoids in an optimal approximation of Gaussian, Rayleigh, and Rice processes with given Doppler power spectral density (PSD) properties. It is shown that the new method can easily be applied to the approximation of various other kinds of distribution functions, such as the Nakagami (1960) and Weibull distributions. Moreover, a quasi-optimal parameter computation method is presented.