STUDY OF VARIOUS MULTIPATH CHANNELS& SMALL-SCALE FADING& ITS PERFORMAMCE IN WIRELESS COMMUNICATION (original) (raw)
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Communication over Multipath Fading Channels: A Time-Frequency Perspective
Wireless Communications, 1997
Dynamics of multipath fading have a major effect on the performance of mobile wireless communication systems. The inherently time-varying nature of the mobile wireless channel makes nonstationary signal processing techniques particularly attractive for system design. Time-frequency representations are powerful tools for time-varying signal processing, and in this paper, we present a time-frequency view of wireless communication over multi path channels. Our discussion is anchored on a fundamental finite-dimensional time-frequency representation of the wireless channel that facilitates diversity signaling by exploiting multi path and Doppler shifts. The substantially higher level of diversity afforded by time-frequency processing over conventional techniques translates into significant gains in virtually all aspects of system performance. We illustrate the utility of the time-frequency framework via novel signaling and receiver structures, and multiuser acquisition and interference-suppression algorithms.
Time-selective signaling and reception for communication over multipath fading channels
IEEE Transactions on Communications, 2000
The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code-division multiple-access systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel, which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the intersymbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the intersymbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudorandom codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels.
A multipath channel model for mobile-radio communications
… , 1992. Proceedings, PIMRC'92., Third IEEE …, 1992
A model for the mobile radio channel is proposed that is intended to apply to wideband signals in the 900 MHz band. The model is specified by a two-variable impulse response h,(t), where x represents the location of the receiver. The model consists of two submodels, one describing the direct line-of-sight path, and the other the signal that reaches the receiver over simple reflections. This geometric model is spatially stationary but time-invariant. Time variation is the result of the receiver's motion that alters the point z. Moreover the model is based on physical reasoning and places no restriction on the transmitted signal.
Influence of multipath radio propagation on wideband channel transmission
International Multi-Conference on Systems, Sygnals & Devices, 2012
The mobile radio propagation environment clearly places fundamental limitations on the performance of radio communication systems. Signals arrive at a receiver via a scattering mechanism and the existence of multiple propagation paths (multipath) with different time delays, attenuations and phases gives rise to highly complex time varying transmission channel. In order of systems engineers to determine optimum methods of mitigating the impairments caused by multipath propagation, it is essential that the transmission channel be properly characterized.
Theory of multipath shape factors for small-scale fading wireless channels
IEEE Transactions on Antennas and Propagation, 2000
This paper presents a new theory of multipath shape factors that greatly simplifies the description of small-scale fading statistics of a wireless receiver. A method is presented for reducing a multipath channel with arbitrary spatial complexity to three shape factors that have simple intuitive geometrical interpretations. Furthermore, these shape factors are shown to describe the statistics of received signal fluctuations in a fading multipath channel. Analytical expressions for level-crossing rate, average fade duration, envelope autocovariance, and coherence distance are all derived using the new shape factor theory and then applied to several classical examples for comparison. Index Terms-Angle of arrival, diversity, fading channels, mobile communications, multipath channels, propagation, scattering. I. INTRODUCTION T HE motion in space of a wireless receiver operating in a multipath channel results in a communications link that experiences small-scale fading. The term small-scale fading describes the rapid fluctuations of received power level due to small subwavelength changes in receiver position [1]. This effect is due to the constructive and destructive interference of the numerous multipath waves that impinge upon a wireless receiver [2]. The resulting signal strength fluctuations affect, in some way, nearly every aspect of receiver design: dynamic range, equalization, diversity, modulation scheme, and channel and error-correction coding. Due to its random unpredictable nature, small-scale fading is always studied as a stochastic process. Numerous researchers have measured and analyzed the first-order statistics of these processes, which mostly involves the characterization of smallscale fading with a probability density function (PDF) [3]-[5]. The autocorrelation statistics of fading processes or secondorder statistics have also been studied [6], [7]. Second-order statistics include measures of a process such as power spectral density (PSD), level-crossing rate, and average fade duration. Second-order statistics are heavily dependent on the angles-of-arrival of received multipath. Traditionally, most second-order statistics have been studied using an omnidirectional azimuthal propagation model [2]. That is, multipath waves are assumed to arrive at the receiver with equal power
Simulation of Multipath Fading Effects in Mobile Radio Systems
RF signals transmitted via wireless mobile channels suffer from several effects like small-scale fading and signal dispersion and distortion. This paper reviews these effects and simulates Rayleigh and Rician multipath fading channels with a comparison between them in terms of the effect of RF signal random fluctuations, average received signal level, outage probability, and effect of Doppler shift. In addition to that, signal dispersion occurring to pulses transmitted through these types of channels has also been discussed and simulated.
In this Research, we exhibit a time-frequency point of view of wireless communications after some time-varying multipath channels. Our discourse is principal with regards to code-division multiple access (CDMA) systems in light of their outstanding capacity to battle multipath blurring. Beginning with a time-frequency portrayal of the mobile wireless channel, we land at a sanctioned nite-dimensional time-frequency portrayal of the channel that will fill in as the foundation of our treatment. The accepted time-frequency-based channel portrayal demonstrates that spread-range signalling after some time-varying multipath channels has extra degrees of flexibility that are not abused by existing communications systems. CDMA systems have an expansive time-data transmission product (TBP) that can be abused to give decent variety against channel blurring.
On the combining of multipath signals in narrowband Rayleigh fading channels
IEEE Transactions on Broadcasting, 1999
A6stracl-This paper deals with the process of constructive and destructive addition of multipath signals in a Rayleigh fading channel. It derives closed formulas associating the number uf multipaths with the Rayleigh statistics of the channel and it displays the accuracy of these formulas by means of simulation. The primary conclusion is that multipaths add constructively with high prohahility, thus the more the multipaths the larger the mean power level experienced at the receiver. In this context, multipath propagation exhibits a beneficial property that could he exploited for designing improved performance receivers. Zrrdex terms-Multipath comhining, Multipath fading, Rayleigh fading, Mobile channels. I. INTIIOLXJCTION IRELESS communication channels are characteri-W zed as multipath propagalion channels because the signal at the rcccivcr is usually composed of several delayed replicas or the transmitted signal. The effects of multipath propagation are hoth detrimental and bcncficial. On tlic one hand, lack of multipath propagation would rcndcr mobile communications almosl impossible in heavily structured cnvironments where line-of-sight (LOS) communication is difficult to achieve. Addilionally, multipath propagation develops a form of time diversity, which in wideband transmission systems can be effectively exploited to provide improved performance. On rhc other hand, multipath propagation introduces severe signal fluctuation, commonly rcfcrrcd to as fading, at a moving receiver in a narrowband transmission system. This fading phcnomenon rcnders mobilc channels particularly hostilc communication cnvironments; it is well known [1,2] that common modulalion schemes p c r h u far worse in a mobilc cnviionmcnt than in a fixed one. The primary source of signal fading is the large variation in relative phase of tlic multipath signals cxpericnccd as the rccciver quickly moves ovcr distances as small as half a wavelength. It is commonly quoted that fading is developed by the constructive and destructivc addition of llie multipath components [ 11. This work has been supported in part by Ihc National Scicnce and Engineering Research Council (NSERC) and the Advanced Systcins lnstitutc (A S) of HC through a Visiting Fellowship Program.