Analysis of multipath channel fading techniques in wireless communication systems
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Multipath fading occurs in any environment where there is multipath propagation and there is some movement of elements within the radio communications system. This may include the radio transmitter or receiver position, or in the elements that give rise to the reflections. The multipath fading can often be relatively deep, i.e. the signals fade completely away, whereas at other times the fading may not cause the signal to fall below a useable strength. Multipath fading may also cause distortion to the radio signal. As the various paths that can be taken by the signals vary in length, the signal transmitted at a particular instance will arrive at the receiver over a spread of times. This can cause problems with phase distortion and inter symbol interference when data transmissions are made. As a result, it may be necessary to incorporate features within the radio communications system that enables the effects of these problems to be minimized. This paper analyses the effects of various types of multipath fading in wireless transmission system.Keywords:
Rake receiver
Fading distribution
Channel state information
Multipath interference
Distortion (music)
Diversity scheme
Multipath fading is one of the major practical concerns in wireless communications. A multipath transmission takes place when a transmitted signal arrives at a receiver by two or more paths of different delay. Such multiple paths may be due to atmospheric reflection or refraction, or reflections from buildings or other objects. A multipath fading channel is usually modeled as a time-variant tapped delay system. RAKE receiver can be used to detect the signal from the multipath fading channel. However, the number of delay taps, the time delay of each path, and the tap weight for each delay path need to be determined. The objective of this thesis is to explore the possibility of using the advance signal processing algorithms to estimate the number of delays, the time delay of each path, and the tap weight for each delay path in multipath channels and to investigate the performance of the RAKE receiver based on chip rate channel estimates in a realistic mobile environment. Simulations show that the new approach outperforms the existing approaches.
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There are two kinds of interference in a communication system: multipath and multiaddress interference. Using A RAKE receiver in a direct sequence spread spectrum system is an efficient method of anti-multipath interference, and multistage interference cancellation technology we can be used for anti-multiaddress interference. Combining a RAKE receiver with multistage interference cancellation, multipath and multiaddress interference can be removed at the same time.
Rake receiver
Multipath interference
Adjacent-channel interference
Rake
Co-channel interference
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We develop an analytical framework to quantify the effects of the spreading bandwidth on spread spectrum systems operating in multipath environments with arbitrary power delay profile. The focus of the paper is to characterize the symbol error probability (SEP) performance of a RAKE receiver tracking the L strongest multipath components in frequency-selective Rayleigh fading. By transforming the physical RAKE receiver with correlated ordered paths into the domain of a "virtual RAKE" receiver with conditionally independent virtual paths, analytical expressions for the SEP are derived in terms of the spreading bandwidth, multipath spread of the channel and the number of combined paths.
Rake receiver
Rake
Diversity scheme
Power delay profile
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Spread spectrum communication over a multipath fading channel usually outperforms narrowband communication because the number of resolvable paths is higher when the signal bandwidth is larger. Unfortunately, the complexity of the optimum RAKE receiver is proportional with the ratio of the channel delay spread to chip duration, which might be very large when the delay spread exceeds the symbol duration. The authors introduce a reduced complexity receiver and compare its the performance with the performances of the optimum RAKE receiver for spread spectrum communication and the receiver for narrowband communication.
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Narrowband
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As wireless data rate requirements increase, multipath delay spread becomes an increasingly significant limitation on the performance of wireless systems. Techniques such as RAKE reception combat time dispersion by combining multipath components. Alternative implementations of RAKE receivers isolate the strongest multipath components and then shift each component to a common timing reference. The optimal timing reference in frequency-selective fading channels remains an open problem. This paper examines the impact pulse shaping and multipath delay spread on both signal-to-noise ratio (SNR) and bit-error rate performance. The receiver being considered achieves symbol synchronization to the strongest multipath component. The performance when synchronization is achieved based on the first multipath component arrival is also found and used to illustrate performance differences. Multipath delay distributions used on the performance calculations are derived from indoor measurements. Pulse shapes considered in the analysis include root-raised cosine, raised cosine, and Gaussian filters. SNR losses are shown to range between 1-6 dB for bit rates of 10 Mb/s. Results show that synchronization of the receiver to the strongest multipath component gives a 1-3 dB advantage over synchronization to the first arriving multipath component.
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Raised-cosine filter
Multipath mitigation
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Third generation mobile communication systems based on the WCDMA air interface employ RAKE reception. Increasing the number of Rake fingers, helps to capture more multipath energy but at the expense of increased receiver complexity. Mobile radio channel measurements at 1945 MHz and 2135 MHz were carried out in Manchester city centre. Statistics of relative power and excess delay of multipath components were analysed to determine the optimum number for Rake fingers based on the percentage of captured multipath energy. Then the probability of bit error for a WCDMA system was predicted for different propagation conditions.
Rake receiver
Rake
W-CDMA
Air interface
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A new signal reception method, multipath interference exchange reduction (MIXR), that reduces multipath interference in direct-sequence code division multiple access (DS-CDMA) systems is described. This method uses a despread signal at a timing correlated with the interference in the conventional despread signal before RAKE combining. It is especially effective in the downlink receiver of DS-CDMA systems in which the downlink signals of all users are received with the same fading channels. One computer simulation showed that the BER of the proposed method is 1-2 dB better than that of a conventional RAKE receiver. Other simulations showed that the MIXR receiver is more sensitive than conventional RAKE receivers.
Rake receiver
Multipath interference
SIGNAL (programming language)
Near-far problem
Rake
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The RAKE receiver used in PCS IS-95 CDMA communication systems has the ability to coherently combine and demodulate individually received multipath components. In order to optimize the settings of the RAKE receiver (i.e. RAKE search window), knowledge of the multipath delay spread characteristics in the area of receiver operation is required. This paper documents analysis of measured multipath characteristics in the dense urban, urban, suburban and rural areas in and around a major metropolitan city. Measured power delay profiles are characterized in terms of RMS delay spread, and number of resolvable multipath components.
Rake receiver
Channel sounding
Rake
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RAKE receiver is a multipath diversity reception technology. The technology is used to solve the multipath delay of signal transmission problems. Delay estimation is responsible for the search and synchronization of the multipath signals. But the delay estimation cannot achieve the complete synchronization of chip; therefore, there is still loss of signal energy. On the basis of delay estimation, this article proposed an improved RAKE receiver with delay adjustment; the technology can improve the synchronization accuracy of multipath signals, and consequently improve the performance of RAKE receiver.
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SIGNAL (programming language)
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Spread spectrum (SS) multiple access techniques have been proposed for third generation broadband wireless access. We develop an analytical framework to quantify the effects of spreading bandwidth on SS systems operating in dense multipath environments in terms of the receiver performance, receiver complexity, and multipath channel parameters. In particular, we consider wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. The focus of the paper is to characterize the combined signal of the RAKE receiver fingers tracking the strongest multipath components. Closed form expressions for the mean and the variance of the total RAKE receiver output signal-to-noise ratio (SNR) are derived in terms of the number of RAKE fingers, spreading bandwidth, and multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain. A representative result indicates that for SS systems with 5 MHz signal bandwidth operating in a channel with constant power delay profile having 5 /spl mu/s spread, the average SNR gain from increasing the number of RAKE fingers from one to three is 3.8 dB and from three to five is 1.5 dB. Furthermore, the reduction in the variation of SNR is 1.1 dB and 0.4 dB for the same increments in the number of fingers.
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Rake
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