|Title:||Performance analysis of coded OFDM-CDMA system in rayleigh fading channel with AWGN|
|Subject:||Mobile communication systems|
Spread spectrum communications
Code division multiple access
Hong Kong Polytechnic University -- Dissertations
Department of Electronic and Information Engineering
|Pages:||xi, 91, 31 leaves : ill. (some col.) ; 30 cm. + 1 computer optical disc|
|Abstract:||To operate future generation multimedia communications systems, high data rate transmission needs to be guaranteed with a high quality of service . The system requirements of the Universal Mobile Telecommunications System (UMTS) set high demands on the air interface of the 3rd generation mobile networks. In Europe, within the Advanced Communications Technologies and Services (ACTS) programme, the FRAMES (Future Radio Wideband Multiple Access Systems) project has the objective to define and specify an UMTS air interface . For instance, the third generation cellular mobile systems should offer a high data rate up to 2 Mbit/s for video, audio, speech and data transmission. Moreover, there are choices for these cellular systems to select the appropriate access scheme among those available. All of these have motivated researchers to carry on investigations on different multiple access systems that can offer higher spectral efficiency. Spread spectrum communication technology, originating from the military application, is used for two purposes primarily: to overcome the effects of strong intentional interface (jamming), and to hide the signal from the eavesdropper (covertness) . Other advantages include high immunity against multi-path distortion, no need for frequency planning, high flexibility, and easier variable rate transmission etc. The capability of minimising multiple access interference is given by the cross-correlation properties of spreading codes. In the case of multi-path propagation the capability of distinguishing one component from others in the composite received signal is offered by the auto-correlation properties of the spreading codes . The performance of such a DS-CDMA system will depend mainly on the number of active users, the channel characteristics, and the number of arms employed in the RAKE receiver. The system capacity is limited by self and multiple access interference, which results from the imperfect auto- and cross-correlation properties of the spreading code under mobile environment. On the other hand, the technique of multi-carrier transmission has recently been receiving wide interest for high data rate applications. One of the advantages of the multi-carrier transmission is its robustness in the case of frequency selective fading channels. This is based on converting the high rate serial data stream into several parallel data streams of lower rate, and by using these sub-streams to modulate different sub-carriers. Since the amount of filters and oscillators is considerable for a large number of sub-carriers, an efficient digital implementation of the special form of the multi-carrier modulation, called orthogonal frequency division multiplexing (OFDM), was proposed in . There, the multi-carrier modulation is easily realised by using the discrete Fourier transform. OFDM, having densely spaced sub-carriers with overlapping spectra of modulating signal, abandons the use of steep band-pass filters to detect each sub-carrier as it is used in the frequency division multiple access schemes. Therefore, it offers a high spectral efficiency. The advantages and success of multi-carrier modulation and the spread spectrum technique motivated many researchers to investigate the suitability of the combination of multi-carrier modulation with spread spectrum, known as multi-carrier spread spectrum for cellular systems. This combination, introduced in 1993 as a multiple access scheme, will allow one to benefit from the advantages of both schemes: higher flexibility, higher spectral efficiency, simpler detection techniques, narrow band interference rejection capability, etc. In this dissertation, the works of conventional OFDM-CDMA  system and adaptive OFDM systems [10, 11, 18] will be combined. The newly proposed adaptive convolutional-coded OFDM-CDMA will be built and its ability to combat against Rayleigh fading channel will be investigated. It is expected that such an "adaptive" system arrangement will have several dB of Eb/No advantage over the conventional convolutional-coded OFDM-CDMA systems.|
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