Digital technology has made complex signal processing possible in communication systems and greatly improved the performance and quality of most modern telecommunication systems. The telecommunication industry and specifically mobile wireless telephone and computer networks have shown phenomenal growth in both the number of subscribers and emerging services, resulting in rapid consumption of common resources of which the electromagnetic spectrum is the most important. Technological advances and research in digital communication are necessary to satisfy the growing demand, to fuel the demand and to exploit all the possibilities and business opportunities. Efficient management and distribution of resources facilitated by state-of-the-art algorithms are indispensable in modern communication networks. The challenge in communication system design is to construct a system that can accurately reproduce the transmitted source message at the receiver. The channel connecting the transmitter and receiver introduces detrimental effects and limits the reliability and speed of information transfer between the source and destination. Typical channel effects encountered in mobile wireless communication systems include path loss between the transmitter and receiver, noise caused by the environment and electronics in the system, and fading caused by multiple paths and movement in the communication channel. In multiple access systems, different users cause interference in each other’s signals and adversely affect the system performance. To ensure reliable communication, methods to overcome channel effects must be devised and implemented in the system. Techniques used to improve system performance and capacity include temporal, frequency, polarisation and spatial diversity. This dissertation is concerned mainly with temporal or time diversity. Channel coding is a temporal diversity scheme and aims to improve the system error performance by adding structured redundancy to the transmitted message. The receiver exploits the redundancy to infer with greater accuracy which message was transmitted, compared with uncoded systems. Sparse graph codes are channel codes represented as sparse probabilistic graphical models which originated in artificial intelligence theory. These channel codes are described as factor graph structures with bit nodes, representing the transmitted codeword bits, and bit-constrained or check nodes. Each constraint involves only a small number of code bits, resulting in a sparse factor graph with far fewer connections between bit and check nodes than the maximum number of possible connections. Sparse graph codes are iteratively decoded using message passing or belief propagation algorithms. Three classes of iteratively decodable channel codes are considered in this study, including low-density parity-check (LDPC), Turbo and repeat-accumulate (RA) codes. The modulation platform presented in this dissertation is a spectrally efficient wideband system employing orthogonal complex spreading sequences (CSSs) to spread information sequences over a wider frequency band in multiple modulation dimensions. Special features of these spreading sequences include their constant envelopes and power output, providing communication range or device battery life advantages. This study shows that multiple layer modulation (MLM) can be used to transmit parallel data streams with improved spectral efficiency compared with single-layer modulation, providing data throughput rates proportional to the number of modulation layers at performances equivalent to single-layer modulation. Alternatively, multiple modulation layers can be used to transmit coded information to achieve improved error performance at throughput rates equivalent to a single layer system / Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

The research conducted for this thesis seeks to understand issues associated with integrating a direct spread spectrum system (DSSS) transceiver on to a single chip. Various types of sequences, such as Kasami sequences and Gold sequences, are available for use in typical spread spectrum systems. For this thesis, complex spreading sequences (CSS) are used for improved cross-correlation and autocorrelation properties that can be achieved by using such a sequence. While CSS and DSSS are well represented in the existing body of knowledge, and discrete bulky hardware solutions exist – an effort to jointly integrate CSS and DSSS on-chip was identified to be lacking. For this thesis, spread spectrum architecture was implemented focussing on sub-systems that are specific to CSS. This will be the main contribution for this thesis, but the contribution is further appended by various RF design challenges: highspeed requirements make RF circuits sensitive to the effects of parasitics, including parasitic inductance, passive component modelling, as well as signal integrity issues. The integration is first considered more ideally, using mathematical sub-systems, and then later implemented practically using complementary metal-oxide semiconductor (CMOS) technology. The integration involves mixed-signal and radio frequency (RF) design techniques – and final integration involves several specialized analogue sub-systems, such as a class F power amplifier (PA), a low-noise amplifier (LNA), and LC voltage-controlled oscillators (VCOs). The research also considers various issues related to on-chip inductors, and also considers an active inductor implementation as an option for the VCO. With such an inductor a better quality factor is achievable. While some conventional sub-system design techniques are deployed, several modifications are made to adapt a given sub-system to the design requirements for this thesis. The contribution of the research lies in the circuit level modifications done at sub-system level aimed towards eventual integration. For multiple-access communication systems, where a number of independent users are required to share a common channel, the transceiver proposed in this thesis, can contribute towards improved data rate or bit error rate. The design is completed for fabrication in a standard 0.35-μm CMOS process with minimal external components. With an active chip area of about 5 mm2, the simulated transmitter consumes about 250 mW&the receiver consumes about 200 mW. AFRIKAANS : Die navorsing wat vir hierdie tesis onderneem is, beoog om kundigheid op te bou aangaande die kwessies wat met die integrasie van ‘n direkte spreispektrumstelsel (DSSS) sender-ontvanger op ‘n enkele skyfie verband hou. Verskeie tipes sekwensies, soos byvoorbeeld Kasami- en Gold-sekwensies, is vir gebruik in tipiese spreispektrumstelsels beskikbaar. Vir hierdie tesis is komplekse spreisekwensies (KSS) gebruik vir verbeterde kruis- en outokorrelasie-eienskappe wat bereik kan word deur so ‘n sekwensie te gebruik. Alhoewel DSSS en KSS reeds welbekend is, en diskrete hardeware oplossings reeds bestaan, is die vraag na gesamentlike geïntegreerde DSSS en KSS op een vlokkie geïdentifiseer. Vir hierdie tesis is spreispektrumargitektuur aangewend met die klem op KSS substelsels. Dit is dan ook die belangrikste bydrae van hierdie tesis, maar die bydrae gaan verder gepaard met verskeie RF-ontwerpuitdagings: hoëspoed-vereistes maak RF-stroombane sensitief vir die uitwerking van parasitiese komponente, met inbegrip van parasitiese induktansie, passiewe komponentmodellering en ook seinintegriteitskwessies. Die integrasie word eerstens meer idealisties oorweeg deur wiskundige substelsels te gebruik en dan later prakties te implementeer deur komplementêre metaaloksied-halfgeleiertegnologie (CMOS) te gebruik. Die integrasie behels gemengdesein- en radiofrekwensie(RF)-ontwerptegnieke – en finale integrasie behels verskeie gespesialiseerde analoë substelsels soos ‘n klas F-kragversterker (KV), ‘n laeruis-versterker (LRV), en LC-spanningbeheerde ossileerders (SBO’s). Die navorsing oorweeg ook verskeie kwessies in verband met op-skyfie induktors en oorweeg ook ‘n aktiewe induktorimplementering as ‘n opsie vir die SBO. Met sodanige induktor is ‘n beter kwaliteitsfaktor haalbaar. Hoewel enkele konvensionele substelsel-ontwerptegnieke aangewend word, word daar verskeie wysigings aangebring om ‘n gegewe substelsel by die ontwerpvereistes vir hierdie tesis aan te pas. Die bydrae van die navorsing is hoofsaaklik die stroombaanmodifikasies wat gedoen is op substelselvlak om integrasie te vergemaklik. Vir veelvoudige-toegang kommunikasiestelsels waar ‘n aantal onafhanklike gebruikers dieselfde seinkanaal moet deel, kan die sender-ontvanger voorgestel in hierdie tesis meewerk om die datatempo en fouttempo te verbeter. Die ontwerp is voltooi vir vervaardiging in ‘n standaard 0.35-μm CMOS-proses met minimale eksterne komponente. Met ‘n aktiewe skyfie-oppervlakte van ongeveer 5 mm2, verbruik die gesimuleerde sender ongeveer 250 mW en die ontvanger verbruik ongeveer 200 mW. / Thesis (PHD)--University of Pretoria, 2011. / Electrical, Electronic and Computer Engineering / unrestricted