DSSS Communication Link Employing Complex Spreading Sequences

Abstract

The present explosion in digital communications and multi-user wireless cellular networks has urged a demand for more effective modulation methods, utilizing the available frequency spectrum more efficiently. To accommodate a large number of users sharing the same available frequency band, one requirement is the availability of large families of spreading sequences with excellent AC and CC properties. Another requirement is the availability of sets of orthogonal basis functions to extend capacity by exploiting all available degrees of freedom (e.g., temporal, frequency and spatial dimesions), or by employing orthogonal multi-code operation in parallel, such as used in the latest 3GPP and 3GPP2 Wide-band Code Division Multiple Access (WCDMA) modulation standards by employing sets of orthogonal Walsh codes to improve the overall data throughput capacity. The generic Direct Sequence Spread Spectrum (DSSS) transmitter developed in this dissertation has originally been designed and implemented to investigate the practicality and usefulness of complex spreading sequences, and secondly, to verify the concept of non-linearly interpolated root-of-unity (NLI-RU) filtering. It was found that both concepts have a large potential for application in point-to-point, and particularly micro-cellular Wireless Local Area Networks (WLANs) and Wireless-Local-Loop (WLL) environments. Since then, several novel concepts and subsystems have been added to the original system, some of which have been patented both locally and abroad, and are outlined below. Consequently, the ultimate goal of this research project was to apply the principles of the generic DSSS transmitter and receiver developed in this study in the implementation of a WLL radio-frequency (RF)-link, and particularly towards the establishment of affordable wireless multimedia services in rural areas. The extended coverage at exceptionally low power emission levels offered by the new design will be particularly useful in rural applications. The proposed WLL concept can for example also be utilized to add a unique mobility feature to for example existing Private Automatic Branch Exchanges (PABXs). The proposed system will in addition offer superior teletraffic capacity compared to existing micro-cellular technologies, e.g., the Digital European Cordless Telephony (DECT) system, which has been consider by Telkom for employment in rural areas. The latter is a rather outdated interim standard offering much lower spectral efficiency and capacity than competitive CDMA-solutions, such as the concept analyzed in this dissertation, which is based on the use of unique large families of spectrally well confined (i.e., band-limited) constant envelope (CE) complex spreading sequences (CSS) with superior correlation properties. The CE characteristic of the new spreading sequences furthermore facilitates the design of systems with superior power efficiency and exceptionally robust performance characteristics (much less spectral re-growth) compared to existing 2G and 3G modulation standards, in the presence of non-linear power amplification. This feature allows for a system with larger coverage for a given performance level and limited peak power, or alternatively, longer battery life for a given maximum communication distance and performance level, within a specified fixed spreading bandwidth. In addition, the possibility to extend the concept to orthogonal multi-code operation provides for comparable capacity to present 3G modulation standards, while still preserving superior power efficiency characteristics in non-linear power amplification. Conventional spread spectrum communication systems employ binary spreading sequences, such as Gold or Kasami sequences. The practical implementation of such a system is relatively simple. The design and implementation of a spread-spectrum communication system employing complex spreading sequences is however considerable more complex and has not been previously presented, nor been implemented in hardware. The design of appropriate code lock loops for CSS has led to a unique design with 3dB performance advantage compared to similar loops designed for binary spreading sequences. The theoretical analysis and simulation of such a system will be presented, with the primary focus on an efficient hardware implementation of all new concepts proposed, in the form of a WLL RF-link demonstrator.