Document Type Master's Dissertation Author Marx, Frans Engelbertius fmarx@worldonline.co.za URN etd-01242006-083415 Document Title DSSS Communication Link Employing Complex Spreading Sequences Degree MEng (Electronic Engineering) Department Electrical, Electronic and Computer Engineering Supervisor
Advisor Name Title Prof L P Linde Keywords
- Multi-Dimensional Direct Sequence Spread Spectrum
- Families of Complex
Date 2005-08-26 Availability unrestricted Abstract The present explosion in digital communications and multi-user wireless cellular networks has urgeda 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.
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