International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California / The purpose of this paper is to present a chronology from a Small Business Innovation Research Program (SBIR) showing the impact of a wireless network architecture on future airborne data acquisition systems. The major advantages and challenges associated with the use of wireless network data acquisition versus wired time division multiplexing systems are rooted in data latency, bandwidth efficient data transmission while maintaining a low bit error rate and not interfering with existing avionics. Many of the issues raised are subtle and complex. It is not the intent of this paper to give these issues the thorough academic and technical analysis they deserve. It is the hope of the authors that this paper will generate awareness and discussion on these issues.

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. / Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

The need for multiple access strategies arises whenever a number of users have to share a communication resource, since it is usually either cost prohibitive or impractical to dedicate a communication channel to a particular user. A need for such algorithms arises in many instances, particularly in applications utilizing wireless systems where all users access a common channel or medium. Such random access techniques as ALOHA and slotted ALOHA have been successfully implemented in a number of wireless applications. One of the major drawbacks of these algorithms is the necessity of a return path from the central station to each system user, which makes their use both inefficient and expensive for applications where one-way communication would suffice. For such applications, a need remained for a random access algorithm which can maximize the probability of successful message transmission in a one-way communication environment. A random access technique that addresses the above-mentioned need is developed. With this technique, each user sends an original message of predetermined length to a central receiver. The user then retransmits the message a specified number of times in a predetermined interval reserved for the retransmission process. The time interval between each successive retransmission of a given message is random. Assuming total annihilation of all colliding messages, the expression for the probability of successful transmission of a given message in terms of the major channel parameters is theoretically formulated. This technique offers a significant improvement, compared to a single transmission, in ensuring that a message is successfully received. The actual message collision dynamics in this system are experimentally studied using two different types of direct-sequence spread spectrum receivers, one employing a sliding correlator and the other using a matched filter. The spreading code in such systems offers extra protection for messages against possible interferers. The results indicate that it is often possible to properly receive a given message in the presence of co-channel interferers, thus significantly improving the overall system performance. These results are subsequently incorporated with the propagation data for several different types of microcells to arrive at a more precise theory of the link. / Ph. D.

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Approved for public release, distribution is unlimited / Direct-Sequence Spread Spectrum (DSSS) modulation is being advanced as the physical-layer basis for Seaweb undersea acoustic networking. DSSS meets the need for channel tolerance, transmission security, and multi-user access. This thesis investigates the performance of subspace-decomposition blind-equalization algorithms as alternatives to RAKE processing of DSSS signals. This approach is tailored for superior performance in time-dispersive and frequency-dispersive channels characteristic of ocean acoustic propagation. Transmitter and receiver structures are implemented in Matlab and evaluated with a statistics-based model of a doubly spread channel with additive noise. Receiver performance is examined using Monte Carlo simulation. Biterror rates versus signal-to-noise ratio are presented for various multipath assumptions, noise assumptions, and receiver synchronization assumptions. / Lieutenant, Hellenic Navy

Third generation mobile communication systems promise a greater data rate and new services to the mobile subscribers. 3G systems support up to 2 Mbps of data rate to a fixed subscriber and 144 Kbps of data rate to a fully mobile subscriber. Code Division Multiple Access (CDMA) is the air interface access scheme widely used in all the 3G communication systems. This access scheme has many inherent advantages m terms of noise immunity, security, coherent combining of multi path signals etc. But all these advantages come at the expense of higher complexity of the receivers. The receivers form the major portion of the processing involved in a base station. The heart of any CDMA receiver is the RAKE. The RAKE receiver separates the different multi-paths received by the antenna by using the properties of the Pseudo Random sequences. The phase and strength of each of these path signals is measured and are used by the coherent combiner, which de-rotates all the signals to a single reference and coherently combines them In general the Base station receivers make use of the top three multi-path signals ranked in terms of their signal energy Hence four RAKE fingers, each catering to single multi-path are needed for receiving a single code channel (3 for coherent combining and one for scanning). One such channel receiver requires a processing power of 860 MIPS (Mega Instructions Per Second). Some of the CDMA standards support up to 90 code channels at the same time. This means that the total processing power required at the base station is about 80 GIPS. This much of processing power will require large number of high end DSPs, which will be a very costly solution. In the current base station architectures these blocks are implemented using ASICs, which are specific to a particular standard and also the algorithms used for the different operations are fixed at the design time itself. This solution is not flexible and is not amenable for SDR (Software defined Radio) architectures for the Base stations. This thesis proposes a Co-Processor solution, which can be attached to a generic DSP or any other processor. The processor can control the Co-Processor by programming its parameter registers using memory mapped register accesses. This co-processor implements only those blocks, which are compute intensive. This co-processor performs all chip-rate processing functions involved m a RAKE receiver. All the symbol-rate functions are implemented through software in the processor. This provides more choices m selecting the algorithms for timing recovery and scanning. The algorithms can be changed through software even after the base station is installed in the field. All the inputs and outputs of the Co-Processor are passed through dual port RAMs with independent read and write clocks. This allows the Co-Processor and the processor to be running on two independent clocks. This memory scheme also increases the throughput as the reads and writes to these memories can happen simultaneously. This thesis introduces a concept of incorporating programmable PN/Gold code generators as part of the Co-Processor, which significantly reduces the amount of memory required to store the Scrambling and Spreading codes. The polynomial lengths as well as the polynomials of the code generator are programmable. The input signal memory has a bus width equal to 4 times the bus width of the IQ signal bus width (4 * 24 = 96 bits) towards the Co-Processor to meet the huge data bandwidth requirement. This memory is arranged as word interleaved memory banks. This can supply one word per memory bank on each clock cycle as long as the accessed words fall in different memory banks. The number of banks is chosen as more than twice that of the number of Correlators/ Rake fingers. This gives more flexibility in choosing the address offsets to different Correlator inputs. This flexibility allows one to use different timing recovery schemes since the number of allowable address offsets for different Correlators is more. The overall complexity of the solution is comparatively less with respect to the generic DSP based solution and much easier to modify for a different standard, when compared to the rigid ASIC based solution. The proposed solution is significantly different from the conventional way of designing the Base station with fixed ASICs and it clearly outweighs the solutions based on conventional approach in terms of flexibility, design complexity, design time and cost.