Approved for public release, distribution is unlimited / Despite the increase in the demand for high speed Internet services, the last-mile solutions currently available neither are inexpensive enough to attract the majority of the population, nor are they available in low density populated areas. This thesis examines Fixed Broadband Wireless (FBW) as an alternative technology to the current last-mile solutions. The analysis shows that LMDS and MMDS are the most promising emerging FBW technologies and that they are able, by utilizing microwave radio as their fundamental transport medium and using high modulation schemes, to provide digital two-way voice, data, video and Internet services. The thesis shows that both technologies are constrained by free space loss and line-of-sight impairments with rain absorption being the most significant cause of attenuation in the LMDS case, while vegetation and multipath fading play a significant role mostly in the MMDS case. Additionally, it is shown that there is a positive association between the data rate achieved and the level of influence due to Additive White Gaussian Noise (AWGN). Based on the analysis and using the coverage areas, the total capacity, the achieved data rates, the weather and line-of-sight limitations as well as the cost as the most important criteria, it is concluded that LMDS is a preferable solution for enterprise end-users in densely populated urban areas outside the reach of fiber networks, while MMDS targets residential end-users in rural or suburban areas that are not able to receive service through high-speed wireline connections. / Lieutenant Junior Grade, Hellenic Navy

Abstract not available

The problems of optimal as well as suboptimal detection for CDMA transmissions over an additive white Gaussian noise (AWGN) channel, have been the focus of study in the recent past. However, CDMA transmissions are frequently made over channels which exhibit fading and/or dispersion; hence receivers need to be designed which take into account this behaviour. In spite of the major research effort invested in multiuser demodulation techniques, several practical as well as theoretical open problems still exist. Some of them are considered in more detail in this thesis. The aim of the thesis is to develop multiuser demodulation algorithms for mobile communication systems in frequency-selective fading channels, and to analyze their implementation complexity. The emphasis is restricted to the uplink of an asynchronous DS-CDMA system where the users transmit in an uncoordinated manner and are received by one centralized receiver. The original work that is undertaken for the MScEng study is the evaluation of a multiuser receiver structure for a frequency-selective fading channel, where there exists a steady specular path and two fading paths. Furthermore, the effect of using selection diversity is investigated by examining the bit error rate, asymptotic multi user efficiency and near-far resistance of the proposed detector structure. These results are confirmed both analytically and by simulation in the thesis. An investigation is also conducted into the application of neural networks to the problem of multiuser detection in code division multiple access systems. The neural network will be used as a classifier in an adaptive receiver which incorporates an extended Kalman filter for joint amplitude and delay estimation. Finally, some open problems for future research will be pointed out in the thesis. Keywords: AWGN channel , DS-CDMA system, frequency-selective, multi user demodulation, asymptotic multiuser efficiency, near-far resistance, neural network, Kalman filter. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.

Approved for public release, distribution is unlimited / Wide local area networks (WLAN) are increasingly important in meeting the needs of next generation broadband wireless communications systems for both commercial and military applications. Under IEEE 802.11a 5GHz WLAN standard, OFDM was chosen as the modulation scheme for transmission because of its well-known ability to avoid multi-path effects while achieving high data rates. The objective of this thesis is to investigate the performance of the IEEE 802.11a WLAN standard receiver over flat fading Nakagami channels in a worst case, pulse-noise jamming environment, for the different combinations of modulation type (binary and non-binary modulation) and code rate specified by the WLAN standard. Receiver performance with Viterbi soft decision decoding (SDD) will be analyzed for additive white Gaussian noise (AWGN) alone and for AWGN plus pulse-noise jamming. Moreover, the performance of the IEEE 802.11a WLAN standard receiver will be examined both in the scenario where perfect side information is considered to be available (optimum receiver) and when it is not (sub-optimum receiver). In the sub-optimum receiver scenario, the receiver performance is examined both when noise-normalization is utilized and when it is not. The receiver performance is severely affected by the pulse-noise jamming environment, especially in the suboptimum receiver scenario. However, the sub-optimum receiver performance is significantly improved when noise-normalization is implemented. / Lieutenant, Hellenic Navy

[Truncated abstract] The transmission of digital information over a wireless communication channel gives rise to a number of issues which can detract from the system performance. Propagation effects such as multipath fading and intersymbol interference (ISI) can result in significant performance degradation. Recent developments in the field of iterative detection have led to a number of powerful strategies that can be effective in mitigating the detrimental effects of wireless channels. In this thesis, iterative detection is considered for use in two distinct areas of wireless communications. The first considers the iterative decoding of concatenated block codes over slow flat fading wireless channels, while the second considers the problem of detection for a coded communications system transmitting over highly-dispersive frequency-selective wireless channels. The iterative decoding of concatenated codes over slow flat fading channels with coherent signalling requires knowledge of the fading amplitudes, known as the channel state information (CSI). The CSI is combined with statistical knowledge of the channel to form channel reliability metrics for use in the iterative decoding algorithm. When the CSI is unknown to the receiver, the existing literature suggests the use of simple approximations to the channel reliability metric. However, these works generally consider low rate concatenated codes with strong error correcting capabilities. In some situations, the error correcting capability of the channel code must be traded for other requirements, such as higher spectral efficiency, lower end-to-end latency and lower hardware cost. ... In particular, when the error correcting capabilities of the concatenated code is weak, the conventional metrics are observed to fail, whereas the proposed metrics are shown to perform well regardless of the error correcting capabilities of the code. The effects of ISI caused by a frequency-selective wireless channel environment can also be mitigated using iterative detection. When the channel can be viewed as a finite impulse response (FIR) filter, the state-of-the-art iterative receiver is the maximum a posteriori probability (MAP) based turbo equaliser. However, the complexity of this receiver's MAP equaliser increases exponentially with the length of the FIR channel. Consequently, this scheme is restricted for use in systems where the channel length is relatively short. In this thesis, the use of a channel shortening prefilter in conjunction with the MAP-based turbo equaliser is considered in order to allow its use with arbitrarily long channels. The prefilter shortens the effective channel, thereby reducing the number of equaliser states. A consequence of channel shortening is that residual ISI appears at the input to the turbo equaliser and the noise becomes coloured. In order to account for the ensuing performance loss, two simple enhancements to the scheme are proposed. The first is a feedback path which is used to cancel residual ISI, based on decisions from past iterations. The second is the use of a carefully selected value for the variance of the noise assumed by the MAP-based turbo equaliser. Simulations are performed over a number of highly dispersive channels and it is shown that the proposed enhancements result in considerable performance improvements. Moreover, these performance benefits are achieved with very little additional complexity with respect to the unmodified channel shortened turbo equaliser.