High Frequency (HF) communication has been shown to be a useful communication technique from the very beginning of World War I and it accelerated during World War II. This is attributed to its simplicity, ability to provide near globe connectivity at low power without repeaters, moderate cost, and ease of proliferation . In fact, the HF communication system utilizes the ionosphere  to refract the skywave signals to a distant receiver. This ionospheric channel has some disadvantages. First, it is a non-stationary channel as the HF frequency propagation is a function of the sun spot activities, solar winds, and diurnal variations of the ionization level . Second, the channel produces distortion in both signal amplitude and phase. As the different ionospheric layers move up or down, independent Doppler shifts on each propagation mode are introduced. Multipath fading  caused by multiple refractions of the signal from the ionosphere with or without ground reflection causes performance degradation in the HF system.
Some techniques have been developed to improve HF performance . One example is Space-Diversity , which uses more than one antenna at distant spaces to combine the received signal. Angle-of-Arrival Diversity that takes advantage of the fact that different modes have different arrival angles at the receiver, and so, highly directional antenna for example, can be used to improve the system performance. Another method of improving HF performance is to use different frequencies to transmit and receive messages. This method is known as Frequency diversity. Using time-diversity, one can add a degree of redundancy to the transmitted message through the use of different types of coding, interleaving, etc.
In the military standard, MIL-STD-188- 110A , a convolutional encoder  followed by interleaver - was used to scramble and transmit the data in different bit rates. In the presence of multipath fading , a training sequence is transmitted in an interleaved fashion with the data symbols with a 50% duty cycle. This has the disadvantage of losing half the bandwidth. At present, the recent advances of the Digital Signal Processing (DSP)  make it possible to reduce the bit-error-rate "BEY and increase the transmission bit rate  through the usage of adaptive equalization [ 19]- which will be the focus of this dissertation.
Equalizers such as, Transversal Equalizer , Blind Equalizer , Training waveform Equalizer , and Minimum Mean Square Error (MMSE)  Adaptive Equalizer have been applied into various communication systems. This proposal work will be to initially apply some of the previous developed equalizer to the HF channel specifically. Thereafter, new adaptive channel equalization , will be developed to compensate for transmission channel impairments due to bandwidth limitations, multipath propagation, and rayleigh fading  conditions in mobile environments. A new technique for frequency offset prediction has been developed and finally, a new approach for MIL-STD-188- 110A high frequency single-tone modem employing orthogonal Walsh-PN codes has been implemented.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Department of Electrical and Computer Engineering
Electrical Engineering and Computer Science
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Doctoral Dissertation (Open Access)
Qahwash, Murad M., "High Frequency Communication System Modeling and Performance Enhancement, Employing Novel Adaptive DSP Techniques" (2002). Retrospective Theses and Dissertations. 1653.
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