Design and Realization of Adaptive Channel Equalizer for Shortwave Communication using TMS320C50 DSP Kit

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Addis Ababa University


The high frequency (HF) band is a cost effective medium for a long distance transmission beyond the horizon, for limited bandwidth of up to 3 kHz. However, the HF channel is simultaneously a time and frequency dispersive environment. Time spread is two fold and results from multiple reflections between earth surface and ionosphere, where the latter is known to have various reflection layers. This multipath behavior results in various received modes at the receiver site corresponding to a time spread of a few milliseconds. Frequency spread is due to motion of electron density distributions in the ionospheric media, and can be as high as 10 Hz. In addition to time and frequency spread the relative movements of ionospheric layers impose a Doppler shift on transmitted signals. The received signal in fading often modeled by a Rayleigh distributed envelope and a uniformly distributed phase. Frequency selective fading channels produce intersymbol interference (ISI) in the received signal. In these channels an increase in the power does not improve performance, since additional power amplifies the ISI in step with the desired signal. The maximum likelihood sequence estimator (MLSE) is the optimal method of canceling the ISI; however, it is prohibitively complex. Suboptimal but efficient method of compensating for the ISI is to equalize the channel impairments by applying a filter at the receiver. In general radio channel characteristics are subject to variation in time, and this leads to the need for adaptive equalizers. In this development, a linear frequency selective slowly fading model is used for the channel. A linear adaptive channel equalizer is considered as a solution to improve the performance of digital communication over this short wave channel. The performance of the equalizer is studied under a software simulated environment using MATLAB programming for a linearly modulated, binary phase shift keyed (BPSK), signals. The TMS320C50 DSK (digital signal processor starter kit) assembler and debugger are used to develop the assembly code so as to realize and implement the filter on TMS320C50 digital signal processor. Finally, the designed equalizer is tested for its performance.



Shortwave Communication