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HF Radio Data Modem TMS320C5000 DSP Code

Description: the HF (High Frequency) radio band - 3MHz to 30MHz - allows signal transmission over hundreds of kms by reflection back from the upper ionosphere. However the individual ionospheric layers cause time-varying multipath modes, leading to fading and, with data, severe intersymbol interference. Background noise and other users contribute further interference to the wanted signal. Hence HF Radio is a difficult medium for data transmission. The software here implements separately the transmit (tx) and receive (rx) functions for communicating digital data over HF radio channels.
         The method adopted uses serial data transmission with a signal bandwidth of 3kHz, the nominal HF channel bandwidth. It allows data rates of either 2.4kb/s, modulated as 2QAM, or 4.8kb/s, via 4QAM. The carrier frequency is 4.8kHz for external mixing to the desired HF channel centre frequency, and the sample rate can be 19.2kHz or 32.0kHz.
         To ensure the receiver can automatically achieve and, during dropouts, re-achieve synchronisation, the transmitter generates a continuous sounding signal alongside the main data signal. The receiver processes involve carrier and clock synchronisation, and continuous adaption of an equaliser powerful enough to combat the intersymbol interference from the channel multipaths which are continually time-varying. The serial data transmission method allows the multipath signal components to be largely resolved. Then the adaptive equaliser can combine the individual components, so utilising the inherent time diversity to maintain signal-to-noise.
         The assembler software is in the form of separate callable subroutines for tx and rx, one each for initialisation and for main-routine processing of input/output sample blocks and data. The user is responsible for interfacing samples and data to hardware. A comprehensive user's guide together with example application files are provided with the code.
         Test results have been made over channel simulations. These quantify the signal-to-noise performance over fixed multipath conditions. A video is available showing adaption to a channel simulation implementing 4 time-varying multipaths.

Performance: SNR for 10-4 Error Rate over fixed paths with 9.6kHz BW noise:
Single-path channel: 2.4kb/s - 3.3dB;    4.8kb/s   6.3dB
3 equal paths:            2.4kb/s - 8.3dB;    4.8kb/s 12.0dB

Processor Load:

#example figures for the TI TMS320C5000(C55x) DSP processor at the 32.0kHz sample rate - for the 19.2kHz sample rate subtract 1.2MIPS and 0.4kbytes program memory

Availability: the software is still under development but would benefit from testing over real links