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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 create multiple transmission paths (multipath). These are time-varying, 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 DSP software here implements separately the transmit (tx) and receive (rx) functions from a proprietary scheme for communicating digital data over HF radio channels.
         The method uses serial data transmission at data rates of either 2.4kb/s, modulated as 2QAM, or 4.8kb/s, via 4QAM. The signal bandwidth is 3kHz, the nominal HF channel bandwidth. 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 intersymbol interference from the time-varying channel multipaths.
         The serial data transmission format allows the multipath signal components to be largely resolved. Then the adaptive equaliser can constructively combine the individual components, so using the time diversity inherent in the multipath signal to maintain signal-to-noise. Additionally the sounding process enables the rx to automatically synchronise to either of the tx data rates. Then for example the tx can change data rates depending on the importance of the data being sent.
         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 is provided with the code.
         Tests have been made using software channel simulations. Over fixed multipath conditions, the signal-to-noise performance has been quantified. Over a channel simulation implementing 4 time-varying multipaths a video is available showing the adaption performance.

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 1.1MIPS and 0.4kbytes program memory

Availability: the system has been optimised in channel simulation but would benefit from testing over real links