Digital IC Functions
The Digital Modem and Control IC formats data for transmit and provides precision signal acquisition and demodulation functions during receive. In addition, it is responsible for programming the various registers of the RFIC and for interfacing with the host microcontroller aboard a scout-craft.
The top-level architecture of the digital IC is shown below. In transmit mode, this IC provides forward error correction (FEC) convolutional coding compatible with Proximity-1 protocols, and optional packet framing functions. Residual carrier BPSK and suppressed carrier BPSK and QPSK are accommodated through CMOS outputs supplied to the RFIC complex modulator.
In receive mode, the IC handles the demodulation task as well as bit/frame synchronization A Discrete Fourier Transform (DFT)-based algorithm allows acquisition of BPSK or QPSK modulated signals over a +/- 50 kHz search range. This extended range is provided to address frequency drift of the TCXO at temperatures below its rated range as well as aging and Doppler issues. In order to provide a modest compromise between accurate frequency estimation and processing complexity, the DFT is structured as a cascade of coarse and fine frequency estimation stages.
Digital Signal Processing Algorithms
A 16 point DFT is employed in the estimation to reduce the frequency uncertainty to less than 10 kHz within the first stage with additional fine-tuning performed in similar follow-on processing. These results correspond to a sampling frequency of 128 kHz of an 8 ksps BPSK signal received with 0 dB Eb/No. For sufficiently long integration intervals, there is good agreement between floating and fixed point performance. The non-monotonic behavior of the fixed-point, maximum error curve is a result of limited simulation trials for short integration periods. Due to the relatively low sampling rates in the frequency estimation processing, hardware re-use of the 16-point DFT is employed resulting in savings in required gate utilization.
Additional functions of the digital IC include controlfor the receiver and synthesizer circuits. At regular intervals, the IC monitors the RSSI output of the receiver and updates receiver gain settings to adjust for signal strength variations during an orbital pass. In addition, the VCO within the RFIC’s synthesizer contains frequency range settings which are periodically monitored and updated to retain lock as IC or host PC-board temperatures vary.