The paper presents software implementation of a receiver forming a part of an adaptive communication system. The system is intended for communication with a satellite placed in a low Earth orbit (LEO). The ability of adaptation is believed to increase the total amount of data transmitted from the satellite to the ground station. Depending on the signal-to-noise ratio (SNR) of the received signal, adaptive transmission is realized using different transmission modes, i.e., different modulation schemes (BPSK, QPSK, 8-PSK, and 16-APSK) and different convolutional code rates (1/2, 2/3, 3/4, 5/6, and 7/8). The receiver consists of a software-defined radio (SDR) module (National Instruments USRP-2920) and a multithread reception software running on Windows operating system. In order to increase the speed of signal processing, the software takes advantage of single instruction multiple data instructions supported by x86 processor architecture.

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The paper is related to an adaptive satellite communication system for data transmission from small, low cost, low Earth orbit satellites. Tests run in a set-up consisting of a number of software-defined radio (SDR) modules operating as a satellite, a ground station, and a satellite channel simulator, have shown that by changing modulation scheme and code rate one can obtain increase of amount of data which can be downloaded from a satellite during a single pass over a ground station approximately by a factor of 2. To determine data rates obtainable in an SDR system using a common personal computer as a digital signal processing device, execution times of particular processing steps involved in the reception process were measured.

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