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Description
High speed data transmission has increasingly become important in satellite communications due to the exponential growth of generated data with the advancement of observation technology such as ultra-high definition 4K cameras. This poses quite a challenge for new space applications, which focus on short development span and cost-effectiveness, by pushing the limits of receiver designs and costs. Moreover, Consultative Committee for Space Data Systems (CCSDS) recommends OQPSK/QPSK modulation for high-rate space-to-earth communication in 401.0-B-32 Blue Book. However, this modulation scheme has an inherent issue of phase ambiguity. Therefore, it is of high importance in new space to design OQPSK/QPSK receiver which can reach high data rates, resolves phase ambiguity in a computationally effective way as well as has low cost and neat design without increasing ground station complexity.
In this study, we present a low-cost software-defined radio (SDR) and GNU radio QPSK receiver solution which is capable of reaching 25M sample rate in real-time using only CPU of a computer. The solution includes Satlab GNU Radio OOT blocks to cope with phase ambiguity problem without disturbing data flow between sequential blocks. For that purpose, Satlab’s Phase Stepper and Node Synchronizer OOT blocks are used such that their PDU inputs are connected to the PDU output of frame synchronizer block which is looking for the sync word. Should there be a successful frame synchronization, these blocks preserve their states. If there is no frame lock within the specified samples, Phase Stepper changes the phase. In addition, our method does not involve any parallel branches in GNU Radio nor multiple flows working at the same time, thereby saving computational power and facilitating mission control. We have performed packet error rate measurements using this receiver and found 1% PER sensitivity to be-100dBm for 1M baud rate (2M bit rate). In addition, we make use of satellite two-line element (TLE) data to compensate for doppler shift so that the main receiver structure does not have to perform frequency tracking -which slows down receiver locking time and causes frame losses. The results of this study demonstrate that real-time high data rates can be achieved by GNU Radio and its versatile OOT block-coding feature without compromising communication link performance.
