FPGA-based Coherent MSK Spread Spectrum Modem for Small Satellites TT&C Transponders

FPGA-based Coherent MSK Spread Spectrum Modem for Small Satellites TT&C Transponders

In low earth orbit (LEO) satellite communication links it is required to employ modulation/demodulation techniques to achieve transmission with minimum power and efficient usage of spectrum (according to international telecommunication union (ITU) and consultative committee for space data systems (CCSDS) regulations and recommendations) with minimum bit error rate (BER) at the receiver. Minimum shift keying (MSK) meets all these requirements. It is required also to measure the varying distance between the satellite and the ground station (range) and the velocity of the satellite (range rate) to facilitate the tracking of the satellite by the ground station antenna tracking system and therefore, a direct sequence spread spectrum (DS-SS) technique is used. Thus, coherent MSK DS-SS modem is chosen in this paper for LEO communication links. This paper investigates design, implementation and testing of a FPGA-based coherent MSK DS-SS modem suitable for small satellites TT&C transponders. The modem includes a MSK modulator, an automatic gain control (AGC), and MSK DS-SS demodulator/synchronizer (where a proposed novel phase ambiguity solver algorithm is presented). Demodulator performance is evaluated by adding band-limited (nearly white within the signal bandwidth) Gaussian noise to the MSK DS-SS modulated signal (resulting in Eb/No near 0dB) and measuring the BER and the phase variances of the synchronized carrier and the clock with its operating Eb/No for the extracted chips, which show good performance.