CDMA communications systems with constant envelope modulation for CubeSats

In this paper a communication system for CubeSats in formation to operate in the vicinity of the Lunar Lagrangian L1 is proposed. CubeSats will collect lunar scientific data and will perform surface observations. An improved low complexity CDMA system for CubeSats for communications between the Lunar L1 and Earth station is considered. The complexity of a coded CDMA transmitter is lower than the complexity of the CDMA receiver with decoder therefore for downlink communications it makes sense to use encoders such as space standard LDPC code followed by a spread spectrum transmitter for CDMA systems for CubeSats. For the uplink an uncoded CDMA system is chosen since the uplink transmit power is expected to be high enough to support the use of uncoded CDMA system. The uncoded CDMA yields receivers for CubeSats that have low complexity implementation. For uplink since there would be no multipath the use of orthogonal spreading codes is more appropriate. The choice of orthogonal codes would reduce the multiuser interference at CubeSats. For the downlink, based on the available bandwidth, and the data rates, a reasonable processing gain could be obtained. Thus the multiuser interference degradation due to the other CubeSats could be made small at the Earth station. In this paper we analyzed and simulated the proposed improved CDMA system for a concept Constellation of CubeSats. All system simulations are done using Simulink Matlab platform. For highly efficient nonlinear power amplifiers we use a filtered offset QPSK with phase modulation which is a CCSDS standard for constant envelope signaling. This allows a nonlinear amplifier at CubeSat to operate at saturation point for the highest efficiency. We compare the difference in performance with our current pulse shaped BPSK (using half-sine wave). Certainly if there is no bandwidth limitation (due to the spectral standard masking) we could as well use unfiltered rectangular pulses which would produce constant envelope signaling. But it seems that rectangular pulses will not satisfy the bandwidth limitation imposed by the spectral standard. Filtered offset QPSK with phase modulation is much more bandwidth efficient scheme.