Optical CDMA for a constellation of CubeSats

In this paper, an end-to-end optical CubeSat Code Division Multiple Access (CDMA) system architecture is developed. The signal processing components required for successful operation are defined. Performance analysis for the uncoded case for N CubeSats is derived. Software simulations are used to obtain the performance of N CubeSats using a coded optical CDMA multiple access scheme. Results from successful completion of this investigation might be used in the future for near-Earth and deep space optical communication scenarios that require science data from CubeSats to be transmitted to Earth. An M-ary Pulse Position modulation (PPM) format is considered. Low phase offset autocorrelation and cross correlation signature sequences are used for spreading. Tradeoffs between the weights, lengths, and maximum number of such sequences are assessed, and their performance with optical PPM modulation is accurately analyzed in the presence of multiuser interference. For a hard decision optical receiver with photon counting detectors, a low rate protograph code is proposed. The code is optimized using the reciprocal channel approximation, which is a one dimensional density evolution. Performance analysis is provided for uncoded and simulated coded optical CDMA under three background noise regimes and for up to N=15 CubeSats and signature sequences of weights 5 and 3. Our results show, for example, that in the case of 2-PPM and using weight 5 signature sequences for a single CubeSat, there is a 7 dB coding gain (with respect to the uncoded case) at a BER of 10
^-4
, whereas for 15 CubeSats there is a 12 to 15 dB coding gain at the same BER depending on the received background noise intensity. We have demonstrated that with the proposed coding scheme, the loss due to multiuser interference is less than 1 dB for 15 CubeSats. Leveraging existing laboratory hardware to test the deep-space optical CDMA concept, an experiment is performed. Four gain-switched lasers near 1550 nm are combined with a WDM multiplexer to generate 3 channels of CDMA signals plus a background noise channel. CDMA modulation signals are encoded across multiple lasers using a multi-channel logic analyzer. Light is attenuated to the single photon level and measured with a superconducting nanowire single photon detector with time-correlated single photon counting electronics.