Wavelength division multiple access for deep space optical communications

In this paper, a high capacity wavelength division multiplexing (WDM) for high data rate optical communications, a WDM with 2-dimensional M-PPM to achieve a higher number of bits per photon transmission, an optical wavelength division multiple access (WDMA) for multiple CubeSats/small spacecraft for deep space optical communications, and a combined optical WDMA with optical code division multiple access (CDMA) are investigated. The signal processing components required for successful operation of above mentioned optical systems are defined. For WDMA, the performance of the uncoded case for N SmallSats/CubeSats is obtained. Software simulations are used to obtain the performance of N SmallSats/CubeSats using a coded optical WDMA multiple access scheme. Results from successful completion of this investigation might be used in the future for deep space optical communication scenarios that require science data from SmallSats/CubeSats to be transmitted to Earth. An M-ary Pulse Position modulation (PPM) format is considered. For a hard decision optical receiver with photon counting detectors, a rate compatible protograph code is proposed. The code is optimized using the reciprocal channel approximation, which is a one-dimensional density evolution. The performance analysis is provided for the uncoded and simulated coded optical WDMA under background noise for up to N=16 SmallSat/CubeSats. Our results show, for example, that in the case of 2-PPM there is a 7 dB coding gain (with respect to the uncoded case) at a FER of 10
⁻⁶
, for a rate 0.8 code, and 12 dB coding gain for a rate 1/6 code, depending on the received background noise intensity. Leveraging existing laboratory hardware to test the deep-space optical WDMA concept, an experiment is performed. Finally the concept of combining WDMA with optical CDMA to support more users is analyzed using short signature sequences and uncoded 2-PPM modulation and simulations are performed for coded system.