Very low rate coded laser communications for noise-limited space optical channels

For an optical communications signal, when the noise power dominates, the capacity scales as the square of signal power over noise power. In this regime, data rate scales inversely as the 4th power of range, and the high data rate advantage of optical communications that results from a narrower transmit beam cannot be achieved. Before a received signal can be decoded, it first must be acquired from the background noise. In noise-limited optical communications, a background power dependent signal power threshold exists below which acquisition cannot occur, and data cannot be recovered. Decreasing the pulse width, corresponding to increasing the modulation bandwidth B, both increases capacity and aids acquisition. Higher modulation bandwidths than about 10 GHz require higher power in the control electronics due to the energy required to charge and discharge circuit capacitances. Higher complexity and higher power are negative attributes for space optical communications. One solution is to use a mode-locked laser, which enables very high effective peak power pulses with minimum complexity. A mode-locked laser utilizes a non-linear inter-cavity element to generate a fixed-rate train of narrow pulses. Communications requires data encoding on the fixed pulse train carrier. One method is to use a mode-locked laser with right or left circularly polarized output. Initial acquisition is aided by the narrow temporal spectrum and high peak power. Once acquired, all of the background noise outside the single data slot of the symbol can be rejected. In the noise-limited regime, capacity now is proportional to signal power over bandwidth-noise power product. The use of the mode-locked laser, versus a conventional pulse-carved continuous wave laser, allows optical communications links to be established in background noise regimes where the signal could not be acquired using conventional modulation schemes. This laser source and modulation scheme has benefits to CubeSat optical communications, and lander/rover Direct-to-Earth communications. In this paper we investigate the design and performance evaluation of a coded mode-locked laser with right and left circular polarization switching. At the optical receiver a polarizing beam splitter and two photo-detectors are used to detect the data. One method for data detection is to compare the number of received photons from two photo-detectors. This comparison produces 0, or 1, or an erasure output. The erasure symbol is produced when the numbers of photons from the two detectors are equal. This method of detection produces an equivalent binary-input ternary-output discrete memoryless channel. Transition probabilities of this channel are computed based on received signal and background noise fluxes. We evaluate the performance of a very low-rate turbo code (rate 1/31) to be used over this channel. We compare this code's performance with the capacity of the underlying optical channel, and with uncoded system performance. We demonstrate that at least 10 dB coding gain can be achieved with respect to the uncoded case. Moreover, we show that the performance of this well designed code is within 1 dB of channel capacity for an information block size of 16384 bits.