Short-Preamble DSP for Downstream Alamouti Coherent-Lite PON Using Gear-Shifted LMS Equalization

We present a short-preamble digital signal processing (DSP) architecture for downstream Alamouti coherent-lite passive optical network (PON) reception using a simplified optical network unit (ONU) receiver. A four-branch gear-shifted least-mean-square (LMS) equalizer uses a large step during quadrature phase-shift keying (QPSK) preamble training and a small step during decision-directed payload tracking. In 50 GBaud 16-ary quadrature amplitude modulation (16-QAM) simulations over 20 km single-mode fiber, a 2048-symbol preamble is the first cold-start point whose mean bit-error rate (BER) falls below the selected uncoded BER target of 10−2. A 20-realization-per-point sweep gives a sub-1 dB penalty for 2048 symbols and a mean offline mean-square-error (MSE) settling time below 100 ns, with low-gain locking events retained. However, frame-level reliability improves with longer training: in an indicative 20-frame Monte Carlo test, 13 of 20 frames met the BER target with a 2048-symbol preamble, compared with 17 of 20 using 4096 symbols. For repeated frames to the same ONU after initial acquisition, warm-start tap reuse allows frames 4–5 in the tested sequence to fall below target with 512-symbol preambles, corresponding to a later-frame nominal coded payload rate of 163 Gb/s instead of 123 Gb/s. The results quantify the preamble/reliability/payload-rate trade-off and indicate that downstream Alamouti coherent-lite reception can support short-training equalizer acquisition after preamble synchronization with one balanced detector and one analog-to-digital converter.