# On the Achievable Rate and Capacity for a Sample-based Practical Photon-counting Receiver

## TL;DR

This paper analyzes the capacity and achievable rate of a practical photon-counting receiver, deriving bounds and approximations for various regimes, and compares its performance to the ideal continuous Poisson channel.

## Contribution

It provides the first comprehensive bounds and asymptotic analysis for the capacity of a non-perfect photon-counting receiver under different conditions.

## Key findings

- Bounds are tight for large peak power with zero background radiation.
- Approximate achievable rate is more accurate in medium SNR regimes.
- Capacity approaches continuous Poisson capacity as symbol duration decreases.

## Abstract

We investigate the achievable rate and capacity of a non-perfect photon-counting receiver. For the case of long symbol duration, the achievable rate under on-off keying modulation is investigated based on Kullback-Leibler (KL) divergence and Chernoff $\alpha$-divergence. We prove the tightness of the derived bounds for large peak power with zero background radiation with exponential convergence rate, and for low peak power of order two convergence rate. For large peak power with fixed background radiation and low background radiation with fixed peak power, the proposed bound gap is a small positive value for low background radiation and large peak power, respectively. Moreover, we propose an approximation on the achievable rate in the low background radiation and long symbol duration regime, which is more accurate compared with the derived upper and lower bounds in the medium signal to noise ratio (SNR) regime. For the symbol duration that can be sufficiently small, the capacity and the optimal duty cycle are is investigated. We show that the capacity approaches that of continuous Poisson capacity as $T_s=\tau\to0$. The asymptotic capacity is analyzed for low and large peak power. Compared with the continuous Poisson capacity, the capacity of a non-perfect receiver is almost lossless and loss with attenuation for low peak power given zero background radiation and nonzero background radiation, respectively. For large peak power, the capacity with a non-perfect receiver converges, while that of continuous Poisson capacity channel linearly increases. The above theoretical results are extensively validated by numerical results.