This paper studies the achievable rate of a relay- assisted optical wireless communication channel, and presents a novel discrete signaling method by our proposed symbol-number filling algorithm. Unlike the radio frequency counterpart, the signaling is designed subject to nonnegative, peak and average power constraints, taking the input-dependent Gaussian shot noise into account. Decode and forward (DF) protocol is adopted by the relay, and is shown to outperform the direct transmission (DT) protocol. Further, the impact of relay placement on the rates is investigated on the two dimensional relay positions. The region where DF is better than DT is given and the maximal rate is shown to be achievable when the relay is placed in the middle of the source and destination link.