A new performance upper bound for convolutionally-encoded direct-detection optical OPPM communications systems

In a convolutionally-encoded direct-detection optical overlapping pulse-position modulation (OPPM) channel, due to absence of orthogonality among the transmitted symbols, the conventional performance upper bounds that are obtained by weight-based transfer function of the convolutional encoder (CE) do not adequately characterize performance. A novel ensemble average upper bound is proposed. This method employs the "chip distance" between symbols, which emphasizes the inherent non-orthogonality of the OPPM signals, to compute the chip distances of all possible path-pairs in the trellis of the convolutionally-encoded OPPM symbols and, subsequently, to arrive at the minimum chip distance. The resulting minimum distance is then used to arrive at an upper bound on the performance of the aforementioned channel. With the aid of simulation, this ensemble average bound is shown to provide an accurate estimate of the performance of a convolutionally-encoded OPPM channel.