Outage Capacity for MISO Intensity-Modulated Free-Space Optical Links with Misalignment

The outage capacity of slow-fading free-space optical channels is analyzed for a multiple-input/single-output configuration in the presence of atmospheric and misalignment fading. A spatial repetition code is considered at the transmitter and a closed-form expression for the outage capacity is developed. In addition, a simple asymptotic closed-form expression is derived at high signal-to-noise ratio. Two methods are considered for system design using the derived outage capacity results with different beam configurations. The outage capacity is optimized over a predetermined set using numerical techniques. Using the asymptotic form of the outage capacity, however, a closed form for the optimum beamwidth is derived. A comparison of simulation results in both cases gives very similar performance indicating the effectiveness of the asymptotic form to provide a near-optimum expression for the beamwidth.