Performance of Alamouti space time codes in an optical uplink channel

The optical uplink, ground station to satellite, is more susceptible to signal fading compared to the downlink channel due to beam wandering experienced at the transmitter in the ground station. Alamouti space time codes have been used to partially mitigate this handicap. With the use of these codes, a bit error rate(BER) of 1 × 10
^-8
can be achieved for a link margin of 6dB. A transmitter beam radius of 47cm and a receiver diameter of 1m are noted to be suitable for the uplink optical system with transmitter at an altitude of 2.5km from the ground and the satellite at a zenith angle of 0°. The performance of the Alamouti codes is simulated in MATLAB 2006b and analyzed for one, two and four receivers and the use of 2 × 2 is proposed in this work, since the deployment of four receivers at the satellite does not seem practical. It is demonstrated that a 2×2 Alamouti space time code achieves a total gain of 6dB when convolutional coding and QPSK modulation are used whereas a gain of about 5dB is achieved by using 8PSK or 16QAM. Link Budget analysis of the uplink scenario suggests that the proposed method reduces the required transmitter power from 24.883kW to 6.2207kW.