Optical Transmitter Diversity With Phase-Division in Bit-Time

The application of free-space optical communication systems in satellite applications has gained increasing attention in recent years. Their high data rates and comparably low size, weight and power requirements make them an attractive solution to increase data throughput in a number of applications, such as for optical GEO feeder links with throughputs beyond 1 Tbit/s. In order to use free-space optical links in such application scenarios, a number of challenges must be overcome, such as link-blockage by clouds and, most importantly, impairments due to atmospheric turbulence. Transmitter diversity makes use of the fact that a spatial separation of about 1 m between two transmitters is sufficient to achieve decorrelated channels. When the transmitted signals are combined on receiver side, a diversity gain can be observed. However, typical transmitter diversity systems make use of different wavelengths to separate the diversity channels in order to avoid interference among those channels. This leads to increased system complexity and is bandwidth inefficient. The transmitter diversity scheme Phase-Division in Bit-Time is a novel concept to avoid interference among multiple channels by adding an additional phase modulation on transmitter side in a free-space optical communication system with intensity modulation and direct detection (IM/DD). It enables using the same wavelength and even the same laser source for multiple transmitters. Furthermore, it is similar to Alamouti's scheme known in RF communication systems, but does not require channel state information.