NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an end-to-end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.
The hardware implementation of a low complexity Low Density Parity Check (LDPC) decoder is described. The design of the LDPC decoder optimized on minimizing the amount of hardware resources necessary for implementation. In addition to implementation details, design tradeoffs considered in the development of the LDPC decoder are discussed. The intended application of the LDPC decoder is a nonlinear satellite communications channel. The nonlinearities and communications signal perturbations include Additive White Gaussian Noise (AWGN), phase noise, phase imbalance, and a model satellite high power amplifier nonlinearity. The LDPC decoder performance is then characterized in the satellite channel.
The hardware implementation of a high throughput Max Log Map Serial Concatenated Convolutional Code (SCCC) turbo decoder for an optical channel employing 64 Pulse Position Modulation (PPM) is described. The Max Log MAP turbo decoder is in contrast to a corresponding optimal log MAP turbo decoder. The Max log MAP decoder is the preferred turbo decoder for applications requiring high throughput. The performance of both the max log MAP decoder and log MAP decoder are compared to the theoretical performance values. Tradeoffs used in the implementation of the high throughput Max Log Map 64 PPM decoder are discussed.
Turbo codes and Low Density Parity Check (LDPC) codes are well known to provide Bit Error Rate (BER) performance close to the Shannon capacity limit. Bandwidth constrained satellite channels could potentially benefit by employing higher order PSK modulations. However, employing higher order PSK modulations may not be practical for satellite amplifiers due to the increased power requirements. The excellent performance of serial concatenated turbo codes could be used to maintain satellite amplifier power levels to those relatively close to the Shannon limit. The performance of the system, however, is dependent on the satellite channel, which typically includes phase noise and some degree of nonlinearity in the satellite amplifier. The performance of various waveforms and PSK modulations employing Serial Concatenated Turbo Codes are investigated using a model of a non-ideal satellite channel. The hardware complexity of the serial concatenated turbo decoder at the ground receiver is also considered.
Conference Committee Involvement (2)
Satellite Data Compression, Communication, and Archiving II
13 August 2006 | San Diego, California, United States
Satellite Data Compression, Communications, and Archiving
31 July 2005 | San Diego, California, United States