Ryan Shoup

Design of an Optical Photon Counting Array Receiver System for Deep-Space Communications

Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical free-space laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radio-frequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the mars laser communications demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.

An end-to-end demonstration of a receiver array based free-space photon counting communications link

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.

Using DVB-S2 over asymmetric heterogeneous optical to radio frequency satellite links

The DVB-S2 coding standard has seen widespread use in many radio frequency (RF) communications applications. The availability of commercial-off-the-shelf (COTS) intellectual property (IP) that can be used to rapidly prototype and field communications systems makes this well-performing, standards-based approach to forward error correction (FEC) coding extremely attractive. In this paper, we evaluate the application of the DVB-S2 coding standard to an asymmetric satellite communications channel. The uplink comprises a fading optical link employing binary differential phase-shift keyed (DPSK) modulation, while the downlink comprises an RF link employing 16-ary amplitude and phase shift keyed (16-APSK) modulation. To simplify the payload implementation, hard-decision uplink demodulation is considered with uplink channel state information transmitted on the downlink for soft-decision decoding in the ground-based receiver. Additionally, we outline many of the tradeoffs in the overall system design, and some performance results of a baseline design are presented.

Hardware implementation of a high-throughput 64-PPM serial concatenated turbo decoder

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.

A peak to peak frame synchronization algorithm for data frames transmitted asynchronously over fading channels

Space-division multiple access communication system architectures employing frame-switching, where ingress frames from one of N ingress ports are switched to one of N egress ports, can support the simultaneous transmission of data from multiple users. If the switched inputs have undergone channel fading, the frame switch may (incorrectly) switch ingress frames to (incorrect) egress ports. When frame switch errors occur, the egress port data streams may contain an incorrect number of data frames and/or erroneous frame sequence numbers resulting in frame synchronization problems at the end receiver associated with that particular egress port. A maximum-likelihood algorithm is employed to run at the end receiver to correctly recover frames and corresponding frames sequence numbers for proper frame synchronization.

Incorporation of uplink channel state information into an end-to-end coded satellite communication system

Satellite communications systems employing forward error correction (FEC) codes are simplified when the code is applied at the sources transmitter and removed at the end users receiver (end-to-end coding), avoiding decoding and re-encoding on board the satellite. Channel capacity can be approached when channel state information from the uplink is incorporated into the soft information input to the downlink decoder. For fading channels, the channel state information is time-varying and must be updated frequently. This paper will describe techniques for both measuring and incorporating practical, bandwidth efficient uplink channel state information in an end-to-end coded satellite communications system. Link performance with and without channel state information will be compared. A technique for estimating channel state information will also be analyzed.

Practical hardware realizable frame discrimination algorithms for deep fading channels in optical communications

Frame discrimination algorithms are discussed that address the problem of differentiating user frames transmitted asynchronously through optical fading channels. In communications systems where multiple users transmit asynchronously, fading channels can render the received frames unidentifiable at the receiver so that intended user destinations for the frames are then unknown. Algorithms that are readily hardware-realizable are discussed that discriminate user frames so that the frames can be properly delivered at the receiver to intended recipients.

Optical M-PPM signaling with binary forward error correction

Orthogonal modulation is a natural choice for free-space optical links due to the relatively simple modulation and demodulation options that are available. In particular, pulseposition modulation (PPM) takes advantage of the inherent average power limitations of some optical transmitters. In practical implementations, two types of optical PPM receivers are of interest: photon counting receivers and optical preamplified receivers. The differences between the two implementations result in different performance as well as different methods of analysis. In this paper, we consider error-correction coding options for both photon counting and preamplified versions of M-ary PPM, where M is the cardinality of the signal set. Of primary interest is the performance of commercially-available binary error-control codes, since such codes have widespread availability and near-optimum performance. We derive a capacity result to define the performance penalty from using binary decoding on M-ary orthogonal signalling.

Hardware implementation and characterization of a low density parity check (LDPC) decoder

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.

Performance of PSK modulation with serial concatenated turbo codes through a nonideal satellite channel

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.