Michael K. Cheng

California Institute of Technology, Pasadena, California, United States

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Publications (9)0.2 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The Optical Communications Telescope Laboratory (OCTL) located on Table Mountain near Wrightwood, CA served as an alternate ground terminal to the Lunar Laser Communications Demonstration (LLCD), the first free-space laser communication demonstration from lunar distances. The Lunar Lasercom OCTL Terminal (LLOT) Project utilized the existing 1m diameter OCTL telescope by retrofitting: (i) a multi-beam 1568 nm laser beacon transmitter; (ii) a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) receiver for 1550 nm downlink; (iii) a telescope control system with the functionality required for laser communication operations; and (iv) a secure network connection to the Lunar Lasercom Operations Center (LLOC) located at the Lincoln Laboratory, Massachusetts Institute of Technology (LL-MIT). The laser beacon transmitted from Table Mountain was acquired by the Lunar Lasercom Space Terminal (LLST) on-board the Lunar Atmospheric Dust Environment Explorer (LADEE) spacecraft and a 1550 nm downlink at 39 and 78 Mb/s was returned to LLOT. Link operations were coordinated by LLOC. During October and November of 2013, twenty successful links were accomplished under diverse conditions. In this paper, a brief system level description of LLOT along with the concept of operations and selected results are presented.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2044087 · 0.20 Impact Factor
  • Philip Tsao · Michael K. Cheng · George Lu · Clayton Okino
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    ABSTRACT: Distributed applications are often faced with a choice between improved throughput or improved reliability but not both. We argue that this is not a strict dichotomy and propose a framework that improves both application performance and reliability by adaptively adjusting source and channel coding parameters. For simplicity, we assume that the source and channel we work with are memoryless and in general behave in a way such that Shannon's separation theorem holds. Although not all sources and channels could be characterized as such, doing so allows us to work this resource allocation problem in parallel. We reduce redundant transmissions and minimize bandwidth utilization through an LZ77 style dictionary based source coding approach. To ensure data integrity, we apply rateless forward error correction techniques at the transport layer. Our algorithm works in conjunction with physical layer forward error correction and generates just enough overhead needed to achieve error free transmission without requiring a heavy use of a reverse channel for acknowledgments. We show through simulations that our combined source and channel approach reduces network traffic in our experimental platform by a measurable amount while maintaining and at times exceeding the Quality of Service (QoS) that is obtained without our technique.
    IEEE Aerospace Conference Proceedings 01/2010; DOI:10.1109/AERO.2010.5446945
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    ABSTRACT: Memory systems used in space applications suffer from radiation-induced errors, either temporary upsets (soft errors) or permanent defects (hard errors or stuck-at errors). Scrubbing is a method to protect memory contents by periodically decoding the stored data to correct those soft and stuck-at errors then rewriting the corrected data back into memory. However, defective cells will remain and accumulate over time. Conventional coding disregards defective cells, however this may be inefficient for memory protection in space. In this study, alternative coding schemes for scrubbing are investigated, where the channel model depends on the cell states, defective or not, and the encoder uses channel state information (CSI) or side information. At every scrubbing, the error correcting code (ECC) decoder provides partial CSI back to the encoder and the encoder uses the CSI to improve the performance of memory systems with scrubbing. Information theoretic limits of the channel with partial CSI are investigated and several coding schemes are introduced to mitigate the effects of defective cells, particularly those caused by stuck-at defects. In addition, coding schemes with partial CSI are concatenated with binary Bose-Chaudhuri-Hocquenghem (BCH) codes to protect memory contents from both soft and stuck-at errors in space radiation environments. Numerical simulation results show that scrubbing with partial CSI improves reliability over the state-agnostic approaches.
  • Source
    Michael K Cheng · Dariush Divsalar · Stephanie Duy
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    ABSTRACT: In this work, we study the performance of structured Low-Density Parity-Check (LDPC) Codes together with bandwidth efficient modulations. We consider protograph-based LDPC codes that facilitate high-speed hardware implementations and have minimum distances that grow linearly with block sizes. We cover various higher-order modulations such as 8-PSK, 16-APSK, and 16-QAM. During demodulation, a demapper transforms the received in-phase and quadrature samples into reliability information that feeds the binary LDPC decoder. We will compare various low-complexity demappers and provide simulation results for assorted coded-modulation combinations on the additive white Gaussian noise and independent Rayleigh fading channels.
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    ABSTRACT: Low-density parity-check (LDPC) codes are the state-of-the-art in forward error correc- tion (FEC) technology that exhibits capacity approaching performance. The Jet Propulsion Laboratory (JPL) has designed a family of LDPC codes that are similar in structure and therefore, leads to a single decoder implementation. The Accumulate-Repeat-by-4-Jagged- Accumulate (AR4JA) code design offers a family of codes with rates 1/2, 2/3, 4/5 and lengths 1024, 4096, 16384 information bits. Performance is less than one dB from capacity for all combinations. Integrating a stand-alone LDPC decoder with a commercial-off-the-shelf (COTS) re- ceiver faces additional challenges than building a single receiver-decoder unit from scratch. In this work, we outline the issues and show that these additional challenges can be over- come by simple solutions. To demonstrate that an LDPC decoder can be made to work seamlessly with a COTS receiver, we interface an AR4JA LDPC decoder developed on a field-programmable gate array (FPGA) with a modern high data rate receiver and mea- sure the combined receiver-decoder performance. Through optimizations that include an improved frame synchronizer and different soft-symbol scaling algorithms, we show that a combined implementation loss of less than one dB is possible and therefore, most of the coding gain evidence in theory can also be obtained in practice. Our techniques can benefit any modem that utilizes an advanced FEC code.
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    ABSTRACT: We describe novel interleaver and deinterleaver architectures that support bandwidth efficient memory access for decoders of turbo-like codes that are used in conjunction with high order modulations. The presentation focuses on a decoder for serially concatenated pulse-position modulation (SCPPM), which is a forward-error-correction code designed by NASA to support laser communications from Mars at mega-bits-per-second (Mbps) rates. For 64-ary PPM, the new architectures effectively triple the fan-in of the interleaver and fan-out of the deinterleaver, enabling parallelization that doubles the overall throughput. The techniques described here can be readily modified for other PPM orders
    International Symposium on Circuits and Systems (ISCAS 2006), 21-24 May 2006, Island of Kos, Greece; 05/2006
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    ABSTRACT: We present a field programmable gate array (FPGA) implementation of a turbo-like decoder for a serially concatenated pulse-position modulation (SCPPM) code. NASA developed this coded modulation scheme for deep space com- munications from Mars. Under a nominal mission condition, the SCPPM coded system can operate within a one dB signal energy gap from capacity. The structure of SCPPM makes direct application of the conventional turbo decoding algorithm very inefficient. Here, we describe techniques to increase the throughput and performance of a hardware SCPPM decoder. Using our optimizations, we demonstrate a 6 mega-bits per second (Mbps) decoder realization on a single FPGA. Extension to a higher data rate decoder using multiple FPGAs is readily achievable. Similar codes designed for the optical channel can benefit from our optimization techniques.
    Proceedings of the Global Telecommunications Conference, 2006. GLOBECOM '06, San Francisco, CA, USA, 27 November - 1 December 2006; 01/2006
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    ABSTRACT: Significant technological advances were made toward utilizing the Hale telescope for receiving the faint laser communication signals transmitted from an optical transceiver on a spacecraft orbiting Mars. The so-called Palomar Receive Terminal design, which would have supported nominal down- link data rates of 1-30 Mbps, is described. Testing to validate technologies for near-Sun (3� from edge of solar disc) daytime operations is also discussed. Finally, a laboratory end-to-end link utilizing a 64-ary pulse-position modulated photon- counting receiver and decoder that achieved predicted near- capacity (within 1.4 dB) performance is described.
  • Source
    Polly Estabrook · Dennis K Lee · Michael K Cheng · Chi-Wung Lau
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    ABSTRACT: The National Aeronautics and Space Administration (NASA) Space Communications and Navigation (SCaN) network consists of the Space Network (SN), the Near Earth Network (NEN), and the Deep Space Network (DSN). Historically, the three NASA networks have used different ground station telemetry receivers since these networks serve different applications and have different service requirements. In order to support the demands of future missions, SCaN has developed plans to support advanced modulation and coding techniques, including Low Density Parity Check (LDPC) codes. This paper presents the measured performance comparison of selected current and future ground station receivers within the SCaN network using LDPC rate ½ short length code. These receivers include the SN Integrated Receiver, the DSN Downlink Telemetry and Tracking (DTT) receiver, and the commercial-off-the-shelf (COTS) User Services Subsystem (USS) Component Replacement (CR) Narrowband Modem prototype receiver. Some of these receivers were not designed for use in the LDPC low E s /N o operating region, but our results show that with proper receiver loop bandwidth optimization and configuration, all of these receivers are capable of providing good performance with this code.