Andrew P. Worthen

Routing and rate control for coded cooperation in a satellite-terrestrial network

We address the problem of high-throughput, delay-constrained communication over a satellite-terrestrial network where terrestrial node mobility leads to intermittent links. Due to the short time-scale of link state durations, standard single-path routing protocols are disadvantaged by the delay incurred in determining that a route is unavailable and then finding a new route. Instead we focus on the approach of sending data over multiple paths simultaneously, and use random linear network coding as a distributed way of sending linearly-independent data on different paths. We present a routing and rate control protocol for coded multipath routing. This protocol specifies the fraction of offered traffic carried on each path, provides a congestion avoidance strategy to limit queueing delays in the network, and adapts quickly to time-varying connectivity. We outline our coded routing and rate-control strategy and also present simulation results from a mobile satellite-terrestrial network.

Rate Control for Network-Coded Multipath Relaying with Time-Varying Connectivity

This paper presents techniques for achieving high throughput in delay-constrained, multihop wireless communication networks with time-varying link connectivity. We develop a rate-controlled, multipath strategy using network coding, and compare its performance with that of multipath flooding and with the performance of traditional single-path strategies. These performance comparisons include both theoretical benchmarks and simulation results from cooperative relay scenarios, which incorporate different sets of link connectivity statistics that are drawn from field tests of mobile satellite communication terminals. The results indicate that with appropriate rate-control, network coding can provide throughput performance comparable to multipath flooding of the network while utilizing bandwidth nearly as efficiently as single-path routing.

Internet Routing in Space: Prospects and Challenges of the IRIS JCTD

Recent technological advances coupled with the collaboration between government and industry will soon make it possible to include IP routers and IP modems on board a commercial geostationary communications satellite. The Internet Protocol Routing in Space (IRIS) Joint Capability Technology Demonstration (JCTD) will introduce a new network capability that is aimed at enhancing military network-centric operations through information access, collaboration and dissemination. This paper describes the IRIS vision and strategy, and the specific goals of the JCTD. It describes the network architecture and technology development aspects for deploying a combined router and modem function as part of a hosted payload within a commercial transponded satellite.

On the Channel Memory-Diversity Tradeoff in Communication Systems

In this chapter we investigate the tradeoff between channel estimation and channel diversity for channels with memory. Block memory channels are analytically tractable yet provide practical models for many communication systems. We show that for finite block length transmission techniques there is a fundamental tradeoff between the channel estimator and the channel diversity. When the channel is operating at rates much below capacity, smaller channel memory is better since channel estimation is not crucial. This is because the code contains more than adequate redundancy to compensate for mismatch due to the potentially poor channel estimate as well as channel errors. However, at rates close to capacity, channel state information is crucial for successful decoding and therefore longer channel memory, which allows better estimates, is better. We draw these conclusions based on analysis of some simple discrete-input, discrete-output channels using the channel reliability function and on simulation results for more realistic channels (soft output) using low-density parity check codes with a joint iterative decoder and channel estimator.

Performance bounds on cooperative radar and communication systems operation

A theoretical framework that embraces the competing objectives of cooperative radar-communication operations is proposed that engages the apparent trade-space in an optimal fashion. Specifically, minimization of a cooperative risk metric that extends the Neyman-Pearson criterion to include communication data rate is explored. Hence, establishing performance bounds for cooperative interaction. Ideal informed reception where simultaneous access to received data from both the radar and communication (comm.) system is shown to yield a structured covariance-based water-filling solution. Unlinked cooperation where codebooks, waveforms, and schedules are known by all, but received data is not relayed between radar and comm. conveys a complex tradespace dependent on the rate of information transmitted by the comm. system relative to the capacity of the radar-comm. data link.

CPM optimization for low-complexity serial concatenated CPM

Serial concatenated continuous phase modulation (SCCPM) combines the constant envelope and bandwidth efficiency of continuous phase modulation (CPM) with the performance of serially concatenated codes making it attractive for satellite communication links with saturated transmitter amplifiers. Iterative demodulation and decoding of SCCPM provides excellent performance with simple outer codes by combining the structures of the modulation and the code. Because of the large space of CPM parameters, finding good SCCPM schemes with desired bandwidth efficiency and reasonable complexity is challenging. We present a systematic, simulation-based approach to this optimization problem with special attention to the interesting trade-off between the CPM bandwidth efficiency and the outer code rate.

MQCC: Maximum Queue Congestion Control for Multipath Networks with Blockage

This paper presents a transport layer protocol for multi-path networks with blockage. Using urban SATCOM as an example, we see from data taken from a 2006 measurement campaign that these blockages are generally on the order of 1 -- 5seconds in length and the links are blocked approximately 33%of the time. To compensate for this type of impairment, we have developed a multipath IP overlay routing algorithm, a random linear coding reliability scheme, and a maximum-queue-based (MQCC) congestion control algorithm. MQCC uses average buffer occupancy as a measure of the congestion in a network (as opposed to packet loss or round trip time (RTT)) and updates the transmission rate of each source to avoid network congestion. This allows us to design a congestion control algorithm that is independent of the channel conditions and can be made resilient to channel losses. The reliability scheme uses selective negative acknowledgments (Snacks) to guarantee packet delivery to the destination. We show through simulation that we can approach the optimal benchmark in realistic loss blockage channels.

Moderate time-scale dynamic spectrum access with wide-area spectrum sensors

Simulation approaches and results are provided for Dynamic Spectrum Access (DSA) with remote wide-area spectrum monitoring equipment and moderate delay in spectrum data dissemination. Issues expected to arise using this approach include information delay time, failure to detect all transmitters, transmitter location errors, poor transmit-power estimates, and errors in the propagation model used to predict radio interference. The system works well if delays do not exceed 10 minutes, location error standard deviations are less than or equal to 2 km, and at least half of the transmitters are detected. It is found to be more sensitive to poor transmitter power and propagation-loss estimates.

Request Protocol Performance Impact for Mobile SATCOM with Dynamic Resource Allocation

Future communication schemes for transponding satellites will use dynamic resource allocation for efficient transmission of multi-media packet traffic. In typical centrally controlled schemes, a controller terminal is designated to collect resource requests from the other terminals and allocate slots. The delay between request generation and the related allocations reduces performance. For communications with terminals on-the-move, occasional blockage of the link is an additional impairment to the request-allocation protocol. This results infrequent lost request and allocation messages which may impair the ability of the terminal to communicate even when it is not blocked. We consider how the request-allocation protocol interacts with the on-the-move SATCOM channel for several allocation algorithms in the context of user-relevant performance metrics

A multipath routing overlay for networks with blockage

Blockage of communication links due to environmental obstructions and the resulting on/off channel behavior present challenges for providing reliable communication in mobile wireless networks. Traditional reliability techniques such as forward error correction and automatic repeat request (ARQ) are not designed to operate at the timescales typically observed in blockage channels. This work presents an approach to overcoming blockage that consists of multipath routing coupled with end-to-end rateless coding. In order to provide compatibility with operational IP networks as well as the possibility of incremental deployment, these techniques can be implemented as a routing overlay. We present algorithms to compute the maximum throughput achievable through this routing overlay approach, as well as a specific scheme to implement a multipath routing overlay in an IP network. Finally, we provide results from testing this approach on a mobile wireless network with satellite communication links. In our test scenario, satellite links suffer blockage from buildings in an urban setting. Our results characterize multipath routing overlay performance, which depends on the responsiveness of the underlay routing scheme.