Chih-Chun Wang

Multiuser Detection of Sparsely Spread CDMA

Code-division multiple access (CDMA) is the basis of a family of advanced air interfaces in current and future generation networks. The benefits promised by CDMA have not been fully realized partly due to the prohibitive complexity of optimal detection and decoding of many users communicating simultaneously using the same frequency band. From both theoretical and practical perspectives, this paper advocates a new paradigm of CDMA with sparse spreading sequences, which enables near-optimal multiuser detection using belief propagation (BP) with low-complexity. The scheme is in part inspired by capacity-approaching low-density parity-check (LDPC) codes and the success of iterative decoding techniques. Specifically, it is shown that BP-based detection is optimal in the large-system limit under many practical circumstances, which is a unique advantage of sparsely spread CDMA systems. Moreover, it is shown that, from the viewpoint of an individual user, the CDMA channel is asymptotically equivalent to a scalar Gaussian channel with some degradation in the signal-to-noise ratio (SNR). The degradation factor, known as the multiuser efficiency, can be determined from a fixed-point equation. The results in this paper apply to a broad class of sparse, semi-regular CDMA systems with arbitrary input and power distribution. Numerical results support the theoretical findings for systems of moderate size, which further demonstrate the appeal of sparse spreading in practical applications.

Efficient network-coding-based opportunistic routing through cumulative coded acknowledgments

The use of random linear network coding (NC) has significantly simplified the design of opportunistic routing (OR) protocols by removing the need of coordination among forwarding nodes for avoiding duplicate transmissions. However, NC-based OR protocols face a new challenge: How many coded packets should each forwarder transmit? To avoid the overhead of feedback exchange, most practical existing NC-based OR protocols compute offline the expected number of transmissions for each forwarder using heuristics based on periodic measurements of the average link loss rates and the ETX metric. Although attractive due to their minimal coordination overhead, these approaches may suffer significant performance degradation in dynamic wireless environments with continuously changing levels of channel gains, interference, and background traffic. In this paper, we propose CCACK, a new efficient NC-based OR protocol. CCACK exploits a novel Cumulative Coded ACKnowledgment scheme that allows nodes to acknowledge network coded traffic to their upstream nodes in a simple way, oblivious to loss rates, and with practically zero overhead. In addition, the cumulative coded acknowledgment scheme in CCACK enables an efficient credit-based, rate control algorithm. Our evaluation shows that, compared to MORE, a state-of-the-art NC-based OR protocol, CCACK improves both throughput and fairness, by up to 20x and 124%, respectively, with average improvements of 45% and 8.8%, respectively.

TCP/IP Timing Channels: Theory to Implementation

There has been significant recent interest in covert communication using timing channels. In network timing channels, information is leaked by controlling the time between transmissions of consecutive packets. Our work focuses on network timing channels and provides two main contributions. The first is to quantify the threat posed by covert network timing channels. The other is to use timing channels to communicate at a low data rate without being detected. In this paper, we design and implement a covert TCP/IP timing channel. We are able to quantify the achievable data rate (or leak rate) of such a covert channel. Moreover, we show that by sacrificing data rate, the traffic patterns of the covert timing channel can be made computationally indistinguishable from that of normal traffic, which makes detecting such communication virtually impossible. We demonstrate the efficacy of our solution by showing significant performance gains in terms of both data rate and covertness over the state-of-the-art.

Cross-layer optimization for wireless multihop networks with pairwise intersession network coding

For wireless multi-hop networks with unicast sessions, most coding opportunities involve only two or three sessions as coding across many sessions requires greater transmission power to broadcast the coded symbol to many receivers, which enhances interference. This work shows that with a new flow-based characterization of pairwise intersession network coding (coding across two unicast sessions), an optimal joint coding, scheduling, and rate-control scheme can be devised and implemented using only the binary XOR operation. The new scheduling/rate-control scheme demonstrates provably graceful throughput degradation with imperfect scheduling, which facilitates the design tradeoff between the throughput optimality and computational complexity of different scheduling schemes. Our results show that pairwise intersession network coding improves the throughput of non-coding solutions regardless of whether perfect/imperfect scheduling is used. Both the deterministic and stochastic packet arrivals and departures are considered. This work shows a striking resemblance between pairwise intersession network coding and non-coded solutions, and thus advocates extensions of non-coding wisdoms to their network coding counterpart.

On The Capacity of Immediately-Decodable Coding Schemes for Wireless Stored-Video Broadcast with Hard Deadline Constraints

Multimedia streaming applications have stringent Quality-of-Service (QoS) requirements. Typically, each packet is associated with a packet delivery deadline. This work models and considers streaming broadcast of stored video over the downlink of a single cell. We first generalize the existing class of immediately-decodable network coding (IDNC) schemes to take into account the deadline constraints. The performance analysis of IDNC schemes are significantly complicated by the packet deadline constraints (from the application layer) and the immediate-decodability requirement (from the network layer). Despite this difficulty, we prove that for independent channels, the IDNC schemes are asymptotically throughput-optimal subject to the deadline constraints when there are no more than three users and when the video file size is sufficiently large. The deadline-constrained throughput gain of IDNC schemes over non-coding scheme is also explicitly quantified. Numerical results show that IDNC schemes strictly outperform the non-coding scheme not only in the asymptotic regime of large files but also for small files. Our results show that the IDNC schemes do not suffer from the substantial decoding delay that is inherent to existing generation-based network coding protocols.

CCACK: Efficient Network Coding Based Opportunistic Routing Through Cumulative Coded Acknowledgments

The use of random linear network coding (NC) has significantly simplified the design of opportunistic routing (OR) protocols by removing the need of coordination among forwarding nodes for avoiding duplicate transmissions. However, NC-based OR protocols face a new challenge: How many coded packets should each forwarder transmit? To avoid the overhead of feedback exchange, most practical existing NC-based OR protocols compute offline the expected number of transmissions for each forwarder using heuristics based on periodic measurements of the average link loss rates and the ETX metric. Although attractive due to their minimal coordination overhead, these approaches may suffer significant performance degradation in dynamic wireless environments with continuously changing levels of channel gains, interference, and background traffic. In this paper, we propose CCACK, a new efficient NC-based OR protocol. CCACK exploits a novel Cumulative Coded ACKnowledgment scheme that allows nodes to acknowledge network coded traffic to their upstream nodes in a simple way, oblivious to loss rates, and with practically zero overhead. In addition, the cumulative coded acknowledgment scheme in CCACK enables an efficient credit-based, rate control algorithm. Our evaluation shows that, compared to MORE, a state-of-the-art NC-based OR protocol, CCACK improves both throughput and fairness, by up to 20x and 124%, respectively, with average improvements of 45% and 8.8%, respectively.

Density evolution for asymmetric memoryless channels

Density evolution (DE) is one of the most powerful analytical tools for low-density parity-check (LDPC) codes and graph codes with message passing decoding algorithms. With channel symmetry as one of its fundamental assumptions, density evolution has been widely and successfully applied to different channels, including binary erasure channels (BECs), binary symmetric channels (BSCs), binary additive white Gaussian noise (BiAWGN) channels, etc. This paper generalizes density evolution for asymmetric memoryless channels, which in turn broadens the applications to general memoryless channels, e.g., z-channels, composite white Gaussian noise channels, etc. The central theorem underpinning this generalization is the convergence to perfect projection for any fixed-size supporting tree. A new iterative formula of the same complexity is then presented and the necessary theorems for the performance concentration theorems are developed. Several properties of the new density evolution method are explored, including stability results for general asymmetric memoryless channels. Simulations, code optimizations, and possible new applications suggested by this new density evolution method are also provided. This result is also used to prove the typicality of linear LDPC codes among the coset code ensemble when the minimum check node degree is sufficiently large. It is shown that the convergence to perfect projection is essential to the belief propagation (BP) algorithm even when only symmetric channels are considered. Hence, the proof of the convergence to perfect projection serves also as a completion of the theory of classical density evolution for symmetric memoryless channels.

Pairwise Intersession Network Coding on Directed Networks

When there exists only a single multicast session in a directed acyclic/cyclic network, the existence of a network coding solution is characterized by the classic min-cut/max-flow theorem. For the case of more than one coexisting sessions, network coding also demonstrates throughput improvement over noncoded solutions. This paper proposes pairwise intersession network coding, which allows for arbitrary directed networks but restricts the coding operations to being between two symbols (for acyclic networks) or between two strings of symbols (for cyclic networks). A graph-theoretic characterization of pairwise intersession network coding is proven based on paths with controlled edge-overlap. This new characterization generalizes the edge-disjoint path characterization of noncoded network communication and includes the well-studied butterfly graph as a special case. Based on this new characterization, various aspects of pairwise intersession network coding are studied, including the sufficiency of linear codes, the complexity of identifying coding opportunities, its topological analysis, and bandwidth- and coding-efficiency.

Beyond the Butterfly - A Graph-Theoretic Characterization of the Feasibility of Network Coding with Two Simple Unicast Sessions

The problem of network coding with two simple unicast sessions is considered for general directed acyclic graphs. An explicit graph-theoretic characterization is provided for the feasibility of whether two symbols at different sources can be simultaneously transmitted to the designated sinks via network coding. The existence of a routing scheme is equivalent to finding edge-disjoint paths. Similarly, in this paper it is proven that the existence of a network coding scheme is equivalent to finding paths with controlled edge overlaps, and the characterization includes the well-studied butterfly graph as a special case. Various generalizations and implications are discussed based on the constructive nature of the flow-based conditions. For example, it is shown that a linear network coding scheme using only six paths is as effective as any non-linear network coding scheme.

On the Capacity of 1-to-

This paper focuses on the 1-to-K broadcast packet erasure channel (PEC), a generalization of the broadcast binary erasure channel from the binary symbol to a finite field G F(q) with sufficiently large q. We consider the setting in which the source node has instant feedback of the channel outputs of the K receivers after each transmission. The main results of this paper are: (i) The capacity region for general l-to-3 broadcast PECs and (ii) The capacity region for two types of 1-to-K broadcast PECs: the symmetric PECs, and the spatially independent PECs with one- sided fairness constraints. This paper also develops (iii) A pair of outer and inner bounds of the capacity region for arbitrary 1-to-K broadcast PECs, which can be easily evaluated by any linear programming solver. The proposed inner bound is proven by a new class of intersession network coding schemes, termed the packet evolution schemes, which is based on the concept of code alignment in GF(q) that is in parallel with the interference alignment techniques for the Euclidean space. Extensive numerical experiments show that the outer and inner bounds meet for almost all broadcast PECs encountered in practical scenarios and thus effectively bracket the capacity of general 1-to-K broadcast PECs with COF.