I-Hsiang Wang

Interference Mitigation Through Limited Receiver Cooperation

Interference is a major issue limiting the performance in wireless networks. Cooperation among receivers can help mitigate interference by forming distributed MIMO systems. The rate at which receivers cooperate, however, is limited in most scenarios. How much interference can one bit of receiver cooperation mitigate? In this paper, we study the two-user Gaussian interference channel with conferencing decoders to answer this question in a simple setting. We identify two regions regarding the gain from receiver cooperation: linear and saturation regions. In the linear region, receiver cooperation is efficient and provides a degrees-of-freedom gain, which is either one cooperation bit buys one over-the-air bit or two cooperation bits buy one over-the-air bit. In the saturation region, receiver cooperation is inefficient and provides a power gain, which is bounded regardless of the rate at which receivers cooperate. The conclusion is drawn from the characterization of capacity region to within two bits/s/Hz, regardless of channel parameters. The proposed strategy consists of two parts: 1) the transmission scheme, where superposition encoding with a simple power split is employed and 2) the cooperative protocol, where one receiver quantize-bin-and-forwards its received signal and the other after receiving the side information decode-bin-and-forwards its received signal.

Interference Mitigation Through Limited Transmitter Cooperation

Interference limits performance in wireless networks and cooperation among receivers or transmitters can help mitigate interference by forming distributed MIMO systems. Earlier work shows how limited receiver cooperation helps mitigate interference. The scenario with transmitter cooperation, however, is more difficult to tackle. In this paper we study the two-user Gaussian interference channel with conferencing transmitters to make progress towards this direction. We characterize the capacity region to within 6.5 bits/s/Hz, regardless of channel parameters. Based on the bounded-gap-to-optimality result, we show that there is an interesting reciprocity between the scenario with conferencing transmitters and the scenario with conferencing receivers and their capacity regions are within a bounded gap to each other. Hence, in the interference-limited regime, the behavior of the benefit brought by transmitter cooperation is the same as that by receiver cooperation.

Two-way interference channels

We consider two-way interference channels (ICs) where forward and backward channels are ICs but not necessarily the same. We first consider a scenario where there are only two forward messages and feedback is offered through the backward IC for aiding forward-message transmission. For a linear deterministic model of this channel, we develop inner and outer bounds that match for a wide range of channel parameters. We find that the backward IC can be more efficiently used for feedback rather than if it were used for independent backward-message transmission. As a consequence, we show that feedback can provide a net increase in capacity even if feedback cost is taken into consideration. Moreover we extend this to a more general scenario with two additional independent backward messages, from which we find that interaction can provide an arbitrarily large gain in capacity.

Interference mitigation through limited transmitter cooperation

Interference limits performance in wireless networks and cooperation among receivers or transmitters can help mitigate interference by forming distributed MIMO systems. Earlier work shows how limited receiver cooperation helps mitigate interference. The scenario with transmitter cooperation, however, is more difficult to tackle. In this paper we study the two-user Gaussian interference channel with conferencing transmitters to make progress towards this direction. We characterize the capacity region to within 6.5 bits/s/Hz, regardless of channel parameters. Based on the bounded-gap-to-optimality result, we show that there is an interesting reciprocity between the scenario with conferencing transmitters and the scenario with conferencing receivers and their capacity regions are within a bounded gap to each other. Hence, in the interference-limited regime, the behavior of the benefit brought by transmitter cooperation is the same as that by receiver cooperation.

Two unicast information flows over linear deterministic networks

We investigate the two unicast flow problem over layered linear deterministic networks with arbitrary number of nodes. When the minimum cut value between each source-destination pair is constrained to be 1, it is obvious that the triangular rate region {(R<inf>1</inf>, R<inf>2</inf>) ∶ R<inf>1</inf>, R<inf>2</inf> ≥ 0, R<inf>1</inf> + R<inf>2</inf> ≤ 1} can be achieved, and that one cannot achieve beyond the square rate region {(R<inf>1</inf>, R<inf>2</inf>) ∶ R<inf>1</inf>, R<inf>2</inf> ≥ 0, R<inf>1</inf> ≤ 1, R<inf>2</inf> ≤ 1{. Analogous to the work by Wang and Shroff for wired networks [1], we provide the necessary and sufficient conditions for the capacity region to be the triangular region and the necessary and sufficient conditions for it to be the square region. Moreover, we completely characterize the capacity region and conclude that there are exactly three more possible capacity regions of this class of networks, in contrast to the result in wired networks where only two rate regions are possible. Our achievability scheme is based on linear coding over an extension field with at most four nodes performing special linear coding operations, namely interference neutralization and zero forcing, while all other nodes perform random linear coding.

Optimizing Quantize-Map-and-Forward relaying for Gaussian diamond networks

We evaluate the information-theoretic achievable rates of Quantize-Map-and-Forward (QMF) relaying schemes over Gaussian N-relay diamond networks. Focusing on vector Gaussian quantization at the relays, our goal is to understand how close to the cutset upper bound these schemes can achieve in the context of diamond networks, and how much benefit is obtained by optimizing the quantizer distortions at the relays. First, with noise-level quantization, we point out that the worst-case gap from the cutset upper bound is (N + log2 N) bits/s/Hz. A better universal quantization level found without using channel state information (CSI) leads to a sharpened gap of log2 N + log2(1 + N) + N log2(1 + 1/N) bits/s/Hz. On the other hand, it turns out that finding the optimal distortion levels depending on the channel gains is a non-trivial problem in the general N-relay setup. We manage to solve the two-relay problem and the symmetric N-relay problem analytically, and show the improvement via numerical evaluations both in static as well as slow-fading channels.

Coding and system design for quantize-map-and-forward relaying

In this paper we develop a low-complexity coding scheme and system design framework for the half duplex relay channel based on the Quantize-Map-and-Forward (QMF) relaying scheme. The proposed framework allows linear complexity operations at all network terminals. We propose the use of binary LDPC codes for encoding at the source and LDGM codes for mapping at the relay. We express joint decoding at the destination as a belief propagation algorithm over a factor graph. This graph has the LDPC and LDGM codes as subgraphs connected via probabilistic constraints that model the QMF relay operations. We show that this coding framework extends naturally to the high SNR regime using bit interleaved coded modulation (BICM). We develop density evolution analysis tools for this factor graph and demonstrate the design of practical codes for the half-duplex relay channel that perform within 1dB of information theoretic QMF threshold.

Quantize-map-and-forward relaying: Coding and system design

Quantize-map-and-forward (QMF) is a relaying scheme that has been shown to achieve the capacity of Gaussian relay networks to within a constant gap. Under QMF the compression indices forwarded by relays are not decoded explicitly. Instead, the message is decoded jointly with the compression indices. In this work we present a practical coding and signaling framework that captures this aspect of QMF. We outline the framework for a simple Gaussian network with a single half-duplex relay. We propose a scheme where binary LDPC codes are used for encoding at the source and at the relay. For suitable choice of codes and quantizer we show that the joint decoding operation for QMF can be reduced to belief propagation over a Tanner graph. Simulation results are given to justify the proposed coding scheme. The proposed framework can be extended for use with high order signal constellations.

Interference mitigation through limited receiver cooperation: Symmetric case

Interference is a major issue that limits the performance in wireless networks, and cooperation among receivers can help mitigate interference by forming distributed MIMO systems. The rate at which receivers cooperate, however, is limited in most scenarios. How much interference can one bit of receiver cooperation mitigate? In this paper, we study the two-user Gaussian interference channel with conferencing decoders to answer this question in a simple setting. We characterize the fundamental gain from cooperation: at high SNR, when INR is below 50% of SNR in dB scale, one-bit cooperation per direction buys roughly one-bit gain per user until full receiver cooperation performance is reached, while when INR is between 67% and 200% of SNR in dB scale, one-bit cooperation per direction buys roughly half-bit gain per user. The conclusion is drawn based on the approximate characterization of the symmetric capacity in the symmetric set-up. We propose strategies achieving the symmetric capacity universally to within 3 bits. The strategy consists of two parts: (1) the transmission scheme, where superposition encoding with a simple power split is employed, and (2) the cooperative protocol, where quantize-binning is used for relaying.

Approximate Capacity of the Dirty Multiple-Access Channel With Partial State Information at the Encoders

In this paper, we consider the K -user Gaussian multiple-access channel with multiple independent additive white Gaussian interferences. Each interference is known to exactly one transmitter non-causally. The capacity region is characterized to within a bounded gap regardless of channel parameters. These results are based on a layered modulo-lattice scheme which realizes distributed interference cancellation.