Tomas Uricar

Non-Uniform 2-Slot Constellations for Bidirectional Relaying in Fading Channels

In this paper we introduce the constellation alphabets suitable for bidirectional relaying in parametric wireless channels. Based on the analysis of hierarchical minimum distance, we present a simple design algorithm for the non-uniform 2-slot constellation alphabets. These novel constellation alphabets outperform traditional linear modulation schemes in Hierarchical-Decode-and-Forward and Denoise-and-Forward relaying strategies in fading channels without sacrificing the overall system throughput.

Design criteria for hierarchical exclusive code with parameter-invariant decision regions for wireless 2-way relay channel

The unavoidable parametrization of the wireless link represents a major problem of the network-coded modulation synthesis in a 2-way relay channel. Composite (hierarchical) codeword received at the relay is generally parametrized by the channel gain, forcing any processing on the relay to be dependent on channel parameters. In this paper, we introduce the codebook design criteria, which ensure that all permissible hierarchical codewords have decision regions invariant to the channel parameters (as seen by the relay). We utilize the criterion for parameter-invariant constellation space boundary to obtain the codebooks with channel parameter-invariant decision regions at the relay. Since the requirements on such codebooks are relatively strict, the construction of higher-order codebooks will require a slightly simplified design criteria. We will show that the construction algorithm based on these relaxed criteria provides a feasible way to the design of codebooks with arbitrary cardinality. The promising performance benefits of the example codebooks (compared to a classical linear modulation alphabets) will be exemplified on the minimum distance analysis.

Superposition coding for wireless Butterfly Network with partial Network Side-Information

Although the 2-source wireless Butterfly Network (BN) represents a natural extension of the well-known 2-Way Relay Channel (2-WRC) scenario, a direct implementation of conventional Wireless Network Coding (WNC)/Physical Layer Network Coding (PLNC) principles in this network is not straightforward. While the information necessary for decoding of the desired data from WNC-coded data at each destination is naturally available in 2-WRC, in the BN the destinations can obtain such information only by overhearing the wireless transmission from the unintended source. Since the amount of such received “Network” Side-Information (N-SI) would be limited by the quality of the corresponding source-destination link, the assumption of perfect side-information is no longer valid in BN, rising many novel research challenges for the design of WNC-based BN. In this paper we propose the Superposition Coding (SC) based scheme as a feasible strategy for relaying in BN. We will show that by a proper optimization of its parameters (rates & power allocation of underlying SC messages), SC is able to achieve the upper bound of two-way rate in BN for both the zero and perfect N-SI case and moreover, it represents a viable solution even for the case where only partial N-SI is available at both destinations.

Non-Uniform 2-Slot Constellations for Relaying in Butterfly Network with Imperfect Side Information

There are two key aspects affecting the performance of butterfly network with wireless network-coded relaying. The first one is the amount of Complementary Side Information (C-SI) at destinations and the second is given by a parametric Multiple-Access channel performance of source constellation alphabets. In this paper we show that theNon-uniform 2-slot (NuT) constellations, proposed originally for bi-directional relaying in the 2-Way Relay Channel (2-WRC), are suitable also for the butterfly network, combining the favorable parametric channel performance with the potential to increase the reliability of required C-SI (necessary for successful decoding at the final destination). Possessing both these qualities, the NuT alphabets surpass again the conventional linear modulation schemes, extending further the scope of their potential application.

Wireless (Physical Layer) Network Coding With Limited Hierarchical Side Information: Maximal Sum-Rates in 5-Node Butterfly Network

Favorable characteristics of wireless channels (including the inherent broadcast and superposition nature) provide a fertile ground for the extension of conventional network coding (NC) principles to wireless communication networks. However, the research of emerging wireless (physical layer) network coding (WNC) techniques have already revealed several non-trivial research problems that do not appear in the conventional (wireline) NC systems, including the sensitivity to channel parametrization and challenging multi-source transmission synchronization. In this paper, we uncover another significant research challenge typical for multi-node WNC systems. We show that the performance of contemporary WNC bi-directional relaying strategies is dominated by the availability of a specific hierarchical side information (HSI), required for the successful decoding of desired information from hierarchical (WNC-coded) data streams. We analyze the impact of unreliable transmission of HSI on the performance of a wireless butterfly network (WBN), and we show that all state-of-the-art relaying strategies must be appropriately modified to avoid the deterioration of WBN performance in the limited HSI regime.

Relaying in Butterfly Networks: Superposition constellation design for Wireless Network Coding

Wireless Physical Layer Network Coding (WNC) techniques exploit the inherent nature of wireless channels to improve the performance of wireless communication systems. Unlike conventional Network Coding, in WNC the transmitted signals constructively interact directly in the constellation space, thus inducing specific requirements on the source constellation design. Suitable multi-source constellations should enable direct decoding of WNC functions of user data (from the observed superimposed constellations at receiving nodes) and simultaneously they should allow delivery of partial information to nodes with worse channel conditions (exploiting the natural broadcast property of wireless channels). Source constellations possessing both the aforementioned attributes simultaneously can be desirable in a WNC-based system and thus proper constellation design can become a relatively challenging task. In this paper we focus on this problem and we introduce a systematic constellation design algorithm for a 5-node Wireless Butterfly Network (WBN) with WNC processing, where the basic principles of multi-source constellation design for WNC systems can be demonstrated. We show that the proposed constellations outperform the conventional approaches over the whole range of SNR conditions in the system.