Capacity Analysis for Gaussian and Discrete Memoryless Interference Networks

Interference is an important issue for wireless communication systems where multiple uncoordinated users try to access to a common medium. The problem is even more crucial for next-generation cellular networks where frequency reuse becomes ever more intense, leading to more closely placed co-channel cells. This thesis describes our attempt to understand the impact of interference on communication performance as well as optimal ways to handle interference. From the theoretical point of view, we examine how interference affects the fundamental performance limits, and provide insights on how interference should be treated for various channel models under different operating conditions. From the practical design point of view, we provide solutions to improve the system performance under unknown interference using multiple independent receptions of the same information. For the simple two-user Gaussian interference channel, we establish that the simple Frequency Division Multiplexing (FDM) technique suffices to provide the optimal sum- rate within the largest computable subregion of the general achievable rate region for a certain interference range. For the two-user discrete memoryless interference channels, we characterize different interference regimes as well as the corresponding capacity results. They include one- sided weak interference and mixed interference conditions. The sum-rate capacities are derived in both cases. The conditions, capacity expressions, as well as the capacity achieving schemes are analogous to those of the Gaussian channel model. The study also leads to new outer bounds that can be used to resolve the capacities of several new discrete memoryless interference channels. A three-user interference up-link transmission model is introduced. By examining how interference affects the behavior of the performance limits, we capture the differences and similarities between the traditional two-user channel model and the channel model with more than two users. If the interference is very strong, the capacity region is just a simple extension of the two-user case. For the strong interference case, a line segment on the boundary of the capacity region is attained. When there are links with weak interference, the performance limits behave very differently from that of the two-user case: there is no single case that is found of which treating interference as noise is optimal. In particular, for a subclass of Gaussian channels with mixed interference, a boundary point of the capacity region is determined. For the Gaussian channel with weak interference, sum capacities are obtained under various channel coefficients and power constraint conditions. The optimalities in all the cases are obtained by decoding part of the interference. Finally, we investigate a topic that has practical ramifications in real communication systems. We consider in particular a diversity reception system where independently copies of low density parity check (LDPC) coded signals are received. Relying only on non-coherent reception in a highly dynamic environment with unknown interference, soft-decision combining is achieved whose performance is shown to improve significantly over existing approaches that rely on hard decision combining

D11.2 Consolidated results on the performance limits of wireless communications

Deliverable D11.2 del projecte europeu NEWCOM#The report presents the Intermediate Results of N# JRAs on Performance Limits of Wireless Communications and highlights the fundamental issues that have been investigated by the WP1.1. The report illustrates the Joint Research Activities (JRAs) already identified during the first year of the project which are currently ongoing. For each activity there is a description, an illustration of the adherence and relevance with the identified fundamental open issues, a short presentation of the preliminary results, and a roadmap for the joint research work in the next year. Appendices for each JRA give technical details on the scientific activity in each JRA.Peer ReviewedPreprin

Performance Limits of Microwave and Dual Microwave/Millimeter Wave Band Networks

Traditionally, wireless networks communicate over the conventional microwave band (sub-6 GHz) as it supports reliable communication over a large geographic area. The ever increasing demand for bandwidth to support the rising number of consumers and services, however, is fast depleting the available microwave spectrum. As such, complementing the microwave spectrum with additional bandwidth from the millimeter-wave (mm-wave) band has been envisioned as a promising solution to this problem. Since transmissions in the mm-wave band are typically achieved with highly directional steerable antenna arrays to counter the severe path-loss in mm-wave frequencies, the resulting mm-wave links are typically rendered highly directional, which can often be modeled as directional point-to-point links. However, mm-wave transmissions are inherently unreliable compared to those in the microwave band. Hence, communicating simultaneously over both bands in an integrated mm-wave/microwave dual-band setup is emerging as a promising new technology. In this dual-band setting, high-rate data traffic can be carried by relatively unreliable high-bandwidth mm-wave links, while control signals and moderate-bandwidth traffic can be communicated over the relatively reliable microwave band. In this thesis, we first study two dual-band multi-user networks that model two important aspects of wireless communication: inter-user interference and relay-cooperation. The broad goal of this study is to characterize information-theoretical performance limits of such networks, which can then be used to obtain insights on the optimal encoding/decoding strategy, effective resource allocation schemes, etc. In the first part of this thesis, we study a two-transmitter two-receiver dual-band Gaussian interference channel (IC) operating over an integrated mm-wave/microwave dual-band. This channel models a setting where a pair of single-transmitter single-receiver links communicate simultaneously, and thus mutually interfere. Here, transmissions in the underlying microwave band are modeled as a two-user conventional Gaussian IC (GIC). In contrast, a transmitter in the mm-wave band is assumed to be capable of communicating to either the desired destination or the interfered destination via a point-to-point direct-link or a cross-link, respectively. The dual-band IC is first classified into 3 classes according to the interference level in the underlying microwave GIC, and then sufficient channel conditions are obtained under which the capacity region of the 3 classes are characterized. For cases in which the sufficient conditions do not necessarily hold, approximate capacity results are obtained that characterizes the capacity region to within 1/2 bit per channel use per user. The performance of the dual-band IC is likely to be impacted significantly by the point-to-point nature and large bandwidth of the mm-wave links, and specifically by whether the mm-wave spectrum is used as direct-links or cross-links. Transmitting in either the direct-links only or the cross-links only is not optimal for all channel conditions, and there exists a non-trivial trade-off between the two modes. To understand the impact of this trade-off on the performance of the dual-band IC, we study the power allocation scheme over the mm-wave direct and cross-links that maximizes the sum-rate of the channel. The resulting power allocation strategy is characterized in closed form, which possesses rich properties and reveals useful insights into the trade-offs in such networks. In the second part of this thesis, we study a fading Gaussian multiple-access relay channel (MARC) over an integrated mm-wave/microwave dual-band, where two sources communicate to a destination with the help of a relay. In the dual-band MARC, transmission in the underlying microwave band is modeled as a conventional Gaussian MARC. However, similar to that in the dual-band IC, a mm-wave transmitter in this channel is modeled as being able to communicate to either the destination or the relay by creating a direct-link or a relay-link, respectively. For dual-band MARC, we characterize an achievable region and a set of rate upper bounds, and then obtain sufficient channel conditions under which its capacity region is characterized. Similar to the dual-band IC, the performance of the dual-band MARC will likely be significantly affected by whether the mm-wave band is used as direct-links or relay-links, and a non-trivial trade-off between the two modes exists in this case as well. To understand this trade-off, we study the transmission power allocation scheme over the mm-wave direct and relay-links that maximizes the sum-rate of the dual-band MARC. The resulting power allocation scheme, characterized in closed form, is observed to have rich structural properties, which reveal insights into the trade-offs in relay cooperation in dual-band networks. While dual-band communication is a promising technology, currently the bulk of the connectivity is still supported by the microwave band. However, the problem of interference mitigation for conventional microwave bands is still open even for the basic case of a two-user IC. Motivated by this, in the third part of the thesis, we study the performance limits of the multiple-access interference channel (MAIC) which models the interference during cellular uplink over the conventional single band. Focusing on the weak interference case, which provides a more realistic model of the inter-cell interference, we characterize an achievable strategy and 3 novel upper bounds on the sum-rate in the partially symmetric case, thereby providing improved sum-rate upper and lower bounds in these cases