Capacity of All Nine Models of Channel Output Feedback for the Two-user Interference Channel

In this paper, we study the impact of different channel output feedback architectures on the capacity of the two-user interference channel. For a two-user interference channel, a feedback link can exist between receivers and transmitters in 9 canonical architectures (see Fig. 2), ranging from only one feedback link to four feedback links. We derive the exact capacity region for the symmetric deterministic interference channel and the constant-gap capacity region for the symmetric Gaussian interference channel for all of the 9 architectures. We show that for a linear deterministic symmetric interference channel, in the weak interference regime, all models of feedback, except the one, which has only one of the receivers feeding back to its own transmitter, have the identical capacity region. When only one of the receivers feeds back to its own transmitter, the capacity region is a strict subset of the capacity region of the rest of the feedback models in the weak interference regime. However, the sum-capacity of all feedback models is identical in the weak interference regime. Moreover, in the strong interference regime all models of feedback with at least one of the receivers feeding back to its own transmitter have the identical sum-capacity. For the Gaussian interference channel, the results of the linear deterministic model follow, where capacity is replaced with approximate capacity.Comment: submitted to IEEE Transactions on Information Theory, results improved by deriving capacity region of all 9 canonical feedback models in two-user interference channe

Wireless Full-Duplex: From Practice to Theory

Full-duplex is the ability of a node to transmit and receive simultaneously in the same band. Ideal wireless full-duplex communication can double the spectral efficiency compared to the traditional half-duplex communication. In this dissertation, we study the challenges in realizing full-duplex communication. We tackle the challenges from two different perspectives: node and network. Node perspective: Simultaneous transmission and reception results in a large self- interference due to the proximity of transmit and receive antennas at the full-duplex node. To establish the feasibility of wireless full-duplex, we develop a wideband real-time physical layer and evaluate its performance on the WARP testbed. Self-interference reduction in our proposed physical layer is achieved through passive suppression and active cancellation. Based on the constraints of the physical layer, we propose a MAC layer protocol which is designed specifically to discover and enhance opportunities to communicate in the full-duplex mode. Experimental evaluation of our physical layer design, as well as several other full-duplex designs proposed in literature, reveal that active cancellation does not push self-interference all the way upto the thermal noise floor. In this dissertation, we explore the bottlenecks limiting active cancellation in full-duplex systems. We show that the amount of active cancellation is limited by transmitter side noise, particularly by the phase-noise in the local oscillator at the transmitter of the full- duplex node. Thus, unlike conventional half-duplex systems where receiver thermal noise is a limiting factor, full-duplex systems are limited by transmitter side noise. As a key by-product of our analysis, we propose a signal model for a wideband MIMO full-duplex system. We use our proposed signal model to study the performance limits of a system where the start of transmission and the start of reception at a full-duplex node are not synchronized. Interestingly, we discover that the bit-error-rate of the communication mode where the start of transmission precedes the start of reception is better than the mode where start of transmission follows reception of a packet at the full-duplex node. Network perspective: In order to extract gains in capacity from full-duplex operation in a multi-user network, we propose to use full-duplex capable nodes to si- multaneously operate uplink and downlink in a network. Such operation results in a new type of interference in the network – internode interference, i.e., the uplink transmission from each mobile user starts interfering with the downlink receptions at all the other mobile users. We show a physical layer coding strategy that aligns interference over time and extracts gains in degrees-of-freedom of the network. Finally, we recognize that larger gains from full-duplex operation are possible by leveraging the fact that the strength of the internode interference channel is often different from the uplink/downlink channel. By analysing the uplink/downlink capacity of a network composed of one base-station and two mobile users, we show that full-duplex not only out-performs half-duplex, but also recovers some of the degrees-of-freedom lost due to lack/delay of channel state information of the network