Physical Layer Network Coding: A Cautionary Story with Interference and Spatial Reservation

Physical layer network coding (PLNC) has the potential to improve throughput of multi-hop networks. However, most of the works are focused on the simple, three-node model with two-way relaying, not taking into account the fact that there can be other neighboring nodes that can cause/receive interference. The way to deal with this problem in distributed wireless networks is usage of MAC-layer mechanisms that make a spatial reservation of the shared wireless medium, similar to the well-known RTS/CTS in IEEE 802.11 wireless networks. In this paper, we investigate two-way relaying in presence of interfering nodes and usage of spatial reservation mechanisms. Specifically, we introduce a reserved area in order to protect the nodes involved in two-way relaying from the interference caused by neighboring nodes. We analytically derive the end-to-end rate achieved by PLNC considering the impact of interference and reserved area. A relevant performance measure is data rate per unit area, in order to reflect the fact that any spatial reservation blocks another data exchange in the reserved area. The numerical results carry a cautionary message that the gains brought by PLNC over one-way relaying may be vanishing when the two-way relaying is considered in a broader context of a larger wireless network.Comment: 6 pages, 11 figures, Proc. of IEEE CoCoNet Workshop in conjunction with IEEE ICC 201

Multiuser Cognitive Radio Networks: An Information Theoretic Perspective

Achievable rate regions and outer bounds are derived for three-user interference channels where the transmitters cooperate in a unidirectional manner via a noncausal message-sharing mechanism. The three-user channel facilitates different ways of message-sharing between the primary and secondary (or cognitive) transmitters. Three natural extensions of unidirectional message-sharing from two users to three users are introduced: (i) Cumulative message sharing; (ii) primary-only message sharing; and (iii) cognitive-only message sharing. To emphasize the notion of interference management, channels are classified based on different rate-splitting strategies at the transmitters. Standard techniques, superposition coding and Gel'fand-Pinsker's binning principle, are employed to derive an achievable rate region for each of the cognitive interference channels. Simulation results for the Gaussian channel case are presented; they enable visual comparison of the achievable rate regions for different message-sharing schemes along with the outer bounds. These results also provide useful insights into the effect of rate-splitting at the transmitters, which aids in better interference management at the receivers.Comment: 50 pages, 15 figures, submitted to IEEE Transactions on Information Theor

Achievable Rates and Scaling Laws for Cognitive Radio Channels

Cognitive radios have the potential to vastly improve communication over wireless channels. We outline recent information theoretic results on the limits of primary and cognitive user communication in single and multiple cognitive user scenarios. We first examine the achievable rate and capacity regions of single user cognitive channels. Results indicate that at medium SNR (0-20dB), the use of cognition improves rates significantly compared to the currently suggested spectral gap-filling methods of secondary spectrum access. We then study another information theoretic measure, the multiplexing gain. This measure captures the number of point-to-point Gaussian channels contained in a cognitive channel as the SNR tends to infinity. Next, we consider a cognitive network with a single primary user and multiple cognitive users. We show that with single-hop transmission, the sum capacity of the cognitive users scales linearly with the number of users. We further introduce and analyze the primary exclusive radius, inside of which primary receivers are guaranteed a desired outage performance. These results provide guidelines when designing a network with secondary spectrum users

Achievable Rates and Scaling Laws for Cognitive Radio Channels

<p>Abstract</p> <p>Cognitive radios have the potential to vastly improve communication over wireless channels. We outline recent information theoretic results on the limits of primary and cognitive user communication in single and multiple cognitive user scenarios. We first examine the achievable rate and capacity regions of single user <it>cognitive channels</it>. Results indicate that at medium SNR (0&#8211;20&#8201;dB), the use of <it>cognition</it> improves rates significantly compared to the currently suggested spectral gap-filling methods of secondary spectrum access. We then study another information theoretic measure, the multiplexing gain. This measure captures the number of point-to-point Gaussian channels contained in a cognitive channel as the SNR tends to infinity. Next, we consider a cognitive network with a single primary user and multiple cognitive users. We show that with single-hop transmission, the sum capacity of the cognitive users <it>scales linearly</it> with the number of users. We further introduce and analyze the <it>primary exclusive radius</it>, inside of which primary receivers are guaranteed a desired outage performance. These results provide guidelines when designing a network with secondary spectrum users.</p