In Defense of Wireless Carrier Sense

Carrier sense is often used to regulate concurrency in wireless medium access control (MAC) protocols, balancing interference protection and spatial reuse. Carrier sense is known to be imperfect, and many improved techniques have been proposed. Is the search for a replacement justified? This paper presents a theoretical model for average case two-sender carrier sense based on radio propagation theory and Shannon capacity. Analysis using the model shows that carrier sense performance is surprisingly close to optimal for radios with adaptive bitrate. The model suggests that hidden and exposed terminals usually cause modest reductions in throughput rather than dramatic decreases. Finally, it is possible to choose a fixed sense threshold which performs well across a wide range of scenarios, in large part due to the role of the noise floor. Experimental results from an indoor 802.11 testbed support these claims

ZigZag Decoding: Combating Hidden Terminals in Wireless Networks

This paper presents ZigZag, an 802.11 receiver that combats hidden terminals. ZigZag exploits 802.11 retransmissions which, in the case of hidden terminals, cause successive collisions. Due to asynchrony, these collisions have different interference-free stretches at their start, which ZigZag uses to bootstrap its decoding. ZigZag makes no changes to the 802.11 MAC and introduces no overhead when there are no collisions. But, when senders collide, ZigZag attains the same throughput as if the colliding packets were a priori scheduled in separate time slots. We build a prototype of ZigZag in GNU Radio. In a testbed of 14 USRP nodes, ZigZag reduces the average packet loss rate at hidden terminals from 82.3% to about 0.7%

Cooperative communication in wireless networks: algorithms, protocols and systems

Current wireless network solutions are based on a link abstraction where a single co-channel transmitter transmits in any time duration. This model severely limits the performance that can be obtained from the network. Being inherently an extension of a wired network model, this model is also incapable of handling the unique challenges that arise in a wireless medium. The prevailing theme of this research is to explore wireless link abstractions that incorporate the broadcast and space-time varying nature of the wireless channel. Recently, a new paradigm for wireless networks which uses the idea of 'cooperative transmissions' (CT) has garnered significant attention. Unlike current approaches where a single transmitter transmits at a time in any channel, with CT, multiple transmitters transmit concurrently after appropriately encoding their transmissions. While the physical layer mechanisms for CT have been well studied, the higher layer applicability of CT has been relatively unexplored. In this work, we show that when wireless links use CT, several network performance metrics such as aggregate throughput, security and spatial reuse can be improved significantly compared to the current state of the art. In this context, our first contribution is Aegis, a framework for securing wireless networks against eavesdropping which uses CT with intelligent scheduling and coding in Wireless Local Area networks. The second contribution is Symbiotic Coding, an approach to encode information such that successful reception is possible even upon collisions. The third contribution is Proteus, a routing protocol that improves aggregate throughput in multi-hop networks by leveraging CT to adapt the rate and range of links in a flow. Finally, we also explore the practical aspects of realizing CT using real systems.PhDCommittee Chair: Sivakumar, Raghupathy; Committee Member: Ammar, Mostafa; Committee Member: Ingram, Mary Ann; Committee Member: Jayant, Nikil; Committee Member: Riley, Georg

Embracing interference in wireless systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2013."February 2013." Cataloged from PDF version of thesis.Includes bibliographical references (p. 169-183).The wireless medium is a shared resource. If nearby devices transmit at the same time, their signals interfere, resulting in a collision. In traditional networks, collisions cause the loss of the transmitted information. For this reason, wireless networks have been designed with the assumption that interference is intrinsically harmful and must be avoided. This dissertation takes an alternate approach: Instead of viewing interference as an inherently counterproductive phenomenon that should to be avoided, we design practical systems that transform interference into a harmless, and even a beneficial phenomenon. To achieve this goal, we consider how wireless signals interact when they interfere, and use this understanding in our system designs. Specifically, when interference occurs, the signals get mixed on the wireless medium. By understanding the parameters of this mixing, we can invert the mixing and decode the interfered packets; thus, making interference harmless. Furthermore, we can control this mixing process to create strategic interference that allow decodability at a particular receiver of interest, but prevent decodability at unintended receivers and adversaries. Hence, we can transform interference into a beneficial phenomenon that provides security. Building on this approach, we make four main contributions: We present the first WiFi receiver that can successfully reconstruct the transmitted information in the presence of packet collisions. Next, we introduce a WiFi receiver design that can decode in the presence of high-power cross-technology interference from devices like baby monitors, cordless phones, microwave ovens, or even unknown technologies. We then show how we can harness interference to improve security. In particular, we develop the first system that secures an insecure medical implant without any modification to the implant itself. Finally, we present a solution that establishes secure connections between any two WiFi devices, without having users enter passwords or use pre-shared secret keys.by Shyamnath Gollakota.Ph.D