Showing papers by "Xinyu Zhang published in 2010"

Chorus: Collision Resolution for Efficient Wireless Broadcast

Abstract: Traditional wireless broadcast protocols rely heavily on the 802.11-based CSMA/CA model, which avoids interference and collision by conservatively scheduling transmissions. While CSMA/CA is amenable to multiple concurrent unicasts, it tends to degrade broadcast performance, especially when there are a large number of nodes and links are lossy. In this paper, we propose a new, drastically different protocol called Chorus that improves the efficiency and scalability of broadcast service with a MAC layer that allows packet collisions. Chorus is built upon the observation that packets carrying the same data can be effectively detected and decoded, even when they overlap in time and have comparable signal strength. It performs collision resolution using symbol-level iterative decoding, and then combines the resolved symbols to reconstruct the packet. This collision-tolerant mechanism significantly improves the transmission diversity and spatial reuse in wireless broadcast, providing an asymptotic broadcast delay that is proportional to the network radius. This advantage is exploited further by Chorus's MAC-layer cognitive sensing and scheduling scheme. We evaluate Chorus with symbol-level simulation, and validate its network-level performance via ns-2, in comparison with a typical CSMA/CA broadcast protocol.

DAC: Distributed Asynchronous Cooperation for Wireless Relay Networks

Abstract: Cooperative relay is a communication paradigm that aims to realize the capacity of multi-antenna arrays in a distributed manner. However, the symbol-level synchronization requirement among distributed relays limits its use in practice. We propose to circumvent this barrier with a cross-layer protocol called Distributed Asynchronous Cooperation (DAC). With DAC, multiple relays can schedule concurrent transmissions with packet-level (hence coarse) synchronization. The receiver then extracts multiple versions of each relayed packet via a collision-resolution algorithm, thus realizing the diversity gain of cooperative communication. We demonstrate the feasibility of DAC by prototyping and testing it on the GNURadio/USRP software radio platform. To explore its relevance at the network level, we introduce a DAC-based MAC, and a generic approach to integrate the DAC MAC/PHY layer into a typical routing algorithm. Considering the use of DAC for multiple network flows, we analyze the fundamental tradeoff between the improvement in diversity gain and the reduction in multiplexing opportunities. DAC is shown to improve the throughput and delay performance of lossy networks with medium-level link quality. Our analytical results are also confirmed by network-level simulation in ns-2.

Spatio-Temporal Fusion for Small-scale Primary Detection in Cognitive Radio Networks

Abstract: In cognitive radio networks (CRNs), detecting small-scale primary devices---such as wireless microphones (WMs)---is a challenging, but very important, problem that has not yet been addressed well. We identify the data-fusion range as a key factor that enables effective cooperative sensing for detection of small-scale primary devices. In particular, we derive a closed-form expression for the optimal data-fusion range that minimizes the average detection delay. We also observe that the sensing performance is sensitive to the accuracy in estimating the primary's location and transmit-power. Based on these observations, we propose an efficient sensing framework, called DeLOC, that iteratively performs location/transmit-power estimation and dynamic sensor selection for cooperative sensing. Our extensive simulation results in a realistic CRN environment show that DeLOC achieves near-optimal detection performance, while meeting the detection requirements specified in the IEEE 802.22 standard draft.