Side Channel: Bits over Interference
ABSTRACT Interference is a critical issue in wireless communications. In a typical multiple-user environment, different users may severely interfere with each other. Coordination among users therefore is an indispensable part for interference management in wireless networks. It is known that coordination among multiple nodes is a costly operation taking a significant amount of valuable communication resource. In this paper, we have an interesting observation that by generating intended patterns, some simultaneous transmissions, i.e., "interference,” can be successfully decoded without degrading the effective throughput in original transmission. As such, an extra and "free” coordination channel can be built. Based on this idea, we propose a DC-MAC to leverage this "free” channel for efficient medium access in a multiple-user wireless network. We theoretically analyze the capacity of this channel under different environments with various modulation schemes. USRP2-based implementation experiments show that compared with the widely adopted CSMA, DC-MAC can improve the channel utilization efficiency by up to 250 percent.
Conference Paper: TiM: Fine-Grained Rate Adaptation in WLANs[Show abstract] [Hide abstract]
ABSTRACT: Channel condition varies frequently in wireless networks. To achieve good performance, devices need rate adaptation. In rate adaptation, choosing proper modulation schemes based on channel conditions is vital to the transmission performance. However, due to the natural character of discrete modulation types and continuous varied link conditions, we cannot make a one-to-one mapping from modulation schemes to channel conditions. This matching gap causes either over-select or under-select modulation schemes which limits throughput performance. To fill-in the gap, we propose TiM (Time-line Modulation), a novel 3-Dimensional modulation scheme by adding time dimension into current amplitude-phase domain schemes. With estimation of channel condition, TiM changes base-band data transmission time by artificially interpolating values between original data points without changing amplitude-phase domain modulation type. We implemented TiM on USRP2 and conducted comprehensive simulations. Results show that, compared with rate adaptation choosing from traditional modulation schemes, TiM can improve channel utilization up to 200%.2014 IEEE 34th International Conference on Distributed Computing Systems (ICDCS); 06/2014
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ABSTRACT: Packet corruption caused by collision is a critical problem that hurts the performance of wireless networks. Conventional medium access control (MAC) protocols resort to collision avoidance to maintain acceptable efficiency of channel utilization. According to our investigation and observation, however, collision avoidance comes at the cost of miscellaneous overhead, which oppositely hurts channel utilization, not to mention the poor resiliency and performance of those protocols in face of dense networks or intensive traffic. Discovering the ability to tolerate collisions at the physical layer implementations of wireless networks, we in this paper propose Coco, a MAC protocol that advocates simultaneous accesses from multiple senders to a shared channel, i.e., optimistically allowing collisions instead of simply avoiding them. With a simple but effective design, Coco addresses the key challenges in achieving collision tolerance, such as precise sender alignment and fine control of the transmission concurrency. We implement Coco in 802.15.4 networks and evaluate its performance through extensive experiments with 21 TelosB nodes. The results demonstrate that Coco is light-weight and enhances channel utilization by at least 20% in general cases, compared with state-of-the-arts protocols.2013 21st IEEE International Conference on Network Protocols (ICNP); 10/2013
Article: Managing Contention with Medley[Show abstract] [Hide abstract]
ABSTRACT: As WLANs achieve gigabit per second speeds, they will need to support users with a wide range of workloads, ranging from VoIP and Web clients to data backup, file transfers, and streaming high-definition video. Unfortunately, channel efficiency degrades severely in these scenarios under existing MAC protocols due to contention and back-off overheads. Moreover, small yet latency-sensitive flows suffer disproportionally as load increases. We present Medley, a system that leverages frequency-based contention to allocate subchannels in an OFDMA-based link layer in a delay-fair manner. In contrast to traditional CSMA schemes in which each node competes uniformly for the channel, Medley ensures that nodes with smaller service rates are served before those with heavier demand; the more bandwidth a node consumes, the larger its packet average delay will become. An initial implementation of Medley on a software defined radio platform demonstrates its feasibility in a small network, while more comprehensive simulation results show its benefits under a wider range of conditions. Medley delivers delay fairness while remaining over 94 percent efficient in the face of massive over-subscription.IEEE Transactions on Mobile Computing 03/2015; 14(3):579-591. DOI:10.1109/TMC.2014.2315822 · 2.91 Impact Factor