Research On Techniques And Applications Of Constructive Interference Based Synchronous Transmissions In Wireless Networks

Constructive interference based synchronous transmissions(CIST) is a nascent trend in wireless networks due to its ability to increase network concurrency, reduce transmission latency, and enhance synchronization accuracy. However, constructive interference(CI) has a precondition to function, i.e., the maximum temporal displacement Δ of concurrent packet transmissions should be less than a given hardware constrained threshold(e.g. 0.5μs, for the IEEE 802.15.4 radio). Under that precondition, the common receiver can decode that packet with high probability while traditionally, it is considered this situation will cause packet collisions. For applications such as network flooding, time synchronization, CIST can greatly alleviate the burden of coordination to determine which node should transfer the packet first. The coordination overhead is non-negligible, especially when the number of potential transmitters is large. That’s why CIST can realize millisecond network flooding latency. The major contributions of this paper are summarized as follows.1) We show the root cause of CI with waveform analysis and theoretical models. We propose the concept of deliberate synchronized constructive interference(Disco), which advances the technique of CI by providing power gains and PRR improvements compared with the original links. We derive the satisfaction condition with closed form formulas of CI and Disco under lossless and lossy links. Moreover, we propose Triggercast, a distributed middleware service, which can generate Disco on real wireless sensor network(WSN) platforms. To synchronize transmissions of multiple senders at the chip level, Triggercast effectively compensates propagation and radio processing delays, and has 95 th percentile synchronization errors of at most 250 ns. Triggercast also intelligently decides which co-senders to participate in synchronous transmissions, and aligns their transmission time to maximize the overall link PRR(Packet Reception Ratio), under the condition of maximal system robustness. Extensive experiments in real testbeds demonstrate that Triggercast significantly improves PRR from 5% to 70% with 7 concurrent senders. Triggercast can control network topology(e.g., increasing new communication links) without changing the original network state(adding new nodes, increasing nodes’ power, etc.), which is attractive for opportunistic routing.2) We theoretically disclose that Glossy suffers the scalability problem. The PRR of intermediate nodes degrades significantly as the density or the size of the network increases. As a special case, we analytically show that Glossy has a PRR lower bound(94.5%) while flooding on the grid topology. Based on this observation, we propose the SCIF(spine constructive interference based flooding) and EACIF(energy adaptive constructive interference based flooding) protocols. The main idea is firstly to construct a spine for a given topology, and then to flood the packet as Glossy on spine nodes and to control leaf nodes to keep silent without retransmissions. Extensive simulations show that the PRR and energy consumption performance of the proposed protocols outperforms that of Glossy. For uniform distributed topology, when the network size grows from 400 to 4000, the PRR of SCIF keeps stable above 96% while the PRR of Glossy is only 26% when the network size is 4000. For the real data traces of the CitySee project, sophisticated simulations show that EACIF reduces 64.0% energy consumption in terms of radio on time compared with Glossy.3) We also leverage CIST in the realm of time synchronization in WSNs, which is often considered time-consuming and error-prone due to random time-stamp delays for MAC layer access and unstable clocks of intermediate nodes. We propose a non-invasive time synchronization protocol DMH(direct multi-hop). As its name stands, the nodes directly use the timestamps of the sink, like virtually synchronizing with the sink. With the ideas of automatic packet switching, working slots prediction, and EACIF, DMH eliminates the invasiveness of Glossy in dimensions of time, space and computation, and reduces energy consumptions. Preliminary simulations show that, with the real data trace of CitySee, the energy consumption(in terms of radio on time) of DMH is 18.5% of Glossy. Moreover, theoretical analysis and testbed experiments show that the influence of instable clock jitters of intermediate nodes is negligible on the following nodes, and the global synchronization error of DMH grows linearly with the increase of hops while the state-of-the-art FTSP exhibits an error that grows exponentially.