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Bringing multi-antenna gain to energy-constrained wireless devices

Leveraging the redundancy and parallelism from multiple RF chains, MIMO technology can easily scale wireless link capacity. However, the high power consumption and circuit-area cost prevents MIMO from being adopted by energy-constrained wireless devices. In this paper, we propose Halma, that can boost link capacity using multiple antennas but a single RF chain, thereby, consuming the same power as SISO. While modulating its normal data symbols, a Halma transmitter hops between multiple passive antennas on a per-symbol basis. The antenna hopping pattern implicitly carriers extra data, which the receiver can decode by extracting the index of the active antenna using its channel pattern as a signature. We design Halma by intercepting the antenna switching and channel estimation modules in modern wireless systems, including ZigBee and WiFi. Further, we design a model-driven antenna hopping protocol to balance a tradeoff between link quality and dissimilarity of channel signatures. Remarkably, by leveraging the inherent packet structure in ZigBee, Halma's link capacity can scale well with the number of antennas. Using the WARP software radio, we have implemented Halma along with a ZigBee- and WiFi-based PHY layer. Our experiments demonstrate that Halma can improve ZigBee's throughput and energy efficiency by multiple folds under realistic network settings. For WiFi, it consumes similar power as SISO, but boosts throughput across a wide range of link conditions and modulation levels.