The last two decades have witnessed the prosperity of various wireless technologies such as RFID (radio frequency identification), Wi-Fi, ZigBee, and Bluetooth. As the key solutions to provide the last-mile connectivity for the Internet of Things (IoT), they are adopted to form complementary wireless systems in terms of power consumption, data rate, and so forth. Unfortunately, coexistence among wireless devices inevitably incurs interference which is a ticking time bomb for this type of “IoTification.” Typically, wireless devices compete for access to the same frequency spectrum. When relatively few devices are operating in an environment, everything works fine. As more devices are added, interference problems begin to crop up, leading to reduced throughput. This necessitates interference harmonization guidelines to achieve better coexistence among such devices.
In this thesis, we propose several novel solutions to solve interference problems for building better wireless coexistence systems. Particularly, we focus on RFID, RF-powered sensor networks (RPSNs), Wi-Fi, ZigBee, and Bluetooth, which are the representative enabling technologies in the IoT era. Regarding RFID and RPSNs, we are concerned about the single-technology interference issues arising when numerous RFID tags and RFpowered sensor devices coexist in an RFID system and an RPSN, respectively. Operating in the same 2.4 GHz unlicensed band, Wi-Fi, ZigBee, and Bluetooth are studied from the perspective of cross-technology interference when they coexist in the same environment.
Firstly, we propose ProTaR, a Probabilistic Tag Retardation-based protocol to address the RFID tag collision/interference issue in the midst of missing RFID tag identification in a large-scale RFID system. ProTaR leverages a mask to distill partial bits from the 96-bit ID of each tag for the characterization of tag uniqueness. In addition, a bit vector is constructed by an RFID reader to inform each tag of the transmissions of others. This idea successfully eliminates the tag collisions and hence makes full utilization of tag responses. Experimental results show that ProTaR achieves 100% identification accuracy regardless of missing tag ratio. Furthermore, extensive simulations present that ProTaR enables a time-efficiency improvement of up to 88% in comparison with existing solutions.
Secondly, we present a subtle MAC (Medium Access Control)-layer design in an RPSN, called FarMac, which aims at interference-free data collection from numerous sensor devices that can be recharged via the emerging wireless power transfer (WPT) technique. FarMac leverages a centralized algorithm to achieve multi source WPT for maximizing the transferred power to a lethargic sensor device that needs energy replenishment before conveying its data. In addition, each lethargic sensor device executes a distributed algorithm to compute its necessary energy harvesting time. Furthermore, FarMac achieves concurrent WPT and data collection via an interference cancellation technique. Simulation results show that FarMac guarantees network resilience and improves network throughput by up to 41% in comparison with a benchmark approach.
Thirdly, we take aim to design a novel Wi-Fi interference-resilient ZigBee decoder, called PolarScout, when low-power ZigBee networks coexist with Wi-Fi. Unlike several existing solutions that need clear signal preamble, tremendous signal strength difference between ZigBee and Wi-Fi, and Wi-Fi interference recognition in prior to ZigBee decoding, PolarScout aims at direct ZigBee decoding in a more generic and challenging case where Wi-Fi interference may feature a wide range of power levels and arises within a ZigBee packet at an arbitrary position. At the heart of PolarScout lies a subtle shell-shaping technique that harnesses a customized pulse to smooth the shell of a group of successive signal samples. Experimental results validate the superiority of PolarScout and its resilience to a wide range of Wi-Fi interference types.
Fourthly, we present BuSAR, a novel approach to account for the coexistence/interference problem between Bluetooth piconets and dense Wi-Fi networks. BuSAR embodies the first work to aim at mitigating the cross technology interference from Bluetooth to highly-dense Wi-Fi networks in a distributed manner. At the heart of BuSAR lies a subtle technique called Bluetooth slot availability randomization, which exploits the redundancy of erroneous Bluetooth packets for Wi-Fi throughput boost. With BuSAR adopted, multiple Bluetooth piconets are guaranteed to operate independently and only a lightweight algorithm is needed to be implemented at each Bluetooth device. Both theoretical analysis and experimental results are provided to show the feasibility of BuSAR.