While interest in Internet of Things (IoT) applications has surged in recent years, the broad diversity in their constraints, such as power consumption, channel bandwidth, link robustness, and packet latency, still challenges state-of-the-art technologies to enable efficient and ubiquitous wireless connectivity for IoT devices in many practical scenarios. In this thesis, we study three sets of primary constraints in developing IoT networks: energy efficiency, spectral efficiency, and physical-layer security.First, this thesis introduces EE-IoT, an energy-efficient wireless communication scheme for IoT networks. The key enabler of EE-IoT is an asymmetric physical-layer design that allows low-complex and single-carrier IoT devices to communicate with multi-carrier-based wireless local-area network (WLAN) access point (AP) at a very low sampling rate, leading to a significant reduction of IoT devices' hardware complexity and power consumption. This thesis further introduces a practical design, termed WiFi-IoT, to enable a transparent coexistence of IoT devices and legacy Wi-Fi devices. WiFi-IoT design allows a multi-antenna AP to simultaneously serve Wi-Fi and IoT devices.℗ Second, to improve the spectral efficiency of dense IoT networks, this thesis presents a practical uplink distributed multiple-input multiple-output (MIMO) scheme, termed UD-MIMO, for WLANs that enables concurrent data transmissions from multiple users to multiple APs in the absence of fine-grained inter-node synchronization. UD-MIMO leverages new co-channel interference management and data decoding techniques that allow WLANs' APs to decode concurrent data packets from asynchronous users. This thesis further introduces a first-of-its-kind multi-antenna long-range (LoRa) gateway, termed MaLoRaGW, that enables multi-user MIMO LoRa communications, marking a significant advancement in LoRa network capabilities. MaLoRaGW features a joint design for uplink packet detection and downlink beamforming, enabling it to concurrently serve multiple LoRa user devices in both uplink and downlink. The key component of MaLoRaGW is a baseband signal design for uplink packet collision recovery, accurate channel estimation, and implicit beamforming.Third, this thesis proposes two jamming-resilient receiver architectures to secure delay-constrained IoT networks against jamming attacks. The proposed schemes leverage multi-antenna technology and new signal detection methods to suppress jamming signals and decode desired signals. It first introduces JammingBird, a multi-antenna receiver design for vehicular ad hoc networks that can tolerate high-power and in-band constant jamming attacks. JammingBird uses two MIMO-based modules, a jamming-resistant synchronizer and a jamming suppressor module, in its physical-layer design. Jamming-resistant synchronizer employs a spatial projection filter that alleviates the impact of jamming signal and allows JammingBird to conduct packet timing and frequency synchronizations. The jamming suppressor module leverages the spatial degrees of freedom provided by multiple antennas to cancel the jamming signal and recover the signal of interest. This thesis further proposes a MIMO-based receiver design to secure ZigBee communications against constant jamming attacks. The enabler of the proposed scheme is a learning-based jamming mitigation method, which can mitigate unknown interference using an optimized neural network.This thesis provides details on the system implementation, experimental setup, and performance evaluation of the proposed schemes in real-world environments. It further delves into an in-depth analysis of the lessons learned and highlights the open issues in the design of efficient and secure wireless IoT communications and networks.
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