The use of millimeter-wave and terahertz frequencies (30 GHz to 1 THz) for wireless links is rapidly emerging as one of the accepted paradigms for future (5G and beyond) wireless networks. Driven by increased spectrum availability, millimeter-wave (mmWave) and terahertz (THz) communications are envisioned as the key building block to realizing the next order of magnitude in data rate and user densities for next-generation wireless networks.

As a key challenge for maintaining directional links, mmWave bands have exploited large antenna arrays to overcome higher path loss and reflection loss and enhance link budget to provide a multi-Gbps data rate for a single point-to-point transmission. Unfortunately, RF chains become the limiting resource in mmWave bands due to numerous hardware and power constraints that make it challenging to have a separate RF chain and data converter for each antenna. Consequently, to date with Wi- Fi, a single RF chain has precluded Multi-User (MU) communication and supports only one stream/ one user at a time. Furthermore, scaling multi- user data rates at THz bands introduce new challenges beyond mmWave: The wider THz spectrum exhibits a unique property of angular dispersion, i.e., frequency-dependent radiation direction with different propagation characteristics, thus requiring different system and node architectures for multi-user communication.

By leveraging the unique capabilities of mmWave/THz wireless signals, namely, the ability to flexibly access a large swath of spectrum, sparse scattering, and the possibility of directionality in small form factors (i.e., large antenna arrays or high-frequency antenna structures), this thesis presents the design, implementation, and experimental evaluation of novel solutions to achieve scalable multi-user directional networking in mmWave to THz spectrum in unprecedented ways.

In this thesis, I design and experimentally demonstrate for the first time how to support MU downlink and uplink in mmWave WLANs using only a single RF chain. To realize this, I propose (i) mmWave WLAN architecture in which the number of supported simultaneous users and streams exceed the number of RF chains, thus removing the RF-chain limitation on MU scaling, (ii) Scalable Multi-User Constellations that enable single beamformed MU transmission and reception at the AP, and (iii) Novel User and Beam adaptation framework that targets aggregate rate maximization with beam training and computation overhead design tradeoffs.

Then, I design and implement the key components of the first multi- user THz WLAN system based on a novel node architecture built on leaky-wave antennas, a promising structure for THz networks. By exploit- ing electromagnetics of antennas to protocol design, signal processing, and end-to-end system design, I demonstrated a contention-free and scheduler- free THz-scale multi-user communication scheme that supports up to 11 simultaneous users in practice, achieving Tb/s aggregate data rates using just a single-element antenna link. Together, these above designs address the key challenges of directional networking and demonstrate scalable multi-user mmWave and THz systems with minimal device cost and power consumption with the potential to enable ultra-high-speed connectivity with millisecond latency and massive scalability, thus yielding a paradigm change in the design and development of next-generation multi-user wireless systems.