THz communication can realize the next order of magnitude in data rate and user densities due to the availability of wide THz-scale spectral bands. Wide bandwidth links can exhibit angular dispersion, i.e., frequency-dependent radiation direction. While angular dispersion has enabled path discovery and dynamic beam steering via frequency tuning, multi-user communication in THz links remains an unaddressed challenge. This paper presents the first study and performance evaluation of multi-user THz WLANs with angularly dispersive links. We employ a single parallel-plate Leaky-Wave Antenna (LWA) for THz directional transmission and present a multi-user communication strategy that exploits angular dispersion and angular separation of users and provides all-spectrum access to users located in different directions with the objective of aggregate rate maximization. With analytical model-driven evaluations and over-the-air experiments that inform our trace-driven emulation of multi-user conditions, we show how the multi-user performance of an angularly dispersive LWA link fundamentally depends on frequency, angle, and bandwidth utilized by users through non-linear mechanisms. As increasing bandwidth yields a larger signal footprint in LWA links, we demonstrate that as compared to the model prediction, not only is the aggregate data rate maximized with wider beams, but the experimental link is far better even for practical irregular beams with side lobes and asymmetry. Our experimental findings reveal the potential of leveraging angular dispersion and users’ angular separation to establish a scalable THz wireless link that offers contention-free or medium access control-free access. Our results demonstrate the feasibility of accommodating up to 11 simultaneous users, making it a promising candidate solution for densely populated user environments.

Wireless channels are vulnerable to eavesdroppers due to their broadcast nature. One approach to thwart an eavesdropper (Eve) is to decrease her SNR, e.g., by reducing the signal in her direction. Unfortunately, such methods are vulnerable to (1) a highly directional Eve that can increase her received signal strength and (2) Eve that is close to the receiver, Bob, or close to the transmitter, Alice. In this paper, we design and experimentally evaluate Multipath Multicarrier Misinformation to Adversaries (M3A), a system for Alice to send data to Bob while simultaneously sending misinformation to Eve. Our approach does not require knowledge of Eve's channel or location and, with multipath channels, randomly transforms Eve's symbols even if Eve is located one wavelength-scale distance from Bob (approximately 10 cm) or if Eve is located between Alice and Bob in their direct path (Eve is approximately 1/3 closer to Alice). In particular, our approach is to move each of Eve's received symbols (over time and across subcarriers), to an independently random transformation as compared to Bob, without Alice or Bob knowing Eve's location or channel. We realize this by modulating Alice's per-subcarrier beamforming weights with an i.i.d. random binary sequence, as if Alice had a separate antenna array for each subcarrier, and could randomly turn antennas in each array on and off. We implement M3A on a real-time Massive MIMO testbed and show that M3A can increase Eve's bit error rate more than two hundredfold compared to beamforming, even if she is positioned approximately a wavelength away, whether above, below, or beside Bob. Finally, to ensure reliability at Bob, we show that with M3A, Bob's bit error rate is approximately an order of magnitude lower than achieved with prior work.

Today’s mmWave WLANs can realize simultaneous multi-user multi-stream transmission solely on the downlink. In this paper, we present Uplink Multi-user Beamforming on single RF chain AP (UMBRA), a novel framework for supporting multi-stream multi-user uplink transmissions via a single RF chain. We design multi-user overlayed constellations and multi-user receiver mechanisms to enable concurrent time-triggered uplink multi-user transmissions received on a single RF chain AP. We devise exemplary beam selection policies to jointly adapt beams at users and the AP for targeting aggregate rate maximization without increasing training requirements compared to single-user systems. We implement the key components of UMBRA using a programmable WLAN testbed using software-defined radios and commercial 60-GHz transceivers and collect over-the-air measurements using phased-array antennas and horn antennas with varying beamwidth. We find that in comparison to single-user transmissions, UMBRA achieves more than$1.45 \times $improvement in aggregate rate regardless of the choice of the user group, geometric separation, receiver beamwidth, and also under LOS blockage.

THz communication can realize the next order of magnitude in data rate and user densities due to the availability of wide THz-scale spectral bands. Wide bandwidth links can exhibit angular dispersion, i.e., frequency-dependent radiation direction. While angular dispersion has enabled path discovery and dynamic beam steering via frequency tuning, multi-user communication in THz links remains an unaddressed challenge. This paper presents the first study and performance evaluation of multi-user THz WLANs with angularly dispersive links. We employ a single parallel-plate Leaky-Wave Antenna (LWA) for THz directional transmission and present a multi-user communication strategy that exploits angular dispersion and angular separation of users and provides all-spectrum access to users located in different directions with the objective of aggregate rate maximization. With analytical model-driven evaluations and over-the-air experiments, we show how the multi-user performance of an angularly dispersive LWA link fundamentally depends on frequency, angle, and bandwidth utilized by users, through non-linear mechanisms. As increasing bandwidth yields a larger signal footprint in LWA links, we demonstrate that as compared to the model prediction, not only is aggregate data rate maximized with wider beams, but that the experimental link is far better even for practical irregular beams with side lobes and asymmetry. Our experiments demonstrate that by exploiting angular dispersion and users' angular separation, we can transmit without contention or medium access control up to 11 simultaneous users.

Multi-user transmission in 60 GHz Wi-Fi can achieve data rates up to 100 Gbps by multiplexing multiple user data streams. However, a fundamental limit in the approach is that each RF chain is limited to supporting one stream or one user. In this paper, we scale multi-user 60 GHz WLAN data rate by overcoming this limit and propose SIngle RF chain Multi-user BeAmforming (SIMBA), a novel framework for multi-stream multi-user downlink transmission via a single RF chain. We build on single beamformed transmission via overlayed constellations to multiplex multiple users’ modulated symbols such that grouped users at different locations can share the same transmit beam from the AP. For this, we introduce user grouping and beam selection policies that span tradeoffs in data rate, training, and computation overhead. We implement a programmable WLAN testbed using software-defined radios and commercial 60 GHz transceivers and collect over-the-air measurements for different indoor WLAN deployments using a 12-element phased antenna array as well as horn antennas with varying beamwidth. We show that in comparison to single-user transmissions, SIMBA achieves$2\times $improvement in aggregate rate and two-fold delay reduction for simultaneous transmission to four users.

Today's mmWave WLANs can realize simultaneous multi-user multi-stream transmission solely on the downlink. In this paper, we present Uplink Multi-user Beamforming on single RF chain AP (UMBRA), a novel framework for supporting multi-stream multi-user uplink transmissions via a single RF chain. We design multi-user overlayed constellations and multi-user receiver mechanisms to enable concurrent time-triggered uplink multi-user transmissions received on a single RF chain AP. We devise exemplary beam selection policies to jointly adapt beams at users and the AP for targeting aggregate rate maximization without increasing training requirements compared to single-user systems. We implement the key components of UMBRA using a programmable WLAN testbed using software-defined radios and commercial 60-GHz transceivers and collect over-the-air measurements using phased-array antennas and horn antennas with varying beamwidth. We find that in comparison to single-user transmissions, UMBRA achieves more than 1.45× improvement in aggregate rate regardless of the choice of the user group, geometric separation, and receiver beamwidth.

Multi-user transmission in 60 GHz Wi-Fi can achieve data rates up to 100 Gbps by multiplexing multiple user data streams. However, a fundamental limit in the approach is that each RF chain is limited to supporting one stream or one user. To overcome this limit, we propose SIngle RF chain Multiuser BeAmforming (SIMBA), a novel framework for multi-stream multi-user downlink transmission via a single RF chain. We build on single beamformed transmission via overlayed constellations to multiplex multiple users' modulated symbols such that grouped users at different locations can share the same transmit beam from the AP. For this, we introduce user grouping and beam selection policies that span tradeoffs in data rate, training and computation overhead. We implement a programmable WLAN testbed using software-defined radios and commercial 60-GHz transceivers and collect over-the-air measurements using phased array antennas and horn antennas with varying beamwidth. We find that in comparison to single user transmissions, SIMBA achieves 2× improvement in aggregate rate and two-fold delay reduction for simultaneous transmission to four users.

We study terrestrial wireless uplink Time of Arrival (ToA) localization systems where multiple infrastructure nodes measure the ToA from the signal transmitted by a user of interest. The measured ToA is shared with a centralized server that then computes the location of the device. While the fundamental issues such as accurate synchronization and accurate location of nodes concerning precise location estimation have been addressed elsewhere, we mainly focus on improving the efficiency of ToA estimation in this work. Using our proposed methodology, a 30 fold improvement in the computational efficiency at no cost of localization accuracy was observed which in practice enables the localization network to track and coordinate many more simultaneous users frequently. We validated our ToA estimates on the over-the-air measurements taken from our on-campus localization network and obtained sub-meter accuracy, indicating that our solution is competitive with conventional high overhead ToA methods.