Muhammad Mahboob Ur Rahman

A Scalable Architecture for Distributed Transmit Beamforming with Commodity Radios: Design and Proof of Concept

We describe a fully-wireless prototype of distributed transmit beamforming on a software-defined radio platform. Distributed beamforming is a cooperative transmission technique that can achieve orders of magnitude increases in range or energy efficiency of wireless communication systems. However, this technique requires precise synchronization of the radio frequency signal from each transmitter. The significance of our prototype is in demonstrating that this requirement can be satisfied using digital signal processing methods on commodity hardware with low-quality oscillators. Our synchronization approach scales to large numbers of transmitters: each transmitter runs independent algorithms based on periodically transmitted feedback packets from the receiver. A key simplification is the decoupling of the algorithms for frequency locking and beamsteering at each transmitter, even though both processes use the same feedback packets. Frequency locking employs an Extended Kalman filter to track the local oscillator offset between a transmitter and the receiver, using frequency offset measurements based on the feedback packet waveform, while the phase adjustments for beamsteering are determined using a one-bit feedback algorithm based on the feedback packet it payload. Our prototype demonstrates that distributed transmit beamforming can be incorporated into wireless networks without requiring hardware innovations, and provides open-source building blocks for future research and development.

Fully wireless implementation of distributed beamforming on a software-defined radio platform

We describe the key ideas behind our implementation of distributed beamforming on a GNU-radio based software-defined radio platform. Distributed beamforming is a cooperative transmission scheme whereby a number of nodes in a wireless network organize themselves into a virtual antenna array and focus their transmission in the direction of the intended receiver, potentially achieving orders of magnitude improvements in energy efficiency. This technique has been extensively studied over the past decade and its practical feasibility has been demonstrated in multiple experimental prototypes. Our contributions in the work reported in this paper are three-fold: (a) the first ever all-wireless implementation of distributed beamforming without any secondary wired channels for clock distribution or channel feedback, (b) a novel digital baseband approach to synchronization of high frequency RF signals that requires no hardware mod-ifications, and (c) an implementation of distributed beam-forming on a standard, open platform that allows easy reuse and extension. We describe the design of our system in de-tail present some initial results and discuss future directions for this work.

Distributed beamforming with software-defined radios: Frequency synchronization and digital feedback

We present an implementation of distributed transmit beamforming using software-defined radios. The transmit nodes synchronize in carrier frequency using a pilot signal sent by a master node, and employ feedback from the receiver to adjust their carrier phases so as to add up coherently at the receiver. Our implementation advances the state of the art for all-wireless distributed beamforming in two important ways. The first is the implementation of extended Kalman filters for frequency synchronization at the slave nodes, which is shown to be effective despite the high local oscillator (LO) offsets typical of software-defined radios, and the low duty cycle of the pilot transmitted by the master node. The second advance is the implementation of the well-known one bit feedback scheme for phase adjustment using digital feedback from the receiver. We present experimental results that show the efficacy of our implementation for frequency synchronization and beamforming.

Demonstrating distributed transmit beamforming with software-defined radios

We present a fully wireless implementation of distributed transmit beamforming using software-defined radios. Distributed beamforming is a cooperative transmission scheme whereby a number of nodes in a wireless network organize themselves into a virtual antenna array and focus their transmission in the direction of the intended receiver, potentially achieving order of magnitude improvements in energy efficiency. The main technical challenge in realizing these gains is in precisely synchronizing the radio frequency signals of the cooperating nodes. This idea has been studied extensively over the past decade, and several techniques and architectures for its practical implementation have been developed. In this work, we demonstrate our recent implementation of distributed beamforming on a standard, open-source, software-defined radio platform, where the low quality oscillators make synchronization particularly challenging. Our demonstration will consist of three cooperating transmitters sending signals that add up constructively at the receiver. Low-rate feedback packets broadcast from the receiver are employed for frequency and phase synchronization at each transmitter in completely distributed fashion.

Consensus Based Carrier Synchronization in a Two Node Network

Abstract Motivated by synchronous communications, we propose a consensus based carrier synchronization algorithm involving two transceiving units. Our algorithm achieves frequency lock globally and exponentially. Further it also achieves phase synchronization in the following sense. Asymptotically it induces the two transmitters to be either in phase, or out of phase by 180 degrees. We provide a simple method for the transmitters to know if they are out phase by 180 degrees. This constitutes a significant advance over existing carrier synchronization technology which is largely based on Phase Locked Loops that only achieve lock locally.

A Scalable Feedback Mechanism for Distributed Nullforming With Phase-Only Adaptation

This paper considers a problem of distributed nullforming, in which multiple wireless transmitters steer a null toward a designated receiver by only adjusting their carrier phases. Since each transmitter transmits at full power, the system maximizes “power pooling” gains for cooperative communication or jamming, while simultaneously protecting a designated receiver. Analysis in a noiseless setting shows that, while the received power at the designated receiver, as a function of the transmitted phases, is nonconvex with multiple critical points, all of its local minima are also global minima. This implies that a null can be formed using a distributed, scalable protocol based on gradient descent: each transmitter adapts its phase based only on aggregate feedback broadcast by the receiver (so that feedback overhead does not increase with the number of transmitters), along with an estimate of its own channel gain (which can be obtained, e.g., via reciprocity). Simulations show that the convergence rate actually improves with the number of transmitters, and that the algorithm is robust to noise, substantial channel estimation errors, and oscillator drift.

PHY layer authentication via drifting oscillators

PHY layer authentication of a wireless sender has gained much interest recently. In this paper, we consider the famous Alice, Bob and Eve model and investigate (for the first time) the feasibility of using time-varying clock offsets for sender-node-authentication at Bob. Specifically, we exploit the fact (and de-facto problem) that clock offset between every node pair is unique; moreover, the two clock offsets between any two node pairs drift independently and randomly over time. Therefore, an explicit mechanism is needed to track the time-varying clock offsets. To this end, we model oscillator drift as brownian motion frequency and phase drift, and present a novel framework which is based on interplay between a hypothesis testing device and a bank of two Kaiman filters; one KF (KFh0) tracks Alices clock while other KF (KFh1) tracks Eves clock. Building on aforementioned framework, we then propose a novel sender-node-authentication method (so-called MHF method) by means of which Bob can automatically accept (reject) a received packet if it is sent by Alice (Eve). Finally, simulation results are presented which corroborate the efficiency of the proposed method.

A distributed consensus approach to synchronization of RF signals

We propose a consensus-based algorithm for the synchronization of carrier signals in a wireless network. This work is motivated by recent progress on distributed beamforming and other cooperative MIMO techniques that require synchronized RF signals among all the cooperating nodes in a network and is aimed at addressing the limitations of the centralized master-slave approach used in previous work in this area. Our proposed algorithm is based on a variation of the classic Kuramoto model for the synchronization of coupled oscillators and is well-suited for a digital baseband implementation. We describe our proposed algorithm in detail and present initial results that show that this algorithm achieves global frequency lock given only that the network is connected i.e. there exists (possibly multi-hop) paths for every node to transmit and receive a signal from every other node.

A Scalable Feedback-Based Approach to Distributed Nullforming

We present a novel approach to the problem of distributed nullforming where a set of transmitters cooperatively transmit a common message signal in such a way that their individual transmissions precisely cancel each other at a designated receiver. Under our approach, each transmitter iteratively makes an adjustment to the phase of its transmitted RF signal, by effectively implementing a gradient descent algorithm to reduce the amplitude of the overall received signal to zero. We show that this gradient search can be implemented in a purely distributed fashion at each transmitter assuming only that each transmitter has an estimate of its own channel gain to the receiver. This is an important advantage of our approach and assures its scalability; in contrast any non-iterative approach to the nullforming problem requires centralized knowledge of the channel gain of every transmitter. We prove analytically that the gradient search algorithm converges to a null at the designated receiver. We also present numerical simulations to illustrate the robustness of this approach.

A Distributed Relay Beamforming-Enhanced TDMA System

Token-passing wireless network protocols (TPWNP) (e.g., EchoRing), designed for hard real-time systems, typically need to provide ultra-low-latency coupled with ultrahigh-reliability guarantees. In this paper, we initiate a study to investigate the feasibility of distributed relay beamforming (DRBF) in a TP-WNP with the aim to enhance its reliability (and latency) performance even further. Specifically, we consider employing N i) amplify-and-forward (AF), ii) decode-and-forward (DF) relays in a wireless network running a TP-WNP. The relays operate in FDD mode, and do distributed transmit beamforming to realize low-latency, highly-reliable communication between each of the M source-destination pairs in the TP-WNP (a.k.a TDMA) system. The enablers/pre-requisites for the proposed DRBF-TDMA system are frequency, phase and timing synchronization among the relay nodes. To this end, we propose a novel distributed method for frequency synchronization among the AF/DF relay nodes operating in FDD mode. Furthermore, for oscillators with drift, we derive a rule of thumb which provides us the maximum relaying delay Tdelay up to which the proposed frequency synchronization method is effective. For phase and timing synchronization, we employ standard techniques from the literature. Our simulation results verify the analytical results, i.e., by means of proposed DRBF using N AF (DF) relays, upto a factor of N (N2) gains in received SNR can be achieved, at each of the M destination nodes in the proposed system.