Multi-Node Open-Loop Distributed Beamforming Based on Scalable, High-Accuracy Ranging

Abstract

We present a distributed antenna array supporting open-loop distributed beamforming at 1.5 GHz. Based on a scalable, high-accuracy internode ranging technique, we demonstrate open-loop beamforming experiments using three transmitting nodes. To support distributed beamforming without feedback from the destination, the relative positions of the nodes in the distributed array must be known with accuracies below λ/15 of the beamforming carrier frequency to ensure that the array maintains at least 90% coherent beamforming gain at the receive location. For operations in the microwave range, this leads to range estimation accuracies of centimeters or lower. We present a scalable, high-accuracy waveform and new approaches to refine range measurements to significantly improve the estimation accuracy. Using this waveform with a three-node array, we demonstrate high-accuracy ranging simultaneously between multiple nodes, from which phase corrections on two secondary nodes are implemented to maintain beamforming with the primary node, thereby supporting open-loop distributed beamforming. Upon movement of the nodes, the range estimation is used to dynamically update the phase correction, maintaining beamforming as the nodes move. We show the first openloop distributed beamforming at 1.5 GHz with two-node and three-node arrays, demonstrating the ability to implement and maintain phase-based beamforming without feedback from the destination.

Distributed Transmit Beamforming: Analyzing the Maximum Communication Range

“…Existing works have proposed and demonstrated many distributed BF approaches. Location based BF towards a known direction in a LOS channel was demonstrated in [6]. DBF was demonstrated using explicit channel phase feedback in [3], [4], [7].…”

Online Expectation-Maximization Based Frequency and Phase Consensus in Distributed Phased Arrays

“…Modern advancements in wireless technologies have enabled more reliable communications between separate wireless systems, and as a result, several new applications involving distributed systems have emerged over recent years. These include distributed beamforming systems [1], distributed automotive radars [2], [3], and distributed massive MIMO systems [4], among others.…”

“…These closed-loop methods are thus only suitable for the wireless communication applications where such feedback from the target destination is feasible. Recently, open-loop synchronization methods have also been proposed [1], [10]- [12], in which the nodes communicate with each other to synchronize their oscillators without using any feedback from the destination. Since the destination need not be an active system, open-loop distributed phased arrays can also be used for radar and remote sensing applications.…”

“…In an open-loop distributed phased array, the electrical states (i.e., the frequencies and phases) of the nodes can be synchronized by either using a centralized approach or a decentralized approach. In [1], [12]- [14], a centralized open-loop distributed phased array was used in which nodes are classified as either a primary node or a secondary node. The primary node transmits a reference signal to the secondary ones which is used to synchronize their electrical states across the array.…”

“…The authors assumed a specific class of the nodes in which the frequencies are only incremented in discrete frequency steps, which limits the frequency resolution of the nodes and increases the residual phase error of the algorithm. In this paper, we consider a more advanced class of nodes, e.g., USRP based nodes as used in [1], for which such frequency quantization errors are negligible. Furthermore, unlike the work in [25], we consider both frequency and phase synchronization of the nodes in distributed phased array, both of which are necessary for distributed beamforming.…”