Practical Considerations in the Implementation of Collaborative Beamforming on Wireless Sensor Networks

Abstract

Wireless Sensor Networks (WSNs) are composed of spatially distributed autonomous sensor devices, named motes. These motes have their own power supply, processing unit, sensors and wireless communications However with many constraints, such as limited energy, bandwidth and computational capabilities. In these networks, at least one mote called a sink, acts as a gateway to connect with other networks. These sensor networks run monitoring applications and then the data gathered by these motes needs to be retrieved by the sink. When this sink is located in the far field, there have been many proposals in the literature based on Collaborative Beamforming (CB), also known as Distributed or Cooperative Beamforming, for these long range communications to reach the sink. In this paper, we conduct a thorough study of the related work and analyze the requirements to do CB. In order to implement these communications in real scenarios, we will consider if these requirements and the assumptions made are feasible from the point of view of commercial motes and their constraints. In addition, we will go a step further and will consider different alternatives, by relaxing these requirements, trying to find feasible assumptions to carry out these types of communications with commercial motes. This research considers the nonavailability of a central clock that synchronizes all motes in the WSN, and all motes have identical hardware. This is a feasibility study to do CB on WSN, using a simulated scenario with randomized delays obtained from experimental data from commercial motes.

Feasibility of a Stochastic Collaborative Beamforming for Long Range Communications in Wireless Sensor Networks

“…Although the CB-DCB proposals are old, little progress has been made in the last 14 years in terms of their practical application in a real WSN deployment [21]. The main problem is the synchronization and the introduction of time delays, the mote cannot introduce the necessary delays for the strict beamforming and electronic steering that an array can perform, and this is because its architecture is very simple and low cost [14]. It needs to be remembered that the hardware of the motes is little more than a microcontroller capable of communicating at short distances with the standard IEEE 802.15.4, [14].…”

“…The main problem is the synchronization and the introduction of time delays, the mote cannot introduce the necessary delays for the strict beamforming and electronic steering that an array can perform, and this is because its architecture is very simple and low cost [14]. It needs to be remembered that the hardware of the motes is little more than a microcontroller capable of communicating at short distances with the standard IEEE 802.15.4, [14]. The fundamental difference between CB or DCB in WSN and an array of antennas is that in an array there is only one oscillator that generates the carrier, and with WSN-CB-DCB independent oscillators are treated, one in each mote, and this imposes many technical limitations [14].…”

“…However, CB transmission has not ceased to be a chimera in the face of the challenges posed to the hardware with which the motes are constituted, as can be seen in [8][9][10][11][12][13], mainly due to inaccuracy of their clocks and their synchronization. We must not forget that they are very economical devices that have not been designed for this purpose, and we must devise the procedure that would allow us to obtain CB with the limitations imposed by its architecture [14].…”

“…In order to achieve the typical pointing of beam-forming applications, delays of the order of a fraction of the wavelength or very small delay times (a small fraction of nanoseconds) are necessary. These small delays are impossible to achieve with the motes currently available on the market [14]. A fixed pointing would be possible if the clocks of the motes could be synchronized, with zero offset for all the motes; the fixed pointing would be achieved with the spatial delays associated with the special separation of the motes.…”

“…This would require a meticulous spatial deployment by adequately controlling the special position of each mote. Even so, it would be difficult due to the errors of synchronization of the motes mentioned above [14] as well as the lack of the perfect location.…”