# Low-Power Beam-Switching Technique for Power-Efficient Collaborative IoT Edge Devices
###### Abstract  
Collaborative beamforming (CB) enables uplink transmission in a wireless sensor network (WSN) composed of sensors (nodes) and far-away access points (APs). It can also be applied to the case where the sensors are equipped with beam-switching structures (BSSs). However, as the antenna arrays of the BSSs are randomly headed due to the irregular mounting surface, some sensors form beams that do not illuminate a desired AP and waste their limited energy. Therefore, to resolve this problem, it is required to switch the beams toward the desired AP. While an exhaustive search can provide the globally optimal combination, a greedy search (GS) is utilized to solve this optimization problem efficiently. Simulation and experimental results verify that under certain conditions the proposed algorithm can drive the sensors to switch their beams properly and increase the received signal-to-noise ratio (SNR) significantly with low computational complexity and energy consumption.  
IoT; low-power; collaborative beamforming; beam switching; greedy search +
Footnote †: 10 February 2021  
[PERSON] et al. proposed the concept of the CB to add the radio signal of sensors constructively in the desired direction, and they demonstrated its remarkable performance over uniformly distributed sensors [41]. Furthermore, [PERSON] et al. and [PERSON] et al. conducted research on Gaussian and arbitrary distribution, respectively [42; 43]. From the discussions of the above and other authors, it has been verified that the CB can form a directional beam, as shown in Figure 3, and is prominently helpful to achieve power improvement [44] in the direction of a desired AP (theoretically, proportional to the square of the number of sensors) [45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57]. Their work also show that the CB can increase the success probability of long-range direct transmission regardless of distribution of sensors.  
Figure 1: The scenario that sensors (colored dots) have (**a**) omnidirectional or (**b**) directional radiators.  
Figure 2: (**a**) The configuration of a beam-switching structure (BSS). (**b**) The beampatterns of sensors (colored dots) having BSSs.  
Although the CB was originally designed for distributed omnidirectional radiators [58], it can also be applied to our scenario due to its simplicity. For the CB, sensors should switch their beams toward the desired AP by exploiting the position information of sensors and an AP, extracted from a reference position system such as the Global Positioning System (GPS). However, due to the irregular surface on which sensors are mounted, it could not be guaranteed that all of the switched beams would illuminate the proper angular range. Accordingly, it is anticipated that a serious reduction of the received power at the AP, resulting from the absence of some sensors in the CB, will occur. To avoid this problem, there are two approaches: (1) each sensor performs the beam switching in turn, or (2) all of the sensors do it simultaneously in a random manner. However, considering the innate restrictions of wireless sensors such as low power capacity and time-variant channel condition, the first approach which switches the beams of the sensors simultaneously is utilized.  
In this paper, the received signal-to-noise ratio (SNR) at the desired AP is investigated to confirm the above speculation first. The algorithm to resolve the beam-switching problem with the goal of improving the SNR above a certain level, which is based on the GS requiring low computing capability, is also designed. Finally, through simulations and experiments, it is verified that the proposed algorithm can increase the SNR considerably, and is applicable in an embedded system having a Micro Controller Unit (MCU), which is widely used for wireless sensors.