Combining RF energy harvesting and cooperative communications for low-power wide-area systems

Abstract

This paper presents the combination of energy harvesting (EH) at the wireless sensor and cooperative communications for low-power wide-area (LPWA) systems. Firstly, the Internet of Things sensor harvests the energy from the power beacon with multiple transmit antennas via radio frequency signals and then uses the harvested energy to transmit signals to multiple gateways with multiple receive antennas. Then, the cooperative communications is applied at the server based on the gateway outputs. By mathematical analysis, we derive the exact closed-form expressions of outage probabilities (OPs), throughput, and symbol error probabilities (SEPs) of the EH-LPWA system over Nakagami-$m$ fading channel in the cases without and with cooperative communications. Our expressions can be considered as the first results applying EH for LPWA systems with mathematical analysis. Then, we obtain the optimal value of the time switching ratio that minimizes the OPs and SEPs and maximizes the throughput of the considered EH-LPWA system. Numerical results have clarified that, the distances, path loss exponent, and data transmission rate have a strong impact on the OPs, throughput, and SEPs. Particularly, using a half of transmission blocks for EH can maximize the system performance. Moreover, when the number of transmit antennas at power beacon is equal to the number of receive antennas at gateways, the system performance can be improved significantly. Finally, the accuracy of the obtained expressions is demonstrated via Monte-Carlo simulations.

Introduction

Recently, energy harvesting (EH) from radio frequency (RF) signals has been widely used to satisfy the energy requirements of wireless communication systems. Hence, it becomes a promising forthcoming technique to be deployed in the fifth generation (5G) and beyond networks [1], [2]. Specifically, EH can supply enough power for sensor in Internet of things (IoT) systems, heterogeneous networks (HetNets), mobile devices, and extremely remote area communications. In addition, wireless devices can transmit the RF signals over the air for a long range, thus, EH from RF signals can be applied for many devices that are located in the restricted areas, where the traditional energy grid is extremely difficult to be deployed. For example, RF EH power supply is very promising for IoT based smart environment monitoring systems with energy efficient Beat sensors and LoRaWAN communications [3]. Consequently, the researches and experiments about EH are developed quickly so that we could apply this technique for the current and future wireless systems soon [4], [5], [6].

In the literature, the linear and non-linear energy harvesters have been proposed to apply the EH technique for wireless devices [1], [7]. In particular, the wireless devices can harvest the energy from base stations [8], [9], [10] or power beacons [11]. The mathematical analysis was used to derive the expressions in terms of outage probability (OP), throughput, and symbol error probability (SEP) of the EH communication systems [8], [11]. It was shown that for a certain EH system, there is an optimal value of the time switching ratio which can minimize the system OP/SEP. Also, using multiple antennas for the power beacon can greatly improve the performance of EH systems because of a significant increase about the amount of harvested energy. In addition, the non-linear characteristics of energy harvesters cause the power ceiling for the harvested energy leading to a error floor of OP/SEP in EH system [12]. Furthermore, EH technique is combined with various new techniques such as full-duplex (FD), cognitive radio (CR), spatial modulation (SM), and non-orthogonal multiple access (NOMA) for enhancements in both energy and spectral efficiencies [8], [11], [12], [13], [14]. In addition to the mathematical analysis, the experimental measurements were also used to investigate the amount of harvested energy and the performance of EH communication systems in practice [7], [15], [16].

On the other hand, the current wireless communication systems such as Long-term Evolution (LTE) and WiFi are usually designed for short-range networks so that these systems can achieve reliable communications and high speed data transmissions [17]. However, the disadvantages of these systems are the high energy consumption and the high deployment cost. Meanwhile, other systems such as ZigBee and Bluetooth can consume the low power but these systems also operate for a short-range with the low speed [18], [19]. Consequently, these systems are not suitable for LoRaWAN communications because of the low power consumption requirement of LoRaWAN systems. In this context, low-power wide-area (LPWA) network technologies have been emerged as the promising connectivity solutions for IoT devices in recent years due to their advantages [17], [20]. Particularly, LPWA technologies can reduce the power consumption with the low delay sensitivity and wide coverage. Therefore, LPWA systems can solve various issues in the current wireless communication networks such as battery life, deployment cost, and coverage [17], [18], [21], [22]. The recent reports observed that, LPWA systems are highly promising for IoT requirements. Specifically, the LoRaWAN is a physical layer technology developed by Semtech, where an adaptive data rate chirp modulation technology is used to allow the flexible long-range communications with the low design cost and low power consumption [18], [21], [23]. Consequently, the LPWA system range depends on the transmission power, coding scheme, and data rate [23]. To provide the robustness performance of the LPWA systems, a type of the chirp spread spectrum with the integrated forward error correction is often used. Also, a special chirp spread spectrum technique can be used in a bidirectional communications [18].

Beside the applications of LPWA systems for IoT devices, the performance of LPWA systems has been also investigated in the literature. In [22], a disruptive approach was proposed to increase the number of users used in LoRaWAN systems. By applying time-power multiplexing, the network capacity was significantly improved because the gateways can transmit more-than-one packets at the same time. Similarly [22], the works in [21] considered LoRaWAN networks with an increase of IoT devices. The uplink OP was investigated over Poisson distributed channels when the interferences between IoT devices were taken into account. It was shown that the OP was greatly impacted by the interferences, distances, and the number of IoT devices. In [24], a theoretical analysis is conducted to investigate the feasibility of the transmission of a LoRa wide-area system via various conditions such as the effects of spreading factor (SF) and heights of transmit antennas. Their test results indicated that the system can transmit and receive at a long distance of 8.33 km. Together with the measured experiments, the mathematical analysis of LPWA systems was firstly performed in [25]. Specifically, [25] derived the OP and bit error rate (BER) expressions of LPWA systems and validated them via computer simulations.

As the above discussions, both EH and LPWA network technologies have many advantages and can be applied for various applications in IoT systems. Also, the benefits of EH technique have been discussed and experimented to clarify the potentials of this technique when being deployed in the LPWA systems [20], [26]. However, the combination of the EH technique and LPWA technology have not been applied in the literature in terms of the mathematical analysis. Meanwhile, this combination is very important due to the fact that EH from RF signals has great potentials to supply the stable energy to low power energy-constrained systems such as wireless sensor networks, IoTs, and extremely remote area communications used in 5G and beyond networks [1], [2]. In particular, the amount of harvested energy can fully satisfy the power requirements of IoT devices in LPWA systems. Therefore, exploiting EH technique for LPWA systems is inevitable in the future. This observation motivates us to consider an EH-LPWA system where the IoT sensors can harvest the energy from power beacon and then use the harvested energy for transmitting signals. By using the mathematical analysis, we obtain the exact closed-from expressions of OPs, throughputs, and SEPs of the considered EH-LPWA system. So far, this is the first work that mathematically analyzes the performance of LPWA systems with EH technique. The main contributions of the paper are summarized as follows:

• We investigate an EH-LPWA system where EH technique is exploited. Specifically, the IoT sensor is located in a restricted area, where the traditional power grid is extremely difficult to be deployed. Thus, it has to harvest the energy from the power beacon before transmitting signals to gateways. In addition, power beacon and gateways are equipped with multiple antennas.

• We obtain the exact closed-form expressions of OPs, throughputs, and SEPs of the considered EH-LPWA system over Nakagami-$m$ fading channels for both cases without and with cooperative communications. Then, we derive the optimal value of the time switching ratio that minimizes the OPs and SEPs and maximizes the throughput of the considered EH-LPWA system. Due to the generic properties of the Nakagami-$m$ fading channels, we can easily derive the OPs, throughputs, and SEPs of the considered EH-LPWA system over other channels such as Gaussian, Rayleigh, and Rician channels by changing the value of $m$. We validate all derived expressions through Monte-Carlo simulations.

• We evaluate the performance of the considered EH-LPWA system for various scenarios. Particularly, the numerical analysis results clarify that the data transmission rates, the distances, the time switching ratio for EH, the number of transmit/receive antennas, and the Nakagami parameters greatly impact on the OPs, throughputs, and SEPs of the system. By using a half of transmission block for EH, the system performance can be optimized. When the total of transmit and receive antennas are constant, we can use the number of transmit antennas at PB equal to the number of receive antennas at gateways to achieve the lowest OP/SEP of the considered EH-LPWA system.

The rest of this paper is organized as follows. Section 2 presents the system and signal models of the considered EH-LPWA system without and with cooperative communication. Section 3 analyzes the system performance by mathematically deriving the OP and SEP expressions for both cases without and with cooperative communication. Section 4 provides the detail calculatiions to obtain the optimal value of the time switching ratio. Section 5 provides numerical results and discussions. Finally, Section 6 concludes this paper.