^{1}

^{2}

^{1}

^{2}

We focus on the design of transmission protocol for energy harvesting wireless sensors. The sensors can harvest the energy from the environment, but they cannot charge and discharge at the same time. We propose a protocol for energy harvesting and wireless transmission, which contains two steps. In the first step, the sensor harvests the energy from environment, and the energy harvesting rate is controlled by the harvested energy power of the energy saving device (ESD). In the second step, some data should be transmitted to the receiver in a certain time. Considering one slot time, the first part of the time is devoted exclusively to energy harvesting, and the remaining time of the slot is for transmitting the information data. Assume that

The energy harvesting in wireless sensor network has become increasingly attractive with sustainable and long lasting power supplies. With development of IOT, more and more sensors are embedded in medical device or building structures to sense the environment [

Among all these methods, radio signal radiated by ambient transmitters became a new source for wireless energy harvesting, and harvesting energy from ambient RF signals can power a wireless sensors network [

There is some progress in the recent research of energy harvesting. For example, the information theoretic capacity with an energy harvesting transmitter was discussed in [

In this work, we consider the design of transmission protocol for energy harvesting wireless sensor system, in which the users transmit their independent information using their individually harvested energy in the uplink. Our work is unlike traditional research on Simultaneous Wireless Information and Power Transfer (SWIPT), which assumes the simultaneous energy and information transmissions to sensors (or users) in the downlink. A major problem of this assumption is that practical circuits cannot charge and discharge at the same time. We assume that the transmitter needs to deliver

The rest of this paper is organized as follows. Section

In this paper, we assume the system includes one pair of single-antenna transmitter (Tx) and receiver (Rx). Suppose

Let

The receiver signal at the destination node in one slot time,

According to the protocol, the total buffered energy at

Denote

In this section, we analyze the instantaneous signal-to-noise ratio (SNR) and derive the probability density function (PDF) of the SNR.

The instantaneous SNR in the destination is given by

For a given

Because

The outage probability is an important criterion of the system performance. In this section, we determine an optimal save-ratio to improve the performance of outage probability. Suppose

Consider the following optimization problem:

In this section, we evaluate the performance of the proposed transmission schemes. First, we will calculate the mutual information

In our simulation, we consider the BPSK modulation communication system. The channel is Rayleigh fading, and the path loss exponent

Figure

The system mutual information.

The relationship of outage probability and the save-ratio

In Figure

Outage probability comparison for optimal system and nonoptimal system.

In this paper, we studied a wireless system under energy harvesting conditions. We established the relationship between the system outage performance and the transmission protocol. Besides, we proposed an optimal method to optimize the system outage performance via finding the optimal save-ratio. We also characterized how the outage probability varies with the save-ratio and compared the outage performance between nonoptimal save-ratio and optimal save-ratio under the assumption of Rayleigh fading channel. Simulation results demonstrated that remarkable gain is achieved for the proposed optimal scheme. If we choose the optimal save-ratio for the transmission, we can achieve the outage performance minimization. And the optimal scheme can provide improved efficiency and reliability of the energy harvesting wireless sensor networks, which means such energy harvesting approaches are promising for further performance improvement.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work is partially supported by the National Natural Science Foundation of China under Grants 61372088, 61271257, and 61328102 and the project funded by the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20130320).