Simultaneous Wireless Information and Power Transfer

Since the RF signal not only carries energy but also is used as a vehicle to transport in- formation at the same time, harvesting energy and receiving information from the same RF signal input can be theoretically realized. An interesting application of the RF signal, namely SWIPT, has drawn significant attentions. SWIPT system can be divided into two

2.1 Fundamental Concepts 14 categories, integrated-SWIPT (shown in Figure 2.1 (a)) and decoupled-SWIPT (shown in Figure 2.1 (b)), based on whether the received signal at the receiver comes from the same BS. In integrated-SWIPT scenarios, the energy signal and the information signal are both carried

Fig. 2.3 Architecture for SWIPT systems.

in the same RF signal. In decoupled-SWIPT scenarios, the information signal and the energy signal, generated by the different BS, are received at the receiver at the same time. This model can overcome the limitation of integrated-SWIPT model, because of receivers can be deployed close to the energy transmitter. However, this model causes serious interference at the receiver side.

Since the information and energy transfer of the system cannot be maximized at the same time, there exists conflict between WPT and WIT in SWIPT systems. The question of how the trade-off between the information and energy transmission impacts the system performance has attracted the attention of the research community. Varshney first investigated the trade-off between the information and energy transmission by exploiting a rate-energy (R-E) function in a point-to-point additive white Gaussian noise (AWGN) channel [21]. Then, Grover et al. extended the work in [21] to frequency-selective channels with the AWGN model to study the trade-off between information and energy [22]. The studies in [21] and [22] were proposed

2.1 Fundamental Concepts 15 based on the assumption of the ideal receiver architecture, which means that receivers in both studies are capable to decode information and harvest energy from the same received signal at the same time. However, this assumption is difficult to implement in practical systems because practical circuits are totally different for ID and EH [23, 24]. In other words, the transmitted information cannot be decoded at the ideal receiver during the EH processing. Inspired by this challenge, three types of receiver architectures are designed to implement SWIPT in wireless communication networks in the current thesis. In the following parts, these practical receiver designs are introduced, namely separated receiver architecture, co-located receiver architecture and integrated receiver architecture.

2.1.2.1 Separated Receiver Architecture

Zhang et al. proposed a separated receiver architecture to simultaneously implement ID and EH at the receiver side. In this practical architecture, the EH circuit and the ID circuit are adopted in two independent receivers, respectively [23]. These two types of receivers can be easily built based on off-the-shelf information receivers and energy receivers. Moreover, the trade-off between the achievable information rate and the harvested energy can be obtained by using optimization techniques.

2.1.2.2 Co-located Receiver Architecture

Different from the separated receiver, the ER and the IR are capable to receive the same signal by sharing the same antenna in the co-located receiver. As the received signal at the EH circuit and the ID circuit comes from the same antenna, the system complexity of using the co-located receiver is smaller than that of the separated receiver. Similar to ideal receivers [21, 22], the question of how EH and ID are simultaneously implemented in the co-located receiver design is still an important issue. To answer this question, Zhang et al.

2.1 Fundamental Concepts 16 [23] proposed two practical receiver models, namely time switching (TS) receiver and power splitting (PS) receiver, to overcome the practical limitation of the co-located receiver design. In the following, these two practical receivers are introduced in orders.

1. Time Switching Receiver. An information receiver and a RF energy receiver are connected to the same antenna in the TS receiver. In this scheme, the transmission block is divided into two orthogonal time slots at the transmitter, i.e., one for information transmission and the other for energy transfer. Based on a time switching sequence, each antenna is capable to switch the received signal to the EH circuit or the ID circuit in two time slots [23]. The perfect time synchronization between the receiver and the transmitter is important to implement SWIPT in the TS receiver. The optimal R-E trade-off can be achieved by optimizing the time switching sequence.

2. Power Splitting Receiver. A passive power splitter is equipped at each antenna to distribute the received signal to the EH circuit and the ID circuit based on different power levels. In fact, the received signal is split into two signal streams with different power. One stream with 0≤ρ ≤1 portion signal power is used for ID, and the other stream is used for EH with 1−ρ power [23, 24]. The different trade-off between the achievable information rate and the harvested energy can be achieved by using different PS ratioρ. When ratioρ =0 orρ =1, the PS receiver becomes as the TS receiver.

2.1.2.3 Integrated Receiver Architecture

An integrated information and energy receiver was proposed by Zhou et al. [24] to implement ID and EH operations by exploiting one-time RF-to-baseband conversion. In this receiver, the received RF signal is first converted to a DC signal by a rectifier at the ER. Then, the DC signal is distributed to the battery and the IR. Note that the DC signal can be considered as

2.1 Fundamental Concepts 17 the baseband signal for information decoding. Therefore, the IR at the integrated receiver does not need any RF band to baseband conversions.