# RF Energy Harvesting Wireless Communications:
## 5 Intermittent Connection Problem in RF-EHWC Networks
### Cause of Intermittent Connections  
The problem of intermittent connections is essentially caused by the slow energy harvesting rate of RF-EHWC system and the randomness of short-term traffic.  
As summarized in Section 2.1, the power density of ambient RF energy is usually thin, which is in the order of \(\mathrm{nW/cm^{2}}\) or even lower. With the limited power supply, an RF-EHWC device can only operate at an extremely low duty cycle (duty cycle is defined as the percentage of time that the transmitter can transmit using the energy received in a unit time). We use a low-power wireless communication module, Texas Instruments CC2500 (Texas Instruments, Dallas, TX, USA), as an example to offer an insight into how the low density of RF energy impacts the duty cycle of RF-EHWC node. Assume an energy harvester is equipped with a 2 dBi antenna receiving energy from the 900 MHz frequency band to power CC2500. Based on the effective aperture of this receiving antenna (i.e., 351 cm\({}^{2}\)[76]) and highest energy density on 900 MHz frequency band (i.e., 1930 nW/cm\({}^{2}\)[23]), the power provided by the harvester is 0.47 mW with 70% power conversion efficiency. When transmitting data at 1 mW (0 dBm), the power consumption of CC2500 is about 46.6 mW (21.2 mA \(\times\) 2.2 V [77]), two orders higher than the energy harvesting rate. In this case, the duty cycle of CC2500 is at most 0.40%. This indicates that the RF energy harvested by an RF-EHWC device is far from enough for continuous data communications. Accordingly, energy harvesting nodes will become temporarily unavailable when the stored energy is depleted after intensive transmissions.  
The randomness of short-term traffic is another reason that causes intermittent connections. It is common for nodes to have heterogeneous and highly dynamic traffic loads due to the nonuniform deployment of the network, dynamic channel quality, and variation of the surrounding environment. Depending on which routing protocol is running in the network, some special nodes, such as a cluster head in the hierarchical routing or a high-energy node in an energy-aware routing, may experience much heavier traffic than other nodes in a short term. This will not be a problem in conventional wireless sensor networks. The battery-powered nodes are resilient to the temporary variation of the local traffic since their lifetime is mainly determined by the long-term energy consumption. For this reason, the energy-aware routing proposed for the conventional wireless sensor network aims to balance the long-term traffic among nodes, whereas the short-term traffic unbalancing can be ignored [78]. In RF-EHWC networks, however, a burst of traffic can drain the harvested energy quickly and causes a node to be temporarily disconnected from the network. For a large-scale RF-EHWC network, the intermittent connections are very difficult to avoid since the short-term traffic is usually random and unpredictable [79]. Therefore, it is crucial to design routing protocols that can balance not only the long-term traffic but also the short-term traffic to mitigate the intermittent connections.