# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 3 BackCom: An Effective Solution
### BackCom Architecture and Protocols  
Figure 6 delineates the architecture of the BackCom tag and shows a passive tag receiving carrier signals, transmitted by the RF source. The energy harvester block of the tag can harvest energy from carrier signals. The accumulated energy can be collected in the capacitors or energy storing components in the storage block. The stored energy can supply consistent power to send information to the decoder unit and the modulation box. Subsequently, the micro-controller in the modulation box can modulate the unique information of the tag on the carrier signals via the backscattering operation [9]. The signal backscattering is caused by changing the impedance intentionally, as shown by the variable impedance block in Figure 6. Similarly, an architecture of the 802.15.4 receiver is presented in [53]. It consumes 361 \(\upmu\)W and is compatible with battery-free applications. We have considered the physical architecture of the tag and its different blocks. Various protocols will now be discussed. Efficient BackCom protocols can maintain consistency between the harvested power and the data transmission via the same antenna in the tag. The passive tags always rely on external power sources to harvest sufficient energy for their operation. However, maintaining uniformity in the power harvesting rate and the information transfer in passive tags is a challenging task. Efficient protocols can address this challenge by maintaining the energy harvesting rate and data reception. The scenario that uses RF tags to harvest energy first and to then utilize the stored power to transmit information from tags to readers is referred to as the harvest-then-transmit (HTT) protocol. The HTT protocol is used to maximize the expected throughput of wireless communications (using direct and relay transmission) with non-dedicated sources [54]. Similarly, a hybrid access point (Hybrid-AP) employs the HTT protocol and initially broadcasts wireless energy to various distributed users in the downlink while allowing the device to transmit data to the Hybrid-AP while working in uplink mode [55]. Similarly, the Flit protocol was also designed to increase channel utilization while reducing energy loss [56]. Flit improves the data rate by exploring idle listening and reduction in collisions between multiple tags. Moreover, Flit offers practical and straightforward coordination mechanism for tags. [PERSON] et al. proposed the QuarkNet stack [57], which divides the BackCom network stack into random smaller units called micro-frames. The micro-frames depend on the remaining energy of the tag. The frame size in QuarkNet could be at least 1 bit in limited energy scenarios and as large as a complete frame in the case of sufficient available energy [57]. Usually, a tag has to wait for carrier signals to be transmitted completely by the RF source. Afterwards, the tag can transmit its complete signal. This symmetry between the transmitter and tag is exploited by [PERSON] et al. [33]. The authors propose a fully asymmetric protocol for BackCom, namely the Laissez-Faire protocol, which allow nodes to transmit data as soon as they sense carrier signals. This technique takes advantage of the ability of the reader to sample data at a rate higher than the tag. Moreover, this method suggests concurrent transfers in asymmetric BackCom. For concurrent transmissions from various devices, the NetScatter protocol is suggested [58]. NetScatter  
Figure 6: Architecture of the BackCom tag. uses a distributed coding mechanism and can decode all concurrent transmissions using only a single FFT operation. Likewise, [PERSON] et al. presented the automatic rate adaption protocol, which adapts the aggregate bit rate of multiple transmitters [59]. However, this protocol could not change the transmission bit rate individually. We have discussed protocols and taking advantage of RF signals in different scenarios. We will now outline multiple configurations of the source, reader, and tag.