Page 25 - ITU Journal, Future and evolving technologies - Volume 1 (2020), Issue 1, Inaugural issue

ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1

Table 1 – Summary of the contributions to backscatter PR tag-to-tag (T2T) links

Article Key contribution Frequency Experimental Results

[9] T2T communication concept 915 MHz T2T link at 10 cm
[11] T2T link with ambient exciter 539 MHz 1 kbps at 0.75 m and -8 dBm
[18] coding technique to extend link distance 915 MHz 0.003 kbps at 6 m and -20 dBm
multi-antenna tag for increased data rate 539 MHz 1000 kbps at 2.1 m and -20 dBm
[17] phase cancellation in T2T link 915 MHz
[13] theoretical analysis of T2T link 915 MHz
[16] demonstration of multi-hop network 915 MHz 5 kbps at 3 m and -20 dBm
[19] M-PSK for increased data rate 539 MHz 20 kbps at 0.75 m
[10] MAC protocol 915 MHz multi-hop T2T links reach 5.65 m

cost of chip area and power consumption. tion between the tags and computation are of the same
order of magnitude. Further, the different energy costs
3.2 Energy harvesting architecture for performing different operations on the tags lead to a
unique power management paradigm for BTTN.
The RF energy harvesting module acquires energy from
the external excitation signal. A power harvesting cir- 4. SCALING FROM A SINGLE LINK
cuit comprises rectification of the incident AC voltage, TO A FULL NETWORK: ROUT-
followed by multiplication and regulation that provides
stable DC supply voltage for the operation of the tag. ING FOR IOT APPLICATIONS
The energy efficiency of the conventional power har- 4.1 From a Link to a Network
vesting circuit is optimized for a certain range of in-
put power. As the input power can exceed the power Extending a single link communication to a tag network
consumption of the tag, the extra energy can be stored is far from trivial [20]. Two issues need to be consid-
using a supercapacitor. This enables the operation of ered: topology formation and routing. The topology
the tag when the harvested energy is lower than the formation involves selection of network links for com-
instantaneous power consumption. The size of the su- munication based on the energy states of the individual
percapacitor is limited by the form factor of the tag. tags. This decision typically involves tags beyond local
The power management logic optimizes the charging of neighborhoods and may require dynamic operation as
the supercapacitor based on the incident RF power and the tags’ energy states continuously vary.
the power needs of the tag operation. For communication across a tag-to-tag link, there must
be enough RF power reaching the Rx tag to power up
Based on the incident RF power, the stored energy and the receiving tag for effective demodulation and then
the operation of the tag, e.g., backscatter, receive or to do any needed post-demodulation computation (e.g.,
compute, the power management module directs the MAC, routing decisions). The power needed for effective
tag’s operation. The operation of such tags powered by demodulation is dependent on the modulation index,
RF harvested energy and low capacity supercapacitors which in turn depends on the wireless channel condi-
introduces some unique challenges compared to those tions that determine the powers reaching the tags. This
of a traditional sensor node. Sensor nodes incorporate is heavily influenced by the tag and exciter locations.
active radios that dominate the power budget. Though As mentioned in Section 3.2, the energy management
significant steps have been made in reducing their power module decides the power split among the various oper-
consumption at the receiver end [1], the transmit power ations. The Rx tag in a weak link may have to decide
still dominates the operation as the radios must gener- whether to receive a packet at all if it may not be able
ate the RF carrier signal used for communication. The to forward it immediately for a lack of enough avail-
principal difference between the power budgets of con- able power. Similarly, a more “energy-rich” tag may be
ventional sensor nodes and the RF tags is that the tags able to take up more responsibilities for routing or MAC
operate at orders of magnitude of lower power consump- protocol decisions.
tion due to the low energy cost of the communication, as
the energy cost of their communication can be orders of 4.2 Routing and MAC
magnitude lower than for the nodes comprising active
radios. This is because tags only reflect (backscatter) The challenges of designing routing and MAC protocols
externally supplied RF signals and do not generate any for BTTN arise from the unique characteristics of the
signal on their own. However, in BTTN tags there is backscattering environment, including the extreme low-
no such dominance – the energy costs for communica- power operation and from the intended BTTN applica-

© International Telecommunication Union, 2020 5