Page 22 - 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

and 236 nW power consumption as in a Bluetooth be used as devices to ubiquitously tag “things” on a
wakeup receiver [2]). large scale.

In summary, the BTTN paradigm utilizing DE or AE
can provide a common medium or language for direct
Passive Receiver(PR) Tag Tag Tag Tag jects of the IoT irrespective of cost, volume or density
Exciter
peer-to-peer communication between all constituent ob-
Backscattering Tag-to-Tag Networks (BTTN)
Ambient
of object population. In a sense, the excitation source
source
in BTTN can be thought of as simply illuminating an
area, and the tags can “see” each other in this “light.”
Whether the illumination comes from a natural source
(AE approach) or a light bulb (DE approach), the com-
Monostatic
munication between the tags is unaltered.
Active Receiver(AR) Active Rx. Bistatic Ambient Tag 1.1 Scalability and practical implications of
Tag
Exciter +
source
the AE approach
Active
Exciter
Active
Generally, AE backscatter systems are considered to be
Rx.
Rx.
extremely scalable because they can theoretically en-
Dedicated Exciter(DE) Ambient Exciter(AE) able communication without any deployment. However,
there are important practical considerations that can
adversely affect this in a real-world IoT deployment.
Fig. 1 – Classification of backscatter systems. All such systems
contain a backscattering transmitter (Tag). They are classified • AEAR systems: There are widespread efforts in the
based on the source of excitation signal and the type of receiver
employed. literature on these systems. The systems are based
on the idea of building passive tags that can use
Combining the above criteria, we classify all backscatter ambient excitation to synthesize backscatter pack-
systems into four types as follows: ets that are compatible with commodity standards
such as WiFi, Bluetooth or ZigBee [4, 5, 7]. A
(i) Dedicated Exciter Active Receiver (DEAR) [3], corresponding commodity receiver can then receive
this signal and communicate with the tag. This
(ii) Ambient Exciter Active Receiver (AEAR) [4, 5, 6, approach significantly complicates the transmit cir-
7], cuitry on the tag. Further, such tags can only be
built to synthesize one kind of backscatter pack-
(iii) Dedicated Exciter Passive Receiver (DEPR) [8, 9, ets using one kind of excitation signal. Under this
10], and approach, the tags cannot communicate with each

(iv) Ambient Exciter Passive Receiver (AEPR) [11]. other.
• AEPR systems: These systems fall under the
This classification is shown in Fig. 1. As pointed out BTTN umbrella. While this, in theory, enables
in [12], DEAR systems can be either monostatic where maximum scalability, it is important to note the
the exciter and receiver functions are on the same device ambient power level requirement. As shown in [13],
(e.g., traditional RFID) or bistatic with these functions in order to enable a practical link distance, the re-
being on different devices. quired power level is of the order of −25 dBm. Most

We focus specifically on passive receiver (PR) backscat- ambient excitation signals in general indoor envi-
ter systems which form the basis of the so-called ronments from various sources including TV tow-
Backscattering Tag-to-Tag Networks or (BTTNs). PR ers, cell phone towers or WiFi APs are far below
backscatter systems present fundamentally different this value.
challenges than AR backscatter systems. The challenges
stem from the need to passively receive the backscat- Thus, a practical implementation of BTTN might in-
ter signals in the presence of the interfering excitation variably call for a dedicated exciter. However, such an
with only an envelope detector. However, if these chal- exciter is simply an autonomous, RF transmitter that
lenges are overcome, then BTTN can fundamentally is not part of the communication network and does not
transform the capabilities of the IoT by enabling all pas- centralize the tag-to-tag communications (refer to the
sively tagged “things” to talk directly with each other light bulb analogy above). Furthermore, the BTTN
without any central active controller or master. In AR backscatter modulator is designed for tag-to-tag com-
systems whether of DEAR or AEAR variety, this capa- munication as opposed to tag to commodity receiver
bility is impractical since the high cost and high power- communication. This keeps the “language” and thereby
requirement of active receivers means that they cannot the design of the transmitter much simpler. Unlike the

2 © International Telecommunication Union, 2020