Page 23 - 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

AEAR approach where backscattering is tuned to one
format of the excitation signal, BTTN tags can talk to
each other irrespective of the excitation signal format
whether it is a CW or a modulated signal. The only Ideal link
requirement is that in the case of a modulated exciter Passive Receiver (PR) link in BTTN
signal, the bandwidth of the tag-to-tag backscatter is
lower than the bandwidth of the excitation signal. Rx Signal Amplitude (mV)
Most of the review papers in the literature on the pas-
sive backscatter communication focus on the backscatter
systems with active receiver(AR) [14] or group both the
AR and PR systems in the same category [12] without
addressing specific issues that exist in PR systems. In Tx to Rx Distance (m)
enabling a single BTTN link and further scaling up to
a larger network, a vast array of challenges needs to be
overcome. These stem mainly from having to process Fig. 2 – Characteristic of backscatter signal amplitude with in-
creasing Tx to Rx distance in BTTN (PR) link compared to an
received signals and mitigate interference in passive re- ideal link without phase cancellation.
ceivers, to operate in an extreme low power regime (e.g.,
[15]), and to communicate in inherently high volume backscatter signal and the excitation signal causes a phe-
and high density networks. In the rest of this paper, we nomenon referred to as phase cancellation. This is seen
provide an overview of recent advances in BTTNs, chal- in Fig. 2 (the red solid line), where the received sig-
lenges, applications of BTTNs, and future directions for nal instead of monotonically decreasing with distance
research. undergoes alternating peaks and nulls, with decreasing
peak values [16, 17]. We note that this phenomenon will
2. THE FUNDAMENTALS also occur in so-called bistatic AR systems [12] where
the exciter and receiver are separate. On the other hand,
in monostatic AR systems like traditional RFID, the re-
One of the most challenging requirements of a PR is ceiver (reader) is able to cancel out the excitation sig-
to receive the inherently weak backscatter signal in the nal and use IQ demodulation for the received backscat-
presence of a much stronger interfering excitation with- ter. In this case the received signal amplitude decreases
out IQ demodulation or carrier cancellation capability. monotonically with distance (the blue dash-dot line).
We illustrate this challenge with a basic BTTN link con- Phase cancellation and low modulation index are two of
sisting of two tags in an area that sees a sufficient level of the most fundamental challenges in enabling basic com-
excitation signal whether DE or AE. In this basic link, munication in a PR BTTN link. The phase cancellation
at any given time, one of the tags transmits (Tx) and can be addressed using a multi-phase backscatter mod-
the other one receives (Rx). All BTTN tags are identi- ulator, while signals with low modulation indices are
cal, and they switch between Tx and Rx roles based on processed with demodulators with innovative architec-
the MAC-layer and network-layer protocols. tures [17, 16]. We note that in a link with AR, there
are alternative ways to avoid phase cancellation – by
The Tx tag generates the modulated backscatter sig- providing a frequency shift to the backscattered signal,
nal by altering the antenna’s reflection cross section. thereby avoiding interference with the excitation signal
The signal seen at the Rx tag is a superposition of the altogether [14].
excitation signal and the modulated backscatter signal
from the Tx tag. In the absence of an on-board radio
transceiver, the Rx tag has to demodulate the backscat- The maximum communication range of a BTTN link
ter signal using envelope detection. The received signal (Tx to Rx distance) depends critically on the excitation
has a very low modulation index due to the small ampli- power available at the Tx tag regardless of whether the
tude of the backscatter signal combined with the much excitation source is DE or AE. Distances up to about
larger magnitude of the exciter signal. Additionally, as 3 m have been reported with −20 dBm power available
the two signals combine at the Rx tag, the modulation at the Tx tag (5 kbps, BER below 10 ) [16]. Further
−3
index is significantly impacted by the relative phase dif- improvement is possible using coding techniques such as
ference between the excitation signal and the backscat- CDMA, but at the expense of data rate. For example,
ter signal. [18] has reported distance up to about 10 m with sim-
ilar power levels but providing much slower bit rates,
In Fig. 2, we see the characteristic of the received base- in the order of 100 bps. Innovations in the demodula-
band backscatter signal amplitudes as a function of the tor design can improve the distance and/or improve the
Tx to Rx distance for an ideal link and a PR backscat- data rate. Other innovations are also possible including
ter link which constitutes BTTNs. In the PR BTTN multiple antennas on the tag [18] or beamforming using
link, the relative phase difference between the received multiple tags via a collaborative arrangement. The con-

© International Telecommunication Union, 2020 3