  Ambient backscatter communication (AmBC) over
orthogonal-frequency-division-multiplexing (OFDM) signals has recently been
proposed as an appealing technique for low power Internet-of-Things (IoT)
applications. The special spectrum structure of OFDM signals provides a range
of flexibility in terms of bit-error-rate (BER) performance, data rate, and
power consumption. In this paper, we study subcarrier-wise backscatter
communication over ambient OFDM signals. This new AmBC is to exploit the
special spectrum structure of OFDM to transmit data over its squeezed
orthogonal subcarriers. We propose a basis transmission scheme and its two
modifications to support a higher data rate with superior BER performance
compared to existing methods. The basis scheme can transmit one bit per
subcarrier using on-off keying (OOK) modulation in the frequency domain. In the
first modification, interleaved subcarrier block transmission model is employed
to improve the BER performance of the system in frequency-selective channels.
It results in a trade-off between the size of the blocks and data rate. Thus,
in the second modification, interleaved index modulation (IM) is employed to
mitigate the data rate decrementation of the former modification. It also
stabilizes and controls the power of the signal to result in interference
reduction for a legacy receiver. Analytical and numerical evaluations provide a
proof to see the performance of the proposed method in terms of BER, data rate,
and interference.
