  A digital-assisted photonic analog wideband radio-frequency multipath
self-interference cancellation (SIC) and frequency downconversion method based
on a dual-drive Mach-Zehnder modulator and the recursive least square (RLS)
algorithm is proposed and demonstrated for in-band full-duplex systems. Besides
the reference for the direct-path self-interference (SI) signal, the RLS
algorithm is used to construct another reference for the residual SI signal
from the direct path and the SI signals from the reflection paths. The proposed
method can solve the performance limitation in the previously reported SIC
methods of constructing the multipath SI signal using a single reference caused
by the limited dynamic range of the digital-to-analog converter when the
direct-path SI signal is much stronger than the sub-weak reflection-path SI
signals. An experiment is performed. When the carrier frequency of the
multipath SI signal is 10 GHz and the direct-path SI signal is much stronger
than the sub-weak multipath SI signal, the cancellation depths of about 26.7
and 26.1 dB are realized with SI baud rates of 0.5 and 1 Gbaud. When the
direct-path SI signal and sub-weak multipath SI signal own closer power, the
corresponding cancellation depths are 24.7 and 20.8 dB, respectively.
