  Blind Null Space Learning (BNSL) has recently been proposed for fast and
accurate learning of the null-space associated with the channel matrix between
a secondary transmitter and a primary receiver. In this paper we propose a
channel tracking enhancement of the algorithm, namely the Blind Null Space
Tracking (BNST) algorithm that allows transmission of information to the
Secondary Receiver (SR) while simultaneously learning the null-space of the
time-varying target channel. Specifically, the enhanced algorithm initially
performs a BNSL sweep in order to acquire the null space. Then, it performs
modified Jacobi rotations such that the induced interference to the primary
receiver is kept lower than a given threshold $P_{Th}$ with probability $p$
while information is transmitted to the SR simultaneously. We present
simulation results indicating that the proposed approach has strictly better
performance over the BNSL algorithm for channels with independent Rayleigh
fading with a small Doppler frequency.
