  A key challenge for a common waveform for Integrated Sensing and
Communications (ISAC) - widely seen as an attractive proposition to achieve
high performance for both functionalities, while efficiently utilizing
available resources -- lies in leveraging information-bearing channel-coded
communications signals (c.c.s) for sensing. In this paper, we investigate the
sensing performance of c.c.s in (multi-user) interference-limited operation,
and show that it is limited by sidelobes in the range-Doppler map, whose form
depends on whether the c.c.s modulates a single-carrier or OFDM waveform. While
uncoded communications signals -- comprising a block of $N$ i.i.d zero-mean
symbols -- give rise to asymptotically (i.e., as $N \rightarrow \infty$) zero
sidelobes due to the law of large numbers, it is not obvious that the same
holds for c.c.s, as structured channel coding schemes (e.g., linear block
codes) induce dependence across codeword symbols. In this paper, we show that
c.c.s also give rise to asymptotically zero sidelobes -- for both
single-carrier and OFDM waveforms -- by deriving upper bounds for the tail
probabilities of the sidelobe magnitudes that decay as $\exp( - O($code rate
$\times$ block length$))$. This implies that for any code rate, c.c.s are
effective sensing signals that are robust to multi-user interference at
sufficiently large block lengths, with negligible difference in performance
based on whether they modulate a single-carrier or OFDM waveform. We verify the
latter implication through simulations, where we observe the sensing
performance (characterized by the detection and false-alarm probabilities) of a
QPSK-modulated c.c.s (code rate = 120/1024, block length = 1024 symbols) to
match that of a comparable interference-free FMCW waveform even at high
interference levels (signal-to-interference ratio of -11dB), for both
single-carrier and OFDM waveforms.
