  This paper addresses an interference channel consisting of $\mathbf{n}$
active users sharing $u$ frequency sub-bands. Users are asynchronous meaning
there exists a mutual delay between their transmitted codes. A stationary model
for interference is considered by assuming the starting point of an
interferer's data is uniformly distributed along the codeword of any user. The
spectrum is divided to private and common bands each containing
$v_{\mathrm{p}}$ and $v_{\mathrm{c}}$ frequency sub-bands respectively. We
consider a scenario where all transmitters are unaware of the number of active
users and the channel gains. The optimum $v_{\mathrm{p}}$ and $v_{\mathrm{c}}$
are obtained such that the so-called outage capacity per user is maximized. If
$\Pr\{\mathbf{n}\leq 2\}=1$, upper and lower bounds on the mutual information
between the input and output of the channel for each user are derived using a
genie-aided technique. The proposed bounds meet each other as the code length
grows to infinity yielding a closed expression for the achievable rates. If
$\Pr\{\mathbf{n}>2\}>0$, all users follow a locally Randomized On-Off signaling
scheme on the common band where each transmitter quits transmitting its
Gaussian signals independently from transmission to transmission. Using a
conditional version of Entropy Power Inequality (EPI) and an upper bound on the
differential entropy of a mixed Gaussian random variable, lower bounds on the
achievable rates of users are developed. Thereafter, the activation probability
on each transmission slot is designed resulting in the largest outage capacity.
