  An information-theoretic approach to irregular repetition slotted ALOHA
(IRSA) is proposed. In contrast with previous works, in which IRSA analysis is
conducted only based on quantities that are typical of collision models such as
the traffic, the new approach also captures more fundamental quantities.
Specifically, a suitable codebook construction for the adder channel model is
adopted to establish a link with successive interference cancellation over the
multi-packet reception channel. This perspective allows proving achievability
and converse results for the average sum rate of IRSA multiple access schemes.
