  Associated with multi-packet reception at the access point, irregular
repetition slotted ALOHA (IRSA) holds a great potential in improving the access
capacity of massive machine type communication systems. Considering the
time-frequency resource efficiency, K = 2 (multi-packet reception capability)
may be the most suitable scheme for scenarios that allow smaller resource
efficiency in exchange for greater throughput. In this paper, we analytically
derive an optimal transmission probability distribution for IRSA with K = 2,
which achieves a significant higher load threshold than the existing benchmark
distributions. In addition, the energy efficiency optimization in terms of the
maximum repetition rate is also presented.
