  This paper proposes an improvement of the random multiple access scheme for
satellite communication named Multislot coded ALOHA (MuSCA). MuSCA is a
generalization of Contention Resolution Diversity Slotted ALOHA (CRDSA). In
this scheme, each user transmits several parts of a single codeword of an error
correcting code instead of sending replicas. At the receiver level, the decoder
collects all these parts and includes them in the decoding process even if they
are interfered. In this paper, we show that a high throughput can be obtained
by selecting variable code rates and user degrees according to a probability
distribution. With an optimal irregular degree distribution, our system
achieves a normalized throughput up to 1.43, resulting in a significant gain
compared to CRDSA and MuSCA. The spectral efficiency and the implementation
issues of the scheme are also analyzed.
