Performance analysis of multi-code spread slotted ALOHA (MCSSA) system

We present an analytical framework to investigate the dynamics of a medium access control (MAC) protocol which is governed by two distinct disturbing elements. The first element concerns the users' random selection from a finite number of resources (i.e., channels or spread spectrum (SS) codes). The second element is the simultaneous transmission of users' packets if the resources happen to be SS codes. These elements are realized through two stages of packet acquisition (Stage-I) and packet transmission (Stage-II). Therefore, a successful transmission depends not only upon the success in Stage-I, but also success in Stage-II. Simply stated, MCSSA combines two existing MAC protocols: multichannel ALOHA and CDMA. An earlier attempt to properly study this protocol by Liu an El Zarki (1994) was unsuccessful. The model developed in our paper is based on a finite-state discrete time Markov chain and a combinatorial approach. A key feature in our model is a recursive expression for the probability of successful channel or code acquisitions given so many simultaneous attempts by the users. Our analytical model is properly constructed and adequately parameterized to reduce to the special cases: slotted ALOHA, multichannel slotted ALOHA, and CDMA. To assess the performance of the MCSSA protocol, we evaluate two commonly used performance metrics: average packet delay and throughput. Preliminary results indicate that, depending on different loading regions (e.g., high offered load level), the MCSSA protocol surpasses that of its CDMA counterpart. This behavior is attributed to the controllability feature of Stage-I which performs similar to a traffic smoother in queueing systems.