  Machine-to-machine (M2M) wireless systems aim to provide ubiquitous
connectivity between machine type communication (MTC) devices without any human
intervention. Given the exponential growth of MTC traffic, it is of utmost
importance to ensure that future wireless standards are capable of handling
this traffic. In this paper, we focus on the design of a very efficient massive
access strategy for highly dense cellular networks with M2M communications.
Several MTC devices are allowed to simultaneously transmit at the same resource
block by incorporating Raptor codes and superposition modulation. This
significantly reduces the access delay and improves the achievable system
throughput. A simple yet efficient random access strategy is proposed to only
detect the selected preambles and the number of devices which have chosen them.
No device identification is needed in the random access phase which
significantly reduces the signalling overhead. The proposed scheme is analyzed
and the maximum number of MTC devices that can be supported in a resource block
is characterized as a function of the message length, number of available
resources, and the number of preambles. Simulation results show that the
proposed scheme can effectively support a massive number of MTC devices for a
limited number of available resources, when the message size is small.
