  In machine-type communication (MTC), random access has been employed for a
number of devices and sensors to access uplink channels using a pool of
preambles. To support different priorities due to various quality-of-service
(QoS) requirements, random access can be generalized with multiple pools, which
may result in low spectral efficiency. In this paper, for high spectral
efficiency, random access with layered preambles (RALP) is proposed to support
devices with two different priorities based on the notion of power-domain
non-orthogonal multiple access (NOMA). In RALP, two groups of devices, namely
type-1 and type-2 devices, are supported with different priorities, where
type-1 devices have higher priority than type-2 devices. Closed-form
expressions are derived for the detection performance of preambles transmitted
by type-1 devices, which can be used for a certain performance guarantee of
type-1 devices of high priority. Low-complexity preamble detection methods are
also discussed.
