  While LTE is becoming widely rolled out for human-type services, it is also a
promising solution for cost-efficient connectivity of the smart grid monitoring
equipment. This is a type of machine-to-machine (M2M) traffic that consists
mainly of sporadic uplink transmissions. In such a setting, the amount of
traffic that can be served in a cell is not constrained by the data capacity,
but rather by the signaling constraints in the random access channel and
control channel. In this paper we explore these limitations using a detailed
simulation of the LTE access reservation protocol (ARP). We find that 1)
assigning more random access opportunities may actually worsen performance; and
2) the additional signaling that follows the ARP has very large impact on the
capacity in terms of the number of supported devices; we observed a reduction
in the capacity by almost a factor of 3. This suggests that a lightweight
access method, with a reduced number of signaling messages, needs to be
considered in standardization for M2M applications. Additionally we propose a
tractable analytical model to calculate the outage that can be rapidly
implemented and evaluated. The model accounts for the features of the random
access, control channel and uplink and downlink data channels, as well as
retransmissions.
