  In this paper, we consider a massive uncoordinated non-orthogonal multiple
access (NOMA) scheme where devices have strict latency requirements and no
retransmission opportunities are available. Each device chooses a pilot
sequence from a predetermined set as its signature and transmits its selected
pilot and data simultaneously. A collision occurs when two or more devices
choose the same pilot sequence. Collisions are treated as interference to the
remaining received signals. We consider successive joint decoding (SJD) and
successive interference cancellation (SIC) under a Rayleigh fading and path
loss channel model. We first derive the expression for the outage probability
for the case where devices transmit at the same fixed rate. Then, we derive the
expression for the maximum achievable throughput for the case where devices
transmit with rateless codes. Thus, their code rate is adaptive to the system
conditions, i.e., load, received powers, and interference. Numerical results
verify the accuracy of our analytical expressions. For low data rate
transmissions, results show that SIC performs close to that of SJD in terms of
outage probability for packet arrival rates up to 10 packets per slot. However,
SJD can achieve almost double the throughput of SIC and is, thus, far more
superior.
