  In this work, we investigate the effect of time asynchrony in non-orthogonal
multiple access (NOMA) schemes for downlink transmissions. First, we analyze
the benefit of adding intentional timing offsets to the conventional power
domain-NOMA (P-NOMA). This method which is called Asynchronous-Power
Domain-NOMA (AP-NOMA) introduces artificial symbol-offsets between packets
destined for different users. It reduces the mutual interference which results
in enlarging the achievable rate-region of the conventional P-NOMA. Then, we
propose a precoding scheme which fully exploits the degrees of freedom provided
by the time asynchrony. We call this multiple access scheme T-NOMA which
provides higher degrees of freedom for users compared to the conventional
P-NOMA or even the modified AP-NOMA. T-NOMA adopts a precoding at the base
station and a linear preprocessing scheme at the receiving user which
decomposes the broadcast channel into parallel channels circumventing the need
for Successive Interference Cancellation (SIC). The numerical results show that
T-NOMA outperforms AP-NOMA and both outperform the conventional P-NOMA. We also
compare the maximum sum-rate and fairness provided by these methods. Moreover,
the impact of pulse shape and symbol offset on the performance of AP-NOMA and
T-NOMA schemes are investigated.
