  Though the concept of non-orthogonal multiple access (NOMA) was proposed
several years ago, the performance of uplink NOMA has only been verified in
theory, but not in practice. This paper presents an over-the-air implementation
of a uplink NOMA system, while providing solutions to most common practical
problems, i.e., carrier frequency offset (CFO) synchronization, time
synchronization, and channel estimation. The implemented CFO synchronization
method adopts the primary synchronization signal (PSS) of LTE. Also, we design
a novel preamble for each uplink user, and it is appended to every frame before
it is transmitted through the air. This preamble will be used for time
synchronization and channel estimation at the BS. Also, a low-complexity,
iterative linear minimum mean squared error (LMMSE) detector has been
implemented for multi-user decoding. The paper also validates the proposed
architecture numerically, as well as experimentally.
