  This paper introduces a hybrid pilot-aided channel estimation technique for
mitigating the effect of pilot contamination for the uplink of multi-cell
multiuser massive MIMO systems. The proposed hybrid pilot is designed such that
it enjoys the complementary advantages between time-multiplexed (TM) pilot and
time-superimposed (TS) pilot, and thereby, allows superior solution to the
conventional pilot schemes. We mathematically characterize the impact of hybrid
pilot on the massive MIMO uplink by deriving a closed-form approximation for
the uplink achievable rate. In large-number-of-antennas regime, we obtain the
asymptotically optimal solution for hybrid pilot by jointly designing the TM
pilot and the TS pilot. It is shown that either TM pilot or TS pilot has the
advantages for large frame-size and limited frame-size transmission,
respectively, while the hybrid pilot scheme can offer a superior performance to
that employing either TM pilot or TS pilot. Numerical results demonstrate the
effectiveness of the proposed design.
