In this paper, superimposed pilots are introduced as an alternative totime-multiplexed pilot and data symbols for mitigating pilot contamination inmassive multiple-input multiple-output (MIMO) systems. We propose anon-iterative scheme for uplink channel estimation based on superimposed pilotsand derive an expression for the uplink signal-to-interference-plus-noise ratio(SINR) at the output of a matched filter employing this channel estimate. Basedon this expression, we observe that power control is essential whensuperimposed pilots are employed. Moreover, the quality of the channel estimatecan be improved by reducing the interference that results from transmittingdata alongside the pilots, and an intuitive iterative data-aided scheme thatreduces this component of interference is also proposed. Approximateexpressions for the uplink SINR are provided for the iterative data-aidedmethod as well. In addition, we show that a hybrid system with users utilizingboth time-multiplexed and superimposed pilots is superior to an optimallydesigned system that employs only time-multiplexed pilots, even when thenon-iterative channel estimate is used to build the detector and precoder. Wealso describe a simple approach to implement this hybrid system by minimizingthe overall inter and intra-cell interference. Numerical simulationsdemonstrating the performance of the proposed channel estimation schemes andthe superiority of the hybrid system are also provided.
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