Spectral and Energy Efficiency of Superimposed Pilots in Uplink Massive MIMO

TL;DR

This paper compares superimposed pilots and regular pilots in uplink Massive MIMO, showing that optimized regular pilots can achieve similar spectral and energy efficiency as superimposed pilots despite their different approaches to pilot contamination.

Contribution

It provides a rigorous analysis of superimposed pilots in multicell Massive MIMO, deriving closed-form achievable rates and comparing them with regular pilots under practical deployment scenarios.

Findings

Optimized regular pilots achieve comparable spectral efficiency to superimposed pilots.

Superimposed pilots' reduction of pilot contamination is offset by increased interference.

Both methods can be optimized to reach similar energy efficiency in practical scenarios.

Abstract

Next generation wireless networks aim at providing substantial improvements in spectral efficiency (SE) and energy efficiency (EE). Massive MIMO has been proved to be a viable technology to achieve these goals by spatially multiplexing several users using many base station (BS) antennas. A potential limitation of Massive MIMO in multicell systems is pilot contamination, which arises in the channel estimation process from the interference caused by reusing pilots in neighboring cells. A standard method to reduce pilot contamination, known as regular pilot (RP), is to adjust the length of pilot sequences while transmitting data and pilot symbols disjointly. An alternative method, called superimposed pilot (SP), sends a superposition of pilot and data symbols. This allows to use longer pilots which, in turn, reduces pilot contamination. We consider the uplink of a multicell Massive MIMO network using maximum ratio combining detection and compare RP and SP in terms of SE and EE. To this end, we derive rigorous closed-form achievable rates with SP under a practical random BS deployment. We prove that the reduction of pilot contamination with SP is outweighed by the additional coherent and non-coherent interference. Numerical results show that when both methods are optimized, RP achieves comparable SE and EE to SP in practical scenarios.