Performance of Synchronized and Unsynchronized Pilots in Finite Massive MIMO Systems

Abstract

Pilot contamination occurs when cells simultaneously transmit the same pilot sequences, creating interference. Unsynchronizing the pilots can reduce pilot contamination, but it can produce data to pilot interference. In this paper, we investigate the impact of pilot contamination and other interference, namely data to pilot interference, on the performance of finite massive MIMO systems with synchronized and unsynchronized pilots. Two unsynchronized pilot schemes are considered. The first is based on the time-shifted pilot scheme in [1], where pilots overlap with downlink data from nearby cells. The second timeshifted method overlaps pilots with uplink data from nearby cells. Results show that if there are small numbers of users, the first time-shifted method provides the best sum rate performance. However, for higher numbers of users, the second time-shifted method has advantage compared to other methods. We also show that time-synchronized pilot is not necessarily the worst case scenario in term of sum rate performance when shadowing effect are considered.

Covariance Matrix Estimation for Massive MIMO

“…It has to be noted that the symbol vector s (n) ℓk can either contain regular 2 , staggered, or superimposed pilots depending on the contents of p ℓk and x (n) ℓk . With staggered pilots, the users in different cells stagger their UL pilot transmissions [18], [19], and with superimposed pilots, the users transmit UL pilots alongside data [11].…”

“…STAGGERED PILOTS The proposed method in Section III can be employed with either regular, superimposed, or staggered pilots. However, we will restrict our attention in this section to staggered pilots since it is a particular case of superimposed pilots and provides an additional degree of freedom by allowing the pilot and data powers to be varied [18]- [20]. The estimated channel will be used in a regularized zero-forcing (RZF) combiner for data detection, which is given as…”

Time Shifted Pilot Signal Transmission With Pilot Hopping To Improve The Uplink Performance of Massive MIMO System For Next Generation Network

“…To improve the performance of MMIMO system, the concept of asynchronous pilot transmission is further carried by other researchers [15], [16], [17] and [18] by using MMSE channel estimation and linear pre-coding techniques such as Matched Filtering and Zero Forcing. Jesper et al [19] introduced a novel scheme in which users transmit pilot sequence synchronously and each user hops to new pilot sequence in each transmission slot.…”

“…In this system, pilot signal transmission is overlapped with the downlink data and channel is estimated using Least Square estimation. The results are analysed and compared with Time-Shifted Pilot signal Transmission (TSPT) [16]…”

“…The Complexity of expectation in equation. 16 is reduced by using Jensen's inequality [16] to calculate estimated achievable uplink data-rate and is given as…”