Design of Orthogonal Uplink Pilot Sequences for TDD Massive MIMO under Pilot Contamination

—Massive MIMO has been acknowledged as a promising technology to counter the demand for higher data capacity for wireless networks in 2020 and beyond. However, each Base Station (BS) requires good enough knowledge of Channel State Information (CSI) on both the uplink and the downlink as massive MIMO relies on spatial multiplexing. In Time Division Duplex (TDD) massive MIMO systems, this CSI is acquired using channel reciprocity. However, the use of nonorthogonal uplink pilot sequence due to limited coherence time leads to pilot contamination in TDD massive MIMO systems that results in inter-cell interference in the downlink data transmission. This paper proposes a design of orthogonal uplink pilot sequences for multi-cell TDD massive MIMO systems. We propose to use Zadoff-Chu Pilot Sequences (ZCPS) and eliminate pilot contamination during channel estimation process. In the proposed design, each BS is assigned with a specific orthogonal code and the set of ZCPS is multiplied element-wise at each BS with BS-specific orthogonal code to generate orthogonality among pilot sequences across neighboring cells. The proposed design eliminates pilot contamination during channel estimation process thus achieves significant sum-rate gains as verified by the simulation results.