Investigation of Two-Dimensional Orthogonal Sequence Mapping to Multi-Layer Reference Signal for LTE-Advanced Downlink

In the LTE-Advanced downlink, to satisfy high-level performance requirements, advanced multi-antenna transmission techniques such as higher-order single-user (SU)-MIMO, multi-user (MU)-MIMO, and coordinated multi-point transmission/reception (CoMP) are key. At the 3GPP RAN WG1 meeting, the demodulation reference signal (DM-RS) was defined to support channel estimation and data demodulation for up to eight transmission layers. A hybrid code division multiplexing (CDM) and frequency division multiplexing (FDM) scheme was adopted as a DM-RS multiplexing scheme. The time-domain orthogonal cover code (OCC) is used for CDM since time domain orthogonality among OCCs is relatively robust against channel variation. However, in a medium-to-high mobility environment, orthogonality distortion occurs among OCCs, which results in performance degradation. In this paper, we propose a two-dimensional (2D) OCC that achieves two-dimensional orthogonality in the time and frequency domains to improve the performance of CDM-based DM-RSs while reducing the peak transmit power of each OFDM symbol where CDM-based DM-RS are mapped. Simulation results show that the 2D OCC is effective in improving the block error rate performance especially in a medium-to-high mobility environment. Furthermore, it is shown that the proposed 2D OCC effectively reduces the peak transmit power compared to the time-domain OCC.