Orthogonal uplink Reference Signal design via subgroup-wise shifts of a CAZAC sequence

In the Single-Carrier Frequency Division Multiple Access (SC-FDMA) system, the orthogonality of uplink Reference Signals (RS) via Code Division Multiplexing (CDM) could be deteriorated by unused guard subcarriers and the consequent time-domain oversampling of Constant Amplitude Zero Auto-correlation (CAZAC) sequence. As a result, the conventional CDM RS structure is sensitive to power control and near-far effect. In this paper, we propose a novel orthogonal uplink RS design via subgroup-wise shifts of a base CAZAC sequence. By using UE-specific shifts and subgroup-specific linear modulation, the RSs of different subgroup will not interfere with each other even in the presence of zero subcarriers. Moreover, by classifying user equipments (UEs) into the different subgroups based on their power levels, the proposed scheme is more robust to imperfect power control and near-far effects.