When facing the challenges of wireless communications beyond 5G, single-carrier waveforms appear as a promising enabler for energy-efficient communications thanks to their low PAPR. Among the single-carrier waveforms, DFT-s-OFDM plays a privileged role because of its commonalities with CP-OFDM, albeit its flexibility to allocate control signals in the frequency domain is much more limited. This paper describes a technique for insertion of Reference Signals in the frequency domain based on the use of padding sequences appended to the data before the Transform Precoding stage, followed by direct reference signal insertion after DFT. We prove that the use of padding sequences can trade off PAPR vs. performance through proper sequence design, and that a dynamic framework for sequence selection based on feedback from the receiver can be advantageous. Simulations confirm the suitability of the proposed technique as an enabler for flexibly allocating distributed reference signals with a reduced overhead compared to other standard approaches.

A 5G mobile network has evolved from a cell centric radio access network to a user centric one. A user-centric no-cell (UCNC) framework has been proposed, which enables physical layer coordination over a large number of transmission and reception points for each mobile user to always experience cell-center-like communications. This paper presents some key technologies in the UCNC framework. The focus is on proof of concept of UL grant-free transmissions in the UCNC architecture. Massive connectivity supporting a huge number of devices is one of the three important scenarios in 5G networks. The challenges to support massive connectivity lie in the cost of signaling overhead and transmission latency. An uplink grant-free transmission based on sparse code multiple access (SCMA) design is proposed. The proposed scheme can provide different levels of overloading to efficiently meet the massive connectivity requirements. Grant-free transmission conducted in RRC connected state can reduce signaling overhead, while in energy conserved operation (ECO) state, it can significantly reduce the user plane transmission latency. Extensive laboratory testing has been conducted in a collaborative project of Huawei and Telefonica to verify the performance matrices. Taking LTE as a baseline for comparison purposes, it is shown that the signaling overhead by using uplink grant-free transmission can be reduced by around 80% and the user plane transmission latency in ECO state can be reduced by around 93%. Moreover, massive connectivity is efficiently supported with the SCMA scheme that yields around 230% gain over OFDMA in terms of supported active users.