Algorithms for NR synchronization layer functions (CFO correction, PSS, SSS)

The 5th Generation technology standard for cellular networks is characterized by the development of a new radio-access technology known as New Radio (NR). It exploits many of the structures and features of 4G Long Term Evolution (LTE), but a rethinking of already consolidated transmission techniques was needed to meet new requirements. This thesis studies the innovative synchronization methods for 5G. First of all, the 5G standard defines a new Synchronization Signal Block (SS-Block), i.e. a block consisting of two synchronization signals and one broadcast physical channel. To speed up the initial access process the NR is allowed to use from 8 to 64 synchronization blocks during a variable time interval (not necessarily each radio frame). This leads also to an increasing system complexity. The synchronization procedure at the receiver side is still based on the correlation function but, to maximize performance and to keep low the complexity, an efficient implementation must be found. The starting point consists in the detection of the Primary synchronization signal (PSS) and the Secondary synchronization signal (SSS). It can be achieved by mean of cross correlation of the received signal with local sequence replicas, taken from a predefined set: the real transmitted one is not known a priori. The frequency offset produced by the oscillators non-idealities must be taken into account, since it can disrupt the good correlation properties of the sequences. Choosing when and how to perform its estimation and compensation may strongly affect the performances. Other procedures follow up, using the PBCH (Physical Broadcast CHannel) and the DMRS (Demodulation Reference Signal) inside the SS-Block, to perform channel estimation, to collect BS cell information and more. It must be considered that NR cells are typically composed by antenna arrays, which are able to perform beamforming and beam sweeping. Thus, the choice of the best beam must be provided, in order to maximize the quality of the transmission. This thesis is developed by proposal and in collaboration with TELECOM ITALIA. The company is developing a Link Level simulator for 5G, built with Matlab. Its purpose is to model a radio interface which is compliant with 3GPP specification and to evaluate the link level performances of 5G-based point-to-point communications. To this pre-existing tool, the new functions for the synchronization described above must be added. The simulator tries to model the transmission link as accurately as possible, taking into account the characteristics of the wireless channel and the many impairments that can affect the synchronization procedure. The scope of the thesis is to study and test the techniques for initial synchronization and to evaluate their improvements with respect to LTE. This new algorithms will try to explore the time-frequency characteristics of the m-sequences used inside the SSB. To get even closer to a real application, the software will be used for FPGA implementations, so a easily portable architecture from the simulator to the hardware implementation is needed (e.g. by considering moving from floating point to fixed point notations).