In order to connect to a network, a terminal device needs to acquire network synchronization information and obtain essential system information. Synchronization signals (SSs) are used for adjusting the frequency of the terminal device relative to the network, and for finding proper timing of the received signal from the network.
In the New Radio (NR) system, a procedure for synchronization and access may involve several signals as follows:
NR-Primary synchronization signal (NR-PSS) that allows for network detection in the presence of a high initial frequency error, up to tens of ppm. Additionally, NR-PSS provides a network timing reference. 3GPP has selected Zadoff-Chu sequences as PSS signals in a Long Term Evolution (LTE) system and m-sequence in the NR system.
NR-Secondary synchronization signal (NR-SSS) that allows for more accurate frequency adjustments and channel estimation while at the same time providing fundamental network information, e.g. cell identifier (ID).
NR-Physical broadcast channel (NR-PBCH) that provides a subset of minimum system information. It will also provide timing information within a cell, e.g. to separate timing between beams transmitted from a cell. The amount of information to fit into the NR-PBCH is of course highly limited to keep the size down. Furthermore, demodulation reference signals (DMRS) are interleaved with NR-PBCH resources in order to receive.
A synchronization signal block (SSB) as proposed for the NR system may comprise the above signals NR-PSS, NR-SSS, NR-PBCH and related DMRS. FIG. 1 shows an illustration of the SSB in which the NR-PBCH is a part of the SSB. In the illustration, two OFDM symbols are reserved for NR-PBCH transmission. The NR-PSS and NR-SSS are defined to be 127 subcarriers wide whereas the NR-PBCH is defined to be 288 subcarriers wide.
A number of SS blocks that are typically close in time constitute an SS burst set. The SS burst set may be repeated periodically, e.g. every 20 ms in default. The terminal device can, by using the SS blocks in the SS burst set, determine downlink timing and frequency offset, and acquire some fundamental system information from the NR-PBCH. It has been agreed that an NR UE in idle mode can expect an SS burst set transmitted once per 20 ms, and the NR UE in connected mode can expect the SS burst sets once per 5 ms. Hence, once the NR UE has obtained downlink synchronization, it knows in which slots to expect the SS block transmissions. The location of the SS block in a SS burst set needs to be provided to the NR UE to derive the subframe level synchronization.