16.1 Synchronization procedures
36.2133GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical layer proceduresRelease 17TS
16.1.1 Cell search
Cell search is the procedure by which a UE acquires time and frequency synchronization with a cell and detects the narrowband physical layer Cell ID.
If the higher layer parameter operationModeInfo indicates ‘inband-SamePCI‘ or samePCI-Indicator indicates ‘samePCI” for a cell, the UE may assume that the physical layer cell ID is same as the narrowband physical layer cell ID for the cell.
The following signals are transmitted in the downlink to facilitate cell search for Narrowband IoT: the narrowband primary and narrowband secondary synchronization signals.
A UE may assume the antenna ports 2000 – 2001 and the antenna port for the narrowband primary/secondary synchronization signals of a serving cell are quasi co-located (as defined in [3]) with respect to Doppler shift and average delay.
16.1.2 Timing synchronization
Upon reception of a timing advance command, the UE shall adjust uplink transmission timing for NPUSCH, and SR if configured with higher layer parameter sr-WithoutHARQ-ACK-Config, based on the received timing advance command.
The timing advance command indicates the change of the uplink timing relative to the current uplink timing as multiples of 16
. The start timing of the random access preamble is specified in [3].
In case of random access response, an 11-bit timing advance command [8], TA, indicates NTA values by index values of TA = 0, 1, 2, …, 1536, where an amount of the time alignment is given by NTA = TA ×16. NTA is defined in [3].
In other cases, a 6-bit timing advance command [8] or the Timing advance adjustment field in DCI format N0 if present [4], TA, indicates adjustment of the current NTA value, NTA,old, to the new NTA value, NTA,new, by index values of TA = 0, 1, 2,…, 63, where NTA,new = NTA,old + (TA −31)×16. Here, adjustment of NTA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing by a given amount respectively.
For a timing advance command reception ending in DL subframe n, the corresponding adjustment of the uplink transmission timing shall apply from the first available NB-IoT uplink slot following the end of n+12 DL subframe and the first available NB-IoT uplink slot is the first slot of a NPUSCH transmission. When the UE’s uplink NPUSCH transmissions in NB-IoT uplink slot n and NB-IoT uplink slot n+1 are overlapped due to the timing adjustment, the UE shall complete transmission of NB-IoT uplink slot n and not transmit the overlapped part of NB-IoT uplink slot n+1.
If the received downlink timing changes and is not compensated or is only partly compensated by the uplink timing adjustment without timing advance command as specified in [10], the UE changes NTA accordingly.
For a UE in a NTN serving cell, using serving satellite higher-layer ephemeris parameters, if configured, the UE determines (defined in [3]) using the serving satellite position and its own position to pre-compensate the two-way transmission delay on the service link. To pre-compensate the two-way transmission delay between the uplink time synchronization reference point and the serving satellite, the UE determines (defined in [3]) based on one-way propagation delay which can be obtained as:
where , , and are given by the higher layer parameters nta-Common, nta-CommonDrift, and nta-CommonDriftVariation respectively, and is the epoch time given by the higher layer parameter epochTime. provides a distance at time between the serving satellite and the uplink time synchronization reference point divided by the speed of light. The uplink time synchronization reference point is the point where DL and UL are frame aligned with an offset given by .
For a NB-IoT UE communicating over NTN, time and frequency pre-compensation is adjusted per uplink segment with a transmission duration of time units, where the quantity is provided by system information, as specified in 3GPP TS 36.331.