Advanced frequency synchronization in a mobile integrated access backhaul deployment

1. An apparatus, comprising:

at least one processor; and

at least one memory including computer program code;

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to:

estimate a change in a propagation delay at two time instants;

measure a time period of the two time instants using a local clock of a network node, wherein the two time instants correspond to a known timing of a received signal;

subtract the change in the propagation delay from a measured time period Δt;

calculate a difference between a subtracted time period and the known timing of the received signal; and

convert the difference to a frequency offset value Δf to synchronize downlink transmission frequency between the network node and a parent node.

2. The apparatus of claim 1 , wherein estimating the change in the propagation delay at two time instants comprises determining a timing offset due to doppler t drift_D by subtracting a propagation delay T P (t 2 ) at a second time instant t 2 from a propagation delay T P (t 1 ) at a first time instant t 1 .

3. The apparatus of claim 1 , wherein the propagation delay T P (t 1 ) at a first time instant t 1 is determined based on a first timing advance value TA 1 and a first reception-transmission timing parameter T_delta 1 signaled from a parent node at or near the first time instant t 1 , and the propagation delay T P (t 2 ) at a second time instant t 2 is determined based on a second timing advance value TA 2 and a second reception-transmission timing parameter T_delta 2 signaled from the parent node at or near the second time instant t 2 .

4. The apparatus of claim 3 , wherein: the propagation delay T P (t 1 ) at the first time instant t 1 is given by the formula T P (t 1 )=TA 1 /2+T_delta 1 , wherein T_delta 1 =− 1/2 *Tg, wherein Tg is a reception-transmission gap at the parent node; and

the propagation delay T P (t 2 ) at the second time instant t 2 is given by the formula T P (t 2 )=TA 2 /2+T_delta 2 , wherein T_delta 2 =− 1/2 *Tg.

5. The apparatus of claim 3 , wherein:

TA 1 =N′ TA1 *T c , wherein N′ TA1 is an integer adjusted by a first timing advance command signaled by the parent node at the first time instant t 1 , and wherein T c is a basic time unit; and

TA 2 =N′ TA2 *T c , wherein N′ TA2 is an integer adjusted by a second timing advance command signaled by the parent node at the second time instant t 2 .

6. The apparatus of claim 5 , wherein N′ TA1 and N′ TA2 are adjusted by the network node during an autonomous correction procedure to generate values N TA1 and N TA2 , respectively, and wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus at least to filter a selection of the first timing advance value TA 1 or the first reception-transmission timing parameter T_delta 1 , or the second timing advance value TA 2 or the second reception-transmission timing parameter T_delta 2 .

7. The apparatus of claim 3 , wherein the first time instant t 1 and the second time instant t 2 are selected by the network node.

8. The apparatus of claim 1 , wherein determining the known timing of the received signal comprises estimating a total timing drift t drift_total of the received signal between a first time instant t 1 and a second time instant t 2 using a local clock reference of the network node.

9. The apparatus of claim 8 , wherein calculating the difference between the subtracted time period and the known timing of the received signal comprises determining a timing change t drift_ref_clock caused by an offset in frequency of the local clock reference by subtracting a time offset due to doppler t drift_D from the total timing drift t drift_total .

10. The apparatus of claim 9 , wherein the frequency offset value Δf of a local clock reference is determined based on the timing change t drift_ref_clock caused by the offset in frequency of the local clock reference, the measured time period Δt of the two time instants using a local clock of the network node, and a frequency of the local clock reference f c .

11. The apparatus of claim 1 , wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus at least to synchronize a downlink transmission timing between the network node and the parent node based on the frequency offset value.

12. The apparatus of claim 1 , wherein the frequency offset value Δf of the local clock reference is given by the formula Δf=t drift_ref_clock /Δt*f c , wherein t drift_ref_clock is a timing change caused by an offset in frequency of the local clock reference, Δt is the measured time period of the two time instants using a local clock of the network node, and f c is a frequency of the local clock reference.

13. The apparatus of claim 1 , wherein:

a first time instant t 1 is a time after receiving a first timing advance value TA 1 and a first reception-transmission timing parameter T_delta 1 , such that the propagation delay T P (t 1 ) approximates an actual propagation delay; and

a second time instant t 2 is a time after receiving a second timing advance value TA 2 and a second reception-transmission timing parameter T_delta 2 , such that the propagation delay T P (t 2 ) approximates the actual propagation delay.

14. The apparatus of claim 1 , wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus at least to measure, by the network node, an exact time drift over a number of frames of a received downlink signal.

15. The apparatus of claim 1 , wherein the operations are performed while the network node is moving.

16. The apparatus of claim 1 , wherein a mobile termination part and a distributed unit part of the network node are running with fixed timing using a local oscillator.

17. An apparatus, comprising:

at least one processor; and

at least one memory including computer program code;

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to:

estimate, by a network node, a frequency offset due to a network node movement corresponding to doppler;

estimate, by the network node, a frequency offset due to an offset in a local clock reference of the network node;

utilize, by the network node, information signaled from a parent node while adjusting a timing reference of the network node;

wherein the information signaled from the parent node indicates the timing reference and/or the frequency reference has been adjusted at the parent node; and

not adjust, by the network node, a frequency of the network node based on a timing advance value and a reception-transmission timing parameter in response to the information signaled from the parent node indicating that the parent node has changed a timing of the parent node,

wherein estimating the frequency offset due to the network node movement corresponding to doppler t drift_D comprises subtracting a propagation delay T P (t 2 ) at a second time instant t 2 from a propagation delay T P (t 1 ) at a first time instant t 1 .

18. The apparatus of claim 17 , wherein the frequency offset due to the network node movement corresponding to doppler t drift_D is determined in part based on a propagation delay T P (t 1 ) at a first time instant determined based on a first timing advance value TA 1 and a first reception-transmission timing parameter T_delta 1 signaled from the parent node at the first time instant, and a propagation delay T P (t 2 ) at a second time instant determined based on a second timing advance value TA 2 and a second reception-transmission timing parameter T_delta 2 signaled from the parent node at the second time instant.

19. An apparatus, comprising:

at least one processor; and

at least one memory including computer program code;

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to:

estimate, by a network node, a frequency offset due to a network node movement corresponding to doppler t drift_D ;

estimate, by the network node, a frequency offset due to an offset in a local clock reference of the network node;

utilize, by the network node, information signaled from a parent node while adjusting a timing reference of the network node;

wherein the information signaled from the parent node indicates the timing reference and/or the frequency reference has not been adjusted at the parent node; and

adjust, by the network node, a frequency of the network node based on a timing advance value and a reception-transmission timing parameter in response to the information signaled from the parent node indicating that the parent node has not changed a timing of the parent node,

wherein estimating the frequency offset due to the network node movement corresponding to doppler t drift_D comprises subtracting a propagation delay T P (t 2 ) at a second time instant t 2 from a propagation delay T P (t 1 ) at a first time instant t 1 .