Apparatus and method for performing call service fallback in wireless communication system

1 . A method of operating a terminal in a wireless communication system, the method comprising:

camping on a first base station belonging to a first network;

receiving a first message including information related to fallback for a call service from the first base station;

transmitting a second message requesting the fallback for the call service to the first base station based on channel quality of the first base station being less than a threshold determined based on the information; and

transmitting a third message to a second base station belonging to a second network,

wherein the information related the fallback comprises information related to a range of the threshold for determining whether to perform the fallback for the call service.

2 . The method of claim 1 , further comprising:

receiving configuration information related to recording of measurement values of channel quality from the first base station.

3 . The method of claim 1 , further comprising:

recording measurement values of channel quality of the first base station.

4 . The method of claim 3 , further comprising:

determining the threshold based on the measurement values and the range.

5 . The method of claim 4 , wherein the determining the threshold comprises, determining the threshold using a Bayesian optimization (BO) technique using an observation including a paging success probability per threshold used in the past and an object function set based on the measurement values and the range.

6 . The method of claim 5 , wherein a Gaussian process is used as a surrogate function for the BO technique, and

wherein an expected improvement (EI) is used as an acquisition function for the BO technique.

7 . The method of claim 1 , further comprising:

transmitting a fourth message to the first base station without the fallback, in case that the channel quality of the first base station is greater than or equal to the threshold.

8 . The method of claim 7 , further comprising:

recording whether paging of the threshold is successful based on whether to maintain the call service in the first network.

9 . The method of claim 7 , wherein the call service provided by the first network comprises a call service in which text data generated from voice data and assistance data representing acoustic characteristics of the voice data are transmitted and received.

10 . The method of claim 9 , wherein the assistance data is transmitted based on a bitrate determined according to channel quality between the first base station and the terminal, and

wherein the text data is transmitted based on a fixed bitrate.

11 . The method of claim 9 , wherein the assistance data includes a style embedding vector representing at least one of a pitch, stress or speed of the voice data.

12 . The method of claim 9 , further comprising:

receiving, from the first base station, information related to a network model for inferring voice data based on the text data and the assistance data.

13 . A method of operating a base station in a wireless communication system, the method comprising:

receiving a first message indicating that a terminated call has occurred from an upper node to a terminal;

transmitting, to the terminal, a second message including information indicating a range of a threshold for determining whether to perform fallback of a call service; and

receiving, from the terminal, a third message requesting the fallback or a paging response message corresponding to the second message.

14 . A terminal in a wireless communication system, the terminal comprising:

a transceiver; and

a processor connected to the transceiver,

wherein the processor is configured to:

camp on a first base station belonging to a first network;

receive a first message including information related to fallback for a call service from the first base station belonging to the first network;

transmit a second message requesting the fallback for the call service to the first base station based on channel quality of the first base station being less than a threshold determined based on the information; and

transmit a third response message to a second base station belonging to a second network,

wherein the information related the fallback comprises information related to a range of the threshold for determining whether to perform the fallback for the call service.