Method and apparatus for initial access in communication system

An operation method of a base station in a communication system, the operation method comprising:

• transmitting a first synchronization signal block (SSB);
• receiving, from a terminal, a physical random access channel (PRACH) preamble in a random access channel (RACH) occasion (RO) associated with the first SSB in a first radio resource to which a first subcarrier spacing (SCS) is applied;
• determining a position of a slot to which the RO belongs in a reference radio resource to which a reference SCS is applied-instead of the first SCS;
• determining a radio network temporary identifier (RNTI) for a random access (RA) procedure based on a slot index corresponding to the position of the slot in the reference radio resource;
• and
• performing a transmission operation of a response for the PRACH preamble by using the RNTI in the first radio resource.

A terminal in a communication system, the terminal comprising:

• a processor;
• a memory electronically communicating with the processor;
• and
• instructions stored in the memory, wherein when executed by the processor, the instructions cause the terminal to:
• receive, from a base station, a first synchronization signal block (SSB);
• determine an SSB index associated with the first SSB based on a different scheme according to a frequency band in which the base station operates;
• determine a random access channel (RACH) occasion (RO) associated with the SSB index;
• and
• transmit a physical random access channel (PRACH) preamble to the base station in the RO,
• wherein when the base station operates in a first frequency band, the SSB index is determined based on a first equation or a second equation, and when the base station operates in a second frequency band higher than the first frequency band, the SSB index is determined based on the second equation,
• the first equation is [an index of a physical broadcast channel (PBCH) demodulation reference signal (DMRS) mod NSSBQCL], the second equation is [an SSB candidate index mod NSSBQCL], the PBCH DMRS is included in the first SSB, the SSB candidate index is associated with the first SSB, and NSSBQCL indicates a number of SSBs having a quasi-co-located (QCL) relation.