An Introduction to 5G

9Random Access

The mobile uses the random access procedure if it needs to contact a 5G cell but has no means of doing so using the physical uplink control channel (PUCCH) or the physical uplink shared channel (PUSCH). There are several triggers. In the state of RRC_CONNECTED, the mobile uses random access during the procedures for handover and dual connectivity to initialize its timing advance. Other applications include recovery from the interruption of a spatially filtered beam. During standalone operation, the mobile also uses random access for initial access to a primary cell from the 5G states of RRC_INACTIVE and RRC_IDLE, while other applications include requests for the transmission of system information.

The procedure begins with an uplink transmission on the physical random access channel (PRACH), and continues with the ensuing signalling. The later parts of the procedure anticipate issues that we will cover in Chapter 11, so we will highlight those towards the end of the chapter.

9.1 Physical Random Access Channel

9.1.1 PRACH Formats

The mobile begins the random access procedure by transmitting one or more preambles on the physical random access channel [1]. A PRACH preamble has its own subcarrier spacing, which can be different from that of the other uplink channels, and which is the reciprocal of the PRACH symbol duration.

There are two types of preamble. Long PRACH preambles occupy 839 subcarriers with a spacing of 1.25 or 5 kHz, and are only used in frequency ...