5G NR- Initial Access/ RACH

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Initial Access means a sequence of processes between UE and gNB (network) for efficiently and dynamically UE to acquire uplink synchronization and obtain a specific ID for radio access communications. The initial access is referred to as RACH(Random Access Channel) procedure. The initial access may mean ‘Downlink Synchronization + RACH’. The main purpose of RACH is to achieve UPlink synchronization between UE and eNB and obtain the resource for message 3 (e.g RRC connection request). When we want to connect a UE to a 5G network, it has to synchronize in downlink as well as in uplink. Downlink synchronization is obtained after successfully decoding SSB, to establish uplink synchronization and RRC connection, UE has to perform RACH random access procedure. 

Random Access Procedure is performed in the following five events:

• Initial Access from RRC_IDLE
• RRC connection re-establishment procedure
• Handover
• DL data arrival during RRC_CONNECTED requiring random access procedure
• UL data arrival during RRC_CONNECTED requiring random access procedure

Types of RACH Procedure 

• Contention based Random Access (CBRA)
• Non Contention or contention-free random access (CFRA) 

Contention-based Random Access

In contention-based random access, UE selects a preamble randomly from a pool of preambles shared with other UEs. UE has a potential risk of selecting the same preamble as another UE and may experience conflict or contention. In contention-based, multiple UE’s attempt to connect to the network at the same time. The eNB is intelligent enough to tackle this situation because every UE should be unique to the network. 

The UE can always send the same preamble ID to the network, thereby resulting in collisions. This type of collision is called “contention” and is known as the “contention-based” RACH process. The network has to go through additional processes to resolve these contentions and this process is known as contention resolution. 

Procedure

In total, there are 64 preambles per cell. The 64 preambles are grouped to indicate the length of the needed resource. The preambles are reserved for contention-free access. 

• A preamble will be selected by UE and transmitted in the available subframe. Based on the correlation, it may detect the access and can measure the timing of the UE transmission. 
• The information on the scheduled resource will be exchanged and a temporary C-RNTI will be assigned. 
• The UE sends its ID. The type of ID depends on the state. 
• The contention resolution is performed

Non Contention or contention-free random access

 In this, the preamble is allocated by the gNB and such preambles are also known as dedicated random access preamble. It is provided to UE either via RRC signaling or Physical layer signaling. when the dedicated resources are insufficient, the gNodeB instructs UEs to initiate contention-based RA. It is also a three-step procedure.   

• Random Access Preamble Assignment
• Random Access Preamble Transmission (msg1) 
• Random Access Response