1. Random Access Procedure

When we switch ON our smartphone for the very first time, it will start to search for network. We are always surrounded by many networks and there are many frequencies from different operators in the air to which our UE can connect. Therefore the UE  needs to synchronize to each frequency and check whether this is frequency from the right operator to which it wants to connect to. This is done through initial synchronisation process. Once synchronized the UE reads the master information block(MIB) and System information blocks(SIB) to check whether this is the right PLMN. When the PLMN is right then UE proceeds to read other SIB's on that cell.

LTE Cell selection/re-selection

 

The Base station continuously broadcast MIB's and SIB's on BCCH. BCCH contains information about the location of the downlink and uplink carrier configuration. When the UE is powered ON, it has to first synchronize with the network at the frame and slot level.

  • The Primary Synchronization Signal (PSS) is responsible for slot synchronization.

  • The Synchronization Signals are sent only on time slots (1st and 11th) of each frame.

  • PSS is send at the last OFDM symbol of the 1st and 11th slot of each frame.

  • Once, when the PSS is identified, Secondary Synchronization Signal (SSS) is always sent at the slot before the PSS is present. The SSS is responsible for frame synchronization.

  • Using both these signals, the UE derives the Physical Cell ID of the corresponding cell.

From PSS: Physical Layer Cell Identity is derived. It carries the values 0, 1 and 2.

From SSS: Physical Layer Cell Identity Group is derived. It can take the value from 0 to 167.

Formula:

Cell ID = (3 x Physical Layer Cell Identity Group) + Physical Layer Cell Identity
 

Once the UE synchronizes with the network, it requires PLMN Id, Tracking Area Id, Cell Id, Radio and Core network capabilities for camping on a cell. These information is transmitted in MIB and SIB's of the cell so the UE proceeds to read them. After the above process the UE is synchronized with the network in the Downlink direction and have read SIB1 and SIB 2. Now, it needs to synchronize in the Uplink direction. For this to happen, RAP (Random Access Procedure) is initiated.

 

There are two types of Random Access Procedure(RAP),

  1. Contention based Random Access Procedure

  2. Non-contention based Random Access Procedure

 

1.1 Contention based Random Access Procedure

In contention based multiple UE's attempt to connect to the network at the same time. As every UE is unique to the network so the eNB is intelligent enough to tackle this situation. The UE's can always send the same Preamble ID to the network, thereby resulting in collisions. This kind of collision is called "Contention" and is known as "Contention based" RACH Process. The network would go through additional process to resolve these contentions and hence this process is called "Contention Resolution".

  1. In the first message the UE sends Random Access Preamble to the network about its resource requirement which includes the Preamble ID and RA-RNTI.

  2. The eNB conveys the resources reserved for this UE along with the Timing Advance (TA), Preamble ID and T-CRNTI (a number generated by eNB and asks the UE to send the RRC connection).

  3. UE sends the RRC connection Request using resources given by the eNB. It also sends the identifier (CRI) to the eNB which is used to resolve the Contention.

  4. The eNB runs an algorithm and generates C-RNTI which will be a permanent ID for the UE till the connection is alive. The eNB sends the UE identifier. In this step, the UE which has received the ID continues while other UE's will back off and try again.

The Contention-based RACH procedure is used in four cases:

  • Initial access from RRC_IDLE state or radio failure 

  • Handover requiring random access procedure 

  • DL or UL data arrival during RRC_CONNECTED after UL PHY has lost synchronization

  • UL data arrival when are no dedicated scheduling request (PUCCH) channels available

1.2 Non-contention based Random Access Procedure

The non-contention based RACH procedure is used only during handover and DL data arrival. This procedure is always initiated from network in case of a handover. For this procedure, the eNB reserves a set of preamble sequence. When this type of scenario is encountered the eNB allocates the set from this reserved pool. This entire procedure is controlled by the eNB. Hence there is no question of collision.

 

In "Random access preamble assignment" the eNB assigns a 6 bit preamble code. In "Random access preamble" the UE transmits the assigned preamble. the eNB accepts the preamble via "Random access response".

2. Backoff indicator

Backoff indicator is a part of MAC sub header parameter and it is indicating the time delay between two PRACH. The UE shall send another PRACH under the following circumstance,

  • UE sent a PRACH but didn’t get Random Access Reponse for some reason

  • The preamble sent by the UE was not detected by the eNB

  • UE sent PRACH and got Random Access Reponse from network but preamble ID is not for the UE (UE lost the contention resolution)

 

The eNB can force the UE to wait a certain time before it tries to send another PRACH. The maximum length of the backoff time is signaled to the UE by the eNB with the backoff parameter. The backoff parameter is sent in the Random Access Reponse and all Random Access Reponses can however be read by all UEs who sent a preamble in step 1 of the random access procedure even though the UE did not get a random access response with its own preamble and use the backoff parameter.

3. Cell Selection Criteria

UE would not start registration even though it has successfully detected a cell and even decoded MIB and SIB's unless the Srxlev > 0 and Squal > 0.

For a cell to be selected, they should meet a certain criteria as follows.

When UE is in HPLMN

Squal = Qqualmeas – Qqualmin > 0

Srxlev = Qrxlevmeas – (Qrxlevmin + QrxlevminOffset) – Pcompensation > 0

 

When UE is in VPLMN

Squal = Qqualmeas – (Qqualmin + QqualminOffset) > 0

Srxlev = Qrxlevmeas – (Qrxlevmin + QrxlevminOffset) – Pcompensation > 0

Out of the variables used in the equation, only Qrxlevmeas and Qqualmeas is the value UE really measures when it turns ON and most of the other parameters are broadcasted in SIB1.