Brief Summary
Alright, so this video is all about the RACH (Random Access Channel) procedure in LTE networks, bhai. It explains why RACH is needed, when it happens, the different types, and how preambles are generated and transmitted. Key takeaways include:
- RACH is essential for uplink synchronization and getting resources for RRC connection requests.
- It occurs during initial access, handover, and radio link failure.
- There are two types of RACH: contention-based and contention-free.
- Preambles are unique signatures used for initial access, and their format affects the maximum cell radius.
- Parameters like root sequence index, high-speed flag, and zero correlation zone are crucial for generating preambles.
Introduction to RACH Procedure
The video starts with a basic intro to the RACH procedure in LTE. RACH stands for Random Access Channel procedure. It's important to understand what RACH is and why it's needed in LTE networks. The video will cover when RACH happens, the types of RACH, and details about preambles.
Why RACH is Required
RACH is needed for two main reasons. First, it helps achieve uplink synchronization between the UE (User Equipment) and the eNodeB (base station). Second, it's used to get the resources needed to send the RRC (Radio Resource Control) connection request, which is message 3 in the connection process.
When RACH Occurs
RACH happens in a few key situations. It's used during initial access when the UE is first switched on. It also occurs during handover, when the UE switches from one cell to another. Additionally, RACH is used in case of radio link failure, to re-establish the connection.
Types of RACH Procedures
There are two types of RACH procedures: contention-based and contention-free. Contention-based RACH is used during initial access, where multiple UEs might try to access the network at the same time, leading to potential collisions. Contention-free RACH is used during handover, where a dedicated preamble is assigned to the UE to avoid contention.
Understanding Preambles
Preambles are unique signatures that the UE uses to start the uplink synchronization process. There are 64 preambles defined in LTE. A preamble consists of a cyclic prefix (CP) and a sequence. The cyclic prefix helps mitigate inter-symbol interference.
Preamble Formats and Structure
There are four preamble formats (0-3) for FDD and one (4) for TDD. The length of the cyclic prefix and the sequence vary depending on the format. The total length of the preamble is the sum of the CP length and the sequence length. A guard time is added to the end of the preamble to prevent overlap between transmissions from different UEs.
Preamble Format and Cell Radius
The preamble format affects the maximum cell radius. For example, preamble format 0 allows a maximum radius of about 14 km. Other formats (1, 2, 3) are used in areas with high interference or where the eNodeB distance is greater. TS (basic time unit) is derived from the sampling rate (30.72 MHz for a 20 MHz system bandwidth).
When and Where to Transmit RACH
The eNodeB reserves specific resource blocks for RACH. Two important parameters are the RACH configuration index and the RACH frequency offset. The RACH configuration index (0-63) determines the preamble format and the system frame number (SFN) and subframe for transmission. The RACH frequency offset specifies the starting resource block for the RACH transmission in the frequency domain.
Generating 64 Preambles
The 64 preambles are generated using parameters like root sequence index, high-speed flag, and zero correlation zone. The root sequence index indicates which Zadoff-Chu (ZC) sequence the cell uses. The zero correlation zone controls how many cyclic shifts a UE creates from one ZC sequence. The high-speed flag helps differentiate between slow and fast-moving UEs.
Root Sequence Index and Cyclic Shift
The root sequence index is the starting point for generating the preamble sequence. The zero correlation zone defines the cyclic shift between sequences. The high-speed flag indicates whether the UE is moving slowly or quickly, affecting the choice of cyclic shift.
Mapping Root Sequence and NCS Value
The 3GPP table maps logical root sequences to physical root sequences (mu). For example, a root sequence index of 100 corresponds to a physical root sequence of 636. The zero correlation zone configuration (NCS) and high-speed flag determine the cyclic shift interval.
RACH Occupied Frequency
RACH occupies 6 Physical Resource Blocks (PRBs). Each PRB has 12 subcarriers, and each subcarrier is 15 kHz. The total occupied frequency is about 1.048 MHz, which is approximately 1.4 MHz as per the specifications.
RACH Procedure Messages
The RACH procedure involves five messages. Message 1 is the RACH preamble sent by the UE, carrying the RA-RNTI (Random Access Radio Network Temporary Identifier). Message 2 is the Random Access Response sent by the eNodeB, including the RA-RNTI, frequency allocation, RB assignment, MCS configuration, and an Uplink Grant. Message 3 is the RRC Connection Request sent by the UE, including the UE identity and establishment cause. Message 4 is the RRC Connection Setup message sent by the eNodeB, where contention resolution occurs. Message 5 is the RRC Connection Setup Complete message sent by the UE, including the attach request and PDN connectivity request.
Contention Resolution and Parameters
Contention resolution happens in message 4, where the eNodeB distinguishes between multiple UEs that might have sent the same preamble. RACH-related parameters include the number of RA preambles, power ramping parameters, preamble transmission maximum, RA response window size, and MAC contention resolution timer. The Preamble configuration includes the root sequence index, preamble configuration index, high-speed flag, zero correlation zone, and preamble frequency offset.
