What are the different types of RACH in 5G?
There are two main types of RACH procedures in 5G: Contention-Based RACH (CBRACH) and Non-Contention-Based RACH (NCRACH).
CBRACH is used for small control-plane data transmissions and relies on a contention-based approach, where devices may collide when accessing the network. NCRACH, on the other hand, is designed for larger data-plane transmissions and eliminates contention by assigning specific preambles to devices. Additionally, there are synchronous and asynchronous versions of CBRACH, offering different trade-offs between contention and latency, allowing for flexibility in network access based on specific requirements.
Let’s delve into the details of each:
Contention-Based RACH (CBRACH):
CBRACH is primarily used for initial access and for transmitting small data packets, often referred to as “control-plane” communication.
It is contention-based because multiple user devices may attempt to access the network simultaneously. When multiple devices transmit their requests at the same time, collisions may occur.
To minimize collisions, CBRACH uses a mechanism similar to the ALOHA protocol. Devices select a random access preamble and transmit it. If there is no collision, the network responds with a contention resolution message, and the device proceeds with the establishment of the connection.
CBRACH is suitable for scenarios with low to moderate traffic and is efficient for small data transmissions.
Non-Contention-Based RACH (NCRACH):
NCRACH, on the other hand, is used for “data-plane” communication, which typically involves transmitting larger data packets.
Unlike CBRACH, NCRACH avoids contention and collisions. When a device needs to transmit data, it selects a reserved preamble that is assigned to it by the network. This eliminates the need for contention resolution.
NCRACH is well-suited for scenarios with high data traffic and reliability requirements. It ensures that devices can transmit data without the risk of collisions.
In addition to these primary RACH types, there are also two forms of CBRACH in 5G:
Synchronous CBRACH:
In this type of CBRACH, devices follow a predefined schedule for accessing the channel. This reduces the chances of contention and collisions but may introduce some latency.
Asynchronous CBRACH:
Asynchronous CBRACH allows devices to access the channel without a strict schedule, which can result in more contention and potential collisions. However, it offers lower latency compared to synchronous CBRACH.
The choice between CBRACH and NCRACH depends on the specific use case and network requirements. CBRACH is efficient for low to moderate traffic and latency-tolerant applications, while NCRACH is preferred for high-data-rate and low-latency applications where collision avoidance is critical. The synchronous and asynchronous CBRACH modes provide flexibility in balancing contention and latency based on the network’s needs.