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RO RACH Occasion

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The Random Access Channel (RACH) is a fundamental component of the Radio Resource Control (RRC) protocol in mobile communication systems such as GSM (Global System for Mobile Communications) and UMTS (Universal Mobile Telecommunications System). The RACH is responsible for establishing the initial connection between a mobile device and the base station, allowing the device to access the network and initiate various types of communication, such as making a phone call or sending data.

In the context of RACH, an "occasion" refers to a specific time interval or window during which a mobile device can access the network. It is used to avoid collisions and ensure orderly access to the network by multiple devices in a cell.

The RACH occasions are divided into two main categories: Access Service Class (ASC) and Access Retry (AR). Let's explore each of them in more detail:

Access Service Class (ASC):

  • Random Access Procedure (RAP): This procedure allows a mobile device to request access to the network by transmitting a random access preamble on the RACH. The preamble is a specific sequence of symbols that helps the base station identify the device.
  • Random Access Channel (RACH): It is a common channel used by multiple mobile devices to send their random access preambles simultaneously. The RACH is divided into different time intervals, known as RACH slots, during which devices can transmit their preambles. The number of available RACH slots in each RACH occasion depends on the specific system configuration and can vary.
  • Contention Resolution Period (CRP): After transmitting the random access preamble, the device waits for a response from the base station. During the CRP, the base station analyzes the received preambles and determines which devices are successfully accessing the network. If multiple devices use the same preamble, a collision occurs, and the base station sends a contention resolution message to those devices, indicating that they need to retransmit their preambles in subsequent RACH occasions.

Access Retry (AR):

  • In some cases, due to collisions or other reasons, a device may not successfully access the network in the ASC RACH occasions. In such situations, the device can utilize the Access Retry (AR) RACH occasions to make additional attempts.
  • The AR RACH occasions are generally provided with less capacity (fewer RACH slots) compared to the ASC RACH occasions. This limitation ensures that devices that fail to access the network in the ASC RACH occasions do not monopolize the RACH, allowing other devices to have fair access opportunities.

The number of RACH occasions and their configuration are determined by the network operator and are influenced by factors such as network capacity, traffic load, and quality of service requirements. The aim is to strike a balance between providing sufficient access opportunities for devices while avoiding excessive collisions and congestion on the RACH.

In summary, the RO RACH occasion refers to the specific time intervals during which a mobile device can access the network using the Random Access Channel. It encompasses the ASC RACH occasions for initial access attempts and the AR RACH occasions for additional access retries if needed. These occasions are designed to facilitate orderly and efficient access to the network by multiple devices, ensuring fair and reliable communication services.