LTE PDSCH Channel: Physical Downlink Shared Channel Explained

This page describes the LTE PDSCH channel, also known as the Physical Downlink Shared Channel, within the context of the LTE system. The PDSCH channel is the primary channel for carrying data, dynamically and opportunistically allocated to users.

The PDSCH is also responsible for transmitting broadcast information that isn’t carried on the PBCH (Physical Broadcast Channel). This includes System Information Blocks (SIBs) and paging, as well as RRC (Radio Resource Control) signalling messages. Furthermore, application data is transferred via the PDSCH.

• Paging messages: These are broadcast using the PDSCH channel. An LTE UE (User Equipment) in RRC IDLE mode monitors the PDCCH (Physical Downlink Control Channel) for paging indications. Upon trigger, it decodes the paging message carried in the PDSCH RBs (Resource Blocks).
• Downlink RRC Signalling messages: The PDSCH carries these messages. Signalling Radio Bearers (SRB) utilize the PDSCH. Each connection typically has its own set of SRBs.

The LTE PDSCH channel employs QPSK, 16QAM, and 64QAM modulation schemes. The LTE eNodeB (evolved NodeB, or base station) selects the appropriate modulation type using an adaptation algorithm. This selection depends on the radio channel conditions and buffer capacity. QPSK, being the most robust scheme, is used for transmitting SIBs and Paging.

As the name suggests, PDSCH is a shared channel, meaning its RBs are shared among all active connections.

LTE PDSCH channel Reference: 3GPP TS 36.211