How Transport Channel Work in LTE?

A transport channel defines how and with what characteristics the information is transmitted.

Inherited from the WCDMA, data on the transport channel is organized into “Transport Blocks”, TBs.
A Transport block can be transmitted every TTI = 1 ms

The “Transport Format”, TF, defines how the blocks can be transmitted:

Transport block size, it depends on the MCS and the number of PRB allocated
Allowed modulation scheme
Antenna mapping

Note: In case of multi-antenna system, there can be 2 TBs for each TTI.

Each TTI, the scheduler decides which chunk to allocate to which user. The chunks are not standardized.

The following transport channels in DL have been defined by the 3GPP:

Broadcast Channel (BCH) characterized by a fixed, pre-defined transport format with a robust modulation to be broadcast in the entire coverage area of the cell.
Downlink Shared Channel (DL-SCH) characterized by: A dynamic link adaptation by varying the modulation, coding and transmit power support for H-ARQ (radio retransmission).
Paging Channel (PCH) characterized by: Requirement to be broadcast in the entire cell.
Multicast Channel (MCH) characterized by: requirement to be broadcast in the entire coverage area of the cell

The following transport channels in UL have been defined by the 3GPP:

Uplink Shared Channel (UL-SCH) characterized by: support for dynamic link adaptation by varying the transmit power and potentially modulation and coding , support for H-ARQ ,support for both dynamic and semi-static resource allocation.
Random Access Channel (RACH) characterized by limited control information, collision risk

Introduction to Transport Channels in LTE

In LTE, transport channels are responsible for carrying data between different layers of the protocol stack, from the MAC (Medium Access Control) layer to the physical layer. They play a key role in managing how data is transmitted over the air between your device (UE) and the network.

Types of Transport Channels in LTE

There are several types of transport channels in LTE, each designed for different purposes:

Dedicated Transport Channel (DCH): Used for user data or signaling for a specific user. It’s dedicated to your device when you are actively connected.
Shared Transport Channel (STCH): Used for common data sent to multiple devices, like scheduling information and random access responses.
Broadcast Transport Channel (BCH): Carries system-wide broadcast information, such as cell identity and system parameters.
Multicast Transport Channel (MCH): Used for transmitting multicast data, such as video streams, to multiple users simultaneously.

How Transport Channels Operate

The transport channel ensures that data is packaged and transmitted efficiently over the physical layer. For example, when you’re browsing the internet, your device will use a dedicated transport channel for user data, while signaling or control information may use a shared transport channel. The choice of transport channel depends on the type of data and how it needs to be transmitted.

Role of Scheduling in Transport Channels

Transport channels are also closely tied to scheduling, which is handled by the MAC layer. The scheduler decides when and how much data can be sent on each transport channel based on available resources, ensuring an efficient use of the network.