WCDMA/UMTS: TB, TBS, TTI, TFC, TFCS, TFI, and TFCI Explained

This tutorial clarifies the WCDMA (UMTS) basics, expanding on topics like UMTS channels, measurements, the PHY layer, TDD vs. FDD, RRC States, Security, Cell search, Bands, UARFCN, and Frame structure.

This page explains the differences between key WCDMA terms: TB, TBS, TTI, TFC, TFCS, TFI, and TFCI.

Transport Formats/Configurations in WCDMA/UMTS

WCDMA/UMTS technologies utilize specific transport formats and configurations for efficient data transmission. Let’s break down these elements:

Transport Block (TB)

• The fundamental unit of data exchanged between Layer 1 (L1, the physical layer) and the MAC (Medium Access Control) layer for physical layer processing. Think of it as the smallest data package the physical layer can handle.

Transport Block Size (TBS)

• Represents the number of bits contained within a single Transport Block (TB). This dictates the capacity of a TB.

Transport Block Set (TBS)

• A collection (set) of Transport Blocks (TBs) that are exchanged between Layer 1 and the MAC layer simultaneously, utilizing the same transport channel. Basically, it’s a batch of TBs sent together.

Transport Block Set Size

• The total number of bits present within a Transport Block Set (TBS). It reflects the overall data volume within the set.

Transmission Time Interval (TTI)

• Defines the periodicity or interval at which a Transport Block Set (TBS) is transferred by the physical layer onto the radio interface. Common TTI values are {10, 20, 40, 80 ms}.
• The MAC layer delivers one TBS to the physical layer every TTI. This dictates the frequency of data transfer.

Transport Format Combination (TFC)

• This refers to a combination of multiple Transport Channels (TrCHs), each having its own Transport Format (TF).
• It’s the authorized grouping of currently valid TFs that can be submitted to Layer 1 on a Coded Composite Transport Channel (CCTrCH), containing one TF from each TrCH.

Transport Format Combination Set (TFCS)

• A set of Transport Format Combinations (TFCs) that exists on a CCTrCH.
• The Radio Network Controller (RNC) produces the TFCS.
• Layer 3 provides the TFCS to the MAC layer for control purposes.
• The MAC layer then selects from the different TFCs available within the TFCS.
• The MAC layer only has control over the dynamic aspects of the Transport Formats. The semi-static aspects, which relate to Quality of Service (QoS), are controlled by the RNC’s admission control.
• The bit rate can be rapidly adjusted by the MAC layer without requiring Layer 3 signaling.

Transport Format Indicator (TFI)

• A label or identifier that corresponds to a specific Transport Format (TF) within a Transport Format Set (TFS).
• This is used for communication between the MAC layer and Layer 1. It tells L1 which format is being used.

Transport Format Combination Indicator (TFCI)

• Used to inform the receiving side (e.g., the mobile device or the base station) about the currently valid Transport Format Combination (TFC) being used. It lets the receiver know how to decode the data.