Traffic Channel / Adaptive Multi-Rate Half Rate Speech

Description

The TCH/AHS is a fundamental logical channel in the GSM system, dedicated to carrying user speech traffic. It operates on a half-rate configuration, meaning it utilizes only half of the standard GSM traffic channel's physical resource (one timeslot every other frame) to transmit a voice call. The core technology enabling this is the Adaptive Multi-Rate (AMR) speech codec. The AMR codec is not a single fixed bitrate; instead, it defines a set of eight source codec modes with bitrates ranging from 4.75 kbit/s to 12.2 kbit/s. The TCH/AHS channel pairs these source rates with corresponding channel coding schemes to create a set of 'codec modes' optimized for different channel conditions.

The operation of TCH/AHS is characterized by its link adaptation and codec mode adaptation mechanisms. Link adaptation selects the most robust combination of speech coding and channel coding based on real-time measurements of channel quality, such as the Bit Error Rate (BER). In poor radio conditions, a lower source rate (e.g., 4.75 kbit/s) is used, freeing up more bits for powerful channel coding (e.g., convolutional coding) to protect the speech data. In excellent conditions, a higher source rate (e.g., 7.95 kbit/s) can be used to deliver superior speech quality. Codec mode adaptation can be controlled by the network based on measurements and traffic load.

Architecturally, the TCH/AHS is mapped onto physical channels in the GSM TDMA structure. It uses a 26-frame multiframe, where specific frames are allocated for the half-rate traffic, and others for associated control channels (SACCH) and idle frames. The channel supports both voice activity detection (VAD) and discontinuous transmission (DTX), which further conserve mobile terminal battery life and reduce overall interference in the network by transmitting only when the user is speaking. The TCH/AHS is a key component in the GSM radio resource management strategy, allowing network operators to trade off speech quality for increased network capacity dynamically.

Purpose & Motivation

The TCH/AHS was introduced to significantly increase the voice capacity of GSM networks without requiring additional spectrum or cell sites. Prior to AMR, GSM used fixed-rate codecs like the Full-Rate (FR) and Half-Rate (HR) codecs, which offered a binary choice between good quality (FR) or double capacity with noticeably lower quality (HR). The fixed HR codec often resulted in poor speech quality that was unacceptable for many users, limiting its practical deployment.

The introduction of the Adaptive Multi-Rate codec, and by extension the TCH/AHS channel, solved this trade-off. It was motivated by the need for a more intelligent system that could adapt to radio conditions. The primary problem it solves is spectral efficiency for voice services. By using a half-rate physical channel and an adaptive codec, it effectively doubles the number of simultaneous calls per radio carrier compared to a full-rate TCH. Furthermore, its adaptive nature ensures that this capacity gain does not come at a constant cost to quality; during good radio conditions, the speech quality can approach that of a full-rate channel, while in poor conditions, it maintains call integrity through stronger error protection. This allowed operators to handle growing subscriber bases and peak traffic loads more efficiently.

Key Features

• Utilizes the Adaptive Multi-Rate (AMR) speech codec with eight source bitrates from 4.75 to 12.2 kbit/s.
• Operates on a half-rate physical channel, doubling voice capacity compared to a full-rate TCH.
• Employs link adaptation to dynamically select the optimal codec mode based on radio channel quality.
• Supports Voice Activity Detection (VAD) and Discontinuous Transmission (DTX) for interference and power savings.
• Uses a 26-frame TDMA multiframe structure with dedicated slots for traffic and associated control signaling.
• Enables network-controlled codec mode adaptation for load balancing and quality management.

Evolution Across Releases

Defining Specifications