Traffic Channel Half Rate Speech

Description

The Traffic Channel Half Rate Speech (TCH/HS) is a specific instance of the half-rate traffic channel in GSM, explicitly dedicated to carrying speech traffic using a half-rate speech codec. It operates within the GSM TDMA frame structure, where a single radio frequency carrier is divided into eight time slots. A TCH/HS occupies the equivalent of half a time slot's capacity per frame, typically by assigning the channel to a user in alternating frames or using a sub-channel within a time slot. This allows the network to multiplex two independent voice conversations onto one physical time slot, effectively doubling the potential number of simultaneous calls in a cell compared to using only full-rate channels (TCH/FS).

Technically, when a TCH/HS is assigned, the mobile station and the Base Transceiver Station (BTS) engage in a specific mode of operation. The speech audio from the user is encoded by a half-rate speech codec, such as the GSM Half-Rate (HR) codec defined in earlier releases or an Adaptive Multi-Rate (AMR) codec operating in a half-rate mode (e.g., AMR 5.90 kbps). The encoded speech frames are then channel coded, interleaved, and mapped onto the designated half-rate physical resource. The associated control signaling, necessary for maintaining the call (like power control, timing advance, and measurement reports), is carried on a Half-Rate Slow Associated Control Channel (SACCH/HS) that is paired with the TCH/HS. This SACCH/HS uses a reduced signaling rate, matching the resource efficiency of the traffic channel itself.

In the network architecture, the decision to assign a TCH/HS is made by the Base Station Controller (BSC) based on algorithms that consider factors like overall cell load, available channels, and the capabilities of the mobile station (not all early phones supported half-rate). The BSC instructs the BTS and the mobile station to configure the link accordingly. The role of TCH/HS is integral to traffic management and capacity optimization in GSM networks. It allows operators to handle more subscribers, particularly in dense urban environments where spectrum is scarce. While the initial half-rate codec offered lower voice quality, the introduction of enhanced codecs like AMR, which can dynamically switch between full and half rates based on radio conditions, allowed TCH/HS to be used more aggressively without significant quality degradation, striking an optimal balance between capacity and service quality.

Purpose & Motivation

TCH/HS was developed to provide a concrete, speech-optimized implementation of the half-rate channel concept within GSM. The core problem it addressed was identical to that of TCH/H: spectrum scarcity and the need for higher voice capacity. However, TCH/HS specifically focuses on the application for speech services, as opposed to data services (which would use TCH/H for other purposes like circuit-switched data at half-rate). Its creation was motivated by the need to standardize exactly how a half-rate channel would be used for voice, including the specific codec mappings and associated control procedures.

The limitation of prior approaches was that networks were either using full-rate channels exclusively, which capped capacity, or employing non-standardized methods for capacity enhancement. TCH/HS provided a standardized, interoperable way to deploy half-rate voice across different vendors' equipment and mobile handsets. This was crucial for large-scale network rollouts and for ensuring that subscribers could use half-rate services seamlessly as they moved between cells and networks. It solved the problem of inefficient spectrum use for voice, which constituted the vast majority of early cellular traffic.

Furthermore, TCH/HS laid the groundwork for more intelligent adaptive rate control. By having a defined channel type for half-rate speech, it became possible for the network to implement algorithms that could downgrade a call from TCH/FS to TCH/HS during congestion, or upgrade it when capacity freed up, all transparently to the user. This dynamic channel assignment, facilitated by specifications like 45.914 (Multirate speech codec), was a significant step towards the quality-of-service and resource management sophistication seen in later 3G and 4G systems.

Key Features

• Dedicated to carrying speech traffic using standardized half-rate speech codecs (e.g., GSM-HR, AMR half-rate modes).
• Doubles voice capacity by multiplexing two calls onto a single GSM TDMA time slot.
• Works in conjunction with a Half-Rate Slow Associated Control Channel (SACCH/HS) for essential in-call signaling.
• Dynamically assignable by network algorithms based on load, improving resource utilization.
• Enables trade-off between network capacity and speech quality, managed via codec adaptation.
• Widely supported across GSM releases and a foundational feature for GERAN capacity management.

Evolution Across Releases

Defining Specifications