Traffic Channel Half rate Speech

Description

The TCH-HS (Traffic Channel Half rate Speech) is a logical traffic channel in the GSM system designed to increase the capacity of the radio access network for voice services. It operates by utilizing a speech codec that compresses voice to approximately half the bitrate of the full-rate codec, typically around 5.6 kbps for the original GSM Half Rate (HR) codec. This allows two separate TCH-HS channels, and thus two simultaneous voice calls, to be multiplexed onto a single physical timeslot within a TDMA frame. This multiplexing is achieved through sub-channelization, where the timeslot is divided into two sub-slots, each assigned to a different mobile station. Similar to the TCH-FS, the TCH-HS operates within a 26-frame multiframe structure, but the allocation of frames for traffic and the associated Slow Associated Control Channel (SACCH) is adapted for the two sub-channels.

From a technical operation perspective, when the network decides to allocate a half-rate channel—often based on traffic load and resource optimization algorithms—it configures the physical channel in half-rate mode. The mobile station and the network transcoder (TRAU) must both support the half-rate codec. The speech signal is encoded at the lower bitrate, and the resulting bitstream is mapped to the assigned sub-slot. The two sub-channels on the same timeslot are carefully synchronized to avoid interference. Each sub-channel has its own associated signaling, with SACCH information for each call being transmitted in designated frames within the multiframe, ensuring both calls can report signal measurements and receive necessary control commands.

Architecturally, the TCH-HS is managed by the BSC, which makes the decision to use half-rate based on cell configuration and current load. Its introduction required enhancements in the BTS to handle the sub-channel processing and in the TRAU to support the half-rate codec. The role of TCH-HS is primarily one of capacity enhancement. It allows network operators to serve more subscribers with the same amount of radio spectrum, which is a critical economic and operational factor. However, this comes at the cost of potentially reduced voice quality compared to the full-rate channel, as the lower bitrate codec is more susceptible to audio artifacts, especially in noisy environments. Therefore, TCH-HS usage is often dynamically managed, potentially upgrading a call to full-rate if quality degrades or network load decreases.

Purpose & Motivation

The TCH-HS was developed to address the critical problem of limited radio spectrum and the growing demand for mobile voice services in the 1990s. Its primary purpose is to increase the capacity of a GSM network without requiring additional radio carriers or cell sites. By allowing two calls to share a single physical resource, it effectively doubles the voice capacity per timeslot, which is a significant improvement in spectral efficiency. This solved the economic and technical challenge of network congestion in high-traffic urban areas.

The motivation stemmed from the success of GSM and the rapid growth in subscriber numbers. The fixed allocation of one timeslot per call in the TCH-FS model was becoming a bottleneck. The TCH-HS introduced a trade-off: accept a reduction in speech quality (using a lower bitrate, more compressed codec) in exchange for serving more users. This was a pragmatic solution for operators facing capacity constraints. It addressed the limitation of the TCH-FS's inflexible resource usage, providing a tool for statistical multiplexing of voice traffic. The deployment of TCH-HS was a key step in optimizing network utilization and delaying costly infrastructure expansions.