Half Rate

Description

GSM Half Rate (HR) is a fundamental channel mode and associated speech codec within the GSM system. At the physical layer, a GSM traffic channel (TCH) normally carries a Full Rate (FR) speech call at a gross rate of 22.8 kbps. The Half Rate mode allows two separate voice calls to be multiplexed onto a single physical TCH by allocating each call a half-slot structure, effectively operating at a gross rate of approximately 11.4 kbps per call. The core of HR is the speech codec (originally defined in GSM 06.20), which uses a Vector Sum Excited Linear Prediction (VSELP) algorithm to compress the speech signal to a net bitrate of about 5.6 kbps, plus channel coding for error protection.

From a network architecture perspective, the HR mode is managed by the Base Station Controller (BSC). The BSC, based on network load and configuration, can decide to assign an incoming call to a Half Rate channel instead of a Full Rate channel. This decision involves signaling to the Mobile Station (MS) and the Base Transceiver Station (BTS) to configure the appropriate channel coding and timeslot usage. The MS must support the HR codec to engage in such a call. The channel coding for HR is more robust than FR to compensate for the lower bitrate, using a higher proportion of bits for error correction to maintain call quality under marginal radio conditions.

The role of HR in the GSM network was primarily capacity enhancement. By enabling two voice calls per physical radio timeslot, it effectively doubled the voice capacity of a cell site without requiring additional spectrum or hardware. However, this came at the cost of reduced voice quality compared to FR, especially in noisy environments. HR was often used as a capacity-boosting tool during peak hours or in dense urban areas, while FR was preferred for better quality. Its specifications have been maintained across 3GPP releases for backward compatibility, even as more advanced codecs like Enhanced Full Rate (EFR) and Adaptive Multi-Rate (AMR) were introduced.

Purpose & Motivation

GSM Half Rate was developed in the early 1990s to address the pressing need for increased network capacity as GSM subscriber numbers grew exponentially. The initial GSM Full Rate codec used one timeslot per call. With limited spectrum and a fixed number of timeslots per carrier, network operators faced rapid capacity exhaustion. HR technology was a direct solution to this spectral efficiency problem, allowing operators to serve more subscribers with the same infrastructure investment.

It addressed the economic imperative of maximizing return on the expensive spectrum licenses and base station deployments. Before the widespread adoption of more spectrally efficient digital techniques like HR, operators would have needed to build more cell sites—a costly and slow process. HR provided a software-upgradable capacity boost. While it sacrificed some voice quality, the trade-off was deemed acceptable for the significant capacity gain, especially for a network primarily focused on voice service. HR paved the way for later, more sophisticated adaptive codecs like AMR, which could dynamically switch between codec modes (including HR) based on network conditions and quality requirements.