Voice services over Adaptive Multi-user Channels on One Slot

Description

VAMOS is a sophisticated feature within the GERAN specifications that fundamentally enhances the capacity of GSM networks for voice services. It operates by allowing two or even four simultaneous voice calls to share a single physical radio timeslot, which is the fundamental unit of time-division multiple access (TDMA) in GSM. This is achieved through advanced signal processing techniques at both the Base Transceiver Station (BTS) and the Mobile Station (MS). The core mechanism involves the use of Orthogonal Sub Channels (OSC), where two users are paired and assigned to the same timeslot but with different, carefully chosen training sequences (TSCs) that are mutually orthogonal or nearly orthogonal. This orthogonality allows the receiver, using advanced interference cancellation algorithms like Single Antenna Interference Cancellation (SAIC) or Downlink Advanced Receiver Performance (DARP) Phase 2 capabilities in the mobile, to separate the two overlapping signals.

The system architecture involves modifications to the BTS to support the generation of a composite downlink signal containing the waveforms for both paired users. The BTS must also handle uplink reception of two simultaneous signals on the same frequency and timeslot. Key network components include the BTS with VAMOS-enabled transceivers (TRXs) and mobile stations supporting DARP Phase 2 or better. The Adaptive Multi-rate (AMR) codec is a prerequisite, as VAMOS leverages its robustness. The 'Adaptive' aspect of VAMOS is crucial: the network can dynamically adjust the power imbalance between the two paired sub-channels based on real-time channel conditions and the capabilities of the paired mobiles. This power control optimization, managed by the Base Station Controller (BSC), is key to maintaining call quality for both users.

In operation, the BSC performs user pairing, selecting two mobiles with compatible receiver capabilities and channel conditions. It then instructs the BTS to activate VAMOS mode for that timeslot. The BTS transmits the combined signal. In the uplink, both mobiles transmit simultaneously, and the BTS, using its multiple receive antennas and interference rejection combining (IRC) techniques, separates the signals. For the four-user mode (VAMOS Level II), the concept is extended using Quadrature Phase Shift Keying (QPSK) modulation instead of Gaussian Minimum Shift Keying (GMSK), effectively creating four sub-channels. VAMOS's role is to provide a cost-effective capacity boost within the existing GSM spectrum, delaying the need for refarming spectrum to other technologies and extending the life of GSM infrastructure for voice and basic data services.

Purpose & Motivation

VAMOS was created to address the critical challenge of increasing voice capacity in GSM networks without requiring additional spectrum. As mobile subscriber numbers grew exponentially and spectrum became a scarce and expensive resource, operators needed a way to serve more users within their existing GSM frequency allocations. Traditional GSM allocates one timeslot per user for a full-rate voice call. Half-rate codecs offered a doubling of capacity but often at a perceptible cost to voice quality. VAMOS was developed as a superior solution that could double or even quadruple spectral efficiency while maintaining voice quality comparable to AMR full-rate, by leveraging advancements in mobile receiver technology.

The historical context is the long evolution of GSM alongside 3G and 4G deployments. Operators often needed to maintain extensive GSM coverage for voice, especially in rural areas and for legacy devices, while reallocating precious spectrum bands (like 900 MHz and 1800 MHz) to UMTS and LTE for high-speed data. VAMOS provided a tool to 'squeeze' more voice capacity out of the reduced GSM spectrum footprint, allowing more aggressive refarming. It solved the problem of spectrum congestion for voice services, enabling operators to support a higher number of concurrent calls in dense urban areas or during peak traffic hours without investing in new spectrum or additional radio sites. It directly addressed the limitations of previous capacity-enhancing techniques like frequency hopping and half-rate coding by offering a more efficient physical layer multiplexing method.

Key Features

• Orthogonal Sub Channel (OSC) pairing of two users on one timeslot
• Support for adaptive power imbalance control between paired users
• Optional four-user multiplexing using QPSK modulation (VAMOS Level II)
• Requires mobile station support for DARP Phase 2 or SAIC
• Dynamic user pairing and mode selection by the network (BSC)
• Backward compatibility with legacy non-VAMOS mobiles on non-VAMOS timeslots

Evolution Across Releases

Defining Specifications