MUROS vs. VAMOS: GSM Capacity Enhancement Techniques

This article explores the differences between MUROS (Multi User Re-using One Slot) and VAMOS (Voice Services Over Adaptive Multi-User Orthogonal Subchannels), two techniques used in GSM networks to improve capacity.

MUROS: Multi User Re-using One Slot

MUROS is a technique designed to enhance the capacity of both data and voice services within a GSM network. It achieves this by creating an orthogonal subchannel, allowing two users to share the same time slot and ARFCN (Absolute Radio Frequency Channel Number), meaning they use the same frequency.

Key Features of MUROS

• TSC (Training Sequence Code) Enhancement: MUROS introduces eight or more new TSCs. This allows each cell to be assigned two or more TSCs without needing to change the RF frequency planning.
• User Assignment: Each GSM Base Station (BTS) utilizes TSCs to assign two or more users to a single time slot. A BTS will be assigned both a legacy TSC and a new TSC.
• Signal Differentiation: The receiver can distinguish its own signal from the other MUROS partner’s signal using a cross-correlation method.
• Interference Cancellation:
  • Uplink: STRIC (Successive Trunk Interference Cancellation), SIC (Successive Interference Cancellation), or JD (Joint Detection) techniques can be employed.
  • Downlink: SAIC (Subchannel Aware Interference Cancellation) can be used.
• Synchronization: Front-end synchronization, including time offset estimation and correction, frequency offset estimation and correction, and channel estimation and equalization, are performed using the respective TSC assigned by the base station.
• Supported Modulation Schemes (Downlink):
  • Linear sum of two GMSK signals (with 90-degree phase separation)
  • QPSK with a linear Gaussian filter
  • QPSK with an RRC filter (with a roll-off factor of approximately 0.3)

VAMOS: Voice Services Over Adaptive Multi-User Orthogonal Subchannels

VAMOS, introduced in 3GPP Release 9, is another technique designed to improve GSM capacity. It allows for the multiplexing of two users simultaneously on the same resource (time slot, ARFCN, TDMA frame).

Key Features of VAMOS

• Application: Applied to both the downlink and uplink in the circuit-switched (CS) domain.
• Logical Channels: The following logical channels are used with the VAMOS feature:
  • TCH/FS (Traffic Channel/Full Rate Speech)
  • TCH/HS (Traffic Channel/Half Rate Speech)
  • TCH/EFS (Traffic Channel/Enhanced Full Rate Speech)
  • TCH/AFS (Traffic Channel/Adaptive Full Rate Speech)
  • TCH/AHS (Traffic Channel/Adaptive Half Rate Speech)
  • TCH/WFS (Traffic Channel/Wideband Full Rate Speech)
• Control Channels: The associated control channels used in VAMOS are SACCH (Slow Associated Control Channel) and FACCH (Fast Associated Control Channel).
• Proposed Techniques: Four techniques were proposed for VAMOS in GSM. The probable techniques among them are OSC (Orthogonal Sub-Channelization), alpha-QPSK, or Co-TCH (Co-channel Traffic Channel).

Key Differences Summarized