Mobile Allocation Index Offset

Description

The Mobile Allocation Index Offset (MAIO) is a fundamental parameter in the GSM physical layer, specifically within the channel assignment and frequency hopping framework. It works in conjunction with the Mobile Allocation (MA), which is the list of radio frequency channels assigned to a cell, and the Hopping Sequence Number (HSN), which defines the pseudo-random hopping pattern. When a Traffic Channel (TCH) is allocated to a mobile station, it is assigned a specific MAIO value. This MAIO, ranging from 0 to N-1 (where N is the number of frequencies in the MA list), determines the mobile's starting offset within the hopping sequence.

The operation is as follows: The system uses the Frame Number (FN) and the HSN to generate a pseudo-random sequence of indices. For each TDMA frame, this algorithm produces an index. The mobile then calculates its actual frequency for that frame by taking this index, adding its assigned MAIO (modulo N), and mapping the result to a specific frequency in the MA list. Crucially, within the same cell and using the same HSN, different mobiles must be assigned different MAIO values. This ensures they are transmitted on different frequencies in any given timeslot, implementing a form of Orthogonal Frequency Hopping and preventing co-channel interference between users in the same cell.

MAIO management is performed by the Base Station Controller (BSC) as part of the channel assignment procedure. When setting up a call or during handover, the BSC's radio resource management algorithm selects an available timeslot and an appropriate MAIO from the pool for that cell and hopping sequence. The chosen MAIO and the MA list are communicated to the mobile station in the IMMEDIATE ASSIGNMENT or HANDOVER COMMAND message. This precise coordination allows GSM to exploit the frequency diversity and interference averaging benefits of frequency hopping, which are critical for improving voice quality and system capacity, especially in interference-limited environments.

Purpose & Motivation

MAIO was created as an integral part of the GSM frequency hopping solution to combat two major radio propagation issues: frequency-selective fading and co-channel interference. In a non-hopping system, a mobile stuck on a channel experiencing deep fading or high interference would suffer poor quality for the entire call. Fixed channel assignment also leads to predictable and persistent interference patterns between cells reusing the same frequency.

Frequency hopping, enabled by parameters like MAIO, HSN, and MA, was introduced to randomize these effects. By having a mobile's transmission 'hop' across different frequencies, a fade or burst of interference on one frequency only affects a single frame, and the error correction coding can recover. The MAIO parameter specifically solves the problem of coordinating multiple users within the same cell. By assigning unique MAIOs, the system ensures that two mobiles in the same cell and timeslot never use the same frequency simultaneously, thus eliminating intra-cell co-channel interference. This allows for tighter frequency reuse patterns, increasing network capacity. Without MAIO to orthogonally separate users, the benefits of hopping would be lost due to users colliding on the same hopping frequency.

Key Features

• Defines a user's unique offset within a frequency hopping sequence.
• Value ranges from 0 to (number of hopping frequencies - 1).
• Assigned by the BSC during channel assignment or handover.
• Ensures orthogonality between users in the same cell using the same HSN.
• Critical for implementing GSM Baseband Frequency Hopping.
• Communicated to the MS via assignment or handover commands.

Evolution Across Releases

Defining Specifications