Mobile Allocation Channel Number

Description

The Mobile Allocation Channel Number (MACN) is a fundamental parameter in the GSM radio resource management, specifically for the General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE). It operates within the framework of frequency hopping and channel allocation. The network defines a 'Mobile Allocation' (MA) list, which is an ordered set of Absolute Radio Frequency Channel Numbers (ARFCNs) that can be used for a connection. The MACN is simply an index into this list. For example, if the MA list contains ARFCNs [23, 45, 67, 89], then MACN=0 refers to ARFCN 23, MACN=1 to ARFCN 45, and so on.

When the network assigns resources for a packet data transfer, it sends assignment messages to the Mobile Station (MS). These messages specify the timeslot(s) and, critically, the hopping pattern. The hopping pattern is defined by the MA list and a Hopping Sequence Number (HSN). To tell the MS which specific frequency to use at a given time, the network provides a starting point called the Mobile Allocation Index Offset (MAIO) and uses the MACN concept within the algorithm. The MS calculates the frequency for any given radio block period using a formula that takes the HSN, the MA list size, and the current block number, effectively cycling through the MACN indices in a pseudo-random order determined by the HSN.

Architecturally, the MACN is a parameter generated and managed by the Base Station Controller (BSC) or Packet Control Unit (PCU). It is communicated to the MS via layer 3 radio resource messages, such as the PACKET TIMESLOT RECONFIGURE or PACKET CHANNEL REQUEST messages. The MS's physical layer uses this information, along with data from its assigned timeslot, to tune its transmitter and receiver to the correct frequency for each TDMA frame. This mechanism allows for efficient frequency hopping, which is mandatory for GPRS/EDGE in many deployments to combat interference and fading, thereby improving link quality and overall spectral efficiency. The use of an index (MACN) rather than the full ARFCN in signaling messages saves valuable bandwidth on the air interface.

Purpose & Motivation

The MACN was developed as part of the GSM standard to enable efficient and dynamic allocation of radio resources for packet-switched data services (GPRS), which was a significant evolution from the circuit-switched voice-centric design of original GSM. In circuit-switched calls, a traffic channel (TCH) was assigned a fixed frequency or a simple hopping sequence for the call's duration. Packet data, with its bursty nature, required a more flexible system where channels could be quickly assigned and released on a block-by-block basis to multiple users sharing the same timeslot.

The use of a Mobile Allocation list indexed by MACN solved several problems. First, it reduced signaling overhead: instead of sending a full ARFCN (which requires more bits) for every assignment or hopping calculation, the network could send a shorter index. Second, it decoupled the logical channel assignment from the physical frequency, allowing the network to manage frequency hopping sequences centrally (via the BSC/PCU) and simply instruct the MS on how to navigate a pre-defined list. This was crucial for implementing interference diversity through frequency hopping in the packet domain, matching the robustness already achieved in GSM voice. The MACN mechanism, therefore, was a key enabler for making GPRS/EDGE a spectrally efficient and reliable wide-area mobile data technology.