Total Frequency Hopping

Description

Total Frequency Hopping (TFH) is an advanced frequency hopping technique implemented in GSM and EDGE networks. In a GSM system, the radio channel is divided into 200 kHz carrier frequencies and time division multiple access (TDMA) frames containing 8 time slots. Frequency hopping involves changing the carrier frequency used for transmission and reception of bursts in a pseudo-random pattern. TFH is characterized by applying this frequency change on *every* burst for a given traffic channel. This means that for a single call or data session, consecutive time slots (bursts) are transmitted on different carrier frequencies as dictated by a hopping sequence number (HSN) and a Mobile Allocation Index Offset (MAIO).

The hopping sequence is derived from parameters broadcast on the Broadcast Control Channel (BCCH) and assigned to the mobile station during call setup. The MAIO ensures that different connections on the same cell use different offsets within the same hopping sequence, minimizing the chance of collision. The set of frequencies available for hopping is called the Mobile Allocation (MA) list. In TFH mode, the Base Transceiver Station (BTS) must be capable of transmitting and receiving on all frequencies in the MA list simultaneously, which typically requires a wideband combiner or a per-transceiver synthesizer hopping capability. This is in contrast to Baseband Frequency Hopping (BFH), where the BTS transmitter frequency remains fixed and hopping is simulated by switching timeslots between different transceivers.

From the mobile station's perspective, it must rapidly re-tune its synthesizer between bursts to follow the hopping sequence. This process improves performance through frequency diversity. Since multipath fading and interference are frequency-selective, hopping spreads the signal's exposure across multiple frequencies. This averaging effect reduces the probability of a deep fade or persistent interference on a single frequency causing a call drop. It also provides interference averaging, distributing co-channel interference more evenly among users in a frequency reuse pattern, which can increase overall network capacity and quality.

Purpose & Motivation

TFH was developed to enhance the performance and capacity of GSM networks beyond the capabilities of non-hopping or slower hopping schemes. Early GSM networks often used fixed frequency assignment or slow hopping, which left connections vulnerable to sustained interference or deep fades at a particular frequency. In dense urban environments with frequency reuse, co-channel interference is a major limiting factor.

TFH directly addresses these issues by exploiting frequency diversity. By forcing the signal to hop across a wide band on every burst, it ensures that if one frequency is in a deep fade or heavily interfered, the next burst will likely be on a better frequency. The error correction coding (and interleaving over multiple bursts) in GSM can then recover the information. This makes the link more robust, allowing for lower transmit power (reducing interference to others) and/or providing better voice quality and data throughput at the cell edge. Furthermore, by randomizing the frequency usage among all users in a cell, TFH turns concentrated co-channel interference into a background noise that is averaged across all connections, leading to a more predictable and manageable interference environment. This allows network planners to use tighter frequency reuse patterns, thereby increasing the spectral efficiency and overall capacity of the network.

Key Features

• Frequency changes occur on every TDMA burst (every 577 µs)
• Requires the BTS to support synthesiser hopping or wideband combining
• Utilises a pseudo-random sequence defined by HSN and MAIO parameters
• Provides maximum frequency diversity and interference averaging gains
• Improves robustness against multipath fading and co-channel interference
• Enables tighter frequency reuse, increasing network capacity

Evolution Across Releases

Defining Specifications