Antenna Hopping

During a call connection, a burst can easily be lost when the mobile station happens to be located in a fading dip for that particular frequency, or if it is subjected to interference. The coding and interleaving scheme in GSM is constructed so that loss of a single burst has minimal influence on the speech quality. The probability that several bursts within a speech frame have poor signal quality is reduced if the bursts are transmitted on different paths. This can be done by using antenna hopping. With antenna hopping, transmit diversity is introduced by changing transmit antenna between bursts.

                                                  Schematic picture of antenna hopping

This transmit diversity scheme mimics frequency hopping in the sense that the fading changes rapidly between bursts due to transmission through different paths.
From a subscriber point of view, antenna hopping gives an improved speech quality.
From an operator point of view, antenna hopping is a very attractive scheme in particular for traffic channels without frequency hopping, but also for traffic channels frequency hopping over a relatively low number of frequencies, where it can provide substantial diversity gains. What makes antenna hopping especially interesting as a transmit diversity scheme is the fact that it can be used without increasing the number of TRXs in a base station. Thus giving the operator the following benefits:

• a possibility to tighter frequency reuse and increase in capacity,
• a more robust radio environment,
• a possibility to give subscribers a more uniform speech quality. 

Antenna hopping can reduce the effect of multipath fading. With antenna hopping, a non-moving mobile will typically not remain in a specific fading dip longer than one TDMA frame. The low signal strength dips in multipath fading are thus leveled out, and the mobile will perceive a more constant radio environment.

Schematic picture of multipath fading at one frequency and antenna hopping between two antennas for a slow/non-moving mobile station 

Antenna Hopping Parameters 

AHOP		: Defines the wanted state for antenna hopping on a transceiver group. The wanted state can be either of ON, ONXC0 or OFF. : In state ONXC0, all channels except those on the BCCH frequency/frequencies will antenna hop.