Frequency Hopping

Frequency Hopping:

                                Frequency Hopping is mechanism in which the system changes the frequency (uplink and downlink) during transmission at regular intervals.  It allows the RF channel used for signaling channel (SDCCH) timeslot or traffic channel (TCH) timeslots, to change frequency every TDMA frame (4.615 ms).  The frequency is changed on a per burst basis, which means that all the bits in a burst are transmitted in the same frequency.  In 1Sec= 217Hopes

Advantages of Frequency Hopping:    Frequency Diversity, Interference Averaging, Increase capacity

There are two types of hopping 1 Base Band FH (BBH) 2 Synthesizer FH (SFH). 

1. Base Band Frequency Hopping: In baseband hopping, each transmitter is assigned with a fixed frequency. At transmission, all bursts, irrespective of which connection, are routed to the appropriate transmitter of the proper frequency. The advantage with this mode is that narrow-band tunable filter combiners can be used.

2. Synthesizer Frequency Hopping (SFH).  Synthesizer hopping means that one transmitter handles all bursts that belong to a specific connection. The bursts are sent "straight on forward" and not routed by the bus. In contrast to baseband hopping, the transmitter tunes to the correct frequency at the transmission of each burst the advantage of this mode is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters. It is possible to hop over a lot of frequencies even if only a few transceivers are installed. A disadvantage with synthesizer hopping is that wide-band hybrid combiners have to be used. This type of combiner has approximately 3 dB loss making more than two combiners in cascade impractical.

Frequency Hopping Parameters

Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over.  Maximum of 63 frequencies can be defined in the MA list.

Hopping Sequence Number (HSN): Determines the hopping order used in the cell.  It is possible to assign 64 different HSNs.  Setting HSN = 0 provides cyclic hopping sequence and HSN = 1 to 63 provide various pseudo-random hopping sequences. 

Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which frequency the mobile starts do transmit on.  The value of MAIO ranges between 0 to (N-1) where N is the number of frequencies defined in the MA list.  Presently MAIO is set on per carrier basis.