Types of Frequency Hopping

There are two ways of implementing Frequency Hopping in a Base Station System, one referred as Base Band Frequency Hopping (BBH) and another as Synthesizer Frequency Hopping (SFH).  Their operation differs in the way they establish the Base to Mobile Station link (downlink), however there is not difference at all between Mobile Station to Base Station link in both types of hopping.  Motorola does not allow BBH and SFH to be used together on the same site

1. Base Band Frequency Hopping

This is accomplished by routing the traffic channel data through fixed frequency DRCUs via the TDM highway on a timeslot basis.  In this case, the DRCU would have fixed tuned transmitters combined either in low loss tuned combiners or hybrid combiners.

·         TRX  always transmits fixed frequency.

·         The information for every call is moved among the available TRX on a per burst basis. (Burst of 577 µs)

·         Call hops between same timeslots of all TRX.

·         Processing (coding and interleaving) is done by digital part associated with TRX on which call was initially assigned.

·         For uplink – call is always processed by TRX on which the call was initially assigned.

·         Number of TRX needed is equal to the number of frequencies in the hopping sequence.

·         BCCH frequency can be included in the hopping sequence.

·         Power control does not apply to BCCH or bursts transmitting BCCH frequency.

·         BCCH, timeslot 0 will never hop.

·         Any timeslot with CCCH will never hop.

·         Timeslot carrying all SDCCHs can hop.

If a network running with fixed frequency plan is switched over to BBH (BCCH included in MA list) without any frequency changes, significant quality improvement can be observed in the network.  As a result drop call rate reduces in the network.  Alternatively, for the existing network quality additional capacity can be provided.  FHI can be used effectively in BBH.  Further details regarding FHI planning are discussed later in the document.

2. Synthesizer Frequency Hopping

This is accomplished by high speed switching of transmit and receive frequency synthesizers of the individual DRCUs.  As a result of dynamic nature of the transmit frequency, broadband (hybrid) combining of the transmitters is necessary.

·         TRX changes transmitting frequency every burst.

·         Call stays on the same DRCU where it started.

·         Remote tune combiners (RTC) are not allowed.

·         Number of TRX is not related to number of frequencies in hopping sequence.

·         BCCH can be included in the hopping sequence:

1. If BCCH is included in the hopping sequence, timeslots 1 to 7 can not be used to carry traffic.  They transmit dummy burst when BCCH frequency is not in the burst.  Whenever BCCH frequency is being transmitted in a burst by DRCU, it will be transmitted at full power.
2. BCCH DRCU will never hop.  It either carries traffic in timeslots 1 to 7 or it transmits dummy bursts.

·         Transmission and reception is done on the same timeslot and same DRCU.

·         Motorola allows to have NBCCH on fixed frequency hopping on the same sector.

Frequency Hopping Parameters

GSM defines the following set of 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.

Motorola has defined an additional parameter, FHI.

Frequency Hopping Indicator (FHI): Defines a hopping system, made up by an associated set of frequencies (MA) to hop over and sequence of hopping (HSN).  The value of FHI varies between 0 to 3.  It is possible to define all 4 FHIs in a single cell. 

Motorola system allows to define the hopping system on a per timeslot basis.  So different hopping configurations are allowed for different timeslots.  This is very useful for interference averaging and to randomize the distribution of errors.

GSM algorithm

GSM has defined an algorithm for deciding hopping sequence.  The algorithm is used to generate Mobile Allocation Index (MAI) for a given set of parameters. 

ARFCN: absolute radio frequency channel number

MA: mobile allocation frequencies.

MAIO: Mobile allocation offset (0 to N-1), where N is the number of frequencies defined in MA.

HSN: Hopping sequence number (0-63)

T1: Super frame number (0-2047)

T2: TCH multiframe number (0-25)

T3: Signaling multiframe number (0-50)

This algorithm generates a pseudo-random sequence of MAIs.  MAI along with MAIO and MA will decide the actual ARFCN to be used for the burst.