Frequency Hopping

Copyright © 1999 by Ericsson Radio Systems AB2882-EN/LZT 123 4038 Rev C

Frequency Hopping


Frequency Hopping

A

A

A

Burst 1

Burst 2

Burst 3

Frequency 1

Frequency 2

Frequency 3

Burst 4


Frequency Hopping


Effect on the Air Interface

• Interference averaging

– Gain depends on transceiver distributionand frequency

• Frequency diversity– Gain depends on number of frequencies

and the bandwidth


Achievements

• More uniform speech quality• A more dependable and predictable radio environment

• Increased capacity (tighter frequency re-use enabled)


C/I Distributions

C/I

MS

dis

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utio

nFHOP

FHOP+BTSPC

FHOP+BTSPC+DTX

Nothing

Acceptable C/Ino FHOP

Acceptable C/IFHOP


Performance of Frequency Hopping

• Hopping on many frequencies gives betterfrequency diversity and interference averaging.However the relative benefit by adding yetanother frequency will diminish.

• Hopping over a wide band decreases thecorrelation between the fading dips of differentfrequencies thus improving the frequencydiversity.

• A high traffic load will reduce the gain ofinterference averaging.

• Usage of DTX and dynamic power control willincrease the gain of interference averaging.


A Higher RxQual Can Be Tolerated

Maximum RxQual providing acceptable speech quality

(approx.)No frequency hoppingNo DTX 4.5

Frequency hoppingNo DTX 5-6


Hopping Sequences - Cyclic and Random

• Cyclic sequence (HSN = 0)

... , I4, I1, I2, I3, I4, I1, I2, I3, I4, I1, I2, ...

• A cyclic sequence is the best sequence for combatingconsecutive fading dips.


• Random sequences (HSN = 1 - 63)... , I1, I4, I4, I3, I1, I2, I4, I1, I3, I3, I2, ...

• The period of a (pseudo) random sequence is 6 minutes long.

• Random sequences give better interference averaging (if the network is planned using frequency groups.)

• Different random sequences (different HSNs) should be assigned to nearby cells having the same frequencies.


MS Problems with Random FrequencyHopping?

• MS problem is of no concern any more!(Some MSs have had problems with random frequencyhopping (Siemens and Motorola, very old MS))


Orthogonal Sequences

• The transmitters in a channel group never use thesame frequency simultaneously.

... , I1, I4, I4, I3, I1, I2, I4, I1, I3, I3, I2,

...

... , I2, I1, I1, I4, I2, I3, I1, I2, I4, I4, I3,

...

... , I3, I2, I2, I1, I3, I4, I2, I3, I1, I1, I4,

...

... , I4, I3, I3, I2, I4, I1, I3, I4, I2, I2, I1,

...


Two Hopping Modes

• Baseband hopping (FHOP = BB)

• Synthesizer hopping (FHOP = SY)


Baseband Hopping

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controllerTRX2

controllerTRX3

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• Baseband hopping : Each transmitter transmits on a fixed frequency.The bursts from the transceiver controller are routed to the differenttransmitters by a bus.

+ A narrow-band filter combiner can be used. To this combiner it ispossible to connect up to 16 TRXs without more than 3dB combinerloss.

- It is impossible to hop on more frequencies than there are TXs.


Synthesizer hopping

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• Synthesizer hopping: The transmitters change frequency for every burst.

+ It is possible to hop on more frequencies than there are transmitters.

- Hybrid combiners must be used. When connecting many transmitters theloss will be big.


Co Filling at Baseband Hopping

• 30 TCHs and hopping on the BCCH frequency.

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• 22 TCHs and hopping on BCCH frequency.• One transmitter acting only as Co filler.• All traffic bursts that are to be sent on Co are routed to Co transmitter.

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Co Filling at Synthesizer Hopping With anExtra Transmitter and BCCH Controller

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• 23 TCHs and hopping on BCCH frequency• One transmitter acting only as Co filler and one “BCCH controller”

makes it possible to use one more TCH compared to the prior case.

• All traffic bursts that are to be sent on Co are routed to thistransmitter.

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Co Filling at Synthesizer Hopping WithTwo Channel Groups

• 30 TCHs and a non-hopping BCCH frequency.

• If utilization of hardware is more important than it is to hop on theBCCH frequency.

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Parameter Summary

Main controlling parameters• HOP is the switch for turning frequency hopping on or off, defined

per channel group. It acts on all channels within the specifiedchannel group. SDCCHs as well as TCHs can hop. A BCCH willnot hop, even if it belongs to a channel group that is configured ashopping.

• HSN is the hopping sequence number, defined per channel group.This parameter specifies which hopping sequence to be used, andacts on all channels within the specified channel group. Alltimeslots in one channel group obtain the same HSN. HSN=0yields a cyclic sequence. HSN=1 to 63 yields (six minutes long)pseudo-random sequences.


Parameters

• FHOP selects which hopping method to be used, basebandhopping (FHOP=BB) or synthesizer hopping (FHOP=SY). Itis defined per transceiver group (normally one base stationis one transceiver group and one cell).

• COMB specifies which combiner type that has beenconnected, a wide-band hybrid combiner (COMB=HYB) ora narrow-band filter combiner (COMB=FLT). It is definedper transceiver group. If a filter combiner is connected, onlybaseband hopping can be used as hopping method.


Value Ranges and Default Values

Parameter name Default value Value range Unit

HOP OFF OFF, TCH, ON

HSN 0 to 63

FHOP BB, SY

COMB HYB, FLT

Documents

Frequency Hopping