General concept General concept of of

Frequency hoppingFrequency hopping

By: Rohit Kumar Singh EGIL-Ranchi

The Flow……

1) What is FH?

2) Why FH?1) Multipath fading2) Interference

3) Different types of FH1) Baseband FH2) Synthesizer FH

4) FH Specifications

5) Fractional Loading

FREQUENCY HOPPINGFREQUENCY HOPPING

**In frequency hopping systems, each call hops between a In frequency hopping systems, each call hops between a defined set of frequencies. So poor signal quality on any defined set of frequencies. So poor signal quality on any specific frequency affects only a small portion of the specific frequency affects only a small portion of the transmission.transmission.

* This makes it much easier to recreate any lost bits and so * This makes it much easier to recreate any lost bits and so preserve overall call quality.preserve overall call quality.

*GSM networks use “slow” frequency hopping; *GSM networks use “slow” frequency hopping;

*A hop occurs before each time slot is transmitted*A hop occurs before each time slot is transmitted(every 4.615 millisecond, or 217 hops per second).(every 4.615 millisecond, or 217 hops per second).

Frequency

Time

* This shows the FH in time frequency domain

* Each block of TDMA frame is transmitted in different frequencies

WHY FH?WHY FH?

During a call, a number of physical effects influence theperceived radio environment between a mobile station and a

base station. One such effect is multipath fading, whichmeans that transmitted signals reach the receiver via multiple

paths. Depending on the difference in path length.Another effect is various types of interference. The

dominating type is normally co-channel interference, butother types, such as adjacent channel interference,

intermodulation products, military sources etc. must beconsidered as well.

Multipath fadingMultipath fading

The destructive interference produced by multipath fading is called “fading-dips”. Fading dips may cause speech quality degradation. For a given position the fading depends on the transmission frequency. This multipath fading particularly impacts slow moving mobiles, as they may stay in one position and hence a fade long enough to suffer information loss.

Rayleigh FadingRayleigh Fading• This phenomenon is due to multipath propagation of the signal.• The Rayleigh fading is applicable to obstructed propagation paths. • All the signals are Non-LOS signals and there is no dominant direct path.• Signals from all paths have comparable signal strengths.• The instantaneous received power seen by a moving antenna becomes a

random variable depending on the location of the antenna.

Ricean FadingRicean Fading• This phenomenon is due to multipath propagation of the signal.• In this case there is a partially scattered field.• One dominant signal.• Others are weaker.

Co-channel interferenceCo-channel interference

The interference situation for a mobile is strongly dependent on which frequency and time-slot that the mobile happens to use.Normally co-channel interference is caused by frequency re-use

What can be achivedWhat can be achived

Frequency diversityInterference averaging

Frequency diversityFrequency diversity

Frequency hopping can reduce the influence of signal strength variations caused by multipath fading.

Multipath fading is frequency dependent. This implies that the fading dips appear at different locations for different frequencies.

Interference averagingInterference averaging

Frequency hopping can also break up persistent interference into periodic occasions of single burst interference.

Changing frequency at each burst offers a way to improve the interference situation described above. The co-channel interference will change at every burst.

The more frequencies that are used in the hopping, the more rare such frequency collisions will be.

Short technical descriptionShort technical description

Baseband frequency hoppingSynthesizer frequency hopping

Baseband frequency hoppingBaseband frequency hopping

At baseband hopping each transmitter operateson a fixed frequency.The advantage with this mode is that narrow-bandtuneable filter combiners can be used.The disadvantage is that it is not possible to use alarger number of frequencies than there aretransmitters.

Baseband frequency hoppingBaseband frequency hopping

ControllerTRX1

ControllerTRX4

ControllerTRX3

ControllerTRX2

Transmitterf1

Transmitterf4

Transmitterf3

Transmitterf2

Bus for routing of burst

Combiner

X

X

X

X

Synthesizer frequency Synthesizer frequency hoppinghopping

The transmitter tunes to correct frequency at transmission of each burst.The advantage is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters.The advantage of Synthesizer over Base band is that we need only as many TRX as the Capacity The disadvantage is that wide-band hybrid combiners have to be used .

Synthesizer frequency Synthesizer frequency hoppinghopping

ControllerTRX1

ControllerTRX4

ControllerTRX3

ControllerTRX2

Transmitterf1,f2,…,fn

Transmitterf1,f2,…,fn

Transmitterf1,f2,…,fn

Transmitterf1,f2,…,fn Hybrid

Combiner

AlgorithmAlgorithm

Hopping sequenceCyclic hoppingRandom hopping

Cyclic hoppingCyclic hopping In cyclic hopping the frequencies are used in aconsecutive order. For instance,the sequence offrequencies for cyclic hopping between fourfrequencies may appear as follows:

... , f 4 , f 1 , f 2 , f 3 , f 4 , f 1 , f 2 , f 3 , f 4 , f 1 , f 2 , ...

A cyclic sequence is specified by setting theparameter HSN (hopping sequence number) to zero.

Random hoppingRandom hopping A random hopping sequence is actually implemented as apseudo-random sequence. 63 independent sequences are defined. When random hopping is used, the frequencies will beused (pseudo-) randomly, and a hopping sequence for fourfrequencies may appear as follows:

... , f 1 , f 4 , f 4 , f 3 , f 1 , f 2 , f 4 , f 1 , f 3 , f 3 , f 2 , ...

The period for a random sequence is 6 minutes.

Implementation with SFHImplementation with SFH

Constrain Separate frequency band for BCCHRe-use patternMA ListHSNMAIOFraction load

ConstrainConstrain

Coverage overlapping constrainFrequency constrain

Coverage overlapping Coverage overlapping constrainconstrain

Due to SFH with 1x1 or 1x3 are tight re-use patterns then coverage control is major constrain.

Homogeneous network is recommended.

Frequency constrainFrequency constrain

Performance of SFH depends on one factor which called “Fractional load”

Maximum fractional load is 50% means number of frequency required is at least 2 time number of TCH Trxs used.

Separate frequency band for Separate frequency band for BCCHBCCH

BCCH cannot handle with high interference asTCH due to : BCCH is not hop with SFH. Power control and DTX are not support on BCCH.

Re-use pattern for SFHRe-use pattern for SFH

Standard re-use pattern1. Re-use 1x12. Re-use 1x33. Multi re-use pattern for SFH (Adhoc AFP

Plan)

Re-use 1x1Re-use 1x1 Define every frequencies to every BTS. Avoid co-channel by MAIO and HSN Consider all frequencies assigned as frequency group A re-

use pattern will be as follow:

GroupA

GroupAGroupA

GroupA

GroupAGroupA

GroupA

GroupAGroupA

Re-use 1x3Re-use 1x3 Separate all frequencies into 3 groups. Define 3 frequency groups to every sites. Avoid co-channel by MAIO and HSN Consider all frequencies assigned as frequency group A,B and C re-

use pattern will be as follow:

GroupA

GroupCGroupBGroupA

GroupCGroupB

GroupA

GroupCGroupB

Multi re-use patternMulti re-use pattern Separate all frequencies into different groups. Define these groups to different sites. Avoid co-channel by MAIO and HSN Consider all frequencies assigned as frequency group G1,G2, G3 and

so on re-use pattern will be as follow:

G1

G2G3G7

G8G9

G4

G5G6

Mobile Allocation MA ListMA List

Set of frequencies the mobile is allowed to hop over. Maximum of 63 frequencies can be defined in the MA list

MAL1= f1,f3,f5,f7,f9MAL2=f2,f4,f6,f8……MAL3=f1,f4,f7,f11…..

Hopping Sequence Number (HSN)

Determines the hopping order used in the cell.  It is possible to assign 64 different HSNs.

Range: 0 to 63Setting HSN = 0 provides cyclic hopping

sequence and HSN = 1 to 63 provide various pseudo-random hopping sequences.  

Mobile allocation index offsetMobile allocation index offset Define the first frequency of group for the first

burst.

Index 0 1 2 3 4 … N-1

Frequency group f1 f2 f3 f4 f5 .. fn

Example of MAIO settingExample of MAIO setting The random sequence of synthesizer hopping will appearas follows for eight frequencies: (HSN = 0)

ControllerTRX1

ControllerTRX4

ControllerTRX3

ControllerTRX2

Transmitterf1, f2, .., f8

Transmitterf1, f2, .., f8

Transmitterf1, f2, .., f8

Transmitterf1, f2, .., f8

Combiner

f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 2)

f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 0)

f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 4)

f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 6)

Index : 0, 1, 2, 3, 4, 5, 6, 7

fn : frequency of the first burstfn : frequency of the second burst

Fraction loadFraction load

Ratio to determine how tight of frequency re-use for SFH.

Define by :Number of frequencies used at a time (per re-use cluster) * 100

Number of frequencies per group

GSM recommends maximum fraction load = 50%

Example of fraction load Example of fraction load calculationcalculation

1x3Number of frequencies : 46Number of frequencies for BCCH : 16Number of TCH frequencies per group : 10Site configuration : 6+6+6 (Tch : 5+5+5)

Fractional load = 5/10 = 50%

Example of fraction load Example of fraction load calculationcalculation

1x1Number of frequencies : 46Number of frequencies for BCCH : 16Number of TCH frequencies per group : 30Site configuration : 6+6+6 (Tch : 5+5+5)

Fractional load = 15/30 = 50%

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