LETS DISCUSS

FH

The Flow

What is FH?Why FH?Multipath fadingInterference Different types of FHBaseband FHSynthesiser FHFH SpecificationsHardware elements supporting FHTips..Capacity gain due to FHOMC spec

FREQUENCY HOPPING

*In frequency hopping systems, each call hops between a defined set of frequencies.

* So poor signal quality on any specific frequency affects only a small portion of the transmission.

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

*GSM networks use slow frequency hopping;

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

Time

Frequency

* This shows the FH in time frequency domain

* Each block of TDMA frame is transmitted in different frequencies

2. Why use frequency hopping?

Frequency hopping mitigates two problems with transmission quality over the air interface:

Multipath fading Interference

Multipath fading

*Usually a radio signal is received as scattered signals travelling over separate paths.

*When the signals combine, they produce an interference pattern of fading.

*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

*With frequency hopping, because the frequencies change, so do the fading patterns associated with them.

*Transmissions on a frequency that is subject to multipath fading will move out of the

fade at the next hop ( frequency diversity).

Interference

*Any given call may suffer interference from calls on neighbour cells transmitting on or close to its frequency.

*This interference will continue through out that call

*When FH is used a call hops with in a set of frequencies , which reduces the the effect of interference.

Different types of FH

Base Band FHSynthesiser hoppingBase Band FH:

In Base band FH a call hops between different TRX of the same sector

In the above fig , a customer will be TRX 1 (f1) for 1 TDMA frame and in the next frame he will be in TRX2 (f2) and next in TRX3 (f3)

f1

f2

f3

Synthesiser FH

In this, the output freq of the TRX changes and the calls will continue on the same Timeslot

The advt of Synthesiser over Base band is that we need only as many TRX as the Capacity ,

but in the case of Baseband Hopping we need 4 TRX in 1 sector eventhoughThe capacity of that sector is very less

f1,f2,f3,f4

FH is described by .

HSN ( Hopping sequence number )MAIO ( Mobile allocation index offset )

*HSN is an algorithm on which the frequencies should be selected with in the predefined group for hopping frequencies

There are 64 algorithm ( HSN ) ie 0-63

*MAIO is the starting frequency of this algorithm.

The value of MAIO can be 0 to N-1 , where N= number of allocated frequencies

*Usually the BCCH RT will not Hop

*If at all we want the BCCH RT also to Hop , then we have to add extra

Hardware called Fill sender, which inturn occupies the space of a TRX

*This Fill sender will not take any call, so effectively the capacity of a

4-4-4 BTS comes down to 3-3-3

* BCCH and TCH are Planned separately.

* Since BCCH is not hopping , we follow a 4/12 pattern for BCCH

* For TCH we make 3 set of frequencies

SecA

For Sec A lets suppose that we have 4 RTs

*First RT will be BCCH , which have a separate Freq

*Now for the 2nd ,3rd and 4th RTs we have a group of

4 freq i.e. f1,f2,f3,f4.

*The same group of freq will be used for RT 2, 3 and 4,

but with different MAIOs

BCCH

RT1

RT2

RT3

RT4

f1,f2,f3,f4

f1,f2,f3,f4

f1,f2,f3,f4

For Sec B

*First RT will be BCCH , which have a separate Freq

*Now for the 2nd ,3rd and 4th RTs we have a group of

4 freq i.e. f5,f6,f7,f8.

*The same group of freq will be used for RT 2, 3 and 4,

but with different MAIOs

* Similarly for Sec C f9,f10,f11,f12 will be for RT 2,3 and 4 , with different MAIOs

BCCH

RT1

RT2

RT3

RT4

f5,f6,f7,f8

f5,f6,f7,f8

f5,f6,f7,f8

Sec B

Site 2

Site 1

*HSN will be same for all 3 sectors of a site.

*No two sites should have same HSN

( same HSN is possible if the sites are so apart that the

signal of one site will no way reachthe area of second site.)

*HSN 0 is for cyclic hopping, which means the frequencies

in the group are selected in the in the cyclic mode for

Hopping

ie if f1,f2,f3,f4 are the freq, then while hopping the freq

are taken in the same order.

HSN

HSN as we said is an Algorithm in which the freqs in the TCH

group should radiate.

For eg if we have f1,f2,f3 and f4 as freq,

HSN =1 can be like :

f1,f3,f4,f1,f2,f4,f3,f2,f4,f1,

No two HSN wil have the same algorithm.

HSN =0 is cyclic hopping which means the 4 freq will be

transmitting in the order, f1,f2,f3,f4,f1,f2,f3,f4,f1,f2,

MAIO

MAIO differentiates between RTs in the same sector

If we have 4 RTs in a sector, one will be BCCH and the other 3 will be Hopping .

These 3 RTs will be hopping with the same freq group, same HSN,

but with different MAIOs . For eg, if we take HSN=1

HSN =1 can be like :

f1,f3,f4,f1,f2,f4,f3,f2,f4,f1,

RT1 will start radiate from f1

RT2 will start radiate from f3 and

RT4 will start radiate from f4

so these RTs will not interfere each other since they will not radiate the

same freq at the same time.( if at all it happens it will last only for 4.615 ms and

after that it selects the other freq.)

Hardware elements supporting FH:

Network element

Baseband

Synthesiser

BCF 2000

X

X

BCE 2000, Ver 4 GSM rel 7

X

BTS 2000

X

X

FLEXENT

X

X

X

X

X

BTS 2000/2c ( Cube )

RBS 900

RBS 2000 with TXFU

X

Tips

*FH is a purchased option.

*The status of this option (whether locked/unlocked ) can be viewed

in the BCE by the command ret-feach

* In BCF log into ./bcf/APP/Default/bin/def_env_vars

newgrp root

bond stdio BCC1

lca-fea:show

*If the option FR HOP ENA is True in the BCE it means that FH option

is enabled

*The minimum no of frequencies needed for a TCH group is 4

*With 4 freq in a group we can go up to 3 ( 4-1) RTs in the same sector

for hopping

* Maximum no of Freq which can be in a TCH group is 8, ie in a sector

max no of RTs which can hop is 7 ( currently Lucent supports only 4 )

Capacity gain due to FH

If we have 31 Freq ( 6.2 Mhz band )

* BCCH and TCH are planned separately .

*12 freq are exclusively used for BCCH plan ( 4/12 patten )

*We need 3 TCH groups , each with 4 freq, which comes to total 12 freq.

*We need a separation of 400Kz ( 2 freq) b/w BCCH and TCH

*So with total 12+2+12 = 26 freq , we can go up to 4-4-4 config

in all sectors in all sites

* With FH enabled, each sector traffic can go up to 21.03Erlang, with out any additional interference

.

*Rest 5 freq ( 31-26 ) can be used for Mico cells , replacement of any freq

which cause interference ,can be used in the sectors where there are Repeators

*Current repeators will support only two frequencies

*So now all the sectors which is being repeated are made into Non hopping mode.

If we want to add more capacity to those sectors by enabling FH,

we have to upgrade the repeators so as to repeat more frequency

*The Min no of frequencies which will be radiated from a sector in FH mode is 5

( 1 BCCH+ 4 TCH freq in a group )

*So repeators also should be ready to repeat 5 freq , which is possible by

adding more Hardware cards

*If Lucent supports 5 RTs in a sector , then also we can have 5-5-5 config

with the existing 31 freq band

* In that case, 12 Freq for BCCH, 2 freq separation, 3 TCH groups

of 5 freq each, ie 15 freq

* So total 12+2+15 =29 Freq, which leaves behind further 2 freq for

Mico cell, interference replacement.

Max: no of freq allocated to one FH group

Max: no of channels assigned to the same FH group

Max: no of different FH group assigned to channels

served by the same RT

Max: no of FHs to which the same freq can be

allocated

8

42

2

2

* With the current freq plan we can have max of two 3-3-3 config

and two 2-2-2 config sites in a cluster.

( A cluster will have 4 sites ie 12 sectors)

ie the Max capacity which we can go in a cluster is 133.38 Erlang

( a 3 RT sector will have 14.03 Er and 2 RT sector with 8.2 Er,

(14.03*6)+(8.2*6)=133.38Er )

* After the FH we can go till 4-4-4 config in all sites, all sectors, in a cluster

ie the Max capacity will be 252.36 Erlang .

( a 4 RT sector will take 21.03 Er , (21.03*12=252.36 Er))

And when Lucent supports 5-5-5 config sites, we can even go to 327.6 Er

in a cluster ( a 5 RT sector will take 27.3 Er ), with the existing 6.2 Mhz band

The OMC spec follows

* Define all the frequencies which are allowed to be used for

hopping in the cell. To do this Specify the frequencies in the CELLALLOC (Cell Allocation)

* For each FH object, define the hopping sequence number in the

Hopping Sequence Number attribute.

* Assign the FH to each channel which you want to be a hopping

channel by setting the Frequency Hopping Relationship attribute

of the CHN object. The modification can be done with the CHN

in unlocked state. However, the FH assigned to the CHN must be

locked.

Note: The initial frequency of the serving RT must be contained

in the allocation list of the referenced FH object.

5. Unlock the FH object. All channels allocated to that FH will be

switched to hopping.

Note: Switching to hopping is delayed for at least 195 seconds.

This is caused by the Frequency Redefinition Procedure to be

started according to GSM.

Modification of Init freq of RT used by an FH object:

*Change the cell allocation with the new freq

*Lock the RT, set the new freq and unlock it

* The same can be done with the script mode-freq-s

Deletion of FH object

The actions to be performed are:

* Lock the FH.

* De-assign the FH for each CHN. This can be done in unlocked

state.

* Delete the FH.

The deinst-fhs-s script carries out the above steps.

THATS IT