Hopping dip analysis

8/11/2019 Hopping dip analysis

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Frequency Hopping

What is Frequency Hopping?

Frequency Hopping is an old technique introduced firstly in military

transmission system to ensure the secrecy of communications and

combat jamming. 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 burstbasis, which means that all the bits in a burst are transmitted in the same

frequency.

Advantages of Frequency Hopping

1. Frequency Diversity

In cellular urban environment, multi-path propagation exists in most

cases. Due to Rayleigh fading, short-term variations in received level are

frequently observed. This mainly affects stationary or quasi-stationary

mobiles. For a fast moving mobile, the fading situation can be avoided

from one burst to another because it also depends on the position of the

mobile so the problem is not so serious. Frequency Hopping is able to

take the advantage due to frequency selective nature of fading to

decrease the number of errors and at the same time they are temporallyspread. As a result, the decoding and de-interleaving processes can more


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effectively remove bit errors caused by bursts received whilst on fading

frequencies (errors will be randomly distributed instead of having long

bursts of errors). This increase in effectiveness leads to a transmission

quality improvement of the same proportion.

Frame Erasure Rate reduces due to 6 dB to 8 dB gain.

Number of reports with rxqual 6 and 7 reduce.

Reported values of rxlev are more concentrated around mean.

2. Interference Averaging


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Interference Averaging means spreading raw bit errors (BER caused by

the interference) in order to have random distribution of errors instead ofhaving burst of errors, and therefore, enhance the effectiveness of

decoding and de-interleaving process to cope with the BER and lead to

better value of FER.

With hopping, the set of interfering calls will be continually changing and

the effect is that all the calls experience average quality rather than

extreme situations of either good or bad quality. All the calls suffer fromcontrolled interference but only for short and distant periods of time, not

for all the duration of the call.

For the same capacity, Frequency Hopping improves quality and for a

given average quality Frequency Hopping makes possible increase in

capacity.

When more than 3 % of the reports have rxqual of 6 or 7 then voice

quality disturbances start to appear.

Gains (reduction in the C/I value needed to satisfy the quality

requirements involved in the criterion) from hopping relative to fixed

frequency operation can be achieved.

1/3 interference: 1 dB gain

i.e. if 1 out of 3 frequencies are experiencing a continuous interference a

gain of 1 dB in C/I requirement is obtained.

Similarly,





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The effective gain obtained with Frequency Hopping is due to the fact that

the interference effect is minimized and it is easier to keep it under

control.


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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.

DRCU always transmits fixed frequency.

The information for every call is moved among the available DRCUs on

a per burst basis. (Burst of 577 s)

Call hops between same timeslots of all DRCUs.

Processing (coding and interleaving) is done by digital part associated

with DRCU on which call was initially assigned.

For uplink call is always processed by DRCU on which the call was

initially assigned.

Number of DRCUs needed is equal to the number of frequencies in the

hopping sequence.


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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


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This is accomplished by high speed switching of transmit and receive

frequency synthesizers of the individual DRCUs. As a result of dynamicnature of the transmit frequency, broadband (hybrid) combining of the

transmitters is necessary.

DRCU changes transmitting frequency every burst.

Call stays on the same DRCU where it started.

Remote tune combiners (RTC) are not allowed.

Number of DRCUs is not related to number of frequencies in hopping

sequence.

BCCH can be included in the hopping sequence:

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 frequencyis not in the burst. Whenever BCCH frequency is being transmitted in a

burst by DRCU, it will be transmitted at full power.

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


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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

valee 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, madeup 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 randomizethe distribution of errors.


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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.


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Planning for Frequency Hopping

1. Frequency Plan:

Frequency Hopping plan differs from the conventional fixed frequency

plan. The plan depends upon the type of Frequency Hopping system

used. In case of SFH including BCCH frequency in hopping sequence is

not a practical option, as it results in loss of traffic channels on BCCH

carrier. A separate frequency plan is prepared for the BCCH

carriers. This planning is very much similar to the conventional fixed

frequency plan with lesser number of frequencies. This plan needs to be

done very carefully as the system monitors cells based on the BCCH

frequency only. Since BCCH carrier radiates continuously without

downlink power control, frequencies used for BCCH on one cell should not

be used as hopping frequencies on other cell. The reason is to avoid

continuous interference from BCCH carriers. The benefits of hopping

increase if more frequencies are available for hopping. Generally the

frequency band is divided into two parts, one used for BCCH frequency

plan and other for hopping frequencies. The division of frequency band

for allocation of BCCH and hopping carriers should be done to maintain

reasonable C/I for BCCH carriers as well as to have enough frequencies

for hopping.

e.g.

consider a network with 31 frequencies, using 12 frequencies for BCCH

and using 18 for hopping with 1 frequency as guard, is the ideal

option. But it may not be practically possible to plan BCCHs with 12

frequencies (4/12 reuse). Using 15 for BCCH plan and 15 for hopping

frequencies is more practical. There always exists a trade-off between

BCCH and hopping plans. Using very less frequencies for BCCH plan


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might result in poor quality on BCCH carrier and the advantages of having

quality improvement on hopping carriers may be lost. The ratio between

hopping and BCCH frequencies should be decided based on the ratio of

number of BCCH and NBCCH carriers in the network.

In case of BBH, generally BCCH carrier is included in the hopping

sequence. The benefits of BBH can be obtained only when most of the

sites in the network are having more than one NBCCH carriers. Benefits

of BBH comparable to SFH can only be obtained by equipping additional

hardware in order to include more frequencies in hopping

sequence. However BBH without additional hardware will result in quality

improvements and provide scope of additional capacity as compared tofixed frequency plan though the benefits may not be as significant as seen

in SFH.

2. Planning of HSN:

HSN allocation to the cells is done in random fashion. Various scenarios

are explained below:

a. MA list is same for all the cells of the site In this case HSN is kept

same for all the cells of the site. MAIO is used on per carrier basis to

provide offset for starting frequency in hopping sequence and avoid hitsamong carriers of the site. Practically it is possible to achieve 0% hit rate


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within the site, as all the cells of the same site are synchronized.

b. MA list is same for the cells of different sites In this case HSN should

be different for all such cells. MAIO can be same or different in this case

as HSN is different.

c. MA list is different for the cells In this case HSN planning is not

important, as there can not be any hits between these cells.

d. HSN is set to 0 This is the case of cyclic hopping. The sequence for

hopping remains same and is repeated continuously. This is not

recommended in the urban environment where frequency reuse is more.

This is because the network is not synchronized so if there is any one hit

it will result in continuous sequence of hits. Cyclic hopping is preferred inrural environment as it provides the maximum benefits of frequency

diversity.

3. Planning of MAIO:

The benefits of MAIO planning can be best achieved only in case when

sectors having same MA list are synchronized. For non-synchronized

sectors MAIO can be the same. In the previous version (GSR2), Motorola

did not provide manual MAIO setting. It was set automatically by the

system. However from GSR3 onwards it is be possible to set MAIO

manually. It has to be changed on a case to case basis. In cases where

there are large numbers of hits, MAIO change can be effective as it addsthe offset in the hopping sequence and hit-rate can be reduced.

4. Planning of FHI:


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This parameter is not specified in GSM. FHI is the Motorola defined

hopping system. It actually means an independent hopping system

consisting of MA and HSN. Total of 4 such hopping systems can be set in

a cell.

FHI can be defined on a timeslot basis.

e.g. consider a cell with 3 carriers i.e. 2 carriers are hopping. It is then

possible to define 4 different FHIs for 16 timeslots. That means timeslot

0 to 3 of 1 carrier can have one FHI and so on.

Benefits and Drawbacks of FHI

Separate FHI can be defined even for each carrier with separate MA

list.

For a fully utilized cell, FHI can be used to control increase in hitrate

during peak hours. This can be done by defining different MA list

associated with a FHI for one of the carriers.

Main benefits of FHI can be obtained in BBH. Consider a cell with 2

carriers using BBH with BCCH included in the hopping sequence. Timeslot0 of BCCH will not hop. A separate FHI (with MA list without BCCH

frequency) has to be defined for timeslot 0 of NBCCH.

Different FHIs in the same cell is not used extensively in Motorola

networks with SFH, where BCCH frequency is not included in hopping

sequence.

One drawback of using FHI on timeslot basis is that it adds more

complexity to the database.


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5. Reuse pattern for hopping carriers:

Conventionally there are 3 main reuse patterns followed for hopping

frequencies.

1 X 1:It means all the cells in the network use the same frequencies forhopping.

e.g. If 15 frequencies are to be used for hopping, then every cell will have

all 15 frequencies in the MA list. This type of reuse is useful in urban

areas, where capacity requirement is large. However there is very less

planning involved and so less control over quality problems.

3 X 9:Three hopping groups are used in 3 sites, one per site. In this

case all the sites should be considered as omni sites for planning

frequency reuse. The advantage of this scheme is it provides better

isolation between sites using same hopping frequencies. The problem

with this method is that, addition of new site may require frequency

replan for the area.

1 X 3:This scheme is very commonly used in Motorola

networks. Hopping frequencies are divided in 3 groups. Each cell on a

site uses one group and it is repeated on all sites. e.g. consider a

network with standard orientation, all V1 sectors will use the same group

and so on. It is very easy to add a site in the network. This reuse

scheme is suitable for homogeneous network with minimum overlapping

areas. The problem with this scheme is in peak hours there may be more

hits.


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Loading Factor: This parameter is a deciding factor for number of

carriers that can be equipped on a sector or a site. Number of carriers

equipped on a site or sector should not be greater than 50% of the

number of frequencies in the MA list of the sector or site. This factor is amajor distinguishing factor between 1 x 1 and 1 x 3.

6. Tools for simulation and drive test:Motorola uses a tool

Handsem which can simulate SFH plan (different reuse patterns and

HSN plan). Latest versions of plaNET and Golf are supposed to supportFrequency Hopping simulation. Drive test tools that display decoded layer

3 information are used for monitoring frequency hopping networks. TEMS

is one of the drive test tools that can be used for the purpose.


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Effect of Frequency Hopping

1. Handovers: When SFH is implemented, BCCH plan is done usinglesser number of frequencies as compared to fixed frequency plan. This

may result in quality degradation. However quality of hopping carriers

improves than before. Also, quality threshold for handovers on hopping

carrier should be increased as compared to fixed frequency plan. In the

present version (GSR3), different quality threshold settings are set BCCH

and NBCCH. By setting lower quality thresholds for BCCH as compared to

NBCCH, number of dropped calls can be controlled. Handover Success

Rate may go down because of the BCCH replan (less frequencies). This

reduction may get compensated due to improvement in quality of hoppingcarriers (improvement in TCH assignment success rate).

2. Call setup:In call setup, SDCCH hopping is also possible. There are

no separate settings required for SDCCH hopping. Since GSR3 allows

control over SDCCH configuration (location of SDCCH on timeslot basis),

SDCCH hopping depends on the location of SDCCH. In case of SFH (with

BCCH not included in MA list), if SDCCHs are on BCCH carrier they will nothop whereas SDCCHs on NBCCH carriers may hop. Generally it is


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preferred to keep SDCCHs on BCCH carrier as SDCCH timeslot is used

continuously and it will increase interference on hopping carriers. Call

success rate will depend on the cleanliness of BCCH carriers. Call Success

Rate may reduce after BCCH replan. This reduction may fet compensated

due to improvement in quality of hopping carriers (improvement in TCHassignment success rate).

3. Frame Erasure Rate (FER):FER indicates the number of TDMA

frames that could not be decoded by the mobile due to interference. This

parameter gives the indication of hit-rate. FER improves (gain of 6 to 8

dB) after implementation of frequency hopping. FER is represented in

percentage terms. FER less than 10% is considered to be good. But thisis a subjective issue and good value should be decided by doing multiple

drives. In future Motorola is planning to include FER as a statistics in the

OMCR.

Implementation of Synthesiser Frequency Hopping (Mumbai

Network):


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1. Frequency Planning:

Channels available: 32 to 62

Frequency band is divided in 2 parts. First 15 channels from 32 to 46 is

to be used as hopping frequencies.

Reasons for such division are:

It is recommended to use lower band for BCCH as it has better

penetration and also it is useful in roaming for logging on to the network.

Hutchison-Max is using channel 31 in hopping in entire network.

BPL Mobile is not allowed to use 32 as BCCH in South and Central

Mumbai (South of Bandra and Sion).

So there is no option to use 32 in hopping otherwise it will be

underutilised.

BCCH plan:

Channel 48 to 62 is used as BCCH. Channel 47, which is guard

channel (between hopping and non-hopping) is used selectively, in an

area where 46 is not in hopping sequence. Possible use of channel 47 in

hopping can also be considered on a case to case basis. (where 4 or 5carriers are required). But then interference from BCCH 48 must be


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considered.

4 x 3 frequency reuse plan is used which theoretically needs 12frequencies. However it is not possible to plan Mumbai network in 12

considering the terrain. So remaining 4 frequencies will be used

selectively. Also there is a plan to reserve these for micro cells. BCCH

plan has been made considering quality as a major criteria. E.g. sector

looking towards Altamount Road has lesser re-used frequency.

NBCCH plan:

1 x 1 plan: all 15 or 16 frequencies in all the cells. As per loading

factor definition there can be 7 NBCCH carriers equipped on a site. This

gives some flexibility to RF Planner to have irregular configuration on each

site. E.g. 3-3-4 or 3-4-3 or 4-3-3 or even 2-5-3 or 2-2-6 configuration

can be used on the site. It is even possible to use 8 NBCCH carriers on a

site but it will result in increase in interference in surroundingsites. However this can be used on a case to case basis. It was decided

to go for 1 x 1 after 1 x 3 implementation.

1 x 3 plan: the band is divided into 3 parts

Set 1 (S1) 32, 35, 38, 41, 44

Set 2 (S2) 33, 36, 39, 42, 45 Set 3 (S3) 34,

37, 40, 43, 46


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These sets are used on a sectorwise basis. Set 1 is used preferably in

sector V1 and so on. The use of these sets is related to the orientation of

the sectors, so that same set is not used on sectors looking at each other.

Since there are 5 frequencies in each sector, as per definition of loading

factor, there can be only 2 NBCCH carriers equipped on each sector. 3-3-

3 is the only configuration allowed in this plan. If configuration like 3-4-3

is required then 4 NBCCH should be in fixed frequency mode. Use of 4th

NBCCH is hopping carrier results in more hits on surrounding sectors that

are using same MA list set. However this can be used on a case to case

basis.

Other issues:

1. Previously in Area A in 129 cells (48 sites) BCCH frequency reuse was

maximum of 9 times (average use 4.4 times). In the new BCCH plan (forSFH) frequency reuse is maximum of 12 times (average use 8.6

times). The main reason for this is we used entire band of 30 channels

for BCCH earlier. However the new BCCH plan is made just from the 15

frequencies.

2. This increase in reuse is definitely going to degrade BCCH carrier

quality as compared to present situation.

3. BCCH carrier is very important for logging on to the network orstaying in the network. Even in the present plan we have observed

problems of logging on to the network (Express towers top floor). This

problem might elevate after new plan.

4. Present version of BSS software assigns SDCCHs on BCCH carrier

only. Since BCCH quality is going to degrade, option of SD location will

have to be purchased.

5. Hopping carriers will have much better call quality than presentfrequency plan. There is a possibility that the quality difference in quality


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on BCCH and NBBCH may be significant. TCH priority (priority to allocate

TCH) option may also be needed.

6. It is presumed that addition of the site is very easy in SFH. It is very

true for NBCCH carriers. But the BCCH plan for new sites is more difficult.

Planned events for implementation:

1. Reduction of overlaps of the existing coverage of all sectors by

antenna optimisation.

2. Preparation of BCCH Plan Area wise and drive test data collection

and analysis

3. Simulation of BCCH plan using NBCCH at full power feature.

4. Implementation of BCCH plan with NBCCH in fixed frequency mode.

5. Optimisation of BCCH plan.

6. Implementation of hopping plan on trial basis in Vashi BSC

7. Optimisation of Vashi BSC in 1 x 3 Hopping plan


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8. Testing of features in Vashi BSC

9. BCCH Plan for the entire network and monitor for 1 week

10. Optimisation of the BCCH Plan

11. Implementtaion of 1 x 3 hopping plan in the network

12. Drive test areawise collection of drive data and anlaysis.

13. GOS monitoring and analysis.

14. Optimisation of 1 x 3 Plan

15. Enable down link DTX and monitor

16. Plan for 1 x 1 plan.

17. 1 x 1 plan in the network

18. Optimise the 1 x 1 plan

Summary of Tests conducted:

a. CellAd Drive test (subjective voice quality - Mean Opinion Score)

b. Monitoring Customer Complaints

c. Addition/Deletion of carrier

d. Addition of Site

e. Parameter Changes MAIO/HSN

f. To try & use Guard Band

g. Install a Repeater


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h. Change in MA List ( No. of Frequencies)

i. Cell Broadcast Facility

j. SDCCH Hopping

k. Enable Uplink DTX

l. Enable combined and non-combined multiframe.

m. Enable Downlink DTx

n. Combination of DTx and multiframes.

o. RCU failures after switchover to hopping.

p. NBCCH full power in hopping environment

q. Extended paging.

r. Various combinations of extended paging, DTx and combined/non-

combined multiframe should be tried out.

Activities in SFH Implementation:

1. The parameters and statistics be monitored at OMCR

RF Loss rate

TCH RF Loss Rate


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SDCCH RF Loss Rate

Handover Failure Rate

Handover Success Rate

TCH Assignment Success Rate

Call Success Rate

Drop Call rate

Drop Call Rate per Erlang

Interference on Idle

Out_ho_cause_atmpt

2. Drive Test

Drive test should be done using Tems and CellAD or Buzzard

(subjective voice quality measurement).

Enough samples of drive tests should be taken before after each

change made to the network (or part of the network)

FICS report should be generated to check the drive test results.

GIMS/Mapinfo should be used to plot the drive test and to identify new

problem areas. Drive test plots should provide RXQual, RxLev,

Handovers, Handover Channel and Drop Calls. This is very useful to

identify the change in any of the areas and the cause. Separate FER plot

should be taken to identify the change.

3. Frequency Planning


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BCCH Plan is very critical for success of SFH

implementation. Motorola provides a feature called NBCCH fullPower. This ensures that NBCCH carrier radiates at full power even if

there is no call on it. This feature can be used to check the planned BCCH

reuse without affecting any of the BCCH carriers. Validation of BCCH plan

can be done using this feature.

Before Final implementation of hopping is should be implemented in

one BSC. Separate BCCH plan is required to be made for that period.

NBCCH Plan also should be kept ready. Initially Loading Factorconstraint (


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g. Change in MA List No Improvements

h. Locking of Boundary site neighbours No Improvements

i. Integration of Koperkhirane Difficulty in BCCH plan &

Ease in NBCCH Plan. Degradation in neighour sites observed

j. Downlink DTx No problems found.

k. Downlink DTx and extended paging Siemens S3+ does not do location

update.

l. Extended Paging No problems found

5. Summary of problems observed after 1 x 3 implementation:

Drive Tests:

Observations: From the drive results following are the areas that had shown Voice Quality

problems.

Area AAlta Mount Road, Walkeshwar, Chowpatty

Area BE.Moses Road, Bandra Kurla Complex, Mahim-Sion Link Rd.

Area CBand Stand, Carter Road, SV Road near Mithibai College, Western Express

Highway near Domestic Airport, Powai

Area D

LBS Road Near Kurla, Eastern Express Highway near Chembur.


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Area EMarve Road, Vasai, Essel World

In the above areas Rx Voice Quality was in levels of 6 & 7. However the call did not drop.

TEMS Drive was repeated to check consistency of the results. In some areas consistency

could not be established. The results are dependent on the traffic (loading) carried during the

drive and also on the quality of BCCH or the hopping carriers.

GOS Statistics:

Observation:In general, it was observed that in all the BSCs the HSR, CSR, DCR have

degraded marginally except in Powai BSC and Thane BSC, where, there was considerable

degradation in HSR.

Following sectors were found to be degraded:

CSR Degradation : HSR Degradation :

1. Flora Fountain V3 1. Cuffe Parade

2. Eros V2 2. Bandra Kurla V3

3. Santacruz V1 3.Mahim Station V1

4. Mahim Station V3 4.Vashi Sect-17 V3

5. Crawford Market V1 5.Belapur V3

6. Juhu Galli V2 6. Kalamboli V1,V3


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7. D Road V1 7. Girgaum V3

8. Kings Circle V3 8. Sewree V3

9. Mira Road V3 9.Sahar V1

10.MIDC V1

11.Powai Hirmdani V1

12.Mumbra V1

13.Kalyan V1

14.Ghodbunder Road V2

15.Charkop V1

Of all the BSCs it was observed that Powai BSC has degraded considerably in HSR, CSR and

DCR. Six sectors have HSR less than 90% in this BSC.

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.

DRCU always transmits fixed frequency.


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The information for every call is moved among the available DRCUs on a per

burst basis. (Burst of 577 s)

Call hops between same timeslots of all DRCUs.

Processing (coding and interleaving) is done by digital part associated with

DRCU on which call was initially assigned.

For uplink call is always processed by DRCU on which the call was initially

assigned.

Number of DRCUs 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.


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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.

DRCU changes transmitting frequency every burst.

Call stays on the same DRCU where it started.

Remote tune combiners (RTC) are not allowed.

Number of DRCUs is not related to number of frequencies in hopping

sequence.

BCCH can be included in the hopping sequence:

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.

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


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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 valee 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.

Hopping dip analysis

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