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 burst basis, 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 temporally spread. As a result, the decoding and de-interleaving processes can more 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
Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having 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 from controlled 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.
1/4 interference: 4 dB gain
1/5 interference: 6 dB gain
2/4 interference: 0 dB gain
2/5 interference: 4 dB gain
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.
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 µs)
· 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.
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
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.
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.
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.
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 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 to fixed 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 hits among carriers of the site. Practically it is possible to achieve 0% hit rate 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 in rural 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 adds the offset in the hopping sequence and hit-rate can be reduced.
4. Planning of FHI:
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. Timeslot 0 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.
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 for hopping.
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.
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 a major 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 support Frequency 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.
Effect of Frequency Hopping
1. Handovers: When SFH is implemented, BCCH plan is done using lesser 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 hopping carriers (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 not hop whereas SDCCHs on NBCCH carriers may hop. Generally it is 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 TCH assignment 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 this is 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):
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.
· 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 5 carriers are required). But then interference from BCCH 48 must be considered.
· 4 x 3 frequency reuse plan is used which theoretically needs 12 frequencies. 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.
· 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 surrounding sites. 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
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.
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 (for SFH) 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 or staying 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 present frequency plan. There is a possibility that the quality difference in quality 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
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
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
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
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
· BCCH Plan is very critical for success of SFH implementation. Motorola provides a feature called ‘NBCCH full Power’. 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 Factor constraint (<50%) must be followed. In cases where Loading Factor constraint can not be followed, those carriers should be kept in fixed frequency mode.
4. Summary of test results
a. SDCCH Hopping No Problem Found
b. Up Link DTX No Problem Found
c. Combined/non-combined multiframe No Problem Found
d. Non Combined & Extended Paging No Problem Found
e. NBCCH at Full Power Quality degradation observed in
some areas but no degradation in FER.
f. 1 X 1 re use plan Some Quality improvement observed
in Vashi Bridge area
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
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 A – Alta Mount Road, Walkeshwar, Chowpatty
Area B – E.Moses Road, Bandra Kurla Complex, Mahim-Sion Link Rd.
Area C – Band 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.
Area E – Marve 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
7. D Road V1 7. Girgaum V3
8. Kings Circle V3 8. Sewree V3
9. Mira Road V3 9.Sahar V1
11.Powai Hirmdani V1
14.Ghodbunder Road V2
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.
In Thane BSC, it was observed that the HSR has gone low in 4 sectors and DCR has gone high in 2 sectors.
· Coverage Reduction:
It was observed that after implementing hopping in the network, following sectors have shown coverage reduction. This was observed during the drive tests and problem statements have been issued to check the BTS power and VSWR.
The sectors are:
1. Andheri V1
2. Bandra Causeway V2
3. IES V1 (for additional 2 carriers)
4. Thane V2
5. Thane Stn V1
6. Vashi V2
7. Kings Circle V1
8. Kalina V2, V3
9. Malad V3
· Power Control in SFH :
Uplink and downlink power control is enabled in the network. No changes have been made in parameters that were working in fixed frequency plan.
If there is degradation in quality the downlink or uplink power increases. But sometimes intracell handover takes place without mobile or BTS radiating at full power. Ideally downlink or uplink must radiate at full power before any handover.
· Idle Channel Interference:
Observation: In the following sectors wehave observed that the Idle Channel Interference was very high during some parts of the peak hour. Due to this there was degradation in call success rate and SD loss factor. In case of Flora Fountain V3 the timeslots on BCCH showing high idle channel interference went out of service.
1. Goregaon V3 – only on BCCH
2. Sher – E – Punjab V3 – only on BCCH
3. JP Road V2 - both
4. Flora Fountain V3 – only on BCCH
The problems gets solved after change of frequency to dummy carrier.
· Loading Problem on sites with 3 hopping carriers:
Observation: In sectors that have 3 NBCCH Carriers, it is observed that there is degradation in performance of surrounding sectors with same MA list during the peak hours. This is due to the fact that 3 NBCCHs are hopping on 5 Frequencies of the MA List. The problem observed in area near Flora Fountain V2 (degradation in Eros V2) and in Santacruz Market V1 (degradation in Santacruz V1). Presently, the 3rd NBCCH hopping carrier is converted to fixed frequency carrier. After this change the performance of the sites have improved. Still in the network following four sectors are with 3 NBCCH hopping carriers: Pancharatna V1, Opera House V3, Vashi BSC V2, Hindu Colony V3.
· Carrier Additions in sites which has 3 Carriers :
Need has come to increase the number of carriers in JP Road V1 from 3 to 5. Due to the MA List of only 5 carriers and also due to the loading problem, further carriers cannot be added in the 1x3 Plan. Hence we had to resort to allocating Dummy frequencies and to put them in Fixed frequencies. Thus 1x3 hopping plan restricts carrier addition in sites that has 3-3-3 carriers.
· Repeater site Visits:
After implementation of SFH, following few repeater sites were visited to check for the coverage.
1. Heera Panna
2. Churchgate BPL Gallery
3. PH Business Centre
4. Kamats, Cuffe Parade
There was not any degradation in signals observed in the above places. The power level and coverage were the same as it was during the Fixed plan.
· Data Call :
On drive data call was made to check the data call feature in Hopping mode. The data was tried both in Mobile to Mobile calls and in Mobile to PSTN Call.
The data call facility worked well in areas where the Rx Qual was upto 6. In areas where there were multiple 6 & 7 the data call dropped.
Also it was observed that for files of size 200KB, the data transmission takes at a speed of 8KB to 8.5KB and the data rate goes down to 5KB. Never the data is transferred at 9600KB.
· Co Channel BCCH and Co Channel BSIC –Care has been taken to avoid Co channel BCCH and Co BSIC problems during implementation of SFH, it was observed that the Co channel BCCH and Co BSIC of neighbours also to be avoided. This problem is not visible on field but it results in degradation in HSR. Hence as a precautionary measure, following BSIC changes were done in the network.
· Downlink DTx
1. Location update tests were done in location area boundary between Sion V1 and Chunabhatti V2 (LAC 122 and 123). These checks were done to check location update from Sion V1 to Chunabhatti V2 and back. Calls were initiated and received on all the phones to check whether location update has taken place.
2. Following phones were used for testing:
Siemens S4, S4 power
Ericsson GH688 (TEMS)
Following Features were tried with different combinations and drive tests were done: BCCH Combined
BCCH Non Combined
SDCCH on BCCH SDCCH on NBCCH Extended Paging
Tests and Results:
1. Drive 1: Sion V1 – BCCH combined, SD on BCCH, no extended paging, DTx enabled.
- Result: No problems found
2. Drive 2: Sion V1 – BCCH non combined, SD on NBCCH (hopping), no extended paging, DTx enabled.
- Result: No problems found
3. Drive 3: Sion V1 – BCCH non-combined, SD on NBCCH (hopping). Extended paging active, DTx enabled.
- Result: Problem with S3+, it did not perform location update. Call origination and termination was not possible. Phone locked to Sion V1 (Reselection was possible without location update) as phone was showing full signal strength. Even after turning off/on, location update was in consistant. This was due to Extended Paging being Active.
4. Drive 4: Sion V1 – BCCH non-combined, SD on NBCCH (hopping), no extended paging, DTx enabled.
- Result: No problems found
5. Drive 5: Sion V1 - BCCH combined, SD on BCCH, no extended paging, DTx enabled.
- Result: No problems found