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.

Example:

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 sector of one 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 ALL sector 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 Sector 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. 

                                  Loading Factor = (BCCH / NBCCH) * 100

                  

In Case Of Reliance Telecom  Loading Factor =12/18 *100 = 66 %

                                                              

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.