Selasa, 12 April 2011

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, multipath 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 ยตs)
· 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 the 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:
  1. 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.
  2. 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.

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 pseudorandom hopping sequences.

Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which frequency the mobile starts to transmit on. The value of MAIO ranges between 0 to (N-1) where N is the number of frequencies defined in the MA list. 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 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 pseudorandom 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.

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 might result in poor quality on BCCH carrier and the advantages of having quality improvement on hopping carriers may be lost.

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 present version (GSR2), Motorola does not provide manual MAIO setting. It is set automatically by the system. However from GSR3 onwards it will 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 very effective as it adds the offset in the hopping sequence and hitrate 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.
6. Effect of Frequency Hopping

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 (GSR2), same quality threshold settings are set for both BCCH and NBCCH. This may result on more drop calls on BCCH carriers. However GSR 3 provides separate settings for BCCH and NBCCH carriers. By setting lower quality thresholds for BCCH as compared to NBCCH, number of dropped calls can be controlled.

Call setup: In call setup, SDCCH hopping is also possible. There are no separate settings required for SDCCH hopping. b 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 hopping carriers as they have better C/I compared to BCCH carriers. Call success rate will depend on the cleanliness of BCCH carriers.

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 hitrate. FER improves (gain of 6 to 8 dB) after implementation of frequency hopping.

7. Tools for simulation and drive test: Motorola uses a tool “Handsem” which can simulate SFH plan (different reuse patters 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.

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