FP Nokia Guide

8/4/2019 FP Nokia Guide

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FH GUIDELINES 1 (21)

Frequency Reuse on Frequency Hopping Network

Since the frequency band is always limited, the frequencies have to be reused in the network. As thereuse distance becomes smaller, there are more frequencies available for each cell. Because each TRXin a cell requires a unique frequency, the capacity potential of a cell is increased, as there are morefrequencies available for each cell. However, when the reuse distance becomes small enough, all thefrequencies available for the cell cannot be utilised because of too severe interference in the cell borderareas. For a conventional non-hopping network this is the practical frequency reuse limit. The BBhopping network has this same limit, but because of frequency hopping gain, somewhat lower reusedistances are allowed before the quality reaches the minimum acceptable limit.

The advantage of RF hopping is that the frequency reuse distance can be set as low as wanted. Thiscan be done, because a RF hopping cell can use more frequencies than there are TRXs installed. Thismeans that the used frequencies are only fractionally loaded as presented in Section 0. For a fractionallyloaded RF hopping network, two reuse figures have to be defined. These are effective reuse and

frequency allocation reuse. They are presented in the following sections.

Effective Reuse

The effective reuse is essentially the same as the conventional frequency reuse distance. It is calculatedas

RN

Neff

freqsTOT

TRXave

= ,

( 5.1 )where:

Reff = effective reuse

NfreqsTOT= total number of used frequencies NTRXave = average number of TRXs in a cell

Since the effective reuse takes the actual number of frequencies together with the number of TRXs intoaccount, it can be also used as a capacity index, provided that the TRXs can be loaded at least to thehard blocking limit as presented in Section 0. The smaller the effective reuse, the higher the capacity interms of the number of TCHs provided by one frequency in the network.

Frequency Allocation Reuse (RF FH only)

Frequency allocation reuse indicates how closely the frequencies are actually reused in a network. Thus,it indicates the severity of a worst case C/I in the cell border. It is calculated as

FARN

N

freqsTOT

freqs MA

=/

,

( 5.2 )

where:

FAR= frequency allocation reuse

NfreqsTOT= total number of used frequencies Nfreqs/MA = average number of frequencies in MA-lists


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If the network doesnt utilise fractional loading, the frequency allocation reuse is the same as theeffective reuse. Example of the reuse calculations for the fractionally loaded RF hopping network is

presented in Figure 0-1.

Frequency Allocation Reuse = Total # offrequencies / # of frequencies in MAL

Effective Reuse = Total # of frequencies/Number of TRXs per cell

Frequency Allocation Reuse Effective Reuse

Total # of freqs = 30

4 TRXs / cell

10 frequencies / cell

32

1

1 12 2

33

1/3

FAR = 30/10 = 3

Eff.reuse = 30/4 =7.5

Example:Example:

Figure 0-1. Example of reuse calculations.

Load on Networks Utilising Fractional Loading (RF FH only)

One of the most essential parameters of the fractionally loaded RF hopping network is the load. The load

on the frequencies is the most important one since it determines the probability of collisions. Collisionmeans that the serving cell and an interfering cell are transmitting at the same frequency at the sametime so that the potential interference becomes reality.

Frequency Load

When designing a network with low frequency allocation reuse, the interference sources are very close.Even a neighboring cell may be an interferer by sharing at least some of the frequencies. In that kind ofsituations the C/I is very low when the collisions occur. In order to guarantee an adequate quality, thecollision probability has to be made low. The closer the interferers, the more infrequent the collisionsmust be in order to maintain a proper quality. The collision probability depends on the load of thehopping frequencies called a frequency load. The frequency load describes the probability that afrequency channel is used for transmission at one cell at one time.

The frequency load is a product of two other loads: the average busy hour TCH occupancy, whichshould in most cases be equal to the hard blocking load that is presented in Section 0, and thefractional load that is presented in Section 0. The frequency load can be written as

L L L freq HW frac= ,

( 5.3 )

where:

Lfreq = frequency load

LHW= the busy hour average hard blocking load


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Lfrac = fractional load

Each frequency allocation reuse corresponds to a different C/I at the cell border, thus requiring adifferent maximum allowed frequency load in order to keep the collision probability low enough.

Hard Blocking Load

Hard blocking means that all the available traffic channels in the cell are in use and all the new callattempts fail because of the lack of available traffic channels. If it is assumed that the call attempts occurrandomly, then the number of call attempts in a time interval is Poisson distributed. If the call attemptsare Poisson distributed and the length of the calls is exponentially distributed, then the hard blockingprobability (that is also known as the grade of service) can be calculated by using the Erlang B formula

B

T

N

T

n

N

TCH

n

n

N

TCH

TCH=

=

!

!0

,

( 5.4 )

where:

B= hard blocking probability

T= offered traffic (Erl)

NTCH = number of TCHs in the cell

In order not to exceed the predefined hard blocking probability, the average busy hour TCH occupancymay not exceed the threshold defined by the offered traffic at the desired blocking probability and thenumber of TCHs. When determining the hard blocking load, only the non-BCCH TRXs should beconsidered as illustrated in Figure 0-2. Thats because the BCCH TRX is non-hopping in RF hopping celland the calculation of the loads is only relevant in soft blocking limited network. Currently soft blockinglimited BB hopping networks should not be designed because of the lack of the gatekeeper algorithm,which prohibits the initialisation of new calls if the load in the network is about to exceed the loadthreshold at the soft blocking limit. The hard blocking load is calculated as

LT

NHW

hopTCH

hopTCH

= ,

( 5.5 )where:

LHW = hard blocking load

ThopTCH= average number of used TCHs in the busy hour NhopTCH = total number of TCHs in the hopping TRXs


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BCCH SDCCH SDCCH TCH TCHTCHTCH

TCH TCH TCH TCH TCHTCHTCH



TRX-1

TRX-2

TRX-3

TRX-4

f1

f2,f3,f4

f3,f4,f2

f4,f2,f3

TCH

TCH

TCH

TCH

Active slots Empty slots

75 % 25 %

Load on the BCCH TRXnot considered, sincethe BCCH frequenciesare planned separately

Figure 0-2. Hard blocking load of 75% on RF hopping TRXs.

The average busy hour TCH load, as defined in Equation (5.5), can be used as the maximum TCHoccupancy. In reality, there are times when the TCH occupancy is over the busy hour average LHW.However this happens randomly and since the LHW limit is an average there is about an equal time inwhich the load is less than the LHW. If the offered traffic is Poisson distributed, the frequency allocationcan be quite safely dimensioned by using the LHW as the maximum TCH occupancy. In an environmentwhere the offered traffic is known not to be randomly generated, a higher figure should be used.

Fractional Load

Fractional loading means that the cell has been allocated more frequencies than there are TRXs asillustrated in Figure 0-3. This is only possible for RF hopping TRXs. The fractional loading is very usefulwhen the number of TRXs is low. By utilising fractional loading, it is possible to provide enoughfrequencies to hop over (to get FH gain) to even a cell with just one hopping TRX. Fractional load can becalculated as

LN

Nfrac

TRX

freqs cell

=/

,

( 5.6 )

where:

Lfrac= fractional load

NTRX = number of TRXs in a cell Nfreqs/cell = number of frequencies allocated to a cell (MA-list length)


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BCCHTRX-1

TRX-2

TRX-3

TRX-4

f1

f2, f3, f4, f5, f6

f2, f3, f4, f5, f6

f2, f3, f4, f5, f6

Active slots Empty slots Frac. load = 3/5 = 0.6

Figure 0-3. Fractional load of 0.6.

In a soft blocking limited network the fractional load is used to tune the frequency load down to a desiredlevel, which is determined by the used frequency allocation reuse.

Trunking Effect and Effective Reuse

For Poisson distributed call attempts, it is characteristic that the hard blocking load providing the sameblocking probability increases as the number of traffic channels increases as presented in Figure 0-4.This is called trunking effect. For a hard blocking limited network this is a real gain since the network isable to serve more traffic with the same grade of service and the same effective reuse. However, for asoft blocking limited network utilising fractional loading the trunking effect doesnt provide any gain. Asthe hard blocking load increases, the fractional load must be decreased in order to keep the frequencyload and thus the collision probability acceptable. Decreasing the fractional load is done by adding morefrequencies than TRXs to the cells. This has a direct effect on the effective reuse. The effective reusecan be rewritten as

RN

N

N

N

N

N

FAR

Leff

freqsTOT

TRX

freqsTOT

freqs MA

freqs MA

TRX frac

= = =/

/.

( 5.7 )

Equation (5.7) shows the fixed relation between the effective and frequency allocation reuses and thefractional load. The required increase in the effective reuse in a soft blocking limited network as thetrunking efficiency increases is presented in Figure 0-5. It should be noted that although the effectivereuse increases, the number of frequencies required to handle a certain amount of traffic stays constant.The effective reuse doesnt take the trunking efficiency into account.


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0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

70.0 %

80.0 %

90.0 %

100.0 %

1 4 710

13

16

19

22

25

28

31

34

37

40

43

46

49

52

55

58

61

64

67

70

73

76

79

82

85

88

91

94

97

100

Number of TCH's

TCHoccupancyatthehardblockinglimit

Hard blocking prob. 5%



Figure 0-4. Average busy hour TCH occupancy at the hard blocking limit.

5

6

7

8

9

10

11

12

2 3 4 5 6 7 8 9 10 11 12

TRX's/cell

effectivereuse

FAR 1 (2% Blocking, Freq.load 7,5% (trialed))FAR 1 (1% Blocking, Freq.load 7,5% (trialed))FAR 3.65 (2% Blocking, Freq.load 30% (trialed))FAR 3.65 (1% Blocking, Freq.load 30% (trialed))FAR 3 (2% Blocking, Freq.load 30% (simulated))FAR 3 (1% Blocking, Freq.load 30% (simulated))

Figure 0-5. Increase of required effective reuse on a soft blocking limited network due to the

better trunking efficiency on bigger cell configurations.


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Frequency Allocation Strategies

When preparing for a frequency allocation, some decisions have to be made concerning the wantedfrequency allocation reuse and the corresponding frequency load. Also, it must be decided whetherto use a separate frequency band for the BCCH carriers or use a common band for both the BCCH andthe normal TCH TRXs.

BCCH Allocation

The BCCH carriers are special in a sense that the transmission to the downlink direction is constant andalways active on them. There are two basic approaches in the BCCH allocation. The BCCH frequenciesmay be allocated from a separate dedicated frequency band or the frequencies for the BCCH TRXsand the TCH TRXs (TRXs not carrying the BCCH) may be allocated from one common band.

Both approaches have been simulated for frequency hopping network in [Kro97]. In this simulation, the

used frequency band was 27 frequencies corresponding to 5.4 MHz. For the dedicated band strategy12 frequencies were dedicated to the BCCH TRXs and the remaining 15 frequencies were used as TCHfrequencies, which were allocated by using a slow Adaptive Channel Allocation (ACA) algorithmpresented in [Alm96]. In the common band case, the BCCH frequencies were first allocated by using areuse of 27. The ACA algorithm was then used to select the TCH frequencies for each cell. The BCCHfrequencies were not changed during this procedure. In both cases three different TRX configurationswere simulated. The cells had 3, 4 or 5 TCH TRXs depending on the case. In every case, the averagereuse is the same in both strategies, so the results are easily comparable.

In the simulation, the signal powers were averaged over a period of 0.48 seconds. During this period, allthe frequencies in the hopping sequence have been used several times. Thus, the fast fading can beassumed to have been removed by averaging. The slow fading was assumed to be constant over theaveraging period. Both, the co-channel and the adjacent channel interference were considered. The

simulated hopping mode was random BB hopping. Frequency diversity effect was not considered. Theused interference limited network consisted of 108 cells in three sectorised configuration having a radiusof 1 km. The mobiles were randomly generated and static. Power control and DTX were used in the bothdirections.

The system performance was measured by determining the 10 percent Cumulative Distribution Function(CDF) value of the C/I ratio. The load measure was defined as the number of served users per cell usingthe time slot one.

The uplink performance as a function of served traffic is presented in Figure 0-6. It can be seen that thecommon band strategy performs better. The improvement is 1-2 dB. The more uniform reuse providedby the common band strategy is more effective, because the continuous transmission on the BCCHTRXs is only employed in downlink direction. The BCCH reuse of 12 forces the reuse on the TCH TRXs

to be very tight. This is unnecessary in uplink direction since the load is about the same on the BCCHand TCH TRXs. The common band strategy is better when the uplink is considered. However, theuplink is not usually the limiting link in interference limited networks, since antenna diversity isnormally utilised at base stations.

The downlink performance on the TCH TRXs as a function of served traffic is presented in Figure 0-7.For a downlink direction the dedicated bands strategy is superior. The improvement is on the orderof 1-5 dB depending on the traffic load. The degradation of the C/I ratio is quite slow as the traffic loadincreases in common bands case. This indicates that the BCCH transmitters are the main interferencesource. It was also shown in additional simulations that the performance gainfromthe power controland the DTX in the common band systems were smaller than in corresponding dedicated band


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systems. This happens, because the BCCH frequencies, which are the dominating interference source,cannot utilise the PC or the DTX.

The downlink performance on the BCCH TRXs as a function of served traffic is presented in Figure 0-8.The downlink performance on the BCCH TRXs is important, because the call initialisation always startson the BCCH frequency and the BCCH frequencies have to be clean enough to guarantee successfuldecoding of the cell identification for handover purposes. The common band strategy performs clearlybetter when the load is small. As the load increases on the interfering TCH TRXs, the performancedegrades rapidly. The dedicated bands strategy provides a very stable behavior as the traffic loaddoesnt have any effect on the performance. In the dedicated band case the C/I of the BCCH frequenciesin the downlink direction is exclusively determined by the used frequency reuse on the BCCH TRXs.Because of the stable and easily predictable behavior on the BCCH frequencies in the downlinkdirection, the dedicated bands strategy is preferable.

Figure 0-6. UL C/I at the 10 % level.

Figure 0-7. DL C/I at the 10 % level.


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Figure 0-8. DL C/I at the 10 % level on the BCCH frequency.

Still one, not common used method is to use separate but not continuous band for the BCCHfrequencies. For example, every 4th frequency is allocated for BCCH. Thus, adjacent channelinterference is avoided between BCCH frequencies. On the other hand, TCH band causes adjacentchannel interference for the BCCH frequencies and vice versa, but the interference might not be toosignificant.

Figure 0-9. Different BCCH allocation strategies.

1.1.1 Selecting the Effective Reuse (BB FH)

With BB hopping, the fractional loading cannot be utilised and the number of hopping frequencies is

always the same as the number of TRXs in a cell, except for TCHs on the zero time slots, whichalways have one hopping frequency less than the other TCHs. Thus, in a BB hopping network thefrequency allocation reuse always equals the effective reuse in the network.

Since frequency and interference diversity gains significantly depend on the number of hopping

frequencies, it is recommended to have at least three hopping frequencies as a minimum

configuration. If the cell TRX configurations are smaller than that, BB FH is not recommended to be

used. In that case, RF FH or IUO might offer a better solution to increase the capacity.

BCCH TCH

BCCH + TCH

BCCH TCH

Dedicated band

Common band

Dedicated mixed band


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Before making the actual frequency plan by using the frequency allocation tool like NPS/X, an estimation

of the minimum effective reuse might be needed, for example in tendering phase. The following Figure

0-10 gives an estimation of an applicable reuse compared to the situation before implementing BB FH.For example, if we have in the non-hopping network reuse 15, after implementing BB FH with 4 TRX

average configuration per cell, we end up to reuse 9. The bigger is the TRX configuration, the smaller

reuse we can use, since the reuse is dependent on the number of hopping frequencies (=TRXs with BB

FH).

MIN Effective Reuses with different TRX configurations in BB

FH case

0

2

4

6

8

10

12

14

16

18

20

3 6 9 12 15 18

Original reuse

New

reuse

No FH

3 TRX

4 TRX

5 TRX

6 TRX

Figure 0-10. Effective reuse after implementing BB FH.

Selecting the Frequency Allocation Reuse and the Frequency Load (RF FH)

If the RF hopping is used, the frequency allocation reuse has a great impact on the required fractional

load and thus, on the number of frequencies allocated to each cell. With BB hopping, the fractionalloading cannot be utilised and the number of hopping frequencies is always the same as the number ofTRXs in a cell, except for TCHs on the zero time slots, which always have one hopping frequency lessthan the other TCHs. Thus, in a BB hopping network the frequency allocation reuse always equals theeffective reuse in the network.

Since frequency and interference diversity gains significantly depend on the number of hoppingfrequencies, it is important to ensure that each cell has enough hopping frequencies. If the cell TRXconfigurations are small, RF hopping with fractional loading makes it possible to still providesufficient number of hopping frequencies to the cells even with small TRX configurations.Fractional loading reduces the average channel utilisation in the network, thus reducing the probabilitythat interference will occur, making it possible to significantly decrease the frequency reuse distance.


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start with a low frequency load and then increase it gradually until the quality threshold isreached.

Important is also to ensure that the effective reuse is not too low to ensure a good quality. The followingTable contains an example of choosing the right F.A. reuse scheme to give the best capacity gain. Ascan be seen, the best capacity is got with the F.A. reuses 2-5. The minimum effective reuse andmaximum frequency load values are still under further consideration. They might be toooptimistic for some environments!

Table 1. Limits for the effective reuse and the frequency load values with different

frequency allocation reuses.

1.1.2 Frequency Sharing by Using MAIO Management (RF FH only)

The MAIO management makes it possible to share the same MA-list between the cells of the same RF

hopping site without co- or adjacent channel collisions. This can be done by utilising the user definableMAIOoffset and MAIOstep parameters presented in Sections Error! Reference source not found. andError! Reference source not found.. MAIOOFFSET helps to avoid the interference between the cellsinside the site, whereas, MAIOSTEP avoids the interference inside the cell. The cell level MAIOoffsetparameter defines the MAIOs for the first TRXs in each cell. The remaining TRXs are given MAIOsaccording to the Equation (3.1). In Nokia implementation the default MAIOstep is 1, but it will beadjustable after the BSC software release S7.

The frequency sharing makes it possible for a cell to hop over all the frequencies allocated to that site aspresented in Figure 0-12. All the cells on a site share the same MA-list. Thus, in a case of a threesectorised site, the site can be allocated three times less frequencies and still the number of frequenciesto hop over in a cell remains the same. Since less frequencies are needed per site, the frequencyallocation reuse distance can be bigger. The bigger reuse distance leads to less interference, so the

fractional loading is not necessarily needed.

Example: 21 frequenciesF.A. reuse MA list length Min. Eff. reuse Max. Freq. load Traffic (Erl) TCHs

1 21.0 8.5 8% 13.4 21

2 10.5 7.5 20% 16.8 25

3 7.0 7 30% 16.8 25

4 5.3 6.5 40% 16.8 255 4.2 7.5 50% 16.8 25

6 3.5 8.5 55% 15.4 23

7 3.0 10.5 60% 14.4 22

8 2.6 12 65% 13.7 21

9 2.3 13 70% 13.1 20


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MA-list: 3 6 9

1

3

2

3

6

9

TDMA frame n-1

1

3

2

TDMA frame n+1

1

3

2

3

6

9

TDMA frame n

6

3

9

Figure 0-12. The principle of frequency sharing.

However, there are some requirements that have to be fulfilled. First of all, the basic requirement is thatthe cells at one site have to be controlled by the same BCF, so that they are frame synchronised.With the current Nokia equipment this requirement limits the maximum TRX configuration to 12 TRXsper site.

The number of frequencies (MA-list length) have to be at least equal (equal if fractional loading is notto be used) to the total number of TRXs in the site. If the MAIOstep parameter is more than one, evenmore frequencies are needed. The requirement can be formulated as follows

stepsiteTRXsitefreqsMAIONN = //min ,

(5.8)

where:

min Nfreqs/site= minimum number of frequencies needed for a site

NTRX/site = total number of TRXs on a site

MAIOstep= the value of the MAIOstep parameter

In Equation (5.8) it is assumed that the MAIO separation between the cells is equal to the used MAIOstep.In that case, the MAIOoffset parameters are allocated as follows

=

=1

1

/

n

i

cellTRXstepcell inNMAIOMAIO ,

(5.9)where:

MAIOoffset n= MAIOoffset for the n th cell in a site MAIOstep= the value of the MAIOstep parameter

NTRX/cell i = number of TRXs in i th cell

If the number of frequencies is less than min. Nfreqs, then co- or adjacent channel interference might occur.Example of this is presented in Figure 0-14. In a normal frequency sharing arrangement, the goal is tominimise the number of frequencies needed per site, so that the frequency allocation reuse distancecan be kept high. For this reason, the MAIOstep should be normally 1. This should be taken into accountin the frequency planning process, because an intracell adjacent channel interference should not beallowed. Since the frequencies have to be in the increasing order in the MA-list, the list may not containadjacent channels if the MAIOstep is 1.


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The cells at one site have to use the same HSN. Otherwise, co-channel interference between the

cells will occur. However, the HSNs should be different in interfering sites in order to ensure

the interference diversity. An example of a correct parameter assignment for frequencysharing is illustrated in Figure 0-13.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MA_LIST1: 1 4 8 10 15 20

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MAI 1 5 1 1 2 3 1 2 0 2 3 4 5 1 5 2 4

TDMA-FRAMES ->

SECTORMA-LISTHSNMAIO TRX 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 161 1 3 1 bcch frequency 1...

0 2 4 20 4 4 8 10 4 8 1 8 10 15 20 4 20 8 151 3 8 1 8 8 10 15 8 10 4 10 15 20 1 8 1 10 20

2 1 3 1 bcch frequency 2 ...2 2 10 4 10 10 15 20 10 15 8 15 20 1 4 10 4 15 1

3 1 3 1 bcch frequency 3 ...3 2 15 8 15 15 20 1 15 20 10 20 1 4 8 15 8 20 44 3 20 10 20 20 1 4 20 1 15 1 4 8 10 20 10 1 85 4 1 15 1 1 4 8 1 4 20 4 8 10 15 1 15 4 10

Figure 0-13. Example of frequency sharing when MAIOstep is 1.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MA_LIST1: 1 4 8 10 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MAI 1 1 1 1 2 3 3 4 0 2 3 0 1 1 1 4 4

TDMA-FRAMES ->SECTORMA-LISTHSNMAIO TRX 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 3 1 bcch frequency 1...0 2 4 4 4 4 8 10 10 15 1 8 10 1 4 4 4 15 151 3 8 8 8 8 10 15 15 1 4 10 15 4 8 8 8 1 1

2 1 3 1 bcch frequency 2 ...2 2 10 10 10 10 15 1 1 4 8 15 1 8 10 10 10 4 4

3 1 3 1 bcch frequency 3 ...3 2 15 15 15 15 1 4 4 8 10 1 4 10 15 15 15 8 84 3 1 1 1 1 4 8 8 10 15 4 8 15 1 1 1 10 105 4 4 4 4 4 8 10 10 15 1 8 10 1 4 4 4 15 15

Figure 0-14. Example of frequency sharing when the site is allocated with too few frequencies

and co-channel interference between sectors exists.

Since the cells on the same site share the same frequencies, all the hopping frequencies are transmittedin every cell on the same site. This has to be taken into account when the frequency planning is done.This can be modeled in NPS/X 3.2 or older by utilising power dividers so that the site has only one cellhaving as many TRXs as there are non-BCCH TRXs in all the sectors of the actual site. The cell isdistributed to multiple antennas forming multiple sectors by using power dividers. Special care has to betaken to compensate the losses of power divider. In frequency allocation phase one common

interference probability is determined for the entire site and the site is then allocated one common set offrequencies that form the MA-list. To avoid interference, the minimum channel separation has to be atleast 1. Since each cell has its own BCCH, the BCCH allocation has to be done separately without thepower divider arrangement.

Simulation results of the performance of a network utilising frequency sharing have been presented in[Nie98]. In this simulation, the network utilising frequency sharing at a nominal reuse of 3/9 wascompared to the RF hopping network using 1/3 frequency allocation reuse at 33 % frequency load. Thereuse on the BCCH carriers was 4/12 in both cases. The served traffic was also the same in both cases.The simulated network consisted of 48 3-sectorised sites. Power control was utilised in DL direction, butthe DTX was not activated. Downlink FER statistics reported by each mobile every 0.48 seconds from


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the non-BCCH carriers were collected for analysis. Mobile speeds of 3 km/h and 50 km/h weresimulated.

The resulting cumulative density functions of DL FER have been presented in Figure 0-15 and Figure0-16. In both mobile speeds, the performance of the two simulated arrangements is very similar until theFER gets close to 10 %. For the mobile speed of 3 km/h the percentage of FER samples indicating FERabove 15 % is 2 % for the frequency sharing case and 3 % for the 1/3 reuse case. For the mobile speedof 50 km/h, the corresponding values are about 1.1 % and 1.5%. The difference in favor of frequencysharing is clear, although not dramatic. However, as higher FER percentages are studied, thedifference gets bigger.

The effect of the mobile speed on the FER distribution can be clearly seen. As the speed increases to50 km/h, the share of both the low FER percentages and the high FER percentages increases. Thehigher mobile speed provides better performance against fast fading. This increases the proportionof low FER. The higher speed also means that the changes caused by slow fading are faster and the

ability of power control to compensate the fluctuations of signal strength is reduced. This alongwith the relatively slow handover algorithm causes the proportion of high FER to increase at the highermobile speeds. However, the mobile speed doesnt have significant effect on the relative performance ofthe network utilising frequency sharing.

It may be concluded according to this simulation that the frequency sharing provides better qualitycompared to the 1/3 reuse case.

0.001

0.01

0.1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

FER

CDF

BCCH reuse 4/12, TCH reuse 1/3

BCCH reuse = 4/12, TCH reuse = 3/9 by using MAIO-management

Figure 0-15. CDF of DL FER for a mobile speed of 3 km/h.

0.001

0.01

0.1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

FER

CDF

BCCH reuse 4/12, TCH reuse 1/3

BCCH reuse = 4/12, TCH reuse = 3/9 by using MAIO-management

Figure 0-16. CDF of DL FER for a mobile speed of 50 km/h.


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Frequency Sharing in the Single MA-list Scheme (RF FH only)

Frequency sharing can also be used to realise the usage of only one MA-list in the networks utilisingsectorised base station configurations. In the single MA-list scheme all the cells use the same set offrequencies. If the cells in one site use the same MA-lists without the frequency sharing functionality,occasional co-channel collisions will happen between the cells of one site. When frequency sharing isused, it can be ensured that no unnecessary co- or adjacent channel collisions will occur provided thatthe cells on the same site use the same HSN.

When the single MA-list scheme is employed, a continuous frequency band is usually allocated to the

cells. In order to avoid intracell adjacent channel interference, the MAIOstep should be set to at least 2.

Preferably, even bigger step should be used, especially if uplink power control is not in use. Because

interference between the cells of the same site is much less likely to occur than intracell interference, a

smaller channel separation can be used between the cells of the same site. Consequently, the number

of needed frequencies is reduced. When this possibility is taken into account, the Equation (5.8) can be

rewritten in more general form as follows

( ) SNMAIONNNsitecellstepsitecellsiteTRXsitefreqs

+= ////min ,

(5.10)

where:

min Nfreqs/site= minimum number of frequencies needed for a site

NTRX/site = total number of TRXs on a site

MAIOstep= the value of the MAIOstep parameter

Ncell/site= total number of cells in the site

S= MAIO separation between cells

A good approach is to set the MAIOstep as high as possible. However, it should be checked that therequirement presented in Equation (5.10) is still fulfilled. An example of a good MAIO plan is presentedin Figure 0-17. In this example, the MAIO separation between cells is 2 and the MAIOstep is set to itsmaximum value, which is 3 in this case. If a MAIOstep of 4 would have been used instead, constantadjacent channel interference would have occurred between the second TRX of sector one and thefourth TRX of sector three as shown in Figure 0-18.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8


1 1 2 1 bcch frequency 1...0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 93 3 4 6 10 6 6 15 8 4 12 13 7 1 12 12 14 10 12

2 1 2 1 bcch frequency 2 ...5 2 6 8 12 8 8 2 10 6 14 15 9 3 14 14 1 12 14

3 1 2 1 bcch frequency 3 ...7 2 8 10 14 10 10 4 12 8 1 2 11 5 1 1 3 14 110 3 11 13 2 13 13 7 15 11 4 5 14 8 4 4 6 2 413 4 14 1 5 1 1 10 3 14 7 8 2 11 7 7 9 5 7

Figure 0-17. Example of frequency sharing when MA-list consists of consecutive frequencies

and MAIOstep is set to 3.


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INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8


1 1 2 1 bcch frequency 1...0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 94 3 5 7 11 7 7 1 9 5 13 14 8 2 13 13 15 11 13

2 1 2 1 bcch frequency 2 ...6 2 7 9 13 9 9 3 11 7 15 1 10 4 15 15 2 13 15

3 1 2 1 bcch frequency 3 ...8 2 9 11 15 11 11 5 13 9 2 3 12 6 2 2 4 15 212 3 13 15 4 15 15 9 2 13 6 7 1 10 6 6 8 4 616 4 2 4 8 4 4 13 6 2 10 11 5 14 10 10 12 8 10

Figure 0-18. Example of too few frequencies compared to the size of the MAIOstep.

Often, it is possible to achieve higher intracell frequency separations, by using bigger MAIOstep and by

not defining the MAIOoffset parameters in increasing order. If this approach is used, the Equations (7.8) -(7.10) are not valid anymore. Instead, each configuration should be evaluated case by case. An exampleof this approach is presented in Figure 0-19. In this example, the used MAIOstep is 6 and the requiredMAIO separation between cells is 2. Compared to the example in Figure 0-17, a bigger MAIOstep cannow be used while the number of required frequencies is still the same.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8


1 1 2 1 bcch frequency 1...2 2 3 5 9 5 5 14 7 3 11 12 6 15 11 11 13 9 118 3 9 11 15 11 11 5 13 9 2 3 12 6 2 2 4 15 2

2 1 2 1 bcch frequency 2 ...4 2 5 7 11 7 7 1 9 5 13 14 8 2 13 13 15 11 13

3 1 2 1 bcch frequency 3 ...0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 96 3 7 9 13 9 9 3 11 7 15 1 10 4 15 15 2 13 1512 4 13 15 4 15 15 9 2 13 6 7 1 10 6 6 8 4 6

Figure 0-19. Example of customised MAIO allocation.

Frequencies and Frequency Hopping

The radio interface of GSM/DCS uses slow frequency hopping. Frequency hopping consists of changingthe frequency used by a channel at regular intervals.

Frequencies used in each transceiver are defined by parameter initialFrequency (1 ... 124 in GSM).When Mobile is in Idle state there are two possible ways to listen BCCH frequencies of adjacent cells.Traditional way is that Mobile listens to the same BCCH frequencies of the adjacent cells of the servingBTS as in Idle mode. An alternative solution to listen BCCH frequencies of adjacent cells is to useimproved list (known as Double-BA list).

This list can be described by the two following parameters:the bCCHAllocationList (1 ... 124) (where GSM up to 124 ARFN can be specified) and theidleStateBCCHAllocation (0, 1 ... 128) (where "0" means that the normal list based on the BCCH ofadjacent cells is considered, and where 1128 means that one of up to 128 possible improved lists canbe considered instead, with frequencies as specified with the previous parameter) when MS is in idlemode .In dedicated mode by the parameter measurementBCCHAllocation (ADJ, IDLE) it is possible to


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specify the list to be used in handover (where ADJ means that the normal BCCH list of the adj cells isconsidered and IDLE means that the BCCH list specified for idle mode is used instead).

There are two different kinds of frequency hopping in the BTSs; Baseband Hopping and SynthesisedHopping, controlled by parameter btsIsHopping (BB, RF, N). Below both of the frequency hoppingmethods are described1.1.3 Baseband Hopping (BB Hopping)

BB Hopping refers to a particular implementation of frequency hopping algorithm in which the basebanddigital signal streams are multiplexed between transmitters and receivers using fixed frequencies. InBaseband Hopping the Base Station is actually changing TRXs.

B

RTSL 0 1 2 3 4 5 6 7

TRX-1

TRX-2

TRX-3

TRX-4

f1 B = BCCH timeslot. It does not hop.

f2

f3

f4

Time slot 0 of TRX-2,-3,-4 hop over f2,f3,f4.

Time slots 1...7 of all TRXs

hop over (f1,f2,f3,f4).

Figure 1. BB hopping on 4 TRXs. Also the BCCH TRX is hopping except on RTSL-0.

1.1.4 Radio Frequency Hopping (RF Hopping)

RF Hopping (Synthesised hopping) refers to a particular implementation of frequency hopping algorithmin which the synthesisers of BTS transmitter and receiver are tuned on every time slot to the frequencyspecified by the hopping algorithm. Number of frequencies to hop over is up to 63.

BTRX-1

Non-BCCH TRXs are hopping over

the MA-list (f1,f2,f3,...,fn) attached to the cell.

TRX-2

B = BCCH timeslot. TRX does not hop.

f1,

f2,

f3,

fn

f1,

f2,

f3,

fn

. . . .

Figure 2. RF hopping in 2-TRX cell.

The BCCH TRX cannot hop because the BCCH frequency must be continuously transmitted in a cell.

In Synthesised Hopping, it is possible to use many frequencies in the same TRX controlled byparameter usedMobileAllocation (0 ... 128) and mobileAllocationList (1 ... 124)(MA). This meansthat the maximum number of hopping frequencies lists that can be specified are 128, and the maximumnumber of frequencies that can be specified within a list is 124 in GSM.

Hopping Sequence Numbers (HSN1 (1 ... 63) for time slot 0, HSN2 (1 ... 63) for time slots 1-7) areneeded in case of both hopping in order to tell hopping sequences. (Chapter 18 Background database).


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1.1.5 Freeform RF-Hopping (S6)

In BSS S6 the frequency hopping for sectorized network can be planned by using MAIO offsetparameter. The parameter is defined that the RF-hopping would be more flexible. If maioOffset (0 ...62) -parameter is used, it is possible to use the same mobileAllocationList(MA) frequency list for twoor more sectors of the site without collisions. The MAIO-Offset parameter defines the lowest hoppingfrequency for the cell and it can be bigger than zero thus synchronising the sectors. (See chapter 18Background database).

The following example will show the principle of maioOffset parameter. A three- sector site with 4 TRXsper cell needs at least nine hopping frequencies in MA (available maios 0...8). The number of availableTRXs for hopping defines the minimum amount of frequencies in MA list. In this case there is 1 BCCH-TRX and 3 RF Hopping TRXs per cell. All three sectors can use the same MA-list when maioOffsetsare set for sectors. HSN must be equal between sectors otherwise collisions will occur regularly. Thefollowing table shows the maioOffsets and maios for TRXs. Note that there are nine RF-hopping

frequencies per sector!

BTS-3

BTS-2

BTS-1

(sector3)

(sector2

)

(sector1)

MAIO

-

0

1

2

-

3

4

5

-

6

7

8

TRX-1 BCCH

TRX-2 TCH

TRX-3 TCH

TRX-4 TCH

TRX-5 BCCH

TRX-6 TCH

TRX-7 TCH

TRX-8 TCH

TRX-9 BCCH

TRX-10 TCH

TRX-11 TCH

TRX-12 TCH

f1

f2

f3

f4f5f6f7f8

f9f10f11f12

MAIOoffset

0

3

6

HSN

3

3

3

Table 1. MAIO values for a 3-sector site, 4 TRXs per sector.

The number of frequencies allowed in a hopping group is increased to 63. This development is new for

III-gen. only; IV-gen. supports 63 frequencies in a hopping group right from the beginning.

The number of TRXs supporting RF hopping in a cell is no more limited by two . RF hopping can be

used only with AFE. This is because wide band combiner is needed with RF hopping.Note: 2nd generation BTS does not support RF hopping.


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FLEXIBLE MAIO MANAGEMENT (S7)

With this feature it will be possible to arrange MAIOs within a cell in a way that using successivefrequency channels becomes possible without continuous in-cell adjacent channel interference.

This functionality is of vital importance for success of RF hopping with tight reuse (so it becomes

essential feature in Intelligent Frequency Hopping, see in IUO chapter) because commonly the operators

will be forced to allocate successive channels in MA list. In order to use RF hopping with more flexibility

the operator needs the management access to all the hopping parameters including MAIOs.

The new parameter added for more flexibility in RF hopping parameters set is MaioStep (MS), with a

range 1..62. With this parameter the MAIOs can be chosen not to be allocated successively for the cell,

but for instance every second or third value.

See the below table for a better understanding:

A 3-sector site, 4 TRXs per sector (i.e., 3 RF hopping TRXs per sector). HSNs for each sector must beequal. MAIO offsets are set as follows: '0' for sector-1, '6' for sector-2 and '12' for sector-3. MAIO steps

are set 2 for all sectors. MA frequency list must contain at least 18 frequencies (available MAIOs:

017).

Sector HNS MAIO-offset MAIO-step TRX MAIO value for all RTSLs

1 N 0 2 TRX-1

TRX-2

TRX-3

TRX-4

BCCH, not allowed to hop

MAIO=0

MAIO=2

MAIO=4

2 N 6 2 TRX-5

TRX-6

TRX-7TRX-8


MAIO=6

MAIO=8MAIO=10

3 N 12 2 TRX-9

TRX-10

TRX-11

TRX-12


MAIO=12

MAIO=14

MAIO=161.1.6 Terminology

Random HoppingFrequencies change according to a pseudo-random sequence. HSN = 1...63.

Cyclic Hopping

Frequencies are used one after another in ascending order in the hopping sequence. HSN = 0.

Slow Frequency Hopping (SFH)The frequency change rate is slower than the modulation rate. In GSM the frequency changes burst byburst (156 bits), thus GSM hopping is clearly slow hopping.

Hopping GroupSet of Radio Timeslots using the same MA and HSN in a cell.

Hopping Sequence Number (0...63) (HSN)


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A parameter used in randomising the hopping sequence. If HSN = 0, it means cyclic hopping, 1...63means random hopping. Each hopping group may have an HSN of its own.

Mobile Allocation (MA)List of Absolute Radio Frequency Channel Numbers, which are used in a particular hopping sequence.

MA-listMobile allocation frequency list. This is an object in the BSC's database. It defines the MA for a RFhopping cell.

Mobile Allocation Index Offset (MAIO)Hopping sequence starting point for each RTSL using the same MA. MAIO synchronises the RTSLs,which use the same MA, to use different frequencies at a time.

MAIO step (MS)

MAIOs can be allocated every second or every third value, for example. Range from 1 (the oldmanagement) to 62.

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