58277511-Planning-Bcch(2)

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GSM Frequency Planning with Band Segregation for the

Broadcast Channel Carriers

F. Galliano

(1)

, N.P. Magnani

(1)

, G. Minerva

(1)

, A. Rolando

(2)

, P. Zanini

(3)

(1) CSELT - Via G. Reiss Romoli, 274 - 10148 Torino (Italy)

(2) formerly with CSELT, now with Mercury One2One

(3) formerly with CSELT, now with Alcatel

Abstract

In this paper we investigate the performance of fixed frequency planning of the GSM system considering

dedicated bands for TCH and BCCH carriers respectively (the BCCH carriers are the ones which host the

BCCH channel). This analysis aims at assessing whether this frequency planning strategy (which in the

following will be indicated as ‘frequency planning with band segregation’) brings to better system performance

with respect to the usual frequency planning strategies, i.e. common band for TCH and BCCH carriers. A

number of representative deployment scenarios are investigated.

1. IntroductionOptimizing the use of the available frequency resources is fundamental in order to increase the overall capacity of a

GSM system. To achieve this, a number of solutions may be adopted, like improved radio resource management

policies, deployment of advanced radio features, introduction of advanced resource planning strategies such as Dynamic

Channel Allocation or Fractional Loading. This paper addresses a fixed frequency planning strategy, based on the use of

dedicated bands for TCH and BCCH carriers respectively. References to this technique can be found in [1] and [2]. In

the following, frequency planning with band segregation is compared to conventional fixed frequency planning with

common TCH/BCCH band. Three cases have been considered:

1] system performance analysis considering an ideal scenario;2] development of frequency plans with and without band segregation with reference to a realistic cell layout;

3] system performance analysis of a realistic scenario taking into account in field cell dimensioning and parameters.

The simulation results reported in case 2 were obtained with the frequency planning tool FREQUENT (FREQUency

assignmENT) developed in CSELT which takes into account the mutual average interference between cells evaluated on

a ‘priori basis’ (e.g., without considering system functionalities nor users’ behavior); these results are therefore relevant

for the planning process. On the contrary, results presented in case 1 and 3 were obtained by means of a software tool

developed in CSELT (TOTO - TDMA Oriented sofTware tOol [3]), which models the main functionalities of the GSM

system and takes into account both propagation data, system functionalities (e.g. power control, DTX), and users'

behavior; these results are therefore relevant for the validation of the frequency plan. The two sets of results, although

referring to two different steps of system deployment and therefore being obtained with different methodologies, show

the same trend.

2. System performance analysis considering an ideal scenario

2.1 Simulation scenario and considered functionalities

Simulations were carried out considering an ideal scenario (i.e., regular layout of hexagonal cells, signal level

distribution according to Hata propagation model), with the same number of transceivers for each cell. Frequency

planning is performed according to a fixed assignment criterion, where frequencies are distributed according to a regular

cluster basis. A pool of 48 carriers was assumed. A few cases were considered, where the number of carriers per cell (N)

was ranging from 4 to 7, depending on the cluster size. For the frequency planning with common band for all carriers

(i.e. with TCH and BCCH sharing the same band), frequencies were assigned to cells according to cluster size 48/N; the

BCCH carriers were assigned to the cells assuming all 48 carries available, in such a way that co-channel cells belonging

to geographically adjacent clusters have different BCCH carriers. In the case of frequency planning with band

segregation, 12 carriers were reserved for exclusive BCCH use throughout the network (i.e., BCCH carriers use a clustersize 12), and TCH carriers used the remaining frequencies (36) with cluster size (36)/(N-1). Table 1 summarizes the

considered simulation scenarios. The simulated network was considered to be synchronous, with low mobility user

terminals, and a traffic load corresponding to an average 2% blocking probability. Discontinuous transmission (DTX),

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handover, Power Control (PC, only quality based case was considered) were taken into account, based on GSM

Specification; adjacent channel interference and frequency hopping were not taken into account.

2.2 Simulation results

Results obtained for the downlink are summarized in figure 1, which shows the values at 10% of the C/I cumulative

distribution versus the number of transceivers per cell. Two cases are considered, corresponding to active PC or PC and

DTX. The frequency plan with band segregation provides better overall performance for cell carrier equipment up to 6

transceivers per cell (corresponding to a reuse 7 for TCH) when both PC and DTX are active. Actually, this is the result

of two opposite effects. As figure 2 shows, the performance of the BCCH carriers worsen when moving from the case

without band segregation to the case with band segregation, due to the shorter reuse distance (with band segregation,

only 12 carriers are dedicated to BCCH whereas without band segregation all 48 carriers are available for BCCH

planning). On the contrary, figure 3 reveals that TCH performance improve substantially with band segregation. In this

case, TCH carriers are not affected by the interference coming from the BCCH carriers (which, according to the GSM

Specifications, are always transmitting with the maximum allowed power). This also allows to maximize the benefits of

system functions such as power control. The introduction of DTX further improves TCH performance. However, if the

number of transceivers per cell increases, thus making shorter the reuse distance of the TCH carriers, the performance

improvement gradually disappears.

In figure 1 it can be noted that the results with 4 TRX/cell are not aligned with other cases, in fact the performance

improvement achievable with dedicated bands is slightly worse than the one achievable with 5 TRX/cell. This is due tothe fact that with 4 TRX/cell the quality of the BCCH carriers is substantially lower than the quality of TCH carriers (see

figure 2 and 3); this implies that almost all values in the lower part of the cumulative distribution (from which the 10%

value is extracted) belong to the BCCH carriers, whose relative weight is therefore definitely higher than in other cases.

If we referred to the 20% values of the C/I cumulative distributions (rather than 10%), this phenomena disappears.

Figure 3 highlights that the TCH quality decreases more rapidly with band segregation when the reuse distance

decreases; in fact, with the common band approach, TCH carriers suffers from the interference coming from BCCH

carriers, which is basically constant, regardless of the number of carriers per cell.

Simulation results do not show any advantage of the frequency planning with band segregation for the uplink. In fact, in

the uplink, the increased interference due to a smaller reuse distance of both TCH and BCCH carriers is not balanced by

any positive effect, since the BCCH carriers in uplink do not behave differently than normal TCH carriers (i.e. PC and

DTX can be applied to BCCH carriers as well as to TCH carriers).

Common Band Band SegregationTRX/cell # carriers Overall reuse # carriers BCCH reuse TCH reuse

4 48 12 48 12 12

5 48(*) 9.6 47 12 9

6 48 8 48 12 7

7 48(**) 6.86 48 12 6

(*) Results obtaind by interpolation of cases with 45 and 50 carriers

(**) Results obtained by interpolation of cases with 42 and 49 carriers

Table 1: TRX assignment scheme

9

10

11

12

13

14

15

16

17

18

3 4 5 6 7 8

TRX/cell

C / I D L 1 0 % - G l o b a l

C/I 10% dif

C/I 10% com

C/I 10% com DTX

C/I 10% dif DTX

Figure 1: Downlink C/I@10% (BCCH and TCH carriers)

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7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

3 4 5 6 7 8

TRX/cell

C / I D L 1 0 % - B C C H

C/I 10% bcch dif

C/I 10% bcch com

C/I 10% bcch com DTX

C/I 10% bcch dif DTX

Figure 2: Downlink C/I@10% (BCCH carriers)

9

10

11

12

13

14

15

16

17

18

19

20

21

3 4 5 6 7 8

TRX/cell

C / I D L 1 0 % - T C H

C/I 10% tch difC/I 10% tch comC/I 10% tch com DTXC/I 10% tch dif DTX

Figure 3: Downlink C/I@10% (TCH carriers)

3. Development of frequency plans with reference to a realistic cell layoutIn case 2 we have built three different frequency plans on a realistic set of cells using the software tool FREQUENT

developed by CSELT to solve frequency allocation problems. The first one is defined using a common band to plan the

BCCH and TCH carriers of the cells, while the second and the last one using two different configurations of band

segregation. The total band available for the frequency allocation was considered as constituted by 50 frequencies

shared as follows:

Frequency plan BCCH band TCH band

1 1o

50 1o

50

2 1o

15 16o

50

3 1,3,5,7,9,11,13,15,

17,19,21,23,25,27,29

2,4,6,8,10,12,14,16,18,20,

22,24,26,28, 30o

50

Table 2: band configurations

The frequency plans accomplished by means of FREQUENT use a set of realistic inputs both in terms of

interferential description and in terms of reuse constraints set.

The description of the interference between the cells of the plan is given by the interference matrix indicating for

each couple of cells (i, j) the C/I induced by the cell j on the cell i when both cells use the same frequency (co-channelinterference); the interference due to the adjacent channels is estimated evaluating the division between the co-channel

C/I and the parameter NFD (Net Filter Discriminator).

The reuse constraints set is constituted by:

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• cell constraint , implicating a minimum distance equal to 3 between frequencies assigned to the same cell;

• site constraint , implicating a minimum distance equal to 2 between frequencies assigned to different cells

belonging to the same site;

• 400 kHz adjacencies constraint , implicating a minimum distance equal to 2 between frequencies assigned to 400

kHz adjacent cells;

• 200 kHz adjacencies constraint , implicating a minimum distance equal to 1 between frequencies assigned to 200

kHz adjacent cells.The area on which the three frequency plans are accomplished is constituted by 622 cells and 2435 carriers, the

elaboration time (CPU time) used for the definition of each plan is equal to 10 hours on a Sun Spark Ultra Workstation.

The different plans are compared as a function of:

• the minimum C/I ratio associated to the BCCH carriers set;

• the minimum C/I ratio associated to the TCH carriers set;

• the number of carriers under the threshold of 9 dB;

• the C/I quality distribution of the BCCH carriers within the cells;

• the distribution of the C/I associated to the BCCH carriers;

• the distribution of the C/I associated to the TCH carriers.

The results are shown in the following tables and diagrams:

Plan 1 Plan 2 Plan 3

Minimum C/I of the BCCH carriers 9.08 dB 7.41 dB 8.62 dB

Minimum C/I of the TCH carriers 9.07 dB 7.13 dB 7.54 dB

Number of under threshold carriers 0 27 20

Table 3: C/I results

1° 2° 3° 4° 5° 6°

Plan 1 100% - - - - -

Plan 2 48.5% 27.8% 15% 6.4% 1.8% 0.5%

Plan 3 37.8% 33.1% 17.5% 8.8% 2.6% 0.2%

Table 4: BCCH quality distribution within the cells (maximum cell equipment is equal to 6 TRX)

622

0 0 0 0 0

302

173

9340

11 3

235206

10955

16 10

100

200

300

400

500

600

700

1° 2° 3° 4° 5° 6°

Quality

B C C H c a r r i e r s

plan 1

plan 2

plan 3

Figure 4: BCCH quality distribution

< 9 dB 9-15 dB 15-20 dB 20-25 dB 25-30 dB 30-35 dB > 35 dB

Plan 1 - 10.4% 33.3% 32.8% 15% 5.1% 3.4%

Plan 2 0.3% 17% 37.6% 28.6% 10.5% 3.9% 2.1%

Plan 3 0.2% 20.6% 41.3% 25.5% 7.1% 3.9% 1.4%

Table 5: C/I distribution of BCCH carriers

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0

65

207 204

93

32 212

106

234

178

65

24 131

128

257

159

4424

9

0

50

100

150

200

250

300

< 9 9-15 15-20 20-25 25-30 30-35 > 35

dB

B C C H

c a r r i e r s

plan 1

plan 2

plan 3

Figure 5: C/I distribution of BCCH carriers

< 9 dB 9-15 dB 15-20 dB 20-25 dB 25-30 dB 30-35 dB > 35 dB

Plan 1 - 29.4% 41% 20.9% 5.6% 1.7% 1.4%Plan 2 1.4% 30.7% 37.6% 20.8% 5.7% 2% 1.8%

Plan 3 1% 30.6% 38.3% 19% 7% 2.4% 1.7%

Table 6: C/I distribution of TCH carriers

0

532

743

379

10231 2625

556

682

378

10436 3219

555

694

345

127

43 30

0

100

200

300

400

500

600

700

800

< 9 9-15 15-20 20-25 25-30 30-35 > 35

dB

T C H c a r r i e r s plan 1

plan 2

plan 3

Figure 6: C/I distribution of TCH carriers

As shown in the set of tables and figures the frequency plan with common band seems to guarantee the best results,

in particular for the BCCH carriers that are chosen after the planning assigning the BCCH channel to the carrier with the

best C/I in the cell. This procedure cannot be performed in case of band segregation because only one frequency

assigned to a generic cell is compatible with the BCCH planning; in case of segregation the first configuration

guarantees the better results for the broadcast channels. As far as the TCH channels are concerned, the results are similar

in case of common and segregated band but in the second and third case these results can become worse if the average

number of carriers per cell increases, owing to the most intensive effects of the interferential condition and

cell/site/adjacency constraints.

4. System performance analysis of a realistic scenario

4.1 Simulation scenario and considered functionalities

In order to evaluate the performance of frequency planning with band segregation under realistic conditions, system

simulations were carried out considering the realistic layout of the central area of an Italian city. Propagation data were

evaluated with a model validated by means of field measurements [4]. Real cell dimensioning was taken into account

(ranging from 2 to 7 carriers per cell, 5 on average). 41 available frequencies were considered; in case of frequencyplanning with dedicated bands, 12 frequencies were reserved for BCCH. Relevant frequency plans were obtained taking

into account all adjacent constraints (see section 3) due to technology by means of the tool FREQUENT. Real measured

data traffic per cell were used for the system simulation. The simulated network was considered synchronous, with low

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mobility user terminals. Discontinuous transmission (DTX), handover, Power Control (PC, only quality based case was

considered) were considered, based on GSM Specifications; adjacent channel interference was taken into account.

4.2 Simulation Results

Simulations have shown an high load in the network characterized by an overall blocking probability higher than the 2%

considered in the ideal case. In this case the overall downlink performance in terms of C/I cumulative distribution do not

improve by adopting the frequency planning with band segregation (see figure 7). Anyway, this result is not in

contradiction with the one obtained by the analysis of the ideal scenario considered in section 2 above, since different

conditions apply. In the realistic scenario studied here, the network load is higher and non-uniformly distributed. Under

these conditions, the interference contributions from BCCH and TCH carriers tend to become more balanced; as a

consequence, the use of dedicated bands do not lead to a more efficient use of functionalities such as power control and

DTX. Moreover, in the scenario considered here, the number of transceivers per cell is quite high (up to 7, 5 on average)

and therefore the reuse factor is quite low. Finally, the frequency planning considered for this realistic scenario was

evaluated taking into account all adjacent constraints due to technology, which were not considered in the ideal case

(where the cluster approach was used) and which effectively reduce the degrees of freedom available for the planning

algorithm (in particular, when the number of available carriers is low, as in the dedicated bands case) with obvious

impact on the final frequency plan. Therefore, in the considered realistic case, the TCH performance in the band

segregation case are not capable of balancing the degradation of the BCCH performance. In the uplink, simulation

results do not show any advantage of the frequency planning with band segregation, in line with results previouslyobtained in the ideal case.

0

10

20

30

40

50

60

70

80

90

100

- 1 0 1 2 3 4 5 6 7 8 9

1 0

1 1

1 2

1 3

1 4

1 5

1 6

1 7

1 8

1 9

2 0

2 1

2 2

2 3

2 4

2 5

2 6

2 7

2 8

2 9

3 0

3 1

3 2

3 3

C/I

P r o b ( C / I < x v a l u e )

COM

SEG

Figure 7: cumulative C/I distribution (BCCH and TCH carriers)

5. ConclusionThe system performance analysis carried out considering an ideal scenario (section 2) has shown that the frequency

planning with band segregation can lead to a better performance in the downlink, reducing the quality of BCCH carriers

and increasing the carrier to interference ration of TCH carriers, thanks to an increased efficiency of network

functionalities such as power control. The system performance analysis carried out in section 4 considering a realistic

scenario under heavy traffic load has shown that the advantages of the frequency planning with dedicated bands

disappear.

This is confirmed by the analysis carried out with reference to the development of frequency plans with and without

band segregation considering a realistic cell layout.

Based on the analysis carried out in this paper, we conclude that, under the hypothesis and assumptions considered

herein, the frequency planning with band segregation can be a viable solution only in case of a small number of

transceivers (e.g, high reuse factor) per cell and under low traffic load.

References[1] M. Madfors et al., “High capacity with limited spectrum in cellular systems”, IEEE Communications Magazine, Aug 1997

[2] F. Kronestedt, M. Frodigh, “Frequency planning strategies for frequency hopping GSM”, Proc. VTC’97, Phoenix, Arizone, USA,

May 4-7 1997, vol III, pp 1862-1866

[3] F. Delli Priscoli, N.P. Magnani, V. Palestini, F. Sestini, “Application of Dynamic Channel Allocation Strategies to the GSM

Cellular Network”, IEEE Journal on Selected Areas in Communications, vol 15, No 8, Oct 1997[4] E. Damosso, F. Grimaldi, M. Sant’Agostino, “Network Planning Tools and Activities in Italy”, Proc. Mobile Radio Conference,

Nov 1991

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