During the autumn 1999 Nokia trialed several reuses in. Brisbane, Australia in co-operation with Cable and. Wireless Optus. The trial area was 14.5 km2 and the.
Comparison of Networks with different Frequency Reuses, DTX, and Power Control using the Effective Frequency Load Approach Jeroen Wigard*, Preben Mogensen*, Per Henrik Michaelsen*,Juan Melero' and Timo Halonen' *Center for PersonKommunikation Fredrik Bajersvej 7A-6, DK-9220 Aalborg Ost, Denmark #Noha Telecommunications, System Performance Group, UK. Abstract - Networks with different reuses, with and without DTX are compared by using the effective frequency load concept (EFL).EFL makes it possible to compare networks under different circumstances, like for example different weekdays or networks using different configurations. The simulations results show that networks using high frequency reuse factors give the best quality, while the gain from DTX is higher than the gain from power control. Both features can be combined to get an even higher gain. Included field uial results show the same tendencies.
I.
The frame erasure rate (FER) for the full rate traffic channel (TCWFS), indicating the percentage of erased frames, is used as a quality measure. Two different FER quality thresholds have been used throughout the article: 5.4% and 10.9%. These thresholds have been used since they are believed to represent respectively good and acceptable quality. The comparisons use the percentage of samples having a FER worse than 5.4 or 10.9% In Section TI the network simulator CAPACITY is described, followed by an explanation of the effective frequency load concept in Section III. Section IV,V, VI and W contain the simulation results, while the field trial results can be found in Section VIII.
INTRODUCTION
Different frequency reuses for the GSM system with and without DTX and power control have been studied in [ 13. However, synthesised frequency hopping has not been considered in those studies.
II.
The capacity simulator CAPACITY has been used for the network simulations. It is able to simulate the factors that affect the performance of the GSM network, like frequency hopping, DTX, and power control and returns the quality experienced by each individual mobile station of every single frame (0.48 seconds) at a given system load. The most important quality measures, which can be extracted are the C/I, BER, FER,RXQUAL, number of blocked calls, dropped calls and blocked handover attempts.
In this paper, networks using synthesised hopping with different frequency reuses and with and without DTX and power control are compared. Field trial results will be shown as well as simulation results. The different frequency reuses are characterised with a reuse factor. The reuse factor indicates the cluster size of cells within which each frequency is used only once. The reuse factor is typically denoted as xlv, where x is the reuse factor for base stations andy the reuse factor for cells. This means that a reuse factor of 3/9, corresponds to a cellular network consisting of 3-sectors per site, and each frequency is only used once within 3 sites per cluster, i.e. once per 9 cells.
In the simulations, both multipath and shadow fading are included. The lognormal fading process is simulated, so the spatial correlation function follows an exponential decaying function, with a decorrelation distance of 110 m. The standard deviation Osh equals 6 dB. The fast fading corresponds to fast fading in a typical urban (TU) channel [3]. The fast fading on adjacent frequencies is assumed to be uncorrelated. Measurements, which are used in the power control and handover algorithm are made for each SACCH multi frame (0.48 sec.). The time resolution of the simulator is 4.615 msec., corresponding to one TDMA burst. The mobile stations are initialised at a uniform distributed random place in the network and they move with a constant speed in a randomly selected direction (uniform distributed). In each simulation, at least 10000 connections are simulated. The presented results are the results of downlink simulations only. The investigated network structure is based on cells with a radius of 2 km and a regular base station grid with 48 3-sector base stations, corresponding to 144 cells. Each cell has 3 transceivers (TRXs), while the BCCH is
When using synthesised hopping the reuse factor can be changed, while maintaining the number of transceivers (TRX) per cell. The interference situation becomes worse, as the reuse factor is decreased, since the interferers come closer. At the same time the gain from frequency hopping is improved, since the number of hopping frequencies is increased and more fractional loading is introduced [2]. To compare the different reuses, the effective frequency load (EFL,)concept has been used. EFL, takes the actual traffic into account and thus enables a fair comparison between different conditions (different week days, different networks, etc.). Basically, EFL describes how much each frequency is loaded in average per cell.
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CAPACITY
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Figure 1 shows the relative frequency of the FER being worse than 5.4% for networks with different bandwidths. All networks use a 1/1 frequency reuse with random frequency hopping. The results are obtained by simulations with CAPACITY.
disregarded. The other simulation parameters can be seen in Table 1. Table 1 Simulation parameters Path loss Shadow fading O s h Shadow fading decorrelation distance Fast fading Call mean hold time Mobile velocity Cell radius Antennas Type of frequency hopping
III.
Lp = 35 log d 6 dB
It can be seen that with the EFL the results from the different bandwidths seem to fit quite well. In other words, networks of different bandwidths seem to give the same curve.
110 m according to the TU channel 80 s (exponential. dist.) 3 50 km/h (uniform dist.) 2 km 65 sectorized
When looking at the 1/1 results, a slight degradation in the quality can be seen when the bandwidth becomes lower than 2 MHz. This is caused by the fact that at lower bandwidths the probability for bursts within the interleaving depth3 using the same frequency is getting higher. This lowers the gain from frequency hopping since the optimal gain is achieved, when the bursts within the interleaving depth use different uncorrelated frequencies [4].
-
Random hopping
EFFECTIVE FREQUENCY LOAD
In reality the fading on adjacent frequencies is not uncorrelated, as it is in the simulator. This will cause that the degradation at small bandwidth increases, since the probability for the bursts within the interleaving depth using the same or an adjacent frequency increases when the bandwidth decreases at a fixed frequency reuse.
Effective Frequency Load (EFL) is defined as:
where L m is the hardware utilisation and Ref is the effective reuse, as defined below.
35,00%
Hardware utilisation indicates the utilisation of the hardware at a given traffic load:
L,
=-
30,0096~ h 25,0096
D
Trafic timeslots
B 20,0096~ e :,
where Traflc is the average traffic carried by the cells measured in Erlang and timeslots is the average number of timeslots available in the cells. For example, with 2 transceivers' in non-hopping mode a cell can carry 9 Erlang (assuming 2% blocking). To carry this traffic 15 timeslots' are offered by the cell. Thus LLw= 9/15.
B
6,096~ 7,096
8,096~ 9,096 10,096 11,0%
12,096 13,0%
Effective FrequencyLoad
Figure 1 The relativejequency of the FER being worse than 5.4% as function of the EFL for the 1/1 Ji-equencyreuse with dyerent bandwidths.
(3)
where NfieqsTOT is the total number of frequencies in the ~ ~ investigated part of the network and N T R is~ the average number of TRXs in a cell The EFL is related to the spectral efficiency by: N M MEFL . (Erlang/kmZ/MHz) Avg - Cellsiie
10,00%
5,0%
The efective reuse is essentially the same as the conventional frequency reuse distance. It is calculated as :
Spectral - eficiency =
15,0096~
ti
(4)
where NMh is the number of channels per MHz (in case of full rate traffic channels in GSM this equals 40) and Avg-Cellsize is the average cell size in km'.
IV.
REUSES
Table 2 shows the network configurations, which have been simulated. The first column contains the frequency reuse pattern, while the number of TRxs can be seen in the second column. The third column shows the number of frequencies per cell, i.e. the number of hopping frequencies, while the last column contains the total frequency spectrum.
'
In GSM one transceiver corresponds to 8 timeslots, which means 8 full rate channels. One timeslot is used by the BCCH.
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The interleaving depth in GSM is 8 bursts
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Table 2 Simulated Network
No. Frequencies
Reuse
113 313 . . 319
3
9 3
that the quality of each network degrades with increasing load. The EFL calculation can be turned around to calculate what the actual average frequency reuse is. This is done by setting the hardware load to 100% and calculate the effective frequency reuse for a given outage with equation (1). When for example only 10% of all mobiles can have to FER higher than 5.4%, the minimum average frequency reuse is respectively 15.4, 14.3, 12.5 and 11.7 for the 111, 113, 212 and 313 frequency reuse patterns.
Total Spectrum
I
5.4MHz 4.8MHz 5.4MHz 5.4 MHz 5.4MHz
W
O (Mobile Allocation Index Offset) management [5] is used, meaning that adjacent and cochannel interference between the sectors and within each sector at a site is avoided.
V.
DTX has been simulated with a DTX factor of 50%. The on - off intervals have an average length of 1 seconds and are exponentially distributed.
Figure 2 and Figure 3 show the relative frequency of the FER being higher than respectively 5.4% and 10.9%. The Figures are generated from the FER samples, where one FER sample is the average FER over 4 SACCH multiframes (1.92 sec) of one connection.
5,0%
6,096
7,0% 8.0% 9.0% Effective Frequency Load
DTX
The results can be seen in Figure 4 and Figure 5. Again it can be seen that higher reuse factors lead to better quality. Also the improvement from using DTX can be seen. At 7% EFL the relative frequency of the FER being worse than 5.4% is decreased with a factor 3 4.5. The highest gains are achieved by the networks using a small reuse. This is due to the fact that the interference is worst in those cases.
10,0%
Figure 2 The relativejkequency of the FER being worse than 5.1% asfirnction of the EFL. EWectlve FruquencyLoad 1O,OO%
Figure 4 The relativejkequenq of the FER being worse than 5.3% asfinction of the EFL when using DTX
400% 3
9
6.00%
ZOO% i I
5,0%
6,0%
7,0%
8,0% Efi.ctin Frequency Lo8d
9,0%
10,0%
Figure 3 The relativejkequency of the FER being worse than IO. 9% asfirnction of the EFL.
It can be seen that networks with high frequency reuse factors give better quality, than networks with low frequency reuse factors, i.e. the probability of the FER being worse than 5.4% or 10.9% is higher for a networks with a low frequency reuse factor. The Figures also show
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Figure 5 The relativefrequency of the FER being worse than 10.9% asfunction of the EFL when using DTX
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The DTX factor could also be taken into account in the EFL calculation, since it indicates the reduction in the frequency load. A 50% DTX factor would mean dividing the EFL by 2. Doing this and comparing the results to Figure 2 and Figure 3 shows that the curves with DTX, where the EFL is divided by 2, seem to fit nicely with the curves without DTX.
When compared to the results without power control it can be seen that the quality in the networks has improved. The gain is less than the gain from using DTX. At 7% EFL the relative frequency of the FER being worse than 5.4% is decreaxd by a factor 1.6-1.8. Again it can be seen that a higher frequency reuse factor gives better quality.
VI.
VII.
DOWNLINK POWER CONTROL
The networks configurations, shown in Table 2, are also simulated with downlink power control. The power control tries to minimise the output power of the base stations based on the measured signal level and the quality of the connection.
DTX AND POWER CONTROL
Power control and DTX can also be combined. The settings from the previous 2 sections are used. When a mobile station is in DTX mode, the reported RXQUAL value is based on an average of 12 bursts4 instead of the 100 bursts. This introduces inaccuracies, as reported in [6], which means that some of the gain from power control is lost.
If the averaged received signal level (AV-RXLEV) is below -95 dBm or if the averaged quality (AVRXQUAL) is above 3 (uncoded BER is higher than 1.6%), the output power at the base station is increased. If neither of the above is true and if either the AVRXLEV is above -80 dBm or if the AV-RXQUAL is below 2 (uncoded BER is lower than 0.4%), the output power at the base station is decreased.
J 5.0%
6.0%
7,040
8.096
10,096
9.0%
E f f d j w Fnguoncy Lord
Figure 8 The retafivejequency of the FER being worse than 5.1% asfunction of the EFL, when using DTXand PC. O,W% 4 5,046
6,046
7,096
8,0%
9,096
10,046
ER.ctive Frequency L a d
Figure 6 The relativejequency of the FER being worse than 5.1% asfunction ofthe EFL, when using PC.
5,0%
6,0%
7,046
8,0%
9,OYo
10,0%
EfkctinFnguncrLord
Figure 9 The relativejequency of the FER being worse than 10.9 % asfunction ofthe EFL, when using DTXand PC. 5,096
6,046
7,0%
8,046
9,046
It can be Seen that the quality of the networks is further improved, compared to just using either DTX or power control. It also can be seen that the 3/3 reuse now gives the best quality, whereas the other curves cross each other. It seems like the small frequency reuses are the
10,0%
mativsFrequency Lord
Figure 7 The relativejequency of the FER being worse than 10.95%asfunction of the EFL, when using PC.
Figure 6 and Figure 7 show the relative frequency of the FER being higher than respectively 5.4% and 10.9%.
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In the silent periods still 12 bursts are transmitted, containing control and comfort noise information.
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best at low EFL, whereas when the load increases the higher frequency reuses give the best quality.
IX.
GSM networks with different reuses with and without DTX and power control have been compared in this article. For the comparison the effective frequency load (EFL) concept is used. It is shown that with EFL networks under different circumstances, like for example different weekdays, and networks with a different setup or frequency reuse, can be compared.
VIII. FIELD TRIAL RESULTS During the autumn 1999 Nokia trialed several reuses in Brisbane, Australia in co-operation with Cable and Wireless Optus. The trial area was 14.5 km2 and the number of cells was 49, leading to an average cell size of 0.27 km’. The average number of TRXs per cell was 2.4. The network setups, shown in Table 3, are trialed5. The power control settings are very similar to the ones used in the simulations. To get points with a different frequency load, the number of frequencies is varied.
Simulation results show that the higher reuses give the best quality, when looking at the relative percentage of FER samples above 5.4 and 10.9%. It is shown that DTX decreases the percentage of FER samples worse than 5.4% at 7% effective frequency load with a factor 3-4.5. The gain of power control is lower: the gain in the same situation is about 1.7. Both features can be combined to get an even higher gain.
Table 3 Network setupf o r trialed networks
I
DTX
Power Control (PC) No No Yes Yes
Frequency Reuse 111 212 212 212
I
717
Nn
Field trial results from Brisbane, Australia show the same tendencies as the simulations: higher reuses give better quality.
No No No Yes
I
I
Nn
In the field trial the dropped call rate instead of the FER has been used a$ quality measure. Figure 10 shows the dropped call rate as function of the effective frequency load. Some polynomial trendlines are shown a$well. It can be seen that also here the network with the greatest reuse factor (313) gives the best quality and also that power control and DTX give a quality gain. So the tendencies found in the simulations are similar to the ones found from the field trial. However, it is hard to compare the results directly to the simulated cases since the EFL is much lower.
A
We like to thank Cable and Wireless Optus for the cooperation during the field trial.
1 92.PC.DTX 33, no PC. no DTX 33, PC,noDTX (111, noPC, no DTX) Poly no PC, no DTX) -Poly (ZE. PC. no DTX)
--- -- -- -Poly
(m,
1 ,oo
1,546
2,096
2,5%
Effective Frequency Load
Figure 10 The dropped call rate asfinction of the EFL for diflerent setups in the trials.
Here only the network setups, which can be compared to the simulated cases, are mentioned.
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It should be noted that the power control settings in the simulations are quite optimal, but not necessarily the most optimal settings. Also notice that the simulated network is completely regular: the base stations are placed in a regular grid and the 3 sectors point in the exact directions 60, 180 and 300 degrees. Also the frequency plans are completely regular. Irregularities in the frequency plan, base station location and/or sector direction are believed to have a greater impact on the higher reuses than on the smaller reuses. Because of this and since real networks are irregular, in reality small frequency reuses might be better than high frequency reuses. This is subject for further study.
ACKNOWLEDGEMENT
111, no PC, no DTX
+
CONCLUSIONS
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LITERATURE J. Wigard, P. Mogensen, J. Johansen and B. Vejlgaard, Capacrty of a GSM network with fractional loading and random frequency hopping, IEEE Proc. PIMRC’ 96, September 1996, Taipei, pp.723-727. H. Olofsson, J. NBslund and J. SkSld, Interference diversity gain in frequencyt hopping GSM, IEEE Proc. VTC’95, May 1995, Chicago pp- 102-106. ETSl TCSMG, GSM 05.05: European digital cellular telecommunication system (Phase 2) - Radio Transmission and Reception, March 1995. Zako B. et al., The GSM radio link performance with space diversity and slow frequency hopping, IEEE Proc. VTC ‘93, pp. 480-482. T.T.Nielsen, J. Wigard, P.H. Michaelsen and P. Mogensen, Slow Frequency Hopping Solutions for GSM Networks of Small Bandwidth, IEEE Proc VTC ’98, May 1998, Ottawa, Canada, pp. 1321-1325. J. Wigard, T.T. Nielsen, S. Skjaerris and P.E. Mogensen, On the influence of Discontinuous Transmission in a PCS1900/GSM/DCS1800 Type of Network, IEEE Proc VTC ‘99, May 1999, Houston, pp 2505-2509.
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