Slow Frequency Hopping Guidelines

Slow Frequency HoppingEdition 01 22.09.1997MCD SFH_PP.DOC 3DF 00995 0000 UDZZA 1/26

SiteLudwigsburg

CELLULAR OPERATIONS DEPARTMENT

Originator(s)U. Birkel

Slow Frequency Hopping

Domain : MCD

Division : Operations

Rubric : Radio Network Planning

Type : Guidelines

Distribution codes :

Predistribution:

D. ADOLPHS ACS / MP H. DERREY AMCFR. KLAHM ACS / SR C. GUETIN AMCFR. COLLMANN ACS / SR2 E. BAUDIENVILLE AMCFK. ECKERT ACS / SR1 J. CHARNOT AMCFK. HEINLEIN ACS / OAD J. XIAO AMCFH.-G. TÜCHLE ACS / OBE

Abstract:This document gives an overview on slow frequency hopping (SFH). The followingaspects will be discussed: Hopping modes, benefits, quality and capacity aspects,simulations, results from field trials, planning strategy, BSS parameter implementationat the Alcatel BSS.

Approval

NameSignature

R. Klahm K. Eckert R. Collmann

NameSignature


Table of contents

1 HISTORY 2

2 REFERENCES 3

3 SCOPE 3

4 INTRODUCTION 4

5 THEORETICAL BACKGROUND 4

5.1 Slow Frequency Hopping 4

5.2 Hopping modes 4

5.3 Benefits of Slow Frequency Hopping 6

5.4 Network improvements 9

5.5 Impact on the number of frequencies 15

5.6 SFH in combination with further mechanism 16

5.7 Frequency hopping in microcells 17

5.8 FER contra RXQUAL regarding voice quality 17

6 FIELD TRIAL RESULTS 17

7 STRATEGIES FOR PLANNING 19

7.1 How to consider frequency hopping in frequency planning 19

8 BSS AND CAE PARAMETER AND IMPLEMENTATION AT THE ALCATEL BSS20

8.1 BSS and CAE Parameter for SFH 20

8.2 Implemented Alcatel BTS concepts 21

9 CONCLUSION 23

10 ABBREVIATIONS 251 History


Date Edition Origin Comments10. June 1997 Draft COR-G Document Creation15. August 1997 Draft COR-G Comments of Readers Review

inserted22. September 1997 01 COR-G Document Release

2 References[1] J. Mezan de Malaric: The1*3 Frequency Reuse: A GSM feature, NORTEL

presentation in China, November 1995[2] A. Kadelka/K. Stocker: Frequency Hopping and Fractional Reuse Techniques,

Position Paper, 29.02.96[3] J.E. Stjernvall: „Calculation of capacity and cochannel interference in a cellular

system“, DMR1 seminar, Espoo 5-7, Feb. 1985[4] C. Brechtmann: Synthesizer Hopping vs. Baseband Hopping, 1994[6] A. Plötz: CAE Data Questionaire, Edition 04,16.05.95[7] G. Schubert: Frequency Hopping und Alcatel micro BTS, 16.05.95[8] Preben Mogensen, Jeroen Wigrad and Frank Frederisksen, „Performance of

Slow frequency Hopping in GSM (link-level)“, COST 231, Poznan, 1995[9] Hakan Olofson, Jonas Naslund, Hohan Skold: Interferer diversity gain in

frequency hopping GSM[10] Jeroen Wigard, Preben Modensen: Capacity of a GSM Network with Fractional

Loading and random Frequency Hopping, PIMRC’96, Taipei[11] IIR Conference, 9-10 July, Singapore, T. Johansson, Ericsson, „Analyzing

performance outcomes of spectrum efficiency approaches“[12] Volker Stuhr, „Effects of Frequency hopping on Interference Probability“, 9/97[13] „Frequency Hopping and Radio Parameter Experimentation in Jakarta“,

Ref: IOF/IER/96.05.10, 10/05/1996

3 ScopeThis document describes the theoretical background of SFH (chapter 5) as well as theresults of field experiments (chapter 6) to verify the aspects of the theoreticaldiscussion. Strategies for planning purposes when introducing SFH will be discussed inchapter 7. Finally the implementation of frequency hopping at the Alcatel BSS and theparameter settings when introducing frequency hopping will be discussed in chapter8.


4 IntroductionGSM introduced antenna diversity and slow frequency hopping (SFH) to improve thetransmission quality, as described in GSM Rec. 04.08, 05.01, 05.02. While antennadiversity combats multipath fading only, frequency hopping additionally averages theeffects of interference.This allows a tighter frequency reuse, thus carrier upgrading can be performed,resulting in an increased capacity while maintaining network performance and quality.Alternatively, if no carrier upgrading is performed, frequency hopping allows qualityand performance to be improved while maintaining capacity.

5 Theoretical Background

5.1 Slow Frequency Hopping

Frequency hopping consists in changing the frequency used by a channel at regularintervals. One distinguishes fast frequency hopping (FFH), where the frequencychanges quicker than the modulation rate, from slow frequency hopping (SFH), whichis used in GSM. GSM uses a TDMA system with 8 timeslots. In slow frequency hoppingeach mobile transmits or receives on a fixed frequency during one timeslot and hopsto another frequency before the next timeslot will be transmitted or received, resultingin 217 hops per second for the MS. This technique can be applied on all traffic andsignalling channels, except the BCCH channel, which must transmit on a fixedfrequency, to make correct RXLEV measurements for the neighbourcell mobilespossible. Moreover, the BCCH channel has to transmit full power on all timeslots. ThusSFH, PC and DTX is not allowed on the BCCH carrier.

5.2 Hopping modes

As shown in figure 1, frequency hopping can be performed in two modes:Cyclic hopping modeRandom hopping mode

While in cyclic hopping mode the same hopping sequence will be used periodically, inrandom hopping mode a pseudo random sequence will be used in order to achieveuncorrelated hopping sequences.

Figure 1: Principle of cyclic and random SFH on Timeslot 1 on F1, F2 and F3

Burst

F1

F1F2F3

Timeslot

F2F3

Cyclichoppingmode

Randomhoppingmode


From the BTS point of view one distinguishesBaseband Hopping (BBH)Synthesizer frequency hopping (or Radio Frequency Hopping = RFH)

In baseband hopping (BBH) each tranceiver (TRX) is transmitting on one fixedfrequency. Hopping is performed by switching the mobiles from burst to burst todifferent TRXs. Within the BTS the baseband part (FU) is separated from the RF-part(CU). Thus SFH is realised by switching the FU to the respective CU via an internalswitch. This concept is implemented in the Alcatel G2 BTS. The amount of hoppingfrequencies N(hop) is determined by the number of TRXs N(TRX): N(hop) <=N(TRX). Note, that a BTS equipped with only one TRX cannot perform basebandhopping.

In synthesizer frequency hopping (RFH) the TRX do not get fixed frequencyassignments, they may change their frequency from TS to TS according to apredefined hopping sequence. In such a system the number of applicable hoppingfrequencies may be larger then the number of equipped TRXs: N(hop) ≥≥ N(TRX).Since the Alcatel micro BTS is usually equipped with one or two TRX, synthesizerfrequency hopping has to be used.

Since no hopping on the BCCH frequency is allowed, synthesizer frequency hoppingmust not be performed on the BCCH TRX. Figure 2 shows both hopping modes. TRX1represents the BCCH frequency, where the BCCH information is transmitted ontimeslot 0. Note that mobiles being connected to TRX1 can only hop in basebandhopping mode. However, the implemented solution at the Alcatel micro BTS (1 TRX),makes synthesizer frequency hopping possible, by transmitting a dummy burst with anadditional transmitter. This special concept will be explained in chapter 8.

0 1 2 3 4 5 6 7TRX1

TRX2

TRX3

TRX4

0 1 2 3 4 5 6 7TRX1

TRX2

Figure 2: Baseband hopping (left) and synthesizer frequency hopping (right)

A drawback of the synthesizer hopping configuration is that the BTS cannot beequipped with remote tunable combiners (RTC), since the tunable filters cannotchange their frequency on a timeslot basis. Therefore a wideband combiner (WBC)has to be used for the connection between transmitter and antenna, resulting in 1.6dB additional downlink loss. One WBC combines max. 2 TRX with an insertion loss of


4.8 dB, one RTC usually combines max. 4 TRX with an insertion loss of 3.2 dB. In abaseband hopping system the transmitter is tuned to a fixed frequency, thus RTCs canbe used in such a configuration. Table 1 shows the achievable coverage ranges indifferent clutter types taking into account an increased downlink path loss of 1.6 dB forthe WBC configuration (which has to be used for synthesizer hopping) in a realisticmacrocellular scenario (GSM, Pcov=95%, 30m antenna height, 46dBm EIRP).

Clutter type urbanflat/hilly

suburbanflat/hilly

forestflat/hilly

open areaflat/hilly

Cov. Range [km]WBC config.

1.50/0.76 2.44/1.38 3.17/2.18 8.45/5.80

Cov. Range [km]RTC config.

1.67/0.85 2.71/1.53 3.52/2.42 9.38/6.44

Table 1: Achievable coverage ranges using WBC and RTC in a macocellular networkApplying synthesizer hopping, no RTCs can be used

5.3 Benefits of Slow Frequency Hopping

Frequency hopping has been included in the GSM specification, mainly in order todeal with two specific aspects, which affect the transmission quality:

Frequency diversityInterferer diversity

5.3.1 Frequency Diversity

Considering urban environments, radio signals reach the receiver due to reflectionsand diffraction on different paths resulting in fading effects. The received signal levelsare varying dependent on the applied frequency and on the receiver location. SlowMS may stay in a fading notch for a long period of time and suffer from a severe lossof information. Frequency hopping introduces frequency diversity and combatsmultipath fading: Different frequencies experience different fadings, thus the mobilewill experience different fadings at each burst and will not stay in a deep minima for along period of time.Fading effects can be reduced by the error correction algorithms of the equalizer. Butthese algorithms only work effectively, if the signal interrupt is shorter than the periodof time over which the codeword is spread with interleaving. Therefore SFH willimprove the transmission quality, because too long fading holes will then be avoided.Since fast moving mobiles do not stay in long and deep fading holes, they do notsuffer severe from this type of fading. Thus especially slowly moving mobiles willbenefit from frequency diversity.The improvement results in an increased receiver sensitivity under fading conditions(not under non fading propagation conditions) and therefore in improved quality inuplink and downlink direction compared to a non-hopping configuration.


5.3.2 Interferer Diversity

Without SFH, some receivers (MS or BTS) are not interfered, while others, receiving onanother frequency, will experience strong interference. This interference can bepermanent such as BCCH frequencies in downlink direction or some fixed interferersincorrectly radiating in the GSM band.With random SFH, the interfering scenario will change from TS to TS, due to ownhopping and uncorrelated hopping of potential interferers. Thus all receivers (MS andBTS) experience an averaged level of interference. This is called interferer averagingor interferer diversity.The average C/I in the network <C/I> remains unchanged but the „standard“deviation σ is reduced (see figure 3). Therefore the number of MS and BTS that willhave a C/I above a certain threshold ( e.g. C/ITHR1=9dB) is increased. Note, that thisis only true, as long as the mean C/I in the network is above the specified threshold. Ifthe mean C/I is below that threshold, SFH will therefore make thesituation worse! Figure 3 illustrates this situation: The diagram shows an arbitraryC/I distribution of eight mobiles.Let us first assume that C/IThr is C/IThr1 (mean C/I is above C/IThr): Without SFH 3 C/Ivalues are below C/IThr1, with SFH no C/I value is below C/IThr1. The situation isimproved, when introducing SFH.Let us now assume that C/IThr is C/IThr2 (mean C/I is below C/IThr): Without SFH 5 C/Ivalues are below C/IThr2, with SFH 7 C/I values are below C/IThr2. The situation isgetting worse, when using SFH.

Figure 3: Improved C/I due to reduced standard deviation based on interferer diversity,as long the mean <C/I> is above the threshold C/IThr1. Left no SFH, right with SFH.

We can therefore conclude, that benefit out of SFH is taken only in well designed andwell tuned networks.Since the benefit of frequency diversity is based on the principle that differentfrequencies experience different depths of fading, it is recommended to separate the

<C/I> <C/I>

C/IThr1

C/IThr2 C/IThr2

C/IThr1

C/I

σσ

C/Iwithout SFH with SFH

MS number

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

MS number


frequencies within the hopping sequence as far as possible. An offset of typically threechannels is automatically defined by the required co-cell separation (e.g. 600kHz = 3channels).

It can further be concluded, that the drawback of cyclic hopping is, that it does notprovide interferer diversity since the interfering and the interfered carriers do not hopuncorrelated. Therefore random hopping is recommened, since more benefit can beexpected from interferer diversity.Figure 4 summarizes the effects of frequency and interferer diversity.

F1F2

MS1BS1

C1

I2

I1

MS2

F2

P F1

F1,F2,F3

F1

F2

MS1BS1 MS2

F2,F3,F1

P

Interference Diversity

Frequency Diversity

NoHopping

FrequencyHopping

Figure 4: Effects of frequency and interferer diversity


5.4 Network improvements

5.4.1 In principle

Frequency diversity and Interferer diversity result in an improved network quality. Thusintroducing SFH in a mature network is a feature for network quality improvement.Network capacity increase can be performed in a second step after the introduction ofSFH. Due to the fact, that the same quality level with a lower C/Idesign ratio can behandled, a SFH network can be planned with a tighter reuse cluster size. Thereforemore carriers per site can be used.In order to evaluate possible capacity increases with SFH, simulations have beenperformed [10]. Within the following section three different simulation results will bepresented and discussed. Approach 1 and 2 are based on [10], whereas the thirdsimulation has been performed with the RNP Tool A955.

5.4.2 Definitions

The following definitions are required for the discussion of the simulation results.

ARCS:A key parameter is the average reuse cluster size ARCS, defined as:

ARCSBandwidth

Average amount of TRX per cell=

Fractional Reuse:Applying synthesizer frequency hopping, additionally fractional reuse techniquescan be applied. The principle of fractional frequency reuse is, to use more frequenciesin a cell than TRX are equipped: N(hop)>N(TRX). This technique can only be realizedby applying synthesizer hopping (RFH).

The following effects of fractional reuse have to be considered in comparison to abaseband hopping system, where only N(hop)<=N(TRX) is possible:

Hopping over more frequencies provides more diversity, whereas only neglectiblefrequency diversity gain can be expected, when going above 4 hoppingfrequencies.The classical definition of ARCS=BW/N(TRX) is no longer valid, a FrequencyAverage Reuse Cluster Size FARCS needs to be defined as:

FARCSBandwidth

Average amount of Frequencies per cell=


Only N(TRX) frequencies are on air at one time (equal to BBH), which can bedefined by the RF Load=N(TRX)/N(hop)=FARCS/ARCS, resulting in a reducedcollision probability for each frequency.The reuse (FARCS) of the frequencies is smaller compared to an according BBHsystem (ARCS).Therefore to contrary effects have to be considered:

1. Fractional reuse increases the level of interference, since FARCS<=ARCS,each frequency is planned with a smaller reuse.

2. Fractional reuse reduces the collision probability (RF Load) for eachfrequency.

Example: BW=36, N(TRX)=3, N(Hop)=12 results in ARCS=36/3=12 (Reuse of BBHsystem) and FARCS=36/12=3 (Reuse of a RFH system with fractional reuse), RFLoad=25%

Hard- and Softblocking:The smaller the ARCS for a constant bandwidth, the more TRX are available per celland accordingly more traffic can be handled. But with increasing amount of TRX percell the level of interference also increases, due to reduced ARCS. Thus the capacity ofa cellular network is basically restricted either by available hardware capacity or byinterference, depending on the ARCS. Accordingly a hard blocking and a soft blockinglimit can be defined as:

1. Hard blocking is determined by the amount of available channels. This type ofblocking occurs in conventional traffic systems, with a low interference probabilitiy. Theblocking is defined by the blocking probability, e.g. Pblock=2%. Mobiles will not getaccess to the network, since all channels are in use (100% traffic load).

2. Soft blocking occurs due to high interference or due to an unacceptable call droprate. This type of blocking occurs in a network design with a low reuse cluster size,resulting in a high level of interference. The soft blocking limit can be defined by thetraffic load, when the quality in the network becomes unacceptable. E.g. when 10% ofthe mobiles will suffer from a C/I < C/IThr or when the call drop rate reaches 5%. Withincreasing traffic load, the capacity will be limited due to soft blocking before the hardblocking limit is reached (traffic load <100%).

5.4.3 Simulation results

Within system simulations the effects of hard blocking and soft blocking have beenevaluated in terms of possible capacity increase [10]. The simulations are based onhexagonal cell structures and the following assumptions: 36 carriers, 8 TS per TRX, 2%(hard) blocking, considering DTX, PC and HO behaviour uniform antenna height,idealistic antenna pattern, homogenious morpho-structure etc.


Two approaches for the definition of the softblocking limit were chosen:

Approach1: Softblocking defined by the traffic load at which 10% of mobiles have aC/I<C/IThrApproach2: Softblocking defined by the traffic load at which the call drop ratereaches 5%

Accordingly two simulations have been performed, as will be described in thefollowing sections. Afterwards a simulation which has been performed with A955 willbe presented.

Simulation results based on approach 1: Softblocking defined by the traffic loadat which 10% of mobiles have a C/I < C/IThr based on [10]

Figure 5 shows the simulated dependancy of maximum achievable capacity and ARCSof a three sector site. Three curves have been evaluated:

Curve 1: Hard blocking,Max. achievable capacity based on installed HW (Erlang B, Pblock=2%)

Curve 2 and 3: Soft blocking/Hopping and No HoppingMax. achievable capacity of a system with and without frequency hopping, taking intoaccount the effect of interference.

It can be seen, that dependent on the ARCS either the hardblocking limit or thesoftblocking limit determines the maximum achievable capacity:

With a cluster size of 12, the networks capacity is limited by the hard blocking limit(point A in figure 5). Introducing SFH does not further improve the maximumachievable capacity, since the hardblocking limit is already reached before thesoftblocking limit is reached. So 100% traffic load is possible without having morethan 10% of MS with a C/I<C/Ithr. Introducing SFH in that case improves the levelof interference, thus the RCS could be further reduced making carrier upgradingfor capacity increase possible.

However in a non hopping network, when we attempt to reduce the cluster sizedown to 9 in order to increase capacity, significant interference problems appearand degrade the capacity of the network due to soft blocking (point C): Thesimulation shows, that as soon as the traffic load is above 63% the quality becomesunacceptable. This is the reason, why non hopping macrocellular networks areusually not planned with an ARCS below 12.

But with the introduction of SFH the effect of interference is reduced and allows a higher capacity (point B). The system reaches still its maximum hardware capacity, defined by the hard blocking limit.


C: 45D: 20

A: 49.8

E: 86.4B: 71.1

0

50

100

150

200

250

3 6 9 12

ARCS

Erl

ang

per

3 se

ctor

site

Hard Block.

Soft Block/No Hopping

Soft Block/Hopping

Figure 5: Theoretical evaluation of capacity increase with SFH, based on simulations [8]

Further reduction of cluster size is impossible for non hopping networks (point D),whereas with frequency hopping, cluster sizes as small as 3 (point E) are theoreticalfeasible and still increase the capacity, but the system is restricted to the softblocking limit. It is doubtful though, that the small (theoretical evaluated) capacityincrease justifies the high investment costs, which are required for such aconfiguration, when performing baseband hopping. In the upper example 12 TRXper BTS would be required, whereas the traffic load is only 30%, when thesoftblocking limit is reached: 70% of the installed hardware is unused. This is notvery cost effective. A possible solution for this problem is to apply fractional reusewith synthesizer hopping, by still planning the frequencies with a reuse of 3, but byinstalling less TRX. The minimum amount of TRX is determined by the traffic, whichhas to be handled, defined by the sofblocking limit.

⇒ Thus when planning with small ARCS, the fractional reuse techniquebecomes interesting for softblocking limited environments.

Table 2 summarizes the discussed results.Configuration <1x3> <3x3> <4x3>Mode Hopp No Hopp Hopp No Hopp Hopp No HoppCapacity [Erl/site] 86.4 20 71.1 45 49.8 49.8Limited by Softbl. Softbl. Hardbl. Softbl. Hardbl. Hardbl.Pint 10% 10% <10% 10% <10% <10%Call drop rate 16% - - - - -


Table 2: Simulation results based on approach 1 („-“ means: no info available)Within this simulation the call drop rate was not a capacity limiting factor and raisedup to 16% for the <1x3> reuse scheme. But in the simulation only a distance HO wastaken into account. If a quality based HO were taken into account the call drop ratewould be smaller.

One can therefore conclude, that at least going from a reuse of 12 down to 9 is asecure approach for capacity increase:The capacity was calculated on 8 TCH per TRX. In order to evaluate a realistic capacityincrease, taking into account a BCCH with a reuse of 12 and taking into accountrequired signalling channels, a possible capacity increase can be evaluated. Whengoing from a reuse of 12 down to 9, a possible capacity increase of 30% becomespossible. Furhter reduction of the ARCS, results in capacity increase of up to 74%,when not taking into account a maximum call drop rate as a limiting factor, whichraised up to 16%. Since the call drop rate is a very important figure, the samesimulation has been performed in [10] by defining a call drop rate of 5% as thesofblocking limit:

Simulation results based on approach 2: Softblocking defined by the traffic loadat which the call drop rate reaches 5% based on [10]. The results are shown in figure6.

Max. Capacity with softblocking based on call drop rate of 5%

0

10

20

30

40

50

60

70

80

<1x1> <1x3> <3x3> <4x3>

Configuration

Erl

ang

per

site

Figure 6: Maximum achievalbe capacity with softblocking based on call drop of 5%

Determining the capacity with the percentage of dropped calls results in a maximumachievable capacity with the <3x3> configuration, whereas the hard blocking limitstill could be reached (traffic load=100%, call drop rate<5%). A further reduction of


the ARCS resulted in a reduced achievable capacity, limited by soft blocking (call droprate=5%, traffic load<100%).

Simulation results achieved with A955:In order to evaluate the effect on interference with fractional reuse, a simulation withAlcatel’s RNP tool A955 has been performed [12]. The following computations havebeen performed:

The interference probability based on different ARCS for a non hopping systemcould directly be calculated: Pint(Nohop)=prob(C/I<C/IThr) for a given ARCSThe interference probability of a BBH system can be calculated, when additionallytaking into account the effect of diversity. A new C/IThr,hop has been used, as afunction of the hopping frequencies, thus: Pint(BBH)=prob(C/I<C/IThr,Hop) for a givenARCSThe interference probability of a RFH system with N(hop)>N(TRX) can be calculatedfor the according FARCS, taking into account the effect of diversity and the fact, thateach frequency is only on air with the probability N(TRX)/N(Hop)=RFLoad (reducedcollision probability), thus Pint(RFH)=prob(C/I<C/IThr,Hop)* N(TRX)/N(Hop) for a givenFARCS

Based on these statistical simplifications, the medium network wide downlinkinterference levels could be calculated [12], as shown in figure 7.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0 1 2 3 4 5 61/RF_LOAD= N(Hop)/N(TRX)1/RF_LOAD= N(Hop)/N(TRX)

Med

ium

Pin

t Net

wor

kwid

eM

ediu

m P

int N

etw

orkw

ide

ARCS=3, 12TRX ARCS=4, 9TRX ARCS=6, 6TRX ARCS=9, 4TRX ARCS=12, 3TRX ARCS=18, 2TRX

No Hopping

BBHwithdiversity

RFH with diversity and decreasing fractional RF_ load

STATIC INTERFERENCE CALCULATIONS WITH A955: BW=36

Figure 7: Static interference calculations with A955


As a result of this investigation, it can be concluded, that the application of fractionalreuse results in an increased level of downlink interference. But, since no dynamicnetwork behaviour and the effect of coding and interleaving could not be taken intoaccount, it still cannot be concluded, that BBH is always superiour to RFH. This has tobe taken into account in further simulations.Furhtermore it has to be carefully analyzed based on the previous simulations, when touse BBH and when to apply RFH.

Conclusion on simulationsSoftblocking begins for nonhopping systems with a reuse below 12 and for hoppingsystems with a reuse below 9. As long as the hardblocking limit is reached, theinstalled hardware is the limiting factor and not the defined quality threshold.Reducing the ARCS below this point, installed hardware will be unused, since thequality level becomes unacceptable before the traffic load is 100%. This is the point,where it might become reasonable to install less TRX and plan the frequencies with atighter reuse (FARCS), then the installed HW (ARCS).Since the upper simulations [10] did not take into account the quality based HO, theresults related to the evaluated call drops have to be considered with care.For a first introduction it is therefore recommended to aim at an ARCS of 9 in ahopping system (with N(TRX)=N(Hop), resulting in a possible capacity increaseof 30%. The BCCH still has to be planned with an according higher,conventional reuse. Further reduction results in an increased level ofinterference and in an increased call drop rate, but can be possible, whencarefully monitoring these figures. This will depend on the individual network(propagation condition, topology, morphology etc.) and has to be empirical evaluatedand can not be decided based on simulations only.

Fractional Reuse with synthesizer hopping (RFH) can be used in the following twocases:

Apply RFH with N(hop)>N(TRX), when a network is planned with an ARCS below 9(softblocking occurs), whereas the amount of installed TRX has to be high enough,that the amount of traffic defined by the softblocking limit still can be carried.

Apply RFH with N(Hop)>N(TRX), for the micro BTS, which is equipped with one ortwo TRX, to take full benefit out of diversity with N(hop)=4.

5.5 Impact on the number of frequencies

The results of theory and experiments show that the achievable gain depends on thenumber of frequencies used in the hopping sequence.Highest improvements are achieved when going from non-hopping to hopping overtwo frequencies. Medium improvements are achieved when hopping on three or fourfrequencies instead of two. But there are only negligible improvements by increasing


the number of hopping frequencies over four. As shown in Figure 8, it can be furtherseen, that less benefit can be taken from fast moving MS (TU50=Typical urban50km/h, TU 3=Typical urban 3km/h). The results are based on a simulation of [8].

TU3

TU506789

101112131415

1 2 3 4 5 6 7 8 9 10 11 12

number of frequencies in hopping sequence

requ

ired

C/I

(dB

)

TU3

TU50

Figure 8: Influence of number of hopping frequencies

5.6 SFH in combination with further mechanism

There are several features available in a GSM system to reduce the overall level ofinterference. Besides propagation conditions the actual produced interference leveldepends on the interferer activity, i.e. the time an interferer is „on air“. The followingmechanism reduce the interferer activity on frequencies not assigned to the BCCH:

Discontinious Transmission (DTX)In speech calls the channel is not busy during speech pauses. Applying DTX, lesssignalling is transmitted and thus the carrier activity is reduced.

Power control (PC)Using power control, the transmission power is reduced, as long as the level and thequality is above a certain threshold, resulting in a reduced level of interference.

Merging all these techniques together, the actual produced average interference levelis reduced and the softblocking limit is improved. Thus the capacity of a network,which is planned with a softblocking strategy, is automatically increased as long as thehardblocking limit is not reached.The benefit of Power Control and DTX is further increased in combination withfrequency hopping, since the reduced level of interference is averaged over severalmobiles. Therefore DTX and PC is recommended in combination with SFH.


5.7 Frequency hopping in microcells

Due to increased signal fluctuations (fading or mobiles turning around the corner etc.)in microcells, more benefit is taken from frequency and interferer diversity than inmacrocells, which suffer less from fading. Thus SFH in microcellular environments willbe more effective.Since microcells usually are introduced in capacity and therefore in interference limitedenvironments, introducing SFH without carrier upgrading can improve the networkquality in microcellular networks very effective.Thus the ARCS can be further reduced after the introduction of SFH. Not only areduction of the ARCS between the microcells becomes possible, but also betweenmacro and microcells.

5.8 FER contra RXQUAL regarding voice quality

In a GSM system, the number of frames that are not erased are sent as an input to thevoice decoder (Figure 9).

DEMOD DECODER

ENCODER

FrameErasureDecision

VoiceDecoder

-

RXQUAL Frame Erasure Rate

DeinterleaveError correct.

Inside the mobile stationAir

Figure 9 - The signal decoding process

Although in non hopping networks the RXQUAL and voice quality are correlated, thisis not the case in hopping networks, where two mobiles with similar RXQUAL can havedifferent voice qualities. The voice quality is rather correlated to the FER. This is due tointerferer averaging and due to the nonlinear mapping of BER to RXQUAL values (formore details see chapter 6). Thus, in SFH networks, the Frame Erasure Rate (FER) is abetter estimate of the voice quality than RXQUAL.

6 Field Trial ResultsIn this chapter, the result of a field trial in Jakarta will be presented [13]. Theevaluation of the performance when introducing SFH was based on the statisticalmeasurements of QoS: Call Drop Rate + Call Establishment Failures. An evaluation ofthe handover behaviour was performed since, as we will see below, the default HOparameters need to be retuned when introducing SFH. Radio measurements, such as


the FER to evaluate the improvement of the voice quality and the RxQual vs RxLevbehaviour in order to determine the improved quality due to interferer averaging wereperformed. Random baseband hopping has been used with this trial with thefollowing configuration: 5 BSC, 60 BTS, whereas 39 BTS a 4TRX, 18 BTS a 3 TRX and3 BTS a 2 TRX were used.The results of the field trials (see figure 11 - 16) can be summarized as follows:Improved FER: 1.4% → 0.6%Reduced Call Drop Rate: 3% → 2.3%Reduced Call Establishment Failure: 6.5% → 5.5%Further it has been evaluated, that better results can be achieved when introducingrandom rather than cyclic hopping and hopping over four, rather than twofrequencies.Since these measurements have been performed before parameter retuning thefollowing HO behaviour occuredIncreased HO rate: 10% ...15%Increased HO rate based on quality: 20%The increased amount of quality HO is based on the higher RXQUAL values, whichoccurs in hopping networks due to interferer averaging and due to nonlinear mappingof the BER to the RxQual values. The measured RxQual vs RxLev diagramm in Figure10 evalulates this aspect, showing that the quality at lower reception levels isdeteriorated in average by one RXQual value, when introducing SFH.

RX QUAL DL = f (RXLEV DL)

0

1

2

3

4

5

6

7

-110

-106

-102 -98

-94

-90

-86

-82

-78

-74

-70

-66

-62

-58

-54

-50

Without Hopping

With Hopping

Figure 10- Higher RxQual values due to introduction of SFH

This is the reason why the amount of quality HO are increased, when introducing SFH.Therefore parameter tuning becomes necessary, by adapting the thresholds for thedefault quality handover parameter accordingly:

L_RxQual_UL(SFH) = L_RxQual_UL(No SFH)+1L_RxQual_DL(SFH) = L_RxQual_DL(No SFH)+1

This will reduce the amount of quality HO and will further improve the quality of thenetwork.


7 Strategies for planning

7.1 How to consider frequency hopping in frequency planning

Frequency planning with A955 is performed within the following steps:

1. Accurate field strength prediction and C/I calculation: Taking into account Topo,Morpho, PC, DTX, Traffic Load, Fading, antenna height etc.

2. Setup an interference matrix based on C/I calculation: Cell A interfers Cell B withan interference probability Pint

3. Calculate a Channel Separation Matrix (CSM) based on a max. allowedinterference threshold (defines QoS), also considering experience, co-site, co-cellseparation etc. Since the BCCH has to be protected in a special way, a lower max.allowed level of interference will be defined for the BCCH.

4. Frequency planning = Solution of this CSM

Due to diversity effects, hopping frequencies can handle lower C/I ratios. Thus ahigher interference threshold can be tolerated for the setup of the CSM in step 3. Thusnine interference thresholds need to be defined for automatic frequency assignment asshown in figure 11 (for each channel type combination, channel types are: the TCH,the hopping TCH (=STCH) and the BCCH). The result will be, that each channel typewill be planned with different ARCS, according to its interferer potential. Hoppingnetworks can be hybrid, having hopping and non hopping carriers within onenetwork, one cell or one BTS.

BCCH TCH

STCH

4%

4%10%

10%6%

6%6%

4%

10%

Figure 11: Nine different interference thresholds for automatic frequency planning

The introduction of SFH is basically performed in three steps:

Introduce SFH without carrier upgrading (N(TRX)=N(hop), e.g. ARCS=12)⇒ Improved Quality of service


Switch on hopping TCH, which were not usable in nonhopping networks, allowinga higher level of interference (ARCS=9)

⇒ Increased capacity, but non optimum dynamic behaviourPerform finetuning after carrier upgrading

⇒ Optimized dynamic network behaviourPossibility of ARCS below 9 should be evaluated as a second step, monitoringinterference and call drop rate, applying N(TRX)<N(Hop)

8 BSS and CAE Parameter and Implementation at the AlcatelBSSIn this chapter an overview about the BSS and CAE parameter, which need to bemodified for introduction of SFH, will be given. Further the implemented Alcatel BTSconcepts will be discussed.

8.1 BSS and CAE Parameter for SFH

Table 2 gives an overview about the relevant BSS parameter and a brief description oftheir meanings.

Parametername DescriptionFHSmax 16 frequ.

Frequency Hopping Sequence: Set of frequencies to be used in thehopping sequence. Example: {F10,F30,F45,F34}

HSNRange:0...63

Hopping Sequence Number: Order of the frequencies defined inthe FHS. Cyclic: HSN = 0, Random: HSN = 1...63

MAIORange: 1...16

Mobile Allocation Index Offset: First frequency to be used in thehopping sequence. Example: MAIO = 2 ⇒ First freq.: F30

FHS_IDRange: 1...16

Identifier of a specific frequency hopping system. E.g. in TS 0 adifferent FHS and thus a different FHS_ID needs to be used than inthe other TS, since the BCCH frequency has to be excluded formthe FHS.

L_RXQUAL_UL_H Lower threshold for triggering a quality HO based on uplinkL_RXQUAL_DL_H Lower threshold for triggering a quality HO based on downlink

Table 3: Relevant BSS Parameter for introduction of SFH

When using random hopping, cells which can potential interfer each other, should usedifferent HSNs in order to provide maximum interferer diversity.When using cyclic hopping, all cells will use the same HSN (HSN = 0).Within the same cells, different MS will use the same HSN but a different MAIO. Thisguarantees, that there will be no collisions within one cell.In general TS 0 must not hop on the BCCH frequency. Thus a different FHS_ID has tobe defined for TS 0. Therefore the Customer Data Population Programs (CDPP) willautomatically populate two frequency hopping systems [6]:


FHS_ID 1 with all associated frequencies of the BTS except BCCH frequency for TS 0FHS_ID 2 with all associated frequencies of the BTS inclusive BCCH frequency.

Once a HSN and the FHSs are defined, the FHS_ID and the MAIO defines thefrequency to use in each TS of each frequency unit. Instead of the ARFCN the FHS_IDand MAIO separated by a comma, will be inserted as the channel value in theaccording OMC screen. The following table illustrates a possible setup with four FUs.

TS 0 TS 1 TS 2 TS 3 TS 4 TS 4 TS 5 TS 6 TS 7FU 1 bc/sd4

orbcch

TCH TCH TCH TCH TCH TCH TCH TCH

fhs_id, maio freq 1, 0 1, 0 1, 0 1, 0 1, 0 1, 0 1, 0 1, 0FU 2 TCH SD/8 TCH TCH TCH TCH TCH TCH TCHfhs_id, maio 2, 0 1, 1 1, 1 1, 1 1, 1 1, 1 1, 1 1, 1 1, 1FU 3 TCH TCH TCH TCH TCH TCH TCH TCH TCHfhs_id, maio 2, 1 1, 2 1, 2 1, 2 1, 2 1, 2 1, 2 1, 2 1, 2FU 4 TCH TCH TCH TCH TCH TCH TCH TCH TCHfhs_id, maio 2, 2 1, 3 1, 3 1, 3 1, 3 1, 3 1, 3 1, 3 1, 3

Table 4: Possible setup for a frequency hopping scenario, at 4 TRX BTS

Since the network quality is deteriorated by one digit, the thresholds ofL_RXQUAL_XX_H need to be tuned, when introducing SFH (increment by on value).

For the implementation of SFH using CAE parameter, the following parameter haveto be defined for each cell (based on [6]):

TO BE DEFINED FOR RFH, BBH

8.2 Implemented Alcatel BTS concepts

Macro BTS:G2 BTS: Maximum 8 carriers, baseband hopping.G3 BTS: Baseband and synthesizer hopping is possible. Real baseband hopping isplanned for a second step, when RTCs will be used instead of WBC.

Micro BTS [7]:

Realisation of synthesizer hopping in the 1 TRX micro BTSAs shown in figure 12 the basic unit of the micro BTS is equipped with 2 transmitter(TCH-TX and BCCH-TX). The BCCH-TX transmits the BCCH information of TS0 anddummy bursts on TS1-7. Traffic data is transmitted on the TCH-TX. Since both


transmitter are equipped with one synthesizer (TXSYN 1 and TXSYN 2) the BCCH andTCH can be tuned on different frequencies. Therefore it is possbile to performsynthesizer hopping on the 1 TRX micro BTS, while transmitting a dummy burst on theBCCH frequency. The hopping synthesizer TXSYN 2 and RXSYN are equipped with twooscillators, making synthesizer hopping possible, switching from TS to TS betweenboth oscillators.If frequency hopping is deactivated, the TCH-TX will not be used. Therefore thecombiner inside the BTS will not be used and the transmission power is increased bythe abscence of combiner loss of 3dB to 26 dBm.

1 and 2 TRX micro BTS: synthesizer hopping:

1 TRX µ-BTS• without frequency hopping (26dBm TX Power)• with synthesizer hopping on TS1-TS7 (23 dBm TX-Power due to combiner for TCH-

TX and BCCH-TX (figure 17))• TS0: BCCH/CCCH Bursts on BCCH-TX, TS1-TS7: Dummy Bursts on BCCH-TX• TS1-TS7: TCHs with frequency hopping on TCH-TX

2 TRX µ-BTS• First TRX: no frequency hopping (BCCH-Frequency)• Second TRX synthesizer hopping (TCH-Frequencies)• TX-Power 26 dBm

RXSYN

ARX

ATX

TCH-TX

BCCH-TX

BCCH/TCH-RX

Tx data

Rx data

TXSYN 1

TXSYN 2

ATX

TX/RX Antenna

C

O

M

B

I

N

E

R

D

U

P

L

E

X

E

R

Figure 12: Architecture of the 1TRX µ-BTS (basic unit), for the realisation of synthesizer hopping


9 ConclusionThe document gave an overview about Slow Frequency Hopping (SFH) in mobile radionetworks, as specified in GSM. Based on the theoretical discussion, simulations andfield experiments, the following conclusions have been drawn:

• The main benefits of SFH are frequency diversity for slowly moving mobiles andinterferer diversity, which occurs due to own hopping and uncorrelated hopping ofthe interferer.

• SFH can be performed in cyclic or random mode, whereas only random hopping

provides interferer diversity and should be prefered to cyclic hopping. • From the BTS point of view, baseband hopping (BBH) and synthesizer frequency

hopping ( = radio frequency hopping = RFH) is possible, whereas only withsynthesizer hopping N(Hop)>N(TRX) is possible.

• Only neglectible frequency diversity gain is expected, when hopping over more

than four frequencies. • With an ARCS below 9, softblocking occurs when using frequency hopping. For non

hopping systems the softblocking limit will occur for an ARCS below 12. • Simulations showed that a capacity gain of up to 74% is possible with an <1x3>

reuse scheme, but with an increased amount of call drops. Defining the call droprate as the softblocking limit, the <3x3> reuse scheme turned out to be optimum interms of capacity.

• An ARCS of 9 should be used for a first introduction of frequency hopping, applying

N(TRX)=N(Hop) as long N(TRX)>=3. The hardblocking limit still can be reached,making a capacity increase of 30% possible, when planning the BCCH e.g. with anARCS of 12. Reducing the amount of TRX by applying fractional reuse in thatconfiguration is not recommended, because then the maximum achievable capacity(determined by hardblocking) will be reduced.

• Further reduction of the ARCS below 9 has to be evaluated in a second step, while

carefully monitoring the effects of softblocking (call drop rate and FER). Fractionalreuse by applying N(TRX)<N(Hop) can be used for such a configuration, especiallyin case of drastic reductions of the frequency reuse (e.g. <1x3>). In this case thesoftblocking limit will already be reached at a traffic load of 30%. Thus, applyingBBH in that case would have the effect, that 70% of hardware capacity is unused.Therefore N(TRX) should be reduced (N(TRX)<N(Hop)), but has to be high enough,that the capacity defined by the softblocking limit still can be carried.


• Micro BTS: Prefer RFH applying Fractional Reuse, to take full benefit out of diversity

with N(hop)=4. • SFH can be taken into account for automatic frequency assignment with a special

interference threshold, the super TCH (STCH). • Power Control and DTX are more effective in hopping networks and thus

recommended. • A drawback of synthesizer hopping configurations is, that no remote tunable

combiners (RTCs) can be used. Thus wideband combiners (WBC) have to be used,which introduce higher losses in downlink direction.

• Due to frequency and interferer diversity, the network quality is improved when

introducing SFH. Thus SFH without carrier upgrading is a possibility for networkquality increase.

• The main results of field trials can be summarized as follows: Improved FER,

Smaller Call Drop Rate and reduced Call Establishment Failure, more benefit istaken from random hopping, optimum number of frequencies to hop over is four.But it also has been found out, that due to interferer diversity and nonlinearmapping of BER to RXQUAL, the parameter for the quality HO need to be adapted.

• SFH should be introduced in well planned, optimized and mature networks, in

order to improve the network quality or in order to increase the capacity.


10 Abbreviations

ARCS Average Reuse Cluster SizeARFCN Absolute Radio Frequency Channel NumberBBH BasebandhoppingBCCH Broadcast Control ChannelBER Bit Error RateBSS Base Station SubsystemBTS Base Transceiver StationC/I Signal to Interferer RatioC/ITHR Threshold of Signal to Interferer RatioCAE Customer Application EngineeringCCCH Common Control ChannelCDPP Customer Data Population ProgrammCU Carrier UnitDTX Discontinious TransmissionFER Frame Erasure RateFFH Fast Frequency HoppingFHS Frequency Hopping SystemFHS_ID Identifier of a FHSFU Frame UnitGSM Global System of Mobile CommunicationHO HandoverHSN Hopping Sequency NumberMAIO Mobile Allocation Index OffsetMS Mobile StationOMC Operation and Maintenance CentrePblock Blocking probability (Hardblocking)PC Power ControlPint Interference ProbabilityQoS Quality of ServiceRFH Radio Frequency HoppingRX ReceiverRXSYN Synthesizer of the ReceiverSACCH Slow Associated Control ChannelSFH Slow Frequency HoppingSTCH Super Traffic ChannelTCH Traffic ChannelTDMA Time Division Multiple AccessTRX TransceiverTS TimeslotTU3, TU50TU2

Typical Urban with 3, 50, 2km/h mobile speed


TX TransmiterTXSYN Synthesizer of the Transmiter

END OF DOCUMENT

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