General concept of Frequency hopping

BackgroundDuring a call, a number of physical effects influence theperceived radio environment between a mobile station and abase station. One such effect is multipath fading, whichmeans that transmitted signals reach the receiver via multiplepaths. Depending on the difference in path length.Another effect is various types of interference. Thedominating type is normally co-channel interference, butother types, such as adjacent channel interference, intermodulation products, military sources etc. must beconsidered as well.

Multipath fadingThe destructive interference produced by multipath fading is called fading-dips. Fading dips may cause speech quality degradation. For different frequencies, the fading dips will occur atslightly different positions in space.

Impact on Network QualityMultipath FadingA result of scattering environmentsLeads to reduced transmission qualityA challenge to mobile communication systems

Co-channel interferenceThe interference situation for a mobile is strongly dependent on which frequency and time-slot that the mobile happens to use.Normally co-channel interference is caused by frequency re-use

What can be achievedFrequency diversityInterference averaging

Frequency diversityFrequency hopping can reduce the influence of signal strength variations caused by multipath fading. Multipath fading is frequency dependent. This implies that the fading dips appear at different locations for different frequencies.

Interference averagingFrequency hopping can also break up persistent interference into periodic occasions of single burst interference. Changing frequency at each burst offers a way to improve the interference situation described above. The co-channel interference will change at every burst. The more frequencies that are used in the hopping, the more rare such frequency collisions will be.

FREQUENCY DIVERSITYProtection Against Deep Fading for Slow Mobiles.Frequency Selective Nature of the Fading: All Frequencies Not Faded at the Same Time and at the Same Position.Corrupted Bursts are Reduced and Temporally Spread: More Effectiveness of Decoding and Deinterleaving Processes.

FREQUENCY DIVERSITY (2)Non HoppingFreq. f1Freq. f1Freq. f2HoppingCorrupted Bursts

FREQUENCY DIVERSITY (3)More Concentration of the Received Level Values Around the Mean (Smaller Deviation).Frame Erasure Rate (FER) is the Right Measure of the Voice Quality (Higher RxQual Values are Tolerated With Frequency Hopping)

Interference in Fixed Networks(Non-hopping)Permanent interference between interfering cellsThe call quality is fixed at either Good or BadLarge separation required to overcome the interferenceAt the expense of frequency reusef1f1Interference

INTERFERENCE AVERAGINGFIXED SYSTEM: Permanent Interference Between Interfering Cells When There is a Call.HOPPING SYSTEM: Variable Interference Depending on the Frequencies Used to Transmit each Burst.The Calls Experience an Average Quality Instead of Extreme Situations of Either Good or Bad Quality.

INTERFERENCE AVERAGING (2)Non HoppingHopping

INTERFERENCE AVERAGING (3)Decoding and Deinterleaving Processes are Able to Cope with a Higher BER => Higher RxQual is Tolerated With F. H. Without Impacting the Quality (Lower FER).Hopping: Bad Perceived Quality => RxQual = 6 or 7Non Hopping: Bad Perceived Quality => RxQual = 5, 6 or 7

INTERFERENCE AVERAGING (4)QUALITY-CAPACITY TRADE-OFF: "For the same capacity FH improves the quality, and for a given average quality FH makes possible increase the capacity".

Interference in Hopping NetworksDiscontinuous InterferenceEffectiveness of channel coding and interleaving enhancedSpreading interference - interference diversityCall experiences better average quality

Short technical descriptionBaseband frequency hoppingSynthesizer frequency hopping

Baseband frequency hoppingAt baseband hopping each transmitter operateson a fixed frequency.The disadvantage is that it is not possible to use a larger number of frequencies than there aretransmitters.

Baseband frequency hoppingControllerTRX1ControllerTRX4ControllerTRX3ControllerTRX2Transmitterf1Transmitterf4Transmitterf3Transmitterf2Bus for routing of burstCombinerXXXX

BASE BAND H0PPING (BBH)The TCUs Always Transmit a Fixed Frequency.The Call "Hops" over the TCUs, Always in the Same Timeslot, on a per Burst Basis.In Reception the Call is Always Processed by the Same TCU (The one where the Call Started).The Number of Frequencies to Hop Over is Limited by the Number of TCUs Equipped in the Cell.The BCCH Carrier Can Hop in Timeslots 1 to 7 (Without Power Control/DTX).

Synthesizer frequency hopping The transmitter tunes to correct frequency at transmission of each burst. The advantage is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters . The disadvantage is that wide-band hybrid combiners have to be used .

Synthesizer frequency hoppingControllerTRX1ControllerTRX4ControllerTRX3ControllerTRX2Transmitterf1,f2,,fnTransmitterf1,f2,,fnTransmitterf1,f2,,fnTransmitterf1,f2,,fnHybridCombiner

SYNTHESISER FREQUENCY HOPPING (SFH)The TCUs are Able to Retune to a New Frequency Each Burst.The Call Always Stays in the Same TCU.One TCU Can Hop up to Over 64 Different Frequencies.Wide-Band Combining Devices (Hybrids) are Required in the Base Station (Cavity Combiners Can Not be used with SFH).

AlgorithmHopping sequenceCyclic hoppingRandom hopping

Implementation with SFHSeparate frequency band for BCCHRe-use patternMAIOHSNFraction load

Coverage overlapping constrainDue to SFH with 1x1 or 1x3 are tight re-use patterns then coverage control is major constrain.Homogeneous network is recommended.

Frequency constrainPerformance of SFH depends on one factor which called Fractional loadMaximum fractional load is 50% means number of frequency required is at least 2 time number of TCH Trxs used.

Separate frequency band for BCCHBCCH cannot cope with high interference asTCH due to :BCCH is not hop with SFH.Power control and DTX are not support on BCCH.

Re-use pattern for SFHStandard re-use patternRe-use 1x1.Re-use 1x3.Mixed.

Re-use 1x1Define every frequencies to every BTS.Avoid co-channel by MAIO and HSNConsider all frequencies assigned as frequency group A re-use pattern will be as follow:GroupAGroupAGroupAGroupAGroupAGroupAGroupAGroupAGroupA

Re-use 1x3Separate all frequencies into 3 groups.Define 3 frequency groups to every sites.Avoid co-channel by MAIO and HSNConsider all frequencies assigned as frequency group A,B and C re-use pattern will be as follow:GroupAGroupCGroupBGroupAGroupCGroupBGroupAGroupCGroupB

Mobile allocation index offsetDefine the first frequency of group for the first burst.

Example of MAIO settingThe random sequence of synthesizer hopping will appearas follows for eight frequencies: (HSN = 0)ControllerTRX1ControllerTRX4ControllerTRX3ControllerTRX2Transmitterf1, f2, .., f8Transmitterf1, f2, .., f8Transmitterf1, f2, .., f8Transmitterf1, f2, .., f8Combinerf1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 2)f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 0)f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 4)f1, f2, f3, f4, f5, f6, f7, f8 (MAIO = 6)Index : 0, 1, 2, 3, 4, 5, 6, 7fn : frequency of the first burstfn : frequency of the second burst

Fraction loadRatio to determine how tight of frequency re-use for SFH.Define by :Number of frequencies used at a time (per re-use cluster) * 100Number of frequencies per groupRecommended fraction load = 35-40%GSM defines maximum fraction load = 50%

Example of fraction load calculation1x3Number of frequencies : 46Number of frequencies for BCCH and GB : 16Number of TCH frequencies per group : 10Site configuration : 6+6+6 (Tch : 5+5+5)

Fractional load = 5/10 = 50%

Example of fraction load calculation1x1Number of frequencies : 46Number of frequencies for BCCH and GB : 16Number of TCH frequencies per group : 30Site configuration : 6+6+6 (Tch : 5+5+5)

Fractional load = 15/30 = 50%

REUSE PATTERNS WITH FH3x3 Reuse Pattern1 Freq./Carrier Base Band Hopping1x3 Reuse Pattern 1x1 Reuse Pattern More Than 1 Freq./Carrier Synthesiser Frequency Hopping

PRACTICAL RULES FOR F.H. IMPLEMENTATIONUse Separate Bands for BCCHs & TCHs.The Larger the Separation of the Frequencies in the Hopping Sequence, the Higher the Benefits of Frequency Hopping. (Frequency Diversity)Quality Handovers Need to Be Optimised (Higher RxQual Threshold).

FREQUENCY HOPPING PERFORMANCE Different Scenario Types:

CASE STUDY1x3 SFH System.1x1 SFH System.

1x3 SFH ON A TYPE D NETWORKScenarioHigh Concentration of Sites in the Busiest Area.Irregular Sector Orientation.Extremely Difficult Planning (Presence of Hills, River, Sea, ...).System Description16 Sites (47 Cells)2/2/2 Configuration in Most of the Sites and 3/3/3 in the Centre40 Frequencies Available

1x3 SFH ON A TYPE D NETWORK (2)SFH System21 Frequencies for BCCHs18 Frequencies for TCHs With SFH1x3 Reuse Pattern Using Sequences of 6 FrequenciesExpansion to 3/3/3 Configuration in Some CellsExceptions to the Regular Pattern due to the Scenario

1x3 SFH ON A TYPE D NETWORK (3)Regular 1x3 Reuse Pattern Hopping Over 6 Frequencies

1x3 SFH ON A TYPE D NETWORK (4)20% Reduction in RFloss on Traffic ChannelsOMC STATS

1x3 SFH ON A TYPE D NETWORK (5)More Successful Handovers10% Reduction in Drop Calls due to Failures in HOHO Suc. Rate94,2394,650102030405060708090100FIXED SYSTEM1x3x6PercentageHO Fail Rate1,031,140,960,9811,021,041,061,081,11,121,14FIXED SYSTEM1x3x6PercentageOMC STATS

1x3 SFH ON TYPE D NETWORK (6)CAPACITY SOLUTIONFIXED SYSTEM:2 Carriers per Cell

SFH SYSTEM:3 Carriers per Cell

82% CAPACITY INCREASE IN A REAL NETWORK

1x1 SFH ON A TYPE D NETWORKCAPACITY AND QUALITY SOLUTION40 Frequencies15 Frequencies for BCCHs7 Frequencies for Micros18 Frequencies for TCHs With SFH1x1 Reuse Pattern Using Sequences of 6 Frequencies4/4/4 Configuration in Most of the CellsMore than 70 Sites

1x1 SFH ON A TYPE D NETWORK (2)IMPLEMENTATIONSame MA for All CellsDifferent HSN per Site (Same for Cells on the Same Site)Non-Adjacent MAIOs on the CarriersInterference Avoidance between Carriers of the Same Site

1x1 SFH ON A TYPE D NETWORK (3)IMPLEMENTATION (2)1x1 SFH: 18 FrequenciesUp to 9 Hopping Carriers per Site Without Interference4/4/4 ConfigurationBCCHTCH Hopping (MAIO)

1x1 SFH ON A TYPE D NETWORK (3)

1x1 SFH ON A TYPE D NETWORK (4)

ADVANTAGESQUALITY IMPROVEMENTReduction in FERImprove Subjective Voice QualityReduction in Drop CallsIncrease Call Success Rate

SummaryQuality ImprovementFrequency diversityEnhanced immunity to multipath fadingReduced spread of signal strengthQuality improvement: FER, voice quality, dropped call rate and handover failure rate - all improved

ADVANTAGES (2)CAPACITY INCREASEThe Same Quality is Achieved With Lower C/I Values.

Reuse the Frequencies in a Tighter Way

ADD MORE CARRIERS& MORE SITES

ADVANTAGES (3)CAPACITY INCREASE (2)Cell Placed in a Non-Location Area BorderTRAFFIC OFFERED

ADVANTAGES (4)CAPACITY INCREASE (3)(*) 3 Frequencies Left(**) One of the Frequencies Reused in a 2x3 Pattern

FREQUENCIESCONFIGURATIONSTRAFFIC OFFERED (E/Km2)CAPACITY INCREASESAVAILABLEFIXEDBBHSFHFIXEDBBHSFHBBH / FIXEDSFH / FIXEDSFH / BBH242/2/22/2/2 *3/3/331.3231.3256.91-82%82%363/3/34/4/4**5/5/556.9183.64111.5447%96%33%484/4/45/5/57/7/783.64111.54164.6433%97%48%605/5/56/6/6 *9/9/9111.54135.99222.3022%99%63%

Quality-Capacity Trade-offFor a given capacity FH improves the qualityFor a given average quality FH increases capacity by implementing AGR

ConclusionsFrequency hopping is a powerful technique to improve transmission quality and enhance capacitySynthesiser frequency hopping is a cost effective and flexible means to maximising capacity using AGRMotorolas wide scale deployment of frequency hopping networks validated the effectiveness of the frequency hopping

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