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Spectrum Planning Report

COMPATIBILITY ASSESSMENT -

800 MHz AMPS SPECTRUM

ADJACENT TOGSM SPECTRUM

Business Directions Group Document: SPP 2/97Spectrum Management Agency Date: April 1997

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Spectrum Planning Report SPP 2/97 -1- April 1997

1 PURPOSE ................................................................................................................ 2

2 BACKGROUND...................................................................................................... 2

3 INTERFERENCE MANAGEMENT FRAMEWORK.......................................... 2

3.1 GSM M OBILE TRANSMITTERS TO AMPS M OBILE RECEIVERS .................................. 33.2 AMPS B ASE TRANSMITTERS TO GSM B ASE RECEIVERS .......................................... 3

4 SUMMARY.............................................................................................................. 4

APPENDICES:

A AMPS/GSM Interference Analysis.........................................................................5

A.1 GSM Receiver Characteristics...............................................................................5A1.1 Receiver Blocking........................................................................................ 5A1.2 Receiver Intermodulation............................................................................ 5

A2 System Assumptions.............................................................................................6A3 Interference Calculations.......................................................................................7

A3.1 Blocking Calculations................................................................................. 8A3.2 Intermodulation Calculations...................................................................... 9

B GSM Base Station Receiver Filter Response........................................................11

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1 PURPOSE

The purpose of this report is to identify the interference situations that occur betweenanalogue AMPS and digital GSM cellular systems and to describe the managementstrategies that have been used to manage such interference. The report also provides ameans of determining the isolation required between AMPS base transmitters and GSMbase receivers to minimise interference between these systems.

2 BACKGROUND

The 900 MHz Band Plan 1 allocated the band 870 - 890 MHz to the analogue AMPSservice, base transmit (BT), and the immediately adjacent 890 - 915 MHz band to theGSM digital service, base receive (BR). The BT, BR designations are specified inrelevant US and ETSI specifications that have been adopted in Australia within relevantAUSTEL standards, and are not able to be changed. The overall spectrumarrangements, including the current operators, are shown in Fig 1.

SAllocated toother services

Allocated toother services

825 835 845 870 880 890 915 935 943.4 951.8 960

AMPSMobile

TransmitBand

AMPS

TransmitBand

BaseTransmit

Band

BaseGSM

TransmitBand

GSMMobile

45 MHzSeparation between send and

45 MHz

TELSTRA

VODAFONE

OPTUS

receive frequenciesSeparation between send and

receive frequencies

906.8898.4

MPA

-A

S

MPA

-B

S

MPA

-A

S

MPA

-B

T

ELSTRA

VODAFONE

OPTUS

Frequency (MHz)

Figure 1: Spectrum Arrangements for AMPS and GSM in the 900 MHz Band

It is not usual spectrum planning practice to allocate adjacent or very close bands of spectrum to services in this way, ie, with opposite transmit / receive designations(opposite site sense), as particularly difficult interference situations arise. In Australiathe situation has been successfully managed in the existing carrier framework, by acombination of regulatory and technical strategies.

3 INTERFERENCE MANAGEMENT FRAMEWORK

The interference situations are:• GSM mobiles interfering with AMPS mobile receivers; and• AMPS base transmitters interfering with GSM base receivers.

1 Statutory Rules 47, 1992.

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3.1 GSM Mobile Transmitters to AMPS Mobile Receivers

The interference mechanism here is one of overloading (blocking) of susceptible AMPShandsets from nearby (<10m) GSM mobiles. This can occur even when the operatingfrequencies are reasonably well separated, but is more severe as the frequencies becomecloser.

This problem became apparent in 1993 with the introduction of GSM services.AUSTEL through its Standards Working Group 4/1, consulted with industry in seeking asolution. The SMA participated in this process.

In summary, it was found that the degree of susceptibility varied greatly between variousmanufacturers handsets from severe to hardly noticeable. Those with superior front-endselectivity and higher 3rd order intercept point performed better. WG 4/1 thendeveloped an Australian Standard 2 to assist manufacturers in determining the degree of susceptibility appropriate for their products, and steps that could be taken to achieve this.This standard was then referenced in an amendment to AUSTEL’s AMPS standard,TS005, as an optional provision.

3.2 AMPS Base Transmitters to GSM Base Receivers

AMPS base transmitters can cause a performance degradation to closely located, nearfrequency GSM base receivers. Sufficient isolation between AMPS base transmittersand GSM base receivers is required to minimise the potential for receiver blocking and tolimit the receiver generated intermodulation interference below an acceptable level. Thismay be achieved by increasing antenna separation both horizontally and vertically, byadditional GSM receive filtering, additional AMPS transmitter filtering, or by reductionof AMPS base transmitter power.

This problem was understood during development of the 900 MHz Band Plan. SMA,Telstra and industry studies at the time showed that for co-located sites, a frequencyseparation of 2 - 3 MHz, together with additional filtering to that normally employedwere necessary to minimise these effects. At frequency separations less than this,separation distances of up to a few kilometres are needed between the systems to avoidinterference. The performance objectives used in these studies were based on the ETSIGSM standards. An analysis of the AMPS/GSM base station compatibility is provided atAppendix A.

The Band Plan allocation strategy took account of this situation and that successfullyallowed both services to operate to the extent possible, and within the intentions of Government policies. These strategies initially included:• the nomination of the lower 10 MHz of GSM spectrum (ie, 890 - 900 MHz) as a

reserved band for later allocation (from 1 January 1995) - this provided an initial de-facto 10 MHz “guard band” between AMPS and GSM; and

2 AS4294(int)-1995, “AMPS Receiver Immunity to Interfering Signals Employing Amplitude Modulation”

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• the designation of the 880 - 890 MHz “AMPS B” bands as secondary in all areas of Australia, except initially in Sydney and Melbourne where primary status applied until1 January 1995. This policy restricted in particular use of the upper part of the Bband in most other areas of Australia, such that the AMPS/GSM problem was largelyavoided.

Subsequent Government decisions led to the sale of the last 2 x 10 MHz of GSMspectrum in March 1995 to the three mobile carriers. The sale necessitated arationalisation of the spectrum allocated to the three mobile carriers and resulted in the

lower third (8.33 MHz) of GSM spectrum being allocated to Telstra, ie, that spectrumadjacent to AMPS B. To optimise the use of both bands, Telstra decided to use the top3 MHz of AMPS as a guard band in Sydney and Melbourne. To achieve this, extrafiltering was required to both the AMPS transmitters and GSM receivers at most co-sited stations, and at some other sites. A description of the characteristics of the filtersemployed on these GSM receivers is provided at Appendix B. (Note: a lesser guardband is possible with the use of higher performance, but more costly, filters).

4 SUMMARY

The SMA has introduced an interference management framework to manage interferencebetween analogue AMPS and digital GSM cellular systems. The fitting of filters at basestations, additional to those used in normal system configurations, is necessary tooptimise the spectrum utilisation for both technologies.

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AMPS/GSM INTERFERENCE ANALYSIS


A.1 GSM Receiver Characteristics

The GSM receiver characteristics are defined as blocking, AM suppression,intermodulation and spurious emissions in the GSM 05.05 standard 3. The two majorcharacteristics that cause site based GSM receiver interference are blocking andintermodulation. Blocking is caused by the presence of high level RF power appearing atthe receiver input causing the receiver to overload. Two or more high level AMPS

signals appearing at the receiver input can mix in the receiver and produceintermodulation products that fall on the desired GSM receive channel. In both cases thereceiver can be desensitised. Sufficient isolation between AMPS transmitters and GSMreceivers is required in order to reduce the receiver generated blocking andintermodulation interference to an acceptable level. This may be achieved by increasingantenna separation both horizontally and vertically, additional GSM receive filtering, orreducing AMPS base transmitter power.

A1.1 Receiver Blocking

The GSM receiver blocking performance stated in GSM 05.05, specifies that referencesensitivity performance should be obtained, with the wanted signal 3 dB above thereference sensitivity level in the presence of an unwanted blocking carrier. The blockinglevels specified in GSM 05.05 are given in Table A1.

In-band (870 - 925 MHz) Level (dBm)

600 kHz < |f-f 0| < 800 kHz -26

800 kHz < |f-f 0| < 1.6 MHz -16

1.6 MHz < |f-f 0| < 3 MHz -16

3 MHz < |f-f 0| -13

Out-of-band0.1 - 870 MHz 8

925 - 12,750 GHz 8

Table A1. Blocking Characteristics of GSM Base Receivers

A1.2 Receiver Intermodulation

The GSM receiver intermodulation performance stated in GSM 05.05, specifies that thethird order intermodulation term generated by two interfering signals at -43 dBm cause a

3 dB degradation in receiver sensitivity. That is, the level of two signals appearing at thereceiver input must be kept below -43 dBm to avoid receiver generated intermodulationinterference when the receiver wanted signal level is at the reference sensitivity level(-104 dBm).

3 GSM 05.05 (ETS 300 577), ”Digital Cellular Telecommunications System (Phase 2); RadioTransmission and Reception”, August 1996.

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The specified requirement only covers the 2 signal 3rd order case. It is complicated bythe fact that intermodulation products will add if they fall on the same frequency. Thissituation is typical of channelised systems, such as AMPS, which have a raster of equallyspaced carriers. The number of intermodulation products falling within the I.F.bandwidth of a GSM base receive channel is a function of the number of regularly spacedAMPS carriers. A reduction in the carrier power may be required to avoid receivergenerated intermodulation interference. The amount of reduction required is the totalpower of the intermodulation products falling within the I.F. bandwidth of a particularGSM base receive channel divided by three.

In an AMPS system with a typical 7/21 frequency reuse pattern (7 cells, 21 channelcarrier spacing), the maximum number of carriers per sector cell is 32 for the entire20 MHz AMPS base transmit segment (approximately half of Telstra’s AMPS basestations use 32 carriers). For the lower part of the GSM base receive band, up to 16 co-incident intermodulation products can occur. Thus the total allowance required is(10log(16))/3 = 4 dB. Note that this is the worst case situation and does not allow forthe probabilistic nature of carriers being present only during conversations. An analysis 4

has shown that there will be approximately 12 co-incident intermodulation productspresent for 95 % of the time. The allowance required for 12 products is 3.6 dB. The

analysis also found that where there is an irregular channel spacing, the allowance isapproximately 2 dB for 95% of the time.

A2. System Assumptions

The following parameters are assumed for the purposes of calculating the isolationrequirements:

• the minimum AMPS transmit to GSM receive horizontal antenna separation is 20 m.This assumption is made as the antenna radiation is in the near field at distances lessthan this and the isolation is not easily determined (empirical data is required);

• only two-signal 3rd order intermodualtion terms are considered for intermodulationanalysis. This assumption has been made as the performance of a GSM base receiverwith more than two off-channel signals is not specified nor accurately known. Theeffects of multi-signal third order products can be approximated using the sum of powers method described in section A1.2 of this report and an appropriate correctionfactor added. Higher intermodulation terms are not considered as they will be at alower level than the 3rd order term. It is assumed that any isolation technique(filtering) will also reduce the power in higher order terms accordingly;

4 Telstra document DCR 0161 “Protection of Existing GSM and AMPS Services from Interference from800 MHz Proposed Spectrum Licence Band Services”, Issue 03, 6 April 1997.

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• the transmit and receive antennas are at the same height and pointing at each other.Antenna discrimination due to the vertical or horizontal radiation pattern of either thetransmit or receive antennas is assumed to be worst case (no discrimination);

• free space propagation conditions exist as both AMPS transmit and GSM receiveantennas are at high sites and blocking and receiver intermodualtion occur at shortdistances;

• GSM system planned to operate with a carrier to interference ratio (C/I) of 13 dB;• GSM base receiver protected from interference for at least 95 % of the time;• GSM base receiver typical antenna gain of 19 dBi;• GSM base receiver typical feeder loss of 3dB;• GSM base receiver sensitivity level of -104 dBm as per GSM 05.05;• GSM base receiver I.F. bandwidth of 250 kHz at the 6 dB points;• GSM base receive filter attenuation as specified in Appendix B of this report; and• the reference point for all receive signal levels is the BSS receiver antenna connector.

A3. Interference Calculations

The following arrangement is used to model the path between the AMPS basetransmitter and GSM base receiver.

AMPS BASETX

GSM BASERX

RX FILTER

EIRP Lp Gr

Lfilt

Lf

Pr

Figure A1. AMPS Base Transmit to GSM Base Receive Model

The calculations are based on the following link budget equation:

or P r = EIRP - L p + G r - L f - L filt ...(1)

where Pr = receive signal level in dBm

EIRP = transmitter radiated power in dBmLP = path loss between tx and rx antennas in dBG r = receive antenna gain in dBiLf = receive feeder loss in dB

Lfilt = receive filter attenuation in dB

Free space path loss can be calculated from the formula:

Lp = 32.45 + 20 log (freq in MHz) + 20 log (dist in km) ...(2)

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A3.1 Blocking Calculations

The isolation required due to blocking can be calculated using equation (1) bysubstituting the blocking levels given in Table A1 into the term P r.

600 kHz < |f-f 0| < 800 kHz, -26 = EIRP - L p + 19 - 3 - L filt or L p = EIRP - L filt + 42

800 kHz < |f-f 0| < 1.6 MHz, -16 = EIRP - L p + 19 - 3 - L filt or L p = EIRP - L filt + 32

1.6 MHz < |f-f 0| < 3 MHz, -16 = EIRP - L p + 19 - 3 - L filt or L p = EIRP - L filt + 32

3 MHz < |f-f 0| , -13 = EIRP - L p + 19 - 3 - L filt or L p = EIRP - L filt + 29

A plot of isolation (L p) versus frequency for 1 watt EIRP is given in Figure A2 below.The chart can be used to determine isolation requirements between the AMPS transmitantenna and GSM base receive antenna for other transmit powers by appropriate scaling.For example, the isolation required at 887 MHz is 18 dB for 1 watt EIRP. For 200 wattsEIRP, the isolation required is 18 dB/W + 23 dBW = 41 dB.

FIGURE A2 - GSM Base Station Isolation Requirement Due to Blocking

-10

0

10

20

30

40

50

60

70

80

870 872 874 876 878 880 882 884 886 888 890

Frequency (MHz)

I s o l a t i o n ( d B / W )

A plot of EIRP versus frequency is given in Figure A3 below for a number of distanceseparations between 20 metres and 1 km. The plot assumes free space propagationconditions and does not include additional isolation due to the transmit or receive

antenna vertical radiation pattern. As an example, the maximum total radiated powerfrom an AMPS base station transmitting on 889 MHz 50 metres away from a GSM basereceiver is 34 dBm or 2.5 watts EIRP.

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FIGURE A3 - Maximum AMPS Base Station EIRP to Avoid Blocking ina Typical GSM Base Station Receiver

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

870 872 874 876 878 880 882 884 886 888 890

Frequency (MHz)

M a x i m u m

E I R P ( d B m

)

dist=20m

dist=50m

dist=100m

dist=200m

dist=500m

dist=1km

A3.2 Receiver Intermodulation Calculations

The isolation required due to receiver intermodulation can be calculated usingequation (1) by substituting the GSM 05.05 specified intermodulation limit of -43 dBminto the term P r.

-43 = EIRP - L p + 19 - 3 - L filt or L p = EIRP - L filt + 59

A plot of isolation (L p) versus frequency for 1 watt EIRP is given in Figure A4 below.The chart can be used to determine isolation requirements between the AMPS transmitantenna and GSM base receive antenna due to 2-signal 3rd order receiverintermodulation for other transmit powers by appropriate scaling. For example, theisolation required at 886 MHz is 47 dB for 1 watt EIRP. For 200 watts EIRP, theisolation required is 47 dB/W + 23 dBW = 70 dB.

FIGURE A4 - GSM Base Station Isolation Due to Receiver Intermodulation

0

10

20

30

40

50

60

70

80

90

870 872 874 876 878 880 882 884 886 888 890

Frequency (MHz)

I s o l a t i o n ( d B / W )

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A plot of EIRP versus frequency is given in Figure A5 below for a number of distanceseparations between 20 metres and 1 km. The plot assumes free space propagationconditions and does not include additional isolation due to the transmit or receiveantenna vertical radiation pattern. As an example, the maximum radiated power from anAMPS base station transmitting two carriers on 887.67 MHz & 888.93 MHz, 50 metresaway from a GSM base receiver is 27 dBm or 0.5 watts EIRP for the 887.67 MHzcarrier and 7 dBm or 5 milliwatts EIRP for the 888.93 MHz carrier.

FIGURE A5 - Maximum AMPS Base Station 2-Signal EIRP to Avoid ReceiverIntermodulation Interference in a Typical GSM Base Station Receiver

-10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

870 872 874 876 878 880 882 884 886 888 890

Frequency (MHz)

M a x i m u m

T w o - S

i g n a l E

I R P ( d B m

)

dist=20m

dist=50m

dist=100m

dist=200m

dist=500m

dist=1km

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GSM Base Station Receiver Filter Response


-70

-60

-50

-40

-30

-20

-10

0

870 872 874 876 878 880 882 884 886 888 890

Frequency (MHz)

A t t e n u a t i o n

( d B )

Frequency (MHz) Attenuation (dB)890 <1889 1

888.5 2888 5

887.5 25887 41886 42885 43884 44883 45.5882 47881 48.5880 50879 52878 54877 56876 58875 60874 61.5873 63872 64.5871 66870 67.5

Note: The filter response was obtained from Telstra 5.

5 Telstra document DCR 0161 “Protection of Existing GSM and AMPS Services from Interference from800 MHz Proposed Spectrum Licence Band Services”, Issue 03, 6 April 1997.

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