process for gsn interference

8/9/2019 Process for GSN Interference

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1. IntroductionIn many cities in India GSM operations already exists. WPC has allocated 824-844 for downlink operation and 869-889MHz for uplink operation for CDMA WLL Services.

It is to be noted that the up-link frequency range of 869-889 MHz causes the interference to theexisting GSM Band, which starts from 890MHz. Currently, CDMA equipment has wide bandfilter with only 20 db attenuation with band pass of 869-894MHz.

2. PurposePurpose of this document is to define a process to nullify the effect of CDMA interference inGSM.

3. Calculation for Safe LimitCalculation of the safe limit of CDMA interference level at GSM receive point is given hereunder:

Assumptions:

Improved GSM Receiver Noise density = (i.e., kTo+ NF)= - 174 +3= -171dBm/HzWhere k : Boltzmanns constant (1.38x10^(-23) J/K)

T : Temperature in absolute scale (300K)NF : GSM BTS Noise Figure (3 dB) (improved)

(This corresponds to an improved GSM Receiver sensitivity of 109 dBm (174+53+3+9) in 200 KHzwith 9 dB as the C/I for the GSM 3/9 frequency re-use)

Interference Level Below Sensitivity Degradation to Sensitivity (dB)20 dB 0.04

15 dB 0.1412 dB 0.277 dB 0.795 dB 1.203 dB 1.760 dB 3.00

Table1

Note: Threshold Degradation due to the CDMA Interference, 7 dB below GSM Rx Noise density iscalculated as: Degradation (dB) = 118 + 10*log[10^(-118/10)+10^(-125/10)] = 0.79 dB

Qualitatively, 0.79 dB has been deemed a tolerable degradation to sensitivity and there fore, the systemwill perform satisfactorily with a CDMA Tx interference level of, which is equivalent to 178 dBm/Hz.Hence, we can ensure satisfactory operation of GSM system, if the spurious interference from CDMAtransmit is restricted within a level of 178 dBm/Hz at the GSM receiver input.

Maximum allowable Receive Interference level (Ig) = 178 dBm/Hz

4. The Specified Emission Limits


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The Transmitter Spurious Emission Limits are defined as:

a. For offset frequency greater than 750 KHz and less than or equal to 1.98 MHz from the CDMAcenter frequency;The overall Spurious Emission power Limit is at least - 45 dBC / 30 KHz

b. For offset frequency greater than 1.98 MHz from the CDMA center frequency;The overall Spurious Emission power Limit is at least - 60 dBC / 30 KHz

(for Pout >= 33 dBm)

5. CDMA BTS transmit Frequency band allocation in India

For Reliance the preferred CDMA BTS transmit band is assumed to be from 869 to 894 MHz. But, theallotted frequency band in India for the 4 operators is 869 to 889 MHz.

Tx3 Tx2 Tx1 0.9 GSM Rx 890.2

889.0

The Center Frequencies of CDMA TX1, TX2 and TX3 are 887.61, 886.38 and 885.15 MHz respectively.The offset frequencies for TX1, TX2 and TX3 from 889 MHz are 1.39, 2.62 and 3.85 MHz. Hence, thespurious emission power limit of 45 dBc / 30 KHz is applicable to TX1 only. For the rest , the spuriousemission power limit can be taken as 60 dBc / 30 KHz.

6. Minimum Isolation requirement

i) Between CDMA BTS Transmit carrier (TX1) 887.61 MHz (Last Channel of CDMA) and GSM BTSRx frequency 890.2 MHz (First channel of GSM):

Assuming, transmit power of CDMA BTS carrier as + 43 dBm (20 watt) at 1.23 MHz;CDMA TX Power density at 30 KHz = + 43 dBm - 10*log(1230/30) = 26.8 dBm/30 KHz.

The CDMA Spurious Emission power Limit is - 45 dBC / 30 KHz; as the offset freq is only 1.39 MHz.The CDMA Tx Spurious Emission in GSM band = 26.8 45 = - 18.2 dBm / 30 KHz

Hence, the CDMA Tx Spurious Emission in GSM band (Sp) = - 18.2 44.8 = - 63 dBm /Hz

As mentioned earlier, this spurious transmission level has to be brought down to a level of - 171 dBm/Hz at GSM BTS receiver input. The total minimum Isolation (Is) required between CDMAand GSM systems is (Is)min. = Sp Ig = - 63 (- 178) = 115 dB. ---- (1)

ii). Between CDMA BTS transmit carrier (TX2) 886.38 MHz (last but one channel) and GSM BTS Rxfrequency 890.2 MHz:

CDMA TX Power density at 30 KHz = + 43 dBm - 10*log(1230/30) = 26.8 dBm/30 KHz.

The CDMA Spurious Emission power Limit is - 60 dBC / 30 KHz; for the offset freq > 1.98 MHz.

Figure 1: 4 operators with 5MHz each

f (MHz)

869 888.51874 879 884

1.8 1.81.80.9

869.91 871.14 872.37 873.6 875.4 876.63 877.86 879.66 880.89 882.12 883.35 885.15 886.38 887.61


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The CDMA Tx Spurious Emission in GSM band = 26.8 60 = - 33.2 dBm / 30 KHz

Hence, the CDMA Tx Spurious Emission in GSM band (Sp) = - 33.2 44.8 = - 78 dBm /Hz

In this case, the total minimum Isolation (Is) required between CDMA and GSM systems is(Is)min. = Sp Ig = - 78 (- 178) = 100 dB ------(2)


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7. Total Isolation availability

It may be noticed that this spurious signal will reach the BTS receiver after suffering a free space pathloss, cable and connector losses at both ends and CDMA Tx and GSM Rx antenna gains and antenna tilts.Path loss depends on the distance of separation between the two systems (GSM and CDMA); cable andconnector losses depend on length and type of cable; antenna gains will depend on the type of antennasand angle of separation between the two.

Down Tilt q

d

m

h Rx. Antenna

Tx. DAntenna

D = Sqrt. (d^2 + h^2)

Down Tilt p m = Arc tan (h/d)

Gt (m+p) and Gr (m-q) are obtained using Table 2

Figure 2:

Table 2: Typical Antenna Gain in Vertical Plane:

Angle in Degrees Gain in dBi0.0 16.52.5 13.55.0 11.07.5 3.510 - 3.512.5 - 5.015.0 and above -7.5

Total Isolation required (Is) is achieved through Free Space Path loss (Xp), Antenna & Feeder cables factor (Xe) and an ExtraIsolation Requirement (Ir) with GSM killer fi lter in CDMA BTS path.

Total Isolation available (Is) = (Xp Xe) + Ir

Free Space path Loss (Xp) = 32.4 + 20 Log (f in MHz) + 20 log (D in Km)

Antenna & Feeder cables Factor (Xe) = Gt (m+p) + Gr (m-q) CDMA BTS TX Feeder cable loss GSM BTS RX Feedercable loss.Characteristics of a typical GSM Killer Filter:(RFS (Celwave) Transmit Filters 5505 TX)

From the response curves of 5505 (as shown in chart 1), we assume;Tx Attenuation @ Fo +/- 2.5 MHz is15.0 dB min;

Fo +/- 5.0 MHz is 52.0 dB min& Fo +/- 6.0 MHz is > 60 dB min.

For a frequency reference point of 890 MHz, the attenuation available by using a GSM Killer Filter (tuned to different centerfrequencies) is depicted in the table given below:

Center Frequency (Fo) Attenuation Center Frequency (Fo) Attenuation

CDMABTS

GSMBTS


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h: vertical separation between antennas expressed in km.d: horizontal separation between sites expressed in km.

STEP 10: Calculate Antenna gains Gt (m+2.0) and Gr (m-2.0) using Table 2 and derive Xe fromthese values. Consider 3.5 dB feeder cable loss each, at both ends.

STEP 11: Calculate total available isolation (Xp Xe) by subtracting STEP 10 from STEP 9.

STEP 12: Calculate the Extra Isolation requirement (Ir) = (Is)min (Xp-Xe). See, if this value of extra Isolation requirement can be achievable using a GSM killer filter or not.

Scenario A :

Both CDMA Tx and GSM Rx antennas are on the same tower and they are vertically separated by adistance of at least 5 meters (to cater the reactive near field effect) with CDMA BTS Transmit Frequencyof 887.61 MHz.

In vertical separation case, to counter the Near field / Fresnel region effects, we need to maintain at least10 wavelengths of separation between the two antennas. In the Transition (near) field, the Power densitydecreases inversely with the distance (not square of the distance, as in the case of Far field).

Free Space path Loss (Xp) = 32.4 + 20 log (887.61) + 10 log (0.005) = 68.3 dB

The angles (m+p) and (m-q) are irrelevant and the gains Gt and Gr are to be taken as 15.0 dBi each. (forvertically separated and exactly collinear antennas)

Antenna and Feeder cables Factor (Xe) = - 15.0 15.0 3.5 3.5 = - 37.0 dB

Available Isolation (Xp-Xe) = 68.3 (-37.0) = 105.3 dB and the Extra Isolation requirement (Ir) is(Is)min (Xp-Xe) = 115.0 105.3 = 9.7 dB only. Using a standard GSM killer filter like 5505 TX or5482 TX, will allow us to tune to fixed step sizes of 1.5 MHz. Specifically designed GSM killer filterwith center frequency fixed to 887.61 MHz, if introduced in the CDMA BTS transmit path, will give atleast 13 dB extra isolation (which is above the required Ir of 9.7 dB). Hence, in this case, we canconclude that there are no interference problems between CDMA and GSM systems.

If it is clear for the band edge CDMA FA of 887.61 MHz, it will also be clear for any frequency below it.There fore, the total CDMA BTS Transmit bands 869-874,874-875, 879-884 and 884-889 MHz are allinterference clear for the GSM Rx band, if the CDMA TX and GSM Rx antennas are collinear andvertically separated by al least 5 meters.

Scenario B:

Where in the CDMA and GSM antennas face each other with h meters of vertical separation and dmeters of horizontal separation (see Figure 1 above).

i). Considering the band edge CDMA BTS FA i.e., 887.61 MHz (TX1):

We use the following procedure to calculate (Xp-Xe):

Free Space path Loss (Xp) = 32.4 + 20 log (887.61) + 20 log (sqrt.(h^2+d^2)) in dB

The Look angle between TX and Rx antennas (m) = Arc tan (h /d) in degrees


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For the angles (m+p) and (m-q), the gains Gt and Gr are calculated using Table 2 above.

Antenna and Feeder cables Factor (Xe) = - Gt(m+p) Gr(m-q) 3.5 3.5 in dB

Extra Isolation requirement (Ir) = (Is)min 1 (Xp-Xe) = 115.0 (Xp-Xe)

Table 3 shows the values of Available Isolation (Xp-Xe) and Table 4 shows the values of Extra Isolation

requirement (Ir) for various values of vertical and horizontal separations. Using the GSM killer Filter,tuned to the center frequency of 888.0 MHz, we get only 8.5 dB extra isolation. By using custom-madeGSM Killer filter with fixed center frequency of 887.61 MHz, we get only 13 dB extra isolation. Hence,band edge CDMA BTS Tx FA is not cleared from interference problems.

ii). Considering the last but one (TX2) CDMA BTS Tx FA i.e., 886.38 MHz:

On the similar procedure as done in case i), we can calculate:Xp = 32.4 + 20 log (886.38) + 20 log (sqrt.(h^2+d^2)), Gt(m+p) and Gr(m-q).

As Xp does not change much with 1.23 MHz frequency change (we see only 0.01 dB difference in Xpvalue), we can take the (Xp-Xe) value, same as that of Table 3 values.

Now, the Extra Isolation requirement (Ir) = (Is)min (2) (Xp-Xe) = 100.0 (Xp-Xe).

Table 5 shows the values of Extra Isolation requirement (Ir) for the given Horizontal and Verticalseparations. By utilizing a standard GSM killer Filter with center frequency of 886.5 MHz, we are gettingaround 30.0 dB Extra Isolation. Where as, if we go for a custom designed GSM killer filter with a fixedcenter frequency of 886.38 MHz, we may get another 3.0 dB Extra Isolation advantage.

Hence, it is observed that the last but one CDMA FA is cleared from the Interference problem, onlywhen there is at least 800 meters of horizontal separation or less than 50 meters of horizontalseparation along with at least 6 meters of vertical separation are ensured between the CDMA BTSTx and GSM BTS Rx antennas.


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HorizontalSeparation (d)

ISOLATION AVAILABLE (Xp-Xe) in dBat the following Vertical Separation (h)

in meters in dB 0 2 4 6 8 10 12 14

1 0.0 11.3 62.8 67.3 70.2 72.4 74.1 75.6 76.8

5 7.0 25.3 70.2 72.4 74.1 75.6 76.8 77.9 78.9

10 10.0 31.3 68.4 76.2 77.4 78.4 79.3 80.1 80.920 13.0 37.3 50.6 74.4 81.6 82.2 82.8 83.4 83.9

30 14.8 40.8 43.9 61.4 77.9 82.4 85.3 85.8 86.2

40 16.0 43.3 43.3 54.2 71.6 80.4 84.8 87.6 87.9

50 17.0 45.3 45.1 55.9 65.8 73.6 82.4 86.6 86.9

100 20.0 51.3 48.5 51.1 54.3 62.0 62.1 71.8 71.9

150 21.8 54.8 54.9 52.1 54.7 54.8 54.9 65.5 65.6

200 23.0 57.3 57.4 54.5 54.6 57.2 57.2 57.3 60.4

250 24.0 59.3 59.3 56.4 56.5 59.0 59.1 59.2 59.2

300 24.8 60.8 60.9 58.0 58.0 58.1 60.6 60.7 60.7

350 25.4 62.2 62.2 62.3 59.3 59.4 59.4 62.0 62.0

400 26.0 63.3 63.4 63.4 60.5 60.5 60.6 60.6 63.1

450 26.5 64.4 64.4 64.4 61.5 61.5 61.6 61.6 64.1

500 27.0 65.3 65.3 65.3 65.4 62.4 62.5 62.5 65.0

550 27.4 66.1 66.1 66.2 66.2 63.2 63.3 63.3 63.3

600 27.8 66.9 66.9 66.9 66.9 64.0 64.0 64.0 64.1

650 28.1 67.6 67.6 67.6 67.7 67.7 64.7 64.7 64.8

700 28.5 68.2 68.2 68.3 68.3 68.3 65.3 65.3 65.4

750 28.8 68.8 68.8 68.8 68.9 68.9 65.9 65.9 66.0

800 29.0 69.4 69.4 69.4 69.4 69.4 69.5 66.5 66.5

850 29.3 69.9 69.9 69.9 69.9 70.0 70.0 67.0 67.0

900 29.5 70.4 70.4 70.4 70.4 70.5 70.5 67.5 67.5

950 29.8 70.9 70.9 70.9 70.9 70.9 70.9 71.0 68.0

1000 30.0 71.3 71.3 71.3 71.4 71.4 71.4 71.4 68.4

1050 30.2 71.7 71.7 71.8 71.8 71.8 71.8 71.8 68.8

1100 30.4 72.1 72.1 72.2 72.2 72.2 72.2 72.2 72.2

1150 30.6 72.5 72.5 72.5 72.6 72.6 72.6 72.6 72.6

1200 30.8 72.9 72.9 72.9 72.9 72.9 73.0 73.0 73.0

1250 31.0 73.2 73.3 73.3 73.3 73.3 73.3 73.3 73.3

1300 31.1 73.6 73.6 73.6 73.6 73.6 73.6 73.7 73.71350 31.3 73.9 73.9 73.9 73.9 74.0 74.0 74.0 74.0

1400 31.5 74.2 74.2 74.2 74.3 74.3 74.3 74.3 74.3

1450 31.6 74.5 74.5 74.6 74.6 74.6 74.6 74.6 74.6

1500 31.8 74.8 74.8 74.8 74.9 74.9 74.9 74.9 74.9

Table 3: Quick reference table for calculation of Isolation (Xp Xe)


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HorizontalSeparation Extra Isolation Requirement (Ir) for various vertical separations

in meters 0 2 4 6 8 10 12 14

5 89.7 44.8 42.6 40.9 39.4 38.2 37.1 36.1

10 83.7 46.6 38.8 37.6 36.6 35.7 34.9 34.1

20 77.7 64.4 40.6 33.4 32.8 32.2 31.6 31.130 74.2 71.1 53.6 37.1 32.6 29.7 29.2 28.8

40 71.7 71.7 60.8 43.4 34.6 30.2 27.4 27.1

50 69.7 69.9 59.1 49.2 41.4 32.6 28.4 28.1

100 63.7 66.5 63.9 60.7 53.0 52.9 43.2 43.1

150 60.2 60.1 62.9 60.3 60.2 60.1 49.5 49.4

200 57.7 57.6 60.5 60.4 57.8 57.8 57.7 54.6

250 55.7 55.7 58.6 58.5 56.0 55.9 55.8 55.8

300 54.2 54.1 57.0 57.0 56.9 54.4 54.3 54.3

350 52.8 52.8 52.7 55.7 55.6 55.6 53.0 53.0

400 51.7 51.6 51.6 54.5 54.5 54.4 54.4 51.9450 50.6 50.6 50.6 53.5 53.5 53.4 53.4 50.9

500 49.7 49.7 49.7 49.6 52.6 52.5 52.5 50.0

550 48.9 48.9 48.8 48.8 51.8 51.7 51.7 51.7

600 48.1 48.1 48.1 48.1 51.0 51.0 51.0 50.9

650 47.4 47.4 47.4 47.4 50.3 50.3 50.3 50.3

700 46.8 46.8 46.7 46.7 46.7 49.7 49.7 49.6

750 46.2 46.2 46.2 46.1 46.1 49.1 49.1 49.0

800 45.6 45.6 45.6 45.6 45.6 45.5 48.5 48.5

850 45.1 45.1 45.1 45.1 45.0 45.0 48.0 48.0

900 44.6 44.6 44.6 44.6 44.5 44.5 47.5 47.5

950 44.1 44.1 44.1 44.1 44.1 44.1 44.0 47.0

1000 43.7 43.7 43.7 43.6 43.6 43.6 43.6 46.6

1050 43.3 43.3 43.2 43.2 43.2 43.2 43.2 46.2

1100 42.9 42.9 42.8 42.8 42.8 42.8 42.8 42.8

1150 42.5 42.5 42.5 42.4 42.4 42.4 42.4 42.4

1200 42.1 42.1 42.1 42.1 42.1 42.0 42.0 42.0

1250 41.8 41.7 41.7 41.7 41.7 41.7 41.7 41.7

1300 41.4 41.4 41.4 41.4 41.4 41.4 41.3 41.3

1350 41.1 41.1 41.1 41.1 41.0 41.0 41.0 41.0

1400 40.8 40.8 40.8 40.7 40.7 40.7 40.7 40.7

1450 40.5 40.5 40.4 40.4 40.4 40.4 40.4 40.41500 40.2 40.2 40.2 40.1 40.1 40.1 40.1 40.1

Table 4: Quick reference table for Extra Isolation Requirement (Ir) in dB for given Horizontal, Vertical separations andCDMA BTS Tx frequency of 887.61 MHz

HorizontalSeparation Extra Isolation Requirement (Ir) for various vertical separations


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RFS (Celwave) 5505 TX filter characteristics

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 1 2 3 4 5 6 7 8 9 10

Frequency in MHz

A t t e n u a

t i o n

i n d B

Scale in X-Axis: 1 division = 2.5 MHz and the Range of the Filter is from 869 to 894 MHz

Center frequency (Fo) is tunable in steps of 1.5 MHz

Figure 2: RFS (Celwave) 5505 TX GSM Killer Filter characteristics


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Isolation Availabilty Chart

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 10 100 1000 10000

Horizontal Separation (d) in Log Scale

( X p - X e

) i n d B

h = 0 m

h = 2 m

h = 4 m

h = 6 m

h = 8 m

h = 10 m

h = 12 m

h = 14 m

Horizontal Separation (d) is in Meters.

Figure 3: Graphical representation of the relationship between (Xp-Xe) in dB and the Horizontal Separation (d) inmeters, for various values of Vertical Separation (h) in meters.


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9. Conclusions

1. When sharing the site and tower with GSM system, a pass band filter (like the GSM killer filtertuned at 887.61 MHz or below) should be used in addition to keeping at least 5 meters of verticalseparation. In this Co-located case, we are able to clear all the CDMA BTS FAs from theInterference problems.

2. When not sharing the site, one should consider maintaining site separations as discussedbelow, in addition to having a GSM killer filter.2.1 Specifically, for the band edge CDMA BTS Tx FA (887.61 MHz), even with the usage

of a GSM killer filter tuned to the center frequency of 887.61 MHz; we are unable toclear the interference problem with reasonable separations. However this needs to beexplored further with use of special or improved designs on the filter

2.2 It is possible to use the last but one CDMA BTS Tx FA (886.38 MHz) with GSMband interference free, if and only if we ensure above 800 meters of horizontalseparation or less than 50 meters of horizontal along with at least 6 meters of verticalseparation between the CDMA BTS Tx and GSM BTS Rx antennas. In this case theGSM killer filter is tuned to the center frequency of 886.38 MHz.

2.3 All other CDMA BTS Tx FAs (885.15 MHz and below) are GSM band interference

free with the usage of a GSM killer filter tuned to the center frequency of 885.15 MHzor below. The sites chosen are assumed to have at least 2 m vertical separation.

3. Above conclusions are derived, considering worst-case scenarios. However, in reality,additional loss in the interfering paths may be available based on local topography. One thingto be noted however is the effect of interference on the subscribers voice quality. Thesubscribers may not normally feel the impact of this kind of interference. This is partially dueto mobiles ability to adjust its transmission power dynamically and also due to mobilesability to leave the weak received signals or blind spot by changing its location.

process for gsn interference

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