An adjacent channel protection ratio of -15 dB, as assumed in this study, is not enough to avoid substantial interference on DVB-T rooftop reception. The potentially affected population is about 18 % in the areas served by channel 60 in Sweden. Assuming a 2 MHz frequency
separation (guard band) between the mobile and the broadcast service and a protection ratio of -30 dB, reduces the potentially affected population significantly, to 2.7 %.
If the mobile network can use vertical polarisation, enabling use of polarisation discrimination and/or the ERP of the mobile network base stations is reduced the potential interference is reduced even further.
It can be argued that an adjacent channel protection ratio of -15 dB, applicable to the use of base station TPC, is to pessimistic when no frequency separation exist between the two services. However, it should be remembered that the ERP of the base stations are average values. In case protection ratios are used applicable to non TPC (about -30dB) the ERP of the base stations would need to be increased further by about 8-15 dB. The potentially affected population would then be the same as previously mentioned.
Generally a protection ratio which is lower than -30 dB is needed to reduce impact of the mobile interference. A frequency separation of 2 MHz between the mobile and the broadcast service seems to be needed to reduce interference. Additionally some of the mitigation techniques discussed in section 11 may be applied in order to further reduce the impact of the interference.
The DVB-T network in Sweden is planned for fixed rooftop reception and provides limited portable coverage. From the results of this study it seems that the interference into portable reception affect larger parts of the coverage area compared to the fixed reception case, while the potentially affected population remains almost the same. However it is not possible to draw the conclusion that portable reception would generally be the most critical case for a DVB-T network which is planned to provide “full” portable reception. Further analysis is needed.
For DVB-T channel 58 and 59 the interference from mobile services seems to be a minor problem. However, before a firm conclusion on this is given, the impact of DVB-T receiver overload would need to be evaluated further.
Generally receiver overload may be a problem within distances of 100-200 meters of a BS. The actual distance depends on the propagation path between the BS and the DVB-T receiving antenna, the antenna positions and antenna patterns.
11 Possible techniques to reduce interference
This section briefly deals with some of the mitigation techniques discussed in [12] and [13], that can be used in order to reduce the interference from the mobile networks. The techniques discussed below cover only the case of interference from the BS (downlink) into DVB-T.
It is difficult to quantify the exact improvement expected by each measure. In a practical situation it would seem realistic to combine several techniques in order to minimize the interference into DVB-T from the new mobile service.
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Site Sharing
If the DVB-T and the mobile network share the same site the interference can be eliminated.
Given the large number of base stations in a mobile network, in particular in urban and built up areas, this does not seem to be realistic, for cost and practical reasons. However, the
calculations also show that the interference can be significantly reduced if the DVB-T transmitter can provide high field strength in the areas where there is interference from the mobile network. This can for example be achieved with one additional DVB-T transmitter positioned in urban areas where channel 60 is used.
Vertically polarized BS antennas
Mobile networks generally used X-pol antennas. This means that no antenna discrimination can be applied for fixed rooftop reception. Use of vertically polarized antenna for the mobile
network in areas where channel 60 is used would significantly reduce the interference into DVB-T rooftop reception, as described in section 9.4.
Reduce output power of BS
Reducing the EIRP of the base station will reduce the interference from the base stations. A power restriction in certain critical areas where DVB-T channel 60 is used is should effectively reduce the interference. This action would also reduce the radius for possible DVB-T receiver overload, when the DVB-T receiver is exposed to high input levels. The impact of use of high EIRP at the BS is studied in section 9.5.
Cut off filters at DVB-T receiver above 790 MHz
This can involve a modification of the DVB-T receiver front end, limiting the frequency range to 470 - 790 MHz. It is however likely that some countries will continue using the sub band for DVB-T, which means that receivers with different input filters need to be provided by the manufactures. Additionally there are already a large number of set top boxes covering the frequency range 470 – 862 MHz, which includes the mobile sub band.
The other alternative would be a separate (passive) cut off filter before the DVB-T front end.
This filter could be installed when needed. This solution is attractive but has the drawback that some sensitivity is lost in the DVB-T receiver. Frequency stability may also need to be
investigated. However such a filter would also reduce impact of receiver overload.
If a 2 MHz guard band is inserted between the broadcast and mobile service it would make the filter implementation easier with respect to manufacturing tolerance and temperature stability.
Insertion loss may also be reduced. Further studies may be needed in this area.
Improved filtering at mobile base stations
While comparing the spectrum mask for UMTS/LTE with the critical spectrum mask for DVB-T it seems clear that there is potential for reduce the out of band interference from the mobile base stations. It is at this stage to early to say what effect this would have upon the protection ratios for adjacent channel interference from mobile services into broadcast. Again further studies are needed.
12 Recommended future studies
This section provides a list of further work recommended
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• When measurements on Protection Ratio between LTE and DVB-T are completed new calculations of interference with these PR is possible
• Use of higher resolution databases in order to be able to locate interference areas more precisely.
• Full evaluation of DVB-T receiver overload for channels 58-60.
• Evaluation of the impact of the mitigations techniques listed in section 12. In particular DVB-T receiver filters and mobile base station output filtering
13 References
[1] Radiation pattern characteristics of UHF Band V television receiving antennas, ITU B/DPI 054, 3 May 2007
[2] Directivity and polarization discrimination of antennas in the reception of television broadcasting, Rec ITU-R BT.419-3
[3] EBU complementary measurements on UMTS uplink interference into DVB-T portable reception, TG4 (07)134
[4] BPN005. Terrestrial Digital Television, Planning and implementation considerations Third issue, 2001, European Broadcasting Union, EBU
[5] NorDig Unified Requirements for Integrated Receiver Decoders for use in cable, satellite and IP-based networks. Version 2.0
[6] RECOMMENDATION ITU-R BT.1368-6* Planning criteria for digital terrestrial television services in the VHF/UHF bands”
[7] Telephone interview with Bo Bergman, Antennservice in Luleå, 2 October 2008.
[8] Protection ratios for DVB systems and sharing with other non broadcast services such as UMTS and WiMAX. ITU- R JTG 5-6 047, EBU, October 2008
[9] ITU-R. Final Acts of the Regional Radio communication Conference for planning of the digital terrestrial broadcasting service in parts of Regions 1 and 3, in the frequency bands 174 – 230 MHz and 470-862 MHz (RRC-06), June 2006
[10] Digital Video Broadcasting (DVB); Implementation guidelines for DVB terrestrial services; Transmission aspects; ETSI TR 101 190 V1.3.1 (2008-10)
[11] CEPT ECC TG4(08) 223 rev 3, Additional measurements on DVB-T protection ratios and overloading thresholds in the presence of a UMTS interfering signal, December 2008
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[12] CEPT Report 22,”Technical Feasibility of Harmonising a Sub-band of Bands IV and V for Fixed/Mobile Applications (including uplinks), minimising the Impact on GE06”, July 2008 [13] CEPT ECC TG4(08) 194. Additional remarks on compatibility between IMT and DVB-T reception, Media Broadcast, September 2008.
[14] SE42(08)093. Utilization of the 800 MHz band (790-862 MHz) band for LTE in sparsely populated areas, Telia Sonera AB, October 2008.
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