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ADJACENT AND CO-CHANNEL INTERFERENCE DISTURBANCES FROM A DIGITAL TERRESTRIAL SIGNAL(COFDM-8K System) ON ANALOGUE PAL SYSTEMS

A. Arrinda ([email protected]), M. Mª Vélez, P. Angueira, D. de la Vega, J. L. Ordiales UNIVERSITY OF THE BASQUE COUNTRY ETSII y IT de Bilbao Alda. Urkijo s/n 48013 BILBAO, Spain

Abstract This paper presents some early results from a measurement campaign in the service area of the digital terrestrial television (COFDM-8K system) experimental network of Madrid (Spain). The purpose of this survey has been to determine the degradation suffered by analogue PAL-G signals in presence of co-channel and adjacent channel digital emissions. The result from this study is a curve that estimates the protection ratios between analogue and digital received power levels.

1. INTRODUCTION 2. TARGETS The introduction of the digital terrestrial This work has focused on the study of television, standardized in Europe by the Digital interference disturbances on an analogue PAL-G (DVB) consortium [1], will be television signal caused by the presence of a gradually carried out in most European countries. digital terrestrial television [2][3] broadcast This progressive development will allow the inside the same coverage area. Some protection consumers to make a gradual access to the new ratio values have been specified by the ITU-R [4]. services, taking into account that they will need to purchase a new IRD. In the same way, The study presented has considered two different broadcasters will have a reasonable time margin types of interference: to be able to update their infrastructures towards the digital terrestrial technologies. . Degradation of a PAL-G signal caused by a COFDM-8K broadcast on the adjacent These facts lead to a simulcast environment frequency channel. during a time period of 8-10 years depending on different countries and estimations, in which . Degradation suffered by a PAL-G signal analogue and digital services will be sharing the interfered by a COFDM-8K broadcast on the same spectrum frequency bands. As an example, same frequency channel (co-channel Spain will be one of the first countries launching interference) digital terrestrial television services by January 2000 and the shut-down of analogue PAL-G These two scenarios will be quite common during services is expected to be completed by the year the co-existence period of digital and analogue 2010. During the transition period, due to the co- services. Channel protection ratios will be existence of both technologies, the minimization necessary to plan digital services without of analogue quality of service degradation caused interfering analogue coverage areas. Up to now, by digital in the same coverage when planning analogue broadcast networks areas will be one of the main factors to take into there have been tight restrictions for the selection account when planning digital services. of frequency channels. It has not been possible to use two adjacent channels in the same coverage area, so these adjacent channels at each From the same site, using an adjacent channel, a site could become suitable candidates PAL-G signal is also being broadcasted with a in order to allocate digital services [5][6]. different . Despite of that difference, the field strength of the analogue 3. EXPERIMENTAL NETWORK signal within the area where measurements have been taken is higher than the minimum values Measurements presented here have been taken recommended by the ITU-R to achieve a good inside the coverage area of the digital terrestrial quality of service [9]. Figure 2 shows the spectra television experimental network located in of both analogue and digital signals from the Madrid (Spain). This network has been designed Navacerrada transmitter captured at one of the for test purposes and allows broadcasting measurement points. according to the European DVB-T standards [1]. This work has been the result of the co-operative There is still another PAL-G signal, within the research and development project VIDITER, measurement area, broadcasted from a developed by the Spanish Government, the transmitter site located at the southern suburbs operator RETEVISION, of Madrid on the same channel as the digital Spanish broadcast and radiofrecuency equipment signal broadcasted from the Navacerrada manufacturers and the University of the Basque transmitter. Country.

Two main sites compose the broadcast infrastructure, one in the middle of Madrid, at the Torrespaña communications tower and the second at the Navacerrada mountain range, 50 Km away from the city at 2256 meters above sea level. The network architecture has been described in detail in [7] and [8].

The interference measurements have been carried out broadcasting DVB-T signals from the Navacerrada transmitter. Four dipole panels with horizontal polarisation and oriented 120 degrees from North direction compose the radiating system installed on this location. The tower height is 20 meters a.g.l. and the gain pattern has been designed to ensure adequate coverage on the north-west urban and suburban areas of Madrid.

Figure 1. Measurement area and studied signals.

fv = 519.25 fs = 524.75

510 MHz 518 MHz 526 MHz CHANNEL 26 CHANNEL 27

Figure 2. Spectrum of the signals emitted from the Navacerrada transmitter. The mentioned set of analogue and digital signals useful to survey co-channel interference with relevant power levels in different points disturbances. Locations where the only relevant within the measurement area has allowed to received signals were the ones broadcasted from study interference caused by digital co-channel the Navacerrada transmitter site, i.e., PAL-G on and adjacent channel signals on PAL-G services. channel 27 and COFDM-8K on channel 26, were Figure 1 shows the geographic location of the included in the second group. Measurements transmitter sites and the measurement area. taken at these points were used to study adjacent channel interference disturbances. Table I shows the main transmission parameters for all the surveyed signals within the All the measurement locations have been selected measurement area. with the restriction of having the Fresnel first ellipsoid of obstacles from the transmitter (or Power transmitters) that was being analyzed at that Transmitter Signal Channel (w) point. Also, the analogue PAL-G received field Navacerrada COFDM-8K 26 250 strength had to be higher than the minimum values recommended by the ITU-R to ensure Navacerrada PAL-G 27 5000 proper signal quality. Madrid PAL-G 26 5000 Two different measurements have been made at Table I. Parameters of the transmitter in the each location. First one consisted on analyzing all area near Madrid. the baseband parameters that characterize the PAL-G image quality. When making this first The DVB-T standards describe two different measurement, the COFDM-8K transmitter was modes of operation (8K and 2K) for digital switched off, so the deviations from ideal values terrestrial . The VIDITER were caused by transmission impairments and project has focused his efforts on the 8K system, propagation path disturbances. The same as this is the choice for future networks in Spain. parameters were captured on the second Other transmission parameters like carrier measurement but this time with the digital , Viterbi coding and guard interval are transmitter switched on. The nature and amount shown on table II [10]. of degradation caused by the presence of the digital signal (on the same channel at some points COFDM 8K System and on adjacent channel at the rest) was analyzed by means of comparing the mentioned two data Number of carriers 6817 sets. 7.61 MHz

Carrier Modulation 64 QAM 5. MEASUREMENT SYSTEM Inner code rate 2/3 Guard interval 1/4 The whole measurement campaign was carried out using a mobile unit specially designed for that Table II. COFDM transmitter operating mode. purpose. It consisted on a van equipped with the following: 4. MEASUREMENT TECHNIQUES . Directive log-periodic with a gain of 7 First step before the measurement campaign was dBi and flat response in the frequency margin to obtain computer aided field strength where digital and analogue channels were distribution simulations over the whole allocated (510-526 MHz). This antenna was measurement area for the three signals situated at the top of a telescopic mast that considered in the interference analysis (PAL-G on allowed taking data at 10 metres above channels 26, 27 and COFDM on channel 26). ground level. These simulations allowed a suitable selection of . Professional PAL-G Signal Demodulator the measurement locations. (Rhode & Schwarz EMFP). . Baseband Video Parameter Analyzer Measurement locations were divided in two (Tektronix VM700A) different groups. The first group was composed by . TV Monitor (SONY BVM-2010P) points where the received signal power was only . Spectrum Analyzer (Hewlett Packard significant for those broadcasted in the channel HP8591A) 26, i.e., COFDM-8K from Navacerrada and PAL- G from the transmitter site at the south of All the measuring equipment was controlled by Madrid. Data obtained from such points was means of data capture and storage software running on a laptop. Figure 3 shows the block The only data analyzed for the COFDM-8K signal diagram of the whole measuring system inside was the received power measured in the 7.6 MHz the mobile unit. bandwidth[11].

In order to obtain the Baseband Unweighted RF Signal to Ratio the luminance bar ANTENNA PAL-G amplitude (which is inserted in one of the test RF Signal Demodulator lines commonly known as Vertical Interval Test Baseband Video lines VIT) is measured and compared with the RF Switch noise rms value of an empty line. The theoretical Video value of this parameter should be infinite as the ideal bar amplitude is 700 mV and the theoretical Analyzer rms value of noise should be zero.

TV Monitor As mentioned before, two different measurements of the Baseband Unweighted Signal to Noise RF Ratio have been obtained, one in presence of the Spectrum digital signal and the other one in absence of it. Analyzer The COFDM-8K signal interference causes CONTROL & DATA ADQUISITION degradation on the analogue baseband signal quality increasing the noise and therefore decreasing the unweighted S/N ratio. The difference between this ratio in presence and Figure 3. Measurement system. absence of the COFDM-8K signal will be referred to as S/N degradation. If this value is high 6. RESULTS AND DISCUSSION enough the interference will produce subjective image quality degradation, i. e., from grade 4 to 3 The digital terrestrial television signal COFDM- in a scale of 1 to 5 levels of image quality. 8K is composed by 6817 carriers equally spaced 1116 Hz from each other. This carrier set occupies Another considered value at each measurement a spectrum range which bandwidth is 7.6 MHz. location is the difference between the received The shape of this signal, due to the closeness of power of the PAL-G signal and the received power the mentioned carriers, is very similar to a flat from the COFDM-8K transmitter. The spectra of response noise spectrum inside that 7.6 MHz both signals have been captured using a bandwidth. Because of that, the expected resolution bandwidth of 100 KHz. The PAL-G interference caused onto an analogue PAL-G received power is (due to the narrowband nature signal broadcasted on the same or adjacent of this signal) approximately equal to the video channel could be an increase of the noise rms carrier received power. value. The digital received power is more complex to Although the highest variation (with and without obtain with a standard spectrum analyzer and the digital COFDM-8K signal presence) was should be measured with a specifically designed 8 expected for the parameters related to the MHz very sharp filter and a baseband S/N ratio, linear and non-linear [12][13]. In this measurement campaign no field parameters were measured during this strength meter was available so the way to obtain campaign. Obtained results have corroborated the received power was made by means of adding former suppositions so the only parameter a correction factor to the measurement done by included in the analysis is the baseband S/N the spectrum analyzer. First, the medium ratio. spectral power density was obtained using a 100 KHz resolution bandwidth filter. The correction The parameters measured corresponding to the factor (k) added to this value was: PAL-G signal at each point were the following: k = 10 log (7600/100) . Baseband Unweighted Signal to Noise Ratio. . Subjective Image Quality. This assumption was possible because the . Received Power. COFDM-8K was approximately flat over the 7.6 MHz bandwidth at the selected measurement locations. The ratio of both measured power levels (analogue signal power to digital signal power) 6.1 Adjacent Channel Interference will be directly related to the amount of S/N degradation. As this ratio increases the S/N On figure 4 the horizontal axis shows the degradation will also increase, until it reaches the difference between the received power on channel zero value. This relationship will provide some 27 (analogue signal) and 26 (digital signal). The reference values for digital terrestrial television vertical axis represents the S/N degradation. As planning in order to avoid disturbances on the power ratio of both signals increases, the S/N existing analogue services [14]. degradation diminishes.

ITU-R grade Image Quality At measurement locations where the power received from the COFDM-8K transmitter was 5 Excellent higher than the one received from the analogue 4 Good one, the S/N degradation appeared to be less than 3 Acceptable 10 dB. The tendency curve shows that higher 2 Poor power differences (around 30 dB) the S/N 1 Bad degradation is unappreciable.

Table III. Image quality scale. As far as the subjective quality of the analogue television signal is concerned, and at most points, The degradation of the PAL-G image quality degradation in image quality is observed when parameters can be appreciated by evaluating the comparing pictures in presence and absence of subjective image quality. The ITU-R establishes the digital signal. However, there have been five different grades for image quality depending locations where no differences were appreciated on the impairments observed on a picture between both situations. displayed by a professional TV monitor. This quality scale is shown in table III. At each of the Table IV shows image subjective qualities at some measurement locations three different persons measurement points compared. The relationship formerly trained to detect common TV image between power ratios and S/N degradation can impairments observed the subjective image also be seen on the same table. quality. The reduced number of observers was imposed by obvious lack of space at the mobile unit.

20

15

Measurements 10

5 degradationS/N

0

-20 -10 0 10 20 30 40 50 60 70

Protection Ratio PAL/DTV (dB)

Figure 4. Relationship between S/N degradation and PAL/COFDM power ratio.

Power S/N ITU-R ITU-R received power is more complex, as there are two Ratio Degradation Quality Quality signals on the same frequency at the same time, PAL (dB) grade grade and the error will also be higher. By now, few COFDM (interfered) data have been obtained but much higher S/N -4,8 dB 12,3 3 3 degradation values are expected. First results -2,8 dB 14,3 3 2 have shown S/N degradations of 15 dB when the -1,3 dB 13,1 4 3 power ratio was 25 dB. -0,8 dB 14,1 4 3 -0,3 dB 6,8 2 2 7. CONCLUSIONS 1,2 dB 5,7 3 2 As a result from the work carried out during the 2,2 dB 7,7 4 3 measurement campaign in the service area of the 9,2 dB 2,9 4 4 VIDITER project experimental network some 14,2 dB 8,9 3 3 curves have been obtained. These curves show the 14,2 dB 6,5 3 3 effects caused by a digital terrestrial television 22,7 dB 7,1 4 3 COFDM-8K on an analogue television PAL-G 39,2 dB 2,2 4 4 signal broadcasted on an adjacent upper channel.

Table IV. Image quality in presence and absence The relationship between the ratio of PAL and of the digital signal. COFDM signal powers and the PAL-G baseband unweighted signal to noise degradation has been Recently, a digital terrestrial television COFDM- obtained as a result of field measurements in a 8K system transmitter has been installed in real broadcast environment. Bilbao (SPAIN) so a few measurements have been made on this transmitter service area to analyze The obtained results are preliminary and more the impact of this emission on adjacent analogue extensive measurement campaigns are needed to PAL-G channels. Data resulting from this survey obtain definitive curves to use when digital have been added to the curve obtained from the terrestrial television planning becomes a reality measurement campaign in Madrid. Data obtained over the whole Europe. in Bilbao match the expected results. Figure 5 shows measurements taken from both networks. Actually, another experimental network is being developed in Bilbao (Spain). The channel assigned 6.2 Co-channel Interference for that emission is the lower adjacent one to one The co-channel interference analysis has been used to broadcast a PAL-G signal. carried out following the same methodology. It should be remarked that calculation of digital

Measurements Madrid

Measurements Bilbao

20

15

10

5 S/N S/N degradation 0

-20 -10 0 10 20 30 40 50 60 70 80

Protection Ratio PAL/DTV (dB)

Figure 5. Relationship between S/N degradation and PAL/COFDM power ratio. (Data from Madrid and Bilbao networks)

A more detailed measurement campaign is now International Television Symposium, Montreux, under design to obtain more data and as a result, 13-17th June 1997. to be able to accurately define the protection ratios to be used in digital terrestrial television [11] ETSI (European Telecommunications planning. Standards Institute] “ Broadcasting (DVB); Measurement guidelines for DVB 8. ACKNOWLEDGEMENTS systems” ETR 290, May 1997.

The authors wish to thank all the partners of the [12] C. Weck, R. Schramm. “RECEIVING DVB-T: VIDITER project and specially RETEVISION for Results of field trials and coverage facilitating this work. considerations” Proceedings of 20th International Television Symposium, Montreux, 13-17th June 9. REFERENCES 1997.

[1] ETSI (European Telecommunications [13] T. de Couasnon et al. “Results of the first Standards Institute) “Digital Video Broadcasting digital terrestrial television broadcasting field- (DVB); Framing structure, channel coding and tests in Germany” IEEE Transactions on modulation for digital Terrestrial television Consumer Electronics, Vol. 39, No. 3, August (DVB-T)” ETS 300 744 March 1997. 1993.

[2] J.H. Stott “Explaining some of the magic of [14] ITU-R. Rec. 500-3, “Method for subjective COFDM” Proceedings of 20th International quality evaluation of TV images” XVI Plenary Television Symposium, Montreux, 13-17th June Session, Dubrovnik, 1986. 1997.

[3] W. Y. Zou, Y. Wu. “COFDM: An Overview” IEEE Transactions on Broadcasting, Vol. 41, No. 1, March 1995.

[4] Rec. ITU-R BT. 1368-1 “Plannig Criteria for Digital Terrestrial Television Services in the VHF/UHF Bands”

[5] A. Vahlin, N. Holte “OFDM for Broadcasting in Presence of Analogue Co-Channel Interference” IEEE Transactions on Broadcasting, Vol. 41, No. 3, September 1995.

[6] S. O’Leary “Digital/Analogue Co-channel Protection Ratio Field Mesaurements” IEEE Transactions on Broadcasting, Vol. 44, No. 4, December 1998.

[7] P. Cañizares, J.L. Torres, J.A. Martínez “VIDITER: Spanish experience on DTT” Proceedings of 20th International Television Symposium, Montreux , 13-17th June 1997.

[8] P. Cañizares, J.L. Torres, S. Mata. “The first Spanish experience on digital terrestrial television broadcasting” IBC (International Broadcasting Convention), IEE No. 428, 1996.

[9] ITU-R BT 417-4 Minimum field strengths for which protection may be sought in planning a television service”. Volume 1997-BT series.

[10] Lis Grete Moller. “COFDM and the choice of parameters for DVB-T” Proceedings of 20th