ADJACENT AND CO-CHANNEL INTERFERENCE DISTURBANCES FROM A DIGITAL TERRESTRIAL TELEVISION 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 Video Broadcasting (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 transmitters 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 transmitter site could become suitable candidates PAL-G signal is also being broadcasted with a in order to allocate digital services [5][6]. different radiation pattern. 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 telecommunications 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 free 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 television broadcasting. 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 modulation, 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. Bandwidth 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 antenna 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 Noise 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.