> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 DRM (Digital Radio Mondiale) local coverage tests using the 26 MHz broadcasting band Jose María Matías, Iker Losada, Pablo Angueira, Member, IEEE, Unai Gil, Juan Luís Ordiales, Member, IEEE, and Amaia Arrinda, Member, IEEE The ability of the ionosphere to refract signals depends, among Abstract—The digital sound broadcasting DRM™ (Digital other factors, on its ionization state. For a given ionization Radio Mondiale™) standard has established worldwide, and the state, the refraction is achieved for frequencies under the MUF number of transmissions on air has increased significantly since (Maximum Usable Frequency). Frequencies over this the approval of the standard. This paper presents the results of the one of the first DRM field trials using the 26 MHz band for threshold will not be refracted back to earth. MUF increases as local broadcasting that were carried out in Mexico. The main the ionization state of the ionosphere increases. objective of these trials was to test the usage of the 26 MHz The 26 MHz band is usually over the MUF, but when the solar frequency broadcasting band to cover a local area in a similar activity is high (which occurs each eleven years), the MUF way to FM broadcasting. This band has been usually used to increases and the transmission is possible. The ionospheric broadcast long distance transmissions mainly under high sun-spot propagation can also be done due to the apparition of the activity. sporadic E layer. This ionospheric layer is not directly related When the 26 MHz band is used for local broadcasting, the tropospheric propagation is the main mechanism instead of the to the sun cycle, and its effect appears more frequently in ionospheric refraction used in HF. At these frequencies the first summer time, being difficult to predict its appearance [5]. For Fresnel ellipsoid is usually obstructed, and multipath at urban these reasons the 26 MHz band has been scarcely used for environments is strong. In this article, practical minimum signal- long distance transmissions, and could be used for local to-noise ratio and field strength levels are calculated for this novel coverage services. However, the ionospheric propagation, due usage of the 26 MHz frequency band. the F and E layers [6] and the sporadic E layer [5] must be Index Terms—Digital radio, digital audio broadcasting, HF taken into account when planning the local use of the 26 MHz radio propagation, urban areas, reception reliability, DRM. band due to the possible interferences that they could produce [7]. The DRM Consortium proposed time ago the possible I. INTRODUCTION alternative use of the 26 MHz broadcasting band [8], and he digital sound broadcasting DRM (Digital Radio hence, a few trials have been done [7], [9]-[11]. The 26 MHz T Mondiale) system is a non-proprietary standard that band has a 430 kHz bandwidth, which could be used for 43 operates in the frequency bands under 30 MHz [1]-[3]. DRM transmissions with 10 kHz bandwidth modes, or 21 The transmissions at those frequencies can propagate by means DRM transmissions using 20 kHz modes. The frequency of groundwave, ionospheric propagation or a combination of planning of this band is coordinated by international both mechanisms [4]. Therefore, the coverage area of a DRM coordination groups, ASBU, ABU-HFC and HFCC, as it is transmission goes from local areas in MF (medium frequency), regulated by Article 12 of Radio Regulations [12]. Each regional areas in MF and LF (low frequency) up to large and country authorities would be in charge of frequency far away areas in HF (high frequency). assignments while the coordinating committees should be The HF bands are usually utilized to broadcast programs to informed. targets far away from the transmitter using ionospheric The tropospheric component is the main propagation propagation. To achieve a successful transmission the mechanism when using the 26 MHz band for local coverage. ionosphere must refract the electromagnetic signal to the earth. The diffraction and the multipath are of great importance. Ionospheric propagation should only be taken into account for interference calculations. Manuscript received April 30, 2006. This paper is a result of the In this article, an extensive and carefully planned field trial collaboration of DRM Consortium, Radio Educacion, Radio Ibero, RIZ Transmitters, DW, TDF and the University of the Basque Country. This work performed during July of 2005 in México D.F. is described. was partially supported by the Spanish Ministry of Education under the MEC The transmissions where performed in the 26 MHz frequency project with code number TIC 2005-0116, by the DRM Consortium and by band with a DRM transmitter and a mean power of 200 Watts. the University of the Basque Country, UPV/EHU. These trials were organized by the DRM Consortium, the The authors are with the Department of Electronics & Telecommunications, Bilbao Engineering College (University of the Basque University of the Basque Country and Radio Educación. Country), Alameda Urkijo s/n. 48013 Bilbao, Spain (e-mail: [email protected]; [email protected]). > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 II. OBJECTIVES TABLE I The ITU provides minimum field strength and signal-to- TECHNICAL CHARACTERISTICS OF THE TRANSMISSION noise ratio levels for different DRM modes and different Transmitter Center: Santa Fe estimated propagation channels [13]. The ITU data suitable for Broadcaster: Radio Educación HF frequencies were obtained considering ionospheric propagation channel models. In consequence, there are not so Coordinates: 99º 15.920' W; 19º 22.071' N far any system threshold values suitable for tropospheric Frequency: 25.620 MHz 2560 m (300 m above the propagation channels. Altitude: The main target of these trials was to test the viability of average height of the city) using the 26 MHz frequency band for local coverage and Mean power: 200 W obtain a first set of planning parameters. This objective Antenna gain: 7 dBi includes the determination of the most suitable DRM Height of mast: 62 m modulation parameters for this propagation channel. Other Height of the antenna: 40 m objectives include: The DRM system has four OFDM modes: A, B, C and D. Obtain the minimum field strength for a correct Theoretical calculi show that the robustness of modes A and B reception for a set of DRM modes. is strong enough to cope with delay spread and Doppler spread Obtain the minimum signal-to-noise ratios for a due to multipath in the coverage area under test. Therefore, correct reception of a set of DRM modes. several combinations of parameters were used with those Obtain the necessary power that would be needed OFDM modes. to cover a city like Mexico City. One of the targets of the trials was to evaluate the 18 and 20 Evaluate the influence of different kind of urban kHz bandwidth DRM transmission modes. Though 20 kHz environments in the reception of the DRM signal. was the preferred signal bandwidth, 18 kHz bandwidth was Test the DRM modes with 18 and 20 kHz used instead, because the acquisition system was optimized for bandwidths. 18 kHz. The results obtained with a bandwidth of 18 kHz can be easily extended to 20 kHz. III. EXPERIMENTAL NETWORK In table II the modulation parameters of each mode, along The transmission centre was located at Santa Fe, in the with the allowed maximum bitrate are summarized. In the southwest outskirts of México D.F.. Santa Fe is a very hilly table, the IDs that will be used to refer to these modes during area 16 km away from the city centre. Santa Fe is 300 meter this document are also presented. Two groups of basic over the mean height of the city. parameters were chosen, one highly protected, mode The mean power of the DRM transmitter was 200 Watts. 18K_B/16/4/0.5, and another less protected one, mode The transmitter could be configured remotely from any 18K_B/64/16/0.6, which allowed higher bitrates. internet enabled PC. As the transmitter was located in the In order to analyze the behavior of other parameters of the outskirts of the city, a directive three element vertical DRM modulation, modes 10K_B/16/4/0.6 and polarization Yagi-Uda antenna was chosen. 18K_A/64/16/0.6 were also chosen. All the tests were This antenna was installed in a tower pointing towards performed using long interleaving because is more appropriate Mexico City center, which was located about 45 degrees for this type of propagation channel [1]. northwest from the transmission centre. The antenna horizontal The 18K_B/16/4/0.5 mode allowed a useful payload bitrate 3 dB beamwidth was 120 degrees and its gain was 7 dBi (data of 21.20 kb/s. This bitrate is high enough to allow the given by the manufacturer). The main lobe of the antenna was transmission of audio with parametric stereo. This coding wide enough to ensure that the gain variation on the scheme is part of the MPEG-4 AAC family and offers an measurement area was negligible. excellent quality for low bitrates [14]. With the high bitrate In table I the main technical characteristics of the modes, i.e. the 18K_B/64/16/0.6 and 18K_A/64/16/0.6, the transmission are summarized. MPEG-4 AAC full stereo mode was used. The 10K_B/16/4/0.6 mode allowed a bitrate of 11.64 kb/s, suitable for CELP coded speech content [1]. As the DRM standard allows up to four audio channels in the DRM signal, the four DRM modes were used with different audio channel configurations.