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THE RADIO NOISE EFFECT ON THE COVERAGE AREA OF DRM BROADCAST TRANSMITTER IN DIFFERENT REGIONS
Oleg Varlamov,
Moscow Technical University of Communications and Informatics, senior staff scientist, Ph.D., Russia, Moscow, vov@mtuci.ru
Keywords: DRM, Digital Radio Mondiale, long wave, medium wave, radio noise, coverage area, preferred frequency.
Digital Radio Mondiale (DRM) is universal, openly standardised digital broadcasting system for all broadcasting frequencies, including LW, MW, SW as well as band I, II (FM band) and III. Present article focuses on effect of atmospheric radio noise on the service area in different regions in the day time (i.e. by ground wave) in LW and MW bands.
Values of maximum expected field strength of atmospheric radio noise exceeded for 2% of the time estimations have been done for different frequencies of LW and MW bands (in the bandwidth of 10 kHz) for different geographical locations (Tiksi, Norilsk, Moscow region, Ho Chi Minh City -rural area excluding city).
The obtained results substantiate the purpose to incorporate the atmospheric radio noise in coverage area calculation of DRM transmitter and described the methodology of calculation. The using of "SNR propagation curves" is also proposed. There are shown examples that make it possible to select the preferred frequency for various geographical conditions. So, frequencies preferred for using in DRM are:
- Lower part of the LW band - in northern latitudes;
- Lower and middle parts of the LW band - in middle latitudes;
- Upper part of the LW band - in tropical regions.
The conducted analysis of radio noise effect in different geographical regions allows to calculate more accurately the coverage area and to choose the preferred frequency for maximum coverage. Digital Radio Mondiale (DRM) is universal, openly stan-dardised digital broadcasting system for all broadcasting fre-quencies, including LW, MW, SW as well as band I, II (FM band) and III. A large number of successful trials and regular broadcasting was carried out in LW and MW bands in various countries including the Russian Federation [1]. However, some aspects of the correct calculation of the DRM transmitter service area are not clear for part of the broadcasters up to now. Present article focuses on effect of radio noise on the service area in different regions in the day time (i.e. by ground wave) in LW and MW bands. The conducted analysis of radio noise effect in different geographical regions allows to calculate more accurately the coverage area and to choose the preferred frequency for maximum coverage.
For citation:
Varlamov O.V. The radio noise effect on the coverage area of drm broadcast transmitter in different regions // T-Comm. 2015. No.2. Pp. 90-93.
Introduction
Digital Radio Mondiale (DRM) is universal, openly standardised digital broadcasting system for all broadcasting frequencies, including LW, MW, SW as well as band I, II {FM band) and III. A large number of successful trials and regular broadcasting was carried out in LW and MW bands in various countries including the Russian Federation [1]. However, some aspects of the correct calculation of the DRM transmitter service area are not clear for part of the broadcasters up to now.
Present article focuses on effect of radio noise on the service area in different regions in the day time (i.e. by ground wave) in LW and MW bands. The conducted analysis of radio noise effect in different geographical regions allows to calculate more accurately the coverage area and to choose the preferred frequency for maximum coverage.
Atmospheric radio noise accounting
There are a lot of publications and presentations on the conferences with information purposes (for example, [2]), where service area of DRM transmitter by ground wave is determined on the basis of minimum usable field strength. Instead of a strict treatment of seasonal variations of the field strength and field strength variations from place to place, which must be conducted in accordance with [3], the author used "necessary field strength for DRM reception": minimum usable field strength + 7 dB (statistic variation of field strength) + 3 dB reserve. These minimum usable field strength are defined in [4, Table 3, 4] as a 46 (40) dBMV/m for LW (MW) frequency bands for mode A, 9 (10) kHz, 64QAM with an average code rate of 0.6.
In some geographical areas, this approach is valid.
At the same time in Note I for the specified tables says [4]: "The derivation of the values in Tables 3 to 6 is based on the intrinsic noise level of a digital receiver as given in the last row of the Table in Appendix I to this Annex. However, when the effect of the external noise is greater than that of the receiver intrinsic noise, then the external noise value should replace the corresponding value of the intrinsic noise in Appendix I to this Annex",
This clear procedure for some reason is not taken into account when the minimum usable field strength is calculated for specific geographic service areas. It can be assumed that the main reason is the difficulty of determining the external noise value. The first approach has been described in [5] is to take into account field strength of atmospheric noise. Based on the data [6] there were evaluated upper limit of the field strength of atmospheric noise exceeded for 2% of the time (for 98% DRM decoding) all over the world as a whole. The results obtained (53.5 dBMV/m for 200 kHz and 38.5 dBMV/m for a frequency of I MHz in bandwidth 10 kHz) are substantially higher than the intrinsic noise of the receiver, recalculated to the field strength, which are 30.5 (24,5) dBMV/m for LW (MW) bands [4]. It should again be emphasized that the above values of atmospheric noise - an assessment of the maximum level of noise throughout the world as a whole.
However, it should be noted that at atmospheric noise distribution of the planet is very uneven. Figure I shows an example of the expected median values of atmospheric radio noise Fr (in dB above kT0b at I MHz [6]) in the northern hemisphere for the summer season and a time block " I 6-20
hours" local time [6]. As can be seen from Fig. I, the maximum value of F. is 85-90 dB (in the tropical latitudes), and the minimum is 25-30 dB (Greenland and Eastern Siberia).
Fig. I. Expected median values of atmospheric radio noise Flm (in dB above kTDb at I MHz) in the northern hemisphere for the summer season and a time block " 16-20 hours" local time
Field strength of atmospheric noise can be defined the in specified geographical point. To do this is necessary to determine the corresponding value Flm at I MHz, and recalculate it to the operating frequency on graph given in [6]. Next step is to determine the expected value of the median deviation of medium voltage (for the bandwidth 200 Hz). Then recalculate it into the necessary bandwidth and determine the probability distributions of the amplitudes for a given percentage of time. This procedure must be carried out for 4 seasons with the 6 time blocks (4 hours) in each and selected the maximum value among them.
Thereafter in accordance with [6] the maximum expected atmospheric noise field strength can be determined in a predetermined location. So, for a short (/; << /.) vertical monopole above a perfect ground plane, the vertical component of the r.m.s. field strength is given by:
En = Fa + 20 log fMllz + B - 95.5 (dBMV/m), where: En - field strength in bandwidth b (Hz), E = 10 log b, and fMHi - centre frequency (MHz).
This procedure involves the use of five graphs for each of the 24 seasonally - time blocks and quite cumbersome. Partially alleviate calculation may be done by freely distributed ITU program «NOISBW».
The example carried out by the above described procedure estimates of values of maximum expected field strength of atmospheric noise, exceeded for 2% of the time, for different frequencies of LW and MW bands (in the bandwidth of 10 kHz) for different geographical locations (Tiksi, Norilsk, Moscow region. Ho Chi Minh City - rural area excluding city) are shown in Fig. 2. For comparison in Figure 2 also are given field strength of industrial noise in rural area, determined in accordance with [6], and the values of the intrinsic noise of the receiver, recalculated to the field strength.
As it seen from Fig. 2, the field strength of atmospheric noise in areas Tiksi and Norilsk is significantly lower than the receiver noise and industrial noise in the rural area. In these regions during calculating the coverage area of DRM transmitter via ground wave should be used the minimum usable field strength, as defined in [4].
Atmospheric noise in different regions
X —X
40C SCO ECO 1000 1200 Frequency (kHz)
n . rnr r. city
Fig. 2. Maximum expected field strength of atmospheric noise, exceeded for 2% of the time, in the bandwidth of 10 kHz, for different geographical locations
In the area of Ho Chi Minh City (tropical zone) field strength of atmospheric noise is close to the maximum, and in the MW band exceeds the level of DRM receiver intrinsic noise on the value of I4...23 dB. When calculating the coverage area, the corresponding values must be added to the minimum usable field strength. Otherwise used in [2] empirical "reserve" by the value of 10 dB would obviously insufficient, and the size of the service area will substantially less than predicted.
Indeed, DRM tests in Vietnam, carried out by Asia-Pacific Broadcasting Union at a frequency of 729 kHz [7] showed that DRM reception was possible with a field strength 57 dBMV/m on the border of the service area. This threshold of signal field strength corresponds to the field strength of atmospheric noise: 57 (dBMV/m ) - 16 (dBSNB) = 41 (dBMV/m). It is on 16.5 dB more then noise floor of the receiver and slightly less than the maximum level of atmospheric noise in this region at this frequency (44 dBMV/m. See Fig. 2).
It should be noted that the elevated level of atmospheric noise in this region is taken into account (on a less detailed level) at planning of AM broadcasting too. In particular, in accordance with [8], the minimum field strength that used in zone "B", which includes Vietnam, is on 10 dB higher than in the middle latitudes.
As it can be seen from Fig. I in Europe and Russian Federation the maximum field strength of atmospheric noise is predicted somewhat south of Moscow. For Moscow region at high frequencies of the MW band the field strength of atmospheric noise is slightly lower that receiver noise, at low frequencies of the MW band - somewhat above them (by up to 5 dB), and at low frequencies of the LW band - above them almost on 14 dB. Therefore, when determining the radius of the coverage area in this region, is also required accounting of atmospheric noise.
These examples show the relevance of performing calculations of the field strength of atmospheric noise when determining the coverage area of the DRM transmitter in each geographic region. However it can be assumed that the field strength of atmospheric noise is below than the intrinsic noise of the receiver in most parts of Russian Federation (in
northern and Asian parts, see Fig. I), Northern and Western Europe as well as North Africa.
It should be noted that during the relatively short-term measurements smaller field strengths of atmospheric noise may be observed. In this case, 100% of DRM signal decoding may be provided at lower field strength than determined in accordance with this technique. Nevertheless, the above factors should be considered when planning the coverage area to provide round the clock and year-round reception with the specified quality.
The analysis of the effects of atmospheric noise onto required field strength for DRM reception on the border of the service area and discussed examples of their distribution in different geographic regions - is not yet a complete answer to the question "Where is DRM will have to live well?", but only one of the terms of this answer.
Frequencies preferred for usage
The field strength of the ground wave decreases with increasing distance from the transmitter in accordance with [9] for different frequencies with different speed, are also depends on the conductivity of the soil. In all cases, the lower frequency has a lower attenuation and higher field strength of the wanted signal. The field strength of atmospheric noise also increases with decreasing of frequency. The rates of change for both processes may be different. Therefore it is necessary to analyze the preferred usage of radio frequencies for DRM, To do this, we transform the ground-wave propagation curves, calculated in accordance with [9] using the freely available ITU program «GRWAVE», in SNR curves, taking into account the frequency dependence of the field strength of atmospheric noise. This conversion is more convenient when we are considering the digital broadcasting in general, because the decoding of DRM signal requires not only certain field strength, but also the required SNR.
For geographical regions in which the field strength of atmospheric noise below the intrinsic noise of the receiver, this transformation will be as addition of a constant (defined by receiver noise ) and the result is quite obvious - the lower the frequency, the greater the radius of the service area.
Let's do this procedure on the example of the Moscow region with parameters S = 0,003 S/m, r. = 4. Recalculated into SNR ground-wave propagation curves for different frequencies of LW and MW bands taking into account prepared above distribution of atmospheric noise by frequency for radiated power of I kW are shown in Fig. 3. As can be seen from Fig. 3, at distances up to 15 km the highest SNR value is provided at a frequency of I MHz, then up to 60 km - at a frequency of 500 kHz. These radiuses of service area may be of interest to the organization of local broadcasting. In most practically cases coverage area of LW and MW transmitters is greater than 100-200 km. At distances from 60 to 150 km highest SNR value is provided at a frequency of 300 kHz and at distances over 300 km - at a frequency of 150 kHz.
Practical usage of these curves is similar to the usage of the ITU curves - from SNR values required for decoding (for example 16 dB) is subtracted transmitter power (in dB relative to I kW), The resulting value is deposited on the vertical axis and is held horizontal line to its intersection with the curve of the appropriate frequency, thereby determining
the radius of the service area. Thus, the 40 kW transmitters (16 dBkW, horizontal line at level "0 dB") will have radius of service area 200 km at I MHz, and 490 km at 150 kHz, which increases the service area up to 6 times.
Ground-wave propagation curves taking into account the distribution of atmospheric noise by frequency for Moscow region (6= 0.003 5>m: e = 4»
HX1
Distance {km)
I- 15fl I-H7 ?GG .H, 3nn -50t> U^ - 1MH/ - 1 MH/
Fig. 3. Recalculated into SNR ground-wave propagation curves taking into account distribution of atmospheric noise by frequency for radiated power of I kW. Moscow region (fl= 0,003 S/m; e= 4)
Calculation examples of radiuses of coverage areas for different geographical regions, taking into account the effect of radio noise are shown in Fig, 4. Examples are given for the transmitter power of 40 kW and the required SNR = 16 dB. They include the point in northern latitudes (Tiksi, with low noise and low ground conductivity 8 = I mS/m), the point in the middle latitudes (Moscow region, average noise level, and 8 = 3 mS/m), and the points in tropical regions with a high noise level and S = 10 mS/m (Ho Chi Minh City and Yola (Nigeria)).
Coverage areas radiuses for different regions
600
400 300 200 100
400 600 800 1000 Frequency (kHz)
1200 1400
rias.Tiksi 1 mSfrn —Tropical, He Chi Minh Cil* ID m3'~
Y'kril" lat ¡tudas Mosncw re^on. !i m f. i m
Tropical, rola (Nlflwla), 10 mSJrn
Fig. 4. Coverage areas radiuses for different geographical regions with taking into account radio noise effect. Transmitter power 40 kW, required SNR = 16 dB
As can be seen from Fig. 4, in the areas with middle and low noise, despite the increase in the field strength of atmospheric noise with frequency decreases, the use of lower frequencies provides a substantially larger radius of the service area.
In tropical regions with high levels of noise, curve has an extremum at the upper part of the LW band.
So, frequencies preferred for using in DRM are:
- Lower part of the LW band - In northern latitudes;
- Lower and middle parts of the LW band - in middle latitudes;
- Upper part of the LW band - in tropical regions.
Thus, LW band, allowed to be used for broadcasting in ITU Region I (Europe, Africa, Russia, etc.) can be used successfully for DRM broadcasting for large areas. This may be particularly advantageous in broadcast area with a low density of population. Problems with LW antennas matching for using in DRM mode also may be successfully solved [ 10-12].
It should be noted that in addition to the above, it is necessary to take into account the discussed above seasonal variations of the field strength of the desired signal for the given climatic conditions and the nature of the vegetation, as well as variations in field strength from place to place (in MW band) in accordance with [3]. However, this procedure may be performed more strictly without adding empirical "reserve".
Conclusions
The obtained results substantiate the purpose to incorporate the atmospheric radio noise in coverage area calculation of DRM transmitter and described the methodology of calculation. The using of "SNR propagation curves" is also proposed. There are shown examples that make it possible to select the preferred frequency for various geographical conditions.
References
1. ITU-R Doc. 6A/228-E. Measurements of DRM coverage area in the mediumfrequency band in the day-time, night-time and in the fading zone. Russian Federation, 2013.
2. Get the most with DRM! 11 September 20I0, IBC, Amsterdam, Transradio Presentation on behalf of the DRM Consortium. http://Ywvw.drm.org/wp-content/uploads/2010/09/DRM_Transradio_ for_IBC_final.pdf Date of access 05.03.20\4.
3. Rec. ITU-R P. 1321-4 (2013-09). Propagation factors affecting systems using digital modulation techniques at LF and MF.
4. Rec. ITU-R BS.I6I5-I (20I I-05). "Planning parameters" for digital sound broadcasting at frequencies below 30 MHz.
5. Varlamov O.V. 2013, 'Peculiarity of frequency-territorial planning of DRM broadcasting networks for LW and MW bands', T-Comm, no. 9, pp. 43-46.
6. Rec. ITU-R P.372-I I (2013-09). Radio noise.
7. ITU-R Doc. WP6E 390. DRM medium wave reception tests in Vietnam. ABU, 2006.
8. Final Acts of the Regional Administrative LF/MF Broadcasting Conference (Regions I and 3) Geneva, 1975.
9. Rec. ITU-R. P. 368-9 (2007-02). Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.
10. Varlamov O.V., Goreglyad V.D. 201 3, 'Bandwidth extension LW transmitting broadcasting antenna systems for operating in DRM mode',T-Comm, no. I, pp. 18-22.
11. Varlam ov O.V. 2013, 'Development of algorithm and software tools for antenna matching circuit design of DRM digital broadcast transmitters', T-Comm, no. 2, pp. 47-50.
12. Gajnutdinov T.A., Garankina N.I., Kocherzhevskij V.G., Gusevd AS. 2014, 'Simple broadband matching devices of long-wave broadcasting antennas', T-Comm, no. II, pp. 33-39.
T-Comm #2-2015
