Urban Indoor Signal and Noise Measurements in the Medium Wave Band

Igor Fernandez, Pablo Angueira, Iratxe Landa, Amaia Arrinda, Juan Luis Ordiales, David De la Vega, Manuel María Vélez

Dept. Electronics and Telecommunications University of the Basque Country

Alda. Urquijo s/n, 48013 Bilbao, SPAIN igor.fernandez@ehu.es pablo.angueira@ehu.es iratxe.landa@ehu.es amaia.arrinda@ehu.es

juanluis.ordiales@ehu.es david.delavega@ehu.es manuel.velez@ehu.es

Abstract— This paper presents a comparison study between Carrier to Noise (C/N) values measured outdoors and indoors in five office buildings of a city in Spain, related to radio broadcasted signals. Measurements were carried out in Bilbao at three frequencies in the Medium Wave Band: 639 kHz, 990 kHz and 1305 kHz. Results show that outdoor mean C/N results are 7.5 dB higher than indoors for 639 kHz and 6.5 dB and 5.9 dB for 990 kHz and 1305 kHz, respectively, with a high spatial variability. These results are mainly due to signal losses when penetrating in buildings, with mean values of 7.5 dB, 8.1 dB and 5.2 dB, respectively. It has also been observed that outdoor noise levels are about 1-3 dB higher than indoors. In some cases noisy spots have been detected where the mean field strength levels are 10 dB higher than outdoors. The measurements provide noise mean field strength levels (both indoors and outdoors) in the range of 65-70 dB�V/m, values higher than the ones suggested in Rec. ITU-R P.372.

I. INTRODUCTION With the deployment of recent digital broadcasting services

in the MW (Medium Wave) such as DRM (Digital Radio Mondiale) [1]-[4], it has become essential to characterize indoor signal distributions in order to carry out proper network planning. In these frequency bands, propagation conditions are very advantageous for certain applications and since new digital services allow transmitting data as well as audio, these frequency bands have acquired a new interest.

The signal and noise behaviour in this band has been studied in the previous years in outdoor tests [5]-[7], but the required information for indoor receiving conditions has not been sufficiently studied yet. A significant number of MW radio listeners, are expected to be using indoor receivers in the following years. In order to plan for these indoor environments, both signal and noise fields have to be studied and their spatial and time variability must be characterized.

This paper presents the first results of an extensive measurement campaign that is being held in Bilbao (Spain). The measurements include Carrier to Noise Ratio (C/N) values inside and outside five office buildings in Bilbao. Results at three different frequencies in the MW band have been obtained. The tests have been based on recording signal and noise levels independently, in order to provide also a description of each component distribution. Accurate space

and time variability is left for future studies, although some preliminary data are presented in this work too.

II. OBJECTIVES The main objective of this work is to compare the indoor

and outdoor C/N values in dense urban environments. Secondary goals are the quantification of outdoor to indoor signal losses and the noise level differences inside and outside the building.

In order to complete the main objective, two parallel studies have been carried out, following an independent methodology to study signal and noise.

III. MEASUREMENTS CAMPAIGN

A. Transmission System The field test has been based on measurements done on

existing AM commercial transmissions in the city of Bilbao. Three different MW broadcasters’ signals have been selected in order to have samples on the low, middle and upper sides of the band: 639 kHz, 990 kHz and 1305 kHz.

The measurements in this paper have been done on analogue AM signals. Nevertheless, the results should be directly applicable to the digital systems (DRM in the case of Europe). The coherence bandwidth in the MW band is higher than 9 kHz (MW channelling in Spain) and so, the frequency response of the MW channel is expected to be flat [8].

B. Measurements System The measurement system was installed on a portable trolley.

A block diagram is presented in Fig. 1:

Fig. 1. Portable measurements system block diagram.

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A calibrated passive antenna ETS Lindgren 3303 was used for measurements. The field strength measurements were done using the R&S ESPI3 meter. This equipment was remotely controlled using software designed for the test. This field meter was used both for signal and noise measurements. The system was previously calibrated and in every case, the measured value was always 10 dB higher than the internal noise value following the procedures described in ITU-R Rec. SM.1753 [9].

The measurement campaign was planned to take place over a wide time period, and the signals under test were commercial AM transmissions. This scenario did not allow a control of the transmission parameters and did not allow for detecting transmitting system changes or transmitter malfunction events that might lead to erroneous conclusions. In order to overcome this fact, a monitoring station at the roof of the University building was installed to monitor continuously the band. This system registered possible transmitted power variations.

C. Methodology The measurements have been carried out in the buildings of

the Bilbao Faculty of Engineering. The campus is located downtown and has five different buildings, each one with its own architectural and construction material features.

The measurements were made in 140 indoor locations and 96 fixed points outside the buildings, so a total number of 236 locations were measured.

The measurement points where planned previously to follow a uniform spatial distribution over each floor of each building. Each building has five floors and data was recorded on every floor. Outdoor measurements were made by leaning out of the windows a non metallic pole to hold the antenna far enough from the building.

The Faculty buildings can be considered as typical office buildings built of concrete, with large corridors, high ceilings and big windows with metallic frames. Two of them (E and D buildings in Fig. 2) have some specific characteristics. E building’s facade is metallic and D building’s facade is made of metallic tinted glass windows.

The measurement methodology consisted of recording four minutes of continuous noise and signal data. The recording time was chosen based on a previous analysis at sample locations where the time evolution of statistical parameters of both and signal remained stable: standard deviation and the difference between median and the value exceeded during 99% of time.

Field meters were configured with 100 Hz resolution bandwidth for AM signal carrier measurements in order to measure only the carrier level and skip any dependency on modulation. Noise measurements were carried out integrating the noise spectral density in 9 kHz.

The monitoring system installed on the roof stored continuously the signal carrier levels of the frequencies under study. The recorded values confirmed that there was no significant change in the transmitted power, so, with respect to the data presented in this paper there was no need to correct any measurement.

The noise measurements at locations inside the Faculty buildings were carried out in three free channels of the MW band. The selected frequencies (not employed in any other national AM or DRM emissions and the closest ones to the other selected three signal frequencies) were: 675 kHz, 1035 kHz and 1242 kHz. It should be noted that the measurements were carried out during daytime so no ionospheric interference was expected.

In order to provide C/N values, the noise measurements were assumed to remain constant over the adjacent channels. For example, the noise level present in 639 kHz was considered to be the same as the one present in 675 kHz (same with the other two frequencies 1035 and 1242 kHz).

Both signal and noise values were recorded using a RMS (Root Mean Square) detector with integration time of 400 ms. This integration period has been used in previous DRM trials and was kept for future comparison purposes [5]-[7].

Fig. 2. Analyzed buildings in Bilbao.

IV. RESULTS The C/N values were calculated using the median C and the

median N associated to each frequency. The authors are aware that this might not be the case at every location, if the variability of C and N would be significant over the time at a certain location. The analysis of time variability around median value is out of the objective of this paper but a preliminary analysis shows that there is not a high variability, as it is shown in Table I, where it can be observed that averaged standard deviations indoors and outdoors are not high. These data suggest that the assumption of independent median calculations for noise and signal are correct.

TABLE I

CARRIER AND NOISE TIME MEAN STANDARD DEVIATIONS (�)

Carrier Noise Frq. (kHz) 639 990 1305 675 1035 1242 �Indoor(dB) 0.5 0.6 0.7 0.6 0.7 0.7 �Outdoor(dB) 0.5 0.6 0.5 0.4 0.6 0.8 The C/N value was calculated for each location as the

difference between the median carrier value and the median noise value in the adjacent channel as follows:

( ) ( ) ( ) ( ) ( ) ( )mVdBNmVdBCdBNC locationmedianlocationmedianlocation /// μμ −= (1)

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Then, as a first indication of the measurement database, the

mean C/N value has been calculated averaging the indoor locations and outdoor locations. It should be noted, that at this stage of the study, the calculations have been made using the mean of dB values, with the aim of evaluating the results, and not to find a physical explanation of the possible statistics that would require calculations with linear samples.

( ) ( ) ( ) ( )�=

=140

1

/140

1/location

locationindoors dBNCdBNC (2)

( ) ( ) ( ) ( )�=

=236

141

/961/

location

locationoutdoors dBNCdBNC (3)

Table II shows that mean C/N measured outdoors is 7.1 dB,

6.5 dB and 5.9 dB higher than indoors at every frequency. It is also observed that a high spatial variability is present.

TABLE II

OUTDOOR AND INDOOR MEAN C/N AND STANDARD DEVIATION

Freq. (kHz) 639 990 1305

C/NOutdoors Mean(dB) 25.8 32.9 16 σ(dB) 10.7 11.5 10.7

C/NIndoors Mean(dB) 18.8 26.4 10.1 σ(dB) 12.8 12.5 11.9

Difference Outdoor – Indoor (dB) 7.1 6.5 5.9

Difference between mean C/N values Outdoors and Indoors

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

639 990 1305

Frequency (kHz)

C/N

Out

door

s - C

/N In

door

s (d

B)

A buildingB buildingC buildingD buildingE building

Fig. 3. Difference between mean C/N values Outdoors and Indoors.

Fig. 3 represents the full dataset mean values (marked as

horizontal lines for each frequency) along with mean values at each building. The first conclusion is the remarkable difference of obtained measurement on different buildings.

On one hand, Table III and Fig. 4 show the behaviour of the signal measured indoors and outdoors.

TABLE III

OUTDOOR AND INDOOR MEAN CARRIER AND STANDARD DEVIATION

Freq. (kHz) 639 990 1305

COutdoors Mean(dB�V/m) 95.5 100.4 80.5 σ(dB) 9.9 11.0 9.4

CIndoors Mean(dB�V/m) 87.9 92.3 75.4 σ(dB) 10.5 10.5 11.2

Difference Outdoor – Indoor (dB) 7.5 8.1 5.2

Difference between mean carrier field strength levels Outdoors and Indoors

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

639 990 1305

Frequency (kHz)

Car

rier O

utdo

ors

- Car

rier I

ndoo

rs (d

B)

A buildingB buildingC buildingD buildingE building

Fig. 4. Difference between mean carrier field strength Outdoors and Indoors.

The horizontal lines on Fig. 4 show the mean value of the

signal received at each frequency and bars provide values for each one of the five buildings where data was recorded. Results suggest that C/N variations are due to signal variations. The only one which changes tendency is the one for building B.

On the other hand, Fig. 5 and Table IV show the difference between mean noise field strength outdoors and indoors. These data show that noise levels outdoors usually are in the range of 1-3 dB higher than indoors. However, in building B, mean noise levels outdoors are 3 dB lower than indoors for 1242 kHz.

TABLE IV

OUTDOOR AND INDOOR MEAN NOISE STRENGTH LEVELS AND STANDARD DEVIATIONS

Freq. (kHz) 639 990 1305

NOutdoors Mean(dB�V/m) 69.6 67.5 64.5 σ(dB) 9.2 9.8 9.8

NIndoors Mean(dB�V/m) 69.2 66.0 65.2 σ(dB) 9.6 9.6 10.1

Difference Outdoor – Indoor (dB) 0.5 1.5 -0.7

Difference between mean noise field strength levels Outdoors and Indoors

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

675 1035 1242

Frequency (kHz)

Noi

se O

utdo

ors

- Noi

se In

door

s (d

B)

A buildingB buildingC buildingD buildingE building

Fig. 5. Difference between mean noise field strength Outdoors and Indoors.

In most cases, the difference between mean C/N outdoors

and indoors is due to the attenuation suffered by signal when penetrating in buildings. The highest differences are obtained in buildings B, C and E and the lowest ones in buildings A and D. These results were foreseeable in the case of building E, since its facade is completely metallic. On the opposite side is building A with the lowest differences, perhaps because it is the narrowest building and it has large corridors and very wide

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windows. Building D also experiences low losses from outdoors to indoors and the main reason could be that its facade is almost completely made by glass, although they are tinted, without concrete parts. Buildings B and C are similar to A, but a bit wider, so mean losses might be expected to be higher.

Observing the mean values of noise field strength in Table IV, noise levels appear to differ significantly from values in Rec. ITU-R P.372. This result was also suggested by previous trials [10]. In this case, mean noise field strength levels outdoors and indoors are in the range of 65-70 dB�V/m.

Finally, in Fig. 6 the floor number dependency of the difference between mean C/N outdoors and indoors is represented. The result is the average of results at the same floor at the five buildings under study. Fig. 7 and Fig. 8 show the similar analysis for signal and noise independently.

Difference between mean C/N Outodoors and Indoors by floors

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

0 1 2 3 4

Floor number

C/N

Out

door

s - C

/N In

door

s (d

B)

639 kHz990 kHz1305 kHz

Fig. 6. Difference between mean C/N outdoors and indoors by floors.

Difference between mean carrier field strength Outodoors and Indoors by floors

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 1 2 3 4

Floor number

Car

rier O

utdo

ors

- Car

rier I

ndoo

rs (d

B)

639 kHz990 kHz1305 kHz

Fig. 7. Difference between mean carrier field strength outdoors and indoors by floors.

Difference between noise field strength Outodoors and Indoors by floors

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

0 1 2 3 4

Floor number

Noi

se O

utdo

ors

- Noi

se In

door

s (d

B)

675 kHz1035 kHz1242 kHz

Fig. 8. Difference between mean noise field strength outdoors and indoors by floors.

The highest difference occurs at ground level, especially for 1305 kHz. The main reason for this especial case is that for this frequency’s adjacent noise channel (1242 kHz), the mean noise indoors is 10 dB higher than outdoors as it is seen in Fig. 9, probably due to data cable installation density in that floor. It could be said that the general tendency in C/N by floors is to have the highest differences from outdoors to indoors at ground level floor, and the lowest ones in floor 1. If signal carrier behaviour is analyzed in Fig. 7, it could be said that there is not a clear tendency.

V. CONCLUSIONS It has been observed that in the urban environments and in

office type buildings where these measurements have been taken, the mean C/N outdoors was 7.5 dB higher than mean C/N indoors at 639 kHz and 6.5 dB and 5.9 dB at 990 kHz and 1305 kHz, respectively. In the other hand, the high values in the standard deviation evidence a high spatial variability with locations. It is also observed that mean noise field strength levels outdoors and indoors are in the range of 65-70 dB�V/m, levels much higher than the ones suggested in Rec. ITU-R P.372. Finally, noise levels outdoors are usually in the range of 1-3 dB higher than indoor levels with some exceptions, where indoor noise levels are up to 10 dB higher than outdoors, perhaps due to the presence of many electric boards and data cables.

ACKNOWLEDGMENT This work has been partially funded by the UPV/EHU and

the Basque Government.

REFERENCES [1] International Telecommunications Union ITU-R Recommendation

BS.1514-1, “System for Digital Sound Broadcasting in the Broadcasting Bands Below 30 MHz”, Oct. 2002.

[2] Digital Radio Mondiale (DRM)-----Part 1: System Specification, International Electrotechnical Commission Std. IEC 62 272-1, Mar. 2003.

[3] Digital Radio Mondiale (DRM); System Specification, European Telecommunications Standards Institute Std. ETSI ES 201 980 v2.2.1, Oct. 2005.

[4] F. Hofmann, C. Hansen, and W. Schafer, “Digital radio mondiale (DRM) digital sound broadcasting in the AM bands,” IEEE Trans. Broadcast., vol. 49, no. 3, pp. 319---328, Sept. 2003.

[5] D. Guerra, G. Prieto, I. Fernandez, J. M. Matías, and P. Angueira, “Medium wave DRM field test results in urban and rural environments,” IEEE Trans. Broadcast., vol. 51, no. 4, pp. 431–438, Dec. 2005.

[6] G. Prieto, M. M. Vélez, P. Angueira, D. Guerra, D. de la Vega, and A. Arrinda, ‘‘Digital Radio Mondiale (DRM). Field trials for minimum C/N requirements,’’ in Proc. of the International Broadcasting Convention, IBC 2005, Amsterdam, The Netherlands, Sept. 8---13, 2005, vol. 1, pp. 43---48.

[7] Universidad del País Vasco, Digital Radio Mondiale (DRM): MW Simulcast Tests in Mexico D.F Tech. Rep. 6E/403-E, Aug. 2006, ITU-R Study Group WP6E.

[8] José maría Hernando Rábanos, Transmisión por Radio, 5th Ed. Editorial Universitaria Ramón Areces 2006.

[9] International Telecommunications Union ITU-R Recommendation SM.1753, “Method for Measurements of Radio Noise”. 2006.

[10] G. Prieto, M. M. Vélez, A. Arrinda, U. Gil, D. Guerra and D. de la Vega, “External Noise Measurements in the Medium Wave Band,” IEEE Trans. Broadcast., vol. 53, no. 2, pp. 553–559, June 2007.

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