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Carrier frequency effects on path loss

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carrier frequency effects on path loss

carrier frequency effects on path loss

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  • 1. Carrier Frequency Effects on Path Loss Mathias Riback, Jonas Medbo, Jan-Erik Berg, Fredrik Harrysson and Henrik Asplund Ericsson Research, Ericsson AB, Sweden Email: {mathias.riback, jonas.medbo, jan-erik.berg, fredrik.harrysson, henrik.asplund}@ericsson.com Abstract- To study the carrier frequency effects on path loss, measurements have been conducted at four discrete frequencies in the range 460-5100 MHz. The transmitter was placed on the roof of a 36 meters tall building and the receive antennas were placed on the roof of a van. Both urban and suburban areas were included in the measurement campaign. The results show that there is a frequency dependency, in addition to the well known free-space dependency 20 log10(f), in most of the areas included in the measurements. In non line of sight conditions, the excess path loss is clearly larger at the higher frequencies than at the lower. A model capturing these effects is presented. I. INTRODUCTION 2MUGTesttrasmitter Test transmitter SHU# GSM900 GSM1800 RF #1I1 SMHU RF #9 5106.75MH. PA RF #14 RF #15 Fig. 1. Schematic overview of the transmitter. Reliable path loss models are essential when planning mobile communication systems. Several path loss models exist that are valid for the frequency bands used today. Since these bands are more or less fully utilized, new bands are considered for future generation mobile systems. Existing net planning tools and models therefore need to be calibrated for the new frequency bands. The question is whether the old models can be used with a simple correction for the free-space frequency dependency 20 log10 (f), or if a more sophisticated model must be used in a real scenario. Several measurements are presented in the literature dealing with the carrier frequency effects on path loss. The results found in the literature are however not consistent. In [1] path loss measurements were performed at 900 and 1800 MHz in different clutter types in Sweden. It was found that in urban areas the frequency dependency was approximately 23 log0(f). In other clutter types such as suburban and semi-open the dependency was found to be approximately 33log10(f). Path loss measurements in the frequency range 450 MHz to 15 GHz for urban areas in Japan are presented in [2]. In this paper it was found that the frequency dependency is approximately 20 log10 (f), which agrees rather well with the results for urban areas in [1]. In [3] path loss measurements at 955 MHz and 1845 MHz were performed in urban areas in Denmark. It was found that the mean difference in path loss between the two frequencies was approximately lOdB which corresponds to a 35log10(f) frequency dependency. This is not consistent with the results from urban areas in [1] and [2]. The same lOdB difference between 900 MHz and 1800 MHz was found in suburban areas in [4]. In [5] the path loss at four frequencies in the range 430-5750 MHz is compared in an urban environment. It is found that the basic transmission loss slope (e.g. how much the path loss increases with distance) increases with frequency. This indicates that a frequency dependency in addition to 20log10(f) exists. 0-7803-9392-9/06/$20.00 (c) 2006 IEEE SplitterCombi-e RF# SplitterCombi- Splitter/Cobi-e R L- RF6 GPIB Fig. 2. Schematic overview of the receiver. Due to this ambiguity in the literature a measurement campaign has been conducted measuring the path loss at four discrete frequencies in the range 460-5100 MHz. II. MEASUREMENTS The path loss was measured at four discrete frequencies (see table I) almost simultaneously. Four parallel transmitter chains were used to transmit CW (continuous-wave) signals. The output power was approximately +28dBm at the highest frequency and the power at the other frequencies was set such that the received power at 1 meter was approximately the same for all frequencies. See figure 1 for a schematic overview of the transmitter setup. At the receiver four antennas were used to receive the different CW signals. These signals were then combined to one signal with wideband combiners and fed to an Agilent E8358A network analyzer via a wideband low noise amplifier (LNA). The LNA had a frequency range of 0.1-6 GHz, 36dB gain and a noise figure of less than 1.3dB over the entire band. The position of the receiver was logged with a GPS unit. See figure 2 for a schematic overview of the receiver setup. The network analyzer used a segmented sweep to measure the received power at the different frequencies. The sweep 2717 PA RF #12 46. RF #13 RF # RF#1 RF 4 RF #4 Att. 0-70dB GPS antenna
  • 2. TABLE I THE FOUR CARRIER FREQUENCIES USED. fl M f820 M f1 MHz f547 460.025 MHZ | 883.200 MHZ 1858.80 MHZ | 5106.75 MHZ TABLE II MEASURED TOTAL ANTENNA GAIN (GCnt = Gt, + Gr,). Gant (fie) Gant (f2 ) Gant (f3C) Gant (f4 ) 3.4dBi 3.7dBi 3.8dBi 3.5dBi consisted of 804 points in total. At each frequency a zero- span sweep (time sweep) of 201 points was performed. The total sweep time was approximately 0.26 seconds and the time between two consecutive sweeps, including data storage, was approximately 0.4 seconds. The sampling frequency within a sweep was approximately 3 kHz and the fast fading could be followed as long as the maximum Doppler frequency was below 1.5 kHz. The time between the last sample of a frequency in one sweep and the first sample of that frequency in the next sweep was 0.75 0.4 = 0.3 seconds. At a normal measurement speed of 15 m/s this corresponds to less than 4.5 meters which means that the slow fading can be followed between the sweeps. The conclusion is that the fast fading was followed within each sweep but not from sweep to sweep which however the slow fading was. A. Antennas and Filters At both the transmitter and the receiver and at all fre- quencies half-wave vertical dipole antennas were used. The advantage of using half-wave dipole antennas is that the elevation antenna pattern will have the same shape at all frequencies. In many other measurement campaigns conducted to investigate the carrier frequency effects on path loss (i.e. [1], [5]) different high gain antennas have been used on different frequencies. This results in different elevation antenna patterns on different frequencies. In this case a measured path loss difference in addition to 20log10(f) can be a true path loss difference but it can as well be a difference in antenna gain at the elevation angle of the incoming rays. The same holds for broadband antennas like practical bicones which will have maximums and minimums in the antenna gain at different elevation angels for different frequencies. By using separate dipoles this uncertainty is removed. In [2] separate dipoles were used but they were places close to each other to make the propagation environment as similar as possible. The problem with that solution is that two antennas placed close to each other have a mutual coupling and the antenna patterns are affected. In our measurements the dipoles were separated at least 1.5 meters at the receiver and 1 meter at the transmitter to reduce the coupling effects. This makes, however, the propagation environment less similar but less coupling is to prefer and by collecting a lot of data over large distances the effects from the difference in propagation environments will average out. At the receiver the four dipoles were isolated using filters before combining the signals. This is to ensure that the measured received power at one frequency is collected by the antenna intended for this frequency. Without the filters the 450 MHz half-wave dipole will, as an example, collect power at 900 MHz since it is a full-wave dipole at that frequency. The transmit antennas were placed 2 meters above the roof of a 36 meter tall building in Kista, outside Stockholm. They were all mounted on a wooden bar approximately 2 meters above the roof. The receive antennas were mounted approximately 1 meter above the roof of a van and the total height above the ground was 2.9 meters. B. Calibration The measurement setup was calibrated by measuring the received power, Pcal, with the transmit cable ends connected directly to the receive cable ends. The antenna gains were also calibrated by measuring the path loss at 1 meter and comparing these measurement results to the theoretical path loss at 1 meter. Both calibration measurements were conducted both before and after the measurement campaign to ensure that it was no drift in the equipment during the measurements. The agreement between the two calibration occasions was very good and mean values from the two calibrations were used in the analysis. See figure 4 (top) for mean measured path loss at 1 meter. Measured total antenna gain is presented in table II and should be compared to the theoretical value for two dipoles which is 4.3dBi. The measured path loss in dB can after the calibration be expressed as Lmeas Pcal -Pmeas + Gant, (1) where Pmeas is the measured power at the receiver in dBm and Gant is the total antenna gain at the receiver and the transmitter. C. Drive routes The measurement campaign was conducted during daytime in the summer with all trees fully leafed. The transmitter was placed on the roof of one of the tallest buildings in a small urban area where most of the buildings are rather tall. The routes covered both the urban area where the transmitter was placed as well as residential and industrial areas in the surroundings. During the drives, the area between the transmitter and the receiver consisted of large open fields, forest areas and residential areas. See figure 3 for maps with all routes marked. III. DATA PRE-PROCESSING During the measurements the maximum speed of the re- ceiver was approximately 25 m/s which corresponds to a Doppler frequency of 425 Hz at the highest frequency. Since the measurement system was able to handle Doppler fre- quencies up to 1.5 kHz, filtering in the Doppler domain was possible. In every sweep the maximum Doppler frequency was 2718
  • 3. Distance [m] Distance [m] Fig. 3. Maps of the measurement area with routes marked in black. The base station is marked with a red cross. Measured Path-Loss at 1 m 40 Theoretical Path-Loss at 1im 430 -5 10 *-i Frequency [MHz] 115W O 135 200 400 600 800 1000 1200 1400 1600 Sample number 0 m --O-- ~~~~~~~~~~~~C(f"fc)I -C, ~~~~~~~~~~~~~~C(f,f') C(f ~1ff1)- c Fig. 6. CDF of C(fi, fj) for the different frequencies. Fig. 4. Top: Theoretical and measured path loss at 1 meter. Bottom: Measured path loss at 5106 MHz, before and after Doppler filtering. 900 100 120 130 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Traveled distance [m] Fig. 5. Example of measured frequency compensated path loss (absolute path loss at f j and 20 log10 (f) dependency removed at all other frequencies). Top: Before smoothening Bottom: After smoothening. determined and the useful part of the signal was filtered out resulting in a SNR gain of 5-10dB. See figure 4 (bottom) for an example of SNR gain. Before the carrier frequency effects on the path loss were analyzed the received power within each sweep was averaged to remove the fast fading. To better visualize the difference in path loss at the different frequencies a running median of length 10 samples was applied to the mean values (one mean value per frequency and sweep). See figure 5 for an example of measured 20 log10 (f)- compensated path loss before and after smoothening. At the highest frequency the received signal power was sometimes close to or below the noise floor. Samples with low signal to noise ratio were removed from the analysis. IV. ANALYSIS When comparing the path loss on the different frequencies the expected free-space difference 20loglo (f) was removed and the difference studied can be expressed as D(fi, fj) = Lmeas (fi) -Lmeas (fj) + 20 logO(to ) (2) 2719 0.90 0.7 m .'. 0.6 vM-0 (-) 0.5 5r, -0 0.4m -0 2 a- 0.3 0.2 q 0.1 LI' 15 20 30 35 40 m 0- -o E o- IL
  • 4. TABLE III EXAMPLE VALUES OF C(fi, fj). Clutter type | C(l 2)| (2 3) | C(3 f C) Typical suburban 30 27 22 Typical urban 24 16 21 All data 30 23 23 where fi and fj are the two carrier frequencies being com- pared and Lmeas(fi) and Lmpns(fj) are the measured path loss on these frequencies in dB. The general trend is that an increase in frequency on average gives an increase in path loss that is larger than the free-space frequency dependency 20 log10 (f). Assume that the measured frequency dependency can be expressed as C(fi, fj) loglo(fj fi). The function C(fi, fj) can then be calculated as C(ft, fj) = 20 D(fi, fj) log (fj)' (3) where D(fi, fj) is the mean value of the difference. Values of C(fi, fj) are presented in table III for one typical suburban and one typical urban route. The result when using all available measurement data is also presented. It is seen from these ex- amples that the frequency dependency is stronger in suburban areas than in urban areas. This trend is consistent with the results found in [1]. Note that dependencies even smaller than 20 log10 (f) was found in urban areas. Cumulative distribution functions (CDFs) of C(fi, fj) from all routes is presented in figure 6. By following how the frequency dependency changes along the routes it is found that the coherence distance for C(fi, fj) is very short. See figure 7 for plots of C(fi, fj) along one route. It is therefore very hard to predict the difference D(fi, fj) in a specific location and to give a physical ex- planation to the difference. Since the difference in addition to 20 log10 (f) does exist, the propagation must include frequency dependent effects such as diffraction and propagation through vegetation. Instead of studying the difference at specific locations the general trends and statistical properties of the difference were further studied. It was found that the difference in path loss is a function of the excess path loss which is defined as Lexcess (f) = Lmeas (f) -Lf (dmeas, ), (4) where Lf (dmeas, f) is the theoretical free-space path loss at distance dmeas (given by the GPS data) and frequency f. The theoretical free-space path loss is given by Lf (d, f) 20 log1o (-) + 20 log1o (d) + 20 log1o (f), (5) where c is the speed of light, d the distance in meters and f the frequency in Hz. The excess loss used in the analysis is the mean excess path loss at all frequencies, Lexcess. In figure 8 is D(fi, fj) (with data from all routes) plotted as a Fig. 7. Example of loglo(f) dependency along one typical urban route. The BS is marked by a red cross. function of Lexcess for the different frequencies. It is seen that D(fi, fj) increases with increased excess path loss. A straight line is fitted to the measurement data in the least- squares sense with the restriction that it should pass through (0, 0). This is intuitive since zero excess path loss should give a 20loglo(f) dependency. This means that only the slope of the line, K, is fitted to the data. A. Model As discussed above the deviation from the 20 log10 (f) dependency is increased with increased excess path loss. In this section a model capturing this behavior is presented. The model converts a known path loss at one frequency to an expected path loss at another frequency. Let the known frequency be fo and the known path loss at this frequency is then L(fo). The expected path loss at frequency f can then be expressed as L (f fo, L(fo)) L(fo) + 20 log1 (f) -K(fo,f)[L(fo)-Lf(fo)], (6) 2720 46OMHz vs. 883MHz Distance [m]
  • 5. f' compared with f' - Fitted Gaussian distribution G=5.4 M_||||i ii _ Histogram of deviation 02 -15 -10 -5 5 10 15 2 f' compared with f'o ~ ~ ~~~~~~~~3 j~*~i Fitted Gaus ian distribution E G=5.7 M ~~1111 Histogram of devito 0.5 ||| 0 0 Z 020 -15 -10 -5 0 5 10 15 20 compared with f' 1 Vit- -_ rli.ri,tiri- itteci (3aussian ciistroDution I M Histogram of deviation 0 5 10 15 20 Deviation from model [dB] Fig. 8. Difference in path loss between ff and the other frequencies in addition to 20loglo(f) as a function of the mean excess path loss at all frequencies. The red lines are fitted to the data in the LS sense and passes through (0,0). 01 0.05 s -0.05 ~~e-0.1 Y-0.15 s, -~~~~~~~~~~ ---o.d_l tednv e mo ng roth ers Q ef i an vled mn g 5r K(utf)es -0.250 '"~~ ~~ ~ us wt han t ro a nroableV -0.3~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ lotdtGeH Fig9kf,fplottedtogetherwith)aMeanovaluesofnKmeasurements. ure 0i s theed oufe dev i one l.3It en wth data hem = t ctan (6 ) inclu de af st al nts ad in thele hgure freq plotteffectso ne hea ve-ren itu ed Fi.9 (l )potdtgteihvalues of K fromthmeasurements.ta andei Is isfou dinthatthe at del ons average sminedases thatlues ofethetconstans apbadcaeproxmtlyGusiantedinstributedI. In Gaussian distributed random variable can be added to (6) to include a statistical spread in the model. V. SUMMARY AND CONCLUSIONS The carrier frequency effects on path loss have been studied and it is found that the path loss on average increases more TABLE IV MODEL PARAMETERS. Fig. 10. Histograms of deviation from the model. than the free space dependency, 20 lglo(f), when the carrier frequency is increased. This additional loss is quite large when going from 450 MHz to 900 MHz which results in a 30loglo(f) dependency on average. When going from 900 MHz to 1800 MHz or from 1800 MHz to 5100 MHz the additional loss is smaller and a 23loglo(0f) dependency is seen on average. In urban areas the deviation seems to be small over the whole frequency range which is consistent with [2]. Further analysis shows that the additional loss also is a function of the excess path loss. A larger excess path loss results in a larger additional loss. A model capturing this behavior is presented. One open issue is whether the model can be extrapolated outside the range of the measurements. Additional measurements are needed to answer this question. Since different results were found in urban and suburban areas the model can be extended to have different parameters for different clutter types. Now the model captures the average effects. More measurement data is however needed to be able to extend the model. It is established that the propagation depends on the carrier frequency but it was not possible to identify the mechanisms. This is due to the fact that the difference in path loss between the different frequencies seems to change very fast which makes it difficult to associate a difference in path loss with a certain propagation phenomena. REFERENCES [1] L. Melin, M. Ronnlund, R. Angbratt, "Radio Wave Propagation - A Comparison Between 900 and 1800 MHz", 43rd IEEE VTC Conference, New Jersey, USA, 1993. [2] Y. Oda, R. Tsuchihashi, K. Tsuenekawa, M. Hata, "Measured Path Loss and Multipath Propagation Characteristics in UHF and Microwave Frequency Bands for Urban Mobile Communications", 53rd IEEE VTC Conference, 2001. [3] P.E. Mogensen, P. Eggers, C. Jensen, J.B. Andersen, "Urban Area Radio Propagation Measurements at 955 and 1845 MHz for Small and Micro Cells", Global Telecommunications Conference (Globecom), pp 1297- 1302, vol. 2, 1991. [4] B. Lindmark, M. Ahlberg, J. Simons, S. Jonsson, D. Karlsson, C. Beckman "Dual Band Base Station Antenna Systems", Nordic Radio Symposium - Broadband Radio Access, pp 69-74, 1998. [5] P. Papazian, "Basic Transmission Loss and Delay Spread Measurements for Frequencies Between 430 and 5750 MHz", IEEE Transactions on Antennas and Propagation, vol. 53, no. 2, Feburary 2005. 2721 ji+KUVLV.0,1iiL 20 30 40 50 60 460MHzvs. 1858MHz 20 30 40 50 60 460MHz vs. 5106MHz 20 30 40 50 60 Excess path-loss [dB] Param. Value Param. Value II Param. Value a T 0.09 ll b | 256 .106 || c 1.8 10 0 -lo -20 -10 0 10 10 6-. 0 -lo -20 -10 0 10 10 0 -lo -20 -10 0 10 -20 -1 5