Long term path attenuation measurement of the 71-76 GHz band in a 70/80 GHz microwave link Jonas Hansryd, Yinggang Li, Jingjing Chen, Per Ligander Ericsson Research, Ericsson AB Fl¨ojelbergsgatan 2A, 431 84 M¨olndal, Sweden
[email protected]
Abstract—The paper presents ten months path attenuation measurements versus hydrometeor events for the 71-76 GHz band of a 1 km GbE 70/80 GHz microwave link. Precipitation and path attenuation agree well with predictions from the ITU-R models. The impact of a finite rain cell across the link is observed.
I. I NTRODUCTION The mobile backhaul network is today one of the most important applications for point-to-point microwave links. With the roll-out of next generation, high bandwidth, mobile data radio access networks (e.g. HSPA+, LTE rel. 8 and LTE rel. 10), the 70/80 GHz band (also known as the E-band) has the potential to become an important complement to the conventional bands used for microwave links (6-38 GHz). However, due to the vulnerability to precipitation, the hop lengths are for the 70/80 GHz in most geographic areas limited to a couple of km’s with telecom grade availability (99.999%). Thus, for network planning, statistics of attenuation due to hydrometeor events (rain, snow, fog) are deeply important. The ITU-R models [1], [2], [3], [4], are well examined and trusted for the conventional microwave bands [5], [6] while there are less empirical data for higher frequency bands [7]. In this work we report results of an ongoing measurement campaign where path attenuation is measured versus different weather conditions for the 71-76 GHz band of an installed 70/80 GHz microwave link with Gbps capacity. The empirical results are compared with predictions from the ITU-R models. II. M EASUREMENT SETUP A 70/80 GHz microwave link developed within Ericsson Research is mounted between the roof of an apartment building (Radio A) and the Ericsson building (Radio B) (see Figure 1 (top)). The distance between the two radios is 1010 m where Radio B’s coordinates are 57.666788,12.017949. A. The microwave link A block diagram of the radio is shown in Figure 1 (bottom) [8]. System parameters are listed in Table I. The radio link supports a full duplex data rate of 1.25 Gbps (GbE) and has an optical GbE interface (1000BASE-LX) with a monitor port for measuring received power. It utilizes a Frequency Division Diplex (FDD) transmission scheme. The transmitter consists of a DBPSK encoder and a 70/80 GHz Tx module generating a RF carrier on either the 71-76 GHz (low) or the 81-86 GHz (high) band depending on which band is used for Tx/Rx.
Ericsson, Mölndal
Radio B
0 101
m
Radio A
Antenna DBPSK encoder
Variable attenuator
70/80 GHz Tx-module
1000BASE-LX transceiver Diplexer DBPSK decoder
Limiting amplifier
70/80 GHz Rx-module
1000 BASE-LX (optical) Baseband Intermediate frequency (IF), 10 GHz
RF Power monitor
RF carrier, 71-76 GHz / 81-86 GHz
Fig. 1. Position of radio test link (top), total hop length is 1010 m. The photo inset shows Radio A mounted at the test site. Bottom figure shows the radio block diagram.
TABLE I S UMMARY OF SYSTEM SETUP PARAMETERS Parameter Transmitter output power Receiver threshold System gain Antenna gain Tx/Rx separation Data rate Modulation scheme Predicted availability
18.6 dBm -58 dBm (BER = 10−12 ) 162 dB 43 dBi 1 km +/- 50 m 1.25 Gbps (GbE) DBPSK 99.9998% (BER < 10−12 )
150 Heavy rain on July 12th and July 18th -10 Mix of rain and snow on Dec 25th and Dec 28th
100
-20
Received power
Heavy snowfall on March 23rd
-30
-40 50
Heavy fog on April 3rd
Rain intensity -50
Rx threshold at BER 10-12
27/01
15/01
03/01
22/12
10/12
28/11
16/11
04/11
23/10
11/10
29/09
17/09
05/09
24/08
12/08
31/07
19/07
25/06
13/06
01/06
Received power in the 71-76 GHz band (blue) versus rain intensity (red)
150 Rain intensity (mm/h)
The maximum output power is 18.6 dBm. On the receiver side a 70/80 GHz Rx module converts the received RF to a 10 GHz intermediate frequency (IF). The RF/IF conversion gain is above 35 dB. A 6 dB directional coupler (Arra P/N 6194-6) tap the 10 GHz IF to a RF power monitor circuit manufactured by Hittite Inc. (HMC-611). The dynamic range of the monitor circuit was measured to 45 dB with an output slope measured to 32.9 mV/dB. A limiting amplifier and a DBPSK decoder circuit detects the signal and converts it to the baseband. The receiver threshold, defined as the minimum received power after the receiver antenna for a BER < 10−12 is -58 dBm. The estimated link availability based on ITU-R models [1], [2], [3], [4], and a BER < 10−12 is 99.9998%, equal to an outage time of 45 seconds per year.
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50
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Fig. 2.
20/05
08/05
26/04
14/04
02/04
21/03
09/03
-60
04:15
04:30
04:45
05:00
05:15
05:30
05:45
04:15
04:30
04:45
05:00
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B. Weather monitoring Weather conditions at Radio B are measured by a visibility meter (AirEye) capable of measuring rain intensity, visibility, snow intensity, rain droplet size and ambient light. Measured parameters are logged with a period of one minute. III. R ESULTS Figure 2 shows rain intensity (red) and received power (blue) at Radio B between March 11th and January 28th. The gaps in the blue line reflect events when the radio link was brought down due to competing measurement campaigns or maintenance. On March 23rd, 2009, a heavy snowfall with intensity higher than 30 mm/h was measured at the site. Here, the received power dropped only a few tenths of a dB. Another two heavy snowfalls were monitored on December 25th and
Fig. 3. Measured rain intensity (top) and path attenuation (bottom) morning July 12th, 2009.
December 28th, these two snow falls were mixed with rain and thus showed higher link attenuation. On April 3rd the visibility was reduced to 160 m due to heavy fog, on this occasion the path attenuation was not affected. The attenuation from both events agreed well with the predicted attenuation from the ITU-R models. The link experienced two very intense rainfall events in July. The heaviest rainfall with a peak rain intensity of 132 mm/h was recorded on the morning of July 12th and a second intense rainfall with a peak intensity of 101 mm/h was recorded on July 18th. The path attenuation, increased on the
30
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25 20 15 10 5 0 20
40 60 80 100 Rain intensity (mm/h)
120
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Normalization scale
1
Fig. 5. Path attenuation versus rain intensity. Solid red line: predicted path attenuation [2]. Pixels: Measured path attenuation versus rain intensity (normalized).
Fig. 4. Measured attenuation versus expected attenuation (top). Error between measured and expected attenuation (red- measurement points with rain, green - all measurement points) (bottom)
first occasion by 31.6 dB causing the received power to fall below the Rx threshold. On the second occasion (July 18th) the path attenuation increased by 29.9 dB. The path attenuation for the measured rain intensities calculated from the ITU-R models [1], [2], [4] assuming a carrier frequency of 72 GHz, V polarization and an effective path length [1] deff = 960 m is 33.6 dB and 27.8 dB respectively. Figure 3 shows the measured rain intensity and path attenuation during the morning of July 12th in closer detail. As shown in the top figure, the peak rain intensity occurred for 1-2 minutes while the peak attenuation lasted for a longer time (3-4 minutes), the reason for this is likely the rain cell moving across the 1 km link. A. Impact of rain cell size The blue dots in Figure 4 (top) shows measured path attenuation between March 11th to January 28th versus expected path attenuation calculated from [4] using the measured rain intensity at Radio B. The red solid line shows the 1:1 correlation between measured and expected attenuation. Figure 4 (bottom) shows the error between measured and expected attenuation. The green area shows the error distribution for all measured points during the campaign (293700 points), while the red area (13961 points) shows the error distribution
when rain was present at Radio B. The width of the green distribution is mainly due to e.g. varying output power caused by temperature variations during a 24h period. The width of the red distribution is caused by a finite rain cell size across the hop. As a result of a finite rain cell, the expected path attenuation for a given rain intensity measured at Radio B will on average be slightly lower compared to a hop where the same rain intensity covers the whole link (an infinite rain cell). This is illustrated in Figure 5 where measured path attenuation is plotted versus rain intensity. Due to the large difference in events for different rain intensities, the number of events for a given rain intensity has been normalized to one (see normalization color grade scale, in bottom of Figure 5). The solid red line shows expected path attenuation versus rain intensity, assuming an infinite rain cell [4]. As shown in the figure, the measured path attenuation follows the predicted attenuation but with slightly lower attenuation and a wide spread. B. Availability Figure 6 (top) shows path attenuation versus time for the measurement period March 11th to January 28th. Blue circles show the actual path attenuation while green dots show attenuation calculated from measured rain intensity [4]. The red solid line shows predicted path attenuation based on the ITU-R models solely [1], [3], [4]. Figure 6 (bottom) shows rain intensity versus normalized time. Green dots shows measured rain intensity at Radio B site and the red solid line shows predicted rain intensity from the ITU-R model [3]. The measured path attenuation is slightly higher than predicted from the ITU models. The ITU-R model (red line) predicts a path attenuation higher than 10 dB for 0.009% of the time. A 10 dB expected path attenuation from measuring
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Fig. 6. Measured path attenuation versus time (top). Measured rain intensity versus time (bottom).
the actual rain intensity occurs 0.02% of the time, while the time the actual measured path attenuation exceeds 10 dB happened 0.05% of the time. The measured data indicates a link availability (path attenuation < 30 dB) of 99.9992% while the availability predicted from the ITU-R models is 99.9998%. The measured rain intensity (Figure 6 (bottom)) is slightly higher than the ITU-R model but follows the predicted precipitation well, a slight variation should be expected over year intervals. IV. S UMMARY We have measured the path attenuation caused by hydrometeor events for the 71-76 GHz band of an installed 1 km 70/80 GHz microwave link. The measured path attenuation follows the predicted attenuation from the ITU-R models with a spread due to an infinite rain cell size. Measured results are with good agreement compared with rain intensity and predicted path attenuation from ITU-R models. R EFERENCES [1] “Propagation data and prediction methods required for the design of terrestrial line-of-sight systems,” International Telecommunication Union/ITU Radiocommunication Sector, Geneva, Switzerland, Tech. Rep. ITU-R:P530-13, Oct. 2009. [2] “Attenuation by atmospheric gases,” International Telecommunication Union/ITU Radiocommunication Sector, Geneva, Switzerland, Tech. Rep. ITU-R:P676-8, Oct. 2009.
[3] “Characteristics of precipitation for propagation modelling,” International Telecommunication Union/ITU Radiocommunication Sector, Geneva, Switzerland, Tech. Rep. ITU-R:P837-5, Jan. 2007. [4] “Specific attenuation model for rain for use in prediction methods,” International Telecommunication Union/ITU Radiocommunication Sector, Geneva, Switzerland, Tech. Rep. ITU-R:P838-3, Jan. 2005. [5] B. Fong, A. Fong, G. Hong, and H. Ryu, “Measurement of attenuation and phase on 26-GHz wide-band point-to-multipoint signals under the influence of rain,” Antennas and Wireless Propagation Letters, IEEE, vol. 4, pp. 20–21, 2005. [6] H. Xu, T. Rappaport, R. Boyle, and J. Schaffner, “38-GHz wide-band point-to-multipoint measurements under different weather conditions,” IEEE Commun. Lett., vol. 4, no. 1, pp. 7–8, Jan. 2000. [7] V. Kvicera, M. Grabner, and O. Fiser, “Frequency and path length scaling of rain attenuation from 38 GHz, 58 GHz and 93 GHz data obtained on terrestrial paths,” in Antennas and Propagation, 2009. EuCAP 2009. 3rd European Conference on, Berlin, Germany, Mar. 2009, pp. 2648–2652. [8] J. Hansryd, Y. Li, J. Chen, and B.-E. Olsson, “A simple DBPSK modem based on high-speed logical gates for a 70/80 GHz GbE microwave link,” in VTC 2010 spring (to be presented), Taipei, Taiwan, May 2010.
