Chapter 8 Microwave Engineering Design
8.2 Design Example
8.2.1 SDH Microwave Circuit
I. 8.2.1.1 Route Selection
A is an sea island subject to B, the distance between A and B is 58km, crossing over the sea. In a project, a hop of SDH microwave is to be established to facilitate the transmission of A. Considering that B is too far way from A, for the sake of microwave transmission quality, another route is needed rather than A—B.
Following is a map about the location of A and B.
Wansong Hill Ruian
Yandun Hill
Wansong Hill Ruian
Yandun Hill
Figure 8.2 location of A, B and C A：Beilu B：Wansong Hill,Ruian C：Yandun Hill
From the map, you can see the nearest place to A is C, and the distance between A and C is 25km. Microwave transmission quality can be assured in
the route A—C. therefore, route A—C is chosen. The cross section of microwave transmission is type D.
II. 8.2.1.2 Determining the Site Known conditions:
(1) There is a microwave site at C with a microwave tower 40m high. The latitude and longitude of the C are: east longitude is 121°07'31.4'', north latitude is 27°50'23.5'', and the altitude is 220m.
(2) From the military map of 1/50000, you can see that on the line from A to C, 3km away from A, there is a small island with altitude of 60m.
Determining the site:
(1) Select the site based on the map, and requirements for the site are as follows:
¾ Microwave route crosses the small island with the altitude of 60m and 3 km away from B.
¾ Ensure that the reflected wave can be prevented by the island (note that the altitude of this point should be controlled at about 60—80m).
¾ Due consideration should be excised for the microwave to be established in other directions when initially choosing the site.
(2) On-Site Survey:
The objective of on-site survey is to check whether transportation of the site is convenient and whether the place is suitable to establish site.
(3) Electrical Test
The objective of electrical test is to check whether the site meet design requirement, that is, whether the receiving level of electrical test is consistent with the receiving level in theory, and whether the reflected wave is prevented (whether the receiving level fluctuate).
These three steps are can be circulated to verify the site.
(4) Determine the site:
Finally, the microwave site of A is determined to be: east longitude 121°11'59.6'', north latitude 27°37'22.5'', altitude 74 m. Specific site is as follows:
Figure 8.3 Site map
A：Beilu island D：Beilong direction C：Dongtou direction small island with altitude of 60m
Note: initially two sites are selected; see the two five-pointed stars on figure 8.3.
Based on on-site survey and electrical test, the two sites both meet the requirements. Considering that microwave may be established from A to D, from the black five-pointed star to D, microwave needs to cross an island of 120m high but the altitude of the black five-pointed star is only 80m. If choosing the place that the black five-pointed star shows, a tower of 60m needs to be built, which may need large investment. While from the place that the red five-pointed star shows to D, microwave only needs to cross an island of 70m, which does not need high tower and saves the cost. Therefore, the place that the red five-pointed star shows is selected as the microwave site.
III. 8.2.1.3 Determining Antenna Mounting Height Known conditions:
(1) From the military map of 1/50000, find there are two obstacles from A to C.
the first obstacle is 75m above the sea level and 280m away from the microwave site of A. and the second obstacle is 52m above the sea level, 3.75km away from the microwave site of A.
(2) The caliber of microwave antenna is 2m, the frequency band of microwave is 6G (U), and that is, the center frequency is 6.775 GHz.
(3) Microwave antenna mounting height of C is 32/22m.
(4) Site of C: east longitude is 121°07'31.4'', north latitude is 27°50'23.5'', and altitude is 220m. Site of A: east longitude is 121°11'59.6'', north latitude is 27°37'22.5'' and altitude is 25.21km.
Calculation:
(1) Calculation of antenna near zone
1. Requirement for the headroom length of the antenna near zone
Headroom length of antenna near zone:
)
2. Requirement for the headroom height of the antenna near zone.
Headroom height of antenna near zone:
)
1. Based on the previous calculation, the first obstacle is located at the near zone of antenna, and the mounting height should meet near-zone headroom requirement.
Microwave antenna minimum mounting height:
)
According to the antenna near-zone headroom requirement, the antenna mounting height should not be less than 10m.
2. For the second obstacle is located at antenna far-zone, antenna mounting height should meet the clearance standard. That is, when k=4/3,
H ≥ H
0,and when k=2/3, H ≥0.
Antenna mounting height of A should be 10m.
Free space clearance:
0 . 577 6 . 86 ( )
Projection height of the obstacle:
Based on calculation, microwave antenna mounting height meets clearance requirement. For the microwave of this hop is cross-sea microwave, site distance is very long, therefore, space diversity is adopted and diversity spacing is 10m. Antenna heights of the two stations are determined as follows:
Antenna height of C is 32/22m and that of A is 20/10m.
(3) Calculation of Reflection Point
Based on the formula in section 6.2.6, when k=4/3, the distance between the reflection point and A is 7.09 km (antenna height of C is 32m and that of A is 20m).
Projection height of the obstacle:
k=4/3: 0.74( )
For reflected wave clearance calculated is a negative value, the reflected wave is prevented by the small island.
(4) Microwave Route Parameter Calculation List
Table 8.1 C—A Microwave route technical parameter list
No. Site name A C
East longitude 121°11'59.6'' 121°07'31.4'' 1 Position
North latitude 27°37'22.5'' 27°50'23.5''
No. Site name A C
2 County Pingyang Dongtou
3 Altitude (m) 74 220
4 Relative mounting height of the primary antenna (m) 20 32 5 Relative mounting height of the secondary antenna (m) 10 22
6 Altitude of the primary antenna (m) 94 252
7 Site distance (km) 25.21
8 Communication azimuth (direct north) 343°06'49'' 163°04'44'' 9 Minus/elevation angle of the primary antenna 0°16'26'' -0°26'39'' 10 Minus/elevation angle of the secondary antenna 0°17'48'' -0°25'17''
11 Frequency band (GHz) 6.775
12 Reflection point (K=4/3) 7.09
13 Altitude of the obstacle (m) 52.0
14 Position of the obstacle: d1(km) 3.75
15 F1(m) 11.89
Clearance (m) 60.77 16 K=4/3 primary antenna
Relative clearance 5.11 Clearance (m) 59.28 17 K=4/3 diversity antenna
(C) Relative clearance 4.99
Clearance (m) 52.26 18 K=4/3 diversity antenna
(A) Relative clearance 4.40
Clearance (m) 56.03 19 K=2/3 primary antenna
Relative clearance 4.71 Clearance (m) 54.54 20 K=2/3 diversity antenna
(C) Relative clearance 4.59
Clearance (m) 47.52 21 K=2/3 diversity antenna
(A) Relative clearance 4.00
(5) Microwave Profile
Figure 8.4 C—A microwave profile
IV. 8.2.1.4 Calculation of Circuit Index
Interruption rate of this microwave circuit selective fading is calculated by linear amplitude dispersion.
Known conditions:
(1) Microwave working center frequency is 6.775 GHz, site distance is 25.14 km, and the cross section type is type.
(2) Microwave equipment configuration is 1+1, space diversity is adopted and the transmission capacity is STM-1.
(3) Microwave transmitting power is 32 dBm, tributary system loss is 5.5 db and the receiving threshold is -73.5 dBm.
(4) Φ2.0m microwave antenna gain is 40.5 db, and total loss of the feeder line is 4.42 db.
Calculation:
(1) Calculation of free space transmission loss
[ ( ) ] 20 lg [ ( ) ] 92 . 44 20 lg 6 . 775 20 lg 25 . 21 137 . 10 ( )
(2) Free space receiving level
)
(4) Calculation of diversity improvement degree Free space diversity
In actual engineering design, maximum free space diversity improvement degree is 200.
Frequency diversity
19
In actual engineering design, maximum frequency diversity improvement degree is 10.
(5) Calculation of flat fading interruption rate (BER<10-4)
10 5
(6) Calculation of selective fading interruption rate (BER<10-4):
For details, refer to the calculation method described in section 2.3.4.2. For the parameter:
Δ f
is 30.1 MHz, L0 is 20dB, and progress error distribution coefficient K1 is 5.Without space diversity:
4
For improvement degree of frequency diversity selective fading, refer to the improvement coefficient of frequency diversity flat fading. Here, it is 10.
Since SDH microwave equipment has decision feedback equalizer which can improve selective fading, here the improvement degree is 2.
(7) Calculation of total interruption loss (BER<10-4)
Based on new national standard, this microwave circuit is evaluated by the local network circuit index: SESR of each kilometer is distributed as 0.002×5%/500=2×10-7. Microwave circuit of this hop allows SESR to be 5.04×10-6.
¾ SESR of the predicted circuit that does not adopt space diversity and frequency diversity is
4
¾ SESR of the predicted circuit that adopts frequency diversity is
5
¾ SESR of the predicted circuit that adopts space diversity is 4.60×10-6.
¾ SESR of the predicted circuit that adopts space diversity and frequency diversity is
From the predicted result, the microwave circuit can meet design requirements only by adopting space diversity and frequency diversity.
(9) Digital microwave transmission index estimate list
Table 8.2 C—A digital microwave transmission index estimate list
No. Site name A C
1 Frequency (GHz) 6.775
2 Arrangement (1+1)*STM-1
3 Modulation 64QAM
4 Site distance (km) 25.21
5 Cross section type D
6 Antenna polarization H
7 Caliber of the primary antenna (m) 2.0 2.0
8 Caliber of the diversity antenna (m) 2.0 2.0
9 Primary antenna gain (dB) 40.5 40.5
10 Diversity antenna gain (dB) 40.5 40.5
11 Transmitting power (dBm) 32
12 Free space loss (dB) 137.10
19 Possibility of flat fading occurrence 54.415
20 Predicted possibility of flat fading interruption rate (BER<1E-4) 6.14E-05
21 Diversity antenna vertical spacing (m) 10 10
22 Frequency diversity spacing (MHz) 80.00
23 Space diversity improvement degree (Isd) 200 200
24 Frequency diversity improvement degree (Ifd) 10
25 Diversity predicted flat fading interruption rate (BER<1E-4) 3.07E-08
26 Predicted selective fading interruption rate 5.00E-06
27 Diversity predicted selective fading interruption rate 1.82E-07 28 Predicted fading interruption rate (BER<1E-4) 2.13E-07
29 Permitted interruption rate (BER<1E-4) 5.04E-06
Table 8.3 digital microwave selective fading index estimate list
No. Site name A C
1 Frequency (GHz) 6.775
2 Arrangement (1+1)*STM-1
No. Site name A C
3 Modulation 64QAM
4 Site distance (km) 25.21
5 Cross section type D
6 Antenna altitude height (m) 94 242
7 Antenna height difference (m) 148
8 Frequency channel bandwidth (MHz) 30.01
9 Progress difference distribution co-efficient (k1) 5
10 Rayleigh fading possibility (F GHz) 10.88%
11 Rayleigh fading possibility (4 GHz) 6.79%
12 Average value of progress error: lm(cm) 23.01
13 Standard deviation of progress error: r1(cm) 30.04
14 Frequency relative coefficient: rf 0.95
15 Linear amplitude dispersion (BER=10^-3) 20
16 Chromatic fading possibility (PLO) 9.19E-04
17 Frequency diversity improvement coefficient 10
18 Decision feedback equalizer improvement coefficient 2 19 Possibility of linear amplitude dispersion exceeding L0 5.00E-06
20 Space diversity S/D
21 Possibility of linear amplitude dispersion exceeding L0 (with diversity) 3.34E-05
22 Predicted selective fading interruption rate 1.82E-07