Chapter 8 Microwave Engineering Design
8.1 Design Method
8.1.1 Overview
Digital microwave relay line engineering design mainly includes three aspects:
¾ Selecting route, determining antenna height, clearance and calculating receiving level
¾ Determining frequency arrangement and polarization
¾ Evaluating the performance of circuit
8.1.2 Route, Site and Antenna Height
I. 8.1.2.1. Route Selection and Technical Requirements Principles of selecting route:
(1) Based on the features of line-of-sight microwave system, the distance between two sites must be within the line of sight.
(2) Avoid water surface and flat wide area, to prevent deep fading caused by strong reflection of the water surface and the ground. If possible, select fluctuated cross sections as the routes, and make full use of the terrains.
(3) If areas with strong reflection cannot be avoided, make antenna of one end highly mounted and the other lowly mounted, and the reflection point can fall on the low end. Note: use obstacles to prevent reflection waves.
(4) To ensure reliable communication, site distance should not be too long. For microwave system with 6–8GH, control it between 30 km and 50 km. Specific distance should be based on actual cross section.
(5) For microwave backbone adopts two-frequency system, try to avoid cross-station interference, and the circuit should be zigzag arranged.
(6) Route selection should be based on the construction scheme of transmission network, and suggestions of construction organization and the existing siting (site position) should be considered.
Principles of selecting site:
(1) The site should be located at the place where the transportation is convenient and reliable power supply is available, rather than the place that is too remote or isolated.
(2) Environment around the site should be safe. You must not locate the site at potential mineral mountain areas, ancient relics and flood-beaten areas.
(3) The site should be located at the place where the soil is homogeneous rather than fault areas, edge of earth slope, ancient watercourse and places with potential land slip and slide. For earthquake-sensitive areas, locate the site at the place favorable against earthquake.
(4) Environment of the site should be quiet. The site should not be located at the place that near industrial factories which emanates harmful gases, dust, smog and other hazardous substances.
(5) Site should meet the requirements of communication security, and firefight.
Methods of selecting routes (1) Indoor line selection
The major task of indoor line selection is to provide route scheme by drawing a map, to guide on-site survey.
¾ On a map (written or electronic) of 1/500000, determine the general layout of the circuit and the possible positions of adding/dropping circuits.
¾ On a military map of 1/50000, from one terminal station to another terminal station, determine the site of teach middle station by carefully calculation and then draw a complete circuit.
¾ List the basic data of each station and each relay section, draw landform profile, calculate the antenna heights of each station and check electric wave propagation, and then determine the tower height.
¾ Select two or more routes, technically and economically compare the routes in an overall view, and then determine the optimal route. For each site, two optional places should be provided, and then engineers can choose during on-site survey.
(2) On-site inspection and survey
In survey, check the transportation, living condition, geological condition, weather condition of the entire line; check whether the actual layout is contradictory to the map, and whether the local construction plan is contradictory to the site plan. Based on these materials, review the indoor line selection which may lay the basis for next design task.
After the survey is done, electrical test is always conducted, but it is not indispensable. According to the previous experience accumulated in similar line design, if new line cannot be estimated, electrical test should be conducted.
Finally, compare the microwave route schemes and make a decision: first check whether the scheme meet technical requirements, second, whether the scheme can assure available living conditions and convenient maintenance. If schemes are up to these two premises, economically compare the schemes in an overall view: total investment, quantity of all the stations on the line, stations that need independent power supply rather than that from city, total tower height, length of the route, length of the cable, and then make the optimal scheme.
II. 8.1.2.2. Determining Antenna Height
Based on the route and sites determined, determine the antenna mounting height by calculating the clearance. If space diversity is adopted, the mounting height of primary antenna and diversity antenna should be determined.
Principles of determining antenna mounting height:
(1) Antenna mounting height should meet the clearance requirement and there should be no obstacles near the antenna (draw the profile and calculate the clearance).
(2) For cross section of C and D types, make sure that he reflection point does not fall on the water surface or areas with larger reflectance or the reflected wave can be prevented by obstacles.
(3) Enlarge the height difference between the receiving and transmitting antennas, which may be helpful to reduce K-shape fading and channel-shape fading.
(4) Height of antennas in microwave relay section should be arranged in a
“high—low—high—low” order.
(5) If conditions are satisfied, microwave antenna should be lowly mounted to save the cost.
8.1.3 Frequency Selection and Polarization Arrangement
Principles of frequency selection and polarization arrangement: make full use of existing frequency resources, and reduce possible RF interference of the system.
There are three solutions for RF channel configuration in actual engineering design. See the following figures.
(a)交替波道配置方案
Figure 8.1 Three solutions of RF channel configuration Alternate channel configuration solution, co-channel band multiplex solution, cross band multiplex solution
In figure 8.1, meanings if symbols are as follows:
XS (MHz): on the same polarization and in the same transmission direction, frequency spacing between center frequencies of adjacent RF channels
YS (MHz): frequency spacing between center frequencies of nearest coming and outgoing RF channels.
ZS (MHz): frequency spacing between the center frequency of the outmost RF channel and frequency band edge. When the upper and lower spacings are different, ZS1 is called lower frequency spacing, and ZS2 is called upper frequency spacing.
DS (MHz): frequency spacing between the center frequencies (such as fn and fn′ ) of each pair of coming and outgoing RF channels.
8.1.4 Circuit Performance Estimate
There are two ways to estimate digital microwave line performance: one is based on unavailability degree, estimate the unavailability degree of the transmission system caused by propagation fading, equipment and power supply fault. Compare the estimate result with defined value to determine whether system design can meet the requirement. For the estimate result of the unavailability degree caused by equipment, power supply fault is relatively large;
it is not used in engineering design. The other is to determine whether interruption rate meet the index requirement based on severe error second (SES) or severe error second ratio (SESR). The interruption rate of the system only needs to consider the influence of electric wave propagation fading.
Following introduces the second method of estimate.
(1) Estimate receiving level based on transmitting power, propagation loss, and antenna gain. Calculate fading margin based on receiving level and receiving threshold level.
(2) Estimate transmission system interruption rate based on fading margin, frequency diversity, space diversity and selective fading margin, and determine whether it matches the index requirement of SES/SESR.
Determining threshold level:
(1) For PDH microwave, bit error performance parameter is established based on BER and the index system of setting second as the basic measurement spacing. Therefore, in engineering design, you only need to predict the possibility of SESR with bit error ratio exceeding 10-3 (that is, calculation of interruption rate described in 2.2.4)
(2) For SDH microwave, the error performance parameter is established based on blocks and block is set as the basic measurement spacing for index system.
The requirement of block is higher than BER; therefore, in engineering design, to predict the possibility of SEBS, you need to predict the possibility of BER exceeding 10-5–10-4 of SESR.
Comparison between Calculation and Target Value:
In actual engineering design, fading caused by gas absorption is not considered and it is considered to be approximately flat fading. Frequency selective fading and the interruption rate caused by interference are independent of each other (that is, coordinate effect coefficient
ξ = 1
). At the same time, considered the difference between actual receiving level and theoretically calculated receiving level, normally, the circuit interruption rate predicted should be 7–10dB less thatthe target value, that is, 5 to 10 times. Specific times should be based on cross section.