Chapter 7 Microwave Engineering Design Requirement
7.2 Basic Requirement of Microwave Path and Cross-section Design
7.2.1 Cross Section and Station Distance
(1) Cross Section
Cross section of relay section of digital microwave relay communication system can be classified into four types based on terrains, weather condition, antenna height and electric wave propagation.
Type A:
The cross section consists of mountains, city buildings or combination of the two, without wide valleys and lakes lying between. The equivalent ground reflection coefficient of this cross section is less than 0.5, that is, the decrease of receiving level caused by ground reflection wave is not more than 6 dB. At the same time, if the cross section has no wide valley and lakes lying between and the gas is dry; multi-path fading is unlikely caused by gas non-homogeneous layer. For example, the cross section, in the area where reflection might be caused, with dry weather and consisting of knife-edge mountains with fluctuations more than 20m, and the cross section of hilly areas, where the elevation angle of electrical wave beam is larger than 0.5 and antennas at the two ends are quite different in height, both belong to type A.
Type B
The cross section consists of hilly areas with tiny fluctuations, without wide valleys and lakes lying between. The equivalent ground reflection coefficient of this cross section is less than 0.7, that is, the decrease of receiving level caused by ground reflection wave is less than 10 dB, and this coefficient cannot be neglected. Though there are no wide valleys and lakes lying between and weather is dry, due to tiny fluctuations, probability of multi-path fading caused by gas non-homogeneous layer cannot be neglected. For example, cross section with dry weather consisting of fluctuated hilly areas which may cause reflection, and plains lines where the weather is dry and the heights of antennas are every different, both belong to cross section of type B.
Type C
Cross section of type C consists of flat grounds and water reticulations, and it refers to humid cross section with many flat grounds, water reticulations and equal grounding reflectance not less than 0.7. For cross section of this type, due to the grounding reflected wave, receiving level undergoes a decrease of more than 10 dB. And because there are many water reticulations and the weather is humid, multi-path fading caused by gas non-homogeneous layer is serious. For example, flat ground lines, where the heights of the antennas are not so different and equal grounding reflectance is more than 0.7, belong to cross connection of type C.
Type D
Cross section of type D is cross-water circuits along coastal paths. Fading caused by water reflection and gas non-homogeneous layer is more serious
than cross sections of type A, B and C. it is a very bad cross section seriously affecting electric wave transmission stability.
In conclusion, the feature of cross section is mainly depending on the influence of grounding reflection and gas non-homogeneous layer on the electric wave transmission. The two factors result in multi-path transmission of electric wave beams, cause frequency selective fading of the receiving signal level at the receiving end, and they seriously affect digital microwave especially the digital microwave system with higher capability and multiple level modulations. On one hand, the two factors can give rise to waveform distortion to the digital signals making BER deteriorate. On the other hand, they reduce polarization discrimination and increase the inter-channel interference of the system.
Therefore, to ensure the transmission quality of digital signals, relay sections of cross sections of type A and type B should be preferred and that of type C and type D should be avoided.
In line design, for the bad cross section and relay sections that seriously affect transmission quality, equalization measure and diversity receiving technology should be adopted to overcome the influence of selective fading.
(2) Choosing Site Distance
The site distance of digital microwave relay communication lines should be determined based on parameters of equipment, geographical conditions, weather, antenna height, electric wave transmission and the technology measure that is adopted.
For relay section with longer or shorted site distance, technology measure should be adopted to ensure that the difference between the free space receiving level that the receiver inputs and the nominal value is not more than 3 dB.
7.2.2 Clearance Standard
(1) For each relay section of SDH microwave relay communication line, considering the range of equivalent earth radius coefficient K, there is certain clearance between electric wave direct rays and the lower obstacle. For single obstacles, the clearance of relay section should meet the following requirements. Clearance of relay section of multiple obstacles, the K should be Kmin, and the electric wave diffraction loss introduced by the obstacles should be not more than 10 dB. If K=4/3, when there is no fading, the receiving level should be not less than the requirement of free space receiving level.
(2) For the relay section that adopts space diversity, path clearance of the main antenna should comply with the following regulations: when path clearance of the diversity antenna meets the requirement of K=Kmin, the electric wave diffraction loss introduced by the obstacles should be not more than 15 dB (for single or multiple obstacles).
(3) Besides meeting the following requirement for clearance, for microwave relay section electric wave rays, the clearance of other sides within
λ / 1 . 17 D
2d ≥
should be not less than 3H0, withind ≤ 17 . 1 D
2/ λ
, the clearance value must meet the antenna headroom requirement. (D is the antenna diameter, λ is working length, and H0 is free space clearance)Table 7.1 Standard for SDH Microwave Relay Section Clearance Range K value
clearance Obstacle type
Kmin 4/3 Description
Knife-edge type H ≥0
H ≥ H
0 K: equivalent earth radius coefficientSmooth ground and others
H ≥ 0 H . 5
0 H0 6m− 5≤H 10 6 −
≤H m
(Note)
Kmin: 0.1% of the statistical K value H0: free space clearance
Note: m=1, 2, 3…, is the number of interference lobe. When K=Kmin, m should be the minimum if possible to avoid the antenna working at lobe of high-order when K=4/3.
7.2.3 Antenna Height and Space Diversity Distance
I. Principles of Selecting Antenna Height
(1) The antenna height should meet the requirement of relay section clearance standard and antenna headroom. For specific requirement, see the following figure. (In the figure, D is the radius of the antenna, λ is the working length).
D1m
20° 1 0 D 20
°
1m H
2
λD L = 1 7 . 1
Figure 7.1 Antenna headroom requirements
(2) Ensure that the antenna height can control the reflection point of the electric wave beam from falling onto the water surface and the area with large reflection coefficient. And at the same time, the difference between the altitudes of the transmitting and receiving antennas should be larger if
possible to reduce the influence of the K fading and channel fading.
II. Principles of Selecting Space Diversity Distance
Space diversity receiving is effective to overcome the influence of electric wave fading.
The principles of determining the distance between diversity receiving antennas are:
(1) For the smooth ground path with grounding reflection coefficient not less than 0.5, overcome K fading
(2) For mountains areas and largely fluctuated ground with grounding reflection coefficient not more than 0.5, overcome channel fading.
In digital microwave engineering design, diversity distance can be adjusted in the range of 200λ—300λ, that is, distance between the diversity antennas can be 8 to 12 m.