Fundamentals of microwave link design
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This document discusses the fundamentals of microwave link design. It covers topics such as the frequency ranges used, types of microwave links based on distance (long haul, medium haul, short haul), components of a microwave link including indoor and outdoor units, antennas, and factors that affect microwave link performance such as multipath fading and rain attenuation. It also provides information on polarization, diversity techniques, link budget calculations, and considerations for deploying microwave links.
Fundamentals of microwave link design
- 1. FUNDAMENTALS OF µ WAVE LINK DESIGN SHIV DUTT RF Lead Engineer Intarvo Technology Pvt Ltd
- 2. Microwaves are electromagnetic radiations in the frequency range 1 GHz to 30 GHz (generally for Telecom).
- 4. Small capacity systems generally employ the frequencies less than 3 GHz while medium and large capacity systems utilize frequencies ranging from 3 to 15 GHz. Frequencies > 15 GHz are essentially used for short-haul transmission.
- 5. Less affected by natural calamities Less prone to accidental damage Links across mountains and rivers are more economically feasible Single point installation and maintenance Single point security They are quickly deployed
- 7. As the frequency increases, the length of the link decreases. Due to the high frequency range (2– 58GHz),the microwave links can be classified into three main categories: (a) Long haul (b) Medium haul (c) Short haul
- 8. The frequency of operation of these links is usually 2–10 GHz. In the best of climatic conditions and frequency of operation, the distance covered by the links could range from 80 km to 45 km. These links are affected by multipath fading.
- 9. The frequency of operation of these links is usually from 11 GHz to 20 GHz. Depending upon the climatic conditions and frequency of operation, the hop length can vary between 40 km and 20 km. These links are also affected by multipath fading and rain fading.
- 10. These links operate in high frequency ranges (23–58 GHz) and thereby cover shorter distances. At lowerfrequency ranges in this band, links are affected by both multipath and rain fading. At higher frequencies,when the hop length is only a few kilometres, the multipath phenomenon does not have a significant effect. However, the impact of rain is quite severe.
- 12. Above figure shows a simple microwave link. Major components that constitute a microwave link are: (a) Indoor unit (b) Outdoor unit (c) Antenna (d) RF low loss cables/Waveguide (e) Microwave tower
- 13. Also called IDU, it usually contains the radio modem; i.e. it acts as a termination point which convert the signal into a radio signal sent across the microwave link, using modulation schemes. Indoor units are usually located in a protected environment, i.e. within cabinets inside a building or similar structure. They are not exposed to the environmental conditions like the outdoor units.
- 15. It converts the modulated low frequency signal into a high frequency radio signal. The OU contains radio frequency transmitters and receivers. Due this feature, it is also known as a radio transceiver. The received signal usually passes the low noise amplifier (LNA), which strengthens the weak signals. This is followed by automatic gain control (AGC), which ensures equality of the signal strength when entering the radio receiver. The OU gets electrical power and the low frequency modulated signal from the IU through coaxial cable/ waveguides connections
- 16. An antenna interacts with free space; hence its understanding becomes critical for the microwave link planning engineers. An antenna is defined as the structure that transfers the electromagnetic energy from the free space into transmission lines and vice versa. There are many types of antennas: horn, parabolic, flat or planar, lens, yagi, array, etc. The application of the antenna depends upon its electrical and mechanical characteristics.
- 17. RF low loss cables are used for 6/7GHz link. Cables are limited to pass the high frequency because its lumped component L and C form LPF circuit. Waveguide are used for 15/18GHz link. It has the capability to pass the high frequency because its depends on the dimension of waveguide.
- 18. Earth’s atmosphere plays a vital role in the propagation of microwaves. the atmosphere contains several minute layers, each having its own pressure, temperature and water vapour content. Refractive index of a particular media can be defined as
- 19. As pressure, temperature and humidity all change with height, the refractive index also changes with height, as refractivity varies with pressure, temperature and humidity. This also means that the radio refractivity N also changes with height: N ∝ η (3.12) η= 1.000 300
- 21. We can define K Factor using above information K-Factor = R / R` where R = Radius of ray beam curvature R` = Radius of earth K=4/3 for earth's atmosphere. Fig. shows value of K according to path traveled by MW wave.
- 22. Fig: K-Factor in MW Link
- 23. • Vertical Polarization is less affected by rain fading. Rain droplets are generally flattened with increase in size and thus Vertical polarization is less affected. • However, horizontal polarization is very much used to avoid interference but they are more prone to rain fading, in case nearby areas are using Vertical Polarization.
- 24. So, vertical polarization is generally used for high frequency links, because high frequencies are less affected by rain fading and horizontal polarization is generally used to avoid interference. However, this cannot be treated as rule. Each operator is free to decide.
- 25. Diversity in MW Links is a sort of redundancy in network. They also help overcome various factors which affect MW links. In regions where multipath fading conditions exist, it is necessary to incorporate diversity into the system design. Two types of Diversity in MW links 1. Frequency Diversity 2. Space Diversity
- 26. • Frequency Diversity calls for use of two different frequencies for same MW link. This is normally avoided because two frequency allocation means double the annual fee payable for frequency. Frequency diversity is generally meant to overcome frequency interferences and various other factors.
- 27. • Space Diversity uses two MW antennas at each side and is best suited to overcome Reflection of MW waves. Signal is received by both antennas called Main Antenna and Diversity Antenna. This diversity also helps a lot in areas of high wind because if one antenna gets misaligned, network can function without fail from another. Note:- Neither space diversity nor frequency diversity provides any improvement or protection against rain attenuation.
- 28. • From the figure above we can see that apart from direct line of sight (LOS) we need to leave some space above and below it to allow deviation of MW wave from its original path. Fresnel zone is nothing but distance below and above a point which should be clear for LOS communication. Fig :MW Communication
- 30. where rn = radius of fresnel zone. Generally we consider n=1 i.e. first fresnel zone clearance. d1 = distance of point from Point A d2 = distance of point from Point B
- 31. ( D=d1 +d2)
- 34. Now we will see link budget of MW link i.e. we will analyze gains and losses and calculate received power at other end. Fig : Link Budget for MW Link
- 35. • From Fig. it can be seen clearly that received power at Point B can be calculated as • RxA = TxA + GA - Lfs - Arain + GB where • TxA = Transmit Power • GA = Gain of Antenna A • Lfs = Free Space Loss • Arain = Attenuation due to rain • GB = Gain of Antenna B
- 36. A microwave link frequently is used to transmit signals in instances in which it would be impractical to run cables. If you need to connect two networks separated by a public road, for example, you might find that regulations restrict you from running cables above or below the road. In such a case, a microwave link is an ideal solution.
- 37. • In areas with lots of rain, use the lowest frequency band to handle such projects. • Microwave hops over or in the vicinity of the large water surfaces and flat land areas can cause severe multipath fading. Reflections may be avoided by selecting sites that are shielded from the reflected rays. • Hot and humid coastal areas