Selective fading, especially at low-to-medium frequencies, can be the limiting factor when determining the maximum link length for point-to-point microwave links. Microwave systems that offer the right countermeasures against selective fading increase the maximum link length for long-haul links. They also help service providers reduce the number of links with expensive space diversity — or 2-antenna — configurations.
Defending against selective fading
As the microwave industry adopts higher modulation schemes, effective countermeasures against selective fading become extremely important. Service providers are already deploying 1024 Quadrature Amplitude Modulation (1024 QAM) systems. And 4096 QAM is on the near horizon. These higher modulation schemes are much more sensitive to selective fading than traditional 128 QAM.
Defending against selective fading is particularly important in long-haul links. With point-to-point radio links, many people think “the higher the transmit power, the longer the radio link.” This is true, but transmit power is only one of the factors that should be considered in long-haul links. There are significant propagation impairments that cannot be mitigated by just increasing the transmit power. Different technologies, such as powerful equalizers, are needed to combat distortion.
Understanding the factors that affect maximum link length and the key countermeasures against selective fading helps service providers evaluate microwave systems.
Factors that affect maximum link length
Rain and snow are the main factors that affect maximum link length. The more intense the rain, the higher the signal attenuation, creating long-haul link deployment challenges. And the longer the link, the higher the attenuation effects of the rain. This type of attenuation is commonly called flat attenuation, or flat fading, because the same degree of fading occurs across the entire frequency. Attenuation due to rain increases at higher frequencies. For example, in a rain rate of 60 mm/h, attenuation is:
- 13 dB/km at 38 GHz
- 0.3 dB/km at 6 GHz
The attenuation is different because the wavelengths are different sizes at high and at low frequencies:
- When the frequency is high, the wavelength is short so the rain attenuates more of the wave energy.
- When the frequency is low, the wavelength is long so the rain attenuates only a portion of the wave energy.
That’s why today we see high-frequency short-haul links in the range of 5 to 20 km while low frequency, long-haul links can reach 150 to 200 km.
Knowing these facts, it seems that microwave link dimensioning is as simple as using the rain intensity and the transmission frequency to calculate the maximum link length. For frequencies higher than 13 GHz, it is that easy. If the transmitted power and the rain attenuation (db/km) are known, it is quite straightforward to calculate the maximum link length.
However, for frequencies that are lower than 13 GHz, other second order phenomena that limit link length must be taken into account. And these phenomena must be considered regardless of the attenuation introduced by rain and the amount of transmitted power.
Multipath reflections cause selective fading
In low-frequency transmissions over long-haul links or across difficult propagation conditions, such as water, the receiver signal is composed of the main signal plus multipath rays. These multipath rays can be reflected by the ground, the water, or the atmosphere. The 3-rays model, also known as the Rummler model, is the universally accepted model to describe the multipath phenomenon in line-of-sight microwave transmission. There are 2 main consequences when multipath rays are reflected:
- The attenuation affects the frequency of the transmitted spectrum in different ways — hence the name selective (Figure 1).
- Mitigation techniques that go beyond increasing the transmitted power or the antenna size are needed to counteract distortion.
Digital equalizers combat distortion
Technologies such as powerful equalizers compensate for and recover the distortion that multipath rays introduce. But not all equalizers are equal in terms of capabilities. It is important to measure the effectiveness of the equalizers and to compare their performance.
The concept of a signature, sometimes called a W-curve, is used to compare equalizer performance. The area of the signature helps to determine the maximum link length. The smaller the area of the signature, the better the performance of the equalizer, and the longer the maximum link length.
The signature shown in Figure 2 indicates that the equalizer can compensate for fading up to 30 dB without errors:
- The x axis shows the system gain including antenna gain.
- The y axis shows the signature depth; the greater the value, the smaller the signature area.
Microwave systems with equalizers that provide a 30 dB signature (in 28 Mhz channel spacing) are the most advanced available today.
System gain plus notch depth determines the maximum link length. If the link system gain is kept constant, an equalizer that improves the signature from 20 dB to 30 dB increases the maximum link length more than 60% from 25 km to 40 km. Figure 3 shows the effect of net system gain and signature notch on maximum link length.
Space diversity has pros and cons
While equalizers help to improve the signature area and increase maximum link length, there are cases where they are insufficient. For example, with long-haul links that are more than 50 km long and are over water, multipath propagation effects cannot be sufficiently reduced by an equalizer.
In these cases, space diversity, which is sometimes called antenna diversity, is a more effective countermeasure against selective fading.
With space diversity, the transmitted signal is simultaneously received at two spatially separated antennas — the main antenna and the diversity antenna. The technique is effective because the correlation between the 2 signals is low if the antennas are well separated. If the signal received by one antenna is of poor quality, the signal received by the other antenna is likely good enough.
However, while space diversity improves link availability, it requires double the number of antennas. More antennas mean higher capital and operational expenditures for service providers.
Again, the power of the equalizer in the microwave system is very important. In many cases, an equalizer that provides a signature of 30 dB lets service providers avoid using space diversity. They can achieve 99.999% availability on long-haul links in the 30 to 40 km range using a single antenna.
Combining is the ideal space diversity method
There are still links where space diversity is the only way to guarantee link availability. For these cases, microwave vendors offer 2 different methods to process the pair of received signals:
- The switching method chooses the better of the 2 signals, ideally at every instant, to minimize the bit error rate.
- The combining method processes the 2 diversity signals, blending their samples with intelligent algorithms.
The digital signal combining method is a more effective approach because it uses the joint contribution from the 2 signals. The switching method could be considered wasteful because it uses only 1 of the 2 signal at any point in time. A space diversity system that uses digital signal combining:
- Contributes up to a 3 dB improvement in the signal-to-noise ratio to counter flat fading.
- Outperforms the switching technique. Even if 1 of the 2 signals suffers from 35 dB of fading, the combined signal is error free. Selecting only 1 of the 2 signals means the system cannot be error-free. As illustrated in Figure 2 above, today’s most advanced equalizers provide a signature depth of 30 dB. This means that 35 dB fading cannot be fully eliminated. As a result, 35 dB of fading on a selected channel in a switched system will create errors.
Evaluate systems carefully
Microwave systems that provide effective countermeasures against selective fading offer an important differentiator. These systems give service providers a deployable 1024 QAM long-haul link; systems that have simply added points to their QAM constellation to reach 1024 QAM do not.
Equalizers are also critical points of comparison when selecting microwave systems, not nice-to-have features. Equalizers can eliminate the need to deploy 2-antenna space diversity systems. Together, equalizers and digital signal combiners improve performance and link length in conditions where space diversity is required. To contact the author or request additional information, please send an e-mail to firstname.lastname@example.org.
-  The Rummler model is named after W.D. Rummler, the researcher who proposed this model in the late 1970s after extensive measurements on a 40-km, 6-GHz link from Atlanta to Palmetto, Georgia (US). Bell Technical Journal 1979 “A new selective fading Model: application to propagation data”, by W.D. Rummler.
-  The signature, or W-curve, concept is described in ETSI EN 302217-2-1.