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What is the real cost of poor quality antennas?

Jim Syme, product line manager, Microwave Systems

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ContentsIntroduction 3

The theory behind compliance 3

The real world 3

Conclusion 5

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IntroductionThere is a lot of pressure today to reduce the costs of their microwave network rollouts. One way some choose to do this is to use low cost antennas without really considering or fully understanding what this can do to total cost of ownership (TCO) due to the implications of poor antenna performance.

In this paper, we will discuss what is the real cost of using a low-cost, low-performance antenna. We will also look at the effect of using a noncompliant antenna, with reference to two earlier case studies, and show how lost traffic (Gb) translates into loss of revenue.

The theory behind complianceThe compliance of a microwave antenna radiation pattern and its associated radiation pattern envelope (RPE) is a critical factor that affects the link’s performance and availability. The noncompliance of the antenna has a direct effect on the link performance and the achieved carrier-to-interference (C/I) ratio. As the level of noncompliance increases, the level of interference in the link increases, decreasing the fade margin and directly affecting the link’s availability. This leads to degradation in the total link performance.

Figure 1 shows the radiation pattern of a class 3-compliant antenna, mapped with the European Telecommunications Standards Institute (ETSI) standard shown as a red line. As you can see, all the radiation pattern is under the ETSI standard, showing absolute compliance.

Figure 1: The radiation pattern of an ETSI Class 3-compliant antenna. The radiation pattern is entirely within the specified boundary.

Figure 2 shows a radiation pattern from a poor quality antenna that breaches the ETSI standard at several angles, showing high levels of noncompliance against the specification published by the manufacturer. This will cause internal and external interference issues, resulting in inefficient radio links, leading to wasted spectrum and hence dramatic cost implications in an interference zone.

Figure 2: The radiation pattern of a non-compliant antenna, showing poor side lobe levels that will lead to interference.

The real worldWe shall now look at this problem in terms of real-world link examples. As shown in figure 3, we have two parallel links that are a distance apart and don’t share any sites. The operating frequency band is 26 GHz, and the antennas used are all 0.3m ETSI Class 3 antennas. Both links are horizontally polarized and operating in the same channel with a bandwidth of 28 MHz. The radio is operating at a modulation scheme of 128 QAM and its throughput is 155 Mb/s.

Figure 3: Two parallel, unconnected microwave links using high-quality, compliant antennas.

The first figure on the left shows 2 parallel links, where A-B is the victim link, while C-D is the interfering link. The angle of interference arrival between them equals 22.9 degrees. The interference case studied here is at site B from site C. Here, the two links are co-polarized and we need to consider the HH RPE of the antenna. The second figure shows the RPE of the HH antenna pattern. This shows that the discrimination at the 22.9 degree angle is 33dB at both sides if the antennas are compliant.

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In figure 4, we can see in the HH RPE figure on the right, at same angle 22.9 degrees, non-compliance has caused a reduction of 10 dB giving a discrimination of only 23dB.

Figure 4: The interference cost of using non-compliant microwave antennas.

The link planner made all the planning considerations based on compliant antenna specifications; but since he actually has noncompliant antennas, the link performance will be severely affected.

Figure 5 shows a summary of the results obtained using Comsearch®’s iQ.linkXG® link planning software. The table compares the compliant and noncompliant antennas. We can see the percentage of degradation, but the question remains: which is giving more interference?

Compliant antenna

Non-compliant antenna

Degradation

Cumulative interference level-site B (dBm) -101.55 -91.59 10%

Threshold degradation site B (dB) 0.7 4.38 525%

Unavailability (sec/year) 106.15 188.21 77%

Traffic lost (Gb/year) 16.4 29.2 78%

Lost revenue *($/year) 328 584 78%

Figure 5: iQ.linkXG planning software reveals the degradation caused by non-compliant antennas.

In the values of cumulative interference, the 10 dB difference between the compliant and non-compliant antennas discussed earlier appears here in the results values. The value of threshold degradation increased dramatically from 0.7 dB to 4.38 dB, which is the main reason for the increase in interference level and a higher probability of the links unavailability. In contrast, the compliant antenna gave better traffic throughput than the non-compliant one which lost a highly significant 78 percent of signal.

This is just one example to illustrate the true cost of degradation. To reveal the actual monetary loss, simply convert these lost traffic volumes to currency. For one link in one year, the traffic lost is equal to 29.2 Gb/year. *Assuming an average cost of $20 per Gb, then this would equate to $584 per year. However, what if we multiply that loss to noncompliance over 100 links over a period of 5 years? Lost revenue climbs to $292,000, and climbs even further when considering more links over longer time spans.

Let’s now look at a second scenario. As shown in figure 6, we have two links that share one single site. The frequency band is 23 GHz, and antennas used are all 0.3m ETSI Class 3 antennas. Link 1 is vertically polarized and link 2 is horizontally polarized. Both links have the same channel frequency with 14 MHz bandwidth. The radio has a modulation scheme of 32 QAM and its throughput is 52 Mb/s.

Figure 6: Two links with a shared site.

The figure on the left of figure 7 shows two links, where A-B is the victim link, while C-D is the interfering link. The angle of interference arrival between them is 50.73 degrees. The interference case studied here is at site B from site C. Here the two links are cross-polarized and we need to consider the VH RPE of the antenna. The second figure shows the RPE of the VH antenna pattern, at angle 50.73 degrees and the discrimination equals 50 dB from both sides if the antenna is compliant.

Figure 7: Two links that share one single site

In the VH RPE on the right hand side of figure 8, non-compliance has caused a reduction of 10 dB giving a discrimination of 40 dB at same angle 50.73 degrees. The noncompliance at AOA has caused a sudden decrease in discrimination, displayed as a sharp rise and fall.

Figure 8: The interference potential of an antenna that is noncompliant at 50.73 degrees.

Figure 9 shows a similar summary to the one explained before for case 1 with the results again obtained by iQ.linkXG software tool from Comsearch. The table compares the compliant and noncompliant antennas.

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Compliant antenna

Non-compliant antenna

Degradation

Cumulative interference level-site B (dBm) -100.3 -90.09 10%

Threshold degradation site B (dB) 1.34 6.8 407%

Unavailability (sec/year) 3,051 6,478 112%

Traffic lost (Gb/year) 159 337 112%

Lost revenue *($/year) 3,180 6,740 112%

Figure 9: iQ.linkXG analysis reveals dramatic losses due to non-compliant antennas.

In the values of cumulative interference, the 10 dB difference between the compliant and noncompliant antennas again appears in the result values. The value of threshold degradation increased dramatically from 1.34 dB to 6.8 dB and is the main reason for the increase in interference level which, in turn, leads to a higher probability of the link unavailability.

The loss is huge at 112 percent and will result in significant loss of revenue. *Again, if we assume the cost of traffic to be $20 per Gb, for one link per one year of traffic, the loss of 337 Gb per year here yields a monetary value of $6,740. If we extrapolate this once again to account for one hundred links over 5 years then the lost revenue would be around $3,370,000.

ConclusionIt is clear that we must focus on what is more important to us in terms of getting the most out of our microwave networks and consider all the aspects of the design before making an antenna selection based on price alone. Using a low-cost, low-quality antenna does not improve TCO; quite the contrary, the modest CapEx saving realized in its purchase price is soon wiped out through lost revenue. Before long, the value curve bends negative and only grows worse with time. Perhaps, even more significantly, customers can and will notice the loss of network quality and reliability—and customers are even harder to replace than non-compliant antennas.

In summary, the best use of limited spectrum and limited budget is to choose a high-quality, compliant antenna solution for a microwave backhaul network.