Non-line-of-sight (NLOS) microwave return in small site deployment

Currently, microwave is the main transmission medium for mobile backhaul, but its application is still limited to the line of sight (LOS) condition. Deploying a small station in a city with messy environment requires close and completely non-line-of-sight scenarios.

Applications without line of sight (NLOS) have been confirmed by wireless access technology, but it is still a new challenge for high-performance backhaul. This article will discuss general principles, main system parameters, simple engineering guidance, and by demonstrating the comparison between Ericsson 28 GHz products and products below 6 GHz, I will question the general viewpoint.

I. background

Point-to-Point Microwave is an economical and effective technology that allows you to flexibly and quickly deploy a backhaul network to almost any point. It is the main return method in mobile networks and remains in this position during the evolution of mobile broadband. Microwave Technology is also developing rapidly, and now supports the return capacity of multiple gibits [1].

The introduction of Small and Medium sites in the wireless access network, the implementation of micro-cell-level engineering will face new challenges for the return network. A typical all-outdoor small station is installed on the street decoration or building surface, 3-6 metres from the street height, and 50-metres from the station. Because of the large number of small sites, they require a more cost-effective and easy-to-install backhaul solution. The solution should support more unified user experience throughout the wireless access network [2]. Traditional backhaul technologies such as cost-effective line-of-sight microwave, optical fiber and copper wire are meeting the new requirements of this solution. However, due to the ceiling height of a building, there will still be a large number of small stations that do not have the conditions to connect through a wired connection or with the other side. NLOS is not a new challenge for microwave return. The existing method can overcome the problem of non-line-of-sight transmission. Passive Reflection and trunk station solutions are used in mountainous terrain. However, this solution is not ideal for cost-sensitive small-site access because more sites are added. In the city, it is very difficult to connect to the ideal site in the changing Buildings every day, and the ideal site is the most effective solution for returning small sites. However, there will be a certain number of sites that are difficult to access, so we need a non-line-of-sight microwave return solution, as shown in 1.

Figure 1 example of non-line-of-sight backhaul deployed on a small site

The ultimate goal of planning the return capacity is to support the full capacity of the honeycomb, that is, the peak capacity of the site and the average site capacity [3]. However, in practice, parameters such as costs and deployment of small site types (capacity or coverage) will determine the final target capacity and availability. The carrier will make a cost balance, which makes the return capacity support at least the expected traffic volume during busy hours and meet the capacity requirements for statistical redundancy in future development. Currently, the target capacity for LTE "Hotspot" small stations should be around 50 Mbps, but it can be flexibly increased to 100 Mbps if needed. These figures are expected to increase as the business continues to grow in a few years, and more small sites will be used. The availability metrics of small station return within the macro cell coverage can be relaxed to 99-99.9%, And the availability metrics of those outdoor stations are 99.9-99.99% [3]. This relatively low availability indicator only requires redundancy for rainfall attenuation at short link distance. Consistently, the return performance must be easy to predict and reliable to ensure low cost of ownership.

Most effective licensing policies for point-to-point microwave spectrum are for Link licenses. However, for small-site network deployment, simplicity and license fees will be very important, so we should consider another spectrum policy. The use of light licensing [3] or technology neutral module license technology neutral block licensing is attractive because this policy will give operators the flexibility of their local networks. Frequency Bands without frequency of application may be attractive because of lower costs, but unpredictable deployment risks. However, the use of the 57-64 GHz band as the International no-need band is expected to have a lower risk than the 5.8G h z band, this is because of its extremely high atmospheric attenuation, sparse initial deployment, and the possibility of using a narrow-beam compact antenna to effectively reduce interference.

In mobile broadband and Wi-Fi networks, non-line-of-sight wireless access is familiar to us in daily life. However, there are a lot of misunderstandings about the non-line-of-sight microwave. For example, one is that the non-line-of-sight microwave is limited to 6 GHz frequencies, the other is that wide-bundle antennas and OFDM-based radio technologies must be used. Even so, Spectrum Based on 6 GHz and above has been used for a long period of time for the study of NLOS [4] frequency [5]. In Region [6], with a 24 GHz spectrum, one-to-50 MHz bandwidth can be used to deploy a 90% small site with a deployment capacity of over 100 Mbps. In this article, we will continue to discuss the general NLOS principle and clarify the misunderstanding of NLOS callback. We will display the NLOS high performance indicators and summarize the guidance and suggestions for NLOS deployment implementation.

Ii. NLOS principle (h1)

Any non-line-of-sight (NLOS) solution can describe three basic propagation phenomena in a combination:

• Diffraction

• Reflection

• Transmission

Diffraction occurs when an electromagnetic wave clicks the edge of a building and is often called a "bending" signal on the edge, as shown in 1. In reality, the wave energy is dispersed in a plane perpendicular to the edge. The diffraction loss increases with the sharpness of "bending" and a higher frequency, and the diffraction loss may be large.

Reflection, especially Random Multi-path reflection, is crucial for Wireless Access Networks Using Wide-bundle antennas. However, using a narrow-beam antenna for single-path reflection is more difficult to construct because you need to find the reflection so that it provides an appropriate angle of incidence, as shown in 1.

Transmission occurs when an object that completely or partially blocks the line of sight is transmitted. The frequency above 2 GHz has poor permeability to most building materials. In reality, transmission is only possible for relatively thin objects, as shown in 1, such as sparse trees.

By understanding these three NLOS propagation principles, we can define simple deployment principles and gain an intuitive understanding of Transmission Performance under any circumstances. However, the diffraction, reflection, and transmission of each point increase the path loss, and there is uncertainty in the calculation of the transmission channel. Therefore, we recommend that you deploy a non-line-of-sight protocol to one or two of the preceding transmission scenarios.

Key features of the NLOS System (h2)

A simple NLOS microwave link index calculation formula can be obtained by adding the non-line-of-sight attenuation (△lnlos) formula to the traditional line-of-sight microwave link index calculation formula:

Here, PRX and PTX are the receiving and transmitting power (dBm); GTX and GRX are respectively the antenna gain (dBi) at the transmitter and receiver end; d is the link distance (km ); f is the frequency (GHz); LF is any Fading Loss (dB); and △lnlos is the additional loss due to the non-line-of-sight propagation (dB ). The above formula is not shown, but it is important to realize the following conclusion: the antenna gain of fixed-size antennas varies with the frequency by 20 log (f, therefore, the actual number of dB at the receiving level will also increase with the increase of the frequency in the relationship of 20 logs (f) (the size of the antenna will not change ). This indicates that the higher frequency of use will bring more transmission advantages in the small station transmission, which is an important component of small antennas.

Two types of microwave return systems are specially studied to illustrate important system performance of line-of-sight (NLOS) transmission. The first system is a commercial product in the unlicensed 5.8 GHz band. Based on TDD and OFDM technology, the product uses a 64-QAM modulation method and uses 2x2 MIMO (cross polarization) configured to provide 100 Mbps full-duplex peak throughput (aggregation 200 Mbps) in a 40 MHz channel bandwidth ). The second system is a licensed 28 GHz Ericsson MINI-link pt 2010 commercial product, based on FDD and single-carrier technology with 512 QAM modulation. It provides 400 Mbps full duplex peak throughput over a 56 MHz channel. Both systems use adaptive modulation to adapt to Throughput Based on the quality of the received signal. At the same time, the two systems use almost the same antenna size. The 28 GHz system uses a 30 cm antenna and the 5.8GHz system uses a 20 cm antenna.

Figure 2 shows the link redundancy between two systems at different link distances, that is, the difference between the acceptance level calculated in Formula 1 and the receiver threshold of a specific modulation method (throughput. If we can predict additional losses in any non-line-of-sight scenario, we can use Figure 2 to predict the expected throughput. Figure 2 shows the advantage of moving the frequency to a higher frequency. In the same antenna size, the link redundancy of the 28 GHz system is about 20 dB higher than that of the 5.8 GHz system.

Figure 2 link redundancy comparison between two systems at different throughput levels

28 GHz (red) frequency, output power: 19dBm, 2x56 MHz channel bandwidth (FDD), 38dBi Antenna Gain

5.8 GHz (blue) frequency, output power: 19dBm, 40 MHz channel bandwidth (TDD), 17dBi Antenna Gain

Iii. Test

3.1 diffraction (h2)

The general misunderstanding is that the diffraction loss of radio waves at frequencies higher than 6 GHz is very high, and it is not suitable for NLOS Radio Wave Propagation in actual operation. However, although the absolute loss of 40 dB is higher than the absolute loss of 34 dB of GHz at the 30 ° angle, the relative difference is only 6 dB loss [8]. The 6dB difference is much less than 28 Ghz and close to 30dB higher than the link redundancy (figure 2 ).

Figure 3 (a) establishes two NLOS return systems in the diffraction scenario. The first transceiver is placed on the top of the office building in the middle of the picture (marked in red ). The second receiver is placed on the automatic elevator. The height of the elevator is 11 meters and the elevator is located behind the 13-meter-tall parking garage. 3 (B ). Figure 3 (c) compares the theoretical receiver level of the blade model diffraction with the measured receiver level when the distance is lower than the distance of LOS. The transmitting power of the two systems is 19dBm, but the gain of the 5.8 GHz antenna is 21 dBi. Therefore, after NLOS transmission, the receiving level is 20 dB weaker than that of the 28 GHz antenna. The theoretical receiving level of 28 GHz is consistent with the measured level although there is a small amount of dB offset. This offset is foreseeable because the model is simple and the actual waves pass through at least five architectural edges, and each edge causes signal loss. In short, the diffraction loss follows the blade-like diffraction model [8]. However, as an empirical rule, we recommend that the additional 10 dB redundancy be added to the loss calculation for prediction purposes based on the ideal model assumption.

Figure 3 testing site (approximately 200 metres) using diffraction line-of-sight (a) Return (B) Mobile lift (c) throughput and receipt level and below-line-of-sight height

Due to the expected high link redundancy, the 28 GHz system maintains full-duplex throughput at a deeper non-line-of-sight ratio than the 5.8 GHz system. The 28 GHz system can transmit a full-duplex Mbps throughput within 6 meters of the line of sight (NLOS) condition, and the corresponding diffraction angle is 30 degrees. The GHz bandwidth can only reach 50 Mbps under the NLOS condition within 3 meters of the line of sight. Link redundancy is the only and most important system parameter of a non-line-of-sight transmission system. With the same antenna size, the 28 GHz system performs much better than the 5.8GHz system.

3.2 reflection

As shown in figure 3 (above), both sets of system performance were tested as metal and brick walls as a single reflection point. The first receiving and sending machine is placed on the roof of the office building in the center of the figure (18 meters above the ground), and the second receiving and sending machine is placed on the 5-meter-high wall on the street of the same office building. The brick wall of the opposite building serves as the reflective surface. The total link length is about 100 meters. The incident angle of the reflection point is about 15 degrees. According to previous research, the △lnlos values of [9] 28 GHz and 5.8GHz are 24 dB and 16 dB respectively. Reflection loss has a very decisive relationship with the reflection material. As a comparison, when the adjacent metal wall is used as the reflection point, the △lnlos of the two systems are about 5 dB. In conclusion, we can assume that the single-point reflection loss of 28 GHz is between 5 and 25 dB, while that of the 5.8GHz system is between 5 and 20 dB. Early studies showed that the surface roughness would lead to pulse diffusion limit [9], but this can be mitigated through a sufficiently long balancer. Figure 4 (bottom) two systems test the throughput for more than 16 hours.

As shown in figure 4, the 28 GHz system shows a stable throughput of 400 Mbps, while the throughput of the 5.8 GHz system fluctuates due to the use of a wider beam antenna, the value fluctuates between 70 Mbps and 100 Mbps. We think this is caused by the wide-beam strong multi-path transmission. OFDM is an effective suppression technology for multi-path propagation. Serious multi-path fading results in a step-by-step reduction of throughput. However, a 28 GHz antenna with a narrow bandwidth and an advanced suppression balancer can effectively suppress the multi-path fading. The MINI-LINK system's single-carrier QAM technology can be used for non-line-of-sight propagation, even using 56 MHz channel bandwidth 512QAM technology.

Figure 4 throughput Mbps () for 28 GHz and 5.8 GHz systems using a reflected NLOS return site ()

3.3 Transmission

The common misunderstanding is that NLOS transmission is supported only under 6 GHz. Figure 5 shows the test performance in the transmission scenario of the two systems. The two receiving and sending machines are respectively placed at the two ends of a tall sparse tree and a short high-density tree. The distance is 150, and the trees cause line-of-sight blocking. Figure 5 tests the influence of leaf density on propagation. Figure 5 emits waves on the left of the tree to pass through sparse trees, and Figure 5 shows sparse and high-density trees on the right.

Figure 5 non-line-of-sight (NLOS) transmission is performed when sparse trees (left) and high-density trees (right) are used. The red circle indicates the receiver position. The above two figures indicate the maximum (green) and the lowest (red) channel frequency response.

As shown in figure 5 (above), the spectrum is a test result with great uncertainty in NLOS loss caused by strong winds and low rainfall. 5 shows that sparse trees increase penetration loss by 6 dB, dense trees increase the penetration loss by 20-40 dB. The conclusion is that for NLOS microwave, sparse trees can be accepted while high-density trees are not recommended as NLOS transmission paths when small stations return high-Reliability Indicators. If the system is 5.8 GHz and 28 GHz, the system performance of the low frequency segment needs to be improved. However, it is still the opposite of the common misunderstanding. It is 28 GHz that can be used in sparse green NLOS to achieve good performance indicators at the same time.

Iv. Deployment Guide

In the previous sections, we have discussed key system indicators for NLOS propagation, diffraction, reflection, and transmission. This section describes how to predict the NLOS callback Deployment scenario and test the actual performance.

Figure 6 performance of line-of-sight (NLOS) return at 28 GHz and GHz. The color area shows the NLOS Transmission Performance and predicted throughput (within the arc is a GHz metric)

The godeburg region in Sweden is a site for testing (figure 6 ). The aggregation site (main site) of the NLOS wireless return system is at the corner of the garage 13 metres above the ground, located south of the test area. This area is mainly composed of 4-6 floors of office buildings. The office buildings are mixed walls of bricks and steel bars, and there is also a 10-meter-wide street from south to north. The street is full of cars and buses. The building wall is composed of bricks, glass and metal.

Table 1 performance experience of △lnlos and bit rate in different non-line-of-sight scenarios

Table 1 summarizes the empirical law of the two test systems in the above-mentioned key scenarios of the non-line-of-sight (NLOS) scenario. As a typical case, it is assumed that the diffraction is 30 °. By using △lnlos as an empirical rule, throughput for each non-line-of-sight scheme is read from figure 2 and summarized in table 1.

Through manual site inspection, Figure 6 shows the expected transmission effect of the tested area over the line of sight (NLOS), and is drawn in different color areas. The scene range includes pure line of sight (green), a single reflection point or part that blocks the line of sight (yellow), a single remote reflection (blue), and dual diffraction or dual reflection (red ). Areas that are not colored indicate that there is no throughput forecast or that it is outside the measurement area. The white dotted line indicates the area for measurement. For the sake of simplicity, the path loss that is partially blocked can be measured in 6 dB.

The receiver placed on the mobile elevator is 3 metres above the ground. The receiver moves along the main street from south to north with the mobile elevator. The street is near the nearest neighbor and parallel to the canyon. The full-duplex throughput between the master station and the receiver is measured. Because the main flap of the 5.8 GHz antenna is wider, no adjustment is required during the measurement process. The main lobe of the 28 GHz antenna is narrow, and the antenna of the main station needs to be adjusted for each test point. However, the alignment of the 28 GHz antenna is relatively simple under non-line-of-sight conditions.

All tests exceed or match the expected performance (color area. For the 5.8 GHz system, the decline of multiple channels, including the movement of mobile vehicles along the canyon sub-district, has a huge impact, but it is clear that the 28 GHz system will slightly reduce the throughput in more difficult scenarios.

V. Summary

Similar to conventional line-of-sight (LOS) return systems, the non-line-of-sight (NLOS) return link mainly relies on large bandwidth and large link redundancy. The 6 GHz band has been proved to be applicable for non-line-of-sight (NLOS) transmission. At the same time, this paper points out that a 6 GHz system is used within 250 meters of the main station, and a moderate size oriented antenna can meet the performance requirements of small station backhaul. Nevertheless, contrary to the traditional concept, Ericsson's MINI-LINK 28 GHz product is theoretically consistent with that of a device that outperforms 6 GHz in most non-line-of-sight conditions. It is mainly because the antenna of the same size is 20 dB higher than the antenna gain, wider bandwidth and stable single-carrier MINI-LINK equipment. During reflection, diffraction, and sparse leaf transmission, the full-duplex throughput of Mbps is demonstrated in actual implementation. The simple project deployment guide supports link prediction and reliable deployment implementation. Microwave backhaul can not only provide the same transmission capacity of multiple gibits as fiber optics, but also effectively support the challenge of approaching or non-line-of-sight small-site backhaul.

References

1) J. Hansryd, J. Edstam, Microwave capacity evolution, Ericsson Review, 1/2011, http://www.ericsson.com/res/docs/review/Microwave-Capacity-Evolution.pdf

2) It all comes back to backhaul, Ericsson white paper, February 2012, http://www.ericsson.com/res/docs/whitepapers/WP-Heterogeneous-Networks-Backhaul.pdf

3) NGMN white paper-Small cell backhaul requirements, NGMN Alliance, June 2012, http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Small_Cell_Backhaul_Requirements.pdf

4) Seidel, S. Y .; arnold, H. W .;, "Propagation measurements at 28 GHz to investigate the performance of local multipoint distribution service (LMDS)," Global Telecommunications Conference, 1995. GLOBECOM '95 ., IEEE, vol.1, no ., pp.754-757 vol.1, 14-16 Nov 1995

5) Rappaport, T. S .; yijun Qiao; Tamir, J. I .; murdock, J. N .; ben-Dor, E .;, "Cellular broadband millimeter wave propagation and angle of arrival for adaptive beam steering systems (invited paper)," Radio and Wireless Symposium (RWS), 2012 IEEE, vol ., no ., pp.151-154, 15-18 Jan. 2012

6) Coldrey, M .; koorapaty, H .; berg, J.-E .; ghebretensa é, Z .; hansryd, J .; derneryd, .; falahati, S .;, "Small-Cell Wireless Backhauling: A Non-Line-of-Sight Approach for Point-to-Point Microwave Links," Vehicular Technology Conference (VTC Fall), 2012 IEEE, vol ., no ., pp.1-5, 3-6 Sept. 2012

7) Fixed service in Europe-current use and future trends post 2012, ECC Report 173, March 2012.

8) Propagation by diffraction, ITU-R P.526

9) Dillard, C. L .; gallagher, T. M .; bostian, C. W .; sweeney, D. G .;, "28 GHz scattering by brick and limestone Wils," Antennas and Propagation Society International Symposium, 2003. IEEE, vol.3, no ., pp. 1024-1027 vol.3, 22-27 June 2003

Side note:

Part A: Abbreviations

FDD Frequency Division Duplex Frequency Division multiplexing

FTTC Fiber to the curb optical Fiber to the junction box

LOS Line-of-sight Visualization

NLOS near-line-of-sight close to visible

NLOS Non-line-of-sight is not visible

OFDM Orthogonal Frequency Division Multiplexing

QAM Quadrature Amplitude Modulation

TDD Time Division Duplex Time Division multiplexing

Part B

Mistaken transfer: non-line-of-sight transmission is only available in 6 GHz Systems

Fact: Although the loss of line-of-sight (NLOS) Propagation increases at a high frequency, the increase in antenna gain ensures superior link performance, such as 28 Ghz.

Part C

False transfer: non-line-of-sight (NLOS) transmission is feasible only when the wide-band antenna system is used.

Fact: the RF signals at both ends of the backhaul network are at a fixed position. The narrow-flap antenna can be easily installed and debugged to find the optimal line-of-sight (line-of-sight) path, high-gain (narrow-flap) antenna, and low-gain (wide-flap) antenna) better guarantee of superior link performance.

Part D

Mistaken transfer: NLOS transmission is only applicable to systems that support OFDM technology.

Fact: Although OFDM is a good mitigation technique to overcome multiple channels, a better solution is to use narrow-Beam Antennas to effectively prevent any multi-path effects. Non-line-of-sight backhaul can use narrow-Beam Antennas to support superior link performance without OFDM.