White PaPer OS-Gemini nOn-Line-Of-SiGht
Motorola OS-Gemini Non-Line-of-Sight Operation
1 Introduction
Only 14% of properties that are 1,500 feet (500 meters) apart have an optical line-of-sight (LoS) path between their roofs. At higher ranges and in low-rise rural environments, this percentage is even lower. These factors lead to an increasing demand for non-line-of-sight (NLoS) wireless Ethernet bridges. The cost and delay of nonwireless methods of connection are very high, and there is increasing interest in unlicensed bands where the speed of connection can be measured in hours rather than months. In the 2.4 and 5.8 GHz frequency bands, many products have been coming into the market that claim NLoS operation when in fact only one or perhaps two of the required features have been included. The actual requirement is for a comprehensive range of features to deliver full NLoS performance. This white paper describes issues affecting NLoS application performance and the features required to fully address them. The radio applications that are enabled by this performance are described in Section 4. The network applications are described in Section 5. The figures in this white paper apply to the USA only.
In this case, there will be additional equipment, rental and maintenance costs. There will also be reduced reliability and security, and increased intoservice time. It is highly desirable to have a direct link in an unlicensed band, removing the need for spectrum license fees. 5.8 GHz is the lowest frequency that fulfills the requirements of bandwidth availability and yet is not overused by other services. 5.8 GHz regulations also allow sufficient link budget to diffract around large obstacles. Table 1 (next page) shows the losses associated with certain obstacles in various environments. As shown here, these losses are much greater for infrared and W-band radio than they are for 5.8 GHz radio. The cost of generating power at the higher frequencies is also greater, so the way in which they have been made to work over long ranges is to restrict the beam-width of the radiators. This narrowing of the width brings problems in the stability of the mounting point. Laser devices often need to be self-adjusting for the movement of tall buildings in the wind.
Contents 1 Introduction 2 Why 5.8 GHz is the Optimum Technology 3 NLoS Radio Propagation Requirements 4 Application Data 5 Network Applications
3 NLoS Radio Propagation Requirements
There are a number of requirements for a radio system that is intended to operate in a Non-Lineof-Sight manner. These are: • Sufficient power budget • Demodulator with sufficient dispersion mitigation • Fading mitigation • Adaptive link characteristics
Power Budget In Motorola’s case, creating
2 Why 5.8 GHz is the Optimum Technology
There are various methods of connecting buildings together, including: • Leased line • Trenching or overhanging wires • Licensed Point-to-Point (PTP) • Unlicensed PTP • Optical PTP Of these, the lowest overall cost is usually the unlicensed point-to-point link because the cost of equipment is normally lower. If, however, the equipment does not have sufficient ability to connect directly between the two buildings, then the link must go via an intermediate point.
sufficient power budget involves transmitting as much power as the regulations in any particular country allow. Of course there is also Transmit Power control to ensure that unnecessary power is not transmitted. On the receive side, the budget is optimized by large antenna gain, low noise figure and low system loss (relative to Shannon).
Table 1: Typical signal losses depend on the environment
O B S ta C L e
Clear, still air Scintillation Birds or foliage Window (double-glazed) Light mist (visibility 400m) Medium fog (visibility 100m) Light rain (25mm/hour) Heavy rain (150mm/hour)
infrareD LiGht (765nm)
-1 dB/km 0 to -3 dB/km Impenetrable -3 dB/km -25 dB/km -120 dB/km -10 dB/km -25 dB/km
W- B a n D r a D i O ( 6 0 G h z )
-15 dB/km 0 dB/km 0 to -20 dB/km -1 dB/km -1 dB/km -1 dB/km -10 dB/km -40 dB/km
5.8 Ghz raDiO
-0.1 dB/km 0 dB/km 0 to -2 dB/km -0.5 dB/km 0 dB/km 0 dB/km 0 dB/km -1 dB/km
0 -5 -10 -15 -20 -25 -30 -35 -40 -45 0 5 10 15 20 25 30 35
(Excess path loss dB)
(Fade margin dB)
Space time coding
to just 15 dB, giving a performance benefit of 25 dB. Another way of viewing this is that, given that only 15 dB is available for fade margin, a conventional system-deployed NLoS solution will only achieve 93% link availability while the Motorola MIMO will achieve 99.99% link availability. Fade rates are variable by path, and depend upon the movement of objects in and around the path. Trees are often the dominant source of fast fading. When the wind is high, the fade rate bandwidth can be up to 4 Hz. In still conditions, the path loss through trees becomes stationary at any random point in the fade cycle. Urban paths above tree level often exhibit very slow fading, and it becomes very difficult to determine the effective mean signal level – and thus the fade margin required – for the site being installed at the time of installation.
adaptive modulation Adaptive modulation,
figure 1: Conventional systems require a 40 dB fade margin to operate successfully in paths that require only 10 dB fade margin for Multiple-Input Multiple-Output (MIMO).
Normal 40
Dispersion. In the type of NLoS channels that
are being used, dispersion can be as much as 5 µS. Equalizing a 10 MHz bandwidth channel with this much dispersion can be a challenge for single-carrier systems. The Motorola OS-Gemini point-to-point systems use a type of Orthogonal Frequency Division Multiplexing (OFDM) that has equalization pilots giving the exact dispersion characteristics. This, in conjunction with forward error correction in the frequency domain, ensures perfect demodulation in any channel.
fading In NLoS paths, multipath fading is
significantly accentuated. Extensive testing has shown that the graph in Figure 1 applies. Using Multiple-Input Multiple-Output (MIMO), the Motorola OS-Gemini 5.8 GHz point-to-point solutions address this problem. Figure 1 shows that, for 99.99% link availability, a conventional system requires 40 dB fade margin for NLoS paths. With MIMO, this requirement is reduced
often used to increase the capacity of point-tomultipoint systems, is also applicable to PTP systems. In this technique, the radio modulation and bandwidth can be modified according to the signal level received. Since the channel often varies in intensity on a sub-second basis, transmitting the maximum amount of data possible means that such a system must be rapidly optimizing itself to the channel conditions. The effect is to increase the data rate capability and to improve the reliability of the system.
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WHITE PAPER: MOTOROLA OS-GEMINI NON-LINE-OF-SIGHT OPERATION
4 Application Data
The result of applying this design method is that Motorola is able to increase – by a very large percentage – the probability of being able to install and operate a link. To give some concrete examples, Motorola has used the ATDI ICS Telecom tool to survey Paris and generate average probabilities of connecting two typical buildings in an urban environment. For illustration, the coverage around some common obstacles is also presented: propagation around a building, propagation over a hill and propagation through trees. All of the following results have been confirmed by trials of the equipment apart from the statistical nature of the heights of equipment in the Paris model.
100 90 80 70
This figure shows that the probability of connecting a device that is capable of reliable NLoS operation is substantially more than a device that only has LoS capability. The ATDI tool used for this analysis used P.526-7 for the detailed propagation model.
(4) (2)
Source (3)
(1)
0
500
meters
1500
figure 3: Motorola OS-Gemini greatly increases the coverage area behind a large building.
Propagation around a Building Figure 3 shows
coverage (%)
60 50 40 30 20 10
the coverage behind a substantial building. The deployment is in a flat area with the normal twostory building height of 8 meters (26 feet) upon which the normal installations must take place. There is an “office block” standing 37 meters (121 feet) high and 120 meters (394 feet) wide. The office block represents a 12-story building in this case. In the lower part of the figure, this stands well above the Fresnel zone. The PTP links are installed on the two 2-story buildings, two meters (7 feet) above the roof. Coverage in the shadow of the building is displayed with the dark blue area (1) showing where the normal data rate is typically at maximum with occasional deviations down to the lowest data rate in moments of extreme fading. The white area (2) is where the OS-Gemini always works at the maximum data rate. The pale blue area (3) is where a LoS system can be deployed. The area of red (4) where the OS-Gemini system cannot be installed (without mounting a relay station) is very small and is in the immediate shadow of the building.
figure 2: Over a range of up to 10 km (6 miles) in an urban setting (Paris), the Motorola OS-Gemini greatly increases the probability of signal coverage.
0 0.1
0.2
0.5
1
2
5
10
range (km)
Detailed Survey of Paris In the detailed survey
of the center of Paris, a Digital Elevation Model (DEM) with 4-meter (13-foot) resolution was used to position 9,000 PTP links. The propagation was taken from the highest point in a 10-meter (32 foot) radius around the property entrance. The probability of LoS or NLoS coverage was then computed, as shown in Figure 2.
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WHITE PAPER: MOTOROLA OS-GEMINI NON-LINE-OF-SIGHT OPERATION
figure 4: In the worst-case scenario, a hill only 18 meters (59 feet) high can block successful signal propagation – shown here between two points 1,000 meters (3,280 feet) apart.
Propagation through trees There is a high degree
1000 m
Propagation Over a hill In the case of hills, the
propagation depends very much upon the shape of the hill being traversed. Reference [1] (see below) shows that the hill drawn in Figure 4 can only be 18 meters (59 feet) high. In practice this is a worst case because any roughness in the path improves the propagation. Buildings that can be seen by both parties improve the propagation further. System performance in this area will mostly be at the full data rate with occasional fades into the lower data rate. This is similar to the performance in the dark blue area of Figure 3.
5.8 GHz point-to-point over round objec t
40 30 20 15 10 7
figure 5: A combination of height and size determines an object’s impact on propagation success.
of variability in the loss associated with trees. The loss varies with the size, thickness, type and wetness and there is an annual component with the cycling of the leaves. Trees are also a very common obstruction in provincial and rural settings. In the case of a single tree, which can be traversed at a loss of about 25 dB, this means that the range is restricted to 8 km (5 miles) if a single tree is the multipoint obstruction.
5 Network Applications
None of the foregoing radio performance would be any use without providing for network applications. The Motorola OS-Gemini* provides up to 43 Mbps aggregate data rate as an Ethernet bridge. Under particularly stressful radio conditions, this may drop to 4.4 Mbps. Typically this will be used to connect together two Ethernet LANs. The device is so inexpensive that it can be efficiently used as a backup for leased line or optical links. If very high reliability is required, one can use two links in parallel with the additional advantage of 86 Mbps aggregate data rate. Further applications include: • Upgrades to existing analog or slower digital links
height (m)
0.01
0.1
1
10
100
1000
• Very long distance wireless links via staging points giving 40 km (25 miles) or more range • Backhaul for mobile cellular networks
radius (m)
Figure 5 shows the maximum height of a smooth hill with a given radius above the optical line-ofsight path of 1 km (0.6 mile). Rough hills should normally give better propagation.
Reference: [1] (2002). RECOMMENDATION ITU-R P.526-7 - Propagation by diffraction. Recommendation P.526, International Telecommunications Union.
For more information about the Motorola Point-to-Point Solutions: Outside of north america: Sales: +44 1364 655500 Tech Support: +44 1364 655656 Sales and tech Support in north america: +1 877 515-0400 www.orthogonsystems.com
MOTOROLA and the stylized M Logo are registered in the US Trademark and Patent Office. All other product or service names are the property of their respective owners. © Motorola, Inc. 2006. MOTOROLA NLoS WP US 15-Jun-06
*formerly
OS-Gemini