Find out information on technology, deployment and applications for modern Digital Microwave Links
Microwave links are widely used for connectivity in modern digital IP networks. With capacities up to 3Gbps and beyond, a modern Microwave Link network can deliver bandwidth in a reliable, cost-effective and flexible manner – without need for disruption and delay caused by digging up streets and avoiding costly leased-line or leased fibre optic alternatives.
On this website you can find more information about radio link deployment and technology. Also we invite you to contact our experts with any questions by sending a message to us on our contact page.
Microwave links are used extensively in 4G/LTE backhaul networks, 2G (GSM) and 3G (UMTS) mobile operators, wireless metropolitan area networks (Wi-MAN) and corporate networks where high performance, flexibility, speed of deployment and low operating costs are required. Key features of links include high spectral efficiency (256QAM, 1024QAM, 2048QAM and 4096QAM), Automatic Transmit Power Control (ATPC) and Adaptive Coding and Modulation (ACM).
Globally, MW radio links are used for around 60% of all mobile backhaul connections due to the compelling technical and commercial arguments in favour of MW radio compared to leased line and trenched fibre alternatives. Speed of deployment and flexibility – the ability to move sites or provision rapidly – are greatly in favour of MW radio over fibre and cabled alternatives.
A link typically features a radio unit and a parabolic antenna, which may vary in size from 30cm up to 4m diameter depending on required distance and capacity. The radio unit is generally either a “Full Outdoor”, “Split Mount” or “Full Indoor” design depending on operator preference, deployment, features and available indoor space for specific sites and installation.
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Introduction to Millimeter Wave Technology for E-Band and V-Band
Millimeter Wave (MMW) is a technology for high speed (10Gbps, 10 Gigabit per second) high capacity wireless links, ideal for urban areas. Using high frequency microwave in the E-Band (70-80GHz) and 58GHZ to 60GHz (V-Band) spectrum, links can be densely deployed in congested cities without interference, and without need for digging for cables and fibre optics, which can be costly, slow and highly disruptive. By contrast, MMW links can be deployed in hours, and moved and reused on different sites as network requirements evolve.
History of MMW
In 2003 the North American Federal Communications Commission (FCC) opened several high frequency millimeter-wave (MMW) bands, namely in the 70, 80, and 90 gigahertz (GHz) ranges, for commercial and public use. Due to the vast amount of spectrum (roughly 13 GHz) available in these bands, millimeter-wave radios have quickly become the fastest point-to-point (pt-to-pt) radio solution on the market. Radio transmission products offering full-duplex data rates up to 1.25 Gbps, at carrier class availability levels of 99.999%, and over distances close to one mile or more are available today. Due to cost-effective pricing, MMW radios have the potential to transform business models for mobile backhaul providers and metro/enterprise “Last-Mile” access connectivity.
The opening of 13 GHz of previously unused spectrum in the 71…76 GHz, 81…86 GHz and 92…95 GHz frequency ranges, for commercial use, and high-density fixed wireless services in the United States in October 2003 is regarded as a landmark ruling by the Federal Communications Commission (FCC). From a technological point of view this ruling allowed for the first time, full line speed and full-duplex gigabit-speed wireless communications over distances of one mile or more at carrier-class availability levels. At the time of opening the spectrum for commercial use, FCC Chairman Michael Powell heralded the ruling as opening a “new frontier” in commercial services and products for the American people. Since then, new markets for fibre replacement or extension, point-to-point wireless “Last-Mile” access networks, and broadband Internet access at gigabit data rates and beyond have been opened.
The significance of the 70 GHz, 80 GHz and 90 GHz allocations cannot be overstated. These three allocations, collectively referred to as E-band, comprise the largest amount of spectrum ever released by the FCC for licensed commercial use. Together, the 13 GHz of spectrum increases the amount of FCC-approved frequency bands by 20% and these bands combined represent 50 times the bandwidth of the entire cellular spectrum. With a total of 5 GHz of bandwidth available at 70 GHz and 80 GHz, respectively, and 3 GHz at 90 GHz, gigabit Ethernet and higher data rates can easily be accommodated with relatively simple radio architectures and without complex modulation schemes. With propagation characteristics being only slightly worse than those at the widely used microwave bands, and well-characterized weather characteristics allowing rain fade to be understood, link distances of several miles can confidently be realized.
The FCC ruling also laid the foundation for a novel Internet based licensing scheme. This online licensing scheme allows fast registration of a radio link and provides frequency protection at a low one time charge of a few hundred dollars. Many other countries around the globe are currently opening the MMW spectrum for public and commercial use , following the landmark ruling of the FCC. Within this paper we will try to explain the significance of the 70 GHz, 80 GHz and 90 GHz bands, and show how these new frequency allocations will potentially reshape high data rate transmission and associated business models.
Target Markets and Applications for High Capacity “Last-Mile” Access Connectivity
In the United States alone, there are roughly 750,000 commercial buildings with 20+ employees. In today’s highly Internet connected business environments the majority of these buildings need high data rate Internet connectivity. While it is certainly true that many businesses are currently satisfied with having a slower speed T1/E1 at 1.54 Mbps or 2.048 Mbps, respectively, or any other form of slower speed DSL connection, a rapidly growing number of businesses are requiring or demanding DS-3 (45 Mbps) connectivity or higher speed fibre connections. However, and here is where the problems starts, according to a very recent study by Vertical Systems Group, only 13.4% of the commercial buildings in the United Sates are connected to a fibre network. In other words, 86.6% of these building have no fibre connection, and building tenants rely on leasing slower speed wired copper circuits from the incumbent or alternative telephony providers (ILECs or CLECs). Such costs for a higher speed wired copper connection like a 45 Mbps DS-3 connection, can easily run to $3,000 a month or more.
Another interesting study conducted by Cisco in 2003 revealed that 75% of the U.S. commercial buildings that are not connected to fibre are within one mile of a fibre connection. However, despite the growing demand for high capacity transmission into these buildings, the cost associated with laying fibre does very often not allow for “closing the transmission bottleneck”. For example, the costs of laying fibre in major U.S. metropolitan cities can run up to $250,000 per mile, and in many of the largest U.S. cities there is even a moratorium on laying new fibre because of the associated massive traffic disruptions. Fibre to commercial building connectivity figures in many European Cities are far worse and some studies suggest that only about 1% of commercial buildings are connected to fibre.
Many industry analysts agree that there is a large and presently underserved market for short-haul wireless “Last Mile” access connectivity provided that the underlying technology allows for carrier-class availability levels. MMW radio systems are perfectly suited to fulfill these technical requirements. Additionally, high capacity and commercially available MMW systems have drastically come down in pricing over the last couple of years. When compared to laying just one mile of fibre in a major metropolitan U.S. or European city, the use of a gigabit Ethernet capable MMW radio can run as low as 10% of the fibre costs. This pricing structure makes the economics of gigabit connectivity attractive because the required capital layout and the resulting Return on Investment (ROI) period are drastically shortened. Consequently, many high data rate applications that could not be served economically in the past due to the high infrastructure costs of trenching fibre can now be served and are economically feasible when using MMW radio technology. Among these applications are:
CLEC and ILEC fibre extensions and replacements
Metro Ethernet backhaul and fibre ring closures
Wireless campus LAN extensions
Fibre backup and path diversity in campus networks
High capacity SAN connectivity
Redundancy, portability and security for Homeland Security and Military
3G cellular and/or WIFI/WiMAX backhaul in dense urban networks
Portable and temporary links for high-definition video or HDTV transport
Why use E-Band MMW Technology?
Of the three frequency bands opened up, the 70 GHz and 80 GHz bands have attracted most interested by equipment manufacturers. Designed to co-exist, the 71…76 GHz and 81…86 GHz allocations allow 5 GHz of full-duplex transmission bandwidth; enough to easily transmit a full-duplex gigabit Ethernet (GbE) signal even with the simplest modulation schemes. The advanced Wireless Excellence design even managed to use the lower 5 GHz band, from 71…76 GHz only, to transport a full duplex GbE signal. Later, a clear advantage is shown in using this approach when it comes to the deployment of MMW technology close to astronomical sites and in countries outside of the U.S. With direct data conversion (OOK) and low-cost diplexers, relatively simple and thus cost efficient and high reliable radio architectures can be achieved. With more spectrally efficient modulation codes, even higher full-duplex transmission at 10 Gbps (10GigE) up to 40Gbps can be reached.
The 92…95 GHz allocation is far more difficult to work with because this part of the spectrum is segmented into two unequal portions that are separated by a narrow 100 MHz exclusion band between 94.0…94.1 GHz. It can be assumed that this part of the spectrum will more likely be used for higher capacity and shorter range indoor applications. This allocation will not be discussed further in this white paper.
Under clear weather conditions, the transmission distances at 70 GHz and 80 GHz exceed many miles due to low atmospheric attenuation values. However, Figure 1 shows that even under these conditions the atmospheric attenuation varies significantly with frequency . At conventional, lower microwave frequencies and up to roughly 38 GHz, atmospheric attenuation is reasonably low with attenuation values of a few tenths of a decibel per kilometre (dB/km). At around 60 GHz absorption by oxygen molecules causes a large spike in attenuation. This large increase of oxygen absorption seriously limits radio transmission distances of 60 GHz radio products. However, beyond the 60 GHz oxygen absorption peak a wider low attenuation window opens up where attenuation drops back to values around 0.5 dB/km. This window of low attenuation is commonly referred to as E-band. The E-band attenuation values are close to the attenuation experienced by common microwave radios. Above 100 GHz, atmospheric attenuation generally increases and in addition there are numerous molecular absorption bands caused by O2 and H2O absorption at higher frequencies. In summary, it is the relatively low atmospheric attenuation window between 70 GHz and 100 GHz that makes E-band frequencies attractive for high capacity wireless transmission. Figure 1 also shows how rain and fog impact attenuation in microwave, millimeter-wave and infrared optical bands that start around 200 terahertz (THz) and that are used in FSO transmission systems. At various and specific rainfall rates attenuation values change slightly, with increasing transmission frequencies. The relationship between rainfall rates and transmission distances will be examined further in the following section. Fog related attenuation can basically be neglected at millimeter-wave frequencies, increasing by several orders of magnitude between the millimeter-wave and the optical transmission band: The main reason why longer distance FSO systems stop working under foggy conditions.
Transmission Distances for E-Band
As with all high-frequency radio propagation, rain attenuation typically determines the practical limits on transmission distances. Figure 2 shows that radio systems operating in the E-band frequency range can experience large attenuation given the presence of rain . Fortunately, the most intense rain tends to fall in limited parts of the world; mainly the subtropical and equatorial countries. At peak times rainfall rates of more than seven inches/hour (180 mm/hour) can be observed for short periods of time. In the United States and Europe, maximum rainfall rates experienced are typically less than four inches/hour (100 mm/hr). Such a rainfall rate causes signal attenuations of 30 dB/km, and generally occurs only during short cloud bursts. These cloud bursts are rain events that appear within relatively small and localized areas and within a lower intensity, larger diameter rain cloud. Since cloud bursts are also typically associated with severe weather events that move quickly across the link, rain outages tend to be short and are only problematic on longer distance transmission links.
The International Telecommunications Union (ITU) and other research organizations have collected decades of rainfall data from all over the world. In general, rainfall characteristics and relationships between rainfall rate, statistical rain duration, rain drop sizes, etc. are well-understood  and by using this information it is possible to engineer radio links to overcome even the worst weather events or to predict the durations of weather related outages on longer distance radio links operating at specific frequencies. The ITU rain zone classification scheme shows the expected statistical rainfall rates in alphabetical order. While areas that experience the least rainfall are classified as “Region A,” the highest rainfall rates are in “Region Q.” A global ITU rain zone map and a listing of the rainfall rates in specific regions of the world is shown in Figure 3 below.
Figure 3: ITU rain zone classification of different regions around the world (top) and actual statistical rainfall rates as a function of the rain event duration
Figure 4 shows a more detailed map for North America and Australia. It is worthwhile to mention that roughly 80% of the Continental US territory falls into rain zone K and below. In other words, to operate at a 99.99% availability level, a radio system’s fade margin must be designed to withstand a maximum rainfall rate of 42 mm/hour. The highest rainfall rates in North America can be observed in Florida and along the Gulf Coast, and these regions are classified under rain zone N. In general, Australia experiences less rain than North America. Huge parts of this country including the more populated Southern coast line are located in rain zones E and F (<28 mm/h).
To simplify, by combining the results of Figure 2 (rainfall rate vs. attenuation) and using the ITU rainfall charts shown in Figures 3 and 4, it is possible to calculate the availability of a particular radio system operating in a certain part of the world. Theoretical calculations based on rainfall data for the United States, Europe and Australia show that 70/80 GHz radio transmission equipment can achieve GbE connectivity at a statistical availability level of 99.99…99.999% over distances close to one mile or even beyond. For a lower 99.9% availability, distances exceeding 2 miles can be routinely achieved. When configuring the network in a ring or mesh topology, effective distances double in some cases for the same availability figure due to the dense, clustering nature of heavy rain cells and the path redundancy that ring/mesh topologies provide.
Figure 4: ITU rain zone classification for North America and Australia
One strong benefit of MMW technology over other high capacity wireless solutions like free space optics (FSO) is that MMW frequencies are unaffected by other transmission impairments such as fog or sandstorms. Thick fog, for example, with a liquid water content of 0.1 g/m3 (about 50 m visibility) has just 0.4 dB/km attenuation at 70/80 GHz . Under these conditions, an FSO system will experience a signal attenuation of more than 250 dB/km . These extreme attenuation values show why FSO technology can only provide high availability figures over shorter distances. E-band radio systems are similarly unaffected by dust, sand, snow and other transmission path impairments.
Alternative High Data Rate Wireless Technologies
As alternatives to E-band wireless technology, there are a limited number of viable technologies capable of supporting high data rate connectivity. This section of the white paper provides a short overview.
Fibre-optic cable offers the widest bandwidth of any practical transmission technology, allowing very high data rates to be transmitted over long distances. Although thousands of miles of fibre are available worldwide and in particular in long haul and inter-city networks, “Last-Mile” access remains limited. Due to substantial and often prohibitively high up-front costs associated with digging trenches and laying terrestrial fibre, as well as right-of way issues, fibre access can be difficult to impossible. Long delays are also frequent, not only because of the physical process of trenching fibre, but also due to obstacles caused by environmental impacts and potential bureaucratic hurdles involved in such a project. For this reason, many cities around the world are prohibiting fibre trenching because of disruption to the inner-city traffic and the general inconvenience the trenching process causes to the public.
Microwave Radio Solutions
Fixed point-to-point microwave radios can support higher data rates such as full-duplex 100 Mbps Fast Ethernet or up to 500 Mbps per carrier in frequency ranges between 4-42 GHz. However, in the more traditional microwave bands the spectrum is limited, often congested and typical licensed spectrum channels are very narrow when compared to the E-Band spectrum.
Figure 5: Comparison between high data rate microwave radios and a 70/80 GHz radio solution.
In general, the frequency channels available for licensing are often no more than 56 megahertz (MHz), but typically 30 MHz or below. In some bands, wide 112MHz channels capable of supporting 880Mbps per carrier may be available, but only in higher frequency bands suited for short distances. Consequently, radios operating in these bands at higher data rates have to employ highly complex system architectures employing modulation schemes up to 1024 Quadrature Amplitude Modulation (QAM). Such highly complex systems result in restricted distances, and throughput is still limited to data rates to 880Mbps in the largest channels. Due to the limited amount of spectrum available in these bands, the wider antenna beamwidth patterns, and the sensitivity of high QAM modulation towards any kind of interference, denser deployment of traditional microwave solutions in urban or metropolitan areas is extremely problematic. A visual spectrum comparison between the traditional microwave bands and the 70/80 GHz approach is shown in Figure 5.
60 GHz (V-Band) Millimeter Wave Radio Solutions
Frequency allocations within the 60 GHz spectrum, and in particular allocations between 57…66 GHz, vary significantly in different regions of the world. The North American FCC has released a wider block of frequency spectrum between 57…64 GHz that provides sufficient bandwidth for full-duplex GbE operation. Other countries have not followed this particular ruling and these countries only have access to much smaller and often channelized frequency allocations within the 60 GHz spectrum band. The limited amount of available spectrum outside of the U.S. does not allow for building cost effective 60 GHz radio solutions at high data rates in European, countries such as Germany, France and England just to mention a few. However, even in the U.S., the regulated limitation in transmission power, coupled with the relatively poor propagation characteristics due to high atmospheric absorption by oxygen molecules (see Figure 1), limits typical link distances to less than half a mile. To achieve carrier-class performance of 99.99…99.999% system availability, for large parts of the continental U.S. territory, the distance is generally limited to a little more than 500 yards (500 meters). FCC has categorized the 60 GHz spectrum as a license-free spectrum. Unlike the higher frequency 70/80 GHz allocations, operation of 60 GHz radio systems does not require legal approval or coordination. On one hand the use of unlicensed technology is very popular among end-users, but at the same time there is no protection against interference, either accidental or intentional. In summary, especially in the U.S., the use of the 60 GHz spectrum can be a potentially viable alternative for short distance deployments, but the technology is no real alternative for link distances beyond 500 meters and when 99.99…99.999% system availability is required.
Free Space Optics (FSO, Optical Wireless)
Free space optic (FSO) technology uses infrared laser technology to transmit information between remote locations. The technology allows transmitting very high data rates of 1. 5 Gbps and beyond. FSO technology is generally a very secure transmission technology, is not very prone to interference due to the extremely narrow transmission beam characteristics, and is also worldwide license-free.
Unfortunately, the transmission of signals in the infrared optical bands is drastically affected by fog, where atmospheric absorption can exceed 130 dB/km . In general, any kind of weather condition that impacts the visibility between two locations (e.g. sand, dust), will also impact the FSO system performance. Fog events and dust/sand storms can also be very localized and difficult to predict, and consequently, the prediction of FSO system availability is more difficult. Unlike extreme rain events, that are very short in duration, fog and dust/sandstorms can also last very long times (hours or even days rather than minutes). This can result in extremely long outages for FSO systems operating under such conditions.
From a practical point of view, and when considering availability numbers of 99.99…99.999%, all of the above can limit FSO technology to distances of only a few hundred yards (300 meters); especially in coastal or fog-prone areas, as well as in regions that experience sand/dust storms. To maintain 100% connectivity when deploying FSO systems in these kinds of environments, an alternative path technology is recommended.
The majority of industry experts agree that FSO technology can offer an interesting and potentially inexpensive alternative in wirelessly connecting remote locations over shorter distances. However, the physics of signal attenuation in the infrared spectrum will always restrict this technology to very short distances.
A short comparison of the discussed and commercially available high data rate transmission technologies and their key performance drivers is shown in Table 1.
Table 1: Comparison chart of commercially available high data rate wireline and wireless transmission technologies
Commercially Available Millimeter-Wave Solutions
The CableFree Millimeter-wave product portfolio includes point-to-point radio solutions operating from 100 Mbps to 10 Gbps (10 Gigabit Ethernet) speeds in the licensed 70 GHz E-band spectrum and up to 1Gbps in the unlicensed 60 GHz spectrum. The systems are available with different antenna sizes to meet the customer’s availability requirements over specific deployment distances at the most competitive price points of any E-band radio manufacturer in the industry. Wireless Excellence’s E-band radio solutions operate in the lower 5 GHz frequency band of the licensed 70/80 GHz E-band spectrum only, rather than simultaneous transmission in both the 70 GHz and the 80 GHz bands. As a result, Wireless Excellence products are not prone to potential deployment restrictions close to astronomical sites or military installations in Europe, where the military is using parts of the 80 GHz band for military communications. The systems are easy to deploy, and due to the low voltage power feed of 48 volts direct current (Vdc), no certified electrician is required for installing the system. Photographs of the Wireless Excellence products are shown in Figure 6 below.
Figure 6: CableFree MMW radios are compact and highly integrated. 60cm antenna version shown
Summary and Conclusions
To solve today’s high capacity network interconnectivity requirements, highly reliable wireless solutions are available providing fibre-like performance at a fraction of the cost of laying fibre or leasing high capacity fibre connections. This is important not only from the performance/cost point of view, but also because fibre connections in “Last-Mile” access networks are still not very widespread and latest studies reveal that in the United States only 13.4% of commercial buildings with more than 20 employees are connected to fibre. These numbers are even lower in many other countries.
There are several technologies in the market that can provide gigabit connectivity to connect remote networking locations. Licensed E-band solutions in the 70/80 GHz frequency range are of particular interest because they can provide the highest carrier-class availability figures at operating distances of one mile (1.6 km) and beyond. In the United States a 2003 landmark FCC ruling has opened this spectrum for commercial use and an Internet based low cost light licensing scheme allows users to get a license for operation within a few hours. Other countries either already have and/or are currently in the process of opening the E-band spectrum for commercial use. Unlicensed 60 GHz radios and free-space optics (FSO) systems can also provide gigabit Ethernet connectivity, but at higher 99.99…99.999% carrier-class availability levels, both of these solutions are only capable of operating at reduced distances. As a simple rule of thumb and for most parts of the United States, 60 GHz solutions can provide these high availability levels only when being deployed at distances below 500 yards (500meters).
ITU-R P.676-6, “Attenuation by Atmospheric Gases,” 2005.
ITU-R P.838-3, “Specific Attenuation Model for Rain for Use in Prediction Methods,” 2005.
ITU-R P.837-4, “Characteristics of Precipitation for Propagation Modeling,” 2003.
ITU-R P.840-3, “Attenuation Due to Clouds and Fog,” 1999.
Implementing Microwave for High Speed Internet ISP Backbones in the Middle East
CableFree FOR3 Microwave links are being installed by ISPs in Iraq for Internet Backbone Connectivity. These links offer 880Mbps full duplex capacity with easy upgrade capability to 2+0 for 1.76Gbps full duplex, and are typically installed on towers or buildings for clear Line of Sight between network node locations.
High Capacity Microwave
CableFree FOR3 can expand to 3.5Gbps and above for ultra high capacity links. Microwave links are fast to install and can be deployed within hours, and distances up to 100km or more on suitable towers.
Microwave is low cost alternative to fibre optic and leased line connectivity and are highly reliable with uptimes of 99.999% or higher possible.
Pictures from Iraq from CableFree regional partner Noor AKD.
CableFree Products are used extensively in the Middle East region with installations in countries including Iraq, UAE, Saudi Arabia, Kuwait, Egypt, Lebanon, Turkey and several others
1024QAM Microwave Links for High Capacity Wireless Transmission
High Capacity Microwave Links from leading vendors use 1024QAM modulation to achieve high capacity, spectral density and efficiency without sacrificing reliability. This technology sets a new benchmark for microwave transmission capacity for operators including 4G / LTE Backhaul links for mobile operators as well as last-mile links, backbone and other applications.
High Capacity Links require High Order QAM modulation
Leading long-haul microwave equipment vendors are now using dependable long-distance transmissions using 1024 QAM. Relative to the industry-standard 256 QAM, this represents a 25% increase in capacity (and up to double the capacity of legacy SDH links), with all other factors the same. Compared to older 4QAM modulation the increase to 1024QAM is five-fold. Operators of long-haul microwave links will certainly enjoy the boost to their capacity with 1024 QAM, especially when these upgrades are relatively painless and generally require only a minor and quick swap of equipment.
Adaptive Coding and Modulation (ACM)
Leading microwave equipment vendors are able to keep their long-haul transmission links operational even in transient fade and noisy conditions. The enabling technology is ACM: Adaptive Coding and Modulation. Microwave links with ACM technology automatically sense the quality of the transmission link and can automatically decrease the modulation technique in case of degraded signal quality due to interference or other microwave propagation problems such as weather. So, if a microwave transmission is operating at maximum capacity using 1024QAM and suddenly encounters interference or high rainfall, a system such as the CableFree microwave system automatically steps down the modulation to lower levels until the transmission network, although at lower capacity now, maintains the ultra high level of link reliability and availability. As the temporary weather effects disappear, the microwave system automatically re-applies more efficient higher-order modulation techniques to regain full capacity.
Overcoming Tradeoffs due to High Order QAM Modulation
With increasing modulation the receiver sensitivity is greatly reduced, and generally transmit power has to be reduced due to linearity constraints in the transmitter. For fixed modulation speeds the result is either increase of antenna size or reduced distances, which may prevent an operator upgrading to higher capacity. The use of ACM allows use of 1024QAM whilst avoiding sacrifice of distance or antenna sizes, by graceful step-down of modulation to lower rates during rare periods of high rainfall.
Use along with other bandwidth-enhancing technologies such as XPIC
1024QAM modulation is fully compatible with other methods to increase capacity such as XPIC (Cross Polar Interference Cancellation). An advanced microwave modem featuring 1024QAM and XPIC can greatly increase capacity. XPIC alone offers double the capacity compared to a single polarised non-XPIC solution.
1024QAM Microwave Summary
These latest advancements in advanced microwave modulation offer network operators an easy and inexpensive upgrade path to higher capacities to meet demand. Advanced modulation technology of 1024QAM is fully shipping and available today and offers a very cost-effective way to boost capacity in long-haul microwave applications.
For Further Information
For More Information on High Capacity Microwave Solutions, Please Contact Us
Adaptive Coding and Modulation or Link adaptation is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link (e.g. the pathloss, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). In a digital Microwave Link ACM uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change.
The Goal of ACM
The goal of Adaptive Modulation and Coding is to improve the operational efficiency of Microwave links by increasing network capacity over the existing infrastructure – while reducing sensitivity to environmental interferences.
Adaptive Modulation means dynamically varying the modulation in an errorless manner in order to maximize the throughput under momentary propagation conditions. In other words, a system can operate at its maximum throughput under clear sky conditions, and decrease it
gradually under rain fade. For example a link can change from 1024QAM down to QPSK to keep “link alive” without losing connection. Prior to the development of Automatic Coding and Modulation, microwave designers had to design for “worst case” conditions to avoid link outage The benefits of using ACM include:
Longer link lengths (distance)
Using smaller antennas (saves on mast space, also often required in residential areas)
Higher Availability (link reliability)
Importance to Operators of ACM
Adaptive Coding and Modulation increases the capacity of microwave links without sacrificing distance or availability, and without requiring larger antennas. The penalty – reduced capacity during heavy fade/rainfall – is usually considered an acceptable trade-off compared to the benefits, especially for IP networks where a variable capacity is generally considered acceptable, compared to legacy PDH (NxE1/T1) and SDH connections which are fixed capacity applications. Conversely, ACM allows operators to minimise costs by using smaller antennas, meet higher availability targets (e.g. 99.999% availability) and customer SLA (service level agreement) and also fit within aesthetic and planning constraints in dense urban areas and regions of natural beauty where large antennas may be prohibited by planners or building owners.
For Further Information on ACM and Microwave Links
For more information on Microwave Links with ACM please Contact Us
Alignment of Microwave Antennas for Digital Microwave Transmission Systems
This article contains generic instructions for alignment of Microwave antennas. Specific products may have different features, in which case please refer to the documentation provided for those products:
Antenna Alignment for Microwave Links
This guide explains how to achieve the optimal antenna alignment of microwave antennas when used with modern digital microwave products. Before attempting to do the alignment it is highly recommended that you read this guide in detail. For specific commands please consult the manual of the product being installed
Step 1: Preparation:
Mount the antenna on the tower according to the antenna installation instructions: Ensure that the adjustment bolts move smoothly and the range of motion is sufficient for the expected angle of up and down (elevation) tilt. Ensure that the mount itself is attached securely and all safety precautions have been taken.
Step 2: Coarse Alignment:
Visually align the antenna with the far end. The most common ways to do this are :
1) If the visibility is good and the sun is in the correct position, have someone at the far end location reflect the sun with a mirror so the location is obvious.
2) If visibility is poor, use GPS coordinates and a GPS compass to aim the antenna coarsely.
Step 3: Fine Alignment.
Before conducting fine alignment, the ODUs at both ends of the link must be attached properly to the antenna via the direct mount or remote mount (using Waveguide) and the far end ODU must be powered on and transmitting. The ODU lightning surge suppressors and grounding provisions should be put in place as well before alignment. The local ODU must be powered on, but need not be transmitting.
1) Frequency of the far end transmitter matches the frequency of the local receiver.
2) The TX output power is not set above the level of the license.
3) ATPC is turned OFF on the far end.
4) Alignment mode is ON for SP ODUs – Display on ODU and IDU will update at 5 times per second.
FINE ALIGNMENT PROCEDURE
1) Adjust the azimuth over a 30 degree sweep by turning the adjustment bolt in increments of 1/10th turn to avoid missing the main lobe. When the highest signal has been found for azimuth, repeat for the elevation adjustment.
2) Turn the local transmitter on to allow alignment at the far end.
3) Move to the far end of the link and repeat step 1.
4) Lock down the antenna so no further movement can occur.
5) Install the antenna side struts supplied with the antenna.
6) Verify the RSSI remains the same and is within 2-4 dB of the expected levels.
The difference between FDD and TDD in Microwave Transmission
Microwave links typically use Frequency-division duplexing (FDD) which is a method for establishing a full-duplex communications link that uses two different radio frequencies for transmitter and receiver operation. The transmit direction and receive direction frequencies are separated by a defined frequency offset.
Advantages of FDD
In the microwave realm, the primary advantages of this approach are:
The full data capacity is always available in each direction because the send and receive functions are separated;
It offers very low latency since transmit and receive functions operate simultaneously and continuously;
It can be used in licensed and license-exempt bands;
Most licensed bands worldwide are based on FDD; and
Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, though not immune to it.
Disadvantages to FDD
The primary disadvantages of the FDD approach to microwave communication are:
Complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, then it may not be possible to deploy the system in that band;
Radios require pre-configured channel pairs, making sparing complex;
Any traffic allocation other than a 50:50 split between transmit and receive yields inefficient use of one of the two paired frequencies, lowering spectral efficiency; and
Collocation of multiple radios is difficult.
TDD compared with FDD
Time-division duplexing (TDD) is a method for emulating full-duplex communication over a half-duplex communication link. The transmitter and receiver both use the same frequency but transmit and receive traffic is switched in time. The primary advantages of this approach as it applies to microwave communication are:
It is more spectrum friendly, allowing the use of only a single frequency for operation and dramatically increasing spectrum utilization, especially in license-exempt or narrow-bandwidth frequency bands ;
It allows for the variable allocation of throughput between the transmit and receive directions, making it well suited to applications with asymmetric traffic requirements, such as video surveillance, broadcast and Internet browsing;
Radios can be tuned for operation anywhere in a band and can be used at either end of the link. As a consequence, only a single spare is required to serve both ends of a link.
Disadvantages of TDD
The primary disadvantages of traditional TDD approaches to microwave communications are:
The switch from transmit to receive incurs a delay that causes traditional TDD systems to have greater inherent latency than FDD systems;
Traditional TDD approaches yield poor TDM performance due to latency;
For symmetric traffic (50:50), TDD is less spectrally efficient than FDD, due to the switching time between transmit and receive; and
Multiple co-located radios may interfere with one another unless they are synchronized.
The term ODU is used in Split-Mount Microwave systems where an Indoor Unit (IDU) is typically mounted in an indoor location (or weatherproof shelter) connected via a coaxial cable to the ODU which is mounted on a rooftop or tower top location.
Often the ODU is direct mounted to a microwave antenna using “Slip fit” waveguide connection. In some cases, a Flexible Waveguide jumper is used to connect from the ODU to the antenna.
The ODU converts data from the IDU into an RF signal for transmission. It also converts the RF signal from the far end to suitable data to transmit to the IDU. ODUs are weatherproofed units that are mounted on top of a tower either directly connected to a microwave antenna or connected to it through a wave guide.
Generally, Microwave ODUs designed for full duplex operation, with separate signals for transmit and receive. On the airside interface this corresponds to a “pair” of frequencies, one for transmit, the other for receive. This is known as “FDD” (Frequency Division Duplexing)
ODU Power and data signals
The ODU receives its power and the data signals from the IDU through a single coaxial cable. ODU parameters are configured and monitored through the IDU. The DC power, transmit signal, receive signal and some command/control telemetry signals are all combined onto the single coaxial cable. This use of a single cable is designed to reduce cost and time of installation.
ODu Frequency bands and sub-bands
Each ODU is designed to operate over a predefined frequency sub-band. For example 21.2 – 23.6GHz for a 23GHz system, 17.7 – 19.7GHz for a 18GHz system and 24.5 – 26.5GHz for a 26GHz system as for ODUs. The sub-band is set in hardware (filters, diplexer) at time of manufacture and cannot be changed in the field.
1+0, 1+1, 2+0 Deployments
Microwave ODUs can be deployed in various configurations.
The most common is 1+0 which has a single ODU, generally connected directly to the microwave antenna. 1+0 means “unprotected” in that there is no resilience or backup equipment or path.
For resilient networks there are several different configurations. 1+1 in “Hot Standby” is common and typically has a pair of ODUs (one active, one standby) connected via a Microwave Coupler to the antenna. There is typically a 3dB or 6dB loss in the coupler which splits the power either equally or unequally between the main and standby path.
Other resilient configurations are 1+1 SD (Space Diversity, using separate antennas, one ODU on each) and 1+1 FD (Frequency Diversity)
The other non-resilient configuration is 2+0 which has two ODUs connected to a single antenna via a coupler. The hardware configuration is identical to 1+1 FD, but the ODUs carry separate signals to increase the overall capacity.
Grounding & Surge Protection
Suitable ground wire should be connected to the ODU ground lug to an appropriate ground point on the antenna mounting or tower for lightning protection. This grounding is essential to avoid damage due to electrical storms.
In-line Surge Suppressors are used to protect the ODU and IDU from surges that could travel down the cable in the case of extreme surges caused by lightning
The specification of a typical Microwave ODU is shown below.
Typical ODU Features and Specifications:
4-42GHz frequency bands available
Fully synthesized design
3.5-56MHz RF channel bandwidths
Supports QPSK and 16 to 1024 QAM. Some ODUs may support 2048QAM
Standard and high power options
High MTBF, greater than 92.000 hours
Software controlled ODU functions
Designed to meet FCC, ETSI and CE safety and emission standards
Supports popular ITU-R standards and frequency recommendations
Software configurable microcontroller for ODU monitor and control settings
Low noise figure, low phase noise and high linearity
Compact and lightweight design
Very high frequency stability +/-2.5 ppm
Wide operating temperature range: -40°C to +65°C
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5G Mobile networks, Microwave Backhaul and future trends in Mobile Networks
With 5G mobile communication becoming available around 2020, the industry has already started to develop a fairly clear view of the main challenges, opportunities and key technology components it involves. 5G will extend the performance and capabilities of wireless access networks in many dimensions, for example enhancing mobile broadband services to provide data rates beyond 10 Gbps with latencies of 1 ms.
Microwave is a key element of current backhaul networks and will continue to evolve as part of the future 5G ecosystem. An option in 5G is to use the same radio access technology for both the access and the backhaul links, with dynamic sharing of the spectrum resources. This can provide a complement to microwave backhaul especially in very dense deployments with a larger number of small radio nodes.
Today, microwave transmission dominates mobile backhaul, where it connects some 60 percent of all macro base stations. Even as the total number of connections grows, microwave’s share of the market will remain fairly constant. By 2019, it will still account for around 50 percent of all base stations (macro and outdoor small cells (see Figure 3). It will play a key role in last mile access and a complementary role the aggregation part of the network. At the same time, fibre transmission will continue to increase its share of the mobile backhaul market, and by 2019 will connect around 40 percent of all sites. Fibre will be widely used in the aggregation/metro parts of the networks and increasingly for last-mile access. There will also be geographical differences, with densely populated urban areas having higher fibre penetration than less populated suburban and rural areas, where microwave will prevail for both short-haul and long-haul links.
Spectrum efficiency (that is, getting more bits per Hz) can be achieved through techniques like higher-order modulation and adaptive modulation, the superior system gain of a well-designed solution, and Multiple Input, Multiple Output (MIMO).
The maximum number of symbols per second transmitted on a microwave carrier is limited by the channel bandwidth. Quadrature Amplitude Modulation (QAM) increases the potential capacity by coding bits on to each symbol. Moving from two bits per symbol (4 QAM) to 10 bits per symbol (1024 QAM) delivers a more than five-fold capacity increase.
Higher-order modulation levels have been made possible through advances in component technologies that have reduced equipment-generated noise and signal distortion. In the future there will be support for up to 4096 QAM (12 bits per symbol), but we are approaching the theoretical and practical limits. Higher-order modulation means increased sensitivity to noise and signal distortion. The receiver sensitivity is reduced by 3 dB for every increased step in modulation, while the related capacity gain gets smaller (in percentage terms). As an example, the capacity gain is 11 percent when moving from 512 QAM (9 bits per symbol) to 1024 QAM (10 bits per symbol).
Increasing modulation makes the radio more sensitive to propagation anomalies such as rain and multi-path fading. To maintain microwave hop length, the increased sensitivity can be compensated for by higher output power and larger antennas. Adaptive modulation is a very cost-effective solution to maximize throughput in all propagation conditions. In practice, adaptive modulation is a prerequisite for deployment with extreme high-order modulation.
Adaptive modulation enables an existing microwave hop to be upgraded from, for example, 114 Mbps to as much as 500 Mbps. The higher capacity comes with lower availability. For example, availability is reduced from 99.999 percent (5 minutes’ yearly outage) at 114 Mbps to 99.99 percent of the time (50 minutes’ yearly outage) at 238 Mbps. System gain Superior system gain is a key parameter for microwave. A 6 dB higher system gain can be used, for example, to increase two modulation steps with the same availability, which provides up to 30 percent more capacity. Alternatively it could be used to increase the hop length or decrease the antenna size, or a combination of all. Contributors to superior system gain include efficient error correction coding, low receiver noise levels, digital predistortion for higher output power operation, and power-efficient amplifiers, among others.
MIMO Multiple Input, Multiple Output (MIMO)
MIMO is a mature technology that is widely used to increase spectral efficiency in 3GPP and Wi-Fi radio access, where it offers a cost-effective way to boost capacity and throughput where available spectrum is limited. Historically, the spectrum situation for microwave applications has been more relaxed; new frequency bands have been made available and the technology has been continuously developed to meet the capacity requirements. However in many countries the remaining spectrum resources for microwave applications are starting to become depleted and additional technologies are needed to meet future requirements. For 5G Mobile Backhaul, MIMO at microwave frequencies is an emerging technology that offers an effective way to further increase spectrum efficiency and so the available transport capacity.
Unlike ‘conventional’ MIMO systems, which are based on reflections in the environment, for 5G Mobile Backhaul, channels are ‘engineered’ in point-to-point microwave MIMO systems for optimum performance. This is achieved by installing the antennas with a spatial separation that is hop distance-and frequency-dependent. In principle, throughput and capacity increase linearly with the number of antennas (at the expense of additional hardware cost, of course). An NxM MIMO system is constructed using N transmitters and M receivers. Theoretically there is no limit for the N and M values, but since the antennas must be spatially separated there is a practical limitation depending on tower height and surroundings. For this reason 2×2 antennas is the most feasible type of MIMO system. These antennas could either be single polarized (two carrier system) or dual polarized (four carrier system). MIMO will be a useful tool for scaling microwave capacity further, but is still at an early phase where, for example, its regulatory status still needs to be clarified in most countries, and its propagation and planning models still need to be established. The antenna separation can also be challenging especially for lower frequencies and longer hop lengths.
Another section of the microwave capacity toolbox for 5G Mobile Backhaul involves getting access to more spectrum. Here the millimeter-wave bands – the unlicensed 60 GHz bands and the licensed 70/80 GHz band – are growing in popularity as a way of getting access to new spectrum in many markets (see Microwave Frequency Options section for more information). These bands also offer much wider frequency channels, which facilitate deployment of cost-efficient, multi-gigabit systems which enable 5G Mobile Backhaul.
Throughput efficiency (that is, more payload data per bit), involves features like multi-layer header compression and radio link aggregation/bonding, which focus on the behaviour of packet streams.
Multi-layer header compression
Multi-layer header compression removes unnecessary information from the headers of the data frames and releases capacity for traffic purposes, as shown in Figure 7. On compression, each unique header is replaced with a unique identity on the transmitting side, a process which is reversed on the receiving side. Header compression provides relatively higher utilization gain for packets of smaller frame size, since their headers comprise a relatively larger part of the total frame size. This means the resulting extra capacity varies with the number of headers and frame size, but is typically a 5–10 percent gain with Ethernet, IPv4 and WCDMA, with an average frame size of 400–600 bytes, and a 15–20 percent gain with Ethernet, MPLS, IPv6 and LTE with the same average frame size.
These figures assume that the implemented compression can support the total number of unique headers that are transmitted. In addition, the header compression should be robust and very simple to use, for example offering self-learning, minimal configuration and comprehensive performance indicators.
Radio Link Aggregation (RLA, Bonding)
Radio link bonding in microwave is akin to carrier aggregation in LTE and is an important tool to support continued traffic growth, as a higher share of microwave hops are deployed with multiple carriers, as illustrated in Figure 8. Both techniques aggregate multiple radio carriers into one virtual one, so both enhancing the peak capacity as well as increasing the effective throughput through statistical multiplexing gain. Nearly 100 percent efficiency is achieved, since each data packet can use the total aggregated peak capacity with only a minor reduction for protocol overhead, independent of traffic patterns. Radio link bonding is tailored to provide superior performance for the particular microwave transport solution concerned. For example, it may support independent behaviour of each radio carrier using adaptive modulation, as well as graceful degradation in the event of failure of one or more carrier (N+0 protection).
Just like carrier aggregation, radio link bonding will continue to be developed to support higher capacities and more flexible carrier combinations, for example through support for aggregation of more carriers, carriers with different bandwidths and carriers in different frequency bands.
The next section of the capacity toolbox is network optimization. This involves densifying networks without the need for extra frequency channels through interference mitigation features like super high performance (SHP) antennas and automatic transmit power control (ATPC). SHP antennas effectively suppress interference through very low sidelobe radiation patterns, fulfilling ETSI class 4. ATPC enables the transmit power to be automatically reduced during favorable propagation conditions (that is, most of the time), effectively reducing the interference in the network. Using these features reduces the number of frequency channels needed in the network and could deliver up to 70 percent more total network capacity per channel. Interference due to misalignment or dense deployment is limiting backhaul build-out in many networks. Careful network planning, advanced antennas, signal processing and the use of ATPC features at a network level will reduce the impact from interference.
Looking to the future, 5G and Beyond
Over the coming years, microwave capacity tools for 5G Mobile Networks will be evolved and enhanced, and used in combination enabling capacities of 10 Gbps and beyond. Total cost of ownership will be optimized for common high-capacity configurations, such as multi-carrier solutions.
Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow or ice, and losses which are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn. Rain fade is not limited to satellite uplinks or downlinks, it also can affect terrestrial point to point microwave links (those on the earth’s surface).
Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, receiving antennas larger (i.e. higher gain) than the required size for normal weather conditions, and hydrophobic coatings.
Two models are generally used for Rain modelling: Crane and ITU. The ITU model is generally preferred by microwave planners. A global map of Rain distribution according to the ITU model is shown below:
Used in conjunction with appropriate planning tools, this data can be used to predict the expected Operational Availability (in %) of a microwave link. Useful Operational Availability figures typically vary from 99.9% (“three nines”) to 99.999% (“five nines”), and are a function of the overall link budget including frequency band, antenna sizes, modulation, receiver sensitivity and other factors.
Another useful Rain Fade map is shown here, showing the 0.01% annual rainfall exceedance rate:
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To deliver a compelling quality of experience for subscribers, you must respond quickly to growing traffic demands. Modern Packet Microwave Mobile Backhaul products help you maximize the network’s performance by enabling rapid deployment of scalable backhaul to cell sites. Modern solutions include a portfolio of microwave products to address the backhaul needs of 2G, 3G, and LTE macro cells and 3G, LTE, and Wi-Fi® small cells. Radio spectrum is maximized using innovative techniques to maximize payload capacity to support the evolution to LTE and heterogeneous networks. Unique, common radio support for indoor and outdoor deployments enhances savings potential.
Packet Microwave Mobile Backhaul is a key component in a modern end-to-end mobile backhaul solution, which provides the flexibility, scale and operational simplicity to lower the total cost of ownership and simultaneously enhance the mobile service experience.
Rapidly support the optimal cell site location
Complete backhaul portfolio for macro cells and small cells
Support for all sites including both end and intermediate cell sites
Space and power efficiency
Full outdoor option to meet different microwave site space requirements
Achieve maximum spectral performance
Maximum bandwidth per band
Advanced quality of service levels supporting subscriber quality of experience
Scale the network cost effectively
Reliably bond radio channels to create larger microwave links
Any topology, any number of microwave link directions
Network awareness for both Carrier Ethernet and/or IP/MPLS networks
Be operationally efficient
Common radio for all cell sites
Evolutionary path from hybrid microwave to packet microwave at the touch of a button
Management beyond basic IP partner integration
Deployment, management, end-user benefits
Grow and retain subscribers by maximizing the mobile experience
Infrastructure support for increased subscriber bandwidth demands
Ability to react quickly to subscriber demand with optimally-located cell sites
Increased capacity that supports high bandwidth data applications
Packet Microwave Mobile Backhaul integrates a modern microwave portfolio with small cell optimized products to provide a complete backhaul offering for small cells and/or macro cells.
Read on in our following pages to find out more about technologies used in mobile backhaul applications