Microwave

Microwave

Microwave was used during World War II in military applications, and because it was successful in that environment, it was introduced into commercial communications. Microwave was deployed in the PSTN as a replacement for coaxial cable in the late 1940s.

As mentioned earlier, twisted-pair and coax both face limitations because of the frequency spectrum and the manner in which they are deployed. But microwave promises to have a much brighter future than twisted-pair on coax. Many locations cannot be cost-effectively cabled by using wires (for example, the Sahara, the Amazon, places where buildings are on mountaintops, and villages separated by valleys), and this is where microwave can shine.

Characteristics of Microwave

Microwave is defined as falling in the 1GHz to 100GHz frequency band. But systems today do not operate across this full range of frequencies. In fact, current microwave systems largely operate up to the 50GHz range. At the 60GHz level, we encounter the oxygen layer, where the microwave is absorbed by the surrounding oxygen. At this point we haven't developed a cost-effective approach to building radios that operate in those higher-frequency bands, but that is very much the intention and the direction wireless communications will take in the future. In fact, when we look forward to exercising more spectrum in order to deliver things such as interactive video to a palmtop, we'll be looking at techniques that use those higher portions of the frequency band, the 60GHz to 100GHz part of the spectrum.

The amount of bandwidth that you can realize out of the very large microwave spectrum is often limited by regulations as much as by technology. Before you can deploy a microwave system, you have to be licensed to operate that system, in all environments except your own private campus. In your own private territory, you can use unlicensed bands, but if you want to cross the public domain using licensed spectrum, you must first be granted approval by your spectrum management agency to operate within a given frequency allocation.

Some communities are very concerned about the potential health hazards of microwave and create legislation or council laws that prohibit placement of such systems. Some communities are very sensitive to the unsightliness of towers and argue that the value of real estate will drop if they are constructed. Therefore, several companies specialize in building camouflaged towers. When you see a tall tree, a church steeple, a light post, or a chimney, it could be a wireless tower disguised to protect your aesthetic balance.

Microwave is generally allocated in chunks of 30MHz to 45MHz channels, so it makes available a substantial amount of bandwidth to end users and operators of telecommunications networks.

Microwave is subject to the uncertainties of the physical environment. Metals in the area, precipitation, fog, rainfall, and a number of other factors can cause reflections, and therefore degradations and echoes. The higher (in elevation) we move away from land-based systems, the better the performance will be because there will be less intrusion from other land-based systems, such as television, radio, and police and military systems.

Repeater spacing with microwave varies depending on the frequency you are transmitting. Remember from Chapter 2 that lower frequencies can travel farther than higher frequencies before they attenuate. Higher frequencies lose power more rapidly. So, in microwave systems that operate in the 2GHz, 4GHz, and 6GHz bands, towers can be separated by 45 miles (72 kilometers). When you get into the higher frequency allocations, such as 18GHz, 23GHz, and 45GHz, the spacing needs to be much shorter, in the range of 1 to 5 miles (1.6 to 8 kilometers). This is an important issue in network design and, depending on the scope over which you want to deploy these facilities, it can have a significant impact on the investment needed.

Another important design criterion is that microwave requires line of sight and is a highly directional beam. Microwave requires a clear, unobstructed view, and it can't move through any obstacles, even things you wouldn't think would be obstacles, such as leaves on a tree. Technologies that depend on line of sight may work brilliantly where you have the appropriate terrain and climate, and they may not perform very well where you have many obstacles or much precipitation. Furthermore, line of sight is restricted by the curvature of the earth, and the curvature of the earth causes you to lose line of sight at about 90 miles (144 kilometers).

The impact of precipitation on microwave can be great. Microwave beams are small, and as you go up into the higher bands, the wave forms get smaller and smaller. Pretty soon they're smaller than a raindrop, and they can be absorbed by a raindrop and then scattered in a million directions. Therefore, in wet atmospheric conditions, there is a great potential for problems with microwave. As a result, practicing network diversity using both terrestrial and nonterrestrial alternatives is critical.

Applications of Microwave

One application associated with microwave is to replace the use of leased lines in a private network. Figure 3.3 shows a simple voice environment that initially made use of dedicated leased lines, also known as tie trunks, to link together two PBXs in two different buildings across town from one another. Because these tie trunks were billed on a monthly basis and they were mileage sensitive, they were going to be a cost factor forever. Instead, a digital microwave system could be purchased to replace the tie trunks. This system would provide capacity between the buildings and do away with the monthly cost associated with the leased lines. This setup is commonly used by multinode or multilocation customers (for example, a health care facility with clinics and hospitals scattered throughout a state or territory, a university with multiple campuses, a retail location with multiple branches, or a bank with multiple branches).

Figure 3.3. Connecting multiple PBXs with microwave

Another key application of microwave is bypassing, which can be interpreted in multiple ways. Initially and this is really how microwave came into the life of the end user this technique was used to bypass the local telecommunications company. With the introduction of competition in the long-distance marketplace, end users in the United States initially had choices about who would be their primary long-distance carrier (that is, interexchange carrier). But to get to that carrier to transport the long-distance portion of the call, we still needed to get special local access trunks that led through the local operator to the competing interexchange provider. That meant paying an additional monthly fee for these local access trunks, and in an attempt to bypass those additional costs, businesses began to bypass the local telephone company by simply putting up a digital microwave system a microwave tower with a shot directly to the interexchange carrier's point of presence.

Bypassing can also be used to circumvent construction. Say that a pharmaceutical company on a large campus has a public thoroughfare, and across the street there's a lovely park where the employees take their lunch and otherwise relax during the day. No one foresaw the fame and fortune the company would achieve with its latest migraine medicine, so it had not planned to build another facility to house the 300 people it now needed to add. Nobody ever provisioned conduit leading to that park across the street. The cost and time to get permission to break ground, lay conduit, pull cable, repave, and relandscape would be cost-prohibitive and time-prohibitive. To bypass that entire operation, microwave could be used between the main campus and the remote park (see Figure 3.4 ). This is essentially the same strategy that wireless local loop is pursuing. Rather than take the time and money to build out a wireline facility, you can do it much more rapidly and much more cost-effectively on a wireless basis. For instance, provisioning a twisted-pair or coaxial cable costs roughly US$1,200 to US$1,500 per subscriber and requires a 12- to 18-month deployment time. Wireless costs US$700 to US$800 per subscriber and requires 3 to 6 months of deployment time. There could always be something that delays the process (for example, contractual problems with the building developer), but, generally, you can deploy a microwave system much more rapidly at a much lower price point. Therefore, these systems are very popular in parts of the world where there is not already a local loop infrastructure.

Figure 3.4. A bypassing construction that uses microwave

Another application for microwave is in the data realm. Say that in your company the buildings that have telephone systems today are going to have LANs as well, and you want to unite the disparate LANs to create a virtual whole. You can use microwave technology as a bridge between two different LANs, giving it the appearance of being one LAN (see Figure 3.5). The main thing that inhibits or potentially slows down the growth of microwave is that only so many people can be operating on the same frequencies in the same area. Therefore, a big limitation of microwave is potential congestion in key metropolitan areas.

Figure 3.5. A LAN interconnect using microwave

Microwave has a disaster-recovery application as well. Because microwave is relatively inexpensive and quick to deploy, it is a good candidate for use after a disaster damages wireline media, systems, or structures.

Advantages and Disadvantages of Microwave

The advantages of microwave are as follows:

         Cost savings Using microwave is less expensive than using leased lines.

         Portability and reconfiguration flexibility You can pick up microwave and carry it to a new building. You can't do that with cables.

         Substantial bandwidth A substantial amount of microwave bandwidth is allocated, so high-speed data, video, and multimedia can be supported.

The the main disadvantages of microwave are as follows:

         Line-of-sight requirement You need to ensure that there are no obstacles between towers.

         Susceptibility to environmentally caused distortions Because the environment (for example, heavy rainstorms) can cause distortion, you need to have backups.

         Regulatory licensing requirement The requirement for regulatory licensing means that you must have time and flexibility to deal with the spectrum agency.

         Potential environmental restrictions Some communities do not allow microwave towers or require that they be camouflaged.

Emerging Applications and Developments in Microwave

A number of microwave applications are emerging. Wireless local loop is one application that is already being used to speed deployment and reduce the cost of bringing in subscriber access. There are also two techniques for supplying broadband access via point-to-point microwave Multichannel Multipoint Distribution Service (MMDS) and Local Multipoint Distribution Service (LMDS) which are introduced here and discussed in more detail in Chapter 13.

MMDS

MMDS essentially started as a digital TV system, and because it is digital, it provides great capacity it enables more than 150 channels. This system operates in the 2GHz to 3GHz band, so it can cover a fairly large area (approximately 30 miles [48 kilometers]). But, again, if the terrain is hostile or environmental conditions are not favorable, the performance you experience may not be comparable to what you would see with wireline approaches. And, of course, the capacity is being shared among all the users receiving service via a given base station. At the moment MMDS has fallen out of favor with most carriers, but remember that its usability depends on the prevailing conditions. With developments in smart antenna design and nonline-of-sight systems, many of MMDS's shortcomings will be addressed in the future (see Chapter 13).

LMDS

LMDS is also referred to as Multipoint Video Distribution service in Europe. It operates over a very large frequency allocation, a 1.3GHz band that's generally located somewhere in the range of 28GHz to 45GHz, depending on the country you're in (some systems in the United Kingdom operate in the 10GHz range). Because it operates at high frequencies, it attenuates rather rapidly compared to MMDS, and it operates over a much smaller area, a microcell, which is 0.5 to 3 miles (1 to 5 kilometers) in range. LMDS is a very popular technique for delivering wireless local loop, particularly in Asia Pacific, Africa, Latin America, and parts of eastern Europe.