FIXED WIRELESS APPLICATIONS

Point-to-Point Microwave

What is Point-to-Point Microwave?

Point-to-point microwave, sometimes called microwave relay, has been around since the mid 1940s. Of course, back then, microwave relay was analog, meaning it used analog modulation to combine the information signal and the RF carrier. What makes point-to-point microwaves new is the use of digital modulation and the new ways in which it is being used.

Point-to-point microwave, as the name implies, is used to communicate, wirelessly, between a single transmitter and a single receiver, both owned by the same entity. This point is important, because when someone is given the right (by the FCC) to communicate using point-to-point microwaves at a certain frequency and in a certain geographical location, everyone else is prohibited from using that frequency in that location. In this respect, it is similar to broadcasting. As you will soon see, there are point-to-point wireless applications where this limitation does not hold.

The tremendous advantage of point-to-point microwave is the user's ability to get information from one point to another without owning the underlying real estate.

Uses for Point-to-Point Microwave

The FCC has allocated many frequency bands for this application, and for many different uses, but by far there are three uses which dominate all others. The first of these uses is referred to as private operational fixed microwave. Most often, the owner of the transmitter and receiver is a private company. In these cases, the wireless systems are used to control (an unattended piece of equipment), to monitor (temperature, pressure, voltage, etc.), or to relay (voice, data, fax, etc.). These systems are especially useful along right-of-ways like highways and railroads.

A second use for point-to-point is relaying large volumes of voice traffic called common carrier microwave. This is one of the ways in which long distance telephone companies get calls from here to there. As will be discussed in the next section, this application is also used by mobile telephony providers to get the phone call from the basestation back to the home office. Common carrier microwave is often used when the terrain over which the signal must travel is severe and laying copper wire (or optical fibers) is impractical.

The third main use for point-to-point microwave is video relay and is referred to as broadcast auxiliary microwave. This is the way in which mobile TV news vans get their signals back to the station. It is also the way the television stations get their signals up to their broadcast antennas, as previously discussed and detailed in Figure 6-4.

Point-to-Point Operations

As mentioned above, microwave relay is used in places where laying copper or optical fibers makes no sense or is impossible, but there is a limit. Our old friend, line-of-sight, ensures that microwave relays are spaced no more than 25 miles apart, which means if a signal is to be transmitted from Los Angeles to San Diego (about 120 miles) using point-to-point microwave communications, at least four microwave relay stations are required. A microwave relay tower with directional antennas is shown in Figure 7-1.

The first thing to notice in Figure 7-1 is that microwave relays use dish antennas (which are covered in this picture for environmental reasons). Recall that dish antennas are highly directional (their antenna pattern has a narrow beamwidth). This makes sense as the microwave relay wants as much RF energy as possible going in only one direction (toward the next receiver). In some instances, microwave relays use horn antennas (shaped like square funnels) to cover wider bandwidths. Most point-to-point communication takes place at high enough frequencies to allow the use of small dish antennas.

As much as the RF system designers desire that all of the RF energy go from one relay to the next, it doesn't. Instead, as the RF energy leaves one relay, the energy spreads out, like water coming out of a hose. Some of the RF energy goes directly to the next relay, some goes off into space, and some bounces off the ground. If that were the end of the story, everything would be fine. Unfortunately, some of the RF energy which bounces off of the ground also makes its way to the next relay, as shown in Figure 7-2. Since the reflected signal has further to travel, it arrives at the next relay later than the direct signal. This situation is called multipath, and it is very similar to the situation in broadcasting in which signals reflect off buildings and cause ghost images in television reception. As a result, sophisticated techniques have been developed to eliminate this problem.

Most of today's high volume point-to-point microwave communications, primarily long distance voice traffic, use digital modulation. The reasons are twofold. First, the RF electronics have become sophisticated enough to implement digital microwave communication. And second, digital modulation allows for cramming more information (more simultaneous telephone conversations) into a given bandwidth. Microwave point-to-point frequency allocations run as low as a few megahertz up to and beyond 38 GHz.

Wireless Local Loop

What is the Local Loop?

In the United States, when you pick up your telephone to make a call, you hear a dial tone, which means you are connected to what is referred to as a Class 5 switching center, also called the end office. You are now connected to your local telephone company. The electrical circuit between you, the end office, and back again is affectionately referred to as the local loop. Whether you know it or not, today (circa 1999) there is a battle being waged over control of the local loop.

Basically, there are only four ways to reach the local telephone company's end office (i.e., there are only four ways to implement the local loop.): copper wire, coaxial cable, fiber optic cable, and wireless communication.

The copper is owned by the local phone company and they try to cram as much information down that poor little piece of wire as is technologically possible. The coaxial cable is owned by the local cable company (if you are lucky enough to have one). They too are trying to cram as much information as is technologically possible down the cable. Today, running fiber optic cable to individual homes is considered infeasible. And then there is wireless.

Why Wireless Local Loop?

In the battle for local loop supremacy, wireless local loop (WLL) is proving to be a very formidable competitor. Wireless local loop technology is simultaneously addressing two very different markets. One market which WLL is addressing is local telephone service in developing nations who do not yet have it, like China and Vietnam. In places such as these, wireless technology is much less expensive and far faster to deploy than laying copper wires in the ground. All of the major wireless infrastructure manufacturers are aggressively pursuing WLL business in developing countries.

The other market being addressed by WLL is in the United States. Why would anyone need WLL in a country where almost everyone has hard-wired local telephone service? Two reasons. First, long distance telephone companies want more than anything else to be local telephone companies too. And they can be, immediately, if they want, but they have to use the local telephone company's equipment and pay them a fee, which sort of defeats the purpose. Wireless local loop technology allows long distance telephone companies to deploy telephone equipment, at a fraction of the cost of laying copper wire, and compete directly with the local telephone companies for local phone business. And with the growth of the Internet, an increasing amount of phone time is considered "local." WLL will some day soon offer most people an alternative to local phone service, probably from their long distance provider.

The other reason for WLL in the United States is increased bandwidth. Everyone wants high speed access to the Internet, and high speed means more bandwidth. There is only so much information that can be stuffed down a copper wire or a coaxial cable, and if more bandwidth is desired than either of them can supply, it must come from somewhere else. And until someone figures out how to lay fiber optic cable inexpensively, the only other solution is wireless, and more specifically, WLL.

Wireless Local Loop Technology

Wireless local loop systems, as envisioned, will generally divide a geographic region into a number of similar sized cells (like cellular telephony). Each cell will be serviced by a basestation, which will communicate with all of the wireless local loop customers within the cell. The basestation may be as simple as a small omnidirectional antenna and control box hanging from the overhead electrical lines. Each customer will be equipped with a transceiver and a small patch antenna. The transceiver may have several outputs: one for a telephone, one for a modem, and maybe even one for a television. The small antenna, which may be inside or outside, will be positioned to communicate with the basestation.

What equipment and frequencies will WLL use? It depends. If the system is to provide basic local telephone service to a developing nation, then the bandwidth requirements will be modest, and almost any infrastructure at any frequency will do. The major wireless infrastructure manufacturers, who provide the systems for mobile telephony, are naturally trying to use that same equipment for WLL to avoid having to develop anything new. Most of the WLL in underdeveloped countries will ultimately be at the same frequencies as the mobile telephony in developed countries and will utilize the same basic equipment, only slimmed down to provide fixed service only.

The Perfect WLL Solution—LMDS

If the WLL system is intended to provide broadband connectivity in the United States, then the bandwidth requirements are more substantial and just any old frequency allocation will not do. However, the FCC has allocated a special frequency band which is tailor-made for WLL called local multipoint distribution service, or LMDS, that resides between 27 and 31 GHz.

The LMDS frequency band was not specifically allocated for WLL. The owner of the frequency band (in a given geographic area) can use it for almost any application. However, there is something very special about the A Block frequency allocation. (LMDS is divided into two different frequency bands: A Block and B Block.) The A Block frequency allocation for LMDS is 1150 MHz wide, which makes it the widest allocated frequency band (by the FCC) in the history of the United States. And since frequency is like real estate (they ain't makin' any more of it), the LMDS A Block is one of the most valuable commodities in the wireless world today.

To get an appreciation for how substantial 1150 MHz of wireless spectrum is, a telephone conversation requires 4 kHz of bandwidth (uncompressed), which means the LMDS A Block can simultaneously transmit over a quarter of a million phone calls. Now you know why most owners are opting to use it for WLL. (Do you think the major long distance carriers might be interested in LMDS?)

Just so you don't think that LMDS is a perfect solution for WLL, it is not without its hurdles to overcome. First, RF electronics at 31 GHz is still relatively expensive, both on the system side and the consumer side. Second, if you recall something from a previous chapter called absorption, you will remember that it limits the distance an RF wave can travel and that the higher the frequency, the worse it gets. Well, at 31 GHz (LMDS's frequency), it is pretty bad. Signal attenuation from absorption (at 31 GHz) requires the receiver to be relatively close to the basestation's transmitter to make use of the LMDS band. That means in a given geographical area there needs to be a lot of basestations for the system to work properly, and a lot of basestations translates to an expensive system. And I won't even mention what happens to a 31 GHz signal during a torrential downpour.

Before you get depressed and give up on LMDS, the systems will begin to appear soon and most will be deployed for wireless local loop. Unfortunately, it will probably not be available at your home anytime soon, because the owners of these expensive LMDS systems actually intend to get a return on their investment. (Can you believe it?) What that means is that the first LMDS systems to appear will most likely be for small businesses (with big wallets), with basestations located near business parks which provide telephone and high speed Internet access. This will serve as the first implementation, in the United States, of wireless local loop.

A New Technology—Spread Spectrum

ISM—Unlicensed Spread Spectrum

The FCC has allocated several frequency bands to applications which are collectively known as industrial, scientific, and medial, or ISM. ISM applications include everything from industrial heating equipment to microwave ovens. What ISM does not include—or it did not include until recently—is wireless communications. The ISM bands were originally intended to allow various electrical and mechanical equipment to radiate unintentional RF energy (at specified frequencies), without interfering with other wireless communication applications. As long as the industrial applications and the wireless applications are operating in their own frequency bands, they don't interfere with each other.

Surprisingly enough, the ISM bands now provide an opportunity for wireless communications. If nothing else, wireless communications operating in an ISM band certainly won't have to worry that their signals will interfere with existing applications. (Microwave ovens don't mind listening in on wireless conversations.) The only problem is, how is it possible to communicate wirelessly in a frequency band with so much unintentional RF energy being radiated? The answer is something called spread spectrum.

Spread spectrum is a technique which allows RF circuitry to distinguish one signal from another when both are operating at the exact same carrier frequency and in the same geographical location. After the development of spread spectrum technology, the FCC recognized an opportunity to make more spectrum available by opening up the ISM bands to wireless communications. And because the FCC is so generous, they decided that as long as their rules are obeyed, no license is needed to operate a wireless system. Operating a wireless system in an ISM band, while transmitting less than one watt, is referred to as unlicensed spread spectrum, which is a very unique wireless application. It is one of the only (if not the only) fixed, point-to-point wireless applications which has different users sharing the same frequency in the same geographical location. Spread spectrum technology is what allows each party to distinguish their signals from the other parties'. Table 7-1 shows the three most popular ISM frequency bands.

Table 7-1. ISM Frequency Bands

Frequency Band	Frequency Allocation
UHF	902-928 MHz
S-band	2.40–2.50 GHz
C-band	5.725–5.875 GHz

Spread Spectrum—Theory of Operation

Recall the analogy of wireless communications being like mailing a letter. In this analogy, the letter is the information signal and the envelope is the RF carrier signal. Modulation is used to combine the letter (information) and the envelope (the carrier). In the previous discussion of point-to-point wireless communications, I assumed that only one party could transmit and receive at a given frequency within a given geographical location. In that version of the analogy, there was no need to address the envelope, because there was only one party who could receive it. (Maybe there was only one other house in the neighborhood.) This is not the case with spread spectrum. With spread spectrum, many parties can transmit and receive at a given frequency within a given geographical location. (There are a lot of houses in the neighborhood and they can all mail letters to each other.) How does spread spectrum ensure that the "envelope" finds its correct destination? It uses an "address."

Spread spectrum is analogous to imprinting an address onto the wireless signal. How does spread spectrum pull off this little magic trick? It modulates the signal again. There are two flavors of spread spectrum. One is called direct sequence spread spectrum or DSSS, and the other one is called frequency hopping spread spectrum or FHSS.

In DSSS, the spread spectrum modulation takes place while the original information signal is still in digital form. (By the way, did I neglect to mention that spread spectrum only works with a digital information signal?) In this case, the party transmitting the signal has a special code, which is nothing more than another digital signal. This (digital) code is used to modulate the (digital) information signal. (Think of it as multiplying the information signal by a secret number.) The trick to using spread spectrum is that the receiving party has the complimentary code. When they receive the signal, they multiply it by their own (complimentary) secret number. What results is the same information signal which was originally transmitted. There are a lot of other parties that receive the same signal, but they all have a different code (i.e., they multiply by a different secret number.) What they receive is undecipherable and therefore ignored.

FSSS is similar to DSSS, except in this case the transmitted carrier frequency is instantaneously and continuously changed according to the special code. Since the receiver also has the same code, it knows what frequencies to look for when receiving. All the other receivers, which have a different code and are therefore looking for completely different frequencies, do not "see" the transmitted signal.

Spread spectrum gets its name from the fact that modulating the signal with the special code "spreads" the bandwidth of the signal over a wider bandwidth. For instance, the human voice requires 4 kHz of bandwidth. After it is modulated with the special code, however, it might cover 4 MHz. The same voice information is still there; it just covers a wider frequency range. After the process is reversed, the signal will return to its original 4 kHz bandwidth.

ISM Spread Spectrum Applications

Because no license is required, spread spectrum in the ISM bands is finding all kinds of applications. One of the most common is cordless telephones. The latest generation of digital cordless phones utilizes spread spectrum in the 900 MHz and 2.4 GHz bands. The great (theoretical) advantage is the lack of interference from other forms of RF radiation. (The phone completely ignores the microwave oven.)

There is at least one company using the ISM band for point-to-point communication. In this case, they put two highly directional antennas (facing each other) on top of towers or street lights, separated by some distance, and use the signal to transmit information. Because the power is limited to one watt, the signal cannot travel as far as licensed point-to-point communications, so this application is limited to relatively short distances.

Another growing application for the ISM band is wireless local area networks or WLAN. (A local area network or LAN is a group of computers in the same location which communicate with each other.) This strategy involves using an omnidirectional antenna, at the server, to cover the entire area of the local area network. In this way, all the computers within the WLAN are assigned their own special code to communicate directly with the server. This application is ideal as the one watt output limitation is sufficient to cover the area of most local area networks.

Finally, the ISM band is being used for wireless local loop (WLL), but not in the way you might think. It is not being used to deliver high bandwidth or even local telephone service to the home. Instead, it is being used by utility companies to read gas, electric, and water meters. In these systems, a basestation transceiver is located somewhere in the neighborhood (maybe on a telephone pole) and is used to periodically communicate with a wireless transceiver installed in the home's meters. If you do not have this feature installed in your home at present, just wait, you will.

Other Applications

FCC Allocations

The FCC continues to allocate (and auction off) bandwidth for wireless applications. Some of these allocations are so new that the auctions are still being conducted (circa 1999) and will continue to be conducted into the foreseeable future. In many instances, the FCC has remained flexible as to the end use of the actual bandwidth itself. Table 7-2 contains a sampling of some of the newer wireless allocations (and applications). Strictly speaking, these are not all fixed wireless applications. In most cases, there is flexibility to provide either fixed or mobile wireless service.

Table 7-2. Some Newer Wireless Applications

Acronym	Wireless Service	Frequency Band	Possible Uses
GWCS	General Wireless Communication Service	4660–4685 MHz	Fixed or mobile communications.
LMS	Location and Monitoring Service	904–927 MHz	To determine the location of mobile radio units.
MMDS	Multichannel Multipoint Communication Service	2596–2690 MHz	Broadcasting.
WCS	Wireless Communication Services	2345 –2360 MHz	Fixed, mobile, radio location, or satellite communications.

One of the implications of this flexibility in end use for these new services is that there may not be a consensus, across the entire country, as to what is implemented. This might severely limit the utility of these new allocations. One of the reasons that cellular phones are so useful is that the entire country uses the same frequency bands for cellular communications. This means that the same cellular phone which works in Los Angeles will also work in Iowa. (Which is pretty useful, if you have to be in Iowa.) This same utility will not exist if the new wireless service providers all decide to implement something different in their geographical location.

If you build it, they will come, does not necessarily apply to wireless applications. Just because somebody has won the right to provide a wireless service does not mean that anybody really wants it (or is willing to pay for it). A prime example is MMDS (see Table 7-2). MMDS was originally slated for wireless cable TV; the idea being to provide the same programming as the local cable provider, only wirelessly. The service provider transmits the programming in the MMDS band from a tower situated at the highest point in the area. To use the service, the consumer would only need to have a small antenna and a TV set top box, and of course pay the monthly fee. The hope was to foster competition to the cable providers, resulting in lower prices for the consumer.

A funny thing happened on the way to wireless (MMDS) cable—nobody (in the U.S.) wanted it. The bad news, for those trying to deploy MMDS, was that right around the time MMDS was coming into being, direct broadcast satellite began to take off and those consumers who were motivated to change, typically went that route. So presently in the United States sits 200 MHz of unused spectrum which somebody is trying to find a use for. The good news for MMDS is that the United States is only part of the equation. In developing countries (those without direct broadcast satellite), MMDS has found a receptive audience.