MOBILE TELEPHONY

Mobile Telephony Choices

Different Technologies

There are several choices for mobile telephone service in the United States. Some of these services differentiate themselves by offering additional features compared to "standard" mobile service, while others are the natural evolution of technology over time, taking advantage of newer digital technology.

One of the lesser known mobile services available is something called Specialized Mobile Radio or SMR. SMR, which operates in two different frequency bands between 806 and 866 MHz, was originally intended for use as a wireless dispatch service (think taxi cabs). Today, it has evolved into a combination dispatch and mobile phone service. This combination service distinguishes SMR from all the other mobile phone services available. Not only can the service be used to make "ordinary" mobile calls in the interconnected mode, it can also be used to conduct wireless teleconferencing in dispatch mode. In this mode, several people using the service can hold a conversation simultaneously. As such, SMR is popular with teams of mobile salespeople who need to conduct spontaneous sales conferences.

The mobile phone service which most people in the U.S. are familiar with, and the one most often referred to as cellular, is something called Advanced Mobile Phone Service or AMPS. AMPS is an analog technology, using frequency modulation, and it is often referred to as first generation cellular technology. AMPS operates in two bands between 824 and 894 MHz. The reason for the two bands is that two different service providers are able to offer cellular service in a given area. The FCC allocated two bands to foster competition, resulting in better pricing for consumers.

The best feature of first generation cellular service is that every service provider throughout the United States uses (or used) the same modulation scheme. This uniformity has led to the concept of roaming (using a cellular phone outside its home area), in which one cellular phone can be used everywhere. The bad news is that it used analog technology, which you should know by now cannot handle the same capacity of phone calls as digital technology and, as such, the analog systems quickly ran out of capacity (think busy signals).

This capacity problem caused service providers to update their systems to the newer digital technology, which provides much more capacity (think profits). These new digital cellular systems are referred to as second generation cellular technology. The good news with the digital technology is that you are much more likely to be able to complete a call in high usage areas. Unfortunately, these upgraded systems led to some unforeseen problems.

The older analog systems did not get upgraded to digital systems instantly. For some period of time, most service providers had (or have) a hybrid system: part analog and part digital. This led to the creation of the dual mode phone. These (expensive) dual mode phones can communicate in both analog and digital (and switch between the two). Of course, to take advantage of the new technology, somebody (you) has to go out and buy a new mobile phone.

By far the biggest problem with upgrading to digital technology stems from the fact that there is more than one digital modulation scheme to choose from. Many of the service providers chose different technologies, for different reasons. You may have already guessed the problem. The roaming feature which is so universal in analog cellular systems is not quite so universal in digital cellular systems. As you will soon learn, there are really only two digital technologies vying for supremacy in the United States. As a result, as long as the two competitors in a given area use these two different technologies, you are assured that your digital phone will work (with at least one of them) in that area while roaming.

Another second generation mobile phone service is called Personal Communications Services or PCS. PCS, or more specifically wideband PCS, is nothing more than second generation (digital) cellular technology at a slightly higher frequency. PCS operates in six bands between 1850 and 1980 MHz. The FCC allocated six different bands to spur competition and to offer entrepreneurs an opportunity to play in the mobile telephone game.

If you have been paying attention, you have observed that in most areas in the United States there are (or will be) 10 different choices for mobile telephone service (2 SMR, 2 cellular, and 6 PCS), not counting satellite-based mobile telephony. The question everyone is asking—or should be asking—is, can the market support 10 different mobile telephone service providers? Who will survive? Stay tuned.

A World of Choices

Just so you do not get the wrong idea, the United States is far from being the only place with mobile telephony. Table 7-3 shows some of the world's major mobile telephone systems. The first thing to notice is that there are a lot of different digital technologies vying for international supremacy. The way things stand today, digital phones which work in the United States will not work anywhere else and vice versa, which can be a problem for those traveling internationally. The good news is that there may be a solution right around the corner.

Table 7-3. Worldwide Mobile Telephone Systems

Acronym	System	Where First Deployed	Technology
AMPS	Advanced Mobile Telephone Service	United States	Analog
CDMA	Code Division Multiple Access	United States	Digital
D-AMPS	Digital Advanced Mobile Telephone Service	United States	Digital
DCS1800	Digital Communication Service	Germany & England	Digital
GSM	Group Special Mobile	80 European countries	Digital
JTACS	Japan Total Access Communications System	Japan	Analog
NADC	North American Digital Cellular	United States	Digital
NMT	Nordic Mobile Telephone	Scandinavian countries	Analog
PCS1900	Personal Communications Services	United States	Digital
PDC	Personal Digital Cellular	Japan	Digital
SMR	Specialized Mobile Radio	United States	Both
TACS	Total Access Communications System	England	Analog

The Next Generation

Somebody got the idea that it would be really nice if there were a single digital technology deployed worldwide which allowed the use of a single mobile phone anywhere in the world (which has service). In steps the International Telecommunications Union ITU. The ITU is the FCC of the world, with responsibility for allocating "international" frequencies. The ITU, along with all the member nations, have started a program called IMT-2000 (International Mobile Telephone). The goal of the program is to develop a single, digital standard which will work all over the world. IMT-2000, which will offer true international mobility, is affectionately referred to as the third generation cellular, or 3G, technology and is situated between 1885 and 2200 MHz.

The two digital technologies which have emerged as the prominent contenders for IMT-2000 are CDMA and GSM (see Table 7-3). The good news is that the two companies backing these technologies (Qualcomm in the United States for CDMA and Ericsson in Sweden for GSM) have begun to cooperate and work together to fulfill the ITU's vision of international mobile telephony. Stay tuned.

The Cellular Concept

Topology

The United States is broken down into a multitude of geographical regions in which the various forms of mobile telephony are authorized. Within the cellular allocation, the United States is broken down into metropolitan statistical areas or MSAs (think city) and rural statistical areas or RSAs (think country). There are two—or, in the case of PCS, six—service providers authorized to provide mobile telephony in each of these areas. The service providers distinguish themselves by being allotted different frequency sub-bands within the overall cellular frequency allotment. Within their assigned geographical region, each service provider breaks up their area into smaller segments called cells.

Each of these cells has an antenna (or antennas) at the center of the cell which projects an antenna pattern, or footprint, covering the entire cell. These antenna patterns provide transmitting and receiving coverage for users within it. Because of the nature of RF behavior, these antenna footprints are circular in shape. However, when RF engineers display a cell pattern on a map, they ordinarily use hexagons to describe the antenna footprints. It is not that hexagons more accurately reflect the antenna patterns, it is that hexagons fit together very nicely into an orderly pattern (see Figure 7-3).

In the world of mobile telephony, there is one major tradeoff constantly taking place. Ideally, the system has a large number of very small hexagons. As you will soon learn, the greater the number of hexagons, the more simultaneous calls the system can handle (think revenue). However, the larger the number of hexagons, the greater the infrastructure required to implement the system (think expenses). As a result, cell coverage is a dynamic activity which is constantly changing in response to increases in capacity requirements.

Infrastructure

At the center of every cell is a cell site or basestation. The cell site contains all of the electronics which enable wireless communication, including all of the RF hardware. At a minimum, cell sites consist of one or more antennas, cables, a transmitter and receiver, a power source, and other control electronics. If the capacity requirements of the cell are small, the cell may employ a single omnidirectional antenna to provide coverage. In situations where more capacity is required, the cell is broken down into three sectors (120 degrees each) and one or more antennas are used to provide coverage for each sector. This is the familiar triangular-top tower often seen by the side of the road and shown previously in Figure 3-5.

At their very simplest, all cell sites provide three functions. Cell sites talk to each other (think mobile to mobile calls), they connect to the public switched telephone network or PSTN (think mobile to landline calls), and they count how many minutes you talk (think money). All three of these functions take place at something called a mobile switching center or MSC, also referred to as a mobile telephone switching office or MTSO.

The MSC is the quarterback for a cellular system. It acts as a hub through which all cellular calls are routed. Figure 7-4 shows a cellular system configuration and the role of the MSC.

As can be seen in Figure 7-4, the MSC is directly connected to each cell site and to the PSTN. When a call is made, it gets routed from the current cell to the MSC and then onto the PSTN (if the other person is on a landline phone) or to another cell (if the other person is on a mobile phone)—and all the while the cash register at the MSC is ringing away.

The MSC is connected to the PSTN by a very high capacity telephone connection. The MSC is connected to each cell site by one of three methods. It uses either a high capacity copper telephone line (called a T1 line), a fiber optic cable, or a point-to-point microwave relay (as discussed in the previous section). The choice of which method is used depends on several things, including the particular cell site's traffic level, how far way the cell is from the MSC, and the terrain between them.

Mobility

The feature which separates mobile telephony from most other wireless applications is the notion that the mobile unit must be able to change what it communicates with dynamically. In fixed wireless communications, there are two transceivers used to establish a single communication link and they remain unchanged during the entire event. In mobile telephony, the mobile transceiver must be constantly changing between transceivers (located at different cell sites) it communicates with as it moves.

Cell sites continuously transmit out a control signal to all the mobile units within their cell. When a mobile phone is first turned on, it shortly receives this control signal and responds by transmitting one of its own. Several cell sites within the area receive this response from the mobile, not just the cell it is in. The key to mobile telephony is power level discrimination. All of the cell sites receive the mobile unit's response, but they all receive different power levels; the cell which receives the highest power response is the cell where the mobile is. Step one is complete: the MSC knows where the mobile unit is.

When the mobile attempts to make a call, it is allocated a small frequency band within the cell to conduct the call. During the call, the signal level (power) is constantly monitored by the MSC by way of the cell site. As the signal level drops, the MSC knows that the mobile is getting ready to leave that cell and enter another cell. Keep in mind that the control signal is still being received by multiple cell sites. It is at this point that the MSC looks to see which adjacent cell site is receiving the most powerful control signal; that cell site is the one which is going to get the call next. How does it make the transition?

At the appropriate time, the MSC conducts an operation called handoff. The handoff process is what is known as a make-before-break connection. In essence, the mobile phone is communicating with two different cell sites for a brief period of time during the handoff. (Otherwise parts of conversations are missing.) This handoff process has its advantages and disadvantages. On the one hand, it provides true mobility. On the other hand, it ties up two cell sites for one call (think lower profits). More will be discussed about this in the next section.

Frequency Reuse and Air Interface

Frequency Reuse

The goal of every mobile telephone service provider is to conduct as many simultaneous calls as possible (think greed). In most wireless technologies, only one party is permitted to transmit a signal, at a given frequency, in a defined geographical location, which works fine for applications like broadcasting. (Having two different stations simultaneously transmitting channel six would really cause a headache.) But cellular technology is different.

In the United States, each cellular provider is allocated 25 MHz of spectrum, 12.5 MHz for transmitting (called the downstream) and 12.5 MHz for receiving (called the upstream). Cellular telephony is a duplex system— both parties can talk at the same time (think husband and wife) because transmitting and receiving are allocated their own frequencies.

In first generation cellular (AMPS) each phone conversation is allocated 30 kHz of spectrum. Therefore, each 12.5 MHz of bandwidth can handle 416 simultaneous phone calls as shown in Figure 7-5. If the cellular service providers were to follow the broadcast model, only 416 total calls could be conducted simultaneously in a given geographical area (an MSA or RSA). Letting only 416 people talk at once in, say, Southern California, would not even satisfy the demands of Beverly Hills.

The good news is that there is no need for cellular service to follow the broadcast model. Since a person on a mobile call only needs their allocated frequency within the cell they are currently in, there is no reason somebody else on the other end of town cannot be using that same exact frequency in an entirely different cell. The concept of multiple users operating at the same frequency, at the same time, and in the same geographic area, is called frequency reuse, and it is what separates mobile telephony from fixed wireless communications.

For frequency reuse to work properly it is imperative that each cell phone only put out enough power to reach the cell site of the cell it is in. If it puts out too much power, it will not only reach the intended cell site, it will reach unintended cell sites, which others may be using at the same frequency for a totally different conversation. This limitation on transmitted power, however, is also an advantage in that low power transmission means that the cellular phone's battery charge will last longer.

Referring back to Figure 7-3, users located in the cells marked with the letter A can both be using the same exact frequency to conduct their own separate conversations. Here is a challenging question: how come adjacent cells cannot conduct different conversations at the same frequency (and the same time)? Imagine that you are a cellular caller on the border between cells and you are communicating with one cell site, but the power level received at the other cell site is almost as great, causing interference to anyone using that frequency in that cell. Because of this potential interference, identical frequencies in adjacent cells cannot be used simultaneously.

One again there is a tradeoff to be made. To avoid the possibility of interference, cells using the same frequency at the same time must be as far away as possible. Conversely, if the cellular provider wants to make as much money as possible (and they do), the cells must be as close together as possible, so more people can talk simultaneously. In practice, the number of cells of separation, which depends on many things, ranges anywhere from four to 21.

Air Interface

As mentioned above, in AMPS each 12.5 MHz of bandwidth is broken down by frequency into 416 different channels, with one conversation per channel. This dividing up of the frequency band is known as frequency division multiple access or FDMA. For AMPS, having 416 different possible conversations at one time (in a given cell) is fine, but what if there were a way to get more than 416 possible simultaneous conversations at one time out of the same 12.5 MHz frequency allocation? With the new digital technologies available, there is.

There are two ways which the new digital wireless technology can increase conversation capacity, and they are referred to as air interfaces. Think of air interface as a second modulation of the RF signal which takes advantage of digital technology to increase capacity.

The first of these air interfaces is known as time division multiple access or TDMA. TDMA takes the same 30 kHz bandwidth which AMPS uses and breaks it down into time slots, as shown in Figure 7-6. Notice that the horizontal axis is labeled with "time." Several conversations can take place simultaneously in the same frequency band because each conversation is periodically allocated a short time slot in which to transmit its message. This obviously requires some sophisticated signal processing, but it does result in higher cell site capacity. Theoretically, each channel can be broken down into six different time slots, which increases the call carrying capacity of the system sixfold.

The other air interface is known as code division multiple access or CDMA. Recall from the previous section on fixed wireless applications, I mentioned a technology called spread spectrum. In essence, spread spectrum stamps an RF signal with a destination address. In this manner, many signals can coexist in the same frequency band at the same time, which is how CDMA operates. In fact, it is a form of direct sequence spread spectrum (DSSS). And because of the miracle of digital technology, more conversations can be crammed into a given bandwidth with CDMA than any other currently employed technology. Figure 7-7 is a graphical depiction of CDMA.

Referring to Figure 7-7, when the RF signal has the CDMA "address" imprinted on it, the spectrum it occupies gets bigger. For instance, a signal which occupies 30 kHz before the address is applied might occupy 1 MHz after the address is applied. This "spreading" of the occupied frequency is why it is called spread spectrum. At first thought, it might seem that having a signal occupy more frequency than it does in its original form is a mistake. However, even though it does occupy a greater frequency band than in its original form, the system can now pile many signals on top of each other because they can all be distinguished by their "address." In this manner, more total signals can fit into a given frequency band, and that is, after all, the goal of every service provider.

Adding Capacity

Within a Cell

Even with all the advances in digital technology macrocells eventually run out of call capacity. (Let's face it, people like to chat.) The service providers like this because it means their cellular infrastructure is being utilized to its fullest. Consumers, on the other hand, get frustrated when they try to make a mobile call and they are greeted with a busy signal. When macrocells run out of call carrying capacity, the only thing the service providers can do—if they want to keep their customers—is to subdivide the macrocell into smaller microcells, as shown in Figure 7-8.

When subdividing a macrocell into microcells each microcell must be capable of communicating directly with the MSC, which means laying copper wire or fiber optic cable or, more frequently, setting up a point-to-point microwave connection. In any event, replacing a macrocell with several microcells is an expensive proposition and the expense must be justified. As a result, microcells only appear in well-traveled corridors, like along a busy freeway.

Occasionally, it even makes sense to further subdivide a microcell into smaller picocells, where mobile traffic is highly concentrated, like a common area in a large city (think Times Square).

Uncovered Areas

When mobile telephone service providers begin to roll out their systems, they naturally place the first macrocells in the highest traffic areas, which means even after the service is up and running there are still areas within the service provider's territory which may not have service. The two places which get call coverage last are the outer fringes of the service provider's territory and places within the territory which suffer from some sort of obstruction. The latter is comprised of tunnels, subways, and the insides of buildings.

The general category of product used to extend a macrocell's coverage is called a repeater. Repeaters come in many shapes and sizes but they all perform one basic function: they extend the wireless range of a macrocell. In that vein, they communicate directly with the macrocell either via copper, fiber optics, or a wireless link. Figure 7-9 shows the layout of a system using a macrocell and a repeater to reach automobiles within a tunnel.

Functionally, there is a very significant difference between using a repeater to extend capacity and breaking down macrocells into microcells to increase capacity. Microcells add capacity because each microcell communicates directly with the MSC. Repeaters, because they communicate with the macrocell itself, actually take away capacity from the macrocell. Every person using the repeater's capacity inside the tunnel in Figure 7-9 means that one less person outside the tunnel can use the macrocell's capacity.

One of the fastest growing uses of repeaters is for in-building applications. In this situation, an antenna is placed on the roof of the building to transmit and receive mobile calls. The signal is then routed from the rooftop antenna, down through the building, to a small repeater on every floor. The signals from the repeater are transmitted and received through an antenna no bigger than a smoke alarm. With in-building repeaters, you can begin a cellular phone call in your car, continue it while you enter the building—even in the elevator—and finish it after you arrive at your desk. (There goes your last excuse to hang up on your mother-in-law.)