The Cellular Concept
Cellular technology requires large geographical regions to be identified and assigned to the various service providers. In the United States, these large geographical regions are identified as Metropolitan Statistical Areas or MSAs (think city) and Rural Statistical Areas or RSAs (think country) in the cellular frequency bands. In the PCS frequency bands, they are identified as Metropolitan Trading Areas or MTAs (large regions) and Basic Trading Areas or BTAs (small regions). While MTAs overlap BTAs, MSAs and RSAs have exclusive territories.
There are two service providers authorized to provide mobile telephony in each of the MSAs, RSAs, and MTAs, and four providers authorized in each of the BTAs. All of these service providers distinguish themselves by being allotted different frequency sub-bands within the overall frequency allotment. Within their assigned region, each service provider breaks up the region into smaller sub-regions called cells.
Each of these cells has an antenna (or antennas) at the center of the cell that 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, 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 71).
Figure 71 Cell pattern covering a geographic area.
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. The greater the number of hexagons, the more simultaneous calls the system can handle (think revenue). However, the greater the number of hexagons, the more infrastructure that is required to implement the system (think expenses). As a result, cell coverage is a dynamic activity that is constantly changing in response to increases in capacity requirements.
Did You Know?
Cells come in three basic sizes: macrocells, microcells, and picocells. There are no exact definitions for each of these except to say that macros are bigger than micros, which are bigger than picos. Macrocells are representative of the first-generation cellular systems. Microcells and picocells are new developments that have resulted from the subdivision of macrocells to add capacity.
At the center of every cell is a cell site or basestation. The cell site contains all of the electronics that 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 usually broken down into three sectors (120þ 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 35.
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 called 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 72 shows the cellular system infrastructure and the role of the MSC.
Figure 72 Cellular system infrastructure.
As can be seen in Figure 72, 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 chapter). The choice of which method is used depends on several things, including the particular cell site's traffic level, how far away the cell is from the MSC, and the terrain between them.
The feature that 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 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 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 that receives the highest power response is the cell that the mobile is in. 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 that 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 go missing.) This type of 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). Transferring the connection from one cell to another is called a hard handoff, while transferring the connection from one sector to another within the same cell is called a soft handoff.
Within a Cell
Because people have fallen in love with mobile phones, macrocells have run out of call capacity. 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 doif they want to keep their customersis to subdivide the macrocell into smaller microcells, as shown in Figure 73.
Figure 73 Dividing up a macrocell into microcells.
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).
When mobile telephone service providers began to roll out their systems, they naturally placed the first macrocells in the highest traffic areas, which meant that even after the service was up and running, there were still areas within the service provider's territory that did not have service. The two places that got call coverage last were the outer fringes of the service provider's territory and places within the territory that 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 74 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 74 means that one less person outside the tunnel can use the macrocell's capacity.
One of the fastest growing uses for 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.
Figure 74 Graphical depiction of a repeater inside a tunnel.
With in-building repeaters, you can begin a cellular phone call in your car, continue it while you enter the buildingeven in the elevatorand finish it after you arrive at your desk. (There goes your last excuse to hang up on your mother-in-law.)