Cell Phone Towers

 Bob Gariano

One of the best places in Lake Forest to sit on an early November afternoon is on the benches in front of Sweets Candy and Ice Cream Store. The benches on the Deerpath Road side of the store face south and, in the early afternoon, their position perfectly captures the warmth of the late autumn sunshine. Looking south from that point even a casual observer will notice the cell phone antenna array over the Deerpath Inn several blocks away. Cell phone towers are an integral part of our digitally formatted and tightly connected life style.

New Engineering

The theory of cell phones is as old as Marconi’s two way radio technology, but the practical engineering considerations of such networks is substantially more recent and the challenges more exigent. Cell phone engineering relies upon digital signals and sophisticated software. It took engineers more than three decades to develop cell phone systems that work as well as they do today.

Just as radio broadcasting companies are assigned specific frequencies to transmit music and news, so cell phone companies have certain frequencies assigned for their use in connecting their customers with voice and data communications. However, cell phone networks are much more complicated and delicate.

Different commercial radio stations in different cities can use the same frequency. They do not interfere with each other because they are spaced far enough apart. The cells in a cell phone network are much closer, generally only one to two miles across, so the opportunity for interference is heightened. In some high density areas, where demand for service is intensive, there may be cell phone towers only 1000 feet apart.

Assigned frequencies are expensive and their cost induces wireless network companies to continuously improve their network technology. The companies try to fit more conversations and data into the same frequency range while still providing reliable service. Of course, unlike radio broadcasts, cell phones must transmit and receive, so the same frequency range does double duty, using capacity for both up links and down links, that is, talking and listening, on the same call.

Signal Codes

One way to get many unencumbered conversations through simultaneously on the same frequency is by using coding like CDMA or code division multiple access. This coding means that several customers can be using the same frequency but each person’s phone only recognizes signals that have that subscriber’s special code.

To over simplify, a digital signal on a particular frequency is a stream of numbers. My call only reacts to the numbers on the frequency that end with the number 8 and my neighbor’s call recognizes numbers that end with a 3. This code is assigned for each call as the particular signal becomes available. Calls are kept separate through coding the signal. Everything is kept straight using software embedded in the phone and coordinated with signal processors in the tower.

There are other coding methods. It is now common for a single frequency to handle scores of telephone calls simultaneously. Compare this with the system in the early 1980s when only ten or fifteen people in an area as large and as populated as Manhattan could access a cell phone connection at the same time.

The engineering challenges of wireless communication do not end with multiple access and coding. Cell phone connections must be reliable in spite of inclement weather conditions, through walls and around corners, and in various terrains. The phones themselves have to be small. There is little room or weight allocated for circuitry or batteries, but the phones must have the functionality and power to win over customers.

Mobile Phones

It is also obvious that cell phones have to work when the customer is moving. That is what mobility is all about. When a transmitter moves, it pushes the waves together in front and stretches them out behind, changing the frequency. This is called the Doppler Effect. It is why a train horn sounds higher pitched as it approaches an observer and then sounds lower pitched as it speeds away.

This change in frequency due to speed could raise havoc with a cell phone signal emanating from a car as the vehicle moves down the highway, changing frequencies or pitch according to the direction of travel. Modern cell phone software corrects for these anomalies without any effort from the caller.

Another challenge of mobility involves hand offs. As a moving cell phone customer in mid conversation starts to leave one cell and enter another, the phone switches signals to the new tower. The first tower automatically hands off the call to the new tower. The cell towers must be positioned and powered so that overlap of signals is precise to enough prevent call interruption during hand offs, but not cause interference during calls.

Even when a cell phone is not being used, it registers with each new cell that it enters by sending that local tower a message. The customer does not have to worry about this automatic function. The processors in the tower communicate back to the subscriber’s base station about that phone’s presence in that cell. That is how a caller can find and connect to a cell phone customer who is traveling outside of their home area.

Future Engineers

This Saturday afternoon, as I was enjoying my Sweets ice cream cone and observing the cell phone tower with its antenna array, signal processors and controls, power backups, and transmitter equipment, a group of junior high school girls walked by. They were busy sending text messages, both thumbs typing away, while talking non stop to each other.

I wondered whether they had any idea of the decades of engineering that enabled each of them to access the wireless system at their finger tips. One of them might become enthralled with the technology and go on to study the systems and network engineering. Perhaps one of these young people will help develop the technology that will lead to future advances in digital communications.

Bob Gariano is President of RGA, an executive search firm that recruits senior executives and board members for public and private companies. Bob can be reached at rgariano@robertgariano.com