5.7 Wireless Mobile Communications

The first analog mobile communication system began as a two-year experiment in Chicago in 1976 and was called the Advanced Mobile Phone Service (AMPS). This is a frequency division multiplexed cellular phone service that transmits calls over the 800 MHz frequencies. It is still widely used in the Americas.

Cordless telephones became the rage in the 1980's. It freed up people to a degree: they could roam around their houses and talk without being confined to stationary phone. Featuring a lightweight cordless handset, cordless phones were relatively inexpensive at under $150 and had no extra service charges. On the downside, cordless telephones were limited to a range of 1,000 feet, were highly susceptible to interference and had very poor security safeguards.

Mobile phone service actually began before cellular service came into being in 1983. Early mobile service was very costly and inefficient and many calls had difficulty getting through or being understandable. Further, cities that offered mobile communication had a long waiting list for the service. An example is 1976 New York City with its metropolitan population of about 20 million. At that time, New York had only 12 channels to serve 543 customers. There were approximately 3,700 customers on the waiting list.

There simply weren't enough frequencies available to service the market. Thus, the cellular concept was born to meet this need.

Cellular Technology

Since the advent of cellular technology, mobile phone service has boomed. In some ways, a cellular telephone is more like a radio than like a phone. Mobile phones are like walkie-talkies that allow you to talk and listen at the same time (full duplex) as well as access more elaborate services like conference calling, voice mail and e-mail. Each phone has a unique identifier with an associated phone number.

Modern cellular networks consist of four major components:

  1. Mobile Telephone Switching Office (MTSO)
  2. Cell sites with a controller and transceiver
  3. System interconnections
  4. Mobile telephone units

In the old system where a city had just one antenna, a very limited number of two-way conversations could be held. With the cellular concept, a city is divided into smaller sections (cells) where each cell has its own antenna and just uses a subset of all the Radio Frequency channels available. The cell site with the strongest signal strength transports the call to the PSTN.

The beauty of the cellular system lies in the fact that frequencies can be reused. A cellular network in a honeycomb-like hexagonal pattern, similar to the one below, is common.

Mobile calls get a little more complex when you are traveling. If you are talking on a cellular phone while driving, you may notice changes as you travel from cell to cell and you may occasionally get cut off. This is due to a technique called cellular handoff. When outside the range of one cell, the phone uses the adjacent cell to continue the call. This all occurs automatically. Study the diagram to see how this works.

Cellular services are extremely successful services that are increasing in exponential numbers the past few years. Because of this rapid expansion, new digital standards have developed to meet demands. At this time, digital cellular systems are more likely to be found in metropolitan areas. Analog systems are still more widespread. TDMA and CDAM are two common digital systems used in the United States.

  1. Time Division Multiple Access (TDMA)
  2. May also be called DAMPS, Digital AMPS (Digital Advanced Mobile Phone Service) or U.S. Digital. TDMA uses time slots within channels to increase capacity with the phones synchronizing to the time slots to avoid transmission collision. TDMA systems operate in the 800 MHz and 1.9 GHz frequency ranges.

  3. Code Division Multiple Access (CDMA)
  4. Uses spread-spectrum transmission scheme and allows multiple conversations on a single channel by assigning codes to individual conversations. This greatly reduces frequency interference. CDMA phones operate at 1.8 GHz and 1.9 GHz frequencies.
Summary of Cellular Services
Tower Spacing
800 MHz
25 miles
1.80 GHz
12 miles
1.90 GHz
12 miles
2.40 GHz
12 miles