Differences in Wireless Technology

Cellular phones have come a long way since they were first introduced in 1981. In 1981, less than 1% of the U.S. population was using cell phones. Now, in 2002, over 40% of the U.S. population owns a cell phone. Cell phones have become so commonplace that people are starting to use these gadgets as their primary mode of communication and completely disconnecting their local home phone service. With these convenient electronics, people can make phone calls from their home or practically anywhere on the planet to anybody in the world. As the world starts to take advantage of cell phones, it is important to get an understanding of how these intriguing instruments work. Most of today's phones and services use digital technologies to communicate. Digital service is a method of encoding information using a binary code of 0s and 1s and transmitting this information as "on" or "off" transmissions. There are three major digital technologies used in the U.S. to transmit digital information, they are Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Global System for Mobile Communication (GSM), which is under the TDMA umbrella. Each technology employs a different way of transmitting information and thus there are many different advantages and disadvantages to each technology.

As the first digital technology to operate in the U.S., TDMA is the crudest technology. TDMA uses the frequency bands available to the wireless network and divides them into time slots with each phone user having access to one time slot at regular intervals. GSM is based on an improved version of TDMA technology. GSM uses wider 200 kHz channels instead of the 30 kHz channels that TDMA utilizes. Also, instead of having only 3 time slots like TDMA, GSM has 8 time slots thus allowing GSM to have more capacity. GSM was created in 1987 and was introduced to the U.S. in 1995. CDMA is based on a form of spread spectrum technology that separates voice signals by assigning them digital codes and spreads them within the same broad spectrum. CDMA technology dates back to the 1940s, when spread spectrum technology was used in military communications systems because it was resistant to interference from enemy signals. Qualcomm Corporation began developing a CDMA wireless system in the late 1980s that went into operation in 1996. The major cell phone providers that provide TDMA, GSM, and CDMA are AT&T, T-Mobile, and Sprint PCS, respectively.

The main difference between TDMA (including GSM) and CDMA is how they share the spectrum. TDMA shares the spectrum by assigning each user a different "slot" in time. Each user on the channel then takes turns transmitting and receiving information during their designated time slot. So, no two users are transmitting or receiving information in one point of time. On the other hand, CDMA shares the spectrum by having everyone on the channel transmitting and receiving at the same time. CDMA is able to achieve this based on a modulation technique called Spread Spectrum. Each user is only transmitting small bits of data that will fit into a small channel. The transmitter takes these small bits of data and spreads them over the entire 1.25 MHz spectrum. CDMA uses an encoding technique that ensures each user's bits do not interfere with other user's bits. The receiver then unspreads the bits and extracts the data that was transmitted. This 1.25 MHz spectrum incorporated by CDMA is a wide channel and can accommodate many different people using it at the same time. CDMA channels are said to have a soft capacity because there is no upper limit on how many users can be on one channel, the only limit is how much interference the user is willing to endure because as more users are on the channel, interference increases. TDMA technology has a definite capacity and is hard coded by the number of slots available in a specific channel. The following are analogies to help better understand how TDMA and CDMA work. Imagine a room full of people trying to carry on one-on-one conversations at the same time. Using TDMA, each couple takes short turns talking by saying only one sentence at a time. Since there is no other person in the room is speaking at the same time, there is no interference. Each couple gets a dedicated time slot to talk. Using CDMA, each couple talks at the same time, but each couple is using a different language. Since nobody else in the room understands any other language except for that of the person they are talking to, there is not much interference. However, as more and more people fill up the room, the excess noise increases and causes more interference (Romaine, 2002). The advantage of TDMA is that each user gets a guaranteed time slot with no interference, but each channel is limited to 3 time slots and therefore the capacity of a TDMA system is limited. In today's world as millions of people are adding new cell phone service, this limited capacity of the TDMA system will cause problems where subscribers cannot get a connection because they cannot find a time slot. The CDMA system offers a higher capacity than the TDMA network. In theory, there