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About this Article
Written by: Caleb Yang
Written on: September 1st, 2003
Tags: communication, computer science, electrical engineering
Thumbnail by: Joshua Davis/SXC
About the Author
In Spring 2003, Caleb was a third year Electrical Engineering student at USC. As a student, he spends his free time reading science-fiction, the collected works of Tom Clancy, and the occasional book on theology. He also enjoys playing video games.
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Volume IV Issue I > Multiple Access Schemes for Mobile Phones
Mobile phones allow users to place calls, send text messages, and receive updates from the internet. Information from a mobile phone is sent and received by way of electromagnetic waves. All information is encoded prior to transmission, decoded upon arrival, and must be sent so that many users can share the cellular system without mutual interference. The latter requirement has been implemented with the instigation of multiple access schemes, which have undergone numerous evolutions since the inception of the industry. Today, there are two major systems which play an important role in both current and future mobile technologies. Those systems are the Global System for Mobile Communications, originated in Europe, and the Code Division Multiple Access Scheme, developed in the United States.

Introduction

In the late 1800's, a Scottish physicist named James Clerk Maxwell formulated a principle that would forever change our world. Maxwell was able to show that the generalized forms of the laws of electricity and magnetism--the laws of Coulomb, Gauss, Biot-Savart, Ampere, and Faraday--suggested the existence electromagnetic (EM) waves [1]. Electromagnetic waves have both an electric and magnetic field component that propagate through space, similar to how a sound wave propagates through air or water. Maxwell's theory has since proven true and has been put to great use. His work catalyzed the development of EM wave transmitters and receivers, eventually leading to the creation of mobile phones.
Joshua Davis/SXC
Figure 1: Cell phones have become a worldwide phenomenon in communication.
For better or worse, the world entered the mobile telephony era in the 1980's and hasn't looked back. The transmission and reception of EM waves are only a part of how the mobile phone networks allow the public to make calls and send text messages (see Fig. 1). The information, whether voice, word, or image, must be encoded and decoded in such a way that the user receives only what he or she was intended to receive. This is accomplished through what are known as multiple access schemes. Numerous such schemes have been developed, each with their own advantages and disadvantages. In the design of a cellular communication system, engineers must consider bandwidth and how many subscribers it can support, which thereby determines how much information can be sent over a given time period. In the world today, there are two multiple access schemes that play an important role in current and future mobile technologies: the Global System for Mobile communication (GSM) and the Code Division Multiple Access scheme (CDMA).