Wireless Communications

Introduction

Wireless is a term used to describe telecommunications in which electromagnetic waves (rather than some form of wire) carry the signal over part or the entire communication path. The first wireless transmitters went on the air in the early 20th century using radiotelegraphy (Morse code). Later, as modulation made it possible to transmit voices and music via wireless, the medium came to be called "radio." With the advent of television, fax, data communication, and the effective use of a larger portion of the spectrum, the term "wireless" has been resurrected.

Common examples of wireless equipment in use today include:

* Cellular phones and pagers

* Global Positioning System (GPS)

* Cordless computer peripherals

* Cordless telephone sets

* Home-entertainment-system control boxes -- the VCR control and the TV channel control are the most common examples

* Remote garage-door openers

* Two-way radios

In this paper we will look at the following technologies used to provide wireless communication:

* Code Division Multiple Access (CDMA) and Global System for Mobil Communications (GSM).

* Satellites

* 802.11X Wireless LAN

* Bluetooth Wireless Technology

* Microwave

CDMA/GSM

In cellular service there are two main competing network technologies: Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA). Today, most major cellular carriers use either CDMA or GSM. There are camps on both sides that firmly believe either GSM or CDMA architecture is superior to the other. This section will explore the technical capabilities of CDMA and GSM; where and how they are used, how security is addressed, advantages and disadvantages, and what the future holds for each of these cellular technology standards.

CDMA was derived as a proprietary standard designed by Qualcomm in the United States, has been the dominant network standard for North America and parts of Asia. CDMA was adopted by the Telecommunications Industry Association (TIA) in 1993. There are currently many different variations of CDMA (cdma2000, 1X EV, 1XEV-DO, and MC 3X, etc.); however the original CDMA is now known as cdmaOne. By May 2001 there were 35 million subscribers on CDMA systems worldwide, now there are over 270 million.

CDMA uses unique spreading codes to spread the baseband data before transmission. It works by transmitting a digitally encoded analog signal using spread-spectrum technology combined with a special coding scheme over a 1.25 MHz channel. It encodes the data using a set of 64 bit Walsh Codes. In theory at least, up to 64 users could use the same channel. The coding used in CDMA is used at the wireless transmission level and makes it difficult for a receiver, not having the correct code, to even receive any intelligible signal (the signal is equivalent to noise) (Dasgupta and Boyd, 2004).

Some major benefits of CDMA are lowered power usage and frequency reuse. A feature that enables efficient use of power is called a Rake correlator (Bryson, Chen and Wan, 2001). This allows CDMA devices to benefit from signal propagation that often occurs when signals bounce off and between buildings. This allows CDMA phones to have lower power output, lengthening battery life. In addition, it allows for more cells to be crammed into urban areas where there is a greater need for more capacity. A second benefit of CMDA is higher frequency reuse. Traditional cell frequencies could not use the same frequencies as the any of the cells directly next to it. This is because the signals would bleed over into each other, creating interference with the neighboring cell. Since CDMA phones have lower power output and because CDMA uses orthogonal codes, it can cancel out the neighboring signals as if they didn't exist (Dasgupta and Boyd, 2004). This allows for about twice the frequency reuse rate of traditional implementations (Unknown, 2003).

CDMA by its very nature is more cryptic than both traditional analog cellular and TDMA. Encoding and decoding CDMA is rather computationally complex, especially considering that this encoding has to be done by a small battery powered phone. In order to listen to a CDMA conversation, it is necessary to know the codes being used. This could probably be looked up in a book; however a computer is needed to do this decoding. CDMA handsets implement encoding and decoding in hardware and someone trying to pick off a CDMA conversation would have to do the decoding on their home computer in software. Of course, hardware encryption is typically several orders of magnitude faster than software. In addition, when CDMA is used, the digital signal being transmitted is encrypted. This means that in order pick off a CDMA phone call, one would have to somehow find the encryption key in addition to decrypting the signal. The evidence to the effectiveness of CDMA's security is that currently, there are few if any reports of people listening in on other conversations or of air-time fraud like there was when analog cellular was most popular (Dasgupta and Boyd, 2004).

The GSM Association is an international organization founded in 1987, dedicated to providing, developing, and overseeing the worldwide wireless standard of GSM. In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and the first phase of the GSM specifications were published in 1990. Today, GSM networks continue to make inroads throughout the world.

GSM differs significantly from its competition in that both signaling and speech channels are digital. There are four different cell sizes in a GSM network - macro, micro, pico, and umbrella cells. The coverage area of each cell is different in different environments. The modulation used in GSM is Gaussian minimum shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal being modulated is smoothened with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (Scourias, 1997).

From the point of view of the consumer, the key advantage of GSM systems has been higher digital voice and other media service quality. The advantage for