Wireless

C H A P T E R

20

Chapter Goals
• • • • •

Identify different types of wireless technologies. Identify different wireless solutions. Introduce quadrature amplitude modulation. Explain wireless systems. Discuss the benefits of using wireless technologies for communications.

Wireless Technologies
Types of Wireless Technology
Eighteen major types of wireless technologies exist,containing a large number of subset technologies that range from ATM-protocol based (which sells at approximately $200,000 per data link, to wireless local-area network (WLAN, which sells at less than $500,000 per data link). Frequencies of the different technologies travel between several hundred feet (wireless LAN) and 25 miles (MMDS). The process by which radio waves are propagated through the air, theamount of data carried, immunity to interference from internal and external sources, and a host of other characteristics varies from technology to technology. Wireless technologies are differentiated by the following:
• • •

Protocol—ATM or IP Connection type—Point-to-Point (P2P) or multipoint (P2MP) connections Spectrum—Licensed or unlicensed

Table 20-1 lists the different wirelesstechnologies.
Table 20-1 Different Types of Wireless Technologies

Broadband1 WAN Licensed Digital Line-of-site
3 2

Narrowband WAN and WLAN Unlicensed Analog Non-line-of-site

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Chapter 20 Introduction to QAM

Wireless Technologies

Table 20-1 Different Types of Wireless Technologies (continued)

Broadband1 Simplex
4Narrowband Half-/full-Duplex Multipoint

Point-to-point
1. Broadband—Data rates that exceed 1.5 Mbps

2. Licensed—Granted by or purchased from the FCC 3. Line-of-site—Direct line of site between two antennae 4. Simplex—One transmitter

Base Station
The base station (also referred to as the hub or the cell site) is the central location that collects all traffic to and from subscribers within acell. The indoor base station equipment consists of channel groups. The channel groups each connect to the existing network, typically with a DS-3 with ATM signaling. The function of the channel group is to effectively act as a high-speed radio modem for the DS-3 traffic. The outdoor base station equipment (Tx/Rx node) modules are located on a tower or a rooftop mount and consist of a frequencytranslation hardware and transmitters/receivers. The Tx/Rx node delivers and collects all the traffic to and from subscribers within a cell or a sector. Additionally, the Tx/Rx node equipment translates the channel group output into the appropriate frequency for over-the-air transmission. Multiple channel groups are used in each sector to meet the traffic demands, thus providing a highly scalablearchitecture.

Introduction to QAM
Many modern fixed microwave communication systems are based on quadrature amplitude modulation (QAM). These systems have various levels of complexity. Simpler systems such as phase shift keying (PSK) are very robust and easy to implement because they have low data rates. In PSK modulation, the shape of the wave is modified in neither amplitude nor frequency, butrather in phase. The phase can be thought of as a shift in time. In binary phase shift keying (BPSK), the phases for the sine wave start at either 0 or 1/4. In BPSK modulation, only 1 bit is transmitted per cycle (called a symbol). In more complex modulation schemes, more than 1 bit is transmitted per symbol. The modulation scheme QPSK (quadrature phase shift keying) is similar to the BPSK. However,instead of only two separate phase states, QPSK uses four (0, 1/2 , ,and 3/2 ),carrying 2 bits per symbol. Like BPSK, QPSK is used because of its robustness. However, because it modulates only 2 bits per symbol, it still is not very efficient for high-speed commun-ications. Hence, higher bit rates require the use of significant bandwidth. Even though QPSK uses no state changes in amplitude, it...