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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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Narrow Band Modulation

Conventional radio systems, such as television and AM/FM radio, utilize narrow band modulation. These systems concentrate all their transmit power within a narrow range of frequencies, making efficient use of the radio spectrum in terms of frequency space. The idea behind most communications design is to conserve as much bandwidth as possible; therefore, most transmitted signals utilize a relatively narrow slice of the radio frequency spectrum. Other systems using the same transmit frequency, however, will cause a great deal of interference because the noise source will corrupt most of the signal. To avoid interference, the FCC requires users of narrow band systems to obtain FCC licenses to properly coordinate the operation of radios. Narrow band products thus have a strong advantage because you can be fairly assured of operating without interference. If interference does occur, the FCC will resolve the matter. This makes narrow band modulation good for longer links covering the geographical size of a metropolitan area.


Figure 2.3  ISM spectrum availability.


NOTE:  

Chapter 3, “Wireless Metropolitan Area Networks,” covers the wireless MAN products that employ narrow band radio frequencies.


Spread Spectrum Modulation

Products that operate according to Part 15.247 of the FCC’s Rules and Regulations must utilize spread spectrum modulation. What is spread spectrum? Spread spectrum modulation “spreads” a signal’s power over a wider band of frequencies (see fig. 2.4). This contradicts the desire to conserve frequency bandwidth, but the spreading process makes the data signal much less susceptible to electrical noise than conventional radio modulation techniques. Other transmission and electrical noise, typically narrow in bandwidth, will only interfere with a small portion of the spread spectrum signal, resulting in much less interference and less errors when the receiver demodulates the signal.


Figure 2.4  Narrow band versus spread spectrum modulation.


NOTE:  

Spread spectrum was initially developed by the U.S. military during World War II to protect communication systems and guided weapons from intentional hostile jamming. One of the principle developers of spread spectrum was Hedy Lamarr, an actress during the 1940s. Hedy had invented the modulation technique to prevent the enemy from jamming or eavesdropping on secret military conversations. One of her first devices was to keep guided torpedoes from being detected or jammed by the enemy. Hedy and George Antheil, a film-score composer who assisted Hedy in perfecting spread spectrum, received a patent for their work in 1940. Actually, spread spectrum was never used during World War II. Sylvania utilized spread spectrum for the first time on ships sent to blockade Cuba in 1962. Hedy Lamarr conceived an excellent modulation technique; however, she never received any compensation for the idea.


Spread spectrum modulators use one of two methods to spread the signal over a wider area: direct sequence or frequency hopping.

Direct Sequence Spread Spectrum

Direct sequence spread spectrum combines a data signal at the sending station with a higher data rate bit sequence, which many refer to as a chipping code (also known as processing gain). A high processing gain increases the signal’s resistance to interference. The minimum linear processing gain that the FCC allows is 10, and most products operate under 20. The IEEE 802.11 Working Group has set their minimum processing gain requirements at 11.

Figure 2.5 shows an example of the operation of direct sequence spread spectrum. A chipping code is assigned to represent logic “one” and “zero” data bits. As the data stream is transmitted, the corresponding code is sent. For example, the transmission of a data bit equal to “one” would result in sequence 00010011100 being sent.


Figure 2.5  The operation of direct sequence spread spectrum.

Many direct sequence products on the market utilize more than one channel in the same area; the number of channels available, however, is limited. With direct sequence, many products operate on separate channels by slicing the frequency band into non-overlapping frequency channels. This results in the potential for several separate networks to operate without interfering with each other. To leave enough bandwidth for moderate to high data rates, however, there can only be a few channels. Proxim’s ProxLink and RangeLAN product families, for example, use direct sequence technology in the 902–928 MHz frequency band. ProxLink incorporates seven different channels, and RangeLAN uses three channels.

Frequency Hopping Spread Spectrum

Frequency hopping works very much like its name implies. It takes the data signal and modulates it with a carrier signal that hops from frequency-to-frequency as a function of time over a wide band of frequencies (see fig. 2.6). A frequency hopping radio, for example, will hop the carrier frequency over the 2.4 GHz frequency band between 2.4 GHz and 2.483 GHz. A hopping code determines the frequencies the radio will transmit and in which order. To properly receive the signal, the receiver must be set to the same hopping code and “listen” to the incoming signal at the right time and correct frequency. FCC regulations require manufacturers to use 75 or more frequencies per transmission channel with a maximum dwell time (time at a particular frequency) of 400 ms. If the radio encounters interference on one frequency, then the radio will retransmit the signal on a subsequent hop on another frequency.

The frequency hopping technique reduces interference because the propagation from narrow band systems will only affect the spread spectrum signal when it is using the frequency of the narrow band signal. Thus, the aggregate interference will be very low, resulting in little or no bit errors.


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