Intro

In this lesson, we look at radio frequency, or RF, spread spectrum technologies, including frequency hopping spread spectrum, or FHSS, direct sequence spread spectrum, or DSSS, and orthogonal frequency division multiplexing, or OFDM. You can download the script for this video from above or at the end of the video. All graphics and tables in this lesson are included in the script.

Electromagnetic Spectrum

RF waves, part of the electromagnetic spectrum, are used to connect devices to networks and to each other without the use of cabling. Each medium used has strength and signal integrity advantages and disadvantages. In addition to selecting the right medium based on the operational environment, it is also important to ensure minimal interference with signals traveling through offices manufacturing floors, and other environments.

RF Ranges and Uses

Radio waves range in frequency between 3 Hz and 300 GHz. Different ranges within the RF spectrum are used for different things. This table generally shows the services that use different ranges. Each frequency range must be assessed before use. This assessment helps to ensure transmitters and receivers are properly placed within organizations. This is largely caused by the susceptibility of high frequencies to walls, equipment, and other barriers, including foliage. For example, organizations that move from 2.4 GHz Wi-Fi to 5 GHz Wi-Fi usually need a coverage assessment to adjust placement of existing or new access points.

Electromagnetic Interference (EMI)

Another consideration during assessments is the effect of other electromagnetic interference, or EMI. As shown, RF is part of the overall electromagnetic spectrum. Signals along the spectrum can disastrously interfere with each other if not properly managed. For example, 2.4 GHz interference is created by Wi-Fi devices, Bluetooth devices, and cordless phones. Switching loads can also cause interference. Causes of these interfering signals are often found on manufacturing floors, and include electric motors, transformers, heaters, lamps, and power supplies. Most countries have regulations that help control EMI emissions, but some interference often slips through. For example, the tool and die maker at a manufacturer I worked for once had a PC sitting on a table next to a metal cutting machine. He complained that his computer's network connection would frequently fail. I asked him for the manual for his cutting machine. On the second page, the manual warned of the emission of EMI. This emission interfered with the signals traveling over the Ethernet drop cable. I replaced the network cable running from the ceiling to his PC with shielded twisted pair. This solved the problem. This same kind of interference also interferes with wireless RF signals.

Spread Spectrum

The specific RF communication methods we discuss now are based on spread spectrum technology. Spread spectrum breaks a message into smaller packets and communicates them simultaneously across multiple RF frequencies. This transmission appears as noise. The receiver correlates the received signals and reconstructs the original message. Spread spectrum is used for multiple reasons, including secure communication, resistance to interference because the same frequency is not continuously used, and resistance to intentional jamming. Basic spread spectrum transmits packets in parallel instead of in series.

Frequency Hopping Spread Spectrum

Frequency-hopping spread spectrum, or FHSS, rapidly changes the carrier frequency while transmitting messages. But instead of parallel transmission, FHSS sends packets in a series as it changes frequencies. Advantages of FHSW include resistance to signal interference, resistance to unwanted data interception, resistance to jamming, and less ability to interfere with other RF signals.

Direct Sequence Spread Spectrum

Direct sequence spread spectrum, or DSSS, is specifically designed to minimize signal interference. The transmission is divided and simultaneously transmitted over as many frequencies as possible within a specified frequency band. This makes DSSS faster than the frequency hopping approach. In addition to the basic advantages of spread spectrum, DSSS also provides a larger coverage range.

Orthogonal Frequency Division Multiplex.

Orthogonal frequency division multiplexing, or OFDM, uses narrowly spaced, overlapping digital subcarriers that can be used at the same time without interfering with each other. OFDM has a greater throughput than the other spread spectrum approaches. Well, that's it for this lesson. If you have questions please ask. ...and until next time be careful what you click.

Outro

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