Radio Frequency (RF) Modulation Techniques Basics

In preparation for managing your wireless networks, you should know something about the different Radio Frequency (RF) modulation techniques that are implemented in IEEE 802.11 networking.

You do not have to know everything about them; just be familiar with the terminology that is used in the following sections because it may be helpful when you are trying to find the source of interference or figure out how your network is being affected by interference.

Frequency-hopping spread spectrum (FHSS)

The FHSS modulation technique uses the available channels to transmit and receive data, but rather than staying on any one channel, it rapidly switches between channels using a pseudorandom pattern that is based on an initial key; this key is shared between the participants of the communication session.

If interference affects only a few of the channels, this interference is minimized because each channel is used only briefly. If the interference is broad, it can still affect all the channels that are in use. This modulation technique requires that the initial seed or key be shared, but after that has happened, it is very difficult to eavesdrop on.

IEEE 802.11 wireless networks use this technique for modulation, while Bluetooth uses an adaptive version of this technique that stops using channels where interference or weak signals exist.

Direct-sequence spread spectrum (DSSS)

Rather than rapidly switching between several channels, DSSS spreads the carrier signal across the entire 22-MHz frequency range of its channel. For example, a device sending over channel 1 would spread the carrier signal across the 2.401- to 2.423-GHz frequencies (the full 22-MHz range of channel 1).

At the same time it is transmitting the data over this channel, it also, at a faster rate, generates a noise signal in a pseudorandom pattern. This noise signal is known to the receiver, which can reverse or subtract the noise signal from the data signal. This process allows the carrier signal to be spread over the entire spectrum.

With the entire spectrum being used, the effect of narrow-spectrum interference is reduced. Also, if the channel is being used by other devices, the effect of their signal is reduced because they are not using the same pseudorandom noise pattern.

DSSS has an advantage over FHSS in that it has better resistance to interference. It is used primarily by IEEE 802.11b networks and cordless phones operating in the 900-MHz, 2.4-GHz, and 5-GHz spectrums. IEEE 802.11g/n networks also sometimes use DSSS, but these newer networks tend to prefer orthogonal frequency division multiplexing (ODFM).

Orthogonal frequency division multiplexing (OFDM)

The slower that data is transmitted, the less likely that interference or line noise will cause a problem with the transmission. Multiplexing allows you to take several pieces of data and combine them into a single unit that can then be sent over the communication channel.

In this case, OFDM takes the data that needs to be transmitted and breaks it into a large number of subcarrier streams (up to 52 subcarriers) that can then all be multiplexed into a single data stream. Because 52 subcarriers exist, the final data stream can be sent at a slower rate, while still delivering more data than other methods in the same time period.

This multiplexing process gives OFDM an advantage over DSSS because it allows higher throughput (54 Mbps instead of 11 Mbps), and it can be used both in the 2.4-GHz frequency range and in the 5-GHz frequency range.

Multiplexing has many uses, and OFDM is used in any technology that needs to send large amounts of data over slower transmission lines or standards. OFDM is used with IEEE 802.11g/a/n networking as well as with ASDL and digital radio.

Multiple-in, multiple-out (MIMO)

MIMO allows multiple antennas to be used when sending and receiving data. The concept of spatial multiplexing allows these multiple signals to be multiplexed or aggregated, thereby increasing the throughput of data.

To improve the reliability of the data stream, MIMO is usually combined with OFDM. When using multiple antennas, you can achieve higher transmission speeds — over 100 Mbps.

MIMO is used in both WiMAX and IEEE 802.11n networks and is the largest reason these networks achieve their high speeds.

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