Modulation: Preparing Signals for Propagation

Modulation creates a radio or light signal from the network data so that it is suitable for propagation through the air. This involves converting the digital signal contained within the computer into an analog signal. As part of this process, modulation superimposes the information signal onto a carrier, which is a signal having a specific frequency. In effect, the information rides on top of the carrier. In order to represent the information, the modulation signal varies the carrier in a way that represents the information.

This is done because it's generally not practical to transmit the information signal in its native form. For example, consider Brian, who wants to transmit his voice wirelessly from Dayton to Cincinnati, which is about 65 miles. One approach is for Brian to use a really high-powered audio amplifier system. The problem with this is that the intense volume would probably deafen everyone in Dayton. Instead, a better approach is to modulate Brian's voice with a radio frequency or light carrier signal that's out of range of human hearing and suitable for propagation through the air. The information signal can vary the amplitude, frequency, or phase of the carrier signal, and amplification of the carrier will not bother humans because it's well beyond the hearing range.

The latter is precisely what modulation does. A modulator mixes the source information signal, such as voice or data, with a carrier signal. The transceiver couples the resulting modulated and amplified signals to an antenna. The modulated signal departs the antenna and propagates through the air. The receiving station antenna couples the modulated signal into a demodulator, which derives the information signal from the radio signal carrier.

One of the simplest forms of modulation is amplitude modulation, which varies the amplitude of a signal in order to represent data. This is common for light-based systems whereby the presence of a 1 data bit turns the light on, and the presence of a 0 bit turns the light off. Actual light signal codes are more complex, but the main idea is to turn the light on and off in order to send the data. This is similar to giving flashlights to people in a dark room and having them communicate with each other by flicking the flashlight on and off to send coded information.

Modulation for RF systems is more complex and covered in the following sections.

Frequency Shift-Keying

Frequency shift-keying (FSK) makes slight changes to the frequency of the carrier signal in order to represent information in a way that's suitable for propagation through the air. For example, as shown in Figure 3-7, modulation can represent a 1 or 0 data bit with either a positive or negative shift in frequency of the carrier. If the shift in frequency is negative, that is a shift of the carrier to a lower frequency; the result is a Logic 0. The receiver can detect this shift in frequency and demodulate the results as a 0 data bit.

Figure 3-7. FSK Makes Use of Changes in Frequency to Carry Information


Phase Shift-Keying

Similar to FSK, some systems utilize phase shift-keying (PSK) for modulation purposes. With PSK, data causes changes in the signal's phase while the frequency remains constant. The phase shift, as Figure 3-8 depicts, can correspond to a specific positive or negative amount relative to a reference. A receiver is able to detect these phase shifts and realize the corresponding data bits.

Figure 3-8. PSK Makes Use of Changes in Phase to Carry Information


Quadrature Amplitude Modulation

Quadrature amplitude modulation (QAM) causes both the amplitude and phase of the carrier to change in order to represent patterns of data, often referred to as symbols. (See Figure 3-9.) The advantage of QAM is the capability of representing large groups of bits as a single amplitude and phase combination. In fact, some QAM-based systems make use of 64 different phase and amplitude combinations, resulting in the representation of 6 data bits per symbol. This makes it possible for standards such as 802.11a and 802.11g to support the higher data rates.

Figure 3-9. QAM Makes Use of Changes in Amplitude and Phase to Carry Information


Spread Spectrum

In addition to modulating the digital signal into an analog carrier signal using FSK, PSK, or QAM, some wireless networks also spread the modulated carrier over a wider spectrum in order to comply with regulatory rules. This process, called spread spectrum, significantly reduces the possibility of outward and inward interference. As a result, regulatory bodies generally don't require users of spread spectrum systems to obtain licenses.

Spread spectrum, developed originally by the military, spreads a signal's power over a wide band of frequencies. (See Figure 3-10.) Spread spectrum radio components use either direct sequence or frequency hopping for spreading the signal. Direct sequence modulates a radio carrier by a digital code with a bit rate much higher than the information signal bandwidth. Frequency hopping quickly hops the radio carrier from one frequency to another within a specific range. Figures 3-11 and 3-12 illustrate direct sequence and frequency hopping, respectively.

Figure 3-10. Spread Spectrum Occupies a Wide Portion of the RF Spectrum


Figure 3-11. Direct Sequence Is a Type of Spread Spectrum


Figure 3-12. Frequency Hopping Is a Type of Spread Spectrum


Most spread spectrum systems operate within the Industrial, Scientific, and Medicine (ISM) bands, which the FCC authorized for wireless LANs in 1975. The ISM bands are located at 902 MHz, 2.400 GHz, and 5.7 GHz. RF systems operating in the ISM band must use spread spectrum modulation and operate below 1 watt transmitter output power. Commercial users who purchase ISM band products do not need to obtain or manage FCC licenses. This makes it easy to install and relocate wireless networks because the hassle of managing licenses is eliminated. Because the ISM bands are open to the public, however, care must be taken to avoid RF interference with other devices operating in the same ISM bands.

Orthogonal Frequency Division Multiplexing

Instead of using spread spectrum, some wireless systems make use of Orthogonal Frequency Division Multiplexing (OFDM). OFDM divides a signal modulated with FSK, PSK, or QAM across multiple sub-carriers occupying a specific channel. (See Figure 3-13.) OFDM is extremely efficient, which enables it to provide the higher data rates and minimize multipath propagation problems.

Figure 3-13. OFDM Sends Multitudes of Data Simultaneously in Parallel


OFDM is becoming popular for high-speed transmission. In addition to being part of both 802.11a and 802.11g wireless LANs, OFDM is the basis for the European-based HiperLAN/2 wireless LAN standards. In addition, OFDM has also been around for a while supporting the global standard for Asymmetric Digital Subscriber Line (ADSL), a high-speed wired telephony standard.

Ultrawideband Modulation

Ultrawideband (UWB) modulation is beginning to take a stronger foothold instead of spread spectrum or OFDM in the wireless networking industry. While it has been used for a while by the military, UWB is now going through the necessary authorizations and developments for public and commercial use. Even though the advancement of UWB has been somewhat slow, UWB becoming a superior technology for many types of wireless networks is a possibility.

UWB uses low-powered, short-pulse radio signals in order to transfer data over a wide range of frequencies. A UWB transmission involves billions of pulses spread over several gigahertz. The corresponding receiver then translates the pulses into data by listening for a familiar pulse sequence sent by the transmitter.

UWB should initially deliver bandwidths from about 40 to 600 Mbps, and eventually data rates could be up to (with higher power). UWB systems also consume little power, around one ten-thousandth of cell phones. This makes UWB practical for use in smaller devices, such as cell phones, PDAs, and even watches that users can carry at all times.

Because UWB operates at such low power, it has little interference impact on other systems. UWB causes less interference than conventional RF systems. In addition, the relatively wide spectrum that UWB utilizes significantly minimizes the impact of interference from other systems.

Concerns still remain, however, about the interference of higher-power UWB systems. The FCC plans to reevaluate UWB in the near future, and they will take a closer look at the issue of higher-power systems. Until then, you're limited to UWB products with short-range propagation.