Spread-Spectrum LANs

Spread spectrum is assigned the industrial, scientific, and medical (ISM) bands of the electromagnetic spectrum. The ISM bands include the frequency ranges of 902 MHz to 928 MHz and 2.4 GHz to 2.484 GHz, which do not require an FCC site license.

Spread spectrum is based on a digital coding technique in which the signal is taken apart or "spread" so that it sounds more like noise, making interception difficult. The coding operation increases the number of bits transmitted and expands the bandwidth used. With the signal's power spread over a larger set of frequencies, the result is a more robust signal that also is less susceptible to impairment from electrical noise and other sources of interference.

Using the same spreading code as the transmitter, the receiver correlates and collapses the spread signal back down to its original form. Spread spectrum is used for wireless Ethernet LANs and is the basis for other advanced wireless transmission techniques such as code division multiple access (CDMA), which is being used to support a variety of services, including emerging personal communications services (PCS).

Spread spectrum is a highly robust wireless data transmission technology that offers substantial performance advantages over conventional narrowband radio systems. As noted, the digital coding technique used in spread spectrum takes the signal apart and spreads it over the available bandwidth, making it appear as random noise. Noise has a relatively flat, uniform spectrum with no coherent peaks and can generally be removed by filtering. The spread signal has a much lower power density, but the same total power.

This low power density, spread over the expanded transmitter bandwidth, provides resistance to a variety of conditions that can plague conventional narrowband radio systems, including:

• Interference: a condition in which a transmission is being disrupted by external sources, such as the noise emitted by various electromechanical devices, or internal sources such as crosstalk.
• Jamming: a condition in which a stronger signal overwhelms a weaker signal, causing a disruption to data communications.
• Multipath: a condition in which the original signal is distorted after being reflected off solid objects.
• Interception: a condition in which unauthorized users capture signals in an attempt to determine its content.

Spread spectrum also is able to achieve a higher transmission rate than narrowband radio systems (nonspread spectrum). Narrowband radio systems transmit and receive on a specific frequency that is just wide enough to pass the information, whether voice or data. By assigning users different channel frequencies, confining the signals to specified bandwidth limits, and restricting the power that can be used to modulate the signals, undesirable crosstalk—interference between different users—can be avoided. These rules are necessary because any increase in the modulation rate widens the radio signal bandwidth, which increases the chance for crosstalk.

For example, current narrowband radio systems generally cannot transmit data beyond 9.6 Kbps without violating the FCC-established narrowband channel spacing of 25 kHz between radio carriers. Figure 1 shows radio systems operating within their own assigned 25-kHz radio channels (top) and a radio system impaired by crosstalk from an adjacent radio system that is exceeding its channel bandwidth to achieve a higher transmission rate (bottom). Some narrowband radio systems operate this way, but they require FCC approval.

Figure 1: How crosstalk impairs the proper operation of a radio system. (a) Proper operation. (b) Channel interference.

Current spread-spectrum LANs provide transmission rates of up to 6 Mbps. The higher speed of spread spectrum over conventional narrowband radio is primarily attributable to the use of very wide channels, which can be up to 500 kHz in the 900-MHz band and 1 MHz in the 2.4-GHz band, versus the 25 kHz channels typically used for narrowband radio.

The main advantage of spread-spectrum radio waves is that the signals can be manipulated to propagate fairly well through the air, despite electromagnetic interference, to virtually eliminate crosstalk. In spread-spectrum modulation, a signal's power is spread over a larger band of frequencies. This results in a more robust signal that is less susceptible to impairment from electrical noise and interference from similar radio-based systems, since they too are spreading their signals, but with different spreading algorithms.

There are several spreading techniques. Direct sequence and frequency hopping are the most common spreading techniques used in the LAN environment.