Spread spectrum is a digital coding technique in which the signal is taken apart or “spread” so that it sounds more like noise to the casual listener, allowing many more users to share the available bandwidth while affording each conversation a high degree of privacy. Actress Hedy Lamarr (Figure S-5) and composer George Antheil share the patent for spread-spectrum technology.
Their patent for a “Secret Communication System,” issued in 1942, was based on the frequency-hopping concept, with the keys on a piano representing the different frequencies and frequency shifts. Lamarr had become intrigued with radio-controlled missiles and the problem of how easy it was to jam the guidance signal. She realized that if the signal jumped from frequency to frequency quickly—like changing stations on a radio—and both sender and receiver changed in the same order at the same time, then the signal could never be blocked without knowing exactly how and when the frequency changed.
Although the frequency-hopping idea could not be implemented at that time because of technology limitations, it eventually became the basis for cellular communication based on Code Division Multiple Access (CDMA) and wireless Ethernet LANs based on infrared technology.
Frequency Assignment
Spread spectrum uses the industrial, scientific, and medical (ISM) bands of the electromagnetic spectrum. The ISM bands include the frequency ranges at 902 to 928 MHz and 2.4 to 2.484 GHz, which do not require a site license from the FCC. 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. The coding operation increases the number of bits transmitted and expands the bandwidth used. Noise has a flat, uniform spectrum with no coherent peaks and generally can 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 narrowband radio systems, including:
- Interference Acondition in which a transmission is being disrupted by external sources, such as the noise emitted by various electromechanical devices, or internal sources such as cross-talk.
- Jamming Acondition in which a stronger signal overwhelms a weaker signal, causing a disruption to data communications.
- Multipath Acondition in which the original signal is distorted after being reflected off a solid object.
- Interception Acondition in which unauthorized users capture signals in an attempt to determine its content.
Non-spread-spectrum narrowband radio systems transmit and receive on a specific frequency that is just wide enough to pass the information, whether voice or data. In 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 cross-talk—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 cross-talk. 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 cross-talk.
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 interference from similar radio-based systems, since, although they too are spreading their signals, they use different spreading algorithms.
Spreading
Spread spectrum is a digital coding technique in which a narrowband signal is taken apart and “spread” over a spectrum of frequencies (Figure S-6). The coding operation increases the number of bits transmitted and expands the amount of bandwidth used. With the signal’s power spread over a larger band of frequencies, the result is a more robust signal that is less susceptible to impairment from electromechanical noise and other sources of interference.
It also makes voice and data communications more secure. Using the same spreading code as the transmitter, the receiver correlates and collapses the spread signal back down to its original form. The result is a highly robust wireless data transmission technology that offers substantial performance advantages over conventional narrowband radio systems. There are two spreading techniques in common use today: direct sequence and frequency hopping.
Direct Sequence In direct sequence spreading—the most common implementation of spread-spectrum technology—the radio energy is spread across a larger portion of the band than is actually necessary for the data. Each data bit is broken into multiple subbits called “chips.” The higher modulation rate is achieved by multiplying the digital signal with a chip sequence. If the chip sequence is 10, for example, and it is applied to a signal carrying data at 300 kbps, then the resulting bandwidth will be 10 times wider.
The amount of spreading depends on the ratio of chips to each bit of information. Because data modulation widens the radio carrier to increasingly larger bandwidths as the data rate increases, this chip rate of 10 times the data rate spreads the radio carrier to 10 times wider than it would otherwise be for data alone. The rationale behind this technique is that a spreadspectrum signal with a unique spread code cannot create the exact spectral characteristics as another spread-coded signal.
Using the same code as the transmitter, the receiver can correlate and collapse the spread signal back down to its original form, while other receivers using different codes cannot. This feature of spread spectrum makes it possible to build and operate multiple networks in the same location. When each network is assigned its own unique spreading code, all transmissions can use the same frequency range yet remain independent of each other.
The transmissions of one network appear to the other as random noise and are filtered out because the spreading codes do not match. This spreading technique would appear to result in a weaker signal-to-noise ratio, since the spreading process lowers the signal power at any one frequency. Normally, a low signal-to-noise ratio would result in damaged data packets that would require retransmission.
However, the processing gain of the despreading correlator recovers the loss in power when the signal is collapsed back down to the original data bandwidth but is not strengthened beyond what would have been received had the signal not been spread. The FCC has set rules for direct sequence transmitters.
Each signal must have 10 or more chips. This rule limits the practical raw data throughput of transmitters to 2 Mbps in the 902-MHz band and 8 Mbps in the 2.4-GHz band. The number of chips is directly related to a signal’s immunity to interference. In an area with a lot of radio interference, users will have to give up throughput to successfully limit interference.
Frequency Hopping In frequency hopping, the transmitter jumps from one frequency to the next at a specific hopping rate in accordance with a pseudo-random code sequence. The order of frequencies selected by the transmitter is taken from a predetermined set as dictated by the code sequence. For example, the transmitter may have a hopping pattern of going from Channel 3 to Channel 12 to Channel 6 to Channel 11 to Channel 5, and so on.
The receiver tracks these changes. Since only the intended receiver is aware of the transmitter’s hopping pattern, then only that receiver can make sense of the data being transmitted. Other frequency-hopping transmitters will be using different hopping patterns that usually will be on noninterfering frequencies. Should different transmitters coincidentally attempt to use the same frequency and the data of one or both become garbled at that point, retransmission of the affected data packets is required.
Those data packets will be sent again on the next hopping frequency of each transmitter. The FCC mandates that frequency-hopped systems must not spend more than 0.4 second on any one channel each 20 seconds, or 30 seconds in the 2.4-GHz band. Furthermore, they must hop through at least 50 channels in the 900-MHz band or 75 channels in the 2.4-GHz band. These rules reduce the chance of repeated packet collisions in areas with multiple transmitters.
Direct sequence spread spectrum offers better performance, but frequency-hopping spread spectrum is more resistant to interference and is preferable in environments with electromechanical noise and more stringent security requirements. Direct sequence is more expensive than frequency hopping and uses more power.
Although spread spectrum generally provides more secure data transmission than conventional narrowband radio systems, this does not mean the transmissions are immune from interception and decoding by knowledgeable intruders with sophisticated tapping equipment. For this reason, many vendors provide optional encryption for added security.
