Newsletter of the Mercurians, in the Society for the History of Technology
Volume 10 No. 1, November 1997
Book Reviews
Tube
Media and Revolution
Antenna: Spread Spectrum
A Capsule Explanation
The term "spread spectrum" (SS) came into use only after years during which innovators in radio signal propagation used other less elegant, but more intuitive, terms. "Pseudonoise," for instance, served quite well in describing this manipulation of signals because, to a receiver not synchronized with the transmitter, incoming signals came through as meaningless noise. But since the apparent noise was generated purposefully, it was false, or "pseudo" noise. The trick in SS is to control the noise so that it protects the data stream (information you want to send) both from interference and uninvited audiences for radar, radio, and television signals.
How does it work? The information in a normal radio signal can be thought of as moving along a range of frequencies (bandwidth) that can be plotted as a rather narrow curve; that curve is called the signal spectrum. The essential feature of spread-spectrum technology is the expansion, or spread, of the bandwidth of the transmitted signal (carrier wave plus data stream) well beyond that at which it incorporates the data stream to be communicated. This bandspreading is controlled by a pattern (or code) which is independent of the data stream, so the transmission combines both the information signal and a pseudonoise pattern. Synchronized reception using the same pattern at the receiver achieves despreading for subsequent recovery of the data-stream (the information).
In other words, spread-spectrum systems use a sequential noise-like signal structure. Today, SS is usually implemented in one of two ways, or a combination of both. In frequency-hoping (FH), the carrier signal, which has a more-or-less constant frequency, has its frequency changed quite often. Modulated by the data stream, the frequency is transmitted for a little time at one frequency, a little time at another frequency, according to a long pattern known only at the transmitter and the intended receiver. After allowing for the time delay for propagation of the signal, the intended receiver listens for a little time at one frequency, a little time at another frequency, according to the same pattern, thereby enabling the receiver to recover the modulated carrier signal and its information.
In the direct-sequence (DS) approach, the carrier signal is noise-like. The phase of the carrier signal, which has a constant frequency, is switched (say, by 180 degrees) at irregular intervals according to a long pseudonoise (PN) pattern before it is modulated by the data stream. After allowing for the propagation-time delay, the intended receiver compares what it receives with a replica of the noise-like carrier (generated using the same PN pattern) and extracts the data stream from it.
Strictly speaking, conventional FM broadcasting utilizes a spectrum-spreading technique, offering a significant amount of interference immunity, as evidenced by the comparatively noise-free reception in the commercial FM band during a lightning storm. In comparison, static plagues reception in the commercial AM band. However, FM can not provide the privacy of spread spectrum transmissions.