When we hear the term spread spectrum we tend to think of it as being something new, a recent development. In reality, this technology was developed almost fifty years ago. It was originally developed and used by the military for secure communications. The military declassified spread spectrum technology in the mid-1980s. Today, spread spectrum technology has the potential to revolutionize wireless communications (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 4).

History

The story behind the birth of spread spectrum technology is quite interesting. The story involves Hedy Lamarr, an actress, once known as "The Most Beautiful Girl in the World", and George Antheil, a musician, once known as "The Bad Boy of Music" (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 1).

Prior to World War II, Hedy was married to Fritz Mandl, an armaments manufacturer, who sold weapons to Hitler and to anyone else who wanted to buy them. Mandl was also researching weapons control systems. Lamarr was exposed to this technology during Mandl's business meetings. Lamarr later left her husband and eventually came to the United States (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 2).

George Antheil was among the first to use the player piano as an instrument for composed music. He was quite famous in Europe, but as Hitler gained power he returned to the United States (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 3).

Lamarr and Antheil met at a party. They both opposed the Nazis and wanted to do something to help the war effort. Lamarr told Antheil about her idea for a "Secret Communications System" that couldn't be intercepted by the enemy. It worked by sending information over multiple radio frequencies in a random pattern. The problem was ensuring that the transmitter and receiver stayed in sync. Antheil solved this problem by using paper rolls similar to those used in player pianos. They patented their invention in 1942, and gave it to the government to help the war effort. However, the military refused to take their invention seriously at that time. The patent however, was later developed as electronic technologies improved. Digital components replaced the paper rolls used for synchronization (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 3).

What is Spread Spectrum?

According to Dr. Robert Price of Consulting in Electronics Systems, spread spectrum refers to radio communications that:

• use cryptographic subsystems.
• use a wide frequency spreading factor.
• and are not dependent on a particular type of tonality in the transmitting wave form (MicroTimes: The Birth Of Spread Spectrum, 1999, p. 3).

How Does it Work?

Spread spectrum communications utilize wide band, noise-like signals operating in one of three frequency bands. These bands are 902 - 928 MHz, 2400 - 2483 MHz, and 5752 - 5850 MHz. It is this noise-like quality that makes the signals hard to detect, intercept, or jam. Because the signals are spread over a wide band they are less likely to interfere with narrow band communications (What is Spread Spectrum Technology, 1999, p. 1).

Types of Spread Spectrum Technology

There are several types of spread spectrum technology. They are as follows:

1. Direct sequence (pseudo-noise).
2. Frequency hopping.
3. Time hopping.
4. Chirp.
5. Hybrid methods (Jachimczyk, 1995, p. 3).

Direct sequence technology codes a message by digitizing it. It then uses a multi-bit pattern to represent each original bit in the message. This multi-bit pattern is called a chip. To the unintended receiver, the signals appear as low-powered wide-band noise and are rejected. This technique uses bandwidth very inefficiently (EIA/TIA-526-14, 1990, chp. 27, p. 11).

In frequency hopping, messages are time-sliced into blocks of fixed or varying lengths. The blocks are then transmitted, in any order, on different frequencies, in a pattern known to the intended receiver. The receiver gets the signals and puts them back in the correct order. To the unintended receiver, the signals appear to be short-duration impulse noise (EIA/TIA-526-14, 1990, chp. 27, p. 11).

Current Uses

The unique characteristics of spread spectrum systems make them suitable for a wide range of applications. We will examine a few of the most common uses of this technology.

1. Cellular Telephone and Packet Data Networks – we are all familiar with this use of technology in the form of cell phones and pagers (Spread Spectrum Technology Study - Applications, 1996, p. 1).
2. Personal Communications Systems – Licenses have recently been issued to several Canadian companies of the development of Personal Communications Systems. These systems will operate in the 1.9 GHz frequency range and will be capable of transmitting both voice and data (Spread Spectrum Technology Study - Applications, 1996, p. 1).
3. Cordless Phones – At the present time there are less than half a dozen cordless phones on the market that utilize spread spectrum technology. Those that do however, offer improved range and signal quality over cordless phones that use other types of technology, but at a significantly higher price (Spread Spectrum Technology Study - Applications, 1996, p. 2).
4. Wireless Modems – These modems can transmit data between two terminals at data rates higher than conventional modems, however, they do not have multiple access capability (meaning they can only connect two terminals). This limits their application (Spread Spectrum Technology Study - Applications, 1996, p. 2).
5. Wireless Bridges – These connect two or more separate networks together. They offer extremely high data rates (Spread Spectrum Technology Study - Applications, 1996, p. 2).
6. Wireless LANs – Wireless LANs typically have lower data rates and are rarely used as a standalone system. They are normally used in conjunction with and to augment a wired network (Spread Spectrum Technology Study - Applications, 1996, p. 2).
7. Global Positioning Systems – These systems communicate with low-orbit satellites to establish the exact location (longitude, latitude, and elevation), of whatever vehicle or device they are attached too (Spread Spectrum Technology Study - Applications, 1996, p. 3).
8. Automatic Vehicle Monitoring Systems – These systems use triangulation to pinpoint the location of a transmitter located in a vehicle. They can be used to recover stolen cars, or to facilitate fleet management (Spread Spectrum Technology Study - Applications, 1996, p. 3).
9. RF Identification Devices – These systems can be used in a wide range of applications such as, automatic toll collection, automatic inventory, and in security systems (Spread Spectrum Technology Study - Applications, 1996, p. 3).
10. Point of Sale Devices – If wireless modems are incorporated in these devices, sales people will be able to process transactions from anywhere in the store (Spread Spectrum Technology Study - Applications, 1996, p. 3).
11. Short-Range Digital-Video Spread Spectrum Systems – This technology uses video-compression techniques to reproduce high-quality color moving pictures. It can be used to augment a licensed-broadcast standard system or to provide coverage for rapid deployment (Spread Spectrum Technology Expands Video Applications, 1996, p. 1).

Spread spectrum technology can be used in situations where ordinary communications systems cannot be used because of the amount of Electromagnetic (EM) energy they give off. Two examples of such situations are in medical facilities that use sensitive diagnostic equipment, and in mining operations use explosives. According to the Strategic Microelectronics Consortium, by using Direct Sequence systems that spread the output power over a large bandwidth, the radiated EM energy is kept low enough for these systems to be used in EM-sensitive environments (Spread Spectrum Technology Study - Applications, 1996, p. 3).

Spread spectrum is an extremely efficient way of using radio waves to communicate. It allows many users to share the same radio frequencies without interference. Spread spectrum has the potential to revolutionize the wireless communications industry and make an enormous contribution to the public good (MicroTimes: The Birth of Spread Spectrum, 1999, p.1). I’m sure that in the future we will find many other uses for this technology.