Sending electromagnetic signals through the air became possible in the later 19th century with wireless telegraphy. The fact that signals were affected by natural interference was soon discovered. By the early 1900s, wireless telegraphy could cross the Atlantic. Intercepting the enemy's radio communications and interfering with their transmission was important in World War II. Messages in code were intercepted and recorded for decryption and translation. Not only can radio signals be detected, but those listening can be detected, since radio receivers also emit a signal.
Above all, since radio direction-finding is widely used in civil and military aviation, ways are needed to reduce interference with these transmissions. The main method is spread spectrum. Spread spectrum systems transmit the message using a bandwidth that is wider than the bandwidth actually needed by the message signal. This spreading of the transmitted signal over a large bandwidth make the resulting wideband signal appear as a noise signal. This resists intentional and unintentional interference with the transmitted signal.
- Direct-sequence spread spectrum transmits the messages and adds chips of noise to the message. The sequence of chips produced by the transmitter is already known by the receiver. The receiver uses the same pseudonoise (PN) sequence to reconstruct the information signal. Benefits:
- It resists intended or unintended jamming
- It shares a single channel among multiple users
- It reduces the signal/background-noise level: this hampers interception
- Determines the relative timing between transmitter and receiver
- Frequency-hopping spread spectrum transmits radio signals by rapidly switching a carrier among many frequency channels. It uses a pseudorandom sequence known to both transmitter and receiver. Its benefits are:
- Spread-spectrum signals can resist narrowband interference.
- Spread-spectrum signals are difficult to intercept. A spread-spectrum signal may simply look like an increase in the background noise to a narrowband receiver.
- Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with little interference.
- Military users can generate the channel sequence with a transmission security key (TRANSEC) that the sender and receiver share in advance. For such critical uses, spread spectrum methods alone do not give enough security.
References[change | change source]
- Debag-Montefiore, Hugh 2000. Enigma: the battle for the code. London: Phoenix. ISBN 0-75381-130-8
- Rappaport, Theodore 2010. Wireless communications principles and practice. 2nd ed, Prentice-Hall, p. 458.
- Shaw, William T. 2006. Cyber Security for SCADA Systems. PennWell Books, p. 76. ISBN 9781593700683