Spread Spectrum Communication Techniques

A means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the bandwidth of the unspread bandwidth. Spread spectrum signals are typically transmitted by electromagnetic waves in free space with usage in both no military and military systems.

Motivation for using spread spectrum signals is based on the following facts:

• These systems have the ability to reject intentional and unintentional jamming by interfering signals so that information can be communicated
• Spread Spectrum signals have a low probability of being intercepted or detected since the power in the transmitted wave is "spread" over a large bandwidth or frequency extent
• Since these signals cannot be readily demodulates without knowing the code or cypher, and it's precise timing, message privacy is obtained
• The wide bandwidth of the spread spectrum signals provides tolerance to multipath (reflected waves that take longer to arrive at the receiver than the direct desired signal)
• A high degree of precision in ranging (distance measuring) can be obtained by using one type of spread spectrum signal, with applications to navigation
• Multiple access, or the ability to send many independent signals over the same frequency band.

There are four generic types of spread spectrum signals:

• Direct Sequence (DS) - The carrier of a DS source stays at a fixed frequency. Narrowband information is spread out into a much larger bandwidth using a pseudo-random chip sequence.
• Pseudo noise (PN)
• Frequency Hopping (FHA)
• Linear Frequency Modulation (chirp)

Chirp Modulation: This is an older spread spectrum method that was developed for radar use. The basic idea is to transmit a long rectangular pulse whose carrier frequency is linearly increased from f1 to f2 (f2>f1). The frequency-modulated signal returned from the target passes through a filter in the receiver at a velocity of propagation proportional to frequency. The result is a pulse that is much shorter in time duration than the transmitted pulse with a larger peak power content. Unchirped pulses such as interference or jamming pulses do not "compress" at the receiver, so that this method yields a processing gain or advantage for the chirpped signal.

Time Hopping: Not normally used alone, is a method in which the transmitted pulse occurs in a manner determined by a pseudorandom code which places the pulse in one of n possible positions per frame. If n is sufficiently large, then the pulse is on only 1/n of the time, and again the transmitted pulse has a processing gain against a full frame jamming pulse of equal energy.

Direct Sequence Systems: Direct sequence systems were once the most prevalent method of communicating in spread spectrum communications. Direct sequence modulation is characterized by phase-modulating a sine wave by an unending string of pseudo noise code chips (symbols of much smaller duration than a bit). This unending string is typically based on a pseudo noise code that generates an apparently random sequence of code chips that repeat only after the code period.