Spread spectrum is a form of wireless communication developed for military and designed to provide secure communication. Spread spectrum was developed during World War II to prevent the Nazis from jamming and interception Allies military signals. It was invented by an actress Hedy Lamarr and a musician George Antheil. Spread spectrum has been further developed over the years. In the 50s engineers from Sylvania Electronics started research into spread spectrum technology. They developed an electronic spread spectrum communication system which was used during Cuban missile crisis for secure communication. Because spread spectrum is hard to detect and intercept it was used by military and kept in secret until 1980s when commercial uses of spread spectrum systems started. Nowadays spread spectrum technology (because of it properties) is used in large variety of different applications such as: GPS (Global Positioning System), mobile and cordless phones, wireless video cameras, LAN (Local Area Networks).
SPREAD SPECTRUM AND ITS TECHNIQUES
Spread spectrum is a noise-like signal which spreads the transmitted signal over a large frequency but the signal PSD (Power Spectral Density - power in the signal per frequency) is very small what makes it hard to detect, intercept or demodulate. In contrast, narrowband signals (where PSD is higher and limited to a very narrow portion of the frequency bandwidth) can be easy jammed and experience interference.
Figure Block diagram of spread spectrum communication system.
Lawrie Brown. Data and Computer Communications. Chapter 9 - Spread Spectrum (online) Available at: http://www.slideshare.net/janudhakad/09-spread-spectrum [Accessed 22 October 2012]
Figure "Comparison of narrowband signal with direct sequence spread spectrum signal."
Bible, Steve. (1995) TAPR Spread Spectrum (online) Available at: http://www.tapr.org/ss_intro.html [Accessed 20 October 2012]
Usage of wide bandwidth by spread spectrum which makes it hard to jam can be described by Shannon's channel capacity theorem:
C - Channel capacity (bps).
B - Required channel bandwidth in hertz (Hz).
S - Signal power (W).
N - Noise power (W).
This equation states that channel capacity is proportional to bandwidth and signal-to-noise ratio. In situation where the noise is "bigger" than the signal the best way to reduce impact of the noise on the signal is to increase the bandwidth.
Because spread spectrum slightly increases noise level and its resistance to interference allows spread spectrum system coexist with narrowband signals which is another advantages of this system.
There are five types of spread spectrum systems: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS) and hybrid spread spectrum.
Frequency hopping spread spectrum (FHSS)
FSSH is used by the original Bluetooth standard and personal area network (PAN). In this system the frequency spectrum is divided into channels. Data signal is split up and transmitted on these channels in pseudorandom pattern which is known by transmitter and receiver only. If there are other collocated networks they will use different pseudorandom pattern or hop code table which allows multiple network coexist in close proximity without interfering. If interference is present on one the channel then receiver and transmitter changes the channel and resends the signal.
There are two types of frequency hopping:
Slow frequency hopping (SFH) where one data symbol is transmitted in the same channel which allows coherent data detection and deploys error correction mechanism.
Fast frequency hopping (FFH) where channel is changed several times during the symbol which increases resistance to jamming and interference, and decreases probability of intercept.
FHSS is difficult to intercepts, decreases narrowband interference, increases signal capacity and it can share frequency with other types of transmissions with slight interference.
All these features listed above make FFSH the best solution for installation designed to cover areas where there is a lot of collocated systems like Broadband Wireless Access.
Figure "Frequency hopping spread spectrum signal example."
Lawrie Brown. Data and Computer Communications. Chapter 9 - Spread Spectrum (online) Available at: http://www.slideshare.net/janudhakad/09-spread-spectrum [Accessed 22 October 2012]
Direct sequence spread spectrum (DSSS)
DSSS is another type of spread spectrum is 3G cellular networks, CDMA based networks and WLANs. DSSS encoder modulates the carrier wave with code sequence which is usually pseudorandom binary code. This code sequence (also known as "pseudo-noise" or PN) changes the carrier phase of the transmitted signal. The receiver must use the same code to decode the signal. DSSS uses more bandwidth than FHSS what increases number of bits in the spreading code and it also has low power density what makes it hard to detect. DSSS generates more bits per second and the signal is spread over a wide range of frequencies. All these features provide good performance and resistance to noise and interference.
Figure "Direst sequence spread spectrum example"
Lawrie Brown. Data and Computer Communications. Chapter 9 - Spread Spectrum (online) Available at: http://www.slideshare.net/janudhakad/09-spread-spectrum [Accessed 23 October 2012]
Time-hopping spread spectrum (THSS)
THSS is a type of SS where turned off and on condition of a carrier is determine by pseudorandom code sequence. In this type of spread spectrum period and duty cycle (pulse duration divided by the pulse period) of a pulsed Radio Frequency carrier are varying by using pseudorandom code sequence. THSS spectrum is often used with FHSS and they form a system called time division multiple access (TDMA) which is used in military
Chirp spread spectrum (CSS)
CSS is a system uses wideband linear frequency modulated chirp pulses which encode the information. A chirp is a sinusoidal signal and its frequency changes over time. CSS as well as other types of SS uses entire allocated bandwidth to broadcast a signal making it robust to channel noise
http://www.ieee.hr/_download/repository/Introduction%20to%20Spread%20Spectrum%20Systems.pdf
http://www.maximintegrated.com/app-notes/index.mvp/id/1890
