By:

NAVNEET BHARDWAJ (1/MTECHIT/USS/2003)JITENDER KUMAR (5/MTECHIT/USS/2003)

• Spread Spectrum (SS) communication was first described on paper by an actress and musician.

•In 1941, Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedoes and received U.S. patent #2.292.387.

•It was not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when the came alive, and has become increasingly popular for applications that involve radio links in hostile environments.

Spread spectrum is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information; the band spread is accomplished by means of a code which is independent of the data, and a synchronized reception with the code at the receiver is used for de-spreading and subsequent data recovery.

The term "spread spectrum" refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which occurs when a key is attached to the communication channel.

•To apply an SS technique, simply inject the corresponding SS code somewhere in the transmitting chain before the antenna. (That injection is called the spreading operation.)•The effect is to diffuse the information in a larger bandwidth. Conversely, you can remove the SS code (de-spreading operation) at a point in the receive chain before data retrieval.•The effect of a de-spreading operation is to reconstitute the information in its original bandwidth. Obviously, the same code must be known in advance at both ends of the transmission channel (In some circumstances, it should be known only by those two parties.)

Uses signal which have transmission bandwidth that has greater magnitude

Provides immunity to all multiple interference and robust multiple access capability

Not bandwidth efficient when used by single user

Provides boon to wireless system designers.

MULTIPLE ACCESS TECHNIQUES

FDMA: Frequency Division Multiple Access.• FDMA allocates a specific carrier frequency to a communication channel• It also allocates the number of different users is limited to the number of slices in the frequency spectrum.• FDMA channel carries only one phone circuit at a time.• If FDMA channel is not in use , then it sits idle and cannot be used by other users to increase or share capacity.• FDMA systems have higher cell site system costs as compared to TDMA systems.• Methods of FDMA access include radio broadcasting, TV, AMPS, and TETRAPOLE.

MULTIPLE ACCESS TECHNIQUES

TDMA :Time Division Multiple Access • In this case the different users speak and listen to each other according to a defined allocation of time slots•It shares a single carrier frequency with several users where each users makes use of non-overlapping time slots•Data transmission for uses of a TDMA system is not continuous•TDMA uses different time slots for transmission and reception, thus duplexers are not required.•Different communication channels can then be established for a unique carrier frequency•Examples of TDMA are GSM, DECT, TETRA, and IS-136

TDMA

There are two main types of SSMA :

Frequency hopped multiple access(FHMA)

Direct sequence multiple access(DSMA)

Digital multiple access systemIt causes the carrier to hop from frequency to frequency over a wide band according to a sequence defined by the PRN.Frequency hopped signal changes channel at rapid intervals.Provides a level of security when a large number of channels are used.

Types of FHMA

Fast frequency hopping (FFHMA): If the rate of change of the carrier frequency is greater than the symbol rate. Low frequency hopping(LFHMA): If the rate of change of the carrier frequency is smaller than the symbol rate

All the users in a CDMA system , use the same frequency and may transmit simultaneously.Unique orthogonal code is assigned to each channelFor recover the signal ,the receiver needs to know the code used by the transmitter.

Channels data rate are very high. The near-far problem occurs at a CDMA receiver if an undesired user has a high speed detected power as compared to the desired user.

Low power spectral density. Interference limited operation. Privacy due to unknown random codes. Applying spread spectrum implies the reduction of multi-path effects. Random access possibilities. Good anti-jam performance.

Research Papers :•Raymond L. Pickholtz, Donald Schilling “ Theory of Spread Spectrum Communications - A Tutorial”, IEEE Vol. 1982•Vladimir Marbukh and Nader Moayeri “A Queueing Model of a Spread Spectrum Multiple Access”, NIST•J. Meel “ Spread Spectrum (SS) Applications”, De Nayer Instituut, Belgium

Books :•Wireless Communications : Principles and Practices by Theodore S. Rappaport• Principles of Spread Spectrum Systems by Don Torrieri

Websites:

•http://www.sss-mag.com/ss01.html•http://www.maxim-ic.com/appnotes.cfm/appnote_number/1890/•http://www.spreadspectrum.co.uk/introductiontospreadspectrum.html