cdma

CDMA

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CONCEPT

• Based upon technology known as Spread Spectrum.

• While CDMA focuses on multiple access, Spread Spectrum offers a lot more.

STORY OF SPREAD SPECTRUM INVENTION

• The frequency hopping technique was invented by the actress Hedy Lamarr, bor in Austria in 1914.

• She married Friedrich Mandl, who was in the Defence Manufacturing business.

• During WW II, she attended many of her husband’s business meetings.

STORY

• She learned about the problems of remote controlled torpedoes used by the marines.

• Wirelessly controlled by English & Americans to direct them against German ships.

• Because the accuracy was not good, manual fine tuning was needed.

STORY

• For the wireless control, a narrow band signal was used.

• Once the German discovered the control signal they could easily jam that signal.

• Hedy Lamarr came up with the idea of multiple frequencies.

STORY

• She said that the comm should take place on one freq at a time and then jump to another channel fast enough so as not to give enough time to tune in the new freq to jam it.

• The change in freq should not be predictable to the enemy.

STORY

• Lamarr’s problem was that this freq hopping pattern had to be synchronized between the sender on the boat and the receiver in the torpedo.

• After some time, Lamarr’s friend George Antheil came up with a solution.

STORY

• Antheil had a bar in Paris with automatic pianos that were played by a piano roll.

• The roll had perforations in it which told the piano which keys had to be played and for how long.

• 88 perforations were possible in parallel.

STORY

• A tracker ball like thing was used to sense the holes and give command to piano.

• The same mechanism was thought of for the torpedo to keep synchronization.

• In 1962, spread spectrum was used by military for the first time in Cuba crisis.

PRINCIPLE OF SPREAD SPECTRUM

• It gained popularity through the needs of military communications.

• In contrast to narrow band comm, it is more resistant to hostile jammers and more difficult to intercept – fundamental conditions for tactile comm.


• For a comm system to be considered as Spread Spectrum, the criteria to be satisfied are:

BW of the transmitted spread signal has to be greater than the information BW (this criterion is also satisfied by FM, PCM and DM)

The spread signal is composed of the information signal and the spreading sequence. The spreading sequence has to be independent from the information in the statistical sense.


• The ratio of the spreading BW Bs to info BW Bi is denoted as the processing gain Gspreading = Bs/Bi of a spread spectrum system.

• The processing gain does not combat white noise with FM and PCM because the spread signal is independent of information signal.


• However, spread spectrum signals offer the following advantages for a wireless comm system:

spread spectrum modulation is capable of dealing with the multi path signals resulting from the radio channel.

These signals can be considered as interference and therefore be suppressed.


the receiver of a spread spectrum system is able to distinguish between different transmitted signals using the spreading sequence. Spreading sequence design is very important for multiple access capability. The spreading sequence is actually the identification for a transmitter-receiver pair.

Other advantages such as low probability Of interception, privacy and anti-jamming capability are more relevant for military needs.

SPREADING TECHNIQUES

• SPREAD SPECTRUM TECHNIQUES

DIRECT SEQUENCE TIME HOPPING FREQ HOPPING HYBRID

WIDEBAND NARROW BAND SLOW FAST

DS/FH DS/TH FH/TH DS/FH/TH


• In DSSS transmitter, the info signal is directly modulated by a spreading sequence.

• Main feature of DSSS is that multiple access capability can be achieved without synchronization between different transmitters.


• FHSS systems periodically change the carrier freq of the modulated information signal in a random fashion.

• During a time interval thop the freq is constant.

• Afterwards both the sender and receiver hop to another frequency.


• The whole BW B is divided into freq slices. The set of available frequencies is called a hop set with N freq slices.

• FHSS systems differ radically from DSSS systems in terms of freq occupancy.

• While DSSS systems occupy the whole BW all the time, FHSS systems use only one freq slice at any point in time.


• If one info bit is transmitted over several freq slices the freq hopper is referred to as fast.

• If several bits are transmitted over one freq slice it is referred to as a slow hopper.

• The relation of hop duration to info bit duration depends upon the number of hops per info bits.


• The adv of FHSS systems over DSSS systems is the less strict synchronization requirement.

• Within THSS, the time axis is divided into frames.

• A single wireless terminal (WT) will use only one slot out of k possible slots within one frame. Within this slot, the WT sends with a k times higher data rate in contrast to the situation in which the WT would transmit within the whole frame.


• Interference among simultaneous wireless terminals can be minimized if coordination between terminals can be achieved.

• This also avoids the near-far effect.

• In the absence of coordination, situations occur in which more than one terminal uses a time slot.

• The receiver will not be able to detect either of the signals correctly.


• Both terminals collide on the wireless link. For such cases, error correction schemes are required.

• Time hoppers have nearly the same acquisition time as that of discrete sequence systems, but their implementation is much simpler than that of a freq hopper.


• HSSS consist of combination of two or more basic SS systems.

• Complexity of Tx and Rx increases but offers a combination of their advantages.


• DSSS:• Advantages:o Best behaviour for multi-path rejectiono Best anti-jam rejectiono Best interference rejectiono No synchronization among terminalso Simple implementationo Most difficult to detect


• DSSS:

• Disadvantages: Near-far problem Require coherent bandwidth Long acquisition time Synchronization of code signal within

fraction of chip time.


• FH:

• Advantages: great amount of spreadingNo need of coherent BWShort acquisition timeInherent securityLess effected by the near-far effect


• FH:

• Disadvantages:

Complex hardware

Error correction is needed


• TH:• Advantages:

High BW efficiency

Less complex hardware

Less affected by the near-far effect


• TH:

• Disadvantages:

Error correction is needed

Long acquisition time

CDMA BASICS

• In FDMA the users are separated in the frequency domain and in TDMA the users are separated in time domain.

• Basically, FDMA and TDMA assign particular frequency or time slices to different wireless terminals.

CDMA BASICS

• When all slices are occupied in the system, no additional wireless terminal can be accommodated.

• Multiple access capability is also provided by CDMA.

• The most common technique of CDMA are FH-CDMA or DS-CDMA.

CDMA BASICS

• CDMA allows multiple users to simultaneously use a common channel for transmission of information.

• A CDMA transmitter will code its information with a code or spreading sequence particularly allocated for that user.

CDMA BASICS

• Afterwards, the transmitter sends the coded signal to the receiver. Using the same code sequence, the receiver decodes the received signal.

• The bandwidth of the coded signal is much larger than the information bandwidth.

CDMA BASICS

• One may say that the information signal was spread, where spread refers to the broadening of the signal spectrum in the frequency domain.

• The coding process is therefore also called spread spectrum modulation while the coded signal is called a spread spectrum signal.

CDMA BASICS

• The spreading of the information signal gives CDMA its multiple access capability.

• The main feature of CDMA system using pseudo noise sequences is that it can tolerate overload in terms of number of simultaneously supported users.

• In CDMA, system capacity is not fixed.

CDMA BASICS

cdma

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