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Old 29th March 2005, 06:39 AM   #1
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Join Date: Aug 2004
Location: California
Default If you love wireless

Then you owe a debt to actress Heddy Lamar.

Spread Spectrum
Spread spectrum is a method commonly used to modulate the information into manageable bits that are sent over the air wirelessly. Spread spectrum was invented by Heddy Lamar, a film actress who still retains the patent to this day and was the relatively recent recipient of a governmental award for this accomplishment.

Essentially, spread spectrum refers to the concept of splitting information over a series of radio channels or frequencies. Generally, the number of frequencies is in the range of about 70, and the information is sent over all or most of the frequencies before being demodulated, or combined at the receiving end of the radio system.

Two kinds of spread spectrum are available:

Direct sequence spread spectrum (DSSS)


Frequency hopping spread spectrum (FHSS)


DSSS typically has better performance, while FHSS is typically more resilient to interference.

A commonly used analogy to understand spread spectrum is that of a series of trains departing a station at the same time. The payload is distributed relatively equally among the trains, which all depart at the same time. Upon arrival at the destination, the payload is taken off each train and is collated. Duplications of payload are common to spread spectrum so that when data arrives excessively corrupted, or fails to arrive, the redundancies inherent to this architecture provide a more robust data link.

Direct sequence spread spectrum (DSSS) is a signaling method that avoids the complexity and the need for equalization. Generally, a narrowband QPSK signal is used. This narrowband signal is then multiplied (or spread) across a much wider bandwidth. The amount of spectrum needed is expressed as: 10(SNR/10) narrowband symbol rate.

Therefore, if a SNR of 20 dB is required to achieve the appropriate BER, the total spread bandwidth needed to transmit a digital signal of 6 Mbps equals 600 MHz.

This is not very bandwidth-efficient. In addition, the sampling rate for the receiver needs to be about 100 times the data rate. Therefore, for this hypothetical system, the sampling rate would also need to be 600 megasamples per second.

With DSSS, all trains leave in an order beginning with Train 1 and ending with Train N, depending on how many channels the spread spectrum system allocates. In the DSSS architecture, the trains always leave in the same order, although the numbers of railroad tracks can be in the hundreds or even thousands.

Code division multiple access (CDMA) is used to allow several simultaneous transmissions to occur. Each data stream is multiplied with a pseudorandom noise code (PN code). All users in a CDMA system use the same frequency band. Each signal is spread out and layered on top of each other and is overlaid using code spreading in the same time slot. The transmitted signal is recovered by using the PN code. Data transmitted by other users looks like white noise and drops out during the reception phase. Any narrowband noise is dispersed during the de-spreading of the data signal. The advantage of CMDA is that the amount of bandwidth required is now shared over several users. However, in systems in which there are multiple transmitters and receivers, proper power management is needed to ensure that one user does not overpower other users in the same spectrum. These power management issues are mainly confined to CMDA architectures.
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