Spread Spectrum Signals (Part 1)
- The transmission bandwidth is bigger than the necessary for a normal transmission. If R is the data rate, a normal modulation would have a bandwidth similar to R Hz. If W is the bandwidth used for the spread spectrum signal, the following relation is true: W/R>>1.
- Spread spectrum is achieved thanks to a pseudorandom sequence, which is similar to noise. Therefore, the transmitted signal will have pseudorandom features, and it could be modulated only if the receiver can generate the same pseudorandom sequence than the transmitter uses.
The specific characteristics of the spread spectrum signal are:
- They are more resistant to interference than the ones used in common transmissions.
- It is a difficult signal to detect, because its power level is low. The SNR is recovered once the signal is demodulated.
- It’s impossible to detect the signal, unless the pseudorandom sequence is detected (useful for undesired receivers).
- The transmission is robust in multipath environments.
- It’s possible the transmission of several signal at the same time though the same channel, because they are uncorrelated (actually, they are orthogonal). So there is no interchannel interference. This technique is known as CDMA.
In the next post, we will see how the spread spectrum system performs!
During the last years, the demand for mobile communication systems has spectacularly increased: at present, there are 6,800 million of mobile devices for a population of 7,000 people million in the world, and it is expected that, by 2014, there will be 7,300 million, according to the ITU. That is the reason why these systems have evolved, developing new technologies more efficient each time. The latest standard in mobile communication systems, Long Term Evolution (LTE) shows this evolution. It incorporates highly efficient techniques such as OFDM (Orthogonal Frequency Division Multiplexing) and other techniques like MIMO (Multiple-Input Multiple-Output). In the next post, we will see the characteristics of the physical layer of LTE, mainly OFDM, in order to justify why its use is strikingly increasing in mobile communications.
Comments and feedback will be welcomed!