Spread Spectrum Signals (Part 2)
- The transmitter modulates the carrier signal with a baseband signal in the traditional way.
- At the same time, a pseudorandom signal is generated from a pseudorandom sequence of binary pulses, which seems random if the receiver doesn’t know the way it was generated. This signal has a wider bandwidth than the band-pass modulated and it called the spread sequence.
- The band-pass signal generated from the first modulation is modulated again with the spread sequence. Depending on the type of system, this modulation can be performed on different ways.
If the second modulation consists in multiplying the bad-pass signal by the spread sequence, the result in the frequency domain is the convolution between both signals. The spectrum of the spread sequence will be bigger, so the result from the modulation will have a bandwidth similar to that one. Despites different types of modulation can be computed, the result will be always a wider bandwidth signal. Therefore, the signal’s energy is split too along the frequency range: the power density function could be smaller than the channel noise, which will make more difficult to detect the signal by a receiver which is unable to perform the dispreading.
Spreading and dispreading computations are made over a pseudonoise signal that comes from a pseudorandom sequence. These sequences have properties very similar to real random sequences. However, pseudorandom sequences are periodic sequences, with a large period which has the same properties as a random sequence:
- It has to be balanced: the difference between the number of zero and ones has to be less or equal to one.
- The consecutive sequences of ones and zero have to distributed on this way: one half has to be of length one, ¼ a length of 2, 1/8 a length of three…and so on!
- It has to be uncorrelated.
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.
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