Question

Explain with the help of block diagram, spread spectrum modulation system.

Answer 1

SPREAD-SPECTRUM COMUNICATION SYSTEM 

Baseband DS spread spectrum communication system model (a)transmitter (b) channel (c)receiver

Explanation:-

In actual DSSS system, the binary data to be transmitted is first carrier modulated using PSK and then this modulated signal is subjected to spreading by multiplying it by the PN sequence. However, in order to discuss the effect of multiplying the data sequence by the PN sequence, for the present we shall consider only a base band signal.

Let the data sequence be denoted by d(t) and the PN sequence be denoted by c(t). Let the data duration be Tb sec and PN sequence duration be Tc sec. In DSSS it is always so arranged that Tc<< Tb

The wave form d(t) is a narrow band signal, while the c(t) wave form is a wide band signal. The product wave form s(t) will have spectrum which is almost like spectrum of c(t), the PN sequence.

In order to illustrate how the spread – spectrum modulation enables us to reject the deterministic interfering signals added to the transmitted signal s(t) during the course of its passage through the channel, we are adding the interfering signal i(t) to the DSSS signal s(t).

Since the interference is additive

The first operation to be performed at the receiver is to de-spread the received signal. For this purpose, it is multiplied by the PN sequence waveform c (t), which is assumed to be in perfect synchronism with the c(t) used at the transmitter side.

We find that when we de-spread the message or data waveform d(t), the interference signal is spread over a wide bandwidth by getting multiplied by the PN sequence waveform c(t).

Thus we find that z(t) consists of a narrow band component d(t) and a wide band component i(t).c(t).

Z(t) is integrated over a period of Tb , data bit duration. The integrator acts as a low pass filter and removes the wide band component thus achieving suppression of the interfering signal.

The output of the integrator gives a voltage v, whose value depend on whether the d(t) was +1 or -1 during interval Tb .

This voltage is given to the comparator which acts as the decision device and says that d(t) was 1 during that Tb if v>0 and that it was a -1 if v<0.

OR

Explanation Of Spread Spectrum Modulation System:-

1. Basic elements of a spread spectrum signal modulation system is shown below.

2. Channel encoder adds extra bits to the information binary sequence for error detection & correction purpose.

3. PN sequence generation at the transmitter & receiver generates identical PN binary valued sequence

4. PN sequence is impressed on the information signal at the modulator (Tx) and remove from the received signal at the Demodulator.

5. Synchronization of the PN sequence generator at the receiver with the PN sequence contained in the incoming received signal is required in order to demodulate the received signal.

Prior to the transmission of information Synchronization may be achieved by transmitting a fixed PN sequence pattern which the receiver will recognize in the presence of interference with high probability.