Synchronization of Spread Spectrum Signals

For the reciever to be able to demodulate the recived signal, the reciever clock and the transmitter clock need to be in sync. So this is why the reciver clock and the transmitter clock are extremely stable and accurate. This would work well if the reciver was right next to the transmitter, but in most cases they are not very close at all. Therefore there is a time uncertainty.

This is the formual for the time delay, where N is the number of chips in the spreading code and Tc is the time taken for on chip.
So if we were to search over the time uncertainty inerval in time steps of 1/2Tc, then the time required to establish initial synchronization is:

Testing with multiple users

Here we have the results of using 2 separate users with different spreading codes and data. The blue coloured plot represents the components for User 1 and the red coloured plots represent the components for User 2. The combined spread signal or 'interference pattern' is purple. The basic method of receiving the original data from the interference pattern is to multiply the pattern with the original spreading code, and then divide the signal by however many users are in the interference signal.

Walsh code spreading

The figure to the left shows how the Walsh code is multiplied with the data to give a spread signal with a higher frequency. The Walsh code and the data were both created using Matlab along with the plotted graphs.  

Pseudo Noise

Pseudo Noise is an important feature of modulating data that is transmitted wirelessly. Also known as Pseudo Random code, it is a sequence of numbers that look random, but can be recreated relatively easily.

The pseudo noise is generated using an algorithm based on inputted numbers and this 'noise' is multiplied with the data that needs to be transmitted to produce a 'spreaded' version of the signal. The spreading of the signal usually transforms the signal into one that is below the natural noise level. This makes the signal hard to trace and intercept or scramble.

Spread Signal below noise level

The pseudo noise is unique to each user, so that users can transmit and receive data simultaneously without the user receiving the wrong data or the data of one user transforming with another because the have the same spreading code (pseudo noise code).

There are a few pseudo noise techniques that are used for CDMA, such as; Gold Codes, Walsh- Hadamard Code, M-Sequence Code, Barker Code and Kasami Code.

We will be focussing mainly on Walsh- Hadamard Code and M-Sequence Code.