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.

Code Division Multiple Access (CDMA)

CDMA is a form of spread spectrum signalling where multiple users can share the same bandwidth simultaneously due to each user being assigned a unique sequence or code.

Above shows how two devices using the same code can communicate at the same time as another two devices due  each pair using their own unique code.

There are two types of CDMA:

• Direct Sequence (DS-CDMA)
• Frequency Hopping (FH-CDMA)

DS-CDMA

This is used in digital cellular systems like 3G. It works by assigning each user with a spreading code which is orthogonal to other codes.

FH-CDMA
The carrier frequencies of the individual users constantly changes with time.

Advantages of CDMA

• The security is better compared to the earlier generations which used FDMA or TDMA, which is Frequency Division and Time Division.
• Due to having individual codes for each user, it has immunity of interference from other users.
• It has the ability to cope with multipath channels.
• It also has good flexibility, so that a user can increase their rate without affecting other users.

Disadvantages of CDMA

• Due to the coding each device needs to be able to process the complex signal.
• If requires a large bandwidth.

Spectrum Allocation

The pie chart below shows that 50% of the spectrum holdings are currently allocated for use by public sector bodies.

The spectrum is an important and finite resource, and the allocation of spectrum is managed by means of licences. Ofcom are the spectrum relegator and they have to strike a balance between the needs of customers, spectrum users and service providers, and the public purse.

This is the overall uk SPECTRUM allocation.

The allocation of spectrum is managed by means of licences. An act called the Wireless Telegraphy Act 1998 meant that auctions were conducted to grant licences. In 2000 the first auction took place, and since then the oversight of the auctions has been passed to Ofcom, due to its formation under the Communications Act 2003.

3G Coverage in the UK

A Comparison of some of the major mobile network providers in the UK and their 3G coverage.

Of the major mobile telecommunication companies in the UK, 3G has the best service in terms of internet speed but the downside on this is that the service coverage is not as widespread as many of the others. Vodafone is noted as having the most consistent service with a wider coverage across the UK. The average price of joining Vodafone is on average more expensive than others however the service that you are provided with is near faultless. More money does mean better quality after all!