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! 

The Journey into Communications Systems

This blog will showcase the twists and turns of our adventure as the amazing group 136 takes a journey into the unknown realm of 3G communication.

Matthew Davies and Hamish Smellie will need to keep one step ahead of the game to produce a first class project in this field of study. Our guide through this journey; Dr Zhu, trained in the art of communications systems engineering, has only a few weeks to turn the young engineers of group 136  into the next generation of super Engineers!