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The advantage of CDMA

Multiple access technologies

The difference between TDMA, FDMA and CDMA

FDMA: Frequency Division Multiple Access

Means dividing the whole spectrum frequency into many single channels (transmit/ receive),

each channel can transmit or receive in one way voice or control information (Analog cellular

system).

TDMA: Time Division Multiple Access

Means that one wireless carrier of bandwidth is divided into many time slot channels in

terms of time, so every user occupies a time slot within can transmit or receive signals in the specific

time slot.

CDMA: Code Division Multiple Access

Means that users can transmit o receive information in the same frequency at the same time

but with different codes. The key of this technology is that every signals before transmission should

be modulated by different Spreading Code then mixed, thus the signal can be reverted only by the

user who have the same code.

Characteristics of CDMA

In CDMA system interference between users or cells is permitted, so adjacent cells can be

deployed with the same frequency, thus spectrum efficiency is very high and the capacity also verylarge, but it caused self-interference, if interference is out of control the quality of the system will be

worse.

The security in CDMA system is granted by the spreading operation because the signal is

close to noise and only the user who have the same code of spreading can revert the signal.

High spectrum efficiency

Soft capacity: Quality, coverage, interference

Self-interference: a UE interfering another UE (Power control needed)


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WCDMA key technologies

Source coding

The coding system used for voice in WCDMA system is AMR (Adaptive Multi rate)

- 8 kinds of coding ratesAdaptive multi rate is used when cell load increases, the system will decrease speech rate of part of

subscribers automatically so as to support more subscribers.

During an AMR communication, the receiver measures the radio link quality and must return to the

transmitter either the quality measurements or the actual codec mode the transmitter should use

during the next frame. That exchange has to be done as fast as possible in order to better follow the

evolution of the channels quality.

WCDMA Channel coding

Enhance the correlation among symbols so as to recover the signal when the interference problem

occurs

We use different kind of codec for different kind of service

- Speech service: convolution code (1/2, 1/3...)- Data service: Turbo code

WCDMA interleaving

Interleaving is used for continuous errors correction, by interleaving two successive bits are nottransmitted near each other.

TERMS

Bit: data after source coding

Symbol: data after channel coding

Chip: data after spreading

Operation of spreading

The operation of spreading is consisted of channelization and scrambling operation


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Channelization:

Transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal.

The number of chips per data symbol is called the Spreading Factor (SF). Channelization codes areorthogonal codes, meaning that in ideal environment they do not interfere each other.

Those codes are generated by WALSH codes based on a tree.

OVSF Code (Orthogonal Variable Spreading Factor) is used as channelization code

SF = chip rate / symbol rate

High data rates low SF code, Low data rates high SF codeWe can use

SF in uplink is from 4 to 256. SF in downlink is from 4 to 512.

Purpose of Channelization Code

Channelization codes are used to distinguish different physicals channels of one transmitter

For Uplink OVSF codes are used to separate different physicals channels of one UE.

For Downlink OVSF codes are used to separate different physicals channels of one cell.

For voice service (AMR), downlink SF is 128, it means there are 128 voice services maximum

can be supported in one WCDMA carrier;


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For Video Phone (64k packet data) service, downlink SF is 32, it means there are 32 voice

services maximum can be supported in one WCDMA carrier.

Scrambling:

SC are used to distinguish different transmitters

For Uplink SC are used to differentiate users in one cell.

For Downlink SC are used to differentiate cells in one carrier.

They are 16 million long SC are used in Uplink for different users those codes are assigned by RNC,

and we have 512 primary codes for downlink.

Primary Scrambling code

Each cell is allocated with only one primary scrambling code

Uplink Transmission on a Cell Level

Uplink Transmission on a Cell Level


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Diversity Technique:

Diversity technique is used to obtain uncorrelated signals for combining

Reduce the effects of fading

Fast fading caused by multi-path

Slow fading caused by shadowing Improve the reliability of communication

Increase the coverage and capacity

Time diversity

Channel coding, Block interleavingFrequency diversity

The user signal is distributed on the whole bandwidth frequency spectrumSpace diversity

Using two antennasUTRAN Network architecture

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