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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