01 wcdma basic knowledge

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Multiple access technology
Duplex technology
Time division duplex (TDD)
Frequency division duplex (FDD)
In third generation mobile communication systems, TD-SCDMA adopts time division duplex (TDD); WCDMA and cdma2000 adopt frequency division duplex (FDD). WCDMA FDD mode has been consolidated with TD-SCDMA.
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Multiple Access Technology and Duplex Technology
Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000
Traffic channels: different frequency bands are allocated to different users,for example, AMPS and TACS
Traffic channels: different time slots are allocated to different users, for example, DAMPS and GSM
FDMA
TDMA
CDMA
User
User
User
User
User
User
Frequency
Time
Power
Frequency
Time
Power
Frequency
Time
Power
Frequency Division Multiple Access: frequency division, sometimes called channelization, means dividing the whole available spectrum into many single radio channels (transmit/receive carrier pair). Each channel can transmit one-way voice or control information. Under the control of the system, any user can be accessed to any of these channels. Analog cellular system is a typical example of FDMA structure. Similarly, FDMA can also be used in a digital cellular system,except that pure frequency division is not adopted. For example, FDMA is adopted in GSM and CDMA.
Time Division Multiple Access means that the wireless carrier of one bandwidth is divided into multiple time division channels in terms of time (or called timeslot). Each user occupies a timeslot and receives/transmits signals within this specified timeslot. Therefore, it is called time division multiple access. This multiple access mode is adopted in both a digital cellular system and a GSM. TDMA is a complex architecture and the simplest case is that a single channel carrier is divided into many different timeslots, each of which transmits one-way burst-oriented information. The key part in TDMA is the user part, in which each user is allocated with one timeslot (allocated when a call begins). The user communicates with a base station in a synchronous mode and counts the timeslot. When his own timeslot comes, the mobile station starts a receiving and demodulation circuit to decode the burst-oriented information sent from the base station. Likewise, when a user wants to send any information, he should first cache the information and waits for his timeslot to come. After a timeslot begins, the information is transmitted at a double rate and next burst-oriented transmission begins to be accumulated.
CDMA is a multiple access mode implemented by Spread Spectrum Modulation. Unlike FDMA and TDMA, both of which separate the user information in terms of time and frequency, CDMA can transmit the information of multiple users on a channel at the same time. That is to say,mutual interference between users is permitted. The key is that every information before transmission should be modulated by different Spread Spectrum Code-Sequence to broadband signal, then all the signals should be mixed and send. The mixed signal would be demodulated by different Spread Spectrum Code-Sequence at the different receiver.Because all the Spread Spectrum Code-Sequence is orthogonal,only the information that was be demodulated by same Spread Spectrum Code-Sequence can be reverted in mixed signal.
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WCDMA: DL data rate=384kbps, symbol rate=960ksps,
chip rate=3.84Mcps
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Source
coding
Channel
coding
Spreading
Modulation
Source
decoding
Channel
decoding
Despreading
Demodulation
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OVSF:Orthogonal Variable Spreading Factor
Channelisation code uses OVSF code, for keeping the orthogonality of different subscriber physical channels. OVSF can be defined as the code tree illustrated in the following diagram.
Channelisation code is defined as Cch,SF, k,, where, SF is the spreading factor of the code, and k is the sequence of code, 0≤k≤SF-1.Each level definition length of code tree is SF channelisation code, and the left most value of each channelisation code character is corresponding to the chip which is transmitted earliest.
SF = 1
SF = 2
SF = 4
Cch,1,0 = (1)
Cch,2,0 = (1,1)
Cch,2,1 = (1,-1)
Cch,4,0 =(1,1,1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,4,2 = (1,-1,1,-1)
Cch,4,3 = (1,-1,-1,1)
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Spreading code
Suppose bit sequence modulated with BPSK is adopted for the subscriber data, with a rate of R, then 1 value is adopted for the bit of subscriber data.
The spreading here means to multiply each subscriber data bit with the spreading code chip including N bits..Assume N=8,then data rate after spreading will be 8R, with same random attribute as the spreading code.We name its spreading factor as 8. And the broad band signal obtained after spreading will be sent to the receiving end via the radio channel.
As the product of signal rate and factor 8 equals to the bandwidth spreading of subscriber data signal,CDMA system is also called the spreading system.
During dispreading, the spread subscriber data will be multiplied, bit duration by bit duration, with the same 8 code chips that are used during the spreading of these bits.If only excellent synchronization can be realized between the spread subscriber signal and the despreading code, can the subscriber bit sequence be retrieved.The despreading operation restores the signal bandwidth to the original value R.
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Data after integration
During the process of receiving the expected correct signal that belongs to the subscriber, complete synchronous despreading codes are adopted for the despreading operation. After obtaining the despreading data, the correlation receiver integrates the resulting products, then get the integration data.
Signals of other subscribers using different spreading codes are actually the interference signals to the first subscriber. In this case, multiply the signals and the despreading code of the first subscriber to get the despreading signal, and then perform integration. Finally, an interference signal with a signal value fluctuating along with 0 will be got.It can be viewed that the signal amplitude of the subscriber increases by 8 times than that of the other interference systems in average. That is to say, the correlation detection increases the expected subscriber signal by the multiple of spreading factor value within the interference of CDMA system.This effect is called “processing gain”, and it is the basic characteristic of the spreading system.Elementarily, this kind of correlation receivers are adopted for the BTS and UE in the WCDMA system.Because the existence of multipath propagation and multi receiving antennas, multiple correlation receivers are necessary for retrieving the signal energy from all the paths or antennas.And the collection of these correlation receivers forms the CDMA RAKE receiver.
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Pp:Ec/IopilotEc/IoTCHSNRpilot-pilotEc/Io
At the receiver, as the codes are different and are known, only the power of the intended user is de-spread.
After despreading (decoding), correct data recovery requires a given value for the Eb to No ratio. Under this Eb/No ratio the noise will generate too many errors. The noise is mainly generated by the other users transmitting at the same time and at the same frequency but using different spreading codes.
Therefore, in order not to cross this maximal noise level, all the users have to share their power: In WCDMA the Time-Frequency plane is not divided among the mobile subscribers as is done in TDMA or FDMA. So the common shared resource is power.
The de-spreading process gives processing gain proportional to the bandwidth of the spreading signal. The larger the spreading factor, the larger the gain.
This means that by using a larger spreading factor, we can reduce the power (and therefore the background noise). Thanks to this property, spread signals can operate at negative signal-to-noise ratios provided that they possess enough gain.
Example:
The narrow-band signal requires an Eb/No of 12 dB to achieve a certain bit-error rate performance. What is the required Ec/No, knowing that the processing gain is 20 dB?
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Broadband signal
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f
P
W
Processing
Gain
Rb
Despreading
Spreading Gain Pg=W/R (W=3.84Mchips/s)
The energy of the signal may be far lower than noise
The lower speed of signal, the higher spreading gain of Pg
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Principle of RAKE Receiver
RAKE receiver help to overcome on the multi-path fading and enhance the receive performance of the system
Receive set
Correlator 1
Correlator 2
Correlator 3
Searcher correlator
Combiner
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Correlator
Code
generators
Q
For the digitized signals input to the baseband, despreading and integration of subscriber data symbols is completed via the correlator and local code generator, specifics are as follows:Channel estimator uses the pilot signal to estimate the channel status; Phase spinner deletes the phase affection caused by the channel from the received signal according to the estimated channel status. The function of delay estimation is to obtain the signal energy distribution at different delay positions via the matching filter, and identify the multipaths with large energy, and allocate their time values to different receive paths of the RAKE receiver.The delay equalizer is to compensate the difference of symbol arriving time for each path.At last, the RAKE combiner adds the symbols after channel compensation to provide multipath diversity to withstand fading.From the aspect of realization, the processing of RAKE receiver can be based on either chip level or symbol level.The correlator, local code generator and matching filter belong to the chip level processing, and this is generally realized via ASIC device; Channel estimation, phase spinning and combination belong to symbol level processing, and this is realized via DSP.Though the realization methods and functions of the RAKE receiver between UE and BTS are different, the principles are complete the same.
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RAKE receiver is adopted
The time diversity effect generated by channel coherence time is efficiently used.
frequency diversity
Low signal transmission power
Great flexibility in carrying multiple services with largely different bit rate and QoS requirement.
Different spreading factors for different services with different data rates
High spectral efficiency
Supporting soft handover and softer handover.
Thank you

01 wcdma basic knowledge

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