Chapter 4 Key Technology of Wcdma

7/28/2019 Chapter 4 Key Technology of Wcdma

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Basic Principles of WCDMA System Error! Style not defined.KeyTechnology of WCDMA

Chapter 4

Key Technology of WCDMAThis chapter introduces the principles of every

part in a WCDMA transceiver, including the

principle and structure of RAKE receiver, radio

frequency (RF) and intermediate frequency (IF)

processing technology, channel codec technology

and multi-user detection technology.

Source source coder channel coder Modulator

Channel

DemodulatorDestination source encoder channel encoder

M Y

RM

Figure 1.1 Block Diagram of Digital Communication System

Figure 1.1 shows a digital communication system

in a common sense, where a WCDMA transceiver

is installed. Channel codec is Convolutional code

or Turbo code. Modem adopts the technology of

CDMA direct spread spectrum communication.

2004-08-26 Confidential Information of Huawei. NoSpreading without Permission

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Channel coding varies with application data, that

is, AMR adaptive multi-rate code for voice, andITU Rec. H.324 series protocols for image and

multimedia services.

4.1 RAKE Receiver

In CDMA spread spectrum system, channel

bandwidth is far larger than channel flattened

fading bandwidth. This is different from traditional

modulating technology which requires balancing

algorithm to eliminate the inter-symbol

interference. CDMA spread spectrum codes

should be highly auto-correlative. In that case,

delay spread in radio channels can be taken as

signal retransfer. If the delay between multi-path

signals exceeds the length of one chip, the CDMA

receiver will take them as non-correlative noise

without balancing again.

Due to the fact that available information was

included in multi-path signals,, CDMA receiver can


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combine multi-path signals to improve signal

noise ratio (S/N) of receive signals. The functionof RAKE receiver is to receive signals in multiple

paths through several related detectors and

combine them together.. As shown in Figure 1.1, it

is a RAKE receiver, which is a typical diversity

receiver specially designed for CDMA system. The

theory behind is that multi-path signals can be

taken as irrelevant ones when the transporting

delay exceeds one chip period.

Q

I

Combination

and addition

I

Delay estimation

Phase

rotation

Channel

estimation

Delay

balance

I

Q

Path 1

Path 2

Path 3

Baseband

input signal

Time value (path location)

Correlator

with DLL

Local

spread

code

Q

I

Combination

and addition

I

Delay estimation

Phase

rotation

Channel

estimation

Delay

balance

I

Q

Path 1

Path 2

Path 3

Baseband

input signal

Time value (path location)

Correlator

with DLL

Local

spread

code

Figure 1.1 Block Diagram of RAKE Receiver

A correlator with DLL is a demodulating correlator


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with phase-locked loop of early-late gate. The

early-late gate and demodulating correlator differfrom each other by 1/2 (or 1/4) chip respectively.

Subtracting the related output of early-late gate

can be used for adjusting code phase. The

performance of delay loop depends on loop

bandwidth.

Owning to fast fading and noise in the channel,

there are great differences between the actual

received phases of various paths and the phases

of original transmitted signals. Therefore, the

phases should be rotated before combination

based on the results of channel estimation.

Channel estimation in the actual CDMA system is

performed based on pilot symbols in the

transmission signals. Depending on sequentialpilot signals in the transmission signals, there are

two ways of phase prediction, one is based on

sequential pilot and the other is based on decision

feedback technology, as shown in Figure 1.2 and


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Figure 1.3.

Correlator

(Pilot channel)

Baseband I/Q signal

LPF

Predicted phase and

amplitude results

I/Q signal

Correlator

(Pilot channel)

Baseband I/Q signal

LPF

Predicted phase and

amplitude results

I/Q signal

Figure 1.2 Channel Estimation Based on Sequential Pilot Signals

Correlator

Baseband I/Q signal

Predicted phase and

amplitude results

I/Q signal

DMUX

Data symbol

Pilot symbol

LPF

interpolation

Symbol

judgingLPFCorrelator

Baseband I/Q signal

Predicted phase and

amplitude results

I/Q signal

DMUX

Data symbol

Pilot symbol

LPF

interpolation

Symbol

judgingLPF

Figure 1.3 Channel Estimation Based on Interrupted Pilot

Condition Using Decision Feedback Technology

LPF is a low pass filter, filtering the noise in

channel estimation output, whose bandwidth is

generally higher than the channel fading rate.

When using interrupted pilot, we should adopt

interpolation technology to perform channel

estimation in the interval of pilot. When using

decision feedback technology, we should first

decide the data symbols in the channel, and then

take the decided results as apriori information


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(similar to pilot) to perform complete channel

estimation, and accordingly obtain good channelestimation results through low pass filtering. The

shortcoming of this way is low accuracy of

channel estimation and big decoding delay in

case of serious noise, due to non-linear and non-

causal prediction technology.

The function of delay estimation is to obtain signal

energy distribution in different time delay locations

through matched filter (as shown in Figure 1.4),

recognize multi-path locations with high energy

and distribute their time value to different receive

paths of RAKE receiver. The measuring precision

of the matched filter can be up to 1/4 ~ 1/2 chip,

but the interval in different receive paths of RAKE

receiver is one chip. In the actual implementation,if the speed of updating delay estimation is very

fast (such as once scores of ms), the phase-

locked loop of early-late gate is not necessary.


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Local spreading codes and scrambling codes

N N-1 0

N N-1 0

Serial input

sampling data

Local spreading codes and scrambling codes

N N-1 0

N N-1 0

Serial input

sampling data

Figure 1.4 Basic Structure of Matched Filter

The major part used for delay estimation is

matched filter, whose function is to correlate the

input data and local codes of different phases and

accordingly obtain correlation energy of different

codes and phases. If the sampling data input in

serial are the same as the phases of local spread

spectrum code and scrambled code, the

correlation energy is the greatest, with a

maximum in the output end of the filter.

Depending on correlation energy, the delay

estimator can obtain multi-path arrival time value.

From the perspective of implementation, there are


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chip level processing and symbol level processing

for the RAKE receiver. For chip level processing,correlator, local code generator and matched filter

are included, while for symbol level processing,

channel estimation, phase rotating and

combination are included. Generally, chip level

processing can be implemented with ASIC

component, while symbol level processing can be

implemented with DSP. Although the

implementation and functions of RAKE receiver of

a mobile station are different from those of a base

station, the principles are just the same.

For several receiver antennas with diversity

reception, we can process multiple paths received

by several receiver antennas in the above way.

RAKE receiver can receive not only multiple pathsof the same antenna but also multiple paths of

different antennas. In terms of RAKE receiving,

the two diversities do not vary essentially.

However, the processing of base-band would get


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more complex as the data of multiple antennas

requires dividing control processing.

4.2 CDMA RF and IF Designing Principles

4.2.1 CDMA RF and IF Architecture

Duplexer

Rx filter

IF demix filter ADCDigital lower

converterBasebandprocessor

Lower converter

Data

I/O

I

Q

IF smooth filter DACDigital upper

converter

I

Q

Upper converter

Tx filter

Local

oscillator

Power amplifier

RF AGC

RF AGC

Local

oscillator

Duplexer

Rx filter

IF demix filter ADCDigital lower

converterBasebandprocessor

Lower converter

Data

I/O

I

Q

IF smooth filter DACDigital upper

converter

I

Q

Upper converter

Tx filter

Local

oscillator

Power amplifier

RF AGC

RF AGC

Local

oscillator

Figure 1.1 Block Diagram of CDMA RF and IF Principles

Figure 1.1 is a block diagram of CDMA RF and IF

principles. For the RF part, it is a traditional

analog structure where valid signals are translated

into IF signals. The downlink channel of RF part

mainly consists of automatic gain control (RF

AGC), receive filter (Rx filter) and down-converter.

The uplink channel of the RF part mainly consists


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of automatic gain control (RF AGC), secondary

up-converter, wideband linear power amplifier andRF transmit filter. The IF part consists of the de-

aliasing filter, the down-converter and the ADC for

downlink processing, and the IF, a smoothing

filter, the up-converter and the DAC for uplink

processing. Regarding WCDMA digital down-

converter, its bandwidth of output base-band

signal is larger than that of the IF signal by 10%,

therefore, called wideband signal, it is different

from the general GSM signal and the first

generation signal.

4.2.2 CDMA RF Designing Performance and

Considerations

As mentioned above, CDMA signal is wideband

signal. Therefore, the RF part must be designed to

be suitable for wideband low-power spectrum

density signal. CDMAs large dynamic range, high

peak factor (due to linear modulating and multi-


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code transmission), and precise high-speed

power control loop are great challenges to thelinearity and efficiency of power amplifier.

CDMA makes very high requirements for the

linearity and efficiency of the RF front end.

Linearity is demanded for strict output spectrum

mask and, at the same time, the great fluctuation

of output signal envelope. To ensure the power

amplifier is efficient enough, we should keep its

operating level around 1 dB point.

To make the mobile station more compact andpower-efficient, one-step direct conversion should

be implemented from baseband to RF or from RF

to baseband at both the transmit end and the

receive end. Such technology is difficult in that the

frequency mixer must be completely linear to

avoid any possible intermodulation product

between two adjacent channels. In addition, input

isolation of the frequency mixer must be good

enough to avoid DC due to self-mixing.


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The performances of AGC and LNA in RF part are

crucial as well. In WCDMA designing, the noiseindex of AGC should be around 80dB, while that

of LNA should be lower than 4dB, because it

decides the overall noise index of the receiver.

Analog RF components cause great RF index

changes and individual diversity. We should

emulate the total receiver performance loss

caused by each RF component in the worst case,

so that a group of stable RF designing parameters

can be obtained. Moreover, according to the latest

designing scheme, the number of analog

components should be made as small as

possible, which makes it necessary to move ADC

and DAC closer to RF part. However, when

considering the present signal processingcapability of the component, the digital IF

technology is a commonly used for designing.


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4.2.3 Digital IF Technology

The sampling law shows that, if we perform equal-

interval sampling at interval of 1/2fHsecond for the

continuous time signal m(t), with a frequency band

limited at (0,fH) Hz, m(t) can be definitely

determined according to the sampling result.

In this case, 2fHis called Nyquist frequency.

Typically, a modern receiver is structured such

that analog-digit conversion and sampling are

performed by the IF component. The specific

process is: IF signal M() with the bandwidth of B

undergoes IF sampling with fs 2B(1 +/n), to get

the resulting signal MS(), which further becomes

the quantized and sampled low IF signal M'S()

after passing the low pass filter H(). The final

signal has the same frequency spectrum as that

of the original one.

It can be seen from the above process that IF

sampling can be done with a frequency lower than


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the highest valve of signal frequency as long as

the frequency meet the specified conditions. In themeantime, frequency conversion can be achieved

through IF sampling, that is, converting the signal

to a lower IF, and multiplying the common

frequencies in the numeric field, and the base-

band diversities I and Q can be deduced.

4.3 Diversity Reception Principle

Radio channel is a random time-specific channel,

whose fading feature will reduce the performance

of the communication system. There are several

measures to avoid fading, such as channel

coding/decoding technology, anti-fading receive

technology or spread spectrum technology.

Diversity reception technology is considered as an

effective and economical anti-fading technology.

As we all know, the signal received in radio

channel is an combination of multi-path

components arriving at the receiver. If the signals


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of different paths obtained at the receive end at

the same time can be combined into the wholereceive signal properly, the effect of fading will be

reduced. That is how diversity is designed.

Literally, the meaning of diversity is to obtain

compound signals separately and then combine

them. The signals, if statistically independent, can

be combined such that the performance of the

system can be improved greatly.

Those received signals, completely or almost

independent of each other, can be obtained by the

following receiving means: different paths,

different frequencies, different angles and different

polarizations.

1) Space Diversity: set several antennas upon the

receive end or the transmit end, and leave

enough space (generally exceeding 10 signal

wavelength) between each two antennas to

ensure the signals sent/received in each

antenna are mutually independent. Figure 1.1


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is an example of dual-antenna transmit

diversity to improve receive signals. Thanks todual-antenna transmit diversity, the

independent receive paths obtained by the

receiver are increased and accordingly

combination effect is gained:

Path 1

Antenna 1

Path 2

Transmitting

diversity

processing

Data flow

Data flow 1

Data flow 2

Restored data flow

Antenna 2

Path 1

Antenna 1

Path 2

Transmitting

diversity

processing

Data flow

Data flow 1

Data flow 2

Restored data flow

Antenna 2

Figure 1.1 Orthogonal Transmit Diversity Principle

The principle of orthogonal transmit diversity is

shown in Figure 1.1. The two antennas transmit

different data: antenna 1 transmits the data in

even location while antenna 2 transmits the data

in odd location. Owing to the irrelevant transmit

data, the data arriving at receiver antenna via

different antenna paths have corresponding

diversity, and accordingly the power of data


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transmission can be reduced. In the meantime,

the reliability of data transmission is greatlyimproved due to lower bit rate of single antenna

transmit data. Therefore, transmit diversity can

increase the data transmission speed of the

system.

2) Polarization Diversity: receive horizontal

polarized wave and perpendicular polarized

wave respectively.

Time Diversity: Another way of diversity is to

combine irrelevant signals transmitted

asynchronously. Frequency Diversity: The same

information is transmitted with several different

carrier frequencies. If frequency difference interval

of each carrier frequency is so large that it

exceeds channel relevant bandwidth, the signalstransmitted with carrier frequencies are irrelevant

of each other. Angle Diversity: Signals are not

related because antennal beams point to different

direction. For example, set several irradiators on


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the microwave antenna to generate little-related

beams. The diversities can be combined inpractice because they are not mutually repulsive.

We can adopt different ways to combine diversity

signals:

1) Selective Combination (SC): Select the signal

with the best S/N from several discrete ones as

the receive signal.

2) Equal Gain Combination (EGC): Combine

several discrete signals by the same branch

gain and take the combined signal as the

receive signal.

3) Maximum Ratio Combination (MRC): Control

each combined branch gain to make them in

proportion to the S/N of the existing branch,

and then combine them to get receive signal.

The above ways are different between the

diversity gains in improving the combined S/N.

Generally, diversity reception is effective to

improve the effect of radio channel reception.


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0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

Max ratio combination

Equal-gain combination

Optimal choice

Improvement(r)

(dB)

Diversity number k

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

Max ratio combination

Equal-gain combination

Optimal choice

Improvement(r)

(dB)

Diversity number k

Figure 3.1 Comparison of different combinations

Figure 3.1 shows the improvement of receiving

effects of different combinations. As the diversity

number K increases, the improvement of SC is

not ideal, while that of EGC and MRC is better,

whose difference is only about 1 dB.

4.4 Channel Coding

Channel coding is adding some extra codes to a

digital sequence M by definite rules so that

irregular information sequence M becomes a

regular digital sequence Y (code sequence). That

is to say, in code sequence, every code of

information sequence is related to extra code. At

the receiving end, the channel encoder encodes


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with this prescient coding rules, or verifies that the

received digital sequence R conforms to the setrule to find out errors in R and then corrects them.

That is the basic idea of channel coding, namely,

verifying and correcting the errors during

transmission based on correlativity.

Generally, digital sequence M is transmitted with K

codes as a group. We call the one with K-code

block an information code block. Channel encoder

adds some extra codes to each information code

block by definite rules, and so the code block with

n-code is constituted. Such n codes are mutually

related, that is, the extra n-k codes are called the

supervising codes of this code block. In terms of

information transmission, supervising codes are

redundant, as it carries no information. However,such redundancy gives the codes some error

detection and correction capability, so the

reliability of transmission is increased and error

rate is reduced. On the other hand, if we require


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the speed of information transmission to remain

constant, after supervising codes are added, theduration of each code in the code block should be

reduced. For a binary code, pulse width should be

also reduced. If the normalized width of each code

pulse is 1 before coding, it should be k/n after

coding, so channel bandwidth should be spread

by n/k times. In this case, bandwidth redundancy

substitutes for reliability of channel transmission. If

the speed of information transmission is allowed

to be slower, the duration of each code after

coding can remain the same. In this case,

bandwidth redundancy substitutes for reliability of

channel transmission.

As shown in Table 1.1, there are great gaps

between coding gains from different codingmethods and the ideal coding gain (up to

Shannon limit).


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Table 1.1 BPSK or QPSK Coding Gain

Coding Adopted Coding

Gain

(dB@BER

= 10-3)

Coding

Gain

(dB@BER

= 10-5)

Data

Speed

Ideal Coding 11.2 13.6

Cascaded Code (RS

and Convolution Code

Viterbi Coding)

6.5 ~ 7.5 8.5 ~ 9.5 moderate

Convolution Code

Sequence Coding (SoftDecision)

6.0 ~ 7.0 8.0 ~ 9.0 moderate

Cascaded Code (RS

and Group Code)

4.5 ~ 5.5 6.5 ~ 7.5 Very High

Convolution Code

Viterbi Coding

4.0 ~ 5.5 5.0 ~ 6.5 High

Convolution Code

Sequence Coding

(Hard Decision)

4.0 ~ 5.0 6.0 ~ 7.0 High

Group Code (Hard 3.0 ~ 4.0 4.5 ~ 5.5 High


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Decision)

Convolution Code

Threshold Coding

1.5 ~ 3.0 2.5 ~ 4.0 Very High

It is observed that, for the same modulating,

coding gains vary with different coding schemes.

The coding schemes we usually adopt are

convolution code, Reed-Solomon code, BCH code

and Turbo code, etc. Convolution code is used for

voice and low-speed signaling in WCDMA, while

Turbo code is used for data encoding.

4.4.1 Convolution Code

The n codes generated by the convolution coder

during any definite time is dependent on K

information bits during this period and the number

of information bits during the former N-1 period oftime. At this moment, supervising code monitors

the information during the N period of time when

the number of codes nN is called constraint

length.


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The decoding schemes of convolution code are as

follows: threshold decoding, hard decision Viterbidecoding and soft decision Viterbi decoding.

Among these decoding schemes, the best one is

soft decision Viterbi decoding, which is usually

adopted. Compared with hard decision Viterbi

decoding, it is not much more complex, but its

performance is better by 1.5 ~ 2 dB.

4.4.2 Turbo Code

We are striving to approach Shannon limit in

coding field, where Turbo code is an innovation

milestone. Grid code is close to Shannon limit in

case of bandwidth-limited channels, while Turbo

code is especially applicable to bandwidth-

unlimited channels, such as deep space

communication and satellite communication.

Theory emulation shows that, in the AWGN

channel with 0.7dB Eb/N0, Turbo code with 1/2

code rate has bit error rate of 10-5.


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Two or more basic coders are cascaded in parallel

via one or more interweavers, and so Turbo codeis constituted as shown in Figure 1.1. Turbo code

is based on the correction of the algorithm and

structure of the traditional cascade code. The

positive feedback of iterative decoding is

cancelled thanks to the introduction of internal-

interweaver. The algorithm of Turbo iterative

decoding involves SOVA (soft output Viterbi

algorithm) and MAP (maximum posterior

probability algorithm) and so on. Thanks to each

iterative performance of MAP algorithm excels

Viterbi algorithm, iterative decoding of MAP

algorithm can get more coding gains.

Interleafer

Convolution encoder 1

Holingmultiplexing

Input m ty t


Output

Interleafer


Holingmultiplexing

Input m ty t


Output

Figure 1.1 Turbo Coder


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4.5 Multi-User Detection Technology

Multi-user detection (MUD) technology can

improve the system performance and increase the

system capacity by canceling the inter-cell

interference. In addition, MUD technology can

effectively release the near/far effect in direct

sequence spread spectrum CDMA system.

As the channels are non-orthogonal and different

users spread spectrum codes are non-orthogonal,

there is mutual interference between users. MUD

is to cancel the mutual interference between multi

users. Generally speaking, for uplink MUD, only

the inter-cell interference can be canceled, while

the intra-cell interference is difficult to cancel due

to lack of necessary information (such as the

detailed information of users in adjacent cells).

For downlink MUD, only the interference in

common channel (such as pilot frequency and

broadcast channel) can be canceled.

The system model of MUD can be shown in


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Figure 1.1: each user transmits data bit 1b , 2b ,, Nb

, frequency is spread by spread spectrum codeword via non-orthogonal fading channel in the

space, and noise n(t) is added, then the user

signals received at the receive end is correlated to

synchronous spread spectrum code word and the

correlation is composed of multiplier and integral

cleaner. The interference between users is

removed with MUD algorithm from the de-spread

result, thus obtaining the estimated value of users

signal 1b , 2b ,, Nb .

It is observed from the following figure that the

performance of MUD depends on synchronized

spread spectrum code word tracing of the

correlator, detection performance of each user

signal, relative energy and the accuracy ofchannel estimation.


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Spreading

code word 1

Noise n (t) Multi-userdetectionalgorithm

Integral

cleaner

Spreading

code word 2

Spreading

code word k

Integralcleaner

Integral

cleaner

Spreading

code word 1

Noise n (t) Multi-userdetectionalgorithm

Integral

cleaner

Spreading

code word 2

Spreading

code word k

Spreading code word 1

Spreading code word k

Integralcleaner

Integral

cleaner

Spreading code word 2

1b

1b 1b

1b

1b

1b

1b

1b

1b

1b1b

1b

2b

kb

1b

1b1b

1b

2b

1b

kb

Figure 1.1 System Model of MUD Technology

According to the uplink MUD, only the intra-cell

interference can be canceled. Provided that the

inter-cell interference energy was f times of the

intra-cell interference energy, the capacity in thecell would increase by (1+f)/f without intra-cell

interference. According to the rule that the

transmit power attenuates linearly by 4 powers of

distance, the inter-cell interference is 55% of intra-

cell interference. Therefore, MUD would ideally

reduce 2.8 times of interference. However in

practice, the validity of MUD is below 100%. The

validity of MUD depends on the detection

methods, traditional receiver estimation accuracy


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and intra-cell user service model. For example, if

there are some high-speed data users in the cell,MUD is adopted to cancel the interference power

caused by these high-speed data users, which

obviously can be more effective to increase

system capacity.

But the shortcoming of it is that noise would be

increased and demodulation signal would be

greatly delayed. Decorrelator is shown in Figure

1.2.

Interference cancellation is to estimateinterference from different users and multiple

paths and then cancel the interference from the

receive signals. Serial interference cancellation

(SIC) is to gradually cancel the interference

caused by the biggest user, while parallel

interference cancellation (PIC) is to

simultaneously cancel the interference cause by

other users.


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R-1

Multipath

delay

estimation

Correlation

calculation

Soft judgingChannel

decoderH 1(t)

H 2(t)

Match filterS1

S2

H K (t)SK

[ ]1 r

r 1R =

Match filter

Match filter

Soft judging

Soft judging

Channel

decoder

Channel

decoder

R-1

Multipath

delay

estimation

Correlation

calculation

Soft judgingChannel

decoderH 1(t)

H 2(t)

Match filterS1

S2

H K (t)SK

[ ]1 r

r 1R =

Match filter

Match filter

Soft judging

Soft judging

Channel

decoder

Channel

decoder

Figure 1.2 Decorrelator

PIC is, for each user, to cancel signal energy

caused by other users at every level of

interference cancellation and demodulate it. The

interference caused by other users can be

basically canceled after 3 ~ 5 times of such

interference cancellation. Note that at every level

of interference cancellation, not all signal energy

caused by other users can be canceled. We just

multiply it by a relatively small coefficient to avoid

the growing error in the traditional receive

detection. The advantage of PIC is that it can

easily implement multi-user interference

cancellation and the delay of it is better than that

of SIC.


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As far as WCDMA uplink MUD is concerned, the

present ideal technology is PIC, because itdemands just 3 to 5 times of resources traditional

receiver demands, and the data path delay is also

small.

As far as WCDMA downlink MUD is concerned, it

focuses on canceling the interference of downlink

common pilot channel, shared channel and

broadcast channel and the interference of

common channels in a co-frequency adjacent

base station.

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Chapter 4 Key Technology of Wcdma

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