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Technology of WCDMA

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             Chapter 4 Key Technology of WCDMA

             This 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.


                                                              M                       Y
                      Source                  source coder           channel coder         Modulator




                                                                                                        Channel


                                                              M                       R
                     Destination             source encoder         channel encoder       Demodulator




                                   Figure 4-1 Block Diagram of Digital Communication System


             Figure 4-1 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.
             Channel coding varies with application data, that is, AMR adaptive multi-rate code for
             voice, and ITU 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 combine multi-path signals to improve signal noise ratio (S/N) of receive
             signals. The function of RAKE receiver is to receive signals in multiple paths through
             several related detectors and combine them together.. As shown in Figure 4-2, it is a
             RAKE receiver, which is a typical diversity receiver specially designed for CDMA
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             system. The theory behind is that multi-path signals can be taken as irrelevant ones
             when the transporting delay exceeds one chip period.

                        Baseband
                        input signal
                                                              I
                                                                                                                        I
                                             Correlator                Phase      Delay                   ∑I
                                             with DLL        Q         rotation   balance



                                            Local
                                                          Channel                                         ∑Q
                                            spread
                                                          estimation
                                            code                                                      Combination
                                                                                        Path 1        and addition

                                                                                        Path 2

                                                                                        Path 3

                                       Time value (path location)

                                         Delay estimation




                                             Figure 4-2 Block Diagram of RAKE Receiver


             A correlator with DLL is a demodulating correlator with phase-locked loop of early-late
             gate. The early-late gate and demodulating correlator differ from 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 sequential pilot 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 4-3 and Figure 4-4.

                                                                                                 Predicted phase and
                          Baseband I/Q signal                                                    amplitude results
                                                   Correlator
                                                                                  LPF                      I/Q signal
                                                  (Pilot channel)


                               Figure 4-3 Channel Estimation Based on Sequential Pilot Signals




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                          Baseband I/Q signal                        Data symbol
                                          Correlator                                Symbol
                                                                                                         LPF
                                                                                    judging
                                                              DMUX

                                                                                      LPF
                                                                     Pilot symbol interpolation
                                                                                                  Predicted phase and
                                                                                                  amplitude results
                                                                                                        I/Q signal


                 Figure 4-4 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 (similar
             to pilot) to perform complete channel estimation, and accordingly obtain good channel
             estimation 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 4-5), 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.

                                                    Local spreading codes and scrambling codes




                                                N       N-1      …        0




                                                                               …
                                                                                        ∑

                        Serial input
                        sampling data
                                                N       N-1      …        0



                                           Figure 4-5 Basic Structure of Matched Filter

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             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 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 paths of 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 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


                                              Rx filter

                                                     Lower converter
                                                                                                                   I
                                                                                                    Digital lower
                     Duplexer                                        IF demix filter       ADC                    Q
                                                                                                    converter
                                         RF AGC
                                                                                                                       Baseband processor




                                                        Local                                                                               Data
                                                        oscillator                                                                           I/O
                          Power amplifier
                                                  Tx filter


                                                                                                                  I
                            Local                                                                  Digital upper
                                                                        IF smooth filter   DAC                   Q
                            oscillator                                                             converter
                                         RF AGC
                                                     Upper converter


                                     Figure 4-6 Block Diagram of CDMA RF and IF Principles


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             Figure 4-6 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 of automatic gain control (RF AGC), secondary up-converter, wideband linear
             power amplifier and RF 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. CDMA’s large
             dynamic range, high peak factor (due to linear modulating and multi-code transmission),
             and precise high-speed power control loop are great challenges to the linearity 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 and power-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.

             The performances of AGC and LNA in RF part are crucial as well. In WCDMA designing,
             the noise index 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 processing
             capability 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 1/2fH
             second 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, 2fH is 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(ω)
             with the bandwidth of B undergoes IF sampling with fs ≥ 2B(1 +α/n), to get the resulting
             signal MS(ω), which further becomes the quantized and sampled low IF signal M'S(ω)
             after passing the low pass filter H(ω). 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 the highest valve of signal frequency as long as the frequency meet the
             specified conditions. In the meantime, 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 of different paths obtained at the
             receive end at the same time can be combined into the whole receive 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 4-7 is an example of dual-antenna transmit diversity
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                  to improve receive signals. Thanks to dual-antenna transmit diversity, the
                  independent receive paths obtained by the receiver are increased and accordingly
                  combination effect is gained:

                                                      Data flow 1                           Restored data flow
                                                                     Antenna 1
                                                                                   Path 1

                      Data flow

                                       Transmitting
                                      发射分集
                                       diversity
                                       processing
                                         处理

                                                                                   Path 2



                                                                      Antenna 2
                                                       Data flow 2




                                     Figure 4-7 Orthogonal Transmit Diversity Principle


             The principle of orthogonal transmit diversity is shown in Figure 4-7. 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 transmission can be reduced. In the meantime, the
             reliability of data transmission is greatly improved 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 signals transmitted 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 the microwave antenna to generate little-related beams. The diversities
             can be combined in practice 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.




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             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.

                                                       12

                                (dB) Improvement (r)
                                                       10
                                                                    Max ratio combination
                                                        8

                                                                                                      Equal-gain combination
                                                        6


                                                        4


                                                        2
                                                                                                    Optimal choice

                                                        0
                                                            1   2    3     4   5    6       7   8      9     10


                                                                         Diversity number k

                                                            Figure 4-8 Comparison of different combinations


             Figure 4-8 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 with this
             prescient coding rules, or verifies that the received digital sequence R conforms to the
             set rule 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

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             reliability of transmission is increased and error rate is reduced. On the other hand, if
             we require the speed of information transmission to remain constant, after supervising
             codes are added, the duration 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 4-1, there are great gaps between coding gains from different coding
             methods and the ideal coding gain (up to Shannon limit).

                                             Table 4-1 BPSK or QPSK Coding Gain

                                                           Coding Gain        Coding Gain
                        Coding Adopted                                                           Data Speed
                                                         (dB@BER = 10-3)    (dB@BER = 10-5)
              Ideal Coding                             11.2                13.6
              Cascaded Code (RS and Convolution
                                                       6.5 ~ 7.5           8.5 ~ 9.5           moderate
              Code Viterbi Coding)
              Convolution Code Sequence Coding (Soft
                                                       6.0 ~ 7.0           8.0 ~ 9.0           moderate
              Decision)
              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
                                                       4.0 ~ 5.0           6.0 ~ 7.0           High
              (Hard Decision)
              Group Code (Hard Decision)               3.0 ~ 4.0           4.5 ~ 5.5           High
              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 of time. At this moment, supervising code monitors the information
             during the N period of time when the number of codes nN is called constraint length.

             The decoding schemes of convolution code are as follows: threshold decoding, hard
             decision Viterbi decoding 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.



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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
                                  -5
             bit error rate of 10 .

             Two or more basic coders are cascaded in parallel via one or more interweavers, and
             so Turbo code is constituted as shown in Figure 4-9. 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.




                                                                                       Holing multiplexing
                           Input m t
                                                                                                             Output y t
                                                       Convolution encoder 1

                                   Interleafer
                                                       Convolution encoder 2


                                                  Figure 4-9 Turbo Coder



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.




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             The system model of MUD can be shown in Figure 4-10: each user transmits data bit

             b1 , b2,…, bN , frequency is spread by spread spectrum code word 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
                                                b1 , b2,…, bN .

             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 of channel
             estimation.

                                                                                                                              ˆ
                                                                                                                              b1
                           b1                                                 Integral
                                                      Noise n(t)              cleaner




                                                                                            Multi-user detection algorithm
                                                                                            Multi-user detection algorithm
                                Spreading                          Spreading code word 1                                     b1
                                code word 1      b1                     b1                                                        ˆ
                           b2                                                Integral                                             b2
                                 b1                                          cleaner
                                 Spreading                         b1
                           b1                                                                                                b1
                                 code word 2                       Spreading code word 2
                                                                        b1
                                 b1                                b1
                           bk                                                                                                     ˆ
                                                                                                                                  bk
                                                                             Integral
                                Spreading                                    cleaner
                           b1   code word k
                                                                   Spreading code word k                                     b1
                                                                             b1
                                  b1                               b1


                                       Figure 4-10 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 the cell 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 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 4-11.

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             Interference cancellation is to estimate interference 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.

                                                                                                         Channel
                                                                                          Soft judging
                                                                                                         decoder
                        S1     H 1(t)                      Match filter
                                                                                          Soft judging   Channel

                        S2     H 2(t)                      Match filter    R -1                          decoder

                                                                                                         Channel
                                                                                          Soft judging
                                                                                                         decoder
                        SK     H K (t)                      Match filter

                                                            Multipath      Correlation
                                                              delay        calculation
                                                            estimation



                                         R=   [ ]
                                               1
                                               r 1
                                                     r




                                                         Figure 4-11 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.

             As far as WCDMA uplink MUD is concerned, the present ideal technology is PIC,
             because it demands 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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