3- Owa324040 Wcdma Ran12 Fde & Ic Features Issue1.00

WCDMA FDE & IC Features

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The multi-path effect is a fundamental characteristic of the UMTS.

In the case of low-rate transmission, the multi-path combination gain obtained by

the RAKE receiver is greater than the inter-path interference, whose value is

insignificant.



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The RAKE receiver, however, is not capable of handling high-rate services, such as

the HSUPA services. The inter-path interference in high-rate transmission is

normally much higher than that in low-rate transmission. As the RAKE receiver

cannot suppress the interference in high-rate transmission effectively, the

throughput of the HSUPA users is limited. As a result, the peak rate of the HSUPA

user may not reach 11.5 Mbit/s, which is the uplink peak rate of 16QAM (as

indicated in R7 of the 3GPP specifications).



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LMMSE receiver:

The LMMSE receiver is capable of suppressing multiple access interference

and near-far occurrences in a CDMA system operating in radio channels in

the multi-path fading environment. The LMMSE receiver is relatively less

complex, and thus more suitable for both uplink and downlink receivers.

The 3G receivers are mainly based on the principle of coherent detection.

Carrier frequency, carrier phase, multi-path profile, path delays, and fading

characteristics are important parameters to perform a coherent detection

of transmitted symbols.

G-RAKE receiver:

The G-RAKE receiver is an equalizer technique used to suppress

interference and combine multiple paths. The G-RAKE receiver is similar to

the RAKE receiver in structure; however, the two receiver types differ in

the parameter settings of the number of paths and Maximal Ratio

Combining (MRC) coefficient. The parameter settings in the RAKE receiver

case are fixed, whereas the parameter settings in the G-RAKE receiver case

are variable. Compared with the RAKE receiver, the G-RAKE receiver has a

larger number of multiple paths and diversified combining coefficients. The

combining coefficients are obtained through the maximal likelihood ratio

equation by converting the interference in the cell into colored Gaussian

noises. The G-RAKE receiver is normally effective when the spreading

factors are not orthogonal on the channel working in specific frequencies.



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The calculation complexity of the FDE is 60% to 70% of the complexity of a

typical LMMSE receiver.

The available equalization techniques, which are used for minimizing the effects

of time dispersion, are as follows:

Time domain equalization technique

Frequency domain equalization technique

When fixed reference channel 8 (FRC8) is used, the result of simulation shows that

the RAKE receiver supports a data rate of only about 4 Mbit/s, while FDE supports

the full data rate of 8.1 Mbit/s.



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Step 1: Pre-processing

A serial to parallel conversion is performed on the received signals.

Step 2: Fast Fourier Transform (FFT)

The signals are converted from the time domain to the frequency domain

by using the FFT algorithm.

Step 3: Spectrum equalization

The uplink FDE is performed to equalize the spectrum in the frequency

domain on the HSUPA E-DPDCH by using the uplink receiver of the NodeB.

After the spectrum equalization, the inter-path interference on the E-

DPDCH is suppressed, and consequently the SNR on the E-DPDCH is

increased.

Step 4: Inverse Fast Fourier Transform (IFFT)

The signals are reverted from the frequency domain to the time domain by

using the IFFT. This is the last operation in the FDE process.

Step 5: Post-processing

A parallel to serial conversion is performed on the processed signals. After

despreading and decoding, the status of the signals is the same as that of

the signals transmitted from the UE.



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In a WCDMA system, all subscribers communicate through radio channels having

the same frequency and timeslot. Subscribers are distinguished by orthogonal

spreading codes. In theory, if the orthogonal spreading codes provide a complete

and perfect orthogonality between channels, co-channel interference can be

avoided. However, in practice, these codes are not completely orthogonal, radio

channels change in time, and multipath fading effect exists. The loss of

orthogonality of the spreading factors in UEs causes fading in the high-rate

transmission scenarios. This in turn leads to severe inter-chip interference (ICI) and

multi-user interference (MUI). Therefore, co-channel interference exists in a

WCDMA system, which characterizes it as a self-interference system.

Far-near effect: As the distances from subscribers to base stations or the fading

degrees are different, strong signals may suppress weak signals.



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In the system, there is interference between channels including intra-cell

interference and inter-cell interference, and between multipaths of signals related

to the same subscriber.



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Interference Cancellation is one of the MUD techniques.

This technique effectively combats the far-near effect, greatly improves system

performance, and increases capacity of the WCDMA system.



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Optimal MUD is implemented by using a matched filter and the Viterbi algorithm

to detect the transmit sequence with the maximum posterior probability for the

received signals. This technique is also called maximum likelihood sequence (MLS)

detection. The Viterbi algorithm has excellent performance. However, the MLS

detection uses the amplitude and phase of the received signals, which are

obtained through estimation, and therefore the optimal MUD actually cannot be

implemented.

Suboptimal MUD can in turn be classified into two types: linear MUD and non-

linear MUD. Non-linear MUD uses the IC technique. It estimates the MAI produced

by different subscribers through decision and reconstruction, and then eliminates

part of or all interference from the received signals. The interference canceller

need not involve the calculation of correlated matrix. It can be expanded without

increasing the calculation complexity. In addition, the interference canceller can

improve the reception performance of low-power signals. Therefore, the IC

technique is often preferred in a third-generation (3G) communications system.



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Parallel interference cancellation (PIC)

PIC performs decision and reconstruction on signals of multiple subscribers

simultaneously to reduce the impact of MAI between subscribers. Currently,

the IC method used in Huawei products is PIC.

Successive interference cancellation (SIC)

SIC performs decision and reconstruction on signals of a single subscriber

at each level and then eliminates interference from the received signals to

reduce the impact of MAI on other subscribers at lower levels. The

operations by levels are performed in descending order of power of

received signals. The SIC operation is performed preferentially on

subscribers with higher power. Therefore, this is most beneficial for

subscribers with lower power.



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The IC technique involves the following operations:

1. One-time demodulation or data regeneration: After the NodeB receives

the real-time antenna data (data of all subscribers), it demodulates the

data of each subscriber and then modulates the data to obtain the

modulated data of each subscriber, that is, regenerating subscriber data

2. IC: The regenerated data is sent to the IC module

3. NodeB demodulation: The NodeB demodulates the data after IC

Real-time antenna data (for cells) : the cell baseband data received by the UL processing module of the NodeB after being processed by the RF module

Time-delay antenna data: the data after IC

Regenerated data (for IC users): means the user baseband data rebuilt on the

basis of the original information about some E-DPDCHs obtained from real-time

antenna data and the features of radio channels



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The E-DPDCH is a physical channel used for the UE to transmit bits (E-DCH

processing results) to the NodeB. Subscriber data is carried on this channel. With

the increase of the HSUPA rate, the UL interference increases.



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Each subscriber has a control channel, which is the major source for the

interference, especially for low-rate and low-activity services. CCPIC is a simplified

MUD technique of the receiver in the NodeB. It is used in the heavy-load scenario.

By eliminating interference from the UL control channel signals in the baseband

data and reducing UL interference, CCPIC improves system capacity and thus

reduces the investment for operators.



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HSUPA provides UL high-rate services. The Radio Access Network (RAN), based

on the wideband code division multiple access technology, is a typical self-

interference system. With the increase of the HSUPA rate, the UL interference

increases. The UL interference is a major factor affecting the UL capacity of the

RAN.

For the baseband data after IC, the interference from the E-DPDCH data is

eliminated and the colored noise in the cell caused by the self-interference feature

of the WCDMA system is reduced. Therefore, the signal-to-noise ratio (SNR) of the

data is improved.



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The RNC obtains the UL load after IC from the time-delay antennas of the NodeB

for more accurate access control and load control.



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3- Owa324040 Wcdma Ran12 Fde & Ic Features Issue1.00

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