KR20070073064A - Adaptive channel estimator in mobile communications system and method therefor - Google Patents

Abstract

An adaptive channel estimation device of a mobile communication system and an estimation method are provided to realize an adaptive channel estimation filter by a simple phase shift checker and an adder without calculation of power measurement and autocorrelation, thereby being suited to a terminal of the mobile communication system. A channel estimation filter(616) inputs accumulated pilot signals, and estimates channel information within a predetermined bandwidth by using the accumulated pilot signals. A phase shift counter(614) tracks a channel phase of the estimated channel information at every unit time, and calculates a phase shift count of the channel phase. A bandwidth selector(612) controls the bandwidth of the filter so that the controlled bandwidth is in proportion to the phase shift count.

Description

ADAPTIVE CHANNEL ESTIMATOR IN MOBILE COMMUNICATIONS SYSTEM AND METHOD THEREFOR}

1 is a diagram illustrating a change in received power due to channel fading.

2 is a diagram illustrating a change in phase due to channel fading.

3 is a diagram illustrating a structure of a synchronous demodulation device including a channel estimation filter according to a preferred embodiment of the present invention.

4 shows a simple configuration of a first order IIR filter.

5 shows a simple configuration of an Nth-order IIR filter.

6 is a diagram illustrating a structure of a synchronous demodulation device including an adaptive channel estimation filter according to a preferred embodiment of the present invention.

7 is a view showing an example of a phase shift count operation when dividing a phase into four.

8 shows another example of a phase shift count operation when dividing a phase into eight.

9 is a diagram illustrating a terminal operation in a discontinuous reception (DRX) mode according to a preferred embodiment of the present invention.

10, 11 and 12 are diagrams showing simulation results of uncoded BER according to fading frequency in a WCDMA system.

The present invention relates to a channel estimation apparatus and method for synchronous demodulation in a mobile communication system, and more particularly, to an adaptive channel estimation apparatus and method for estimating the fading speed of a mobile communication channel environment to control the bandwidth of a channel estimation filter.

Mobile communication systems are evolving into high-speed, high-quality wireless data packet communication systems to provide data services and multimedia services beyond voice-oriented services. High Speed Downlink Packet Access (HSDPA) and 1xEV-DV (Evolution in Data and Voice) standardization, which is currently being conducted around 3rd Generation Partnership Project (3GPP) and 3GPP2, is a high-speed, high quality of 2Mbps or more in 3G mobile communication systems. This is a representative proof of the effort to find a solution for the wireless data packet transmission service of the.

In the mobile communication system performing such wireless communication, a high order modulation method used for high-speed and high-quality data service and a factor that inhibits data reception at a low code rate are largely due to a channel environment. The channel environment for the wireless communication, in addition to the white noise, the received Doppler effect due to the change in the received signal power due to channel fading, shadowing, the movement of the terminal and frequent changes in speed, other users and multipath signals They often change due to interference caused by them, and as shown in FIGS. 1 and 2, not only the power of the reception but also the phase of the channel is continuously changed. A device for correcting a signal distorted by such a channel environment into an original signal form is called a channel estimator.

In a base station of a third generation mobile communication system such as Code Division Multiple Access (CDMA) 2000 or wideband CDMA (WCDMA), or a 3.5 generation mobile communication system such as 1x-EVDV or High Speed Downlink Packet Access (HSDPA), a data channel may be used in advance. A pilot signal consisting of a promised pilot sequence or pilot symbols is transmitted, and the UE estimates the state of the channel, in particular, the phase change using the pilot signal, and compensates the estimated phase to coherent demodulation. Do this. However, when the white noise is mixed, the random nature of the white noise makes the channel prediction difficult. In order to solve this problem, a noise canceling filter is placed at the end of the channel estimator to mitigate random characteristics, and a significant effect can be obtained. Infinite impulse response (IIR) is widely used as a noise removing filter used in a mobile communication system.

As is known, the current output of the IIR filter consists of the sum of the previous outputs and the current output, so that the current output depends on the previous outputs, causing a delay. That is, a lagging occurs due to the signal fed back from the output unit. This delay may occur even more rapidly in a channel that changes at high speeds and degrades the demodulation performance of the receiver. On the other hand, when the channel environment changes at a low speed, it is desirable to reduce the bandwidth of the channel estimation filter, and the unnecessarily wide bandwidth has the side effect of reducing noise immunity.

Therefore, although the bandwidth of the channel estimation filter preferably has an adaptive bandwidth according to the channel environment, the channel estimation filter generally operates at a fixed bandwidth regardless of the channel environment. Channel estimation filters with fixed bandwidths are limited in overcoming changing channel environments, particularly fading, in mobile communication systems.

In order to cope with the changing channel environment in a mobile communication system, various techniques have been proposed for measuring the frequency at which the received power crosses a specific level and estimating the fading frequency therefrom to change the bandwidth of the channel estimation filter. . On the other hand, in a system in which high-speed power control is performed, such as a CDMA mobile communication system, it is difficult to set measurement criteria because a change in received signal power due to power control occurs along with power change due to fading.

Another related technique is to adjust the bandwidth of the channel estimation filter by using the autocorrelation of the pilot signal. Since autocorrelation is a measure of the similarity between two signals with time difference, it is determined that the channel variation is small when the autocorrelation value is large, so that the bandwidth of the channel estimation filter is small, and when the autocorrelation is small, the channel variation is large. Increase the bandwidth of the channel estimation filter. To increase the accuracy of fading frequency estimation by autocorrelation, the number of samples of the signal used for autocorrelation should be large. However, storing enough samples of the past pilot signal for fading rate measurement increases the complexity of the terminal.

Similar techniques have been proposed to use covariance, but similarly enough samples are needed to obtain statistical properties. A technique for adjusting the characteristics of the channel estimation filter in consideration of more various channel environments and complex factors has also been proposed, but it has a disadvantage in that it is not easy to apply to a mobile communication terminal because the complexity of calculation and implementation is not simple.

Accordingly, the present invention, which was devised to solve the problems of the prior art operating as described above, provides an apparatus and method for adaptive channel estimating to increase the demodulation performance of a receiver in a terminal of a mobile communication system.

The present invention provides an adaptive channel estimating apparatus and method for estimating a fading rate in a simple method of implementation in a terminal of a mobile communication system.

According to a preferred embodiment of the present invention, in an adaptive channel estimation apparatus for a mobile communication system,

A channel estimation filter that receives a pilot signal detected by a pilot pattern and accumulated for a predetermined period, and estimates channel information within a predetermined bandwidth by using the accumulated pilot signal; and the channel information is used to compensate a received data signal. Used,

A phase shift counter for tracking the channel phase of the channel information estimated by the channel estimation filter every unit time and calculating a phase shift count of the channel phase;

And a bandwidth selector for controlling the bandwidth of the channel estimation filter to be proportional to the phase shift count.

Another embodiment of the present invention, in the adaptive channel estimation method for a mobile communication system,

Receiving a pilot signal detected by a pilot pattern and accumulated for a predetermined period, estimating channel information within a predetermined bandwidth by using the accumulated pilot signal, and the channel information is used to compensate the received data signal. ,

Calculating a phase shift count of the channel phase by tracking the estimated channel phase of the channel information every unit time;

And controlling the bandwidth such that the phase shift is proportional to the count.

Hereinafter, with reference to the accompanying drawings will be described in detail the operating principle of the preferred embodiment of the present invention. In the following description of the present invention, detailed descriptions of well-known functions or configurations will be omitted if it is determined that the detailed description of the present invention may unnecessarily obscure the subject matter of the present invention. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

3 illustrates a structure of a synchronous demodulation device including a channel estimation filter according to a preferred embodiment of the present invention.

Referring to FIG. 3, the received pilot signal is multiplied by a multiplier 304 with a conjugate complex value through the conjugate calculator 302 of a pilot pattern that is already known, and is converted into an unmodulated signal, which is then converted into an accumulator 306. By accumulating for a predetermined period and then inputting to the channel estimation filter 308. The received data symbol is compensated by the channel compensator 310 by the distorted phase of the channel using the channel information estimated by the channel estimation filter 308 and then output as a demodulated data symbol. When the channel estimation filter 308 outputs channel coefficients as the channel information, the channel compensator 310 performs a conjugate complex product between the channel coefficients and the received data symbols.

When a first order IIR filter is used as a filter for channel estimation using a pilot signal, the input / output relation is as follows.

4 shows a simple configuration of the first order IIR filter. As shown, the input signal is multiplied by the coefficient b by the multiplier 410 and then output through the adder 412. The output signal is delayed by one cycle by delay 416 at branch point 418, then multiplied by coefficient a by multiplier 414 and then added to the adder 412 to sum with the next input signal. Is provided.

Frequency characteristics of the first-order IIR filter configured as described above are as follows.

일 때의 값이므로 다음과 같다.The direct current (DC) gain of the first order IIR filter is zero frequency,

This is the value when is as follows.

의 조건을 만족할 때 DC 이득은 1로 정규화(normalization)된다. therefore

The DC gain is normalized to 1 when the condition of.

5 shows the structure of a more generalized Nth-order IIR filter.

Referring to FIG. 5, the input signal is multiplied by the coefficient b by the multiplier 510 and then output through the adder 512. The output signal is delayed by one cycle each by N delays 516a through 516n at branch point 518, and then multiplied by coefficient vectors a ₁ through a _N by multipliers 514a through 514a. It is then provided to the adder 512 to sum with the next input signal. In the case of applying the N-th order IIR filter as described above, the input-output relational expression is expressed by Equation 4 below.

The DC gain of the Nth-order IIR filter is expressed by Equation 5 below.

Then, in order to normalize the DC gain to 1, the condition to be satisfied is shown in Equation 6 below.

Table 1 below shows examples of coefficients a and b of the first-order IIR filter according to the Doppler frequency and the vehicle speed when the sampling frequency of the signal input to the channel estimation filter is 7.5KHz and the carrier frequency is 2GHz. to be. Among them, a coefficient corresponding to the moving object speed that the system intends to support is selected.

a b (= 1-a) 3dB Cut-Off Frequency (Doppler frequency) Vehicle speed 1/4 3/4 2024 Hz 1,093 Km / h 1/2 1/2 862 Hz 465 Km / h 3/4 1/4 346 Hz 197 Km / h 7/8 1/8 159 Hz 86 Km / h 15/16 1/16 80 Hz 41 Km / h

However, in order to obtain an optimal filter coefficient according to the Doppler frequency, the Doppler frequency must be estimated first. However, a mobile communication terminal requiring less power consumption and a smaller circuit area requires a less complicated estimation method. Therefore, the following describes a structure for adjusting the characteristics of the channel estimation filter in a more simplified structure.

In the preferred embodiment of the present invention, the channel estimation filter 308 of FIG. 3 is configured as shown in FIG. 6 to adapt to the fading speed of the mobile communication channel environment. In FIG. 6 the apparatus 608 will be referred to as an adaptive channel estimation filter.

Referring to FIG. 6, the received pilot signal is multiplied by a multiplier 504 with a conjugate complex value through the conjugate calculator 502 of a pilot pattern that is already known, is converted into an unmodulated signal, and accumulated in the accumulator 506. By accumulating for a predetermined period and then inputting to the adaptive channel estimation filter 508. The received data symbol is also compensated by the channel compensator 510 by the distorted phase of the channel using the channel information estimated by the adaptive channel estimation filter 508. In general, the channel compensator 510 performs a conjugate complex product between the channel information estimated by the channel estimation filter 616 within a predetermined bandwidth, that is, the channel coefficient and the received data symbol.

The adaptive channel estimation filter 608 includes a bandwidth selection unit 612, a phase shift counter 614, and a bandwidth controllable channel estimation filter 616. The bandwidth of the channel estimation filter 616 is determined according to the phase shift count indicating the number of phase shifts per unit time. Hereinafter, the principle of changing the bandwidth of the channel estimation filter 616 according to the channel environment in the adaptive channel estimation filter 608 will be described. The bandwidth of the channel estimation filter 616 is a major factor in determining how quickly channel characteristics can be estimated in response to channel changes.

The phase shift counter 614 of the adaptive channel estimation filter 608 tracks the estimated change in the channel phase at each predetermined unit time, and sets the phase shift count by one whenever the amount of change in the channel phase exceeds a certain magnitude. Increase. As an example, referring to FIG. 7, the entire phase region is divided into four portions 702, 704, 706, and 708, and the channel phase estimated by the channel estimation filter 616 is divided into one quadrant region (0 degrees to 90 degrees). Each time a transition is made to the third quadrant region (180 degrees to 270 degrees) 706, that is, when the amount of change in the channel phase exceeds 90 degrees, the phase shift count is increased by one. In this case, it is possible to check the phase shift only by the real and imaginary codes of the channel coefficients. Specifically, the real and imaginary codes of the channel coefficients estimated in the current unit time are compared with the real and imaginary codes of the channel coefficients estimated in the previous unit time, respectively. As a result of the comparison, only when both the real and imaginary codes are different, the phase shift count is increased by one.

According to another embodiment shown in FIG. 8, the full phase region is divided into eight portions 802 to 816, and the channel phase estimated by the channel estimation filter 616 is changed to another region other than the adjacent region. That is, when the amount of change in the channel phase exceeds 45 degrees, the phase shift count is increased by one. Then, the phase shift can be examined by using the real and imaginary codes of the channel coefficients and the ratio of the magnitudes of the real and imaginary components.

In the embodiment of Figs. 7 and 8, not increasing the phase shift count when the channel phase changes to an adjacent region, so that the phase shift count is continuous when a ping-pong phenomenon occurs near the boundary of each region. This is to prevent the increase.

Table 2 below shows a simulation of phase shift counts per second when the entire phase region is divided into four sections. The phase shift count is directly proportional to the moving body speed as follows.

Vehicle speed (Km / h) 3 30 60 120 250 Phase Transition Frequency (Hz) 6.2 60.6 120.8 245.5 489.7

The bandwidth selector 612 of the adaptive channel estimation filter 608 receives a phase shift count generated per unit time from the phase shift counter 614 and selects a bandwidth as shown in FIG. 9. 9 illustrates the relationship between the phase shift count and the bandwidth of the channel estimation filter 616 according to an exemplary embodiment of the present invention. When the phase shift count per unit time is large, it is preferable that the channel change is fast and the phase tracking speed is high. Therefore, the bandwidth of the channel estimation filter 616 is selected to a predetermined large value. On the other hand, if the phase shift count per unit time is small, the change of the channel is slow. Therefore, the bandwidth of the channel estimation filter 616 is selected as a predetermined small value in order to lower the phase tracking speed and increase the noise immunity.

For this purpose, the bandwidth selector 612 selects the appropriate bandwidth for the mobile speed by appropriately setting a plurality of corresponding bandwidth values, for example in a look-up table, for the possible values of the phase shift count per unit time. In this case, the thresholds may be determined in consideration of the frequency error and the amount of noise between the base station and the terminal. Since the adaptive channel estimation filter 608 is implemented by a simple phase shift check and a simple counter, the adaptive channel estimation filter 608 is suitable for application to a terminal of a mobile communication system.

10, 11, and 12 are uncoded according to fading frequency (ie, Doppler frequency) when the moving vehicle speed is 3Km / h, 30Km / h, and 120Km / h in the WCDMA system, which is a third generation mobile communication system. (uncoded) Shows the simulation result of the bit error ratio (BER). Here, the common pilot channel (CPICH) chip energy to interference ratio (Ec / Ior) is -10dB, Ior / Ioc is 0dB and there is only a single path. In this case, intra-cell interference (Ior) refers to interference occurring in a cell, and inter-cell interference (Ioc) refers to interference occurring outside the cell. For fading frequencies of 80 Hz, 160 Hz, 350 Hz, 860 Hz, and 2000 Hz, BERs according to chip energy to interference ratio (Ec / Ior) of a dedicated physical channel (DPCH) are shown.

Referring to FIG. 10, the use of a bandwidth of 160 Hz shows the best performance when the vehicle speed is 3 Km / h. Referring to FIG. 11, the use of the bandwidth of 860 Hz is best when the vehicle speed is 30 Km / h. Performance is shown, and referring to FIG. 12, it is best to use a bandwidth of 2000 Hz when the moving speed is 120 km / h. If a fixed bandwidth of any one of the fading frequencies is used, eventually, optimized performance is not obtained according to channel conditions.

As an example of the bandwidth that is variably selected by using the adaptive channel estimation filter 608 according to the preferred embodiment of the present invention, if the phase shift count per 0.5 second is 68 or more, 2000 Hz, 67 or less 23 or more 860 Hz, 22 or less 8 or more 350Hz, 7 or less, 3 or more, 160Hz, 2 or less, 80Hz. The BER performance (denoted as "Proposed") in the case of using the adaptive bandwidth according to the present invention then shows a lower BER than in the case of using the fixed bandwidth. As described above, it can be seen that the adaptive channel estimation method according to the preferred embodiment of the present invention exhibits demodulation performance close to optimum for various fading frequencies.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims, but also by those equivalent to the scope of the claims.

In the present invention operating as described in detail above, the effects obtained by the representative ones of the disclosed inventions will be briefly described as follows.

The adaptive channel estimation filter according to the present invention can be implemented as a simple phase shift checker and an adder without the need for calculation of power measurement and autocorrelation.

Claims (10)

In the adaptive channel estimation apparatus for a mobile communication system, A channel estimation filter that receives a pilot signal detected by a pilot pattern and accumulated for a predetermined period, and estimates channel information within a predetermined bandwidth by using the accumulated pilot signal; and the channel information is used to compensate a received data signal. Used, A phase shift counter for tracking the channel phase of the channel information estimated by the channel estimation filter every unit time and calculating a phase shift count of the channel phase; And a bandwidth selector for controlling the bandwidth of the channel estimation filter to be proportional to the phase shift count. The method of claim 1, wherein the phase shift counter, The phase shift count is increased by 1 whenever the amount of change in the channel phase exceeds a specific magnitude. The apparatus of claim 2, wherein the amount of change is 90 degrees, and the phase shift counter selectively increases the phase shift count according to a real code and an imaginary code of the channel information. 3. The method of claim 2, wherein the amount of change is 45 degrees, and the phase shift counter selectively increases the phase shift count according to a ratio of a real code and an imaginary code and a magnitude of a real component and an imaginary component of the channel information. Adaptive channel estimation apparatus. The method of claim 1, wherein the bandwidth selector, And storing bandwidth values corresponding to the plurality of values of the phase shift count, selecting a bandwidth value corresponding to the value of the phase shift count provided from the phase shift counter, and providing the bandwidth value to the channel estimation filter. Channel estimation device. In the adaptive channel estimation method for a mobile communication system, Receiving a pilot signal detected by a pilot pattern and accumulated for a predetermined period, estimating channel information within a predetermined bandwidth by using the accumulated pilot signal, and the channel information is used to compensate the received data signal. , Calculating a phase shift count of the channel phase by tracking the estimated channel phase of the channel information every unit time; And controlling the bandwidth such that the phase shift is proportional to a count. The method of claim 6, wherein the phase shift count is The adaptive channel estimation method of claim 1, wherein the amount of change in the channel phase is increased by one whenever the amount of change in the channel phase exceeds a specific magnitude. 8. The adaptive channel estimation method of claim 7, wherein the amount of change is 90 degrees and the phase shift count is selectively increased according to a real code and an imaginary code of the channel information. 8. The adaptive channel estimation method of claim 7, wherein the change amount is 45 degrees, and the phase shift count is selectively increased according to a ratio of a real code and an imaginary code and a magnitude of a real component and an imaginary component of the channel information. . The method of claim 6, wherein the controlling of the bandwidth comprises: Storing bandwidth values corresponding to a plurality of values of the phase shift count, selecting a bandwidth value corresponding to the value of the phase shift count provided from the phase shift counter, and estimating the channel information in a next time interval; Adaptive Channel Estimation Method, characterized in that used.

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