Apparatus and method for estimating channel in broadband wireless access (BWA) communication system

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-1 APPARATUS AND METHOD FOR CHANNEL ESTIMATION IN A WIRELESS BROADBAND ACCESS COMMUNICATION SYSTEM DESCRIPTION

REFERENCE TO RELATED PATENT APPLICATION BACKGROUND OF THE INVENTION

Field of Invention

The present invention relates to a channel estimation apparatus and method in a Broadband Wireless Access (BWA) communication system. More specifically, the present invention relates to an apparatus and a method for a more accurate estimation of an Infinite Impulse Response (IIR, Infinite Impulse Response) type channel, by selecting a necessary filter factor in a receiver of a system. of communication with orthogonal frequency division multiplexing (OFDM, Orthogonal Frequency Division Multiplexing).

Description of the Prior Art

According to an OFDM scheme, a transmitter Quadrature Phase Shift Key (QPSK) or Quadrature Amplitude Modulation (QAM) modulates digital information in the frequency domain, and converts the information modulated into a signal in the time domain using an Inverse Fast Fourier Transform (IFFT) operation. In doing so, the transmitter typically sends a pilot subcarrier between a preamble symbol and a data subcarrier, so that a receiver can easily estimate the channel. The receiver estimates the channel for the data subcarrier using the preamble symbol and the pilot subcarrier, and uses the estimated value for demodulation.

When an OFDM scheme is adopted in a mobile communication, the reception performance at the receiver depends mainly on the precision with which the receiver estimates the changing channel while it is traveling.

With reference to Figure 1, a frame structure is illustrated in an orthogonal frequency division multiple access (OFDMA, Orthogonal

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-2Frequency Division Multiple Access) 802.16, of the Institute of Electrical and Electronics Engineers (IEEE, Institute of Electrical and Electronics Engineers). Figure 1 depicts a pattern of preamble and pilot subcarriers, of a downlink in an initial interval of a frame, in a changing channel estimate while it is moving.

With reference to Figure 1, a frame structure is illustrated in an 802.16 Orthogonal Frequency Division Multiple Access (OFDMA) scheme from the Institute of Electrical and Electronics Engineers (IEEE). Engineers). Figure 1 depicts a downlink pilot and preamble subcarrier pattern in an initial interval of a frame in a conventional OFDMA system.

The downlink of the OFDMA system carries frames that each include preamble and pilot subcarriers per frame, for the receiver. The receiver estimates a channel on the neighboring data subcarrier using the received frame, as shown in Figure 2.

Fig. 2 is a flow chart of channel estimation in a conventional channel estimation apparatus, in the OFDMA system. Upon receipt of the frame, in step 200 the conventional channel estimation apparatus estimates channel estimation values ​​for each subcarrier of symbol index 0, interpolating preamble subcarriers along a frequency axis, and estimates values ​​of neighbor symbol subcarrier estimation, using time axis interpolation in step 202.

In step 202, the subcarrier channel estimate value of symbol index 1 corresponding to the pilot subcarrier position k of symbol index 2 is calculated by interpolating the estimate value of the neighboring symbol subcarrier channel along the time axis, based on equation (1).

In equation (1), Hj denotes an estimate value of the jth subcarrier of an i-th symbol index.

<img file="ES2348622T3_D0001.tif" />

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-3 The channel values ​​of the subcarriers of symbol index 2 can be obtained in the same way. Therefore, equation (1) can be generalized as follows, that is, the estimate value of the subcarrier of the symbol index n corresponding to the pilot subcarrier position k of the symbol index n + 1, can be acquired by interpolating the value of Subcarrier estimation of the neighboring symbol, along the time axis, based on equation (2).

// „(O = | //„ -. (O + | // „₊i (O. --- (2)

In equation (2), Hj denotes an estimate value of the jth subcarrier of an i-th symbol index.

Next, the conventional channel estimation apparatus estimates the channel for each subcarrier of the symbol index, by applying interpolation on the frequency axis using the subcarrier estimation values ​​acquired using the pilot symbols and interpolation on the time axis at step 204, and determines in step 206 whether the channel has been estimated to the end of the frame. When the channel estimation is not completed in step 206, the conventional channel estimating apparatus returns to step 202. Otherwise, the conventional channel estimating apparatus ends the process of FIG. 2.

However, disadvantageously, the channel estimation method of Figure 2 performs the one-dimensional interpolations of the time / frequency axis regardless of the channel condition, and makes use of the estimation value of the previous symbol as a preamble for the Channel estimation of the next symbol, even when the temporal change of the channel is not very large (for example, low speed environment).

European patent application EP 1367790 proposes an improved OFDM equalizer that can equalize an OFDM signal according to a state of a transmission channel, in which a channel state estimation unit is used to estimate a according channel state with variations of the magnitude of pilots interpolated on the time axis, and an averaging unit is used to selectively calculate an average of each pilot interpolated on the time axis, depending on the status of the channel.

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-4 Accordingly, there is a need for an improved apparatus and method for channel estimation by selecting a necessary filter factor when the receiver of the BWA communication system estimates a channel of an adaptive IIR type, which can provide an estimation value much more accurate channel adjustment by quickly adapting to channel change.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention address at least the above problems and / or disadvantages, and provide at least the advantages described above. Accordingly, one aspect of an exemplary embodiment of the present invention is to provide a channel estimation apparatus and method in a broadband wireless access (BWA) communication system.

Another aspect of exemplary embodiments of the present invention is to provide an apparatus and method for estimating a channel using an adaptive infinite impulse response (IIR) filter, in a receiver of a BWA communication system.

Another aspect of exemplary embodiments of the present invention is to provide an apparatus and method for estimating a channel, selecting a necessary filter factor for an adaptive IIR channel in a receiver of a BWA communication system.

The above exemplary embodiments are achieved by providing an apparatus for estimating a channel in a BWA communication system, including a subcarrier demapping device for extracting pilot symbols at predetermined positions for channel estimation, and a channel estimator for acquiring values ​​of subcarrier channel estimation through IIR filtering, using the pilot symbols provided by the subcarrier demapping device and a predetermined filter factor.

In accordance with one aspect of exemplary embodiments of the present invention, a method for estimating a channel in a BWA communication system includes estimating preamble subcarrier channels when a frame is received; estimating subcarrier estimation values ​​of a neighboring symbol through interpolation on the time axis; IIR filtering of the estimated subcarriers through interpolation on the time axis, using a

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-5 default filter factor; and estimating channels of all subcarriers of a symbol index through interpolation on the frequency axis, using the estimation values ​​of the filtered subcarrier.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objectives, features, and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a diagram of a preamble and pilot subcarrier pattern of a downlink in an initial interval of a frame, according to a conventional OFDMA system;

Figure 2 is a flow chart of a channel estimation of a conventional channel estimation apparatus, in the OFDMA system;

Figure 3 is a block diagram of a receiver in a BWA communication system, estimating a channel using IIR filtering, in accordance with an exemplary embodiment of the present invention;

Figure 4 is a flow chart of a channel estimation of a channel estimation apparatus in the BWA communication system, in accordance with an exemplary embodiment of the present invention;

Figure 5 is a diagram of a frame when the channel estimating apparatus of an OFDMA system estimates the channel using an IIR filter, in accordance with an exemplary embodiment of the present invention;

FIG. 6 is a flow chart of a channel estimation of a channel estimation apparatus in a BWA communication system, in accordance with another exemplary embodiment of the present invention;

FIG. 7 is a flow chart of a necessary filter factor selection when the channel estimating apparatus of the BWA communication system estimates the channel, in accordance with an exemplary embodiment of the present invention; and Fig. 8 is a diagram of the channel estimation value estimation, for the first symbol subcarriers when the channel estimation apparatus of the BWA communication system uses symbols after the preamble, in accordance with an exemplary embodiment of the present. invention.

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Through the drawings, it will be understood that the same reference numerals in the drawings refer to the same elements, features and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The subjects defined in the description, as well as a detailed construction and detailed elements, are provided to aid in a comprehensive understanding of the embodiments of the invention, and are merely exemplary. Accordingly, those skilled in the art will recognize that various changes and modifications can be made to the embodiments described herein, without departing from the scope of the invention. Also, for clarity and conciseness, descriptions of known functions and constructions are omitted.

Exemplary embodiments of the present invention provide an apparatus and method for estimating a channel by selecting a necessary filter factor, when a receiver of a broadband wireless access (BWA) communication system estimates the response type channel. infinite on impulse (IIR). The apparatus is described with reference to figure 3.

Figure 3 is a block diagram of a receiver in a BWA communication system, estimating a channel using IIR filtering, in accordance with an exemplary embodiment of the present invention. Herein, in relation to the receiver, a base station can be a transmitter in terms of an uplink, and a terminal can be a transmitter in terms of a downlink.

The receiver of Figure 3 includes a Radio Frequency (RF) processor 300, an Analog / Digital Converter (ADC) 302, an Orthogonal Frequency Division (OFDM) demodulator 304 Multiplexing), a subcarrier demapping device 306, a filter factor selector 308, a channel estimator 310, an equalizer 312, a demodulator 314, and a decoder 316.

The RF processor 300 includes components such as a head unit and a filter to convert an RF signal passing through a radio channel into a baseband signal. The ADC 302 converts the analog baseband signal output from the RF processor 300 to a digital signal.

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-7 The OFDM demodulator 304 extracts a cyclic prefix (CP, Cyclic Prefix) from the data output from the ADC 302 and delivers data in the frequency domain, through a Fast Fourier Transform (FFT, Fast Fourier Transform).

Subcarrier demapping device 306 extracts actual data symbols from the data output from OFDM demodulator 304, and provides the extracted data symbols to equalizer 312. Subcarrier demapping device 306 extracts symbols at predetermined positions (eg, pilot symbols) for channel estimation, and provides the extracted symbols to filter factor selector 308 and channel estimator 310.

Filter factor selector 308 selects a required filter factor per frame for IIR filtering, using pilot symbols provided from subcarrier demapping device 306, and provides channel estimator 310 with the selected filter factor. The filter factor selector 308, which is optional, may not be included in the receiver. The filter factor selector 308 will be described in greater detail with reference to FIG. 7.

Channel estimator 310 estimates a channel using pilot symbols fed from subcarrier demapping device 306. The channel estimator 310 estimates a subcarrier estimate value of a neighboring symbol, through an interpolation on the time axis, filters in IIR the estimated subcarrier through the interpolation on the time axis using the filter factor selected by the filter factor selector 308, estimates a channel for each subcarrier of the symbol index, by applying an interpolation on the frequency axis using the filtered subcarrier estimate value, and provides equalizer 312 with the channel estimate values. If filter factor selector 308 is absent, a predetermined experimental filter factor is used.

Equalizer 312 channel compensates for data symbols delivered from subcarrier demapping device 306, using channel estimate values ​​delivered from estimator 310. Specifically, equalizer 312 compensates for various noises that are generated on the radio channel. .

The demodulator 314 delivers coded data, demodulating the symbols delivered from the equalizer 312 using a modulation scheme of the

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-8transmitter. Decoder 316 restores the original information data by decoding the encoded data output from demodulator 314, using a transmitter encoding scheme.

Next, with reference to the drawings, a channel estimation method in the BWA communication system is described. FIG. 4 is a flow chart of the channel estimation method of the channel estimation apparatus in the BWA communication system, in accordance with an exemplary embodiment of the present invention. The descriptions in Figure 4 are based on Figure 5. Figure 5 depicts a frame, when the channel estimating apparatus of an OFDMA system estimates the channel using an IIR filter, in accordance with an exemplary embodiment of the present invention.

After receiving a frame, the channel estimation apparatus estimates a channel estimation value for each subcarrier of symbol index 0, interpolating preamble subcarriers along the frequency axis in step 400, and estimates subcarrier estimation values of a neighboring symbol, interpolating along the time axis in step 400.

For example, in step 402, the subcarrier channel estimate value at symbol index 1 corresponding to pilot subcarrier position k at symbol index 2, is acquired by interpolating the subcarrier channel estimate value of symbol neighbor, along the time axis, based on equation (1).

The subcarrier estimate value at symbol index n corresponding to pilot subcarrier position k at symbol index n + 1, can be acquired by interpolating the subcarrier estimate value of the neighboring symbol along the time axis, as a function of equation (2).

In step 404, in the channel estimation apparatus IIR filters the estimated subcarrier through interpolation on the time axis, using the predetermined experimental IIR filter factor.

In a mobile WiMAX system, which is the representative example of the IEEE 802.16 OFDMA system, the transmission power of the pilot subcarrier is greater than that of subcarrier 2, by 2.5 dB, and the transmission power of the preamble subcarrier it is greater than that of the pilot subcarrier, by 6.5 dB.

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-9 Since the transmit power of the preamble subcarrier is the maximum in many cases, the gain can be expected in terms of performance when IIR filtering is applied based on equation (3) after interpolation on the time axis ( or after interpolation on the time axis and interpolation on the frequency axis). In this document, IIR filtering is run for each

V subcarrier j given. To acquire the filtered channel estimate value in

V

H (/)

IIR for the i-th symbol, in the estimate value 'of the current symbol a certain proportion of the estimated channel values ​​is accumulated

<img file="ES2348622T3_D0002.tif" />

of the above symbols. As previously described, IIR filtering is defined as a linear system that feeds back the previous output as the input, to acquire the current output. IIR filtering has little time delay because it can perform channel estimation and data demodulation in sequence, along the time axis. Although in step 404 a series of previous values ​​can be accumulated in the current output in the IIR filtering, only the previous channel estimate value is used as the feedback input, as expressed in equation (3) to facilitate comprehension.

<img file="ES2348622T3_D0003.tif" />

In equation (3), denote the filtered channel estimate value in IIR for the j-th subcarrier of the i-th symbol index, and γ is a weighting value to establish the accumulation range of the estimate values ​​of channel of previous symbols. γ is determined based on the condition of the channel.

When 2 feedback inputs are fed to the extended equation (3), equation (4) can be obtained. In this way, it is possible to extend the concept of multiple feedbacks.

V v V

Ηn {1} = (1 - / or - /!) Hn-2 (7) + H „-i (7) + γΗ„ (7), n&gt; 2 ... (4)

Next, the channel estimation apparatus estimates the channel of each subcarrier of the symbol index by applying frequency axis interpolation using the filtered subcarrier estimation values, in step 406, and

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-10 determines whether the channel estimation has been completed to the end of the frame, in step 408. When the channel estimation has not been completed in step 408, the conventional channel estimation apparatus returns to step 402. When channel estimation is completed in step 408, the channel estimating apparatus ends the process of FIG. 4.

Since γ in equation (3) greatly affects the reception characteristic based on the channel condition, it is necessary to select an eigenvalue based on the channel change. In the static channel, the small IIR filter factor γ from equation (3) exhibits the best performance. In contrast, at severe channel change, the large γ filter factor performs best. For example, when the terminal is traveling rapidly at a speed greater than 60 km / h, the best performance is achieved with the filter factor γ of 1. When the filter factor γ is 1, equation (3) does not substantially filter in IIR the previous channel estimation values, which implies that only the conventional interpolations are maintained on the time axis and on the frequency axis. That is, it is advantageous that the previous estimate value is seldom reflected in the next estimate value, under severe channel change. Conversely, in the vicinity of an Additive White Gaussian Noise (AWGN) channel, the filter factor γ performs best. In other words, when the channel has the smallest time shift, it is advantageous to reflect the preamble channel estimate value of the largest relative power in subsequent symbols.

Next, referring to FIG. 6, a channel estimation method according to another exemplary embodiment of the present invention is explained, which performs IIR filtering by selectively selecting the filter factor as a function of the channel condition. FIG. 6 is a flow chart of the channel estimation method of a channel estimation apparatus of a BWA communication system, in accordance with another exemplary embodiment of the present invention.

Upon receipt of a frame, the channel estimation apparatus estimates channel estimation values ​​for each subcarrier of symbol index 0 by interpolating the preamble subcarriers along the frequency axis in step 600, estimates subcarrier estimation values of the neighbor symbol through

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-11 of interpolation on the time axis as a function of equation (2) in step 602, and selects an IIR filtering filter factor in step 604. Step 604 will be described in more detail with reference to FIG. 7.

Next, in step 606 the channel estimation apparatus filters in IIR the estimated subcarriers through interpolation on the time axis using the filter factor mentioned in step 604 as a function of equation (3), estimates channels of each subcarrier of the symbol index by applying interpolation on the frequency axis using the filtered subcarrier estimate values ​​in step 608, and determines in step 610 whether the channel estimation has been completed until the end of the frame. When the channel estimation has not been completed in step 610, the channel estimation apparatus returns to step 602. When the channel estimation has been completed, the channel estimation apparatus ends the process of FIG. 6.

The following describes the filter factor measurement for IIR filtering, referring to Fig. 7. Fig. 7 is a flow chart of a necessary filter factor selection, when the channel estimating apparatus of the communication system BWA estimates the channel, in accordance with an exemplary embodiment of the present invention.

Filter factor selector estimates channel estimate values ​​from preamble H₀ in step 700. When the preamble subcarriers are present in the subcarrier slots such as in the preamble structure of Figure 1, the filter factor selector first estimates the channels of the pilot subcarriers, and then estimates the channels of the other subcarriers using interpolation on the frequency axis.

In step 702, the filter factor selector selects from among the classified filter factors, a filter factor whose moving average for the error accumulation value is not measured. Herein, the classified filter factors are candidates for the predetermined filter factor. When the filter factor is selected in the range of 0 to 1, for example, the filter factor candidates in the range 0.25 can be (0.25, 0.5, 0.75, 1.00). Specifically, yj denotes the j-th filter factor of the ranked filter factor candidates. When the IIR filter has the degree greater than the first degree with the inputs of

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-12 multiple feedbacks, as shown in equation (4), the filter factor can be an appropriate combination within the limited candidate sets, by extending a filter factor. However, this is derived from the individual filter factor. In an exemplary embodiment of the present invention, the individual filter factor is illustrated as an example.

In step 704, the filter factor selector estimates the channel estimate value of the first symbol using the filter factor selected in step 702 and the symbols after the preamble (the symbol index 0). More specifically H i HA, the selection of the filter factor estimates the value¹ channel estimation for the k-th subcarrier of the first symbol (the symbol index 1) by applying IIR filtering using the j-th filter factor and j selected for the symbols excluding the preamble, without using the preamble.

Figure 8 is a diagram of the channel estimation value estimation, for the subcarriers of the first symbol when the channel estimation apparatus of the BWA communication system uses symbols after the preamble, in accordance with an exemplary embodiment of the present invention. .

Figure 8 illustrates step 704. The second symbol subcarrier channel estimate value corresponding to the first symbol pilot subcarrier position is acquired by interpolating the second symbol pilot subcarrier. Next, the channel estimate values ​​for the pilot subcarriers of the first symbol are acquired by IIR filtering the channel estimate values ​​of the same subcarriers in the second symbol (the symbol index 2). Unlike equation (3), this procedure proceeds in the direction of decreasing symbol index, as expressed in equation (5). In general, there is no need to limit the subcarrier to the pilot subcarrier position of the first symbol by error, and the IIR can be applied to two or more symbols as shown in Figure 7. In this case, the IIR filtering is applied by repeating equation (5).

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-13 In equation (5), Hi (j) denotes the jV th subcarrier estimate value of the i-th symbol index, denotes the IIR filtered channel estimate value for the j-th subcarrier of the i symbol index -th, and γ₇· Denotes the j-th filter factor of the ranked filter factor candidates.

In step 706, the filter factor selector accumulates the errors between the preamble estimated in step 700 and the first symbol estimated in step 704. For reasons of precision and computational complexity, only the error corresponding to the position can be accumulated. pilot subcarrier of the first symbol. The accumulation error value can be measured based on equation (6).

S^jW = Y \ H₀(k) -Hák) ... (6) k

In equation (6), δ (^η) denotes the error accumulation value of the nth frame when using the jth filter factor γ₇Of the classified filter factor candidates, H₀(k) denotes the channel estimate value for the k-th subcarrier (k) of the preamble, and ¹ denotes the channel estimate value for the kth subcarrier of the first symbol.

Equation (6) squares and accumulates the errors to measure the error accumulation value. However, in the current implementation, instead of the square function, for computational simplicity the sum of the absolute values ​​of the real part and the imaginary part can be used. Despite the large computational difference, there is little difference between the two calculations in terms of operation.

At step 708, the filter factor selector measures the moving average, which averages the error accumulation values ​​for the selected filter factor, between the current frame and the previous frame, by the current number based on equation ( 7).

Δ⁷ = + ₊ ... ₊ ^⁽ⁿ-^{A, +}”) ...(7)

N

In equation (7), A⁷ denotes the moving average when the j-th filter factor γ is used among the ranked filter factor candidates₇·, N denotes a

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-14 preset number of recent frames to measure moving average, and 8 (ⁿ) denotes the error accumulation value of the nth frame using yj.

In step 710, the filter factor selector determines whether the moving average is measured for the error accumulation values ​​of each of the ranked filter factors. When there is a moving average filter factor not measured in step 710, the filter factor selector returns to step 702.

When in step 710 there is no unmeasured moving average filter factor, the filter factor selector selects in step 712 the smallest moving average filter factor among the moving averages of the ranked filter factors. The filter factor selected in step 710 is applied to the IIR filtering of the current frame or the next frame.

In step 714, the filter factor selector determines if there is a next frame. When the next received frame exists, the filter factor selector returns to step 700. When there is no next frame, the filter factor selector ends the process of FIG. 7.

As discussed above, the apparatus and a method for channel estimation by selecting the necessary filter factor when the BWA communication system receiver estimates the adaptive IIR type channel can provide a much more accurate channel estimation value. , through immediate adaptation to the change of the channel.

While the invention has been shown and described with reference to certain preferred embodiments thereof, those skilled in the art will understand that various changes in shape and detail may be made therein, without departing from the scope of the invention as defined. by the appended claims and their equivalents.