KR100317384B1 - Channel characteristic compensation method - Google Patents

Abstract

The present invention relates to a channel characteristic correction method, and in particular, to perform signal demodulation using the characteristics of a pilot signal to improve the performance of a receiver compared to the case of using only channel characteristics from a conventional OFDM symbol. There is a purpose. For this purpose, the present invention provides each channel characteristic function from OFDM training symbol (T1) (T2). ) ( ) And the channel characteristic function ( ) ( Mean value of ) And the average value ( ) And channel characteristics ( ) ( Obtaining an error value between and comparing the minimum error value, and if the error value is greater than the minimum error value, obtains channel characteristics of a subcarrier adjacent to the pilot signal, Step).

Description

Channel characteristic correction method {CHANNEL CHARACTERISTIC COMPENSATION METHOD}

The present invention relates to wireless LAN (LAN) technology, and more particularly, to a channel characteristic correction method.

Conventional wireless LANs communicate at a transmission rate of 1 to 2 Mbps using a spread spectrum method.

However, with the development of communication technologies, various transmission demands have increased the demand for higher transmission rates.

For example, if you want to transmit HDTV or DVD signal, you need a transmission speed of several Mbps to 20Mbps.

Accordingly, international standards such as IEEE 802.11a, HIPERLAN, and MMAC have been enacted, and efforts have been made to integrate the mutual standards.

For example, the IEEE 802.11a standard adopts orthogonal frequency division multiplex (OFDM) as a modulation method that supports high-speed transmission of several tens of Mbps and is strong in channel characteristics.

The OFDM transmission frame structure of the IEEE 802.11a standard is shown in the exemplary diagram of FIG.

Here, t denotes a short symbol, T denotes a long symbol, and GI denotes a guide interval.

In the transmission frame, ten OFDM training symbols are transmitted during t1 to t10, and two OFDM training symbols are transmitted during T1 and T2.

Next, a symbol (SIGNAL) having modulation parameter information of the transmission signal is transmitted, and the actual transmission data (Data1, Data2) are divided into predetermined sizes and transmitted.

The OFDM training symbol is used for signal detection, automatic gain adjustment, ADC adjustment, receive antenna selection when using two or more antennas, initial synchronization tasks (frequency error estimation, timing synchronization), and the like. It consists of 12 subcarriers and the data allocated to the subcarriers uses Quadrature Phase Shift Keying (QPSK) modulation as shown in Equation (1) below.

--(One)

The transmission signal at this time is expressed as in Equation (2) below.

---- (2)

here, Is a window.

The OFDM training symbol is used for channel characteristics, more accurate frequency error estimation, frame synchronization, etc., and is composed of 52 subcarriers (excluding DC components). Binary Phase Shift Keying (BPSK) is used.

--- (3)

The transmission signal at this time is expressed as in Equation (4) below.

---- (4)

here, Is a window.

The reason why the amplitudes are different in the above formulas (1) and (3) is to make the transmission power the same.

In addition, the number of subcarriers of the short OFDM training symbol is 1/4 compared to the long OFDM training symbol and the symbol interval is short as 1/4.

On the other hand, OFDM modulation uses an Inverse Discrete Fourier Transform (IDFT) as shown in FIG.

The symbol having the modulation parameter (SIGNAL) is configured as shown in Fig. 3 and contains the important transmission information such as modulation method, channel coding method, frame length, so that the strongest modulation parameter (1/2 convolutional code, BPSK modulation, the lowest data rate.

Modulation parameters according to the rates of the symbols SIGNAL are shown in the table of FIG. 4.

In order to perform synchronous demodulation strongly on the frequency offset and phase noise, four pilot signals are transmitted for each symbol SIGNAL and data Data1 and Data2, and the pilot signals are subcarriers as shown in FIG. Insert at position 21, -7, 7, 21 using BPSK.

The polarity of the pilot subcarrier in BPSK modulation is symbol Starting with the expression (5) The cycle is repeated.

----- (5)

In addition, the same signal is not modulated in four pilot signals for each OFDM symbol, but the polarity of one of the pilot subcarriers is reversed as shown in Equation (6) below.

---- (6)

For example, in the OFDM symbol corresponding to the data Data1 following the symbol SIGNAL, the equation (5) Therefore, subcarriers -21, -7, and 7 transmit '1' and subcarriers 21 transmit '-1' to BPSK.

In general, a receiver receiving a transmission signal of the IEEE 802.11a standard detects a signal in 10 OFDM training symbols, performs an automatic gain adjustment function according to signal strength, and determines a receiving antenna when using antenna diversity and frequency. Perform tasks such as synchronization and timing synchronization.

Next, symbol synchronization, channel characteristic estimation, and precision estimation of frequency error are performed in two incoming OFDM training symbol intervals.

The estimated channel characteristics compensate for the channel characteristics at the time of demodulation of the received signal to eliminate distortion of the signal generated on the transmission path. A channel characteristic estimation method using a long OFDM training symbol is as follows.

52 known transmit signals (T; Tone) are transmitted to the channel to be measured, and the received signal (R) is received and the channel characteristics ( ) Can be obtained.

----- (7)

Where N is noise.

At this time, the transmission signal (T) and the reception signal (R) for each subcarrier is known and the channel characteristics ( ), The channel characteristics of the transmission channel can be obtained as frequency characteristics.

And, if you want to know the time characteristic of the channel characteristic, you can obtain it by IFFT.

The channel characteristics obtained in this section are used for channel distortion correction when demodulating all subsequent symbols.

Channel characteristics obtained from an OFDM training symbol for each subcarrier in the OFDM symbol demodulation (R) Signal corrected for the received signal R distorted by the channel characteristics on the transmission path ( ) Is obtained as in Equation (8) below.

---- (8)

The correction signal ( By performing demodulation using), better reception performance can be obtained.

However, the transmission channel characteristic may not be constant during one frame period due to fading.

That is, channel characteristics may vary within one frame according to the movement of the transmitter or receiver or the movement of people around.

Accordingly, there is a problem in that the bit error probability or the packet error probability of the receiver output may be greatly reduced by demodulating the entire frame only with the channel characteristics measured at the beginning of the frame.

Therefore, in order to improve the conventional problem, the present invention can improve the performance of a receiver by performing signal demodulation using the characteristics of a pilot signal rather than using only the channel characteristics from a conventional OFDM symbol. It is an object of the present invention to provide a channel characteristic correction method.

1 is an exemplary view showing the structure of an IEEE802.11a OFDM transmission frame.

2 is an exemplary diagram showing signal input and output for OFDM modulation.

3 is an exemplary diagram showing the assignment of a symbol (SIGNAL) in an OFDM transmission frame.

FIG. 4 is a table showing rate bit allocation of a symbol (SIGNAL) in FIG.

5 is an exemplary diagram showing frequency allocation of a pilot signal in a subcarrier.

6 is a signal flow diagram for one embodiment of the present invention.

7 is an exemplary view showing window settings in an embodiment of the present invention.

8 is a signal flow diagram for another embodiment of the present invention.

In order to achieve the above object, the present invention provides each channel characteristic function from OFDM training symbol (T1) (T2). ) ( ) And the channel characteristic function ( ) ( Mean value of ) And the average value ( ) And channel characteristics ( ) ( Obtaining an error value between and comparing the minimum error value, and if the error value is greater than the minimum error value, obtains channel characteristics of a subcarrier adjacent to the pilot signal, ) Is performed.

In addition, the present invention provides each channel characteristic function from the training training signal (T1) (T2) to achieve the above object. ) ( ) And its channel characteristic function ( ) ( Mean value of ) And a channel characteristic function ( ), The channel characteristic average value of the pilot signal for the ) And the channel characteristic average value ( ) ( Obtaining an error value of the subcarrier adjacent to the pilot signal when the error value is greater than the minimum error value, and calculating the channel characteristic of the subcarrier adjacent to the pilot signal. ) Is performed.

In the above-described step for channel characteristic correction, the channel characteristic function ( ) And the channel characteristic function ( Channel characteristic estimate of adjacent subcarriers within a predetermined window ) And the channel characteristic estimate ( ) Multiplied by weight gives the initial channel characteristic function ( And a process of correcting by summing).

On the other hand, the step for the channel characteristic correction in the channel characteristic function from the pilot signal ( Estimates the channel characteristics of adjacent subcarriers within a predetermined window ), But the channel characteristic function for a given symbol ( Mean value of ) Can be used.

That is, the step for channel characteristic correction is performed from the pilot signal to the channel characteristic function ( ) And channel characteristic functions due to noise Function from 1 to Nu to correct the error ) And the channel characteristic function averaged for each Nu symbol by dividing by Nu. ) And the channel characteristic function within a predetermined window ( ) Are added together to estimate the channel characteristics of adjacent subcarriers within a predetermined window ( ), And channel characteristic estimates on desired subcarriers among four pilot values. ) Is multiplied by the weight to obtain the channel characteristic function ( ), And then divide by (1 + weight) to estimate the channel characteristics used for receiver demodulation ( ) Can be made.

Hereinafter, the present invention will be described in detail with reference to the drawings.

An embodiment of the present invention is made in the same process as the signal flow diagram of FIG. 6, which will be described below.

First, from the two OFDM training symbols, that is, the symbols T1 and T2, the channel characteristic function ( ) ( )

After this, the channel characteristic function ( ) ( ) By summing and averaging the channel characteristic function of the long OFDM training symbol )

That is, one channel characteristic function ( or In order to obtain excellent performance in noisy or fading environment, the channel characteristic function ( ) ( ) Add up to the average ( Channel characteristic average value ( )

Thereafter, the channel characteristic average value ( ) And channel characteristic functions ( ) ( The error value of) is calculated as in Equation (9) below.

----- (9)

The error obtained in Equation (9) may vary depending on noise, fading, and modulation scheme.

That is, the greater the noise and the larger the delay spread, which indicates the degree of fading, the larger the error value. Also, the error is related to the modulation scheme. Becomes smaller.

In this case, it is determined whether the channel characteristic correction using the pilot information is compared by comparing the minimum error value (Threshold) required for the correction of the channel characteristic for each modulation scheme with the error value (error) of the current frame.

Accordingly, when the error value of the current frame is smaller than the minimum error value, it is determined that the channel characteristic correction is not necessary. To demodulate the received signal.

On the contrary, when the error value of the current frame is larger than the minimum error value, it is determined that channel correction is necessary, and thus the average value of the channel characteristics is determined by using information obtained from the pilot signal. By correcting the ) And its correction value ( Demodulating the received signal using the following description.

Average value of channel characteristics ( 2 OFDM training symbols T1 and T2 transmit fixed tone signals to all subcarriers except the DC component, but the next OFDM symbol SIGNAL transmits specific tone signals to only 4 pilot subcarriers and the remaining 48 There is no new transmission of information on the channel characteristics in the subcarrier.

Therefore, in order to newly estimate channel characteristics for every OFDM symbol, new channel characteristic information ( )

However, as can be seen from the relationship between delay spread and coherence bandwidth of a transmission channel as shown in Equation (10), the aggregation bandwidth is proportional to the inverse of the delay width of the transmission channel.

----- (10)

In other words, the larger the delay spread of the transmission channel, the smaller the coherence bandwidth, which is considered to have the same channel characteristic, and conversely, the smaller the delay width of the transmission channel, the larger the characteristic bandwidth.

Therefore, the channel characteristic function obtained from the pilot signal ( Channel characteristic function of subcarriers adjacent to the pilot signal ) Can be estimated to some degree, first the channel characteristic function ( ) Is determined by referring to the error value of Equation (9).

Subsequently, channel characteristic information of adjacent subcarriers ( Determine the calculation cycle of).

However, the channel characteristic function ( ) May be distorted due to noise, fading, etc., but the channel characteristic information for the remaining subcarrier channels other than the pilot subcarrier is still the most accurate channel characteristic information.

Therefore, the channel characteristic function ( ) ( ), Different weights, or weights, are determined.

Here, the weight depends on the error value and the modulation scheme.

Thereafter, the channel characteristic function () is determined in a state where a window, a calculation period, and a weight are determined. ) ( ) ( ) Is calculated.

First, the channel characteristic function Function from 1 to Nu to correct the error ) By adding up the sum and dividing by Nu, and then averaging the channel characteristic function ).

----- (11)

Thereafter, as shown in Fig. 7, the channel characteristic function from the pilot signal ( Channel characteristic information of an adjacent subcarrier by the window value determined above from Calculate

Where channel characteristic information ( ) Can be calculated for every OFDM symbol or a channel characteristic function ( ) Is changed by the influence of noise and so on. It can be calculated using), and the appropriate value is determined according to the error value.

That is, the channel characteristic function ( ), And add the sums of the appropriate window to estimate the channel characteristics of adjacent subcarriers ( Will be calculated.

------ (12)

Accordingly, as shown in Equation (13) below, the estimated channel characteristic on the desired subcarrier among four pilot values ( ) Is multiplied by the weight to obtain the channel characteristic function ( ), And then divide by (1 + weight) to estimate the channel characteristics used for receiver demodulation ( Get)

----- (13)

In addition, the channel characteristic function ( ) To adjust the channel characteristic correction value ( There are many ways to get).

Meanwhile, another embodiment of the present invention can demodulate a received signal by performing the same process as the signal flow chart of FIG. 8 to determine the necessity of channel characteristic correction.

First, the channel characteristic function obtained from the OFDM symbols T1 and T2 ( ) And calculate the channel characteristic function from the pilot ) To obtain the channel characteristic mean value for each interval (Nu) ) Is performed in the same process as in the signal flowchart of FIG.

The channel characteristic function of the pilot subcarrier is then ) And the channel characteristic mean value from the pilot signal ( Error between the signals, i.e., the channel characteristic function ) And the channel characteristic function obtained from the OFDM training symbols (T1, T2) Error is calculated with the channel characteristic corresponding to the pilot subcarrier, and it is determined whether the error of the transmission channel is greater than the minimum error value.

Accordingly, if the error value of the transmission channel is not larger than the minimum error value, channel characteristic correction is not necessary, and thus the channel characteristic average value from the OFDM training symbols T1 and T2 ( ) Demodulate the received signal.

On the contrary, if the error value of the transmission channel is larger than the minimum error value, the channel characteristic correction is necessary and the channel characteristic estimation value ( In order to obtain), the same process as in the signal flowchart of FIG. 6 is performed.

Here, the channel characteristic function for several symbols rather than one pilot signal ( Using) can further improve the demodulation performance of the received signal.

As described in detail above, the present invention provides a demodulation performance of a received signal by correcting channel characteristics using a long OFDM training symbol and a pilot signal present in each symbol in a wireless LAN reception system according to the IEEE 802.11a standard. Has the effect of improving.

Claims (7)

In a method for demodulating a received signal using channel characteristics obtained from an OFDM training symbol, each channel characteristic function is derived from the OFDM training symbols T1 and T2. ) ( ), And the channel characteristic function ( ) ( ) To average the initial channel characteristics ( ) Is obtained, and the average value ( ) And channel characteristics ( ) ( A third step of obtaining an error value between and comparing the minimum error value to the minimum error value, and obtaining a channel characteristic of a subcarrier adjacent to a pilot signal when the error value of the transmission channel is greater than the minimum error value. ( And performing a fourth step of correcting ()). The method of claim 1, wherein the fourth step is performed from a pilot signal to a channel characteristic function (i.e. ), And the channel characteristic function ( Channel characteristic estimate of adjacent subcarriers within a predetermined window ), And the channel characteristic estimate ( ) Multiplied by weight gives the initial channel characteristic function ( And a third process of correcting the sum by adding a). The method of claim 1, wherein the fourth step is performed from a pilot signal to a channel characteristic function (i.e. ), And the channel characteristic function ) And add the average ) And the channel characteristic function ( Channel characteristic estimates of adjacent subcarriers in a predetermined range ( And a third process of calculating the channel characteristic estimation value ( ) Multiplied by weight gives the initial channel characteristic function ( ) And its initial channel characteristic function ( Channel characteristic correction method, characterized in that it comprises a fourth process of correcting. The method of claim 3, wherein the average value of the second process ( ) Is the channel characteristic function ( Channel characteristic correction method, characterized in that it is calculated by the following calculation to correct the error. 4. A channel characteristic estimate according to claim 3, ) Is calculated by the following equation. 4. The method of claim 3, wherein the fourth process comprises performing a channel characteristic correction value for demodulating the received signal using an operation such as Channel characteristic correction method. In a method for demodulating a received signal using channel characteristics obtained from an OFDM training symbol, each channel characteristic function is derived from the OFDM training symbols T1 and T2. ) ( ) And its channel characteristic function ( ) ( Mean value of ) And a channel characteristic function of the pilot signal ( ), The channel characteristic average value of the pilot signal for the ), And the channel characteristic average value ( ) ( The third step of obtaining the error value (error) of the signal and compare with the minimum error value (Threshold), and if the error value (error) is greater than the minimum error value, the channel characteristics of the subcarrier adjacent to the pilot signal to obtain the function ( And performing a fourth step of correcting ()).