KR101084146B1 - How to measure frequency error - Google Patents

Abstract

The present invention provides a frequency error measuring method applied to a system for transmitting a signal through two or more antennas using a plurality of orthogonal subcarriers, the method comprising: receiving 0 and 1 pre-symbol symbols transmitted from each of the two or more antennas. ; Measuring a Signal to Noise Ratio (SNR) of a signal transmitted through the two or more antennas; Obtaining time synchronization using the 0 prefix symbol to sample the precode symbols; Sampling the predetermined number of prefix symbols if the SNR is equal to or less than a preset reference value; And calculating an expectation of the sampled precode symbol phase values, wherein the expected value of the precode symbol phase values is the frequency error. According to the present invention, by performing the frequency error measurement according to the SNR, there is an effect that can measure the frequency error with better performance.

Frequency offset, precode, SNR, OFDM

Description

Method for Measuring Frequency Error {Method for Estimating Frequency Offset}

1 is an explanatory diagram for explaining a method of allocating a training symbol of a transposition code;

2 is a diagram illustrating an embodiment of a transposition code used for OFDM.

3 is a diagram illustrating an embodiment of a method for transmitting a signal encoded according to FIG. 2;

Figure 4 is an embodiment explanatory diagram showing the reception state of the prefix.

5 is a diagram illustrating an embodiment of a multiple transmit antenna prefix.

6 is an exemplary explanatory diagram showing a frequency error measurement result according to an embodiment of the frequency estimation method.

7 is a flowchart illustrating a process of using different frequency error estimation methods according to SNR.

8 is a diagram illustrating an example of measuring a frequency error using two methods according to SNR.

The present invention relates to a method for measuring frequency error, and more particularly, to a method for measuring frequency error with better performance in a system to which orthogonal frequency division multiplexing is applied.

First, Orthogonal Frequency Division Multiplexing (OFDM) will be described below. OFDM is a special form of multicarrier transmission, in which one data string is transmitted on a subcarrier having a lower data rate. The channel environment for wireless communication has a multipath due to obstacles such as buildings. In a multi-path wireless channel, delay-spreading occurs due to multipathing, and if the delay spreading time is larger than the time when the next symbol is transmitted, Inter-Symbol Interference (hereinafter, referred to as 'ISI') Will occur.

In this case, in the frequency domain, frequency selective fading occurs. When using a single carrier, an equalizer is used to remove inter-symbol interference components. However, as the speed of data increases, the complexity of the equalizer also increases. In an OFDM system, high-speed data is transmitted in parallel using a plurality of subcarriers, whereby a low-speed transmission effect on each subcarrier can be obtained.

Meanwhile, a multi-input multi-output (hereinafter referred to as 'MIMO') will be described below. The existing wireless wireless communication system mainly used for voice service, and mainly used channel coding to overcome poor wireless channel environment. However, as high-quality multimedia services are required, it has changed from conventional voice-oriented communication to data-oriented communication. Thus, to realize this, a technique for sending a large amount of data faster and with less error was required. However, the mobile communication environment greatly distorts the signal due to the effects of multipath, shadowing, attenuation, and interference. In particular, fading due to multipath results in severe distortion of the signal due to the sum of signals having different magnitudes and phases received over different paths. MIMO system has been proposed to overcome this fading phenomenon.

Unlike conventional single input single output (SISO) systems, the MIMO system uses multiple antennas on the transmitting side and the receiving side. Multiple antennas transmit and receive multiple signals, allowing data to be transmitted more efficiently than conventional systems without having to extend bandwidth.

On the other hand, the timing synchronization (Timing Synchronization) will be described as follows. Accurate synchronization ensures reliable communication in a variety of communication environments and ensures faster and more accurate communication. The OFDM system does not affect the performance of the time delay within the protection period through the Cyclic Prefix (CP). However, if the time delay goes out of the protection interval, the orthogonality is lost, resulting in performance degradation. In general, the receiver determines a total frame synchronization by measuring a correlation between a predetermined prefix symbol and a received signal.

Since the precode transmitted from the MIMO system has a different characteristic from the reception signal of the receiver using the prior art, it is possible to efficiently apply a method different from the conventional method in the frequency error estimation method. In other words, the conventional single input-output-orthogonal frequency division transmission method can be used to estimate the frequency error by transmitting the transposition code, but it is sensitive to the signal-to-noise ratio (ie, 'SNR'). There is a problem. Therefore, in the case of a cell edge with a low SNR, it is difficult to estimate the frequency error, so that accurate channel estimation is impossible at the receiver, resulting in performance degradation in signal detection. Therefore, there is a need for a method capable of efficiently performing frequency error measurement even at a location with low SNR as described above.

It is an object of the present invention not only to measure frequency error even in a region with low SNR, but also to provide a method of varying a frequency error measuring method according to SNR.

In order to achieve the above object, the present invention provides a method for measuring a frequency error applied to a system for transmitting a signal through two or more antennas using a plurality of orthogonal subcarriers, the signal to noise ratio of the received signal (hereinafter referred to as' SNR '), and if the SNR of the received signal is less than or equal to the reference value, sampling at least a predetermined number of precode symbols and calculating an expectation of the sampled precode symbol phase values. Is done.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention relates to an efficient method of time synchronization and frequency error estimation using an orthogonal prefix code transmitted in a wireless communication system of a multi-input-orthogonal frequency division transmission scheme. Orthogonal Frequency Division Multiplexing (OFDM) Uses a multi-carrier modulation scheme. Orthogonal frequency division transmission scheme uses a cyclic prefix that is copied in front of the OFDM symbol by replicating the signal of the last part of the OFDM symbol to compensate for the multipath when the signal is transmitted. Using the prefix, frequency error measurement and time synchronization are obtained.

First, the first frequency error measuring method applied when the SNR is high is as follows.

Frequency offset is caused by the frequency difference between the oscillators of the transmitter and receiver. Frequency errors caused by differences in oscillators can cause phase shifts and degrade communication performance. To measure the general frequency error, we use a prefix that is repeated twice at D intervals in time. That is, the frequency error may be measured using Equation 1.

은 수신된 전치부호 심볼을 의미하고,

은 전송된 전치부호 심볼을 의미한다. 한편, N 은 전치부호 심볼의 총 샘플의 수를 나타내며,

는 추정된 주파수 오차를 나타낸다. In Equation 1,

Means received prefix symbol,

Denotes the transmitted prefix symbol. On the other hand, N represents the total number of samples of the prefix symbol,

Denotes the estimated frequency error.

FIG. 1 is a diagram for explaining an embodiment of a method of allocating a training symbol of a prefix. First, the total number of subcarriers used is determined by one of 128, 512, 1024, and 2048. 1 illustrates an example of using 1024 subcarriers. In addition, a portion of subcarriers located at both edges of the subcarriers in the frequency domain are allocated as guard bands, and as shown in FIG. 1, one per three subcarriers is trained (trained) as shown in Equation 2 for the remaining subcarriers. ) As a symbol.

In Equation 2, n is the number of exponential prefix carrier sets, and has any one of 0, 1, and 2, and k has a value of 0 to 283 as a continuous index of the prefix code carrier.

Accordingly, segment 0 uses preamble carrier set 0 (PreambleCarrierSet0), segment 1 uses precode carrier set 1 (PreambleCarrierSet1), and segment 2 uses presigned carrier set 2 (PreambleCarrierSet2).

Table 1 shows the PN sequences transmitted in the prefix.

index IDcell Segment Modulation Sequence (Hexadecimal Format) 0 0 0 A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77
065C7EE7D0ADBEAF One One 0 668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D6
4AB4A33C467001F3B2 2 2 0 1C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE0
3E9B5AB5E1D3F98C6E ：
： ：
： ：
： ：
：

)로 변환시킨 다음, 최상위 비트(Most Significant Bit; 이하 'MSB')로부터 최하위 비트(Least Significant Bit; 이하 'LSB')로

를 매핑한다. 예를들어, 0 은 +1 로, 1 은 -1 로 매핑하면, 인덱스가 0 인 0 번 세그먼트에서

는 110000010010... 이므로, 변환된 PN 코드값은 -1 -1 +1 +1 +1 +1 +1 -1 +1 +1 -1 +1 ... 이다. The PN sequence used is determined by the segment number and the IDcell parameter value. Each defined PN sequence is mapped to a presigned subcarrier in ascending order. In the table, the PN sequence is expressed in hexadecimal format. To get the corresponding PN code value, replace the suggested hexadecimal sequence with a binary sequence (

), And then from the most significant bit (MSB) to the least significant bit (LSB)

Map. For example, if 0 maps to +1 and 1 maps to -1, then segment 0 has index 0

Since 110000010010 ..., the converted PN code value is -1 -1 +1 +1 +1 +1 +1 -1 +1 +1 -1 +1 ....

As described above, the prefix is a signal used for synchronizing the transmission timing between two or more systems in a wireless communication and correcting a frequency error caused by an oscillator between transceivers. The precoding code, which plays such a role, configures the training symbol to have a symmetric structure in time to synchronize the transmission timing and to correct the frequency error.

When operating in the MIMO-OFDM transmission scheme, the orthogonal prefix is used for the detection of the multi-input-output signal, and after the detection of the received signal, the multi-input-orthogonal frequency division transmission scheme is used. It will work. Therefore, when receiving a transposition code having orthogonality, frequency error estimation can be performed using the characteristics of the signal with the same performance regardless of the change of the SNR.

In the multi-input-orthogonal frequency division transmission scheme, the second frequency error measurement method is described as follows when the SNR is low and an orthogonal prefix is used.

2 is a diagram illustrating an embodiment of a transposition code used for OFDM. The encoding is performed as shown in FIG. 2 so as to multiply an existing prefix code transmitted by a single transmitter by an orthogonal code and to transmit two symbol intervals through two antennas.

3 is a diagram illustrating an embodiment of a method of transmitting a signal encoded according to FIG. 2. As shown in FIG. 3, encoding is performed using an orthogonal code for each antenna and an OFDM symbol unit, and each signal is used as a precode sequence of each antenna.

4 is an exemplary explanatory diagram illustrating a reception state of the prefix code. The precoded signal encoded according to the method of FIG. 3 is received as shown in FIG. 4 in an ideal environment without fading channel and thermal noise (AWGN). FIG. 4A shows the received power of the first prefix symbol, and FIG. 4B shows the phase of the first prefix symbol.

As shown in FIG. 4, the 0 prefix symbol and the 1 prefix symbol are received, and the received signal of the 1 prefix symbol is received with a value of 0 + 0j. That is, since the symbols of the first prefix symbol transmitted from each antenna are opposite to each other, they are canceled and received with a value of 0 + 0j. Accordingly, timing synchronization is performed using the 0 prefix symbol of the received signal as described above, and the frequency error is measured using the 0 prefix symbol and the 1 prefix symbol. That is, the frequency error may be measured using the first prefix symbol received with a value of 0 + 0j.

In a real communication environment, since the received signal is affected by the fading fading of the channel, the thermal noise, and the frequency error between the transceiver, the nth specific sample of the first prefix symbol received may be expressed as Equation 3 below.

는 초기에 0 번 전치부호 심볼의 첫번 샘플이 수신될 경우의 위상을 나타내고,

및

은 0 번 및 1 번 안테나에서 전송되는 신호 (

,

)가 겪게 되는 다중경로 페이딩을 나타낸다. 한편, N 은 수신기의 열잡음을 나타내고

는 0 번 전치부호 심볼이 겪게 되는 주파수 오차를 의미한다. 수학식 3 에서는, 1024-point FFT를 사용하는 OFDM 심볼에서 1024 * 1/8 = 128 샘플의 CP 구간을 가지는 경우의 예이다.In Equation 3,

Denotes the phase when the first sample of the 0 prefix symbol is initially received,

And

Is the signal transmitted from antennas 0 and 1 (

,

) Represents the multipath fading experienced. On the other hand, N represents the thermal noise of the receiver

Denotes the frequency error experienced by the 0 prefix symbol. In Equation 3, it is an example of having a CP interval of 1024 * 1/8 = 128 samples in an OFDM symbol using a 1024-point FFT.

The phase error of the first prefix symbol received as described above may be obtained to calculate a frequency error according to Equation 4.

와, 0 번 및 1 번 안테나에서 전송되는 신호 (

,

)가 겪게 되는 다중경로 페이딩인

및

, 의 위상, 수신기의 열잡음 N 의 위상을 모두 반영하여

으로 나타내었다.

와

의 값은 주파수 오차인

와 더해져 수신신호의 위상으로 나타난다. 이때,

값은

에 비해 매우 작은 값이므로, 결국 주파수 오차

값을 알아낼 수 있다. 이 때, 매우 작은 값으로 수렴하게 되는

값은 주파수 오차 추정의 간섭 요인으로 작용할 수 있다.In Equation 4, it is initially a phase corresponding to the first sample of the 0 prefix symbol.

And signals transmitted from antennas 0 and 1 (

,

) Is the multipath fading

And

Reflects the phase of, and the phase of thermal noise, N

As shown.

Wow

The value of is the frequency error

And are added to indicate the phase of the received signal. At this time,

The value is

Is a very small value compared to

Can figure out the value. At this time, the convergence to a very small value

The value can act as an interference factor in frequency error estimation.

FIG. 5 is a diagram illustrating an embodiment of a multiple transmit antenna prefix. FIG. The above-described frequency error estimation method is applicable to all pre-symbol symbols as shown in FIG. 5, and the pre-code structure of all orthogonal characteristics in which signals transmitted from each antenna are canceled with each other when received are orthogonal. All can be applied to.

6 is an explanatory diagram illustrating a frequency error measurement result according to an embodiment of the frequency estimation method. As shown in FIG. 6, the frequency estimation method shows better performance at cell edges with low SNR. That is, by measuring the frequency error using the above method, since the phase of the signal is estimated regardless of the signal strength of the thermal noise N, the same performance can be obtained regardless of the change of the SNR, and thus, the edge of the cell having a low SNR. It can exhibit excellent performance at.

7 is a flowchart illustrating a process of using different frequency error estimation methods according to SNR. Referring to FIG. 6, first, an SNR of a received signal is measured (S61). When the SNR is compared with a predetermined reference value (S72) and larger than the reference value, a method showing better performance when the SNR is high as described above is used. On the other hand, when it is smaller than the reference value, a method showing better performance when the SNR is low as described above is used. The reference value can be, for example, 15 dB.

8 is a diagram illustrating an example of measuring a frequency error using two methods according to SNR. As shown in FIG. 8, it can be seen that better performance is obtained when using different frequency error measurement methods based on the case where the SNR is 15 dB.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

The present invention has the effect of measuring the frequency error with better performance by performing a frequency error measurement in accordance with the SNR.

Claims (6)

In the frequency error measurement method applied to a system for transmitting a signal through two or more antennas using a plurality of orthogonal subcarriers, Receiving 0 and 1 prefix symbols transmitted from each of the two or more antennas; Measuring a Signal to Noise Ratio (SNR) of a signal transmitted through the two or more antennas; Obtaining time synchronization using the 0 prefix symbol to sample the precode symbols; Sampling the predetermined number of prefix symbols if the SNR is equal to or less than a preset reference value; And Calculating an expectation of the sampled precode symbol phase values; The expected value of the pre-symbol symbol phase values is the frequency error measuring method. delete delete delete The method of claim 1, Wherein each first prefix symbol has a different code at the same subcarrier index for the two or more antennas. The method of claim 1, If the SNR of the received signal is equal to or more than the predetermined reference value, the frequency error measuring method characterized in that for measuring the frequency error by using two prefixes having the same value at regular time intervals.

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