KR101128143B1 - Communication system and method for reducing overhead of data used in channel estimation - Google Patents

Abstract

The present invention relates to a communication system and method for reducing the overhead of data used for channel estimation. To this end, when the transmitter modulates and transmits a training sequence including valid data, the present invention demodulates the training sequence received by the receiver, modulates the demodulated training sequence, and inverts the modulated training sequence. The channel response estimate is calculated by multiplying the sequence by the received training sequence. Accordingly, the present invention can calculate the channel response estimate even when using a training sequence including valid data instead of the known data, thereby reducing the overhead incurred by the data used for channel estimation.

Training sequence, valid data, OFDM

Description

TECHNICAL SYSTEM AND METHOD FOR REDUCING OVERHEAD OF DATA USED IN CHANNEL ESTIMATION

The present invention relates to a communication system and method for estimating a channel using a training sequence. Specifically, the training sequence is composed of valid data to reduce overhead caused by the training sequence used for channel estimation. A communication system and method for enabling channel estimation from a training sequence comprising a.

Signals transmitted over a wireless channel are subject to multi-path interference by various obstacles between the transmitter and the receiver. The characteristics of a wireless channel in which multipath exists is defined by the maximum delay spread of the channel and the transmission period of the signal. If the transmission period is longer than the maximum delay spread, no interference occurs between successive signals. However, in the case of high-speed transmission using broadband, the transmission period is shorter than the maximum delay spread, and thus interference (that is, intersymbol interference) occurs between successive signals. In general, an equalizer is required in a single carrier transmission scheme to remove inter-symbol interference. However, as the bandwidth of the communication system increases to support various services, there is a problem that the complexity of the equalizer is increased by increasing the interference between symbols. As an alternative for solving the equalization problem in the wideband single carrier transmission scheme, an Orthogonal Frequency Division Multiplexing (OFDM) system has been proposed.

Typically, the OFDM scheme may be defined as a two-dimensional access scheme in which time division access and frequency division access technologies are combined. In transmitting data by the OFDMA scheme, each OFDMA symbol is divided and transmitted in a plurality of sub-carriers. That is, the OFDM scheme is a multi-carrier modulation scheme of converting serially input data in parallel, modulating each of them into a plurality of sub-carriers having orthogonality. It is a kind.

The OFDM scheme has good spectral efficiency because the sub-carriers overlap each other while maintaining mutual orthogonality, and the sub-carrier uses an Inverse Fast Fourier Transform (IFFT) and a Fast Fourier Transform (FFT). The hardware configuration of the carrier is simple and efficient digital implementation of the modulation / demodulation part is possible.

In such an OFDM system, each sub-carrier has a different signal noise ratio (SNR).

The receiver of the OFDM system determines a digital modulation scheme of each sub-carrier according to the signal-to-noise ratio of the sub-carrier. For example, if the wireless channel environment is good due to the high signal-to-noise ratio, a modulation method with a high data rate such as 256QAM or 512QAM may be used. You can select the modulation scheme with the rate.

Currently, a training sequence is used to estimate the channel by measuring the signal-to-noise ratio of each sub-carrier. The training sequence is information already known at the transmitter and the receiver, and is composed of a plurality of frames. In the plurality of frames constituting the training sequence, the same frame is repeated many times, and a PN (Pseudo Noise) sequence is typically used.

Hereinafter, a structure of a transmitter used in an OFDM system will be described with reference to FIG. 1. 1 illustrates that a PN sequence is used as a training sequence and has a structure corresponding to one sub-carrier.

Prior to initial transmission of the actual data, first, the PN sequence generator 10 sequentially generates the PN sequence. The PN sequence generated by the PN sequence generator 10 is input to the modulator 20. The modulator 20 modulates the PN sequence according to a predetermined modulation scheme.

The PN sequence modulated by the modulator 20 is input to the inverse fast Fourier transform unit 30 and inverse fast Fourier transform. The cyclic prefix adder 40 then adds a cyclic prefix to the inverse fast Fourier transformed PN sequence. The cyclic prefix is inserted between symbols to prevent intersymbol interference, and means a guard interval longer than the maximum delay spread. The PN sequence with the cyclic prefix added is amplified and transmitted through the antenna.

The receiver demodulates the training sequence sent from the transmitter to estimate the channel state of the sub-carrier. Since the training sequence is a known signal, the channel state can be estimated by comparing the demodulated training sequence with the reference training sequence. However, as mentioned above, since the training sequence is simply used to estimate channel conditions, it is an overhead in terms of transmission efficiency. Accordingly, there is a demand for a technology development that can increase transmission efficiency by minimizing overhead caused by data used for channel estimation.

In order to solve this problem, a technique of performing training using sub data used to convey control information such as power control and channel allocation command has been proposed.

A technique for performing training using conventionally proposed sub data will be described. First, since the sub data carries only certain information, it includes one of a finite number of codewords and is known as a codeword set on the receiver side.

The transmitter uses a modulation scheme having a lower data rate than the main data modulation scheme when encoding sub data. In this way, the receiver can decode the sub data without error.

Accordingly, the receiver decodes the received sub data, and even if some symbols of the sub data are not correctly decoded, the receiver can identify the codeword closest to the decoded sub data of the codeword set.

Thereafter, training is performed by comparing the decoded sub data with the identified codeword, and the channel parameter is determined.

However, since the receiving side modulates the sub data using a modulation scheme having a low data rate for error-free decoding, the transmission efficiency is very low. Therefore, the transmission efficiency due to data transmission for training is still not improved.

In addition, since the data used for training can only be selected and used among a set of codewords known to the receiver, the data that can be used is limited.

Accordingly, an object of the present invention is to provide a communication system and method capable of increasing transmission efficiency by reducing overhead caused by data used for channel estimation.

In addition, the present invention is not limited to the data known to the receiver when using the data for use in channel estimation, and a communication system and method for widening the data use restriction range by allowing a part of the data to be transmitted from the transmitter to the receiver The purpose is to provide.

According to an aspect of the present invention, there is provided a communication system for reducing overhead of data used for channel estimation, comprising: a transmitter for modulating and transmitting a training sequence including valid data; And demodulate the demodulated training sequence, invert the modulated training sequence, and multiply the inverted training sequence by the received training sequence to calculate a channel response estimate.

The present invention also provides a transmitter for reducing the overhead of data used for channel estimation, comprising: a modulator for modulating a training sequence including valid data, and an inverse fast Fourier transform of a training sequence modulated by the modulator; And a fast Fourier transform unit, a cyclic prefix adder for adding a cyclic prefix to the inverse fast Fourier transformed training sequence, and an antenna for transmitting a training sequence to which the cyclic prefix is added.

The present invention also provides a receiver for reducing overhead of data used for channel estimation, comprising: an antenna for receiving a training sequence including valid data and comprising a plurality of frames, a demodulator for demodulating the training sequence; A modulator for modulating the demodulated training sequence, an inverter for inverting the modulated training sequence, a multiplied by the inverted training sequence and a training sequence received through the antenna, and outputting a radio channel environment for each frame And a multiplier and a channel estimator for calculating a channel response estimate for the sub-carrier based on the radio channel environment for each frame output through the multiplier.

The present invention also provides a method for reducing overhead of data used for channel estimation in a communication system including a transmitter and a receiver, the transmitter modulating and transmitting a training sequence including valid data; Receiving and demodulating the training sequence, inverting the modulated training sequence after the receiver modulates the demodulated training sequence, and receiving, by the receiver, the inverted training sequence and the received training sequence. And multiplying the channel response estimate.

The present invention also provides a method for reducing the overhead of data used for channel estimation in a transmitter of a communication system, the method comprising: modulating a training sequence including valid data, and performing an inverse fast Fourier transform on the modulated training sequence. And adding a cyclic prefix to the inverse fast Fourier transformed training sequence, and transmitting a training sequence to which the cyclic prefix is added.

In addition, the present invention provides a method for reducing the overhead of data used for channel estimation in a receiver of a communication system, comprising: receiving a training sequence including valid data and comprising a plurality of frames, and demodulating the training sequence And a process of modulating the demodulated training sequence, inverting the modulated training sequence, and multiplying the inverted training sequence and the received training sequence to calculate a radio channel environment for each frame. And calculating a channel response estimate for the sub-carrier based on the calculated wireless channel environment for each frame.

According to the present invention, since the channel estimation can be performed while using valid data instead of using the known meaningless data as the training sequence to estimate only the channel state, the overhead of data generated by using the meaningless data as the training sequence is reduced. It can be effective.

In addition, according to the present invention, since there is no need to modulate the data used for channel estimation as a modulation method having a low data rate as in the prior art, it is possible to increase the transmission efficiency according to data transmission for training compared to the conventional method. have.

In addition, according to the present invention, data for use in channel estimation is not limited to data known to the receiver as in the prior art, and there is an advantage in that the data use range is not limited because valid data does not matter what data is used.

In addition, according to the present invention, by using the medium access control information as valid data included in the training sequence, there is an effect that the data access path can be quickly set by demodulating the physical address for data transmission in advance.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Specific details described below are provided to aid the overall understanding of the present invention. In describing the present invention, detailed descriptions of related known functions or configurations will be omitted when it is determined that the gist of the present invention may be unnecessarily obscured.

The present invention proposes a method for channel estimation using a training sequence including valid data, which is actually required data, without using a separate training sequence for channel estimation like a PN sequence.

2 shows a structure of a transmitter according to an embodiment of the present invention.

Referring to FIG. 2, the transmitter includes a serial / parallel converter 110, a modulator 120, an inverse fast Fourier transform 130, a parallel / serial converter 140, a cyclic prefix adder 150, and the like. Include.

When the actual data to be transmitted is input in the form of a bit stream, the serial / parallel converter 110 converts the serial bit streams in parallel and transmits them to the modulator 120. Here, when the number of sub-carriers is N, X _N is X ₁ X ₂ ... … Is converted to X _n .

The modulator 120 modulates the received parallel data by the modulation scheme set for each sub-carrier. The modulation scheme of each sub-carrier is set based on the channel response estimate described later. Here, the channel response estimate is a value calculated for each sub-carrier at the receiver, and the radio channel environment H is reflected in the value. The channel response estimate calculation operation of each sub-carrier will be described in the following receiver configuration. In addition, the receiver pre-sets a modulation scheme according to the channel response estimate in order to efficiently transmit data errors without causing errors in the wireless channel environment. Accordingly, the modulation scheme of each sub-carrier is determined based on the channel response estimate of each sub-carrier calculated at the receiver, and the modulator 120 transmits the transmission data using the modulation scheme of each sub-carrier determined as described above. The inverse fast Fourier transform unit 130 performs inverse fast Fourier transform on the modulated data. Since the inverse fast Fourier transform unit 130 is implemented in software in a digital signal processor, hardware configurations such as a band pass filter (BPF), a mixer, and a frequency generator (oscillator) required for each sub-carrier are necessary. You do not need to use it.

The parallel / serial converter 140 converts the inverse fast Fourier transform data input in parallel to the cyclic prefix adder 150. The cyclic prefix adder 150 inserts a cyclic prefix between the serially input data. The data with the cyclic prefix inserted is amplified and transmitted to the receiver through the transmit antenna.

In the transmitter according to the embodiment of the present invention, the modulator 120 receives the media access control information from the media access control layer (MAC layer) 100 before transmitting the actual data. The medium access control information is transmitted in a header of the conventional data, but in the embodiment of the present invention, the medium access control information is used as a training sequence.

In the embodiment of the present invention, although the medium access control information is used as the training sequence, other valid data may be used in addition to the medium access control information.

When the medium access control information transmitted from the medium access control layer 100 is input to the modulator 120, the medium access control information is modulated by the modulation scheme set for each sub-carrier, and then the inverse fast Fourier transform process and the parallel / serial conversion as described above. Process, cyclic prefix addition process, etc., to the receiver.

3 shows a structure of a receiver according to an embodiment of the present invention.

Note that the receiver shown in FIG. 3 has a structure corresponding to one sub-carrier. In addition, in the receiver of FIG. 3, a portion for removing a cyclic prefix and a portion for performing fast Fourier transform are omitted for convenience of description.

Referring to FIG. 3, the training sequence transmitted from the transmitter is received through the receiving antenna and then input to the synchronizer 200. The training sequence received at the receiver is a training sequence including medium access control information as described above with reference to FIG. 2. Here, the training sequence received through the reception antenna is a signal reflecting the radio channel environment (H).

4 is a diagram illustrating a distribution of a signal in which the radio channel environment H is reflected.

4 illustrates a signal distribution in the QPSK scheme, and a signal corresponding to points distributed around −1, j, −j, and 1 is a signal in which the radio channel environment H is reflected. In other words, if the received signal has no noise, it corresponds to any one of -1, j, -j, 1, but if there is noise, it corresponds to a point around the non--1, j, -j, 1 do.

When the training sequence is input through the receiving antenna, the synchronization unit 200 adjusts the synchronization using the synchronization header of the training sequence and outputs the synchronization to the demodulator 210.

Since the training sequence is composed of a plurality of frames, the receiver processes signals in units of frames, but for the convenience of description, the following description will describe one training sequence.

The demodulator 210 demodulates the training sequence and then outputs the demodulated training sequence to the modulator 220. In addition, the demodulator 210 outputs the demodulated training sequence, that is, the medium access control information to the medium access control layer 280. In the embodiment of the present invention, since the valid data included in the training sequence is assumed to be described as the medium access control information, the demodulator 210 outputs the demodulated training sequence to the medium access control layer 280. If you use, the valid data will be printed out where it should be used.

The modulator 220 receiving the demodulated training sequence modulates the training sequence again and outputs the modulated training sequence to the inverting unit 230. At this time, the modulator 220 modulates in the same manner as the modulation method of the received training sequence. The inverter 230 inverts the modulated training sequence and outputs the inverted training sequence to the multiplier 240.

The multiplier 240 multiplies the inverted training sequence by the training sequence received through the receiving antenna and outputs a radio channel environment H for one frame.

The radio channel environment (H) for each frame is sequentially input to the channel estimator 250 through the multiplier 240, and the channel estimator 250 is based on the radio channel environment (H) for each frame. Compute a channel response estimation result for the sub-carrier of.

The channel estimator 250 may calculate a channel response estimate for one sub-carrier through Equation 1 below.

Where N is the number of frames included in the training sequence and K is the number of sub-carriers. Thus, H (k) is the channel response estimate for the kth sub-carrier and Hnk means the radio channel environment of the nth frame of the training sequence on the kth sub-carrier. For example, when the number of frames of the training sequence is four, the channel response estimate H (5) for the fifth sub-carrier is (H ₁₅ + H ₂₅ + H ₃₅ + H ₄₅ ) / 4.

The signal-to-noise ratio estimator 260 calculates the signal-to-noise ratio of one sub-carrier based on the channel response estimate calculated by the channel estimator 250.

The calculated signal-to-noise ratio of the sub-carrier is used to determine the modulation scheme of the transmitter. To this end, the receiver pre-stores the modulation scheme of the transmitter in a table form based on the signal-to-noise ratio of the sub-carrier. Such a table may be illustrated as shown in FIG. 6. Here, the table of FIG. 6 is a result derived through simulation, which is merely an example and is not limited thereto.

Thereafter, the receiver checks the table of FIG. 6 to determine a modulation scheme of the transmitter corresponding to the calculated signal-to-noise ratio of the sub-carrier. Referring to FIG. 6, when the calculated signal-to-noise ratio of the sub-carrier is 24 dB, the modulation scheme for the corresponding sub-carrier will be determined by the 32QAM scheme.

Referring back to FIG. 3, the time / frequency offset estimator 270 is applied to the signal-to-noise ratio for each sub-carrier output from the signal-to-noise ratio estimator 260 and to each sub-carrier output from the channel estimator 250. A time / frequency offset value is computed based on the channel response estimate for. Since the time / frequency offset estimator 270 can be calculated using a general time / frequency offset estimation algorithm, a detailed description thereof will be omitted.

In the transmitter and the receiver configured as described above, the state of the training sequence according to the performing operation for channel estimation will be described in detail with reference to FIGS. 2 and 3 along with the state diagram of FIG. 5.

The training sequence consists of a plurality of frames, each frame consisting of a plurality of source bits. 5 is a case where each frame is composed of 2 bits of source bits and QPSK modulation is used. In the state diagram of FIG. 5, for convenience of description, two bits of source bits are used, but of course, n bits of source bits may be used.

Referring to FIG. 5, if the source bit “01” corresponding to the frame of the training sequence is transmitted (col. 1), “01” is modulated to “j” through the modulator 120 (col. 2). The source bits modulated by the modulator 120 are transmitted through a transmission antenna via an inverse fast Fourier transform unit 130, a parallel / serial converter 140, and a cyclic prefix adder 150. The signal transmitted through the transmitting antenna is received through the receiving antenna of the receiver is distorted to "jⓧH" by the influence of the wireless channel environment (H) while passing through the wireless channel (col. 3).

“JⓧH” received through the receiving antenna is demodulated to “01” in the demodulator 210 (col. 4). Here, it is assumed that there is no error during demodulation. The demodulated “01” in the demodulator 210 is modulated to “j” through the modulator 220 (col. 5). Thereafter, “j” is inverted to “−j” through the inversion unit 230 (col. 6).

In this case, the receiver post-modulation signals (four signals “1”, “j”, “-1”, “-j”) have a 90 ° phase difference with each other, and may be expressed as the sum of the real part and the imaginary part. Since the present invention has been described as an example of 2 bits of source bits, a 90 ° phase difference occurs for each signal. However, if an n bit source bit is used, a phase difference of 360 ° / n will occur.

In addition, the signal after each modulation may be expressed as Equation 2 below.

cosθ + jsinθ

If the θ value is assumed to be 0 °, the signal with θ = 0 ° is “1” according to the phase difference of 90 °, respectively. For example, a signal having θ = 270 ° may be represented by “-j”.

At this time, if θ is inverted and calculated as -θ, the signal with -θ = 0 ° is "-1", the signal with -θ = -90 ° is "-j", and the signal with -θ = -180 ° With "-1", a signal with -θ = -270 ° is represented by "-j".

That is, the inversion in the present invention means the inversion of the phase, so if the signals "1", "j", "-1", "-j" is inverted, "1", "-j", "-1" , "J".

The multiplier 240 multiplies "j_H" received through the receiving antenna and "-j" inverted by the inverting unit 230 to calculate a radio channel environment H (col. 7). Here, the radio channel environment (H) means a radio channel environment for one frame included in the training sequence. The channel response estimate for one sub-carrier is calculated taking into account the radio channel environment for all frames included in the training sequence. That is, the channel response estimate for the sub-carrier may be calculated by Equation 1 above.

Although embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may implement the present invention in other specific forms without changing the technical spirit or essential features thereof. I can understand that.

Therefore, since the embodiments described above are provided to completely inform the scope of the invention to those skilled in the art, it should be understood that they are exemplary in all respects and not limited. The invention is only defined by the claims.

In a conventional communication system, the training sequence increases overhead by using data used only for channel estimation, such as a PN sequence. However, the present invention has an advantage of reducing overhead by using valid data as a training sequence. In addition, the present invention can be widely applied to a communication system that needs to improve transmission efficiency by reducing data overhead.

1 is an internal configuration diagram of a receiver of a typical OFDM system.

2 is an internal configuration diagram of a transmitter according to an embodiment of the present invention.

3 is an internal configuration diagram of a receiver according to an embodiment of the present invention.

4 is a diagram showing data received at a receiver in accordance with an embodiment of the invention.

5 is a state diagram illustrating a state change of a training sequence according to an performing operation for channel estimation in a transmitter and a receiver according to an embodiment of the present invention.

FIG. 6 illustrates a table in which a modulation scheme of a transmitter is mapped based on a signal-to-noise ratio of a sub-carrier according to an embodiment of the present invention. FIG.

<Explanation of symbols for the main parts of the drawings>

100, 280: media access control layer 110: serial / parallel conversion unit

120: modulator 130: inverse fast Fourier transform unit

140: parallel / serial conversion unit 150: cyclic prefix addition unit

200: synchronization unit 210: demodulation unit

220: modulator 230: inverter

240: multiplier 250: channel estimator

260: signal to noise ratio estimation unit 270: time / frequency offset estimation unit

Claims (14)

delete delete A transmitter for reducing the overhead of data used for channel estimation, A modulator for modulating a training sequence including valid data; An inverse fast Fourier transform unit for inverse fast Fourier transform of the training sequence modulated by the modulator; A cyclic prefix adder that adds a cyclic prefix to the inverse fast Fourier transform training sequence; And an antenna for transmitting a training sequence to which the cyclic prefix is added. The method of claim 3, And said valid data is medium access control information. A receiver for reducing the overhead of data used for channel estimation, An antenna for receiving a training sequence comprising valid data and comprising a plurality of frames; A demodulator for demodulating the training sequence; A modulator for modulating the demodulated training sequence; An inversion unit for inverting the modulated training sequence; A multiplier for multiplying the inverted training sequence with the training sequence received through the antenna and outputting a radio channel environment for each frame; A channel estimator configured to calculate a channel response estimate for the sub-carrier based on the radio channel environment for each frame output through the multiplier; The valid data is medium access control information, and the demodulator outputs medium access control information obtained by demodulating the training sequence to a medium access control layer. delete The method of claim 5, And a signal-to-noise ratio estimator for calculating a signal-to-noise ratio of a sub-carrier using the channel response estimate calculated through the channel estimator. A method for reducing the overhead of data used for channel estimation in a communication system comprising a transmitter and a receiver, Transmitting, by the transmitter, modulating a training sequence including valid data; Receiving and demodulating the training sequence by the receiver; Inverting the modulated training sequence after the receiver modulates the demodulated training sequence; And the receiver multiplying the inverted training sequence by the received training sequence to calculate a channel response estimate. The method of claim 8, And the valid data is medium access control information. A method for reducing overhead of data used for channel estimation in a transmitter of a communication system, Modulating a training sequence including valid data; Inverse fast Fourier transforming the modulated training sequence; Adding a cyclic prefix to the inverse fast Fourier transform training sequence, Transmitting a training sequence appended with the cyclic prefix. 11. The method of claim 10, And the valid data is medium access control information. A method for reducing the overhead of data used for channel estimation at a receiver in a communication system, Receiving a training sequence comprising valid data and comprising a plurality of frames, Demodulating the training sequence; Modulating the demodulated training sequence; Inverting the modulated training sequence; Calculating a radio channel environment for each frame by multiplying the inverted training sequence and the received training sequence; Calculating a channel response estimate for the sub-carrier based on the calculated wireless channel environment for each frame. 13. The method of claim 12, And the valid data is medium access control information, and further comprising outputting the medium access control information obtained by demodulating the training sequence to a medium access control layer. 13. The method of claim 12, And calculating a signal-to-noise ratio using the calculated channel response estimate for the sub-carrier.

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