KR101403109B1 - Method for compensating channel error and method for determining modulation and coding scheme - Google Patents

Abstract

The channel error compensation method obtains channel information experienced by a received symbol and predicts an error rate of the symbol using a channel correlation coefficient considering the channel information and the mobility of the user. Even when there is the mobility of the user, the channel error can be compensated for and the frequency efficiency can be increased.

Channel error, AMC, MCS, channel correlation coefficient, OFDM

Description

[0001] The present invention relates to a channel error compensation method and a modulation and coding method determining method,

1 is an exemplary diagram showing a mobile communication system.

2 is a block diagram illustrating a transmitter according to one embodiment of the present invention.

3 is a block diagram illustrating a receiver in accordance with an embodiment of the present invention.

4 is a graph showing the transmission rate versus SNR according to the simulation result.

5 is a graph showing the FER versus SNR according to the simulation result.

DESCRIPTION OF REFERENCE NUMERALS OF THE MAIN PARTS OF THE DRAWINGS

110: channel encoder

120: interleaver

130: mapper

150: AMC controller

The present invention relates to a channel error compensation method and a modulation and coding scheme determination method, and more particularly, to a channel error compensation method and a modulation and coding scheme determination method for compensating a user's mobility.

Currently, various transmission and reception schemes are emerging in a wireless communication system aiming at high-quality, high-capacity data transmission using limited frequency resources. In addition, there is an increasing demand for an effective countermeasure against the fading phenomenon occurring in a wireless channel for such high-speed multimedia data transmission.

In recent years, there has been proposed a MIMO (Multiple Input Multiple Output) technique using a multi-transmission / reception antenna to be applied to a next generation mobile communication system for high-speed multimedia data transmission and an OFDM (Orthogonal Frequency Division Multiplexing) Research is proceeding in various ways.

In order to efficiently transmit high-speed data in a wireless channel, frequency selective fading caused by intersymbol interference occurring in high-speed transmission or multi-path interference of a wireless channel must be overcome. The OFDM scheme can effectively remove the frequency-selective nature of the channel. In addition, it is possible to increase the spectral efficiency by using multi-carriers having mutual orthogonality and to perform the modulation and demodulation process at the transmitting / receiving end by using IFFT (Inverse Fast Fourier Transform) and FFT (Fast Fourier Transform) .

In addition, a closed-loop system structure providing a return channel from a receiving end to a transmitting end has been attracting attention as means for further increasing the performance of the system. When channel state information (CSI) is returned to the transmitter, the transmitter can maximize performance by adjusting various system parameters using this information. Adaptive Modulation and Coding (AMC) is a technique for increasing link performance by adjusting transmission power level, modulation level, and / or code rate at the transmitter using current CSI. If the channel condition is good, the data transmission rate is increased and if there is deterioration of the channel, the transmission rate is lowered, so that efficient transmission can be supported. As a result, the average transmission rate can be increased.

The AMC scheme optimizes the amount of data transmitted every moment based on accurate channel information. If the channel information used by the transmitter and receiver is not accurate, the AMC scheme experiences severe performance degradation. If the channel error is very large, the performance of the AMC scheme is lower than that of the open- Fall off.

In the wireless mobile communication system, the channel environment changes flexibly due to the mobility of the user. Particularly, the next generation communication system has a great interest in whether it can provide a high-capacity and high-quality service even in an environment where the user is very mobile . It is required to minimize the performance deterioration of the AMC scheme in a time-varying channel environment in which the user is mobile.

SUMMARY OF THE INVENTION The present invention provides a channel error compensation method for compensating for a channel error caused by a user's mobility.

According to another aspect of the present invention, there is provided a method of determining a modulation and coding scheme considering mobility of a user.

According to an aspect of the present invention, a channel error compensation method is provided. The channel error compensation method estimates an error rate of the symbol using a channel correlation coefficient considering the channel information and the mobility of the user. Even when there is the mobility of the user, the channel error can be compensated for and the frequency efficiency can be increased.

According to another aspect of the present invention, there is provided a method of determining a modulation and coding scheme. The modulation and coding scheme determination method estimates a symbol error rate for each given Modulation and Coding Scheme (MCS) using a channel correlation coefficient that considers the mobility of a user, and calculates an MCS having a maximum frequency efficiency for the error rate Select. It is possible to prevent performance deterioration of the AMC scheme by determining the MCS in consideration of the mobility of the user.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals designate like elements throughout the specification.

1 is an exemplary diagram showing a mobile communication system.

1, a mobile communication system includes a base station (BS) 10 and a plurality of UEs 20. The mobile communication system is widely deployed to provide various communication services such as voice, packet data, and the like.

The base station 10 generally refers to a fixed station that communicates with the terminal 20 and may also be referred to as another term such as a node-B, a base transceiver system (BTS), an access point, terminology.

The terminal 20 may be fixed or mobile and may be referred to by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device,

Hereinafter, downlink refers to communication from the base station 10 to the terminal 20, and uplink refers to communication from the terminal 20 to the base station 10. In the downlink, the transmitter may be part of the base station 10, and the receiver may be part of the terminal 20. Conversely, in the uplink, the transmitter may be part of the terminal 20, and the receiver may be part of the base station 10. The base station 10 may include a plurality of receivers and a plurality of transmitters, and the terminal 20 may include a plurality of receivers and a plurality of transmitters.

2 is a block diagram illustrating a transmitter according to one embodiment of the present invention.

Referring to FIG. 2, a transmitter 100 includes a channel encoder 110, an interleaver 120, a mapper 130, an OFDM modulator 140, and an AMC controller 150.

The channel encoder 110 encodes the input information bits according to a coding scheme determined by the AMC controller 150 to form coded data. The channel encoder 110 may add error detection bits, such as cyclic redundancy check (CRC), to the information bits and add extra code for error correction. The error correction code may be, for example, a convolutional code or a turbo code. The interleaver 120 mixes the encoded data to reduce noise effects from the channel.

The mapper 130 modulates the interleaved data according to a modulation scheme determined by the AMC controller 150, and provides modulation symbols. That is, the coded data is mapped by the mapper 130 to modulation symbols representing positions according to amplitude and phase constellation. The modulation scheme may be m-quadrature phase shift keying (m-PSK) or m-quadrature amplitude modulation (m-QAM). For example, m-PSK may include BPSK or 8-PSK as well as QPSK. m-QAM may include 16-QAM or 64-QAM as well as 256-QAM.

The OFDM modulator 140 converts the input symbols into OFDM symbols. The OFDM modulator 140 may perform Inverse Fast Fourier Transform (IFFT) on the input symbols to convert the time-domain samples. A cyclic prefix (CP) may be added to the time domain samples. The OFDM symbol output from the OFDM modulator 140 is converted into an analog signal and transmitted through the antenna 160. [

The AMC controller 150 determines a modulation scheme and a coding scheme using feedback information received from the receiver 200 (FIG. 3), and transmits the determined modulation scheme and coding scheme to the channel encoder 110 and the mapper 130. In one embodiment, the feedback information may include an MCS index. At this time, the AMC controller 150 can determine the MCS corresponding to the MCS index through the MCS table stored in the memory (not shown). The MCS table is a look-up table in which modulation and coding schemes are determined according to the MCS index. In another embodiment, the feedback information may be estimated channel information. At this time, the AMC controller 150 can determine the MCS using the channel information according to an MCS determination method, which will be described later.

3 is a block diagram illustrating a receiver in accordance with an embodiment of the present invention.

3, the receiver 200 includes a channel estimator 220, an OFDM demodulator 230, a demapper 240, a deinterleaver 250, a channel decoder 260, and an AMC controller 270.

The signal received from the receive antenna 210 is digitized and the channel estimator 220 estimates channel information from the received signal. The signal is then transformed into frequency domain symbols by an OFDM demodulator 230. The OFDM demodulator 230 may remove the CP from the input signal and perform Fast Fourier Transform (FFT). The de-mapper 240, the de-interleaver 250 and the channel decoder 260 decode the signal processing techniques performed by the channel encoder 110, the interleaver 120 and the mapper 13 of the transmitter 100, Perform the inverse process. That is, the demapper 240 demaps the signals output from the OFDM demodulator 120 and outputs a bit-by-bit LLR (Log Likelihood Ratio) signal. The deinterleaver 250 performs deinterleaving on the signal output to the demapper 240. The channel decoder 260 decodes the signal output from the deinterleaver 250 and outputs original data.

The AMC controller 270 determines the MCS level in consideration of the channel information estimated by the channel estimator 220 and the mobility of the user. And sends the index of the MCS level as the feedback information to the transmitter 100 through the feedback channel.

The MCS determination method will be described below.

It is assumed that the receiver 200 knows channel information correctly. We also consider a baseband signal model based on a zero mean complex value and a discrete time frequency selective fading OFDM channel model.

To determine the MCS level, the AMC controller 270 of the receiver 200 first performs an error rate prediction on an OFDM symbol basis. Hereinafter, the error rate is referred to as a bit error rate in order to clarify the explanation. However, the technical idea of the present invention can be applied to a frame error rate or a block error rate as it is. After determining the error rate of a symbol for each given MCS level, a maximum spectral efficiency satisfying a given quality of service (QoS) condition is determined, and the corresponding MCS index is transmitted to the transmitter 100 do.

In order to predict an accurate error rate in the AMC controller 270, a PEP (Pairwise Error Probability) analysis of the system should be performed. To reduce the computational complexity of PEP P (d, H ), first let all subchannels use a fixed modulation level with a magnitude of M = 2 ^m (m is an integer).

Assuming gray mapping, the bound of PEP P (d, H ) can be simplified.

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는 다음 수학식 5와 같이 나타낼 수 있다. Using gray mapping, the symbol transition probability

Can be expressed by the following equation (5).

Where Q _{1, n} and Q _{2, n} are the transition probabilities between neighboring symbols whose first and second bits have different values, respectively. Q _{1, n} and Q _{2, n} can be obtained by the following equations (7) and (8).

Here, σ _s is a signal variance, and σ _n is a noise variance. In a mobile communication system, the channel environment can be changed flexibly due to the mobility of a user. In the case of a user having a high mobility, there is a time delay between the feedback channel and the data transmission channel, so that the performance of the AMC scheme may deteriorate due to an error between the two channels. It is difficult to compensate the channel error due to the time delay with Q _{1, n} and Q _{2, n according} to Equations (7) and (8) in which the mobility of the user is not taken into consideration.

Hereinafter, a method for compensating for a channel error due to mobility of a user will be described. Q _{1, n} and Q _{2, n} are obtained considering the mobility of the user.

The following channel model can be used in a mobility-enabled channel environment.

는 송신기(100)에서 귀환받는 채널,

은 수신기(100)에서 송신하는 시점에서의 채널,

은 귀환되는 동안의 시간 지연으로 인한

와

사이의 채널 에러이다. 수학식 9는 시변 채널 환경에서 피드백 채널과 데이터 송신 채널 사이의 시간 지연으로 인하여 발생하는 에러를 고려하여 데이터를 전송하는 시점에서의 OFDM 채널을 모델링한 것이다. here,

A channel to be fed back from the transmitter 100,

A channel at the time of transmission in the receiver 100,

Due to time delay during the return

Wow

Lt; / RTI > Equation (9) is a model of an OFDM channel at the time of data transmission considering an error caused by a time delay between a feedback channel and a data transmission channel in a time-varying channel environment.

의 상호분산(covariance) 행렬을 갖는 가우시안 벡터로 모델링될 수 있다. σ_h ²는 채널의 평균 에너지이다. The channel error Ξ

Lt; RTI ID = 0.0 > covariance < / RTI > σ _h ² is the average energy of the channel.

와 같이 구할 수 있다. ρ는 사용자의 이동성을 나타내며, 그 값은 사용자의 이동성이 커질수록 작아지고, ρ=1 은 완벽한 채널 정보를 나타낸다. ρ is a correlation coefficient between channels and can be obtained according to the well-known Jake's channel model. The channel correlation coefficient ρ is a parameter obtained by using the Bessel function in consideration of the Doppler frequency (f _d ) and the time delay (τ _d ) in a time varying channel environment

As shown in Fig. ρ denotes the mobility of the user, and the value decreases as the mobility of the user increases, and ρ = 1 indicates complete channel information.

와 Ξ만큼의 차이를 갖는 에러 채널

이다.According to the model of Equation (9), the channel predicted by the receiver (200)

And an error channel having a difference of?

to be.

If the channel is predicted to be larger than the actual value, the MCS corresponding to a value larger than the frequency efficiency that can be supported by the actual channel capacity is selected. This causes a loss of the frame error rate and consequently a large performance deterioration Bring it. Conversely, if the channel is predicted to be less than the actual value, the MCS is chosen to be lower than the frequency efficiency that the actual channel capacity can support, and thus likewise suffers from performance degradation of the transmission rate.

In order to compensate for the error due to the channel error, Q _{1 , n} and Q _{2 , n} are obtained as shown in the following equations (10) and (11).

의 성분들로 구성된다. 분모에는 채널 상관 계수와 전송 파워 그리고 잡음 파워의 함수로 설정하여 사용자의 이동성을 보상한다. The channel coefficients of the molecules at Q _{1 , n} and Q _{2 , n} are calculated according to the channel model of equation

. ≪ / RTI > The denominator is set as a function of channel correlation coefficient, transmission power and noise power to compensate user mobility.

In the present invention, the error rate is predicted through the channel correlation coefficient considering not only the estimated channel information but also the mobility of the user. Thus, when determining the MCS, the channel error due to the mobility of the user can be compensated.

Also, one of the methods to compensate the effect of channel error in the AMC scheme is to reduce the received SNR by a certain amount when performing the algorithm for selecting the MCS. In this method, the capacity of the channel is reduced from the actual predicted value, thereby performing a conservative AMC scheme. When? _sn =? _s /? _n , Equations (10) and (11) can be expressed by the following Equations (12) and (13), respectively.

Since σ _s is the signal variance and σ _n is the noise variance, σ _sn is equivalent to the received SNR. That is, when the error rate is compensated, a certain performance can be obtained by considering the channel correlation coefficient and SNR together.

Meanwhile, the purpose of AMC scheme is to maximize the system throughput while maintaining a certain level of quality of service while adjusting the data rate according to channel conditions. In the present invention, an error rate at each moment is calculated for each given MCS, and an MCS having the highest frequency efficiency is selected within a range satisfying a given QoS.

As described above, according to the present invention, it is possible to guarantee a large amount of performance in the conventional case in which there is no mobility irrespective of the channel correlation coefficient and the received SNR value in a channel environment in which the user is mobile.

The simulations are computed by Monte-Carlo simulations, and the MCS tables used are shown in Table 1 below, using the Rate Compatible Punctured Convolutional (RCPC) codes in Table 2.

l One 2 3 4 5 6 R _T One 2 3 4 4.5 5 R _C 1/2 1/2 3/4 2/3 3/4 5/6 Modulation QPSK 16-QAM 16-QAM 64-QAM 64-QAM 64-QAM

v R _C d _H p Nv (d), d = d H, ..., d H +5 One 1/2 10 3 108, 0, 633, 0, 4212, 0 2 2/3 6 2 3, 70, 285, 1276, 6160, 27128 3 3/4 5 3 42, 201, 1492, 10469, 62935, 379546 4 5/6 4 5 92, 528, 8694, 79453, 791795, 7369828

Each channel has an independent Rayleigh fading channel and assumes an indoor channel model with a 5-tap power delay profile with exponentially decreasing fading characteristics. This channel has a root mean square (RMS) delay spread of approximately 100 ns. When AMC is applied, the quality of service required by the system is assumed to be 0.1%.

4 is a graph showing the transmission rate versus SNR according to the simulation result.

Referring to FIG. 4, in the environment where the channel correlation coefficient corresponds to 0.5 (p = 0.5), the prior art system experiences transmission rate degradation of 50% or more. However, according to the present invention, it is confirmed that the performance is compensated up to about 70% -80% of the total transmission rate. Also, it maintains constant performance at all SNRs.

5 is a graph showing the FER (Frame Error Rate) versus SNR according to the simulation result.

Referring to FIG. 5, there is severe performance degradation in the prior art. FER increases by 40% to 50%, which is a performance that is difficult to support even voice communication systems as well as data communication. However, it can be seen that a substantial amount of FER is restored by applying the channel error compensation scheme according to the present invention.

Although a single-input single-output (SISO) having one transmit antenna and one receive antenna has been described above, the technical idea of the present invention is to provide a multiple-input multiple-output (SIN) MIMO) systems.

The present invention may be implemented in hardware, software, or a combination thereof. (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microprocessor, and the like, which are designed to perform the above- , Other electronic units, or a combination thereof. In software implementation, it may be implemented as a module that performs the above-described functions. The software may be stored in a memory unit and executed by a processor. The memory unit or processor may employ various means well known to those skilled in the art.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention. You will understand. Therefore, it is intended that the present invention covers all embodiments falling within the scope of the following claims, rather than being limited to the above-described embodiments.

As described above, according to the present invention, it is possible to compensate for a channel error even when there is a user's mobility, thereby increasing the frequency efficiency. Therefore, performance deterioration of the AMC scheme can be prevented.

Claims (10)

A method for determining a modulation and coding scheme (MCS) Estimating an error rate of a symbol for a plurality of MCS levels based on a channel model at a receiver; And Determining at the receiver one MCS level based on an error rate of the symbol, The channel model is expressed by the following equation, ≪ Equation &

remind

Represents a channel to be fed back from the transmitter, remind

Represents a data transmission channel at the time of transmission by the receiver, remind

Due to time delay

Wow

Lt; / RTI > The channel error

Is modeled as a Gaussian vector having a covariance matrix, The MCS level is determined by compensating an error due to the channel error based on the following two variables,

remind

A channel correlation coefficient,

A signal variance,

Is a noise variance, M is a modulation level, The channel correlation coefficient

Is a variable calculated by using a Bessel function considering a Doppler frequency (fd) and a time delay (? D) in a time varying channel environment. delete delete delete delete delete delete delete delete delete

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