KR100584090B1 - A method for pre-processing for an uplink of OFDM communication system - Google Patents

Abstract

The present invention relates to a preprocessing method for uplink in an orthogonal frequency division multiplexing communication system.

In the present invention, in an orthogonal frequency division multiplexing communication system in which one AP estimates channels for a plurality of wireless terminals, a modulation scheme determiner for a subchannel is placed in front of an inverse fast Fourier transformer (IFFT), and each modulation is different according to channel gain. Use the method. In detail, a channel for a frame periodically transmitted from the AP is estimated, and the wireless terminal performs adaptive modulation based on the channel estimate. After the modulation, the data is preprocessed through a preequalizer and transmitted. Thereafter, the AP finds a modulation scheme for each subchannel from the information on the determined modulation scheme and uses the demodulation signal.

According to the present invention, since the pilot and the preamble for channel estimation are not required in the uplink and the channel can be estimated with the downlink pilot and preamble broadcasted by the AP, the frequency efficiency can be increased.

Wireless communication system, OFDM, subchannel, adaptive modulation, pre equalizer

Description

A method for pre-processing for an uplink of OFDM communication system

1 is a block diagram illustrating a configuration of a system for adaptive modulation after preprocessing in an adaptive modulated OFDM TDD system according to an embodiment of the present invention.

2 is a diagram illustrating a simple model of an OFDM system in a frequency domain according to an embodiment of the present invention.

3 is a diagram illustrating a structure of a long preamble for uplink and downlink channels according to an embodiment of the present invention.

4 is an exemplary diagram of a synchronization scenario of an uplink channel in a preprocessing OFDM TDD system according to an embodiment of the present invention.

5A and 5B are diagrams illustrating performance evaluation of pre / post equalizers and fixed / adaptive modulation for an ideal channel response in a preprocessing OFDM TDD system according to an exemplary embodiment of the present invention.

6A and 6B are performance evaluation diagrams of pre / post equalizers and fixed / adaptive modulation for estimated channel responses in a preprocessing OFDM TDD system according to an embodiment of the present invention.

7 is a diagram comparing the performance of the case where a fixed / adaptive modulation order is used only for a selected subchannel in a preprocessing OFDM TDD system according to an embodiment of the present invention.

8 is a performance evaluation diagram of pre / post / partial equalizer and fixed / adaptation modulation for an ideal channel response in a preprocessing OFDM TDD system according to an embodiment of the present invention.

The present invention relates to a preprocessing method for uplink in an orthogonal frequency division multiplexing (OFDM) communication system. More specifically, the present invention relates to a single access point for channel estimation in a receiver of a wireless communication system. The present invention relates to a preprocessing method for an uplink of an orthogonal frequency division multiplexing communication system capable of preventing the complexity of estimating channels for a plurality of wireless terminals (WTs) in an AP).

As a prior art, a paper entitled "Pre-equalization for the Up-link of TDD OFDM System" is published by K. Witrisal et al. In order to estimate a channel in the uplink of the present invention, a method of pre-equalization, which is a method of precompensating channel gains for each sub-carrier in the frequency domain before the IFFT of the transmitter, is disclosed.

More specifically, according to the above paper, when TDD OFDM is applied to a WLAN environment, it can be seen that the channel state hardly changes with time, and it is also assumed that the channels have reciprocal reciprocal. It is possible to reduce the complexity of the AP by estimating only the channel between itself and the AP at each WT rather than estimating the channel with all the wireless terminals (WTs) at the access point. In addition, since pilot / preamble for channel estimation is not required in uplink and downlink pilot / preamble channel estimation is broadcasted by the AP, frequency efficiency ( spectral efficiency can be increased. In this case, if the downlink channel estimated by the WT is the same as the uplink channel, the WT already knows the uplink channel before transmitting and uses it to compensate the channel gain for each subcarrier in the frequency domain before the IFFT of the transmitter. Phosphorus pre-equalization was applied.

On the other hand, as the importance of image and data transmission has increased recently, the need for high-speed data transmission is increasing, but frequency resources for this are relatively insufficient, so efficient use of frequency is essential.

However, when the adaptive modulation scheme is used in an existing OFDM system, an excessive number of pilots and preambles are required to provide a certain level of performance in estimating a channel, and an excessive amount of frequency is wasted to implement the implementation. it's difficult. In addition, there is a problem in that the complexity of estimating channels for a plurality of WTs in one AP is very high, so that implementation is difficult.

 Therefore, the technical problem to be achieved by the present invention is to solve the conventional problems, and when estimating a channel in a receiver of a wireless communication system, it is necessary to estimate channels for a plurality of wireless terminals (WT) in one access point (AP). To provide a preprocessing method for the uplink of the orthogonal frequency division multiplexing communication system that can prevent the complexity.

In particular, the technical problem to be achieved by the present invention is to apply a sub-channel adaptive modulation method to select the sub-channel to use and after the pre-processing, to reduce or eliminate the preamble that is excessively used to use the adaptive modulation or fixed modulation method An object of the present invention is to provide a preprocessing method for an uplink of an orthogonal frequency division multiplexing communication system.

According to an aspect of the present invention for achieving the above technical problem, a preprocessing method for an uplink of an orthogonal frequency division multiplexing communication system includes a plurality of wireless terminals (WTs) at one access point (AP). A preprocessing method for uplink in an orthogonal frequency division multiplexing communication system for estimating a channel for a channel, the method comprising: a) performing, by the wireless terminal, channel estimation on a plurality of frames periodically transmitted from the AP; b) determining a channel state according to the channel estimated value, and performing adaptive loading to adjust the number of bits for each subchannel according to the channel state; And c) pre-processing and transmitting the modulated signal, and transmitting information on the modulation scheme performed for each subchannel. In addition, the AP may further include detecting a modulation method for each subchannel based on the information, and demodulating the transmitted signal according to the detected result.

Meanwhile, in step c), the information on the determined modulation scheme may be transmitted to the AP using a separate control channel.

In addition, step c) designs a pre-equalizer based on a channel gain value, and informs the AP of normalization element information used in the normalization process for using the pre-equalizer before the signal transmission. Here, the AP may calculate the normalization element information using mutual symmetry of up / down channels. In addition, the normalization element information may be transmitted to the AP through a separate control channel together with the information on the determined modulation scheme.

In step b), a predetermined subchannel may be selected and a fixed modulation scheme may be used for the selected subchannel.

According to the present invention, in order to improve performance while reducing the complexity of the conventional scheme, the downlink uses a preamble and applies pre-equalization only for the uplink, thereby uplink frequency of the OFDM adaptive modulation system. Can improve efficiency.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

An embodiment of the present invention is a dictionary for avoiding the complexity of estimating channels for a plurality of wireless terminals (WT) in one access point (AP) in an OFDM (Orthogonal Frequency Division Multiplexing) TDD system. Disclosed is a method of pre-processing.

In general, in the case of a TDD system, it can be assumed that the uplink / downlink channels have mutuality (Reciprocity), in which case the channel between itself and the AP in each WT rather than the AP estimates all WT channels. By estimating only the channel of the uplink channel, the complexity of the AP can be reduced. Also, the pilot and preamble for channel estimation are not required in the uplink, and the downlink pilot and preamble channel estimation by the AP is performed. It is possible to increase the frequency efficiency. The present invention is a method for precompensating the channel gain for each subchannel in the frequency domain before the IFFT of the transmitter using the same, and includes a procedure and an algorithm using a preequalizer.

1 is a block diagram illustrating a configuration of a system for adaptive modulation after preprocessing in an adaptive modulated OFDM TDD system according to an embodiment of the present invention.

According to an embodiment of the present invention, an adaptive modulated OFDM TDD system includes an access point (AP) 100 and a plurality of wireless terminals 200 and 300. The AP 100 includes a modem 110, The RF terminal 120 and the antenna 130, each of the plurality of wireless terminals (200, 300) is a modem (210, 310), pre-processor (220, 320), RF stage 230, 330 and antennas 240 and 340.

In the adaptive modulated OFDM TDD system according to an embodiment of the present invention, each subchannel undergoes frequency non-selective fading without interference with each other due to the IFFT of the transmitting end and the FFT of the receiving end, similar to the general OFDM system. The model may be represented as shown in FIG. 2.

FIG. 2 is a diagram illustrating a simple model of an OFDM system in a frequency domain according to an embodiment of the present invention. For example, the multipliers 21, 23, and 25 and the adders 22, 24, and 26 may be formed. In addition, the IFFT of the transmitting end and the FFT of the receiving end cause each subchannel to undergo frequency non-selective fading without interference with each other.

Meanwhile, the conventional adaptive OFDM system allocates an optimal modulation scheme to a subchannel according to a desired data transmission rate according to the channel gain of the subchannel. In order to design a synchronization unit of such an adaptive OFDM system, it is generally possible to design by adding a preamble.

3 is a diagram illustrating a structure of a long preamble for uplink and downlink channels according to an embodiment of the present invention. The structure of the long preamble used for channel estimation in the uplink and downlink may be designed as shown in FIG. 3. .

4 is an exemplary diagram of a synchronization scenario of an uplink channel in a preprocessing OFDM TDD system according to an embodiment of the present invention, and shows a synchronization scenario of an uplink between an AP 41 and a WT 42 in a system of the present invention. One example may be represented as shown in FIG. 4.

On the other hand, in the adaptive OFDM system, the accuracy of channel estimation depends on the performance of the system. In order to increase the accuracy of the channel estimate, the length of the preamble becomes long. For the performance degradation of 0.1 kHz, about 40 preambles are required. This would reduce the efficiency of the channel too much. In order to solve this problem, the present invention applies a prior equalizer technique.

In the embodiment of the present invention, it is assumed that the application to the adaptive OFDM / TDMA / TDD system. Therefore, the modulation scheme determinator for the subchannel is placed in front of the IFFT to use different modulation schemes according to the channel gain. Since the TDMA scheme is used, the channel estimation for the downlink may use a conventional scheme using a preamble.

In addition, when the TDD scheme and the wireless LAN environment are considered, the uplink and the downlink radio links are for systems having mutual symmetry (Reciprocity). In order to increase the accuracy of the channel estimation value for the downlink through the frame periodically transmitted from the AP, it was estimated over several frames.

Based on these channel estimates, the WT applies adaptive modulation and pre-processes and transmits them through a pre-equalizer. The information on the modulation scheme thus determined must be transmitted to the receiving end using a separate control channel, and the receiving end finds a modulation method for each subchannel from the information and uses it for demodulation.

Hereinafter, the principle of the preprocessing method for the uplink of the orthogonal frequency division multiplexing communication system according to an embodiment of the present invention will be described in detail.

(

)이고, 각 부채널에 수신된 신호를

이라 할 때, 다음과 같은 수학식 1을 만족한다.First, when a simple model of the OFDM system for N is represented as shown in FIG. 2, the channel gain value for each subchannel is

(

) And the signal received on each subchannel

In this case, the following Equation 1 is satisfied.

는 송신기의 IFFT 전의 변조된 입력신호를 나타내고,

을 AWGN 수신 잡음을 나타낸다. 적응변조 기법은 채널의 상태가 좋으면 하나의 부채널에 많은 비트를 전송하고, 부채널 이득이 나쁘면 적은 비트를 전송하는 것이다. 각 부채널에 대한 채널 응답을 보상을 하기 위하여 다음 수학식 2와 같은 사전 등화기

를 사용할 수 있다.here,

Denotes the modulated input signal before the IFFT of the transmitter,

Represents the AWGN reception noise. The adaptive modulation technique is to transmit many bits in one subchannel if the channel is in good condition, and to transmit fewer bits in case of poor subchannel gain. In order to compensate the channel response for each subchannel, the pre-equalizer shown in Equation 2 below.

Can be used.

을 다음 수학식 3 및 4에 의하여 SNR 손실을 개선할 수 있다.At this time, the signal detected by the receiving end

It is possible to improve the SNR loss by the following equations (3) and (4).

At this time, it is necessary to reduce the load on the power amplifier by keeping the transmission power limitation of the transmitter constant in the preprocessing. In order to solve this problem, first, a subchannel to be used for transmission is selected using an adaptive loading scheme for one OFDM symbol. The pre-equalizer is designed to go through a normalization process as shown in Equation 5 for the selected subchannel.

를 다음 수학식 6과 같이 정의하고, n은 적응변조 기법에 의해서 선택된 부채널의 번호만을 의미한다.In this case, the normalization factor

Is defined as in Equation 6 below, and n denotes only the number of subchannels selected by the adaptive modulation scheme.

을 다음 수학식 7 및 8에 의하여 SNR 손실을 개선시킬 수 있다.At this time, the signal detected by the receiving end

Can be improved by the following equations (7) and (8).

)를 수신단 AP에서 사전에 알고 있어야 한다. 이것을 위하여 2가지 방법이 가능하다.To use this pre-equalizer, the normalization factor (

) Must be known in advance at the receiving AP. Two methods are possible for this.

)와 결정된 변조 방식에 대한 정보를 별도의 제어 채널을 이용하여 수신단으로 전송하는 방법이 있는데, 이들 중에 어느 하나를 사용하거나 두 가지 모두를 사용할 수도 있다.Firstly, it can be obtained at the receiving end using channel symmetry, or secondly, normalization factor (

) And the information on the determined modulation scheme are transmitted to the receiving end using a separate control channel. Any one or both of them may be used.

In addition, after a preprocessing process as shown in Equation 5 for the selected subchannel using the adaptive allocation algorithm, it is also possible to use a fixed modulation technique instead of adaptive modulation.

5A and 5B are diagrams illustrating performance evaluation of pre / post equalizers and fixed / adaptive modulation for an ideal channel response in a preprocessing OFDM TDD system according to an embodiment of the present invention, respectively. It shows the result of comparing the performance between the case of using the order and the case of using the different modulation order for each subchannel obtained through the adaptive modulation technique for the known channel response, and also the case of applying the pre-equalizer and the post-equalizer. The performance was compared. FIG. 5A shows a case where the average modulation order in an OFDM symbol is QPSK, and FIG. 5B shows a case where 16QAM.

5A and 5B, the o-line shows the performance when a pre-equalizer is applied with a fixed modulation order. And-uses different modulation orders for each subchannel obtained through the adaptive modulation method, and shows the performance when the unused subchannel information is excluded from normalization by using the obtained information.

6A and 6B are diagrams illustrating performances of pre / post equalizers and fixed / adaptive modulation for estimated channel responses in a preprocessing OFDM TDD system according to an embodiment of the present invention. In this case, the channel is estimated, and in the uplink, the channel is estimated using two symbols in one slot.

FIG. 7 is a diagram comparing the performance of the case where fixed / adaptive modulation orders are used only for selected subchannels in a preprocessing OFDM TDD system according to an embodiment of the present invention, and are differently allocated according to modulation orders obtained through an adaptive modulation scheme. The subchannels used and the unused subchannels are distinguished from each other, and the results of comparing the performance of the case where the fixed modulation order is used only for the subchannels used are shown.

In FIG. 7, adaptive modulation is performed at an average modulation order of QPSK of 512 bits (number of bits), 324 for a fixed modulation order of QPSK for a selected subchannel, 908 for using 16QAM, and an average modulation order of QPSK. When adaptive modulation is performed such that is 16QAM, 1024 can be obtained.

FIG. 8 is a performance evaluation diagram of pre / post / partial equalizer and fixed / adaptation modulation for an ideal channel response in a preprocessing OFDM TDD system according to an embodiment of the present invention. In this case, o- shows the performance of using different modulation orders for each subchannel obtained through the adaptive modulation technique, and-shows the unused subchannel information obtained by the adaptive modulation technique. The case where a fixed modulation order is applied only to the subchannel portion to be used is shown.

While the invention has been described above, these examples are intended to illustrate rather than limit this invention. It will be apparent to those skilled in the art that various changes, modifications, or adjustments to the above embodiments are possible without departing from the technical details of the present invention. Therefore, the scope of protection of the present invention will be limited only by the appended claims, and should be construed as including all such changes, modifications or adjustments.

As described above, according to the present invention, when the preprocessing method is applied in the indoor data communication system operating in the OFDM method, it is necessary to estimate the channels for the plurality of WTs in one AP as compared to the existing application modulation system. Reduce complexity

In addition, according to the present invention, since the pilot and the preamble for channel estimation are not required in the uplink and the channel can be estimated with the downlink pilot and preamble broadcasted by the AP, the frequency efficiency can be increased.

Claims (8)

A preprocessing method for an uplink in an orthogonal frequency division multiplexing communication system that performs channel estimation on a channel between an access point (AP) and a wireless terminal (WT), a) the wireless terminal performing channel estimation on a plurality of frames periodically transmitted from the AP; b) selecting a subchannel to be used for one symbol and performing adaptive loading for adjusting the number of bits based on a channel state determined according to a channel estimated value for the selected subchannel; And c) pre-processing and transmitting the adaptive allocation processed signal, and transmitting information on the modulation scheme performed in the subchannel together. Including; In step c), a pre-equalizer is designed based on a channel gain value, and normalization element information including information on the selected subchannel is used in the normalization process to use the pre-equalizer, before the signal transmission. Preprocessing method for uplink notifying AP.  A preprocessing method for an uplink in an orthogonal frequency division multiplexing communication system that performs channel estimation on a channel between an access point (AP) and a wireless terminal (WT), a) the wireless terminal performing channel estimation on a plurality of frames periodically transmitted from the AP; b) selecting a subchannel to be used for one symbol and performing adaptive loading for adjusting the number of bits based on a channel state determined according to a channel estimated value for the selected subchannel; c) pre-processing and transmitting the adaptive allocation processed signal, and transmitting information on the modulation scheme performed in the subchannel together. Including; The step c) is used in the normalization process to use the pre-equalizer and transmits normalization element information including information on the selected subchannel and information on the modulation scheme to the AP using a separate control channel. A preprocessing method for uplink, characterized in that. The method according to claim 1 or 2, And detecting, by the AP, a modulation scheme for each subchannel based on the information, and demodulating the transmitted signal according to the detected result. delete The method of claim 1, The AP normalization element information is obtained from the AP using the symmetry of the uplink / downlink channel, the pre-processing method for the uplink. The method of claim 1, And the normalization element information is transmitted to the AP through a separate control channel together with information on the determined modulation scheme. The method according to claim 1 or 2, Step b) is a pre-processing method for the uplink to determine the different modulation scheme for each sub-channel according to the channel gain before performing the IFFT for the signal to be transmitted. The method according to claim 1 or 2, The step b) selects a predetermined subchannel and uses a fixed modulation scheme for the selected subchannel.

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