JP4516505B2 - OFDM receiver - Google Patents

Description

  The present invention relates to an OFDM reception technique, and more particularly to a propagation path estimation technique based on an OFDM reception signal.

  FIG. 8 is a functional block diagram showing a configuration example for obtaining the frequency response of each subcarrier in a general communication system using an OFDM signal. In FIG. 8, reference numeral 101 is an FFT (Fast Fourier Transform) section, reference numeral 102 is a code multiplication section, reference numeral 103 is an IFFT (Inverse FFT) section, reference numeral 104 is a time window section, reference numeral 105 is an FFT section, and reference numeral 106 is a CE code. The generating unit 107 is a complex conjugate unit.

  The received OFDM symbol for propagation path estimation and the complex conjugate signal of the code frequency-converted by the FFT unit 101 and assigned to each subcarrier of the OFDM symbol generated by the CE code generation unit 106 and the complex conjugate unit 107. Multiplication is performed in the code multiplier 102, and the frequency response of the propagation path is once obtained. Thereafter, IFFT is performed in IFFT section 103, and an impulse response is obtained. The impulse response required here is concentrated in the low frequency range of IFFT output because the delay wave in the propagation path is limited to some extent. For this signal, noise or interference is reduced by limiting the signal in the time window unit 104, that is, by setting the wide area part to zero. After that, by converting the frequency again into the frequency by the FFT unit 105, a highly accurate frequency response can be obtained (see Patent Document 1).

Japanese Patent Laid-Open No. 5-75568 (Equipment for Coherent Demodulation of Digital Data Multiplexed in Time Frequency Domain with Evaluation of Frequency Response of Channel and Limit Determination)

The general frequency response estimation method as described above has been proposed for a system such as a digital TV, and can obtain a frequency response with high accuracy. Various proposals have also been made for the optimization of the time window shown in FIG.
When a time window is used in a region where the noise power ratio is low, propagation path estimation accuracy can be improved by narrowing the window width.

  By the way, when the above-described propagation path estimation method is applied to a one-frequency repetition cellular system, it is important to consider interference in the cellular system, so that the influence of interference components cannot be sufficiently suppressed. was there.

  An object of the present invention is to improve the accuracy of frequency response estimation while suppressing interference components in an OFDM receiver.

  According to an aspect of the present invention, there is provided a frequency response estimation unit that obtains an impulse response between a transmitter and a receiver, limits the impulse by a time window width, and obtains a frequency response for each subcarrier. The frequency response from (m-1) transmission sources or transmission antennas among the transmission sources or transmission antennas of integers of 2 or more is obtained, replicas of signals from the respective transmission sources or transmission antennas are created, and the received signals Subtracting the replica created from the above, and obtaining the frequency response from any one of the (m−1) transmission sources or transmission antennas or transmission antennas not included in the transmission antenna. An OFDM receiver characterized by having a time window width controller for adjusting is provided. A channel (propagation path) is also included between the transceivers. According to the above apparatus, by adjusting the time window width, it is possible to control noise that is suppressed when the time window width is narrowed and distortion of the signal waveform that increases.

  The time window width control unit performs control to change the window width, which is wide when the transmission source or the transmission antenna transmits a desired wave, and narrow when transmitting an interference wave. Thereby, the noise by an interference wave can be suppressed and distortion of the signal waveform of a desired wave can be reduced.

  According to the present invention, there is an advantage that the frequency response of a desired signal can be obtained with high accuracy.

  In this specification, the “time despreading method” is a method of multiplying a frequency response obtained by FFT of a propagation path estimation symbol by a complex conjugate of a code used on the transmission side, and an impulse response on the time axis by IFFT Convert to This signal refers to a method of obtaining a frequency response by performing noise filtering by a time window and performing FFT again.

  In the present embodiment, it is assumed that the desired wave and the interference wave are synchronized at a certain level. A certain level means that when an OFDM symbol synchronization is performed on a desired wave at the receiver and an FFT window is applied, the OFDM symbol does not change within the window for each interference wave. However, there is no problem even if the delay wave exists at a low level and the delay wave changes within the window. As one of such cases, a situation in which the base station forms a plurality of sectors and the terminal station is at the sector edge can be considered.

A downlink access method in which a terminal station receives data from a base station is called OFDMA (Orthogonal Frequency Division Multiple Access). The OFDMA system uses OFDM, TDMA (Time Division Multiple Access) and FDMA (Frequency Division)
Multiple Access).

  In order to improve the communication efficiency in this OFDMA system, it is necessary for each terminal station to access through a subchannel with a good reception state. This means increasing the user diversity effect. Moreover, it is preferable that the handoff can be performed smoothly at the sector edge.

In order to obtain such an effect, it is necessary to accurately estimate the propagation path from each base station (each antenna in the case of a sector edge). An embodiment of the present invention is to provide a technique for accurately estimating a propagation path even in the presence of an interference wave in a system using OFDM.
Examples of the present invention will be specifically described below.

  In the following example, in order to simplify the explanation, the interference wave number is K-1. FIG. 1 is a functional block diagram illustrating a configuration example for estimating a propagation path in a receiver. In FIG. 1, reference numeral 1 is an FFT (Fast Fourier Transform) section, reference numeral 2 is a reception memory, reference numeral 3 is a subtraction section, reference numeral 4 is a code multiplication section, reference numeral 5 is an IFFT (Inverse FFT) section, and reference numeral 6 is a time window section. , Code 7 is a time window width control unit, code 8 is an FFT unit, code 9 is a code multiplication unit, code 10 is a CE code generation unit, code 11 is a complex conjugate unit, codes 12_1 to 12_K-1 are replica memory units, Reference numeral 13 denotes an adder that adds all replicas.

  FIG. 3 is a flowchart showing a process flow based on the configuration shown in FIG. FIG. 2 is a diagram simply showing an output example of the IFFT unit 5 in FIG. Hereinafter, the present embodiment will be described with reference to FIGS. 1 to 3.

  When a propagation path estimation symbol (hereinafter referred to as CE: Channel Estimate) is received (Start), the memory of the replica memory unit 12 is cleared and PW = A is set (step S1). This is set in the time window control unit 7. PW will be described later. Next, the received CE is subjected to FFT in the FFT unit 1 to obtain data converted on the frequency axis and stored in the reception memory 2 (steps S2 and S3).

  First, a CE replica of the first interference base station is created (step S5). A detailed method of creating a replica is to receive the complex conjugate of the CE code used by the first interference base station (interference antenna in the case of a sector edge) output from the CE code generation unit 10 and the complex conjugate unit 11. A frequency response is obtained by multiplying the data in the memory by the sign multiplier 4 (step S4). Then, IFFT unit 5 performs IFFT on the signal to obtain an impulse response.

  FIG. 2A is a conceptual diagram of the impulse response obtained here. In FIG. 2A, the vertical axis represents the power of each impulse, and the horizontal axis represents the FFT point. In general, the impulse response obtained here is a certain length (in FIG. 2A, a region where a and b are combined, the region a is mainly a delayed wave, and the region b is a preceding wave. Otherwise, noise and interference components occupy the majority. In the time window unit 6 of FIG. 1, a process of setting this noise component to 0 is performed. In the present embodiment, the aforementioned PW is defined as the number of samples of PW = a + b. Then, the frequency response from the first interfering base station is obtained by performing the FFT process again in the FFT unit 8 on the signal subjected to the time window in the time window unit 6. By multiplying the obtained response by the code CE used in the first interference base station, a replica of the CE of the first interference base station can be calculated. This replica is stored in the replica memory 12_1 (step S5). The same processing is performed for all the interference base stations until k = K−1 (steps S6 and S7).

  When all replicas are stored in the replica memory (yes in step S6), the time window control unit 7 sets the time window PW = B (B ≧ A is satisfied) (step S8). As shown in FIG. 2C, an impulse remains wider than in the state of FIG. Thereafter, in step S9, the replica is added and subtracted from the received memory value (subtraction unit 3). In step S10, the propagation path of the desired wave is estimated. Thus, the frequency response from the desired base station is calculated in the same procedure as the procedure for obtaining the frequency response from the interference base station. With the above procedure, the frequency response from the desired base station can be obtained with high accuracy.

  In this embodiment, the window width is narrowed when the frequency response from the interfering base station is obtained, and the window width is widened when the frequency response from the desired base station is finally obtained. As a result, when calculating the frequency response of the interfering base station, there is a lot of interference. Since the interference component has already been removed, the window width is increased in order to extract as many signal components as possible. Thus, there is an advantage that the frequency response of the desired signal can be obtained with high accuracy by controlling the time window width at the time of frequency response estimation.

  In the first embodiment, the window width is changed between obtaining the frequency response of the interference wave and obtaining the frequency response of the desired wave. In this case, it is considered that the accuracy is low when estimating the frequency response of the interference wave. In the receiving technique according to the second embodiment, a configuration and operation that can further improve the estimation accuracy of interference waves will be described.

  FIG. 4 is a functional block diagram showing a configuration example of the receiving apparatus according to this embodiment, and FIG. 5 is a flowchart showing the flow of the operation. 4 and 5, corresponding blocks and operation steps having the same configurations as those in FIGS. 1 and 3 are denoted by the same reference numerals and description thereof is omitted.

  The difference between the configurations of FIG. 1 and FIG. 4 is that a replica memory CK12_K is added and the adder 14 adds a replica other than k.

  Regarding processing, replicas up to the interference wave K-1 are created and stored in the respective replica memories as in the first embodiment. In this embodiment, a replica is similarly created for the desired wave K and stored in the replica memory 12_K. In the flowchart shown in FIG. 5, step S4 in FIG. 3 is changed to step S20 to calculate the frequency response of the desired wave and interference wave k, and step S6 in FIG. 3 is changed to step S21 until k = K. The frequency response calculation process is repeated. Note that the number of the desired wave is K for convenience.

  Next, processing for obtaining a response such as the frequency of the interference wave is performed again. At this time, the frequency response replica other than the interference wave obtained by the adding unit 14 and the frequency response replica of the desired wave are added and subtracted from the reception memory (step S30). Then, the frequency response of the interference wave is calculated again (step S31) and stored in the replica memory CK12_K (step S32). This is performed for all interference waves (steps S33 and S34).

  After all the interference wave replicas are obtained (yes), the addition value of the interference wave replica is subtracted from the reception memory (step S35), and finally the frequency response of the desired wave is obtained (step S10).

  The above processing also improves the replica accuracy of the interference wave, so that it is possible to estimate the frequency response of the desired wave with higher accuracy than in the first embodiment.

  In the third embodiment, a case where the calculation is further repeated will be described. FIG. 6 is a functional block diagram showing a configuration according to the third embodiment, and FIG. 7 is a flowchart showing an operation flow. In FIG. 6 and FIG. 7, the same numbers are given to the previously described blocks and operations, and description thereof is omitted.

  In FIG. 6, the difference from the configurations of the first and second embodiments is that K blocks from M-1 to M-K are prepared for obtaining a frequency response, that is, blocks from 4 to 11. This is that K adders 14 are provided from 14_1 to 14_K.

  In the flowchart shown in FIG. 7, since the frequency responses from all base stations can be obtained simultaneously with the configuration of FIG. 6, loop processing based on the change of k is not necessary, and the number of iterations related to replica creation is reduced. Operation steps S40 to S42 according to r are added. Here, the repetition is represented by r, and the maximum number of times is R.

  First, in step S40, r = 1 is set, and then PW = A, and frequency responses from all base stations are calculated (see FIG. 2B) and stored in the replica memory. Thereafter, as PW = B (A> B), the frequency responses other than the frequency response from the base station to be obtained are subtracted from the frequency waveform of the received wave, and the frequency responses from all the base stations are obtained again (see FIG. 2C). Thereafter, it is determined whether r = R (step S41). If R is not satisfied, the same operation is repeated until R is reached (step S42). When r = R (yes), the process proceeds to step S35, the frequency response from the desired base station is calculated again, and the process ends (end). By repeatedly performing calculations in this way, the frequency response from the desired base station can be obtained with high accuracy.

  In the present embodiment, an example has been described in which the number of blocks for obtaining the frequency response is provided for the number of base stations. However, the calculation can be repeated even in the configuration of the second embodiment. However, in that case, it is necessary to repeatedly perform the process of obtaining the propagation path by the number of base stations × the number of repetitions.

  As described above, in the first to third embodiments, an example in which replicas of all interference waves are created has been described. For example, it is possible to perform processing that excludes interference waves with a clearly small power from replica creation targets. By doing so, the characteristics are further improved.

  In addition, even when there are a plurality of interference waves, an example is shown in which the same PW (window width) is applied when the first frequency response is obtained and when the frequency response is obtained by repetitive calculation. It is not necessary, and it is also possible to change it according to the reception intensity from each base station, SIR (Signal power to Interference power Ratio) or SINR (Signal power to Interference and Noise power Ratio).

  Since the window width has a noise suppression effect, the window width at the time of obtaining the first frequency response is narrow for an interference wave having a poor reception state, that is, an interference wave having a low reception intensity or an interference wave having a low SIR or SINR. It is preferable to control the interference wave so that the window width is increased from the beginning.

  The present invention is applicable to an OFDM receiver.

It is a functional block diagram which shows the structural example for estimating the propagation path in a receiver. FIG. 2A is a diagram simply showing an output example of the IFFT unit in FIG. 2B is an output example when the window width A is applied, and FIG. 2C is a diagram illustrating an output example when the window width B (B <A) is applied. It is a flowchart figure which shows simply the flow of the process based on the structure shown in FIG. 6 is a functional block diagram illustrating a configuration example of a receiving device according to Embodiment 2. FIG. FIG. 5 is a flowchart showing an operation flow based on the configuration shown in FIG. 4. FIG. 10 is a functional block diagram illustrating a configuration according to a third embodiment. It is a flowchart figure which shows the flow of the operation | movement based on the structure shown in FIG. It is a functional block diagram which shows one structural example for calculating | requiring the frequency response of each subcarrier in the general communication system using an OFDM signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... FFT, 2 ... Reception memory, 3 ... Subtraction part, 4 ... Code multiplication part, 5 ... IFFT part, 6 ... Time window part, 7 ... Time window width control part, 8 ... FFT part, 9 ... Code multiplication part, DESCRIPTION OF SYMBOLS 10 ... CE code production | generation part, 11 ... Complex conjugate part, 12_1-12_K-1 ... Replica memory, 13 ... Addition part.

Claims (8)

Obtaining a response of an impulse between the transceiver, limiting the impulse by a time window width, and having a frequency response estimation means for obtaining a frequency response for each subcarrier,
Of m transmission sources or transmission antennas (m is an integer of 2 or more), frequency responses from (m−1) transmission sources or transmission antennas are obtained, and replicas of signals from the respective transmission sources or transmission antennas are obtained. An OFDM receiver that creates and subtracts the replica created from the received signal, and obtains a frequency response from either the (m−1) transmission sources or the transmission antennas not included in the transmission antenna,
Have a time window width control unit for adjusting the time window width,
The OFDM receiver according to claim 1, wherein the time window width control unit performs control to change the window width every time the transmission source or the transmission antenna is changed . The time window width control unit performs control to change a window width, which is wide when the transmission source or the transmission antenna transmits a desired wave and is narrow when an interference wave is transmitted. 1. The OFDM receiver according to 1 . The time window width control unit is left with each time window width Ak (an integer satisfying 1 ≦ k ≦ m−1) when obtaining frequency responses from (m−1) transmission sources or transmission antennas. 3. The OFDM receiver according to claim 1, wherein the relationship with the time window width B when obtaining a frequency response from a transmission source or a transmission antenna satisfies Ak <B for all k. Obtain the frequency response of m transmission sources or transmission antennas, create a replica again,
When subtracting a replica other than the transmission source or the transmission antenna that finally obtains the frequency response from the received signal and setting the time window width when finally obtaining the frequency response as C, Ak <C for all k OFDM receiving apparatus according to any one of claims 1 to 3, characterized in that meet. When creating a replica, OFDM receiving apparatus according to any one of the portion of the signal from claim 1 characterized in that it does not create a replica to 4. 5. The OFDM receiving apparatus according to claim 3, wherein Ak is constant for all k.   An OFDM receiving apparatus having a plurality of frequency response estimation means and capable of estimating a frequency response in parallel, wherein the time window width in each frequency response estimation means can be set to a different value. 2. The OFDM receiver according to 1. A frequency response estimation method for obtaining an impulse response between a transceiver, limiting the impulse by a time window width, and obtaining a frequency response for each subcarrier,
Obtaining a frequency response from (m−1) transmission sources or transmission antennas among m transmission sources or transmission antennas while adjusting the time window width;
Creating a replica of the signal from each transmission source or transmission antenna;
Subtracting the replica created from the received signal, have a determining a frequency response from any of the (m-1) number of transmission source or transmission transmission source or transmission antennas not included in the antenna,
An OFDM receiving method , wherein when adjusting the time window width, control is performed to change the window width every time the transmission source or the transmission antenna is changed .

Images