JP2013046131A - Method for estimating time-varying and frequency-selective channels - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for estimating time-varying and frequency-selective channels.SOLUTION: Time-varying and frequency-selective channels 130 in an orthogonal frequency division multiplexing (OFDM) network 120 are estimated by first storing, in a buffer at a receiver 122, a received signal corresponding to a set of pilot tones 100 of a set of OFDM symbols. The pilot tones are predetermined and inserted in frequency subcarriers and time slots of the OFDM symbol. A covariance matrix of the received signal is estimated. A diagonal matrix is estimated on the basis of the covariance matrix and a variance of noise in the received signal. The diagonal matrix indicates delays of non-zero paths in a time domain. A channel impulse response (CIR) for each OFDM symbol is estimated using the diagonal matrix and the received signal. Then, the CIR is transformed to the frequency domain to obtain the channel frequency response (CFR).

Description

  The present invention relates generally to communication networks, and more particularly to orthogonal frequency division multiplexing (OFDM) and estimated channel state information (CSI), which are channel impulse response (CIR) estimates.

  In the communication network, instantaneous channel state information (CSI) is required at the receiver for synchronous detection. In practice, to accomplish this, the transmitter transmits a pilot signal containing pilot tones that are predetermined and known on the receiver side. Thereafter, the receiver evaluates the CSI based on the received signal.

  In OFDM, pilot symbol assisted modulation (PSAM) can be used to estimate CSI with pilot tones inserted on subcarriers. Some or all of the subcarriers can be assigned to pilot tones. The more pilot tones, the better the accuracy of CSI. However, pilot tones consume bandwidth and reduce the effective data rate.

  Due to multipath transmission (multipath), the wireless channel has random variations in the frequency domain, thereby making the channel frequency selective. Also, since mobility (variability) may result in a Doppler effect, the channel becomes time-varying. With the combination of multipath transmission and time variation, the wireless channel becomes doubly selective with variations in both time domain and frequency domain.

  One way to estimate a doubly selective channel is to insert pilot tones in the time and frequency domains. The two-dimensional filter can then be configured as a CSI estimator that processes pilot tones in time and frequency. However, this requires high computational complexity and additional processing delay.

  The number of pilot tones inserted can be related to channel selectivity in the time and frequency domains. If the channel has high selectivity in the frequency domain, more pilot tones can be allocated in the frequency domain. Similarly, more pilot tones can be assigned in the time domain for time-varying channels.

  In practice, block-type pilot tone assignment, where all subcarriers of a particular OFDM symbol are allocated to pilot tones, is useful for slow fading and frequency selective channels. In contrast, comb-type pilot assignment, where specific subcarriers (frequency) are assigned to pilot tones, is appropriate for fast fading channels.

  Channel estimation can be performed in the frequency domain as well as in the time domain. In the frequency domain, the channel frequency response (CFR) is estimated. In the time domain, the channel impulse response (CIR) is estimated, where the channel frequency response is found by a discrete Fourier transform of the CIR.

  The present invention provides a method for estimating a channel in a wireless communication network using pilot tones. The transmitter transmits pilot tones that are periodically inserted into frequency carriers and time slots.

  A method for determining the number of pilot tones and their allocation in the frequency and time domains at the transmitter is described. In addition, the channel estimation process at the receiver is described.

  Specifically, a time-varying and frequency-selective channel in an orthogonal frequency division multiplexing (OFDM) network first receives a received signal corresponding to a set of pilot tones in a set of OFDM symbols into a receiver buffer. Estimated by storing, the pilot tones are predetermined and inserted into the frequency carrier and time slot of the OFDM symbol. A covariance matrix of the received signal is estimated. A diagonal matrix is estimated based on the variance matrix and the noise variance in the received signal. The diagonal matrix shows the non-zero path delay in the time domain. The channel impulse response (CIR) for each OFDM symbol is estimated using the diagonal matrix and the received signal. The CIR is then transformed into the frequency domain to obtain a channel frequency response (CFR).

Compared with conventional channel estimation, the present invention has the following advantages.
a. The present invention can reduce the required number of pilot subcarriers in a doubly selective channel by exploiting the correlation in their structure of delay paths between symbols.
b. The present invention does not depend on frequency selectivity in the frequency domain. Thus, fewer pilot tones are required even in very frequency selective channels.
c. The present invention does not require a priori knowledge of channel statistics such as the channel covariance matrix.
d. The present invention can reduce computational complexity by detecting non-zero path delays for multiple symbols.

It is the schematic of the wireless network by Embodiment 1 of this invention. It is a block diagram of the orthogonal frequency division multiplexing (OFDM) symbol by Embodiment 1 of this invention. It is a flowchart of the method of estimating the channel by Embodiment 1 of this invention.

Embodiment 1 FIG.
As shown in FIG. 1, an embodiment of our invention uses pilot tones 100 to estimate channel 130 at receiver 122 of orthogonal frequency division multiplexing (OFDM) wireless communication network 120. A method 300 is provided. The transmitter 121 periodically transmits pilot tones.

Random Pilot Tone Assignment As shown in FIG. 2, a set of pilot tones 100 is inserted into frequency carrier 101 and time slot 102 of orthogonal frequency division multiplexing (OFDM) symbol 200. The example symbol uses a set of 10 time slots and 8 frequencies in each OFDM symbol.

  The number of pilot subcarriers in a set of pilot tones in a single OFDM symbol can depend on the number of non-zero (important) delay paths in channel 120. Based on the number of frequencies, the transmitter assigns a set of pilot tones to the assigned frequencies uniformly and randomly. The number of pilot tones and their frequencies are known and known at the receiver.

  The number of occurrences of pilot tones in the time domain can depend on Doppler spread (spreading), receiver mobility, or the environment in which the receiver operates. For example, in an indoor environment, pilot tones cannot be transmitted more frequently than in an outdoor (outdoor) or mobile (mobile) environment.

Channel estimation at the receiver Each channel 130 between the transmitter and the receiver is modeled as an impulse response:

Here, α _l is a complex gain, τ _l is a delay corresponding to the l-th path, and is a T _s sampling interval. There are also non-zero (important) delay paths.

として表わすことができる。ここで、Ｆは離散的フーリエ変換（ＤＦＴ）行列であり、

はＣＦＲのベクトルである。 Expressing h as a vector of channel impulse response (CIR), the channel frequency response (CFR) is

Can be expressed as Where F is a discrete Fourier transform (DFT) matrix,

Is a CFR vector.

として表わすことができる。ここで、

は、それぞれ、ＣＦＲ、伝送されたパイロット信号、およびｋ番目のＯＦＤＭシンボルでのｎ番目の副搬送波に対する付加的なガウスノイズ、である。 When a predetermined number of pilot tones are transmitted, the received signal is

Can be expressed as here,

Are the additional Gaussian noise for the CFR, the transmitted pilot signal, and the nth subcarrier in the kth OFDM symbol, respectively.

である。ここで、Ｆ_Ｐはパイロット搬送波に対応する行を含むＤＦＴ行列の小行列であり、また、

は独立し且つ一様分布したガウスノイズのベクトルであり、また、

はｋ番目のＯＦＤＭシンボルのＣＩＲである。 Using the above CFR, the received signal is

It is. Here, F _P is the submatrix of the DFT matrix containing row corresponding to the pilot carrier, also,

Is a vector of independent and uniformly distributed Gaussian noise, and

Is the CIR of the kth OFDM symbol.

を推定することができる。圧縮検知は、当該技術において知られているように、予備的知識、例えば信号の構造や冗長性、を利用するスパース（疎な）すなわち圧縮可能な信号を取得し再構成する。 When the number of pilot tones is insufficient, a compression detection process (such as basis pursuit (BP), matching pursuit (MP), or orthogonal matching pursuit (OMP)) ( Compressed sensing process)

Can be estimated. Compression detection, as known in the art, acquires and reconstructs a sparse or compressible signal that utilizes preliminary knowledge, such as signal structure or redundancy.

の推定は、多数のＯＦＤＭシンボルに亘って共同で行われる。ここで、瞬時チャネル利得が時間変化的である場合でも、チャネルのパワー（電力）遅延プロファイル（ＰＤＰ）が固定されていると仮定する。すなわち、

は次式のような２つの成分に分離される。

Are estimated jointly over a number of OFDM symbols. Here, it is assumed that the channel power (power) delay profile (PDP) is fixed even if the instantaneous channel gain is time-varying. That is,

Is separated into two components:

はｋ番目のＯＦＤＭシンボルでの実際の係数利得のベクトルである。上記の仮定で、行列Ｑおよび

の統計量は、多数のＯＦＤＭシンボルに対して固定されている。 Where the diagonal matrix Q represents whether each path has a zero coefficient or a non-zero coefficient, and

Is a vector of actual coefficient gains in the kth OFDM symbol. With the above assumptions, the matrix Q and

Is fixed for a large number of OFDM symbols.

  For the kth OFDM symbol, the received signal vector is expressed as:

  The covariance matrix of the received signal is determined as follows:

は受信信号にけるノイズ（雑音）のバリアンス（分散）である。 here,

Is the variance of noise in the received signal.

  Thus, the following equation is obtained.

は多数のＯＦＤＭシンボルから次式のように決定することができる。 actually,

Can be determined from a number of OFDM symbols as:

は、受信機で利用できないＰＤＰを表わす。その場合、ＰＤＰは、一定または指数関数的に減衰する関数であると仮定することができる。

Represents a PDP that is not available at the receiver. In that case, the PDP can be assumed to be a constant or exponentially decaying function.

  As defined above, the diagonal component in matrix Q represents whether the delay path has zero or non-zero coefficients. Accordingly, detection of a non-zero (important) delay path corresponds to detection of a non-zero diagonal component of the matrix Q.

  Various compression sensing processes, BP, MP or OMP can be used to detect the diagonal components of the matrix Q.

  After detection of the non-zero delay path, the coefficient corresponding to the delay path in the kth OFDM symbol can be estimated from the following equation.

はＤＦＴ行列の小行列であり、それは単に非零遅延経路に対応する列のみを有しており、また、

は単に非零遅延経路のみを有する。推定される

は、最小自乗（ＬＳ）推定を使用して見つけることができる。 here,

Is a sub-matrix of the DFT matrix, which has only columns corresponding to non-zero delay paths, and

Has only a non-zero delay path. Presumed

Can be found using least squares (LS) estimation.

  FIG. 3 shows a method 300 for estimating a channel according to an embodiment of our invention.

The received signal corresponding to the set of N _p pilot tones is stored 311 in the buffer 310.

  After receiving a set of K OFDM symbols, where K is predetermined based on the channel environment, the covariance matrix of the received signal is determined 312. The frequency carrier, time slot and number of OFDM symbols are predetermined and known at the receiver.

  The diagonal matrix Q is estimated 313 based on the covariance matrix and the noise variance (variance). The matrix Q shows the non-zero (important) path delay in the time domain. The matrix Q is estimated using a compression detection process such as BP, MP or OMP.

The CIR for the kth OFDM symbol is estimated 314 using matrix Q and received signal Y [k].
The CIR is then transformed 315 to the frequency domain to obtain CFR.

Compared with conventional channel estimation, the present invention has the following advantages.
a. The present invention can reduce the required number of pilot subcarriers in a doubly selective channel by exploiting the correlation in their structure of delay paths between symbols.
b. The present invention does not depend on frequency selectivity in the frequency domain. Thus, fewer pilot tones are required even in very frequency selective channels.
c. The present invention does not require a priori knowledge of channel statistics such as the channel covariance matrix.
d. The present invention can reduce computational complexity by detecting non-zero path delays for multiple symbols.

  While this invention has been described by way of illustration of preferred embodiments, it is to be understood that various other modifications and changes may be made within the spirit and scope of this invention. Accordingly, it is the object of the appended claims to cover all modifications and variations that fall within the true spirit and scope of this invention.

Claims (9)

A method for estimating time-varying and frequency-selective channels in an orthogonal frequency division multiplexing (OFDM) network, comprising:
A received signal corresponding to a set of pilot tones of a set of OFDM symbols is stored in a buffer at a receiver, where the pilot tones are predetermined and inserted into the frequency carrier and time slot of the OFDM symbol And
Estimating a covariance matrix of the received signal;
Estimating a diagonal matrix based on the covariance matrix and a variance of noise in the received signal, where the diagonal matrix indicates a non-zero path delay in the time domain;
Estimating a channel impulse response (CIR) for each OFDM symbol using the diagonal matrix and the received signal;
Transforming the CIR into the frequency domain to obtain the channel frequency response (CFR);
A method comprising:   The method of claim 1, wherein the set of pilot tones is uniformly and randomly inserted into a transmitted signal at a transmitter.   The number of pilot subcarriers in the pilot tone set of a single OFDM symbol depends on the number of delay paths in the channel, where the delay paths are non-zero and significant. 1 method.   The method of claim 3, wherein the number depends on a Doppler spread in the channel.   4. The method of claim 3, wherein the number depends on the mobility of the receiver.   4. The method of claim 3, wherein the number depends on the environment in which the receiver operates.   The method of claim 1, wherein the CIR is estimated using a compression detection process.   The method of claim 1, wherein estimating the CIR is performed jointly over the set of OFDM symbols.   The method of claim 1, wherein the power delay profile of the channel is fixed even if the instantaneous channel gain is time-varying.

Images