JP4501071B2 - Single carrier block transmission receiver and reception method - Google Patents

Description

  The present invention relates to a single carrier block transmission (SCBT).

The single carrier block transmission scheme is a block transmission scheme in which a signal block composed of a plurality of information symbols is transmitted as a transmission signal, and equalization or demodulation processing is performed in block units on the reception side. This is a transmission method in which a guard interval (GI) is added and discrete frequency domain equalization is performed on the receiving side.
The signal to be transmitted is a band signal obtained by modulating a single carrier. This is different from the OFDM (Orthogonal Frequency Division Multiplex) scheme in which transmission is performed using a plurality of subcarriers for each block.

The single carrier block transmission method can perform effective frequency domain equalization by using discrete Fourier transform on the receiving side.
The equalizer weight when performing frequency domain equalization can be represented by a diagonal matrix Γ. The diagonal components of the diagonal matrix Γ are represented by {γ ₁ ,. . . , Γ _M }. Here, M represents the number of discrete frequencies, that is, the FFT size.

Conventional equalizer weights include zero-forcing (ZF) -based weights and minimum-mean-square-error (MMSE) weights.
The ZF equalizer weight is
γ _i = 1 / λ _i (i = 1, ..., M)
It is represented by Here, λ _i is a frequency transfer function of the transmission line.

The MMSE equalizer weight is
γ _i = λ ^* _i / (| λ _i ² | + σ _n ² / σ _s ² ) (i = 1, ..., M)
It is represented by Here, σ _n represents noise variance, σ _s represents signal variance, and * represents complex conjugate.
Although the MMSE equalizer has excellent BER (bit error rate) characteristics of the receiver, a means for estimating noise variance or CNR is required. The estimation of the variance value cannot be obtained without a large number of samples. Therefore, when transmission path characteristics such as wireless transmission change with time, it is difficult to realize a circuit.
US Patent Publication No. 2004/0076239 A1 K. Hayashi and S. Hara, “A New Spatio-Temporal Equalization Method Based on Estimated Channel Response,” IEEE Transactoins on Vehicular Technology, Vol. 50, No.5, p.1250-1259, 2001. Mitsuo Yokoyama "Spread Spectrum Communication System" Science and Technology Publishers p.393, 6.3 PN series J. Cioffi and JAC Bingham, “A Data-Drien Multitone Echo Canceller,” IEEE Transactions on Communications, Vol.42, No.10, p.2853-2869, 1994. Kazunori Hayashi “Fundamentals of Modulation / Demodulation and Equalization Technologies” Proc. MWE2004, pp.523-532, 2004

The ZF equalizer has a simple circuit configuration, but has a problem of noise enhancement.
Noise enhancement is a phenomenon in which when the transfer function λ _i of the transmission line takes a value of 0 or a value close to 0 at a certain frequency, the weight γ _{i at} that frequency takes a very large value and the noise is amplified. It is. As a result, the receiver cannot accurately restore the received signal, and the BER characteristics are deteriorated.

SUMMARY OF THE INVENTION An object of the present invention is to provide a single carrier block transmission receiver and reception method that employs a ZF equalizer with a simple circuit configuration and is less likely to cause noise enhancement.

A receiver for single carrier block transmission according to the present invention includes a Fourier transform unit that performs Fourier transform on a received signal, an equalization unit that performs frequency domain equalization on the Fourier transformed signal, and an equalized signal. An inverse Fourier transform unit that performs an inverse Fourier transform, and the equalization unit weights each Fourier-transformed signal using a zero-forcing (ZF) standard using a frequency transfer function of a transmission path. A weighting processing unit to be performed, and an amplitude suppression unit that compares the amplitude of the signal output from the weighting processing unit with a threshold and suppresses the amplitude of the signal when the threshold is equal to or greater than the threshold .

As in this configuration, weighting is performed on the basis of zero forcing (ZF) and amplitude suppression, so that the frequency transfer function λ _i of the transmission line takes 0 or a value close to 0, and at that frequency. Since the amplitude of the signal can be suppressed even when the weight γ _i is increased, the problem of noise amplification is eliminated. Therefore, the received signal can be accurately restored and the deterioration of the BER characteristic can be avoided.

The receiving method of the present invention is a receiving method according to the invention which is substantially the same as the invention of the receiver for single carrier block transmission described above.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram of a single carrier block transmission scheme (hereinafter referred to as an SCBT scheme). This transmission method includes a transmitter side, a transmission line, and a receiver side.
First, the processing on the transmitter side will be described in mathematical form.

The transmission signal symbol s (n) is blocked every M. n is a discrete time.
s (n) = [s ₁ (n), s ₁ (n), ..., s _M (n)] ^T
A pilot signal is inserted into each block of the transmission signal s (n) that has been blocked.
For example, FIG. 3 of Patent Document 1 shows an example in which a Pilot signal is inserted between CP and data. In FIG. 3 of Non-Patent Document 1, an example of superimposing a suppressed pilot channel on a data channel is shown. In this document, a PN sequence is used as a signal transmitted through a pilot channel.

This method is also applied to the single carrier block transmission of this embodiment.
Next, a cyclic prefix (CP) copied from the last part of the block at the head is added.
Next, a preamble (Preamble) is generated. Examples of the preamble include a PN (Psuedorandom Noise) signal series and a chirp signal. Here, see Non-Patent Document 2 for details of the PN signal. The chirp signal is “a signal whose frequency continuously increases or decreases”, and its generation method is described in Non-Patent Document 3, p.2866.

The preamble generated in this way is added to the block. The preamble and block added in this way are called “frame”.
FIG. 2 shows a specific example of the frame configuration. In FIG. 2, one preamble is attached to a plurality of blocks to form one frame. However, one preamble may be attached to one block to form one frame.

This frame is modulated to a radio frequency and transmitted to the transmission path.
Next, processing on the receiver side will be described.
FIG. 3 is a flowchart showing the flow (outline) of reception processing.
The received signal is A / D converted to detect the beginning of the frame (step S1). Next, a block (including a preamble) is cut out from the frame (step S2).

It is determined whether the extracted block is a data block or a preamble (step S3). If it is a preamble, a discrete Fourier transform is performed on the preamble signal (step S5). Based on the signal obtained by the discrete Fourier transform, the frequency transfer function λ _i (i = 1,..., M) of the transmission path is estimated (step S6).
On the other hand, if it is a data block, the data part is extracted from the block, and the cyclic prefix CP is removed from the data (step S8).

Next, a pilot signal is extracted from the data (step S9). The pilot signal is subjected to discrete Fourier transform (step S10), and the frequency transfer function λ _i is updated using the result of the discrete Fourier transform (step S7). This is because the transmission path is expected to change even after estimating the frequency transfer function of the transmission path when receiving the preamble.
Next, the received signal block after cyclic prefix removal is subjected to discrete Fourier transform (step S11).

Then, a frequency domain equalization process is performed based on the frequency transfer function (if updated, the updated frequency transfer function) (step S12). Frequency domain equalization is a process of multiplying a weight Γ for each frequency component in the transform domain and returning the signal to the time domain signal again by discrete inverse Fourier transform. Details of this will be described later.
Finally, discrete inverse Fourier transform is performed on the equalized signal (step S13) to convert it into a time function. Then, this time signal is determined (step S14).

The contents of the reception process described above will be described in detail using mathematical expressions.
Assuming that the cyclic prefix length is _LCP , the transmission signal block s _e (n) to which the cyclic prefix is added is as follows.

However,

Matrix size: (M + L _CP ) × M
In and, 0 _{LCPx (M-LCP)} is the zero matrix of _{L CP × (M-L CP} ), I LCP represents respectively a unit matrix of L _CP × L _CP.
If the impulse response of the transmission line is {h ₀ , h ₁ , ..., h _Lh } and the receiver noise is n (n) = [n ₁ (n), ..., n _M (n)} ^T Receive signal block

It is represented by However, the matrix H representing the convolution with the impulse response of the transmission path is as follows.

The matrix size of H is (M + L _CP ) × 2 (M + L _CP ).
Furthermore, H is a submatrix of (M + L _CP ) × (M + L _CP )

The received signal block is

Matrix size: (M + L _CP ) × 1
Can be written. Here, the first term on the right side is a signal component from the (n−1) -th transmission signal block, and represents a component of inter-block interference (IBI).
On the receiving side, the cyclic prefix is removed (step S8). This is expressed as follows:

The matrix size is M × 1. However,

It is. If L _CP ≧ L _h at this time, R _CP H ₁ = 0 _MxM regardless of the transmission signal,

It becomes. Now, when R _CP H ₀ T _CP of this equation is expanded, it becomes as follows.

A matrix having such a structure is called a circulant matrix (Circulant Matrix), and unitary similarity transformation is possible using a discrete Fourier transform (DFT) matrix. See Non-Patent Document 4 for details.
Using the nature of the cyclic matrix,

Can be written. However the _{Λ λ 1, λ 2, ...} , a diagonal matrix with lambda _M diagonal _{elements, λ 1, λ 2, ...} , λ M is calculated as follows.

Thus, the received signal r (n) after cyclic prefix removal can be written as follows.

In frequency domain equalization, the received signal block after cyclic prefix removal is subjected to discrete Fourier transform (step S11), weight Λ is multiplied for each frequency component in the transform domain, and the signal is again returned to the time domain signal by discrete inverse Fourier transform. Thus, equalization (step S12) is realized.
The weights in the discrete frequency domain are {γ ₁ ,. . . , Γ _M }. If the diagonal matrix with this diagonal component is Γ, the signal at the equalizer output is

It becomes.
Here, the equalizer weight Γ is a diagonal matrix whose diagonal components are {γ ₁ ,. . . , Γ _M }. γ ₁ , γ ₂ , γ ₃ ,... are derived from λ _i (i = 1,..., M).
For example, if the equalizer weight type is ZF standard,
γ ₁ = 1 / λ ₁ , γ ₂ = 1 / λ ₂ , ...
It becomes.

FIG. 4 is a block diagram showing a flow of processing of the ZF reference equalizer 1.
The ZF reference equalizer 1 includes a weighting processing unit 2 for applying the equalizer weight Γ, and an amplitude suppressing unit 3 for suppressing a frequency component with enhanced noise. The input of the weighting processing unit 2 after the Fourier transform is represented as X ₁ , X ₂ , X ₃ ,... (X _i when represented), and the output of the weighting processing unit 2 is represented by Y ₁ , Y ₂ , Y _3. ,... (Y _i when represented) and the output of the amplitude suppression unit 3 as Z ₁ , Z ₂ , Z ₃ ,... (Z _i when represented).

FIGS. 5A to 5C show examples of input / output amplitude characteristics of the amplitude suppression unit 3.
5 (a) is the amplitude of the Y _i | Y _i | if there were more than the threshold value, showing input and output amplitude characteristics of the amplitude suppression unit 3 to the amplitude of the output Z _i to zero.
FIG. 5 (b), the amplitude of the Y _i | Y _i | if there were more than the threshold value, input-output amplitude characteristic of the amplitude suppression unit 3 to fix the amplitude of the output Z _i to the value at the threshold input Indicates.

FIG. 5 (c), Y _i amplitudes of | Y _i | if there were more than the threshold value, the amplitude suppression unit 3 to fix the amplitude of the output Z _i to a value smaller than the value at the threshold input Indicates input / output amplitude characteristics.
FIG. 6 is a flowchart for explaining the processing in the amplitude suppressing unit 3 in FIG. For the Fourier-transformed equalizer weight input X _i (step T1), Y _i is obtained for each discrete frequency (step T2) (step T3) and compared with a threshold A (step T4). If it is less than the threshold A, a complex number Z _i having the same phase amplitude as Y _i is output (step T5). If YI is greater than or equal to the threshold value A, 0 is output as Z _i (step T6).

The above processing is performed until i reaches the FFT size M (steps T7 and T8). When the processing is completed up to FFT size M, inverse Fourier transform is performed (step T9).
The processing in the amplitude suppression unit 3 in FIG. 5A has been described above, but the processing in the amplitude suppression unit 3 in FIGS. 5B and 5C can be described in the same manner.
FIG. 7 is a diagram illustrating the phase relationship of X _i , λ _i , Y _i , and Z _i in the processing of the amplitude suppression unit 3 in FIG.

In the figure, Y _i is obtained by multiplying the input X _i by 1 / λ _i , and the amplitude is suppressed by the amplitude suppression unit 3 to become Z _i . If the amplitude of the transmission path transfer function λ _i of the discrete frequency is small, Y _i becomes an excessive value, but it is shown that the insertion of the amplitude suppression unit 3 suppresses the Z _i to an appropriate amplitude. ing.
As described above, since the amplitude suppressing unit 3 performs processing for suppressing only the amplitude while preserving the phase, the frequency transfer function λ _i of the transmission path takes 0 or a value close to 0, and the weight γ _{i at the} frequency is Even if it becomes large, the amplitude of the signal can be suppressed, so the problem of noise enhancement is eliminated. Therefore, the received signal can be accurately restored and the deterioration of the BER characteristic can be avoided.

Although the embodiments of the present invention have been described above, the embodiments of the present invention are not limited to the above-described embodiments. For example, as the amplitude suppression characteristics, compared with the signal amplitude and the threshold, if it is above the threshold, an example of suppressing the amplitude, absolute weight gamma _i is the weight when the above threshold The value may be suppressed, or the absolute value of the weight may be suppressed when the transfer function λ _i is equal to or less than a threshold value. A nonlinear curve that gradually saturates as the signal amplitude increases may be applied. In addition, various modifications can be made within the scope of the present invention.

As shown in FIG. 5A, the amplitude suppressing unit 3 is assumed to set the output to 0 when there is an input equal to or greater than the threshold value.
In the following, assuming the radio propagation model, common parameters in the simulation are shown.

FIG. 8 is a graph in which the horizontal axis represents the discrete frequency, and the vertical axis represents the output Y (relative scale) after weighting processing (no amplitude suppression).
In the graph, the absolute value of the frequency transfer function of the transmission line is increased by chance in the vicinity of the discrete frequency 900, indicating that a large weight is applied.
When the received signal is subjected to inverse Fourier transform without passing through the amplitude suppression unit 3, the constellation is as shown in FIG. According to this, for example, even when the original signal I (real part) is + and Q (imaginary part) is +, some signals are erroneously determined to be on the negative side. As a result, the bit error rate deteriorates.

When the inverse Fourier transform is performed after passing through the amplitude suppression unit 3 with the threshold A = 100, the constellation becomes as shown in FIG. The received signal is converged to a point in each of the ± I and ± Q regions, and the received signal can be correctly restored. Therefore, the effect of suppressing the amplitude of the frequency component with enhanced noise appears.
When the bit error rate BER was calculated based on this result, the graph shown in FIG. 11 was obtained. The horizontal axis represents C / N (dB), and the vertical axis represents the bit error rate of the detected equalizer output signal.

In the graph of FIG.
a: When using MMSE equalizer
b: When using a conventional ZF equalizer
c: When the amplitude suppression unit 3 of the present invention is applied, the threshold value A = 1000
d: When the amplitude suppression unit 3 of the present invention is applied, the threshold A = 300
e: When the amplitude suppression unit 3 of the present invention is applied, the threshold value A = 100
f: Theoretical value (no delay, time variation is only Rayleigh fading)
Represents. When the threshold A = 1000, characteristics close to those of the conventional ZF equalizer appear, but as the thresholds 300 and 100 decrease, the characteristics of the MMSE equalizer become closer. Therefore, it can be said that the amplitude suppression effect of the present invention appears.

It is a block diagram which shows the outline | summary of a SCBT system. It is a frame block diagram of a transmission signal. It is a flowchart which shows the flow of a reception process. 4 is a block diagram of a ZF equalizer including an amplitude suppression unit 3. FIG. 3 is a graph showing input / output amplitude characteristics of an amplitude suppression unit 3; It is a flowchart for demonstrating the process in the amplitude suppression part 3 with the input-output amplitude characteristic of Fig.5 (a). X _i, λ _i, Y _i in the amplitude suppression unit 3, a diagram showing the phase relation of Z _i. It is a graph showing an example of the output Y after a horizontal axis discrete frequency and a vertical axis weighting process (no amplitude suppression). FIG. 6 is a constellation diagram of a received signal that has passed through a conventional ZF equalizer that does not have an amplitude suppression unit 3; FIG. 4 is a constellation diagram of a received signal that has passed through a ZF equalizer of the present invention having an amplitude suppression unit 3. It is a graph showing the presence or absence of the amplitude suppression part 3, and the bit error rate BER according to the kind.

Explanation of symbols

1 ZF standard equalizer 2 Weighting processing unit 3 Amplitude suppression unit

Claims (2)

It is used for a single carrier block transmission system that transmits a signal block composed of a plurality of information symbols on the transmission side and performs discrete frequency domain equalization in units of these blocks on the reception side,
A Fourier transform unit that performs Fourier transform on the received signal, an equalization unit that performs frequency domain equalization on the Fourier transformed signal, and an inverse Fourier transform unit that performs inverse Fourier transform on the equalized signal,
The equalization unit weights each Fourier-transformed signal using a frequency transfer function of a transmission path on a zero forcing (ZF) basis, and outputs from the weighting processing unit A single carrier block transmission receiver comprising: an amplitude suppression unit that compares the amplitude of a signal to be processed and a threshold value and suppresses the amplitude of the signal when the signal is equal to or greater than the threshold value . It is used for a single carrier block transmission system that transmits a signal block composed of a plurality of information symbols on the transmission side and performs discrete frequency domain equalization in units of these blocks on the reception side,
A single carrier block transmission reception method comprising a step of performing a Fourier transform on a received signal, a step of performing frequency domain equalization on the Fourier transformed signal, and a step of performing an inverse Fourier transform on the equalized signal. And
The step of performing the frequency domain equalization weights each Fourier-transformed signal using a zero-forcing (ZF) standard using a frequency transfer function of a transmission line, and outputs the weighted signal. A method of comparing the amplitude of a signal to a threshold value and suppressing the amplitude of the signal when the signal is equal to or greater than the threshold value.

Images