JP2007081504A - Channel characteristic interpolation method and apparatus in OFDM receiver - Google Patents

Abstract

When interpolating a transmission path characteristic of a symbol in which no SP is arranged from received SP signals distributed in the time direction, the transmission path characteristic is obtained by interpolating from received SPs before and after the symbol to be interpolated. The delay time of several symbols in the time interpolation method for performing the interpolation is reduced.
When interpolating transmission path characteristics from a reception SP, a plurality of reception SPs received in the past are multiplied by different predetermined coefficients, and then each result is complex-added to obtain a first-order extrapolation or reception SP vector. Is interpolated in the transmission path characteristics of symbols in which pilot carriers are not arranged by time direction extrapolation processing such as combining at a predetermined coefficient ratio.
[Selection] Figure 1

Description

  The present invention relates to a relay device that receives a terrestrial digital broadcast modulated by an Orthogonal Frequency Division Multiplexing (hereinafter referred to as OFDM) scheme and relays a broadcast wave.

  In recent years, in the field of wireless devices, the OFDM method has attracted attention as a modulation method that is strong against multipath fading and mobile transmission, and many applied studies are being promoted in countries such as Europe and Japan.

  In Japan, digital terrestrial broadcasting in the UHF band using the OFDM method was started in December 2003. This development trend and method are described in the Journal of the Institute of Image Information and Television Engineers, 1998, Vol. 52, no. 11 (see Non-Patent Document 1).

  Now, in order to broadcast this terrestrial digital broadcast all over Japan, a broadcast wave relay device that receives the UHF band main broadcast from the master station and transmits the terrestrial digital broadcast again at a frequency different from the reception channel or at the same frequency. Is used.

  The former repeater that retransmits at a different frequency is mainly used in an MFN (Multi Frequency Network) environment. The latter repeater for retransmitting at the same frequency is mainly used in an SFN (Single Frequency Network) environment, and the frequency utilization efficiency is increased.

Below, the outline | summary of a MFN repeater and a SFN repeater is demonstrated.
First, FIG. 2 shows the configuration of a conventional MFN repeater, and the features of the MFN repeater will be described with reference to FIG.

  The signal received by the receiving antenna 21 is sampled by the A / D 22, and then converted from the time axis signal to the frequency axis signal by the FFT unit 23, and the frequency axis signal r (t, k) (where t is the symbol number, k: carrier number) is output. The transmission path characteristic interpolation unit 25 interpolates the transmission path characteristics from the master station to the reception antenna based on the pilot signal included in the reception signal.

  The transmission path characteristic interpolation algorithm will be described with reference to FIGS. 3 and 4. FIG. FIG. 3 is an explanatory diagram of pilot signals adopted in the terrestrial digital broadcasting system. Carriers indicated by black circles in the figure are pilot carriers whose amplitude and phase are known, and are distributed in the frequency (carrier) direction and time (symbol) direction. Hereinafter, this pilot carrier is referred to as SP (Scattered Pilot). When interpolating the transmission path characteristics of the symbol indicated by the arrow, it is possible to interpolate the received SP directly as the transmission path characteristics for the carrier in which the SP indicated by the black circle is inserted, but the SP is arranged. It is necessary to interpolate from the SPs received before and after in time for carriers indicated by gray circles that are not.

The present invention is a method related to this interpolation algorithm, and an interpolation method that is often used now will be described in detail.
Since the SPs are distributed and arranged, as described above, it is necessary to interpolate from the preceding and succeeding SPs for a carrier in which no SP is present. An insertion or 0th-order hold method is generally used.

  The primary interpolation is a method in which SPs before and after the symbol to be interpolated are linearly interpolated, and the 0th-order hold substitutes the SP to be interpolated with the SP of the symbol closest in time to the symbol to be interpolated. It is a method.

The transmission path characteristic is interpolated using SP by the interpolation method described above, and the interpolation transmission path characteristic h (t, k) is output from the transmission path characteristic interpolation unit 25.
The equalization unit 26 performs complex division on the output signal of the delay correction unit 24 provided for correcting the delay with the transmission line characteristic interpolation unit 25 by the interpolation transmission line characteristic h (t, k), thereby obtaining the amplitude, Perform phase equalization.

  The equalized signal is subjected to IFFT processing in the remodulator 27 and converted again to a time axis signal, and an OFDM transmission signal is generated by adding a guard interval. The OFDM transmission signal is converted into an analog signal by the D / A 28 and transmitted from the transmission antenna 28 through a channel different from the reception channel.

Next, the configuration of the SFN repeater is shown in FIG. 5, and the features of the SFN repeater will be described using this.
A signal received by the reception antenna 21 is transmitted from the transmission antenna 29 via the A / D 22, the adaptive filter 51, and the D / A 28. Here, since the transmission frequency and the reception frequency are the same, a part of the signal output from the transmission antenna 29 reaches the reception antenna 21 as a sneak wave. The adaptive filter 51 cancels this wraparound by adaptively controlling the filter coefficient. A major reason for performing waveform equalization by applying an adaptive filter to the time axis region is to reduce the propagation delay time from the receiving antenna to the transmitting antenna. This is because if the delay time between the received signal and the transmitted signal is increased, the transfer function of the entire repeater becomes unstable, and the probability that waveform distortion, oscillation, etc. will occur increases. In order to solve this problem, it is necessary to shorten the delay time from the reception antenna to the transmission antenna as much as possible.

  As the filter coefficient updating method, the output signal of the adaptive filter 51 is converted into a frequency axis signal by the FFT unit 23, and the transmission path characteristic interpolation is performed by the transmission path interpolation section 25 as described above. The error calculation unit 52 calculates a residual error between the interpolated SP signal and the known SP signal, and the IFFT unit 53 converts the frequency axis signal into a time axis signal again.

  The filter coefficient update unit 54 sequentially updates the coefficients so that the residual error is minimized. By doing so, the inverse function of the transmission path function from the transmitting antenna to the receiving antenna is formed by the adaptive filter, and the sneak signal can be canceled.

In addition, in an OFDM receiver, interpolation is first performed in the frequency direction, and then linear extrapolation is performed in the time direction to interpolate transmission path characteristics.
Journal of the Institute of Image Information and Television Engineers 1998 Vol. 52, no. 11 JP 2002-300094 A

  Although the repeater described above is used on a fixed line, the received signal fluctuates due to reflected waves and antenna fluctuations during storms. This variation in the received signal is a variation in transmission path characteristics. In the MFN repeater and the SFN repeater, compensation is performed by interpolating the transmission path transfer function from the reception SP and multiplying by the inverse characteristic of the interpolated characteristic. However, in such an environment with many fluctuations, when the SP time direction interpolation using the 0th-order hold method as described above is performed, an error in the interpolated transmission path characteristic becomes large and the equalization characteristic is deteriorated. . Further, in the interpolation method using the primary interpolation method, since interpolation is performed from the front and back SPs, a delay of at least 3 symbols occurs in the 4-symbol SP interval used in the digital terrestrial broadcasting method. In a terrestrial digital broadcasting system parameter, since one symbol is about 1 ms, a delay of about 3 ms occurs.

  In the MFN repeater, this delay time is added to the main route of the repeater, and cannot be ignored in a system with a high demand for low delay. In addition, since the SFN repeater uses the interpolated transmission line characteristics for updating the coefficient of the adaptive filter, this delay time becomes a factor that reduces the stability of the system when high-speed fluctuations in the transmission line characteristics occur. . That is, if a delay element is included in the feedback loop formed for coefficient update, the system loses stability, and in the worst case, it oscillates.

Furthermore, since it is necessary to store data of several symbols before and after, the required memory capacity increases and the hardware scale also increases.
In addition, when interpolating the transmission path characteristics of symbols in which SPs are not arranged from received SP signals distributed in the time direction, interpolation is performed from preceding and following received SPs in the time direction of the symbol to be interpolated, and thereafter Transmission line characteristics were interpolated by first-order extrapolation in the time direction of symbols. However, the method that precedes this time interpolation has a problem that a delay time of several symbols occurs.

  In the present invention, as a first method for solving the above problems, in an OFDM receiver that receives OFDM modulated signals in which pilot carriers having known amplitudes and phases are distributed in the time direction, the past M (M is an integer) ) N (N is an integer) received pilot carriers received within a symbol period are multiplied by independent predetermined coefficients, and then the respective results are combined to transmit a symbol transmission path in which no pilot carrier is arranged. Interpolate by extrapolating the characteristics in the time direction.

  As a second method, the pilot extrapolation is performed by performing the extrapolation on the two pilot carriers received at the past time point closest in time to the symbol to be interpolated. Interpolate the transmission path characteristics of symbols that are not.

  As a third method, for the three pilot carriers received at the past time point closest in time to the symbol to be interpolated, the time extrapolation method described above has a predetermined coefficient for each vector between pilots. Then, the respective results are combined and extrapolated in the time direction to interpolate the transmission path characteristics of symbols on which pilot carriers are not arranged.

  Further, an OFDM receiver and an OFDM repeater having a transmission line interpolation method using the first to third extrapolation methods described above are used.

  In the present invention, when interpolating transmission path characteristics, a plurality of reception SPs received in the past are multiplied by different predetermined coefficients, and then the respective results are combined and extrapolated in the time direction, thereby delay time. Can be shortened.

  Also in a broadcast wave relay device that receives and relays a broadcast wave, the relay delay time can be shortened, and further, a received signal with a fast fluctuation speed can be followed.

  A first embodiment of the present invention will be described in detail with reference to FIG. The configuration shown in FIG. 1 is a configuration in which the transmission line characteristic interpolation unit 25 is replaced with the time extrapolation transmission line characteristic interpolation unit 11 in the MFN repeater according to the prior art shown in FIG. It has a function.

  First, the signal received from the receiving antenna 21 is sampled by the A / D 22 to obtain a time-axis received sample value series. A time window in which symbol interference does not occur is provided on the time axis signal, and the signal within the time window is converted into a frequency axis signal r (t, k) (where t: symbol number, k: carrier number) by the FFT unit 23. Converted. The frequency axis signal r (t, k) is input to the time extrapolation transmission line characteristic interpolation unit 11 to calculate the interpolation transmission line characteristic h (t, k).

The present invention relates to the time extrapolation transmission line characteristic interpolation unit 11, and the function of the time extrapolation transmission line characteristic interpolation unit 11 will be described in detail below.
FIG. 6 shows a configuration of the time extrapolation transmission line characteristic interpolation unit 11. The frequency axis signal r (t, k) from the FFT unit 23 is input to the delay unit 61 and the SP extraction unit 65. The delay unit 61 outputs a signal r (t−1, k) obtained by delaying the frequency axis signal r (t, k) by one symbol. The output signal r (t−1, k) from the delay unit 61 is input to the delay unit 62 and to the SP extraction unit 65. The operation of the delay unit 62 is the same as that of the delay unit 61, and outputs a signal r (t−2, k) delayed by two symbols. Similarly, the delay unit 63 and the delay unit 64 output a 3-symbol delayed signal r (t−3, k) and an N-symbol delayed signal r (t−N, k).

In the SP extraction unit 65, the time extrapolation unit 66 in the subsequent stage interpolates the transmission path characteristics from the received SP carrier, so that the SP is derived from the frequency axis signals received at the present time or the past time as output signals of the delay units 61 to 64. The carrier is extracted and output. Specifically, assuming that the insertion interval in the time direction of the SP carrier is 4 symbols, for example, the signals r (t, k), r (t−1, k), r (t−2) from the delay units 61 to 64 are used. , K), r (t−3, k)... R (t−N, k)
r (t ′, k), r (t′−4, k), r (t′−8, k), r (t′−12, k),... r (t′−4 × i , K) Equation (1)
It becomes. Here, t ′ indicates a symbol time point closest to t in which SP exists in the past N symbols in the k-th carrier at the time point t symbol to be interpolated. Since the SP to be extracted is a signal received in the past, the condition is t ′ ≦ 0.

  For example, an example in which the carrier number is 6 (k = 6) and the symbol number is 0 (t = 0) will be described with reference to FIG. In the carrier r (0, 6) indicated by the arrow in the figure, the symbol that has received the SP in the past in the past is at the time t = -2, and therefore t ′ in equation (1) is t ′ = − 2. Become. Since an SP is arranged every 4 symbols, the SP extracted by the SP extraction unit 65 is r (−2,6), r (−6,6), r (−10,6). R (-14,6)...

  Similarly, SP signals extracted when the carrier number is 6 (k = 12) and the symbol number is 0 (t = 0) are r (0,12), r (−4,12), r (−8). , 12), r (-12, 12)...

The SP extraction unit 65 inputs the SP signal extracted by the equation (1) to the time extrapolation unit 66. The time extrapolation unit 66 interpolates transmission path characteristics at the time of t symbols based on the extracted SP signal. Therefore, each extracted SP signal is multiplied by a predetermined coefficient, and the complex addition result is used as an interpolation SP signal. When the interpolation SP signal is r ′ (t, k) and the predetermined coefficient to be multiplied is α,
r ′ (t, k) = Σα _i × r (t′−4 × i, k) Equation (2)
It becomes.

This will be described in more detail with reference to FIG.
FIG. 8 is a diagram showing the reception SP and the interpolation SP on the complex plane. A black circle indicates a past SP carrier extracted by the SP extraction unit, and a white circle indicates an ideal SP reception point when it is assumed that an SP is received at the time of the symbol t to be interpolated. The asterisk indicates the SP signal interpolated by the equation (2). The solid line indicates the past transmission path characteristics, and the dotted line indicates the future transmission path characteristics. The SP signals indicated by black circles are each multiplied by a predetermined coefficient α _i and subjected to complex addition to calculate the signals indicated by asterisks as interpolation SPs. At this time, an interpolation error component may occur for an ideal SP signal indicated by white circles due to the influence of noise components and high-speed fluctuations. The error component can be at a very small level.

Next, a specific design example of the predetermined coefficient α will be described.
First, as a first specific example, a primary extrapolation method will be described with reference to FIG.
This is a system in which the number of SPs extracted by the SP extraction unit is 2 at the maximum, these SP signals are connected by a straight line, and an interpolated SP signal is arranged on the extension line. Therefore, when taking the terrestrial digital broadcasting system dispersedly arranged every four symbols as an example, the interpolated SP signal r ′ (t, k) indicated by an asterisk can be realized by performing the calculation of Expression (3). .

r ′ (t, k)
= R (t ′, k) + c / 4 × {r (t ′, k) −r (t′−4, k)}
Formula (3)
Here, c has a relationship of c = t−t ′, and r (t, k) and r (t ′, k) indicate that they are c symbols apart. In the 4-symbol distributed SP, c takes a coefficient from 0 to 3.

A second specific example is shown in FIG. In this method, the maximum number of SPs to be extracted is 3, and an interpolation SP is calculated from these SP signals. In the method of the first specific example, the interpolation error becomes small when the fluctuation speed of the transmission path characteristic is slow, but the error amount becomes larger as the fluctuation speed becomes faster. Therefore, the second specific example aims to perform highly accurate interpolation even in an environment where the fluctuation speed is fast by using a reception SP that is earlier in time. The general formula of this second method is synthesized after the three SP signals are multiplied by coefficients a _{0 to} a ₂ as shown in formula (4).

r ′ (t, k)
= A ₀ × r (t ′, k) + a ₁ × r (t′−4, k) + a ₂ × r (t′−8, k)
Formula (4)
Various values can be considered for selecting the coefficients a _{0 to} a ₂ , but as values suitable for the digital terrestrial broadcasting system in which SP carriers are arranged every 4 symbols, equations (5) and (6) are considered. It is done.
a ₀ = 1, a ₁ = 0, a ₂ = 0 (when c = 0)
_{a 0 = 11/8, a} 1 = -4 / 8, a 2 = 1/8 ( For c = 1)
a ₀ = 14/8, a ₁ = −8 / 8, a ₂ = 2/8 (when c = 2)
a ₀ = 17/8, a ₁ = −12 / 8, a ₂ = 3/8 (when c = 3)
Formula (5)
a ₀ = 1, a ₁ = 0, a ₂ = 0 (when c = 0)
a ₀ = 12/8, a ₁ = −6 / 8, a ₂ = 2/8 (when c = 1)
a ₀ = 16/8, a ₁ = −12 / 8, a ₂ = 4/8 (when c = 2)
a ₀ = 20/8, a ₁ = −18 / 8, a ₂ = 6/8 (when c = 3)
Formula (6)
As described above, the extracted SP signals r (t ′, k), r (t ′, k−4), and r (t′−8, k) are expressed by equations (4) to (6). By multiplying by a coefficient, a value indicated by an asterisk in the figure is calculated as an interpolation SPr ′ (t, k).

  The time extrapolation unit 13 can perform interpolation with high accuracy and no delay even for a carrier on which no SP is arranged by the interpolation method described above. In the terrestrial digital broadcasting system parameters, SPs arranged every 12 carriers can be equivalently made every 3 carriers by this interpolation.

  The frequency interpolation unit 14 interpolates the transmission line characteristic h (t, k) of the data carrier on which the SP is not arranged by providing an interpolation filter that interpolates the carrier between the SP carriers interpolated by time extrapolation. . As the interpolation filter, a high-order FIR filter is generally used, but it can be realized even with a simple method such as linear interpolation.

  Further, according to the sampling theorem, it is possible to equalize to a multipath having a longer delay time as the equivalent SP carrier interval is closer. In principle, multipath up to the delay time shown in Equation (7) can be equalized.

Equalizable multipath delay time [sample]
= Effective symbol length [sample] / equivalent SP carrier interval
Formula (7)
Therefore, as described above, the extrapolation in the time direction is performed, the equivalent SP carrier interval is narrowed, and the interpolation in the frequency direction is performed to equalize the multipath with a long delay time. It becomes possible.

The interpolated transmission line characteristic h (t, k) is calculated by the time extrapolation transmission line characteristic interpolation unit 11 described above.
The functions of the delay correction unit 24, equalization unit 26, remodulation unit 27, D / A 28, and transmission antenna 29 shown in FIG. 1 are the same as those shown in FIG.

Next, a second embodiment of the present invention will be described with reference to FIG.
In the second embodiment, the transmission path characteristic interpolation method by time extrapolation according to the present invention is applied to the SFN repeater, and the interpolation method is the same as that described in the first embodiment.

  The configuration shown in FIG. 10 is a configuration in which the transmission line characteristic interpolation unit 25 is replaced with the time extrapolation transmission line characteristic interpolation unit 11 in the SFN repeater according to the prior art shown in FIG. Have. As described above, in the SFN repeater, the output from the adaptive filter 51 passes through the FFT unit 23, the transmission path characteristic interpolation unit 11, the error calculation unit 52, the IFFT unit 53, and the filter coefficient update unit 54 to the adaptive filter 51 again. It forms a feedback loop for input. When the fluctuation speed of the input signal of the adaptive filter 51 is fast, the delay time of the feedback loop loses the loop stability, and signal degradation such as oscillation occurs. Therefore, by using the transmission path characteristic interpolation method by time extrapolation according to the present invention, the delay time can be shortened and a signal with a fast fluctuation speed can be followed.

  In addition to the first and second embodiments, the application range of the present invention can be applied to a diversity repeater having a plurality of antennas, a directivity control repeater using an adaptive array antenna, or a simple receiver. .

The block diagram which shows the structure by the 1st Example by this invention. Block diagram showing the configuration of the MFN repeater Schematic diagram showing the arrangement of SP Block diagram showing transmission path characteristic interpolation method Block diagram showing the configuration of the SFN repeater The block diagram which shows the structure of the time extrapolation transmission line characteristic interpolation part 11 of this invention The schematic diagram which shows operation | movement explanatory drawing of SP extraction part 65 of this invention The schematic diagram which shows the interpolation system from receiving SP of i point of this invention The schematic diagram which shows the interpolation system by the primary extrapolation of this invention The schematic diagram which shows the interpolation system from three reception SP of this invention The block diagram which shows the structure by 2nd Example by this invention.

Explanation of symbols

11: time extrapolation transmission line characteristic interpolation unit, 21: receiving antenna, 22: A / D,
23: FFT section, 24: Delay correction section, 25: Transmission path characteristic interpolation section, 26: Equalization section, 27: Remodulation section, 28: D / A, 29: Transmission antenna, 51: Adaptive filter section, 52: Error calculation unit, 53: IFFT unit, 54: filter coefficient update unit,
61: Memory unit, 62: SP extraction unit, 63: Time extrapolation unit, 64: Frequency interpolation unit

Claims (7)

In an OFDM receiver that receives OFDM modulated signals in which pilot carriers having known amplitudes and phases are distributed in the frequency direction and the time direction, N (N is an integer) received in the past M (M is an integer) symbol period. (2) An OFDM receiving method characterized by extrapolating the two-dimensional data transmission path characteristics of the current data carrier in the time direction using the two-dimensional data transmission path characteristics of one reception pilot carrier. In an OFDM receiver that receives OFDM modulated signals in which pilot carriers having known amplitudes and phases are distributed in the frequency direction and the time direction, N (N is an integer) received in the past M (M is an integer) symbol period. ) Multiply the two-dimensional data transmission channel characteristics of the received pilot carriers by independent predetermined coefficients, and add the respective results in a complex manner to obtain the two-dimensional data transmission channel characteristics of the data carrier on which no pilot carrier is arranged. An OFDM receiving method, wherein extrapolation is performed in the time direction. 3. The OFDM reception method according to claim 1, wherein the temporal extrapolation is linear extrapolation for two pilot carriers received at a past time point close in time to a symbol to be interpolated. An OFDM reception method characterized by interpolating transmission path characteristics of symbols in which pilot carriers are not arranged. 3. The OFDM receiving method according to claim 1, wherein said time extrapolation interpolates vectors between pilots for three pilot carriers received at a past time point close in time to a symbol to be interpolated. An OFDM receiving method characterized by multiplying a predetermined coefficient with respect to each other and then complex-adding the respective results to interpolate by extrapolating in a time direction the transmission line characteristics of symbols on which pilot carriers are not arranged. 5. The OFDM reception method according to claim 1, wherein after performing the extrapolation in time, the interpolation is performed in the frequency direction. In an OFDM receiver that receives OFDM modulated signals in which pilot carriers having known amplitudes and phases are distributed in the frequency direction and the time direction, N (N is an integer) received in the past M (M is an integer) symbol period. ) Multiply the two-dimensional data transmission channel characteristics of the received pilot carriers by independent predetermined coefficients, and add the respective results in a complex manner to obtain the two-dimensional data transmission channel characteristics of the data carrier on which no pilot carrier is arranged. Perform either waveform equalization based on channel characteristic interpolation by extrapolation in the time direction, or diversity combining based on channel characteristic interpolation by time extrapolation interpolation, and the equalized signal or diversity combined signal Characterized in that it has a broadcast wave relay function that converts the signal again into a modulated signal and retransmits it as a broadcast wave Communication apparatus. In an OFDM receiver that receives OFDM modulated signals in which pilot carriers of known amplitude and phase are distributed in the frequency direction and the time direction, a broadcast wave signal is received by a receiving antenna, and the received signal is past M (M is (Integer) The two-dimensional data transmission path characteristics of N (N is an integer) received pilot carriers received within a symbol period are each multiplied by an independent predetermined coefficient, and the results are complex-added to arrange pilot carriers. Broadcast wave relay that performs waveform equalization based on channel characteristic interpolation by extrapolation interpolation in the time direction for two-dimensional data transmission line characteristics of unsupported data carriers, and transmits the signal after waveform equalization as a broadcast wave again An OFDM receiver characterized by having a function.

Images