CN110831147B - Carrier frequency synchronization method suitable for millimeter wave intersatellite link - Google Patents

Carrier frequency synchronization method suitable for millimeter wave intersatellite link Download PDF

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CN110831147B
CN110831147B CN201911090059.3A CN201911090059A CN110831147B CN 110831147 B CN110831147 B CN 110831147B CN 201911090059 A CN201911090059 A CN 201911090059A CN 110831147 B CN110831147 B CN 110831147B
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CN110831147A (en
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崔高峰
赵月
田丁
童建飞
王卫东
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Beijing University of Posts and Telecommunications
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W56/00—Synchronisation arrangements
    • H04W56/0035—Synchronisation arrangements detecting errors in frequency or phase
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
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    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
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Abstract

The invention discloses a carrier frequency synchronization method suitable for a millimeter wave inter-satellite link, and belongs to the field of inter-satellite communication systems. Firstly, constructing a millimeter wave inter-satellite link communication scene, and transmitting information by any two satellites through inter-satellite links; then, aiming at the current transmitting satellite, the high-frequency wireless frame reaches the receiving end of another satellite through the millimeter wave inter-satellite link, after sampling, the digital signal with carrier frequency offset is obtained, and the STF sequence r of the current wireless frame is extractedstf[n]For the sequence rstf[n]Performing demodulation, and extracting a phase difference to obtain a coarse frequency offset estimation value of the current wireless frame; and finally, performing fine estimation on the frequency offset estimation value again and compensating to the current wireless frame to realize carrier frequency synchronization of the current frame. Sequentially selecting the next radio frame, and repeatedly extracting the STF sequence rstf[n]And performing coarse estimation and fine estimation of frequency offset, and compensating the final estimation value into each frame, thereby realizing carrier frequency synchronization of each frame. The invention can obtain a more stable and accurate frequency offset estimation value, and effectively improve the system performance.

Description

Carrier frequency synchronization method suitable for millimeter wave intersatellite link
Technical Field
The invention belongs to the field of inter-satellite communication systems, relates to the problem of carrier frequency synchronization of a receiving end, and particularly relates to a carrier frequency synchronization method suitable for a millimeter wave inter-satellite link.
Background
The millimeter wave communication system has a large available bandwidth and can therefore support high speed data rate transmission. However, long-distance large-capacity millimeter wave communication needs to ensure accurate acquisition and demodulation of signals. Carrier synchronization is one of the key technologies in receiver digital communication modems.
In an inter-satellite communication network, an inter-satellite link is an important channel for information transmission. However, due to the high-speed relative motion between the satellites, the high-speed relative motion of the satellites generates severe doppler shift, so that there is a carrier frequency deviation with a high dynamic range in the receiver signal, which is very large relative to the carrier frequency deviation of the terrestrial communication system, and this greatly affects the reception, demodulation, and decision of data, and if this deviation cannot be cancelled, it will bring a great influence to the communication. In addition, the signal-to-noise ratio of the inter-satellite link is low due to limited satellite transmit power.
The current carrier synchronization technology applied to a ground communication system and a satellite-ground communication system mainly utilizes a known sequence of a wireless frame to extract a phase difference for frequency offset estimation, and the estimation range and the estimation precision of the frequency offset cannot be simultaneously ensured.
Therefore, how to obtain better carrier synchronization performance in the inter-satellite communication with large frequency offset and low signal-to-noise ratio becomes an important issue for research.
Disclosure of Invention
Aiming at the problems, the invention provides a carrier frequency synchronization method suitable for a millimeter wave inter-satellite link, which can effectively resist huge carrier frequency offset generated by inter-satellite link Doppler frequency shift, obtain better frequency offset estimation and compensation effects under a lower signal-to-noise ratio and ensure the correctness of data demodulation of a receiving end.
The method comprises the following specific steps:
step one, constructing a millimeter wave inter-satellite link communication scene formed by a plurality of satellites, wherein any two satellites transmit information by means of the inter-satellite link;
secondly, aiming at the current transmitting satellite, the high-frequency wireless frame reaches a receiving end of another satellite through a millimeter wave inter-satellite link, and the receiving end obtains a digital signal with carrier frequency offset after sampling;
the radio frame structure includes a Preamble (Preamble), a Header (Header), and a Data Block (Data Block).
The Preamble comprises STF and CE;
the Header specifies specific parameters of a physical layer protocol data unit;
each Data Block consists of a guard interval of length 64 and 448 Data symbols.
The specific process is as follows:
step 201, in the STK, according to the relative distance, elevation angle and azimuth angle of two satellites establishing the inter-satellite linkCalculating the Doppler frequency shift f generated by the inter-satellite link at the time td(t)；
The doppler shift calculation formula is as follows:
Figure BDA0002266579360000021
fcis the carrier frequency; vd(t) is the relative velocity between two satellites; and c is the speed of light.
Step 202, calculating the Doppler shift fd(t) induced phase noise;
the phase noise is represented as:
θ(t)＝2π·Δf·t＝2π·fd(t)·t＝2π·(fd(0)+fa(t))·t
Δ f is the carrier frequency offset, fd(0) Is the initial carrier frequency offset, fa(t) is the rate of change of doppler generated by the inter-satellite link at time t;
step 203, calculating a single carrier signal received by a receiving end according to the phase noise;
the single carrier signal calculation formula is as follows:
Figure BDA0002266579360000022
s (t) is a baseband analog signal of the transmitting end; theta0Is the initial phase; w (t) is the complex signal of noise, with the mean and imaginary components being zero and the variance being
Figure BDA0002266579360000023
White gaussian noise.
Step 204, for a sampling period TsAt t ═ nTsSampling a single carrier signal r (t) at a time (n ═ 0,1, 2.) to obtain a digital signal with carrier frequency offset;
the digital signal is represented as:
Figure BDA0002266579360000024
where s [ n ] is the sampled baseband digital signal, θ [ n ] is the sampled phase noise signal, and w [ n ] is the sampled noise complex signal.
Step three, aiming at each frame in the digital signal with carrier frequency offset, extracting the STF sequence r of the current wireless framestf[n]For the sequence rstf[n]Modulating by using autocorrelation and adjacent point difference to obtain a coarse frequency offset estimation value of the current wireless frame:
the method comprises the following specific steps:
firstly, the STF sequence of the current wireless frame is received and demodulated to obtain a modulation sequence zstf[n]；
The formula is as follows:
Figure BDA0002266579360000025
wherein s isstf[n]Is a local STF sequence and is,
Figure BDA0002266579360000026
wstf[n]is an STF sequence rstf[n]The additive noise complex signal carried, L1Is the length of the truncated STF sequence.
Then, for the modulated sequence zstf[n]Performing autocorrelation operation to obtain autocorrelation value Rstf[m]；
The calculation formula is as follows:
Figure BDA0002266579360000031
wherein the content of the first and second substances,
Figure BDA0002266579360000032
is a new noise signal generated by operation, K ═ L1/2。
Continuing, for the autocorrelation value Rstf[m]Carrying out adjacent point difference to obtain a difference value M;
the calculation formula is as follows:
Figure BDA0002266579360000033
wherein γ m is a smoothing coefficient.
Finally, extracting the phase of the difference value M to obtain a coarse frequency offset estimation value f1(ii) a The calculation formula is as follows:
Figure BDA0002266579360000034
wherein arg {. is used to obtain the phase of the complex value.
And step four, using a Kalman filter to perform fine estimation on the frequency offset estimation value again and supplement the frequency offset estimation value to the current wireless frame, so as to realize carrier frequency synchronization of the current frame.
For the current l-th frame, the output f (l) of the kalman filter is:
f(l)＝fpredict(l)+Kg(l)[fobserve(l)-fpredict(l)]
fpredict(l) Using the output result of the kalman filter of the previous frame for the predicted value of the frequency offset of the l-th frame, namely: f. ofpredict(l)＝f(l-1)。
Kg (l) is the filter gain, which is calculated as:
Figure BDA0002266579360000035
wherein the content of the first and second substances,
Figure BDA0002266579360000036
covariance as the estimation error;
Figure BDA0002266579360000037
r is the variance value of the system noise and Q is the variance value of the observed noise.
By means of the covariance
Figure BDA0002266579360000038
Update intoAnd (3) performing next Kalman filtering:
Figure BDA0002266579360000039
fobserve(l) Taking the frequency offset observed value of the l-th frame as a coarse frequency offset estimation result, namely: f. ofobserve(l)＝f1(l)。
Step five, sequentially selecting the next wireless frame and repeatedly extracting the STF sequence rstf[n]And modulating and finely estimating the coarse frequency offset estimation value and compensating the coarse frequency offset estimation value into each frame, thereby realizing the carrier frequency synchronization of each frame.
The invention has the advantages that:
1) the carrier frequency synchronization method suitable for the millimeter wave intersatellite link can estimate the coarse frequency offset of a system by performing autocorrelation operation and adjacent point difference on a known sequence of a wireless frame.
2) The carrier frequency synchronization method suitable for the millimeter wave intersatellite link is used for carrying out secondary optimization on the rough estimation result through Kalman filtering, and a more stable and accurate frequency deviation estimation value can be obtained.
3) Simulation shows that the method can effectively improve the estimation range and the estimation precision of frequency deviation, and effectively improve the system performance.
Drawings
FIG. 1 is a schematic diagram of a carrier frequency synchronization method for an inter-millimeter wave link according to the present invention;
FIG. 2 is a flow chart of a carrier frequency synchronization method for an inter-millimeter wave link according to the present invention;
FIG. 3 is a diagram illustrating a structure of a radio frame according to the present invention;
FIG. 4 is a diagram illustrating the STF structure in a radio frame according to the present invention;
FIG. 5 is a frequency offset estimation range simulation diagram of a carrier frequency synchronization method applicable to an inter-millimeter wave link according to the present invention;
FIG. 6 is a simulation diagram of normalized mean square error of a carrier frequency synchronization method applicable to an inter-millimeter wave link according to the present invention;
fig. 7 is a bit error rate simulation diagram of a carrier frequency synchronization method applicable to an inter-millimeter wave satellite link according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings and examples.
In the prior art, the available bandwidth of the millimeter wave communication system is large, so that high-rate transmission of data can be supported. However, long-distance large-capacity millimeter wave communication needs to ensure accurate acquisition and demodulation of signals. Carrier synchronization is one of the key techniques of the receiver. However, due to the high speed of relative motion between the satellites, the inter-satellite link will suffer from severe doppler shifts. In addition, the signal-to-noise ratio of the inter-satellite link is low due to limited satellite transmit power. Therefore, how to accurately estimate and compensate the huge frequency offset of the inter-satellite link without increasing the transmission power and ensure the transmission performance of the system is the main problem solved by the scheme.
The method firstly carries out modeling simulation on the Doppler frequency shift of the inter-satellite link, analyzes the inter-satellite link characteristics by taking a Walker constellation as an example in the STK, selects the Walker constellation configuration 3/3/1, and has the orbit height of 2000km and the orbit inclination angle of 55 degrees. The inter-satellite link of the two satellites W11 and W22 of the constellation is analyzed, the starting time is set to be 14Nov 201804: 00:00.000UTCG, and the ending time is set to be 15Nov 201804: 00:00.000 UTCG. Wherein W11 is the transmitting star and W22 is the receiving star.
In the Walker constellation of 3/3/1, the doppler shift of two low earth orbit satellites can reach 1 GHz. In order to obtain better carrier synchronization performance in huge doppler shift, as shown in fig. 1, the algorithm proposed by the present invention is divided into two steps: firstly, an STF sequence with a certain length is used for autocorrelation and adjacent point difference to obtain a coarse frequency offset estimation value. The autocorrelation operation can extract carrier frequency offset from the phase difference generated by the operation, and the adjacent point difference can improve the estimation range of the algorithm. Secondly, storing the coarse frequency offset estimation value of the first step in a buffer by using a Kalman filter based on multi-frame iteration to serve as an observation value of the second step, and taking the estimation result of the previous frame as a predicted value to carry out Kalman filtering to obtain the final frequency offset estimation result of the frame; and a Kalman filtering algorithm is adopted to inhibit random noise, so that an estimation result is stabilized, and the purpose of tracking frequency offset change is achieved.
As shown in fig. 2, the specific steps are as follows:
step one, constructing a millimeter wave inter-satellite link communication scene formed by a plurality of satellites, wherein any two satellites transmit information by means of the inter-satellite link;
secondly, aiming at the current transmitting satellite, the high-frequency wireless frame reaches a receiving end of another satellite through a millimeter wave inter-satellite link, and the receiving end obtains a digital signal with carrier frequency offset after sampling;
the wireless frame experiences serious Doppler shift when reaching a satellite receiving end through an inter-satellite link, and the wireless frame structure of the system adopts a ground IEEE 802.11ad (60GHz Wi-Fi) protocol, so that the system has the data transmission characteristics of high capacity and high rate. The structure of the radio frame is shown in fig. 3:
the radio frame structure includes a Preamble (Preamble), a Header (Header), and a Data Block (Data Block).
The Preamble comprises STF and CE; the STF is a short training sequence, mainly used for frequency offset estimation; as shown in fig. 4, there are 16 consecutive Ga128And a-Ga128And (4) forming. Ga128Is a gray sequence with 128 bits and has good autocorrelation.
CE is a long training sequence, mainly used for channel estimation.
The Header specifies specific parameters of a physical layer protocol data unit;
each Data Block consists of a Guard Interval (GI) of length 64 and 448 Data symbols.
The specific process is as follows:
step 201, in the STK, according to the relative distance, elevation angle and azimuth angle changes of two satellites establishing the inter-satellite link, calculating the doppler frequency shift f generated by the inter-satellite link at the time td(t)；
The doppler shift calculation formula is as follows:
Figure BDA0002266579360000051
fcis the carrier frequency; here at 60 GHz. Vd(t) is the relative velocity between two satellites; and c is the speed of light.
Step 202, calculating the Doppler shift fd(t) induced phase noise, the doppler shift having a linear variation within a frame;
the phase noise is represented as:
θ(t)＝2π·Δf·t＝2π·fd(t)·t＝2π·(fd(0)+fa(t))·t
Δ f is the carrier frequency offset, fd(0) Is the initial carrier frequency offset, fa(t) is the rate of change of doppler generated by the inter-satellite link at time t;
Figure BDA0002266579360000052
adis the relative motion acceleration of the satellite.
Step 203, calculating a single carrier signal received by a receiving end according to the phase noise;
the invention aims at a flat fading channel model with Additive White Gaussian Noise (AWGN), and provides a single carrier signal received by a millimeter wave inter-satellite communication system receiving end after matched filtering by the following formula:
Figure BDA0002266579360000053
s (t) is a baseband analog signal of the transmitting end; theta0Is the initial phase; w (t) is the complex signal of noise, with the mean and imaginary components being zero and the variance being
Figure BDA0002266579360000061
White gaussian noise.
Step 204, for a sampling period TsAt t ═ nTs(n ═ 0,1, 2..) time for single carrier signal r (t)Sampling to obtain a digital signal with carrier frequency offset;
the digital signal is represented as:
Figure BDA0002266579360000062
where s [ n ] is the sampled baseband digital signal, θ [ n ] is the sampled phase noise signal, and w [ n ] is the sampled noise complex signal.
Step three, aiming at each frame in the digital signal with carrier frequency offset, extracting the STF sequence r of the current wireless framestf[n]For the sequence rstf[n]Modulating by using autocorrelation and adjacent point difference to obtain a coarse frequency offset estimation value of the current wireless frame:
the method comprises the following specific steps:
firstly, the STF sequence of the current wireless frame is received and demodulated to obtain a modulation sequence zstf[n]；
The formula is as follows:
Figure BDA0002266579360000063
wherein s isstf[n]Is a local STF sequence and is,
Figure BDA0002266579360000064
wstf[n]is an STF sequence rstf[n]The additive noise complex signal carried, L1Is the length of the truncated STF sequence.
Then, for the modulated sequence zstf[n]Performing autocorrelation operation to obtain autocorrelation value Rstf[m]；
The calculation formula is as follows:
Figure BDA0002266579360000065
wherein the content of the first and second substances,
Figure BDA0002266579360000066
is a new noise signal generated by operation, K ═ L1/2。
Continuing, for the autocorrelation value Rstf[m]Carrying out adjacent point difference to obtain a difference value M;
the calculation formula is as follows:
Figure BDA0002266579360000067
wherein γ m is a smoothing coefficient.
Finally, extracting the phase of the difference value M to obtain a coarse frequency offset estimation value f1(ii) a The calculation formula is as follows:
Figure BDA0002266579360000068
wherein arg {. is used to obtain the phase of the complex value.
And step four, using a Kalman filter based on multi-frame iteration to perform fine estimation on the frequency offset estimation value again and supplement the frequency offset estimation value to the current wireless frame, so as to realize carrier frequency synchronization of the current frame.
For the current l-th frame, the output f (l) of the kalman filter is:
f(l)＝fpredict(l)+Kg(l)[fobserve(l)-fpredict(l)]
fpredict(l) Using the output result of the kalman filter of the previous frame for the predicted value of the frequency offset of the l-th frame, namely: f. ofpredict(l)＝f(l-1)。
Kg (l) is the filter gain, which is calculated as:
Figure BDA0002266579360000071
wherein the content of the first and second substances,
Figure BDA0002266579360000072
covariance as the estimation error;
Figure BDA0002266579360000073
r is the variance value of the system noise and Q is the variance value of the observed noise.
By means of the covariance
Figure BDA0002266579360000074
Updating and carrying out next Kalman filtering:
Figure BDA0002266579360000075
fobserve(l) Taking the frequency offset observed value of the l-th frame as a coarse frequency offset estimation result, namely: f. ofobserve(l)＝f1(l)。
Step five, sequentially selecting the next wireless frame and repeatedly extracting the STF sequence rstf[n]And modulating and finely estimating the coarse frequency offset estimation value and compensating the coarse frequency offset estimation value into each frame, thereby realizing the carrier frequency synchronization of each frame.
Compared with the classic two-step autocorrelation and autocorrelation accumulation (autocorrelation) schemes, the carrier synchronization algorithm provided by the invention has a larger estimation range and higher estimation precision, and can effectively improve the system performance. The performance analysis is shown in the following figure:
as shown in fig. 5, the abscissa is the simulated doppler frequency shift, and the ordinate is the frequency shift estimated value obtained by using three frequency shift estimation algorithms, so that it can be seen that when the set doppler frequency shift exceeds 1500kHz, the algorithm provided by the present invention can correctly perform frequency shift estimation, and has a large estimation range.
As shown in fig. 6, the abscissa is the signal-to-noise ratio (SNR) set by simulation, and the ordinate is the Normalized Mean Square Error (NMSE) of the frequency offset estimation value obtained by using three frequency offset estimation algorithms, it can be seen that NMSE decreases with the increase of SNR, and compared with other algorithms, the algorithm proposed by the present invention has the lowest NMSE under the same signal-to-noise ratio.
As shown in fig. 7, the abscissa is the signal-to-noise ratio (SNR) of the simulation setting, and the ordinate is the W Bit Error Rate (BER) obtained by using three frequency offset estimation algorithms, it can be seen that the BER decreases with the increase of the SNR, and compared with other algorithms, the algorithm proposed by the present invention has the lowest BER under the same signal-to-noise ratio.

Claims (2)

1. A carrier frequency synchronization method suitable for a millimeter wave inter-satellite link is characterized by comprising the following specific steps:
step one, constructing a millimeter wave inter-satellite link communication scene formed by a plurality of satellites, wherein any two satellites transmit information by means of the inter-satellite link;
secondly, aiming at the current transmitting satellite, the high-frequency wireless frame reaches a receiving end of another satellite through a millimeter wave inter-satellite link, and the receiving end obtains a digital signal with carrier frequency offset after sampling;
the specific process is as follows:
step 201, in the STK, according to the relative distance, elevation angle and azimuth angle changes of two satellites establishing the inter-satellite link, calculating the doppler frequency shift f generated by the inter-satellite link at the time td(t)；
The doppler shift calculation formula is as follows:
Figure FDA0002731134550000011
fcis the carrier frequency; vd(t) is the relative velocity between two satellites; c is the speed of light;
step 202, calculating the Doppler shift fd(t) induced phase noise;
the phase noise is represented as:
θ(t)＝2π·Δf·t＝2π·fd(t)·t＝2π·(fd(0)+fa(t))·t
Δ f is the carrier frequency offset, fd(0) Is the initial carrier frequency offset, fa(t) is the rate of change of doppler generated by the inter-satellite link at time t;
step 203, calculating a single carrier signal received by a receiving end according to the phase noise;
the single carrier signal calculation formula is as follows:
Figure FDA0002731134550000012
s (t) is a baseband analog signal of the transmitting end; theta0Is the initial phase; w (t) is the complex signal of noise, with the mean and imaginary components being zero and the variance being
Figure FDA0002731134550000013
White gaussian noise of (1);
step 204, for a sampling period TsAt t ═ nTsSampling a single carrier signal r (t) at a time (n ═ 0,1, 2.) to obtain a digital signal with carrier frequency offset;
the digital signal is represented as:
Figure FDA0002731134550000014
wherein s [ n ] is a sampled baseband digital signal, θ (n) is a sampled phase noise signal, and wn is a sampled noise complex signal;
step three, aiming at each frame in the digital signal with carrier frequency offset, extracting the STF sequence r of the current wireless framestf[n]For the sequence rstf[n]Modulating by using autocorrelation and adjacent point difference to obtain a coarse frequency offset estimation value of the current wireless frame;
the method comprises the following specific steps:
firstly, the STF sequence of the current wireless frame is received and demodulated to obtain a modulation sequence zstf[n]；
The formula is as follows:
Figure FDA0002731134550000021
wherein s isstf[n]Is a local STF sequence and is,
Figure FDA0002731134550000022
wstf[n]is an STF sequence rstf[n]The additive noise complex signal carried, L1Is the length of the truncated STF sequence;
then, for the modulated sequence zstf[n]Performing autocorrelation operation to obtain autocorrelation value Rstf[m]；
The calculation formula is as follows:
Figure FDA0002731134550000023
wherein the content of the first and second substances,
Figure FDA0002731134550000024
is a new noise signal generated by operation, K ═ L1/2；
Continuing, for the autocorrelation value Rstf[m]Carrying out adjacent point difference to obtain a difference value M;
the calculation formula is as follows:
Figure FDA0002731134550000025
wherein γ [ m ] is a smoothing coefficient;
finally, extracting the phase of the difference value M to obtain a coarse frequency offset estimation value f1(ii) a The calculation formula is as follows:
Figure FDA0002731134550000026
wherein arg {. is used to obtain a complex-valued phase;
step four, using a Kalman filter to perform fine estimation on the frequency offset estimation value again and supplement the frequency offset estimation value to the current wireless frame to realize carrier frequency synchronization of the current frame;
for the current l-th frame, the output f (l) of the kalman filter is:
f(l)＝fpredict(l)+Kg(l)[fobserve(l)-fpredict(l)]
fpredict(l) Using the output result of the kalman filter of the previous frame for the predicted value of the frequency offset of the l-th frame, namely: f. ofpredict(l)＝f(l-1)；
Kg (l) is the filter gain, which is calculated as:
Figure FDA0002731134550000027
wherein the content of the first and second substances,
Figure FDA0002731134550000028
covariance as the estimation error;
Figure FDA0002731134550000029
r is the variance value of the system noise, and Q is the variance value of the observation noise;
by means of the covariance
Figure FDA00027311345500000210
Updating and carrying out next Kalman filtering:
Figure FDA00027311345500000211
fobserve(l) Taking the frequency offset observed value of the l-th frame as a coarse frequency offset estimation result, namely: f. ofobserve(l)＝f1(l)；
Step five, sequentially selecting the next wireless frame and repeatedly extracting the STF sequence rstf[n]And modulating and finely estimating the coarse frequency offset estimation value and compensating the coarse frequency offset estimation value into each frame, thereby realizing the carrier frequency synchronization of each frame.
2. The method for synchronizing carrier frequency of a millimeter wave inter-satellite link according to claim 1, wherein the radio frame structure in step two comprises a Preamble, a Header and a Data Block;
the Preamble comprises STF and CE;
the Header specifies specific parameters of a physical layer protocol data unit;
each Data Block consists of a guard interval of length 64 and 448 Data symbols.
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