CN102957658A - Time synchronization method and device for OFDM (Orthogonal Frequency Division Multiplexing) communication system - Google Patents

Abstract

The invention discloses a time synchronization method and a device for an OFDM (Orthogonal Frequency Division Multiplexing) communication system. The method comprises the following steps of: determining the timing metric of each antenna of a distributed antenna system; generating a detected antenna set and a missing detection antenna set by using the timing metric of each antenna and the preset threshold; determining an initial time synchronization point of the antenna in the detected antenna set; if the missing detection antenna set is empty or the quantity of the antennae in the detected antenna set is smaller than 2, taking the initial time synchronization point of each antenna in the detected antenna set as a respective final time synchronization point, and otherwise, judging whether the false alarm probability of the distributed antenna system is great than or equal to the preset false alarm probability or not; and if so, taking the initial time synchronization point of each antenna in the detected antenna set as the respective final time synchronization point, and otherwise, determining a final time synchronization point of the antenna in the missing detection antenna set and determining the respective final time synchronization point according to a collaborative threshold of the antenna in the detected antenna set and the respective initial time synchronization point.

Description

Time synchronization method and device of OFDM communication system

Technical Field

The present application relates to the field of communications, and in particular, to a time synchronization method and apparatus for an OFDM communication system.

Background

Currently, Orthogonal Frequency Division Multiplexing (OFDM) technology is widely applied to wireless communication systems such as cellular communication and digital broadcast television, and a corresponding time synchronization scheme of an OFDM communication system with multiple distributed antennas means that a receiving antenna of the OFDM communication system synchronizes with a transmitting antenna in the OFDM communication system, and the following two tasks need to be completed: detecting whether a signal has been transmitted by a transmitting antenna; based on the receiving antenna, the position of the time starting point (i.e. the time synchronization point) of the signal transmitted by the transmitting antenna is determined, i.e. the correct starting point of the Fast Fourier Transform (FFT) window.

The OFDM communication system has a high requirement for time synchronization accuracy, and currently, time synchronization can be achieved by two methods: based on a Cyclic Prefix (CP) and a training sequence, wherein the training sequence is a sequence that is completely known in the time or frequency domain.

The time synchronization is realized based on the CP, and the time synchronization is mainly carried out by the characteristic that the CP is the repetition of the tail end of the OFDM time domain signal, however, the time synchronization is realized based on the CP, the synchronization speed is slow, the synchronization timing range is rough, and the anti-noise performance is poor. Therefore, in practical applications, it is more common to implement the time synchronization of the OFDM communication system by using the following scheme based on the training sequence.

The time synchronization of the OFDM communication system is realized based on the training sequence, and a repetitive training structure, for example, a preamble structure specified in the ieee802.16d protocol, is mostly adopted in the time domain, and the time synchronization is realized by searching a correlation peak-to-peak value generated by correlation operation of the repetitive training structure. As shown in FIG. 1, the training sequence is composed of two identical data blocks, each of which has a length of N/2, where N is the length of the training sequence. Assume that the received signal samples are r_nThe operations of conjugate, square and demod are respectively operated by the operation sign (g)^*、(g)²And | g |)²Meaning "CP" denotes a cyclic prefix, with a detection threshold (or threshold) of T_hHere, the timing metric (timing metric) m (d) can be expressed as:

$M\left(d\right)=\frac{{\left|P\left(d\right)\right|}^2}{{\left(R\left(d\right)\right)}^2}$

wherein P (d) and R (d) are respectively:

thus, by using the timing metric M (d) and the threshold T_hA comparison is made one by one to find the position where the FFT window starts, i.e. the time synchronization point. Wherein, in the scheme for realizing the time synchronization of the OFDM communication system based on the training sequence, the false alarm probability of the detection is required to be not more than the preset false alarm probability P_FAAssuming that the variance of the noise is s²Then, the detection threshold can be obtained by the following formula:

$T_h=\sqrt{-2s^2\ln \left(P_{\mathrm{FA}}\right)}$

wherein the noise variance s²Can be carried out through actual conditionsAnd (4) measuring.

As can be seen from the above scheme for implementing time synchronization of an OFDM communication system based on a training sequence, detecting whether a transmitter antenna has transmitted a signal is a primary task for implementing time synchronization of the OFDM communication system. However, the accuracy of time synchronization based on the training sequence is still not high, and especially in the case of low signal-to-noise ratio, it is impossible to detect the presence of the transmitted signal well, which results in low detection accuracy and high missed detection probability.

Disclosure of Invention

In view of this, the present application provides a time synchronization method and apparatus for an OFDM communication system, which are used to solve the problems that when the time synchronization of the OFDM communication system is implemented based on a training sequence, the accuracy is low, and especially, in the case of low signal-to-noise ratio, whether a transmitted signal exists cannot be detected well, which results in low detection accuracy and high missed detection probability.

The application provides a time synchronization method of an OFDM communication system, wherein the OFDM communication system comprises a distributed antenna system with at least 2 antennas, and the method comprises the following steps:

determining a timing metric for each antenna in the distributed antenna system based on signals received by the antenna, the timing metric comprising a plurality of timing metric values;

generating a detected antenna set and a missed detection antenna set by using each timing metric value of each antenna and a preset threshold value, and determining an initial time synchronization point of each antenna in the detected antenna set;

wherein the timing metric values of the antennas in the detected antenna set are greater than or equal to the threshold value, and each timing metric value of the antennas in the missed antenna set is less than the threshold value;

judging whether a preset cooperative cognitive time synchronization condition is satisfied, if so, taking an initial time synchronization point of each antenna in the detected antenna set as a respective final time synchronization point, otherwise, acquiring a false alarm probability of the distributed antenna system, judging whether the acquired false alarm probability is greater than or equal to a preset false alarm probability value, if so, taking the initial time synchronization point of each antenna in the detected antenna set as the respective final time synchronization point, otherwise, determining the final time synchronization point of each antenna in the missed antenna set, and determining the respective final time synchronization point according to a cooperative threshold value of each antenna in the detected antenna set and the respective initial time synchronization point;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

In the above method, preferably, the generating a detected antenna set and a missed-detection antenna set by using each timing metric of each antenna and a preset threshold, and determining an initial time synchronization point of each antenna in the detected antenna set includes:

respectively and sequentially retrieving the timing metric values of the antennas;

acquiring an antenna with a timing metric value larger than or equal to a preset threshold value, placing the antenna into a preset detected antenna set, and taking a time point corresponding to the timing metric value larger than or equal to the threshold value, which is retrieved for the first time by the placed antenna, as an initial time synchronization point of the placed antenna;

and obtaining antennas of which the timing metric values are all smaller than the threshold value, and placing the antennas into a preset missed detection antenna set.

In the above method, preferably, the determining a final time synchronization point of each antenna in the missed antenna set includes:

respectively reading the timing metric values of all the antennas in the missed detection antenna set in sequence;

acquiring an antenna with a timing metric value larger than or equal to a cooperative threshold value of the antenna, and taking a time point corresponding to the timing metric value which is read by the acquired antenna for the first time and is larger than or equal to the cooperative threshold value of the antenna as a final time synchronization point of the acquired antenna;

and determining the antennas of which the timing metric values are all smaller than the cooperative threshold value, and marking the determined antennas as the antennas which do not receive the signals.

In the above method, preferably, the determining the respective final time synchronization point according to the cooperation threshold and the respective initial time synchronization point of each antenna in the detected antenna set includes:

reading timing metric values before the timing metric values corresponding to the initial time synchronization points of the antennas in the detected antenna set in sequence;

searching the antenna of which the timing metric value before the timing metric value corresponding to the initial time synchronization point is greater than or equal to the cooperative threshold value of the antenna, and taking the time point corresponding to the timing metric value which is read by the searched antenna for the first time and is greater than or equal to the cooperative threshold value of the antenna as the final time synchronization point of the searched antenna;

and marking the antennas of which the timing metric values before the timing metric values corresponding to the initial time synchronization points are all smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as final time synchronization points of the antennas.

The present application also provides a time synchronization apparatus of an OFDM communication system including a distributed antenna system including at least 2 antennas, the apparatus including:

a timing metric determining unit, configured to determine a timing metric for each antenna in the distributed antenna system according to a signal received by each antenna, where the timing metric includes multiple timing metric values;

an initial time synchronization unit, configured to generate a detected antenna set and a missed-detection antenna set by using each timing metric of each antenna and a preset threshold, and determine an initial time synchronization point of each antenna in the detected antenna set;

wherein the timing metric values of the antennas in the detected antenna set are greater than or equal to the threshold value, and each timing metric value of the antennas in the missed antenna set is less than the threshold value;

a final time synchronization unit, configured to determine whether a preset cooperative cognitive time synchronization condition is satisfied, if so, take an initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, obtain a false alarm probability of the distributed antenna system, determine whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, take the initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, determine the final time synchronization point of each antenna in the missed antenna set, and determine its respective final time synchronization point according to a cooperative threshold of each antenna in the detected antenna set and its respective initial time synchronization point;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

In the above apparatus, preferably, the initial time synchronization unit includes:

a timing measurement retrieval subunit, configured to retrieve the timing measurement values of each antenna once;

the initial time synchronization subunit is used for acquiring an antenna with a timing metric value greater than or equal to a preset threshold value, placing the antenna into a preset detected antenna set, and taking a time point corresponding to the timing metric value greater than or equal to the threshold value, which is retrieved for the first time by the placed antenna, as an initial time synchronization point of the placed antenna;

and the missed detection set generation subunit is used for acquiring antennas of which the timing metric values are all smaller than the threshold value and placing the antennas into a preset missed detection antenna set.

The above apparatus, preferably, the final time synchronization unit includes:

the first judgment subunit is used for judging whether a preset cooperative cognitive time synchronization condition is established or not, if so, triggering the first final synchronization subunit, and otherwise, triggering the false alarm probability acquisition subunit;

the first final synchronization subunit is configured to use the initial synchronization time point of each antenna in the inspected antenna set as its respective final time synchronization point;

the false alarm probability obtaining subunit is configured to obtain a false alarm probability of the distributed antenna system, and trigger the second determining subunit;

the second judging subunit is used for judging whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, the first final synchronization subunit is triggered, otherwise, the second final synchronization subunit is triggered;

the second final synchronization subunit is configured to determine a final time synchronization point of each antenna in the missed antenna set, and determine a respective final time synchronization point according to a cooperation threshold of each antenna in the detected antenna set and a respective initial time synchronization point thereof;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

The above apparatus, preferably, the second final synchronization subunit includes:

and the first synchronization module is used for respectively reading the timing metric values of all the antennas in the missed antenna set in sequence, acquiring the antennas with the timing metric values larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the acquired antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as final time synchronization points of the acquired antennas, determining the antennas with the timing metric values smaller than the cooperative threshold value of the antennas, and marking the determined antennas as the antennas which do not receive the signals.

And the second synchronization module is used for respectively reading timing metric values before the timing metric values corresponding to the initial time synchronization points of all the antennas in the detected antenna set in sequence, searching the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the searched antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as final time synchronization points of the searched antennas, marking the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as the final time synchronization points of the antennas.

It can be known from the above solutions that, in the time synchronization method and apparatus for an OFDM communication system provided in the present application, timing metrics of antennas in a distributed antenna system are determined, a detected antenna set and a missed antenna set are generated by using the timing metrics of the antennas and a preset threshold, an initial time synchronization point of each antenna in the detected antenna set is determined, and then when a preset cooperative cognitive time synchronization condition (the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2) is satisfied, the initial time synchronization point of each antenna in the detected antenna set is taken as its respective final time synchronization point, otherwise, it is determined whether a false alarm probability of the distributed antenna system is greater than or equal to a preset false alarm probability value, if so, the initial time synchronization point of each antenna in the detected antenna set is taken as its respective final time synchronization point, otherwise, determining a final time synchronization point of each antenna in the missed antenna set, and determining respective final time synchronization points according to the cooperative threshold and respective initial time synchronization points of each antenna in the detected antenna set, thereby realizing time synchronization of all antennas in the distributed antenna system, i.e. through cooperative cognition of the distributed antennas, the missed antennas can detect the transmitted signals to a certain extent, thereby reducing the missed detection probability of time synchronization and improving the correct detection probability of time synchronization. Meanwhile, the cooperative cognition of the detected antennas is utilized, the false alarm probability of the missed detection antennas is not more than the preset false alarm probability, and therefore the false alarm probability of each distributed receiving antenna is ensured.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings can be obtained by those skilled in the art without creative efforts.

Fig. 1 is a schematic structural diagram of a training sequence in a conventional scheme for implementing time synchronization of an OFDM communication system;

fig. 2 is a flowchart of a time synchronization method of an OFDM communication system according to an embodiment of the present application;

fig. 3 is a schematic distribution diagram of each antenna in a distributed antenna system according to an embodiment of the present application;

FIG. 4 is a partial flow chart of a preferred embodiment of the present application;

FIG. 5 is another flow chart of a preferred embodiment of the present invention;

FIG. 6 is a flow chart of another preferred embodiment of the present invention;

fig. 7 is a schematic structural diagram of a time synchronization apparatus of an OFDM communication system according to a second embodiment of the present application;

FIG. 8 is another schematic structural diagram of a second embodiment of the present application;

fig. 9 is another schematic structural diagram of the second embodiment of the present application.

Detailed Description

The technical solutions in the embodiments of the present application are clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

Referring to fig. 2, a flowchart of a time synchronization method of an OFDM communication system according to an embodiment of the present application is shown, where the OFDM communication system includes a distributed antenna system with at least two antennas, and the method may include the following steps:

step 201: determining a timing metric for each antenna in the distributed antenna system based on signals received by the antenna, the timing metric comprising a plurality of timing metric values.

It should be noted that the distributed antenna system includes at least two antennas, as shown in fig. 3, which is a schematic distribution diagram of each antenna in the distributed antenna system. The signals received by each antenna in the distributed antenna system refer to the transmitted signals received by the receiving antenna of the receiver of the OFDM communication system after the transmitted signals are transmitted by the transmitting antenna of the transmitter of the OFDM communication system, where the receiving antenna is a distributed antenna, that is, the transmitted signals are received by the distributed antenna. For example: the transmitter may be a handset user or a base station, and the receiver may be a base station equipped with distributed antennas or a plurality of handset users participating in collaboration. The content of the above signals is mainly training sequences transmitted by a receiver antenna and a transmitter antenna, and the structure of the above signals is shown in fig. 1.

Wherein the determining the timing metric of each antenna may be implemented by:

computing a timing metric M for the ith (i =1, …, M) antenna_i(d) I.e. by

$M_i\left(d\right)=\frac{{\left|P_i\left(d\right)\right|}^2}{{\left(R_i\left(d\right)\right)}^2}$

Wherein,

and M is the number of antennas in the distributed antenna system.

Step 202: generating a detected antenna set and a missed detection antenna set by using each timing metric value of each antenna and a preset threshold value, and determining an initial time synchronization point of each antenna in the detected antenna set;

and the timing metric value of the antenna in the detected antenna set is greater than or equal to the threshold value, and each timing metric value of the antenna in the missed antenna set is smaller than the threshold value.

The above step 202 can be implemented by:

and comparing each timing metric value of each antenna with a preset threshold value in sequence respectively to generate a detected antenna set and a missed detection antenna set, and determining an initial time synchronization point of each antenna in the detected antenna set.

Step 203: judging whether a preset cooperative cognitive time synchronization condition is established or not, if so, executing a step 204, otherwise, executing a step 205;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

It should be noted that, the fact that the missed-detection antenna set is empty means that the number of antennas in the missed-detection antenna set is less than 1. Namely: after the step 202, or the number of antennas in each of the detected antenna set and the missed antenna set, when the number of antennas in the missed antenna set is less than 1 or the number of antennas in the detected antenna set is less than 2, the step 204 is executed, otherwise, the step 205 is executed.

Step 204: and taking the initial time synchronization point of each antenna in the detected antenna set as the respective final time synchronization point, and ending the time synchronization of each antenna in the detected antenna set.

Step 205: and acquiring the false alarm probability of the distributed antenna system.

Wherein, the step 205 may specifically include:

determining the number of antennas of the distributed antenna;

calculating the false alarm probability of the distributed antenna system according to the number of the distributed antennas:

$P_{\mathrm{FA},C}=\frac{M!}{M_{\det }!(M-M_{\det })!}{\left(P_{\mathrm{FA}}\right)}^{M_{\det }}{(1-P_{\mathrm{FA}})}^{M-M_{\det }}$

wherein, P_FA,CIs the false alarm probability, M, of the distributed antenna system_detAnd M is the number of antennas in the checked antenna set, and M is the number of antennas in the distributed antenna system.

Step 206: and judging whether the acquired false alarm probability is greater than or equal to a preset false alarm probability value, if so, executing the step 204, otherwise, executing the step 207.

Step 207: and determining a final time synchronization point of each antenna in the missed antenna set, and determining respective final time synchronization points according to the cooperative threshold value of each antenna in the detected antenna set and the respective initial time synchronization points.

In step 207, determining the final time synchronization point according to the cooperation threshold and the initial time synchronization point of each antenna in the detected antenna set is an optimization process of the time synchronization points of the antennas in the detected antenna set.

It can be known from the foregoing solutions that, in the time synchronization method for an OFDM communication system provided in this embodiment of the present application, timing metrics of antennas in a distributed antenna system are determined, a detected antenna set and a missed antenna set are generated by using the timing metrics of the antennas and a preset threshold, an initial time synchronization point of each antenna in the detected antenna set is determined, and then when a preset cooperative cognitive time synchronization condition (the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2) is satisfied, the initial time synchronization point of each antenna in the detected antenna set is used as its respective final time synchronization point, otherwise, it is determined whether a false alarm probability of the distributed antenna system is greater than or equal to a preset false alarm probability value, and if so, the initial time synchronization point of each antenna in the detected antenna set is used as its respective final time synchronization point, otherwise, determining a final time synchronization point of each antenna in the missed antenna set, and determining respective final time synchronization points according to the cooperative threshold and respective initial time synchronization points of each antenna in the detected antenna set, thereby realizing time synchronization of all antennas in the distributed antenna system, i.e. through cooperative cognition of the distributed antennas, the missed antennas can detect the transmitted signals to a certain extent, thereby reducing the missed detection probability of time synchronization and improving the correct detection probability of time synchronization. Meanwhile, the cooperative cognition of the detected antennas is utilized, the false alarm probability of the missed detection antennas is not more than the preset false alarm probability, and therefore the false alarm probability of each distributed receiving antenna is ensured. In addition, in the embodiment of the present application, a cooperative cognitive technology is utilized to optimize and update the time synchronization point of the detected antenna, so that the time synchronization accuracy of the detected antenna is improved while the detected antenna provides cooperative processing for the missed antenna.

Preferably, the generating a detected antenna set and a missed detected antenna set in step 202, and determining the initial time synchronization point of each antenna in the detected antenna set may be implemented by:

respectively and sequentially retrieving the timing metric values of the antennas;

acquiring an antenna with a timing metric value larger than or equal to a preset threshold value, placing the antenna into a preset detected antenna set, and taking a time point corresponding to the timing metric value larger than or equal to the threshold value, which is retrieved for the first time by the placed antenna, as an initial time synchronization point of the placed antenna;

and obtaining antennas of which the timing metric values are all smaller than the threshold value, and placing the antennas into a preset missed detection antenna set.

Taking the ith antenna as an example in the distributed antenna system, the foregoing can be implemented by the following steps, as shown in fig. 4:

step 401: and acquiring a first timing metric value of the timing metric of the ith antenna as a current timing metric value.

Wherein i =1, 2, …, M is the number of antennas of the distributed antenna system.

Step 402: and judging whether the current timing metric value is greater than or equal to a preset threshold value, if so, executing step 403, otherwise, executing step 404.

It should be noted that the preset threshold of the ith antenna may be obtained by the following formula:

$T_{h,i}=\sqrt{-2s_i^2\ln \left(P_{\mathrm{FA}}\right)}$

wherein the variance of the noise

Can be measured by actual conditions, and P_FAIs a preset false alarm probability whereby the threshold is predetermined and can be set based on practical circumstances and experience.

Step 403: and placing the ith antenna into a preset detected antenna set, and taking a time point corresponding to the current timing metric value as an initial time synchronization point of the ith antenna.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

Step 404: and judging whether the current timing metric value is the last timing metric value of the ith antenna, if so, executing the step 405, otherwise, executing the step 406.

Step 405: and placing the ith antenna into a preset missed detection antenna set.

Step 406: and acquiring the next timing metric value of the current timing metric values as the current timing metric value, and returning to execute the step 402.

It should be noted that, other antennas in the distributed antenna system may all implement set attribution and initial time synchronization point determination through the scheme shown in fig. 4.

Preferably, the determining the final time synchronization of each antenna in the missed-detection antenna set in step 207 includes:

respectively reading the timing metric values of all the antennas in the missed detection antenna set in sequence;

acquiring an antenna with a timing metric value larger than or equal to a cooperative threshold value of the antenna, and taking a time point corresponding to the timing metric value which is read by the acquired antenna for the first time and is larger than or equal to the cooperative threshold value of the antenna as a final time synchronization point of the acquired antenna;

and determining the antennas of which the timing metric values are all smaller than the cooperative threshold value, and marking the determined antennas as the antennas which do not receive the signals.

Taking the ith antenna in the missed-detection antenna set as an example, the foregoing can be implemented by the following steps, as shown in fig. 5:

step 501: and acquiring a first timing metric value of the ith antenna timing metric in the missed antenna set as a current timing metric value.

Wherein i =1, 2, …, M_miss，M_missThe number of antennas in the missed antenna set is detected.

Step 502: and judging whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna, if so, executing step 503, otherwise, executing step 504.

It should be noted that, the cooperative threshold T of the ith antenna is_h,c,iDetermined by the expected probability of correct time synchronization, i.e. requiring:

wherein,

for measuring timing M_i(d) Probability density function of，P_b,correctThe probability requirement for correct time synchronization (estimated time synchronization point, i.e. FFT starting point, within the CP range of the OFDM symbol), for example, engineering requirement, is 0.9.

Step 503: and taking the time point corresponding to the current timing metric value as the final time synchronization point of the ith antenna, and finishing the time synchronization of the ith antenna.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

Step 504: and judging whether the current timing metric value is the last timing metric value of the ith antenna, if so, executing step 505, otherwise, executing step 506.

Step 505: marking the ith antenna as an antenna that did not receive the signal.

Step 506: and acquiring the next timing metric value of the current timing metric values as the current timing metric value, and returning to execute the step 502.

It should be noted that, all the other antennas in the missed-detection antenna set can achieve the determination of the final time synchronization point or the marking that the signal antenna is not received through the scheme shown in fig. 5.

Preferably, the step 207 of determining the respective final time synchronization point according to the cooperation threshold and the respective initial time synchronization point of each antenna in the detected antenna set includes:

reading timing metric values before the timing metric values corresponding to the initial time synchronization points of the antennas in the detected antenna set in sequence;

searching the antenna of which the timing metric value before the timing metric value corresponding to the initial time synchronization point is greater than or equal to the cooperative threshold value of the antenna, and taking the time point corresponding to the timing metric value which is read by the searched antenna for the first time and is greater than or equal to the cooperative threshold value of the antenna as the final time synchronization point of the searched antenna;

and marking the antennas of which the timing metric values before the timing metric values corresponding to the initial time synchronization points are all smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as final time synchronization points of the antennas.

Taking the ith antenna in the inspected antenna set as an example, the foregoing can be implemented by the following steps, as shown in fig. 6:

step 601: and acquiring a first timing metric value before a timing metric value corresponding to the initial time synchronization point of the ith antenna in the detected antenna set as a current timing metric value.

Wherein i =1, 2, …, M_detect，M_detectThe number of antennas in the inspected antenna set.

Step 602: and judging whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna, if so, executing step 603, otherwise, executing step 604.

It should be noted that, the cooperative threshold T of the ith antenna is_h,c,iDetermined by the expected probability of correct time synchronization, i.e. requiring:

wherein,

for measuring timing M_i(d) Probability density function of (1), P_b,correctThe probability requirement for correct time synchronization (estimated time synchronization point, i.e. FFT starting point, within the CP range of the OFDM symbol), for example, engineering requirement, is 0.9.

Step 603: and taking the time point corresponding to the current timing metric value as the final time synchronization point of the ith antenna, and finishing the time synchronization of the ith antenna.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

Step 604: and judging whether the current timing metric value is the last timing metric value before the timing metric value corresponding to the initial time synchronization point of the ith antenna, if so, executing the step 605, otherwise, executing the step 606.

Step 605: and taking the initial time synchronization point of the ith antenna as the final time synchronization point of the ith antenna.

Step 606: and obtaining the next timing metric value of the current timing metric values as the current timing metric value, and returning to execute the step 602.

It should be noted that, other antennas in the detected antenna set can achieve the determination of the final time synchronization point or the marking that the signal antenna is not received through the scheme shown in fig. 6.

Referring to fig. 7, which shows a schematic structural diagram of a time synchronization apparatus of an OFDM communication system according to a second embodiment of the present application, where the OFDM communication system includes a distributed antenna system having at least 2 antennas, the apparatus may include:

a timing metric determining unit 701, configured to determine a timing metric for each antenna in the distributed antenna system according to a signal received by each antenna, where the timing metric includes multiple timing metric values.

It should be noted that the distributed antenna system includes at least two antennas, as shown in fig. 3, which is a schematic distribution diagram of each antenna in the distributed antenna system. The signals received by each antenna in the distributed antenna system refer to the transmitted signals received by the receiving antenna of the receiver of the OFDM communication system after the transmitted signals are transmitted by the transmitting antenna of the transmitter of the OFDM communication system, where the receiving antenna is a distributed antenna, that is, the transmitted signals are received by the distributed antenna. For example: the transmitter can be a mobile phone user or a base station, and the receiver can be a base station or a mobile phone user. The content of the above signals is mainly training sequences transmitted by a receiver antenna and a transmitter antenna, and the structure of the above signals is shown in fig. 1.

Wherein, when determining the timing metric of each antenna, the timing metric determining unit 701 may be implemented as follows:

computing a timing metric M for the ith (i =1, …, M) antenna_i(d) I.e. by

$M_i\left(d\right)=\frac{{\left|P_i\left(d\right)\right|}^2}{{\left(R_i\left(d\right)\right)}^2}$

Wherein,

and M is the number of antennas in the distributed antenna system.

An initial time synchronization unit 702, configured to generate a detected antenna set and a missed-detection antenna set by using each timing metric of each antenna and a preset threshold, and determine an initial time synchronization point of each antenna in the detected antenna set;

and the timing metric value of the antenna in the detected antenna set is greater than or equal to the threshold value, and each timing metric value of the antenna in the missed antenna set is smaller than the threshold value.

The initial time synchronization unit 702 is connected to the timing metric determination unit 701.

A final time synchronization unit 703, configured to determine whether a preset cooperative cognitive time synchronization condition is satisfied, if so, take an initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, obtain a false alarm probability of the distributed antenna system, determine whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, take the initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, determine the final time synchronization point of each antenna in the missed antenna set, and determine its respective final time synchronization point according to the cooperative threshold of each antenna in the detected antenna set and its respective initial time synchronization point;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

It should be noted that the final time synchronization unit 703 is connected to the initial time synchronization unit 702. And the missed-detection antenna set is empty, which means that the number of antennas in the missed-detection antenna set is less than 1.

When obtaining the false alarm probability of the distributed antenna system, the final time synchronization unit 703 may be implemented in the following manner:

determining the number of antennas of the distributed antenna;

calculating the false alarm probability of the distributed antenna system according to the number of the distributed antennas:

$P_{\mathrm{FA},C}=\frac{M!}{M_{\det }!(M-M_{\det })!}{\left(P_{\mathrm{FA}}\right)}^{M_{\det }}{(1-P_{\mathrm{FA}})}^{M-M_{\det }}$

wherein, P_FA,CIs the false alarm probability, M, of the distributed antenna system_detFor the number of antennas in the inspected antenna set,m is the number of antennas of the distributed antenna system.

It can be known from the foregoing solution that, in the time synchronization apparatus of an OFDM communication system provided in the second embodiment of the present application, timing metrics of antennas in a distributed antenna system are determined, a detected antenna set and a missed antenna set are generated by using the timing metrics of the antennas and a preset threshold, an initial time synchronization point of each antenna in the detected antenna set is determined, and then when a preset cooperative cognitive time synchronization condition (the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2) is satisfied, the initial time synchronization point of each antenna in the detected antenna set is used as its respective final time synchronization point, otherwise, it is determined whether a false alarm probability of the distributed antenna system is greater than or equal to a preset false alarm probability value, and if so, the initial time synchronization point of each antenna in the detected antenna set is used as its respective final time synchronization point, otherwise, determining a final time synchronization point of each antenna in the missed antenna set, and determining respective final time synchronization points according to the cooperative threshold and respective initial time synchronization points of each antenna in the detected antenna set, thereby realizing time synchronization of all antennas in the distributed antenna system, i.e. through cooperative cognition of the distributed antennas, the missed antennas can detect the transmitted signals to a certain extent, thereby reducing the missed detection probability of time synchronization and improving the correct detection probability of time synchronization. Meanwhile, the cooperative cognition of the detected antennas is utilized, the false alarm probability of the missed detection antennas is not more than the preset false alarm probability, and therefore the false alarm probability of each distributed receiving antenna is ensured. In addition, in the second embodiment of the present application, the cooperative cognitive technology is utilized to optimize and update the time synchronization point of the detected antenna, so that the time synchronization accuracy of the detected antenna is improved while the detected antenna provides cooperative processing for the missed antenna.

Preferably, referring to fig. 8, it shows another schematic structural diagram of the second embodiment of the present application, wherein the initial time synchronization unit 702 includes:

a timing metric retrieving subunit 721, configured to retrieve the timing metric values of each antenna once;

the initial time synchronization subunit 722 is configured to acquire an antenna with a timing metric value greater than or equal to a preset threshold value, place the antenna into a preset detected antenna set, and use a time point corresponding to the timing metric value greater than or equal to the threshold value, where the timing metric value is first retrieved by the placed antenna, as an initial time synchronization point of the placed antenna.

It should be noted that the initial time synchronization subunit 722 is connected to the timing metric retrieval subunit 721.

And the missed detection set generation subunit 723 is configured to acquire antennas whose timing metric values are all smaller than the threshold value, and place the antennas into a preset missed detection antenna set.

It should be noted that the missed-detection set generation subunit 723 is connected to the initial time synchronization subunit 722.

Taking the ith antenna in the distributed antenna system as an example, the initial time synchronization unit 702 may be implemented as follows:

acquiring a first timing metric value of the timing metric of the ith antenna as a current timing metric value, wherein i =1, 2, …, M is the number of antennas of the distributed antenna system;

and judging whether the current timing metric value is greater than or equal to a preset threshold value, if so, placing the ith antenna into a preset detected antenna set, and taking a time point corresponding to the current timing metric value as an initial time synchronization point of the ith antenna, otherwise, judging whether the current timing metric value is the last timing metric value of the ith antenna, if so, placing the ith antenna into a preset missed detection antenna set, otherwise, acquiring the next timing metric value of the current timing metric value as the current timing metric value, and returning to judge whether the current timing metric value is greater than or equal to the preset threshold value.

It should be noted that the preset threshold may be obtained by the following formula:

$T_h=\sqrt{-2s^2\ln \left(P_{\mathrm{FA}}\right)}$

wherein the noise variance s²Can be measured by actual conditions, and P_FAIs a preset false alarm probability whereby the threshold is predetermined and can be set based on practical circumstances and experience.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

It should be noted that, in the distributed antenna system, other antennas can all implement set attribution and initial time synchronization point determination through the above-mentioned scheme.

Preferably, referring to fig. 9, it shows another schematic structural diagram of the second embodiment of the present application, wherein the final time synchronization unit 703 includes:

a first determining subunit 731, configured to determine whether a preset cooperative cognitive time synchronization condition is satisfied, if so, trigger the first final synchronizing subunit 732, otherwise, trigger the false alarm probability obtaining subunit 733;

the first final synchronization subunit 732, connected to the first determining subunit 731, configured to use the initial synchronization time point of each antenna in the detected antenna set as its respective final time synchronization point;

the false alarm probability obtaining subunit 733, connected to the first determining subunit 731, is configured to obtain a false alarm probability of the distributed antenna system, and trigger the second determining subunit 734;

the second determining subunit 734 is connected to the false alarm probability obtaining subunit 733 and the first final synchronization subunit 732, and is configured to determine whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, trigger the first final synchronization subunit 732, otherwise, trigger the second final synchronization subunit 735;

the second final synchronization subunit 735 is connected to the second determining subunit 734, and configured to determine a final time synchronization point of each antenna in the missed antenna set, and determine a respective final time synchronization point according to a cooperation threshold of each antenna in the detected antenna set and a respective initial time synchronization point thereof;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

Preferably, the second final synchronization sub-unit 735 includes:

and the first synchronization module is used for respectively reading the timing metric values of all the antennas in the missed antenna set in sequence, acquiring the antennas with the timing metric values larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the acquired antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as final time synchronization points of the acquired antennas, determining the antennas with the timing metric values smaller than the cooperative threshold value of the antennas, and marking the determined antennas as the antennas which do not receive the signals.

Taking the ith antenna in the missed-detection antenna set as an example, the first synchronization module may be implemented in the following manner:

obtaining the missing detection antenna setThe first timing metric value of the ith antenna timing metric is taken as the current timing metric value, where i =1, 2, …, M_miss，M_missThe number of antennas in the missed antenna set is used;

and judging whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna, if so, taking a time point corresponding to the current timing metric value as a final time synchronization point of the ith antenna, finishing the time synchronization of the ith antenna, otherwise, judging whether the current timing metric value is the last timing metric value of the ith antenna, if so, marking the ith antenna as an antenna which does not receive the signal, otherwise, acquiring the next timing metric value of the current timing metric value as the current timing metric value, and returning to judge whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna.

It should be noted that, the cooperative threshold T of the ith antenna is_h,c,iDetermined by the expected probability of correct time synchronization, i.e. requiring:

wherein,

for measuring timing M_i(d) Probability density function of (1), P_b,correctThe probability requirement for correct time synchronization (estimated time synchronization point, i.e. FFT starting point, within the CP range of the OFDM symbol), for example, engineering requirement, is 0.9.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

It should be noted that, by using the above scheme, the other antennas in the missed antenna set can determine the final time synchronization point or not receive the mark of the signal antenna.

And the second synchronization module is used for respectively reading timing metric values before the timing metric values corresponding to the initial time synchronization points of all the antennas in the detected antenna set in sequence, searching the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the searched antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as final time synchronization points of the searched antennas, marking the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as the final time synchronization points of the antennas.

Taking the ith antenna in the inspected antenna set as an example, the second synchronization module may be implemented as follows:

acquiring a first timing metric value before a timing metric value corresponding to an initial time synchronization point of an ith antenna in the detected antenna set as a current timing metric value;

wherein i =1, 2, …, M_detect，M_detectThe number of antennas in the inspected antenna set;

and judging whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna, if so, taking a time point corresponding to the current timing metric value as a final time synchronization point of the ith antenna, finishing the time synchronization of the ith antenna, otherwise, judging whether the current timing metric value is the last timing metric value before the timing metric value corresponding to the initial time synchronization point of the ith antenna, if so, taking the initial time synchronization point of the ith antenna as the final time synchronization point, otherwise, acquiring the next timing metric value of the current timing metric value as the current timing metric value, and returning to judge whether the current timing metric value is greater than or equal to the cooperative threshold value of the ith antenna.

It should be noted that, the cooperative threshold T of the ith antenna is_h,c,iDetermined by the expected probability of correct time synchronization, i.e. requiring:

wherein,for measuring timing M_i(d) Probability density function of (1), P_b，correctThe probability requirement for correct time synchronization (estimated time synchronization point, i.e. FFT starting point, within the CP range of the OFDM symbol), for example, engineering requirement, is 0.9.

The time point corresponding to the current timing metric value refers to a time point of a sampling point in a training sequence corresponding to the current timing metric value.

It should be noted that, other antennas in the detected antenna set can achieve the determination of the final time synchronization point or the marking that the signal antenna is not received through the scheme shown in fig. 6.

Finally, it should also be noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The time synchronization method and apparatus for an OFDM communication system provided in the present application are described in detail above, and the above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. A method for time synchronization in an OFDM communication system, the OFDM communication system comprising a distributed antenna system having at least 2 antennas, the method comprising:

determining a timing metric for each antenna in the distributed antenna system based on signals received by the antenna, the timing metric comprising a plurality of timing metric values;

generating a detected antenna set and a missed detection antenna set by using each timing metric value of each antenna and a preset threshold value, and determining an initial time synchronization point of each antenna in the detected antenna set;

wherein the timing metric values of the antennas in the detected antenna set are greater than or equal to the threshold value, and each timing metric value of the antennas in the missed antenna set is less than the threshold value;

judging whether a preset cooperative cognitive time synchronization condition is satisfied, if so, taking an initial time synchronization point of each antenna in the detected antenna set as a respective final time synchronization point, otherwise, acquiring a false alarm probability of the distributed antenna system, judging whether the acquired false alarm probability is greater than or equal to a preset false alarm probability value, if so, taking the initial time synchronization point of each antenna in the detected antenna set as the respective final time synchronization point, otherwise, determining the final time synchronization point of each antenna in the missed antenna set, and determining the respective final time synchronization point according to a cooperative threshold value of each antenna in the detected antenna set and the respective initial time synchronization point;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

2. The method of claim 1, wherein the generating a detected antenna set and a missed-detected antenna set by using the timing metric values of the antennas and a preset threshold value, and the determining the initial time synchronization point of each antenna in the detected antenna set comprises:

respectively and sequentially retrieving the timing metric values of the antennas;

acquiring an antenna with a timing metric value larger than or equal to a preset threshold value, placing the antenna into a preset detected antenna set, and taking a time point corresponding to the timing metric value larger than or equal to the threshold value, which is retrieved for the first time by the placed antenna, as an initial time synchronization point of the placed antenna;

and obtaining antennas of which the timing metric values are all smaller than the threshold value, and placing the antennas into a preset missed detection antenna set.

3. The method of claim 1, wherein the determining the final time synchronization point for each antenna in the set of missed-detection antennas comprises:

respectively reading the timing metric values of all the antennas in the missed detection antenna set in sequence;

acquiring an antenna with a timing metric value larger than or equal to a cooperative threshold value of the antenna, and taking a time point corresponding to the timing metric value which is read by the acquired antenna for the first time and is larger than or equal to the cooperative threshold value of the antenna as a final time synchronization point of the acquired antenna;

and determining the antennas of which the timing metric values are all smaller than the cooperative threshold value, and marking the determined antennas as the antennas which do not receive the signals.

4. The method according to claim 1, wherein said determining respective final time synchronization points according to the cooperative threshold of each antenna in the detected antenna set and its respective initial time synchronization point comprises:

reading timing metric values before the timing metric values corresponding to the initial time synchronization points of the antennas in the detected antenna set in sequence;

searching the antenna of which the timing metric value before the timing metric value corresponding to the initial time synchronization point is greater than or equal to the cooperative threshold value of the antenna, and taking the time point corresponding to the timing metric value which is read by the searched antenna for the first time and is greater than or equal to the cooperative threshold value of the antenna as the final time synchronization point of the searched antenna;

and marking the antennas of which the timing metric values before the timing metric values corresponding to the initial time synchronization points are all smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as final time synchronization points of the antennas.

5. A time synchronization apparatus for an OFDM communication system, wherein the OFDM communication system includes a distributed antenna system having at least 2 antennas, the apparatus comprising:

a timing metric determining unit, configured to determine a timing metric for each antenna in the distributed antenna system according to a signal received by each antenna, where the timing metric includes multiple timing metric values;

an initial time synchronization unit, configured to generate a detected antenna set and a missed-detection antenna set by using each timing metric of each antenna and a preset threshold, and determine an initial time synchronization point of each antenna in the detected antenna set;

wherein the timing metric values of the antennas in the detected antenna set are greater than or equal to the threshold value, and each timing metric value of the antennas in the missed antenna set is less than the threshold value;

a final time synchronization unit, configured to determine whether a preset cooperative cognitive time synchronization condition is satisfied, if so, take an initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, obtain a false alarm probability of the distributed antenna system, determine whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, take the initial time synchronization point of each antenna in the detected antenna set as its respective final time synchronization point, otherwise, determine the final time synchronization point of each antenna in the missed antenna set, and determine its respective final time synchronization point according to a cooperative threshold of each antenna in the detected antenna set and its respective initial time synchronization point;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

6. The apparatus of claim 5, wherein the initial time synchronization unit comprises:

a timing measurement retrieval subunit, configured to retrieve the timing measurement values of each antenna once;

the initial time synchronization subunit is used for acquiring an antenna with a timing metric value greater than or equal to a preset threshold value, placing the antenna into a preset detected antenna set, and taking a time point corresponding to the timing metric value greater than or equal to the threshold value, which is retrieved for the first time by the placed antenna, as an initial time synchronization point of the placed antenna;

and the missed detection set generation subunit is used for acquiring antennas of which the timing metric values are all smaller than the threshold value and placing the antennas into a preset missed detection antenna set.

7. The apparatus of claim 5, wherein the final time synchronization unit comprises:

the first judgment subunit is used for judging whether a preset cooperative cognitive time synchronization condition is established or not, if so, triggering the first final synchronization subunit, and otherwise, triggering the false alarm probability acquisition subunit;

the first final synchronization subunit is configured to use the initial synchronization time point of each antenna in the inspected antenna set as its respective final time synchronization point;

the false alarm probability obtaining subunit is configured to obtain a false alarm probability of the distributed antenna system, and trigger the second determining subunit;

the second judging subunit is used for judging whether the obtained false alarm probability is greater than or equal to a preset false alarm probability value, if so, the first final synchronization subunit is triggered, otherwise, the second final synchronization subunit is triggered;

the second final synchronization subunit is configured to determine a final time synchronization point of each antenna in the missed antenna set, and determine a respective final time synchronization point according to a cooperation threshold of each antenna in the detected antenna set and a respective initial time synchronization point thereof;

and the cooperative cognitive time synchronization condition is that the missed antenna set is empty or the number of antennas in the detected antenna set is less than 2.

8. The apparatus of claim 7, wherein the second final synchronization subunit comprises:

the first synchronization module is used for respectively reading the timing metric values of all the antennas in the missed antenna set in sequence, acquiring the antennas with the timing metric values larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the acquired antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as the final time synchronization points of the acquired antennas, determining the antennas with the timing metric values smaller than the cooperative threshold value of the antennas, and marking the determined antennas as the antennas which do not receive the signals;

and the second synchronization module is used for respectively reading timing metric values before the timing metric values corresponding to the initial time synchronization points of all the antennas in the detected antenna set in sequence, searching the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being larger than or equal to the cooperative threshold value of the antennas, taking the time points corresponding to the timing metric values which are read by the searched antennas for the first time and are larger than or equal to the cooperative threshold value of the antennas as final time synchronization points of the searched antennas, marking the antennas with the timing metric values before the timing metric values corresponding to the initial time synchronization points being smaller than the cooperative threshold value of the antennas, and taking the initial time synchronization points of the marked antennas as the final time synchronization points of the antennas.

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