WO2018098791A1 - 适用于多跳无线传感器网络的时钟同步频率偏移估计方法 - Google Patents
适用于多跳无线传感器网络的时钟同步频率偏移估计方法 Download PDFInfo
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- the invention belongs to the technical field of wireless sensor networks, and relates to a clock synchronization frequency offset estimation method suitable for a multi-hop wireless sensor network.
- Time synchronization technology is an indispensable part of wireless sensor networks. It can provide a standard time reference for network-wide devices. It is the basic support for network data fusion, energy management, transmission scheduling and other services. In practical wireless sensor network applications, the network is usually composed of a large number of battery-powered nodes deployed in different environments, so energy consumption is a critical issue to consider. In addition, communication is the main source of energy consumption in wireless sensor networks, and an effective clock synchronization parameter estimation algorithm can reduce the number of resynchronizations in the network, prolong the network synchronization period, thereby reducing the communication overhead of the network and saving the energy of the nodes.
- the invention mainly considers the whole network synchronization of a multi-hop wireless sensor network adopting a layered synchronization structure.
- the synchronization between two points cannot be directly extended to the multi-hop wireless sensor network, because the delay error of the intermediate node is introduced into the whole network, which affects the clock synchronization performance of the entire network.
- an object of the present invention is to provide a clock synchronization frequency offset estimation method suitable for a multi-hop wireless sensor network, in which the node can correct the local clock and synchronize the synchronization of the node during each synchronization message interaction.
- the error is always maintained at a low level.
- the present invention provides the following technical solutions:
- a clock synchronization frequency offset estimation method suitable for multi-hop wireless sensor networks is divided into two methods: a forwarding method and a transparent transmission method.
- a forwarding method is adopted and the node requires synchronization accuracy
- the transparent transmission method is adopted.
- the time message in the multi-hop hierarchical synchronization network is propagated down by the root node step by step.
- the intermediate layer node synchronizes with the node to be synchronized as the node to be synchronized, and uses two timestamps after each interaction. Difference As the adjustment time of the node time, the local clock is corrected. After the interval is fixed, the source node participates in the synchronization of the next-level node until it is spread to the node to be synchronized, repeats the above process, and uses the statistical signal after multiple synchronization cycles.
- the method of processing estimates the relative frequency offset of the node;
- Transparent transmission method in the multi-hop hierarchical synchronization network, the processing time of the intermediate layer node is regarded as the additional introduced fixed delay And random delay
- the message passing process of the intermediate node is regarded as transparent until the node to be synchronized obtains the resynchronization time of the root node and utilizes the difference between the two timestamps.
- the calibration is completed, and the above process is repeated. After a plurality of cycles, the relative frequency offset is estimated.
- the source node A serves as a clock source reference node
- the L-th node A L is a node to be synchronized
- the synchronization step thereof include:
- the time message in the network is propagated step by step by the source node, and the child node receives the data frame sent from its parent node, and obtains the sending timestamp and the local receiving timestamp, which are respectively versus
- Node A L uses the difference between two timestamps Get the adjustment And at a later time Adjust your local time, ie subtract the adjustment from the current local time New time as node A L ;
- each child node in the multi-hop network updates the local clock in the new cycle, so a compensation amount ⁇ i needs to be added, assuming that the node starts from receiving the data frame of the parent node.
- X is a fixed value, so that it is t', then there is
- the synchronization step specifically includes:
- Node A L utilizes the difference between the two timestamps Get the adjustment And at a later time Adjust your local time, ie subtract the adjustment from the current local time New time as node A L ;
- the clock synchronization frequency offset estimation method applicable to a multi-hop wireless sensor network provided by the present invention, after a plurality of synchronization messages are exchanged by a node, the relative frequency offset is estimated at one time, and the synchronization precision is maintained when the crystal oscillator is unchanged. In a certain range, there is no need to perform synchronous message interaction for a period of time, thereby saving energy.
- the present invention provides a clock synchronization frequency offset estimation method suitable for a multi-hop wireless sensor network, in which each node can correct its local state during each clock synchronization period.
- the clock keeps its synchronization accuracy error at a low level, thereby overcoming the problem that the synchronization accuracy is uncontrollable during the operation of the synchronization algorithm.
- the present invention provides two methods for estimating the clock synchronization frequency offset of the forwarding method and the transparent transmission method.
- the former is suitable for multi-hop wireless sensor networks with high synchronization accuracy requirements
- the latter is suitable for multi-hop wireless sensor networks with general accuracy requirements.
- FIG. 1 is a schematic diagram of synchronization of a multi-hop node provided by the present invention
- FIG. 2 is a comparison diagram of a corrected and non-corrected type in a multi-hop network provided by the present invention
- FIG. 3 is a flowchart of a method for estimating a frequency offset of a multi-hop network according to the present invention
- FIG. 4 is a performance comparison diagram of a frequency offset estimation result of a forwarding method according to the present invention.
- FIG. 5 is a performance comparison diagram of the frequency shift estimation result of the transparent transmission method provided by the present invention.
- FIG. 1 is a schematic diagram of synchronization of a multi-hop node provided by the present invention.
- a root node A serves as a clock source reference node
- an L-th node A L is a node to be synchronized. They are the send timestamp and the receive timestamp respectively.
- the time message in the multi-hop hierarchical synchronization network is propagated down by the root node step by step.
- the intermediate layer node synchronizes with the node to be synchronized as the node to be synchronized, and uses two timestamps after each interaction. Difference As the adjustment time of the node time, the local clock is corrected. After the interval is fixed, the source node participates in the synchronization of the next-level node until it is spread to the node to be synchronized, repeats the above process, and uses the statistical signal after multiple synchronization cycles.
- the processing method estimates the relative frequency offset of the node. The specific steps are as follows:
- Node A 1 uses the difference Adjust local time, and Indicates the time after node A 1 is adjusted.
- node A 1 Since the node in the multi-hop network adopts a step-by-step synchronization mechanism, then node A 1 sends a time message to its child node A 2 , similar to the above process, there is
- a 1 represents a transmission time stamp of the node, the node A 2 after the new clock offset adjustment for
- each child node in the multi-hop network updates the local clock in the new cycle, so a compensation amount ⁇ i needs to be added, that is, in the second cycle, Have
- the new clock offset after the second cycle adjustment can be expressed as
- N (N> 1) cycle starts, the clock source node single-hop scenario different, since each child node in a multi-hop network will correct their time in each cycle, it is necessary to add a compensation amount ⁇ i .
- the time X is a fixed value, so that it is t'.
- N is the number of times of time synchronization.
- Transparent transmission method in the multi-hop hierarchical synchronization network, the processing time of the intermediate node level is regarded as the additional introduced fixed delay And random delay
- the message passing process of the intermediate node is regarded as transparent until the node to be synchronized obtains the current time of the root node and utilizes the difference between the two timestamps.
- the calibration is completed, and the above process is repeated. After a plurality of cycles, the relative frequency offset can be estimated.
- node A 2 is the node to be synchronized, and A is the root node.
- the specific synchronization steps are as follows:
- t 0 be the synchronization start time
- the transmission timestamp and the reception timestamp are respectively For the first cycle
- Node A 2 is then Use the difference between the two timestamps to adjust the local time, Indicates the time after node A 2 is adjusted, and Can get
- FIG. 2 is a comparison diagram of correction and non-correction in the multi-hop sensor network provided by the present invention, wherein FIG. 2(a) is a correction synchronization process, and FIG. 2(b) is a non-correction synchronization process.
- FIG. 3 is a flowchart of a method for estimating a frequency offset of a multi-hop sensor network according to the present invention.
- This embodiment provides a clock offset estimation method suitable for a multi-hop wireless sensor network. As shown in the figure, the method includes the following steps:
- C2 ⁇ C4 The node sends a local timestamp to the next-level child node to determine whether the node is the root node. If it is the root node, it enters the next stage C5. Otherwise, it enters C4 to use the adjustment amount to correct the local time.
- C5 ⁇ C6 The child node receives and records the timestamp, and uses the difference between the transmission timestamp and the reception timestamp to find the compensation amount ⁇ T.
- C7 ⁇ C8 Determine whether the node is the node to be synchronized. If yes, proceed to the next stage C9. Otherwise, proceed to process C3 to prepare the local timestamp for the next level node.
- C9 ⁇ C11 It is determined whether the number of synchronization cycles reaches the set value N. If it has been reached, the relative frequency offset of the node is estimated. Otherwise, the process proceeds to the process C3 to continue the message transmission process.
- Figure 4 shows a comparison of the performance of the forwarding method frequency offset estimation with its CRLB.
- the figure shows the mean square error curve and the curve is very close CRLB proved node 2
- a frequency offset estimate is valid, and the estimated near optimum estimate.
- FIG. 5 is a comparison diagram of the results of the transparent transmission frequency offset estimation and the corresponding CRLB performance provided by the present invention. It can be seen from the figure that the two curves are very close, and the mean square error curve is always above the CRLB curve, indicating that the estimation result is approximated as the least variance unbiased estimate.
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Abstract
本发明涉及一种适用于多跳无线传感器网络的时钟同步频率偏移估计方法,属于无线传感器网络技术领域。在本方法中,根据节点对同步精度的需求分为转发法和透传法两种:A、转发法,中间层节点在每次校正时间后,间隔同等的时间再发起向下一层次节点的同步交互;B、透传法,将中间层节点的处理时间视为额外引入的固定时延和随机时延,节点直接与根节点建立同步关系。本发明所提供的估计方法,在每次同步消息交互过程中,节点能够校正本地时钟,使节点的同步精度误差始终维持在较低水平,从而克服同步参数估计过程中同步精度不可控的问题。
Description
本发明属于无线传感器网络技术领域,涉及一种适用于多跳无线传感器网络的时钟同步频率偏移估计方法。
时间同步技术是无线传感器网络中不可或缺的部分,可为全网设备提供一个标准的时间参考,是网络进行数据融合、能源管理、传输调度等服务的基础支撑。在实际的无线传感器网络应用中,网络通常是由大量的电池供电的节点部署在不同环境中组成,因此能耗是需要考虑的至关重要的问题。此外,在无线传感器网络中通信是能量消耗的主要来源,而有效的时钟同步参数估计算法,可以减少网络内的再同步次数,延长网络同步周期,进而降低网络的通信开销和节省节点的能量。
在现有技术中,已提出一些时钟同步参数估计算法,通过对时钟同步的过程进行分析和建模,利用统计信号处理技术估计时钟偏移和频率偏移。然而,这些算法中节点的同步参数估计是在多个同步交互周期之后进行的,且节点在算法运行的过程中不能调整自己的本地时钟,因此在算法运行这段时间之内时钟同步精度无法保证,这在一些对同步精度要求很高的应用场景中是不可接受的,例如ISA100.11a等网络。另外,将两点间的同步扩展到多跳无线传感器网络中时,主要可分为分层式和代理式两种方案。
本发明主要考虑了采用分层式同步结构的多跳无线传感器网络的全网同步。此外,不能直接将两点间的同步扩展到多跳无线传感器网络中,因为这样会将中间节点的时延误差等引入到全网,进而影响全网的时钟同步性能。
发明内容
有鉴于此,本发明的目的在于提供一种适用于多跳无线传感器网络的时钟同步频率偏移估计方法,该方法在每次同步消息交互过程中,节点能够校正本地时钟,使节点的同步精度误差始终维持在较低水平。
为达到上述目的,本发明提供如下技术方案:
一种适用于多跳无线传感器网络的时钟同步频率偏移估计方法,在该方法中,根据节点对同步精度的需求分为转发法和透传法两种,在节点对同步精度要求较高时,采用转发法,而节点对同步精度要求一般时,则采用透传法。
转发法:多跳分层同步网络中的时间消息由根节点依次逐级向下传播,中间层节点既作为待同步节点与上一级节点进行同步,每次交互后都利用两个时间戳的差值作为节点时间的调整量校正自己的本地时钟,间隔固定的时间后,又作为源节点参与下一级节点的同步,直到扩散至待同步节点,重复以上过程,经过多个同步周期后利用统计信号处理的方法估计出节点的相对频率偏移;
透传法:在多跳分层同步网络中,把中间层次节点的处理时间视为额外引入的固定时延和随机时延即将中间节点的消息传递过程看成透明的,直至待同步节点获得根节点的再同步时间并利用两个时间戳的差值完成校正,重复以上过程,经过多个周期后,估计出相对频率偏移。
进一步,本发明采用的转发法,当网络中第L级节点AL需要与源节点实现时间同步时,源节点A作为时钟源参考节点,第L级节点AL是待同步节点,其同步步骤包括:
3)从第N(N>1)个周期开始,多跳网络中的各个子节点在新的周期会更新本地时钟,因此需要加入一个补偿量μi,假定节点从接收父节点的数据帧开始到向下一级子节点发送数据帧这段时间X为固定值,令其为t',则有
4)重复步骤1)、2),利用数组存入时间戳,N个周期后,可以估计出相对频率偏移,公式如下:
进一步,本发明采用的透传法,其同步步骤具体包括:
3)重复步骤1)、2),利用数组存入时间戳,经过N个同步交互周期后,可估计出时钟频率偏移,公式如下
本发明的有益效果在于:
1、本发明提供的适用于多跳无线传感器网络的时钟同步频率偏移估计方法,节点通过多次同步消息交互后,一次性估计出相对频率偏移,在晶振不变时,使同步精度维持在一定范围内,在一段时间内无需再进行同步报文交互,从而节省能量。
2、与现有的多跳无线传感器网络时间同步方案相比,本发明提供的适用于多跳无线传感器网络的时钟同步频率偏移估计方法,在每个时钟同步周期,节点能够校正自己的本地时钟,使其同步精度误差维持在较低水平,从而克服同步算法运行过程中同步精度不可控的问题。
3、根据多跳无线传感器网络对时钟同步精度的需求,本发明提供转发法和透传法两种时钟同步频率偏移估计方法可供选择。前者适用于对同步精度要求较高的多跳无线传感器网络,后者适用于对精度要求一般的多跳无线传感器网络。
为了使本发明的目的、技术方案和有益效果更加清楚,本发明提供如下附图进行说明:
图1为本发明提供的多跳节点的同步示意图;
图2为本发明提供的多跳网络中校正与非校正式的对比图;
图3为本发明提供的多跳网络频率偏移估计方法流程图;
图4为本发明提供的转发法频率偏移估计结果性能对比图;
图5为本发明提供的透传法频率偏移估计结果性能对比图。
下面将结合附图,对本发明的优选实施例进行详细的描述。
转发法:多跳分层同步网络中的时间消息由根节点依次逐级向下传播,中间层节点既作为待同步节点与上一级节点进行同步,每次交互后都利用两个时间戳的差值作为节点时间的调整量校正自己的本地时钟,间隔固定的时间后,又作为源节点参与下一级节点的同步,直到扩散至待同步节点,重复以上过程,经过多个同步周期后利用统计信号处理的方法估计出节点的相对频率偏移,具体步骤如下:
由于该多跳网络中节点采用逐级同步机制,随后节点A1向其子节点A2发送时间消息,类似上述过程,有
对于两跳节点,第二个周期调整后的新的时钟偏移量可表示为
从第N(N>1)个周期始,与单跳场景中的时钟源节点不同,由于多跳网络中的各个子节点会在每一个周期校正自己的时间,因此需要加入一个补偿量μi。设节点从接收父节点的数据帧开始到向下一级子节点发送数据帧这段时间X为固定值,令其为t',
重复步骤A0、A1,利用数组存入时间戳,N个周期后,可以估计出相对频率偏移,公式如下:
为了验证结果的有效性,求出的克拉美罗下限(Cramer-Rao Lower Bound,CRLB)
透传法:在多跳分层同步网络中,把中间节点层次的处理时间视为额外引入的固定时延和随机时延即将中间节点的消息传递过程看成透明的,直至待同步节点获得根节点的当前时间,并利用两个时间戳的差值完成校正,重复上述过程,经过多个周期后,可估计出相对频率偏移。
对于两跳节点,节点A2为待同步节点,A为根节点,具体同步步骤如下:
两式相减得到节点A2调整后的新的时钟偏移量
对于第二个周期,节点A2的时间调整量
重复上述步骤,第i个周期的时间调整量为
根据频率偏移估计公式
图2为本发明提供的多跳传感器网络中校正与非校正式的对比图,其中,图2(a)为校正式同步过程,图2(b)为非校正式同步过程。
实施例:
图3为本发明提供的多跳传感器网络频率偏移估计方法流程图。本实施例提供了适用于多跳无线传感器网络的时钟偏移估计方法,如图所示,具体包括以下步骤:
C1:同步过程开始。
C2~C4:节点向下一级子节点发送本地时间戳,判断节点是否为根节点,若为根节点则进入下一阶段C5,反之则进入C4利用调整量校正本地时间。
C5~C6:子节点接收并记录时间戳,利用发送时间戳和接收时间戳的差值求出补偿量ΔT。
C7~C8:判断节点是否为待同步的节点,若是则进入下一阶段C9,反之则进入流程C3准备向下一级节点发送本地时间戳。
C9~C11:判断同步周期数是否达到设定值N,若已达到则估计出节点的相对频率偏移,反之则进入流程C3继续重复消息传递过程。
C12~C13:利用估计出的相对频率偏移补偿本地时钟,同步过程结束。
图4给出了转发法频率偏移估计结果与其CRLB的性能对比图。由图可知,均方误差曲线和CRLB曲线非常接近,证明节点A2的频率偏移估计是有效的,且估计性能接近最优估计。
图5为本发明提供的透传法频率偏移估计结果与对应的CRLB性能对比图。由图可知,两条曲线十分接近,且均方误差曲线始终位于CRLB曲线上方,说明估计结果近似为最小方差无偏估计。
最后说明的是,以上优选实施例仅用以说明本发明的技术方案而非限制,尽管通过上述优选实施例已经对本发明进行了详细的描述,但本领域技术人员应当理解,可以在形式上和细节上对其作出各种各样的改变,而不偏离本发明权利要求书所限定的范围。
Claims (3)
- 一种适用于多跳无线传感器网络的时钟同步频率偏移估计方法,其特征在于:在该方法中,根据节点对同步精度的需求分为转发法和透传法两种,在节点对同步精度要求较高时,采用转发法,而节点对同步精度要求一般时,则采用透传法;具体如下:转发法:多跳分层同步网络中的时间消息由根节点依次逐级向下传播,中间层节点既作为待同步节点与上一级节点进行同步,每次交互后都利用两个时间戳的差值作为节点时间的调整量校正自己的本地时钟,其中分别为相邻节点AL-1的发送时间戳和AL的接收时间戳,间隔固定的时间后,又作为源节点参与下一级节点的同步,直到扩散至待同步节点,重复以上过程,经过多个同步周期后利用统计信号处理的方法估计出节点的相对频率偏移;
- 根据权利要求1所述的适用于多跳无线传感器网络的时钟同步频率偏移估计方法,其特征在于:采用转发法,当网络中第L级节点AL需要与源节点实现时间同步时,源节点A作为时钟源参考节点,第L级节点AL是待同步节点,其同步步骤包括:3)从第N(N>1)个周期开始,多跳网络中的各个子节点在新的周期会更新本地时钟,因此需要加入一个补偿量μi,假定节点从接收父节点的数据帧开始到向下一级子节点发送数据帧这段时间X为固定值,令其为t',则有4)重复步骤1)、2),利用数组存入时间戳,N个周期后,估计出相对频率偏移,公式如下:
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