CN1180591C - 四相移相键控信号的定时同步及相位/频率校正 - Google Patents

四相移相键控信号的定时同步及相位/频率校正 Download PDF

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CN1180591C
CN1180591C CNB008149623A CN00814962A CN1180591C CN 1180591 C CN1180591 C CN 1180591C CN B008149623 A CNB008149623 A CN B008149623A CN 00814962 A CN00814962 A CN 00814962A CN 1180591 C CN1180591 C CN 1180591C
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���ɵ¡�W�����˶�
唐纳德·W·贝克尔
El
威廉·E·L·利
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Abstract

用于数字解调QPSK信号的方法和装置包括利用多个预定的定时假设对数字取样的数据子帧再取样的第一部分。根据对再取样数据的分析确定定时偏移。接着根据定时估算对数字取样的数据子帧再取样。然后通过将复数I/Q对平方两次来消除再取样数据子帧的调制。对已消除了调制的数据进行线性调频Z变换以便将该数据移入频域。使用最高频谱功率确定频率偏移。确定相位偏移并且根据相位偏移和频率偏移对再取样的数据子帧去旋和消除相移。

Description

四相移相键控信号的定时同步及相位/频率校正
技术领域
本发明涉及无线通信系统，具体地说，本发明涉及四相移相键控(QPSK)信号的数字解调。
背景技术
采用跟踪环方案可以实现QPSK信号的数字解调。跟踪环产生再取样定时，并且还用于从输入的数据符号中消除频率和相位偏移，也称为残余频率和相位。通常，该系统一次对一个新的取样对进行操作。该系统采用反馈环路跟踪和校正输入数据流中的定时，频率和相位偏移。
这些反馈环路需要仔细设定的跟踪环增益。另外，它们需要反馈环路有时间将其锁定的捕获周期。这种已知的系统需要在每个新数据发送的开始使用相对长的报头，以便提供用于锁定的反馈环路时间。该已知系统通常还使用可能导致增加误差率的定位环路量度(通常在几个数据取样的范围)来估算定时，频率和相位。此外，由于跟踪环中使用的本地判定，在数字信号处理器芯片中实现该系统限制了该系统可运行的处理速度。
因此，需要一种克服这些缺陷的数字解调QPSK信号的方法和装置。
发明内容
用于数字解调QPSK信号的方法和装置可包括在多个预定定时的假设下对数字化取样的数据子帧(data burst)再取样的第一部分。确定每个假设的最大功率。将具有最大功率的假设用于内插再取样定时估算。然后用再取样定时估算对数据子帧再取样。然后通过对复数I/Q对(Z＝I+J*Q)两次平方来消除再取样数据子帧的调制。该Z4数据表示是Z数据的频率和相位四倍的频率和相位。然后对消除了调制的数据进行线性调频Z(chirp-Z)变换以便将数据转到频域。
然后确定线性调频Z数据变换的数据集的频谱功率。确定并平方第内插最高频谱功率。该内插值是残余解调频率的四倍。
通过用以负4倍旋转残余频率(四倍于在反向旋转的频率)的数据矢量对Z4数据去旋(derotate)来估算数据的相位。用于去旋的矢量的起始相位为0和幅度为1。将得到的去旋复数数据在该数据集的范围相加。得到的总和的反正切是所期望的起始相位的4倍。然后使用频率估算和相位估算对再取样数据去旋和移相，得到定时，频率和相位校正的再取样数据。
线性调频Z变换在用于估算QPSK解调中的残余频率时能够提供很多优点。QPSK信号的数字解调通常采用频率估算消除来自输入数据符号信息的残余频率。通常使用快速傅立叶变换(FFT)或一系列小重叠FFT执行该估算。
本发明的一个实施例将线性调频Z变换方案用于处理频率估算，它提供了三种优于FFT和直接傅立叶变换(DFT)方案的主要优点。
1)可以指定执行该估算的任意频率范围。而FFT则需要频率范围等于从-Fs/2到Fs/2的取样率(Fs)。
2)可以指定任意数量的频率估算点(并且因此产生任意频率估算解)。而FFT则需要频率估算点的数量等于输入点的数量(N)。因此在FFT中，频率估算分解定为Fs/N个。
3)与DFT处理(具有与线性调频Z变换估算相同的灵活性)相比较，线性调频Z变换估算器的运算对97点估算比DFT快5.6倍，对193点估算比DFT快9.7倍，对385点估算比DFT快17.8倍。
附图说明
本发明的这些和其它特征，目的及优点将通过下面结合附图的详细描述变得显而易见，图中相同的部分始终用相同的参考标号标出。
图1是表示基于卫星的通信系统的方框图；
图2是接收器的方框图；以及
图3是解释用于解调QPSK信号的过程和装置的示意图。
具体实施方式
图1是说明实施本发明的实例系统的方框图。图1中的系统通过卫星链路提供高速，可靠的因特网通信业务。
特别是，在图1中，内容服务器100耦合到因特网102，因特网102又耦合到中枢站(hub station)104，以使中枢站104能够从内容服务器100请求和接收数字数据。中枢站104还通过卫星106与多个远程单元108A-108N通信。例如，中枢站104通过前向上行链路110向卫星106发射信号。卫星106从前向上行链路110接收信号并且通过前向下行链路112将这些信号转发。前向上行链路110和前向下行链路112统称为前向链路。远程单元108A-108N监视包含前向链路的一个或多个信道，以便从中枢站104接收指定的远程单元并且广播消息。
用同样的方式，远程单元108A-108N通过反向上行链路114向卫星106发射消息。卫星106从反向上行链路114接收该信号并通过反向下行链路120将这些信号转发。反向上行链路114和反向下行链路120统称为反向链路。中枢站104监视包含反向链路的一个或多个信道，以便从远程单元108A-108N提取消息。
在示范系统的一个实施例中，每一个远程单元108A-108N与多个系统用户耦合。例如，在图1中，远程单元108A表示为与局域网116耦合，局域网116又耦合到一组用户终端118A-118N。用户终端118A-118N可以是多种局域网节点中的一种，如个人或网络计算机，打印机，数字计量读取设备等。当通过供用户终端118A-118N之一使用的前向链路接收消息时，远程单元108A通过局域网116将该消息前转到合适的用户终端118。同样，用户终端118A-118N可通过局域网116向远程单元108A发射消息。
在示范系统的一个实施例中，远程单元108A-108N为多个用户提供因特网服务。例如，假设用户终端118A是一台为了访问万维网(WorldWide Web)而执行浏览器软件的个人计算机。当浏览器接收到来自用户访问网页或嵌入对象的请求时，用户终端118A根据熟知的技术生成请求消息。用户终端118A同样利用熟知的技术通过局域网116将请求消息前传送到远程单元108A。根据该请求消息，远程单元108A通过反向上行链路114和反向下行链路120中的信道生成并发射一个无线链接请求。中枢站104通过反向链路接收到该无线链接请求。根据该无线链路请求，中枢站104通过因特网102将请求消息传递给合适的内容服务器100。
对此响应，内容服务器110通过因特网102将所请求的网页或对象传送到中枢站104。中枢站104接收所请求的网页或对象并且生成无线链接响应。中枢站通过前向上行链路110和前向下行链路112中的信道发射该无线链接响应。
远程单元108A接收该无线链接响应并通过局域网116将相应的响应消息传送到用户终端118A。这样，就在用户终端118A和内容服务器100之间建立起双向链路。
在象上面结合图1所述的系统中，远程单元趋于产生突发数据。突发数据的特征在于具有高峰值与平均的业务量比。这意味这数据块在很短的时间周期内传送，在相当长的空闲时间内中断。远程站的发射就是这里被称为数据子帧。
中枢站104向远程终端118A-N提供通信资源。中枢站104中的通信资源可以根据一种或多种已知技术量化成一系列通信资源中。中枢站104可以包含或实现一个或多个能够执行本发明功能的过程。这些过程，例如可以在诸如数字信号处理器，专用集成电路(ASIC)之类的一个或多个集成电路中实施，和/或可以在微处理器或诸如数字信号处理器之类的其它处理器执行的存储在中枢站104中的软件或固件程序中实施。
通信资源可以被分成一系列码分多址(CDMA)信道。在CDMA系统中，可以由近似正交序列的一系列伪随机定义信道。该系列中的每个序列定义可由远程单元用来与中枢站通信的分离的通信资源。作为替换，该系统可以使用时分多址(TDMA)时隙信道来再划分通信资源。在TDMA的系统中，向远程单元分配用于发射的时隙。通过将发射限制在分配的时隙内，远程单元可以共享中枢站提供的通信资源。另外，系统可以利用TDMA和频分多址(FDMA)的组合。在任何这些或其它多址技术中，在数据子帧中发射的数据可以被编码为四相移相键控符号集。
图2是用于通过反向下行链路120接收信号的中枢站104(图1)中接收器部分的方框图。另外，接收器部分可以用于通过前向下行链路112(图1)接收信号的远程单元108A-N或用于其它无线系统。接收器部分包括接收信号和将信号发射给模拟处理部分204的天线202。模拟处理部分204根据处理过程并使用本领域普通技术人员已知的技术对信号进行模拟处理，如下行变频，功率控制以及滤波。经过模拟处理后，该信号传递到模拟数字转换器206。模拟数字转换器206对模拟信号取样并完成带通滤波。数字正交调谐器(DQT)208从模拟数字转换器206接收数字取样的信号。DQT 208对希望的频率进行微调并且滤出其它滤波器。DQT 208还将数据速率改变成大约是符号速率的两倍。然后将数字化的QPSK信号传递到解调器300。作为替换，可以在提供给解调器300之前对数字化信号进行进一步处理。在一个实施例中，解调器300是执行所存储程序的数字信号处理器。
图3是说明解调数字化QPSK信号的方法和装置的方框图，可由解调器300执行。如上面指出的，图3表示的过程可以通过在处理器，例如数字信号处理器中运行的软件或固件来实现。图3中的每个方框可以作为软件或固件的一部分或作为硬件来实现。另外，方框所代表的功能可以组合成较大的软件，固件或硬件部分。
在方框310中，解调器300接收输入的数据子帧或已数字化取样的QPSK符号分组并将它们保存在存储单元。在一个实施例中，数据子帧以QPSK发射的符号速率的两倍进行取样。然后，如方框312所示，在四个分离的定时假设在再取样部分中对整个数据子帧进行再取样。在一个实施例中，将再取样实施为使用多相匹配滤波器的四个分离相位的四个滤波功能。四个滤波功能对应符号定时的偏差，例如，-1/2，-1/4，0，+1/4。另外，也可以使用不同的定时假设。可以通过系统模拟来确定一个指定系统的定时假设的最佳数量和它们的偏差。
如方框314A-D所示，对每个定时假设累加整个数据子帧的结果I2+Q2。整个数据子帧的结果I2+Q2表示每个定时假设的能量值。该能量值表示每个定时假设和数据子帧的相关性。换句话说，与实际输入数据子帧的定时偏差最相关的定时假设对数据子帧具有最高能量值。
如方框316所示，检查来自方框314A-D的四个能量以确定哪个定时假设具有最高相关性。一旦确定具有最高相关性的能量，则正交内插具有最高相关性的能量与它的两个邻近点以产生定时估算。定时估算也受方框318表示的多相位滤波器的颗粒性或分辨率制约。方框318表示的多相位滤波器利用来自方框316的定时估算对数据子帧再取样。该再取样以每个符号一个复数取样的有效取样速率产生了由复合I/Q取样组成的数据集。方框312，314和316表示的定时估算的确定和每个符号一个取样的和后续再取样可以减小剩余处理模块的计算负担，因为利用定时估算，从该点向前只处理在每个符号的一个取样所取样的数据。
接着，由方框320表示的复数QPSK符号的再取样数据子帧将消除频率偏差和相位偏差。首先，如方框324所示，将具有Z＝I+J*Q的复数I/Q对平方两次(Z4)来消除数据调制。该操作具有把所有复数数据置于相同象限，从而分解象限多重性的效果。Z4数据表示频率和相位是Z数据频率和相位的四倍。
频率偏差估算通常由方框322表示。如方框326所示，利用线性调频Z变换将得到的Z4数据的集合变换到频域。线性调频Z变换允许整个频谱的小的，高分辨率部分的变换。FFT不直接提供频谱的高分辨率估算。
本发明的实施例中使用的线性调频Z变换频率估算算法执行如下：
1)选择要执行的频率范围(freq_range)，并且选择输入点的数量(N)和估算输出点的数量(K)。通常是选择N和K以便能够以适当的FFT大小执行滤波器卷积步骤。因此，选择FFT_SIZE＝(N+K-1)作为2的最接近的乘方。
频率估算分辨率等于
Phi_0＝freq_range/(K-1).
用于线性调频Z轮廓的单位圆的起点是
Theta_0＝-(K-1)/2*Phi_0.
用于线性调频Z轮廓的单位圆的终点是
Theta_1＝(K-1)/2*Phi_0.
2)为处理过程中使用而设计和存储三个数据矢量。复数值A和W定义为：
A＝exp(j*2_PI*Theta_0)＝cos(2_PI*Theta_0)+j*sin(2_PI PI*Theta_0)和
W＝exp(j*2_PI*Phi_0)＝cos(2_PI*Phi_0)+j*sin(2_PI PI*Phi_0).
a)第一数据矢量vec_1(n)由N个点(n＝0...N-1)组成，定义为：
vec_1(n)＝A**(-n)*W**(n**2/2)＝
exp(-2_PI*n*Theta_0)*exp(j*2_PI*Phi_0*(n**2/2))＝
{cos(2_PI*n*Theta_0)-j*sin(2_PI*n*Theta_0)}*
{cos(2_PI*Phi_0*(n**2/2))+j*sin(2_PI*Phi_0*(n**2/2))}.
b)第二矢量是滤波器Filt_2(n)并且由FFT_SIZE个组成点。前N个点(n＝0...N-1)定义为：
Filt_2(n)＝W**(-(n**2/2))＝
exp(-j*2_PI*Phi_0*(n**2/2))＝
cos(2_PI*Phi_0*(n**2/2))-j*sin(2_PI*Phi_0*(n**2/2)).
最后的(K-1)个点(n＝N...(FFT_SIZE-1))定义为：
Filt_2(n)＝W**(-((FFT_SIZE-n)**2/2))＝
exp(-j*2_PI*Phi_0*((FFT_SIZE-n)**2/2))＝
cos(2_PI*Phi_0*((FFT_SIZE-n)**2/2))-j*sin(2_PI*Phi_0*((FFT_SIZE
-n)**2/2)).
该滤波器应对来自前一个矢量相乘的数据卷积，实现它的最快方法在频域。因此该滤波器通过快速傅立叶变换转换到频域并且作为FFT_SIZE频域滤波器值存储在存储器中。
c)包括K个点(k＝0...K-1)的第三数据矢量vec_3(k)定义为
vec_3(k)＝W**(k**2/2)＝
exp(j*2_PI*Phi_0*(k**2/2))＝
cos(2_PI*Phi_0*(k**2/2))+i*sin(2_PI*Phi_0*(k**2/2)).
3)现在线性调频Z变换将N个复数输入点与复数矢量vec_1相乘，将这N个点填塞到K-1个零中使其长度等于FFT_SIZE，并且对该数据集执行FFT。该频域数据与Filt_2的频域版本逐点相乘。该乘积是下一个反快速傅立叶变换(IFFT’d)，它完成vec_1与Filt_2预相乘的数据的快速卷积。现在该滤波后的数据输出与vec_3逐点相乘，产生初始数据集的线性调频Z变换。
如方框328所示，然后确定线性调频Z变换数据的数据集的频谱功率。在方框330中，确定最高频谱功率并且用其最近的两个邻近点正交内插以确定残余频率。该插入值是4倍于残余解调频率的最佳估算。
为估算相位偏差，如方框332所示，用以负4倍于残余频率旋转的数据矢量对在由方框324表示的过程中已消除了调制的复数数据对的集合去旋。负4倍于残余频率的值是由方框330所示的过程确定的。用于去旋的矢量的起始相位为0，幅度为1。然后在方框332所示的过程中将复数数据相加。在方框334中，确定得到的复数总和的反正切，并且表示所期望的相位偏差估算的4倍。
如方框336所示，分别用在方框334和330所示的过程中确定的负1倍的相位估算和负1倍的频率估算使用单位幅度的矢量对来自方框320的再取样数据子帧去旋，该单位幅度具有负1倍的相位估算的起始相位和负1倍的频率估算的旋转。如方框338所示，去旋产生为定时，频率和相位校正的再取样数据。
本领域的普通技术人员应该理解，可以以数字通信中通常使用的和本领域技术人员已知方式从消息分组的报头部分确定消息的开始和突发数据解调的象限锁定方面。
在不脱离本发明精神或实质特征的情况下可以用其它具体形式实施本发明。在各方面考虑的所述实施例仅是说明性的而不是限制性的，因此，本发明的范围由所附的权利要求表示，而不是由上述说明表示。落入权利要求等同物的含义和范围内的所有改变都包括在其范围内。

Claims (15)

1.用于解调数字取样数据子帧的解调器，包括：
再取样部分，用于在多个定时偏差对数字取样的数据子帧再取样；
评估部分，用于评估由再取样部分在多个定时偏差进行的再取样的相关性并根据与数字取样的数据子帧具有最佳相关性的定时偏差以确定定时估算；
多相滤波器，用于根据评估部分确定的定时估算对数字取样的数据子帧再取样并产生再取样的数据；
调制消除部分，用于从多相滤波器接收再取样的数据并通过对再取样数据的复数对两次平方以消除数据调制；
频率估算部分，用于接收来自调制消除部分的再取样数据并通过对再取样数据执行线性调频Z变换以及对数据集合的线性调频Z变换后的数据的频谱功率进行评估来确定频率偏移估算；
相位估算部分，用于接收来自调制消除部分的再取样数据以及接收来自频率估算部分的一个值，并利用从频率估算部分接收到的所述值为再取样数据确定相位偏移；和
相位/频率校正部分，用于接收来自调制消除部分的再取样数据，并通过利用具有一起始相位和一旋转的单位幅度的向量对再取样数据子帧去旋，从而调节再取样数据的相位和频率，其中起始相位是基于相位估算部分确定的相位估算的，旋转是基于频率估算部分确定的频率估算的。
2.根据权利要求1所述的解调器，其特征在于所述再取样部分包括多相滤波器。
3.根据权利要求1所述的解调器，其特征在于评估部分包括用于确定在多个定时偏移进行的再取样的功率的装置。
4.一种用于解调四相相移键控数据的数字取样数据子帧的解调器，该解调器包括：
第一再取样部分，用于在多个定时假设对数字取样的数据子帧再取样；
定时假设相关部分，用于为数据子帧的每个取样确定一个能量值，并利用所述能量值确定多个定时假设中的哪一个与实际的输入数据具有最佳相关性，以及使用该最高相关性根据所述能量值确定定时估算；
第二再取样部分，用于根据定时估算对数字取样的数据子帧再取样并由此产生复数I/Q取样；
数据调制消除部分，用于接收复数I/Q取样并将它们全部移入相同象限，由此产生Z数据；
线性调频Z变换部分，用于将Z数据变换到频域；
频率估算部分，用于通过确定并评估线性调频Z变换数据的数据集合的频谱功率利用调频Z变换的数据估算频率偏移；
相位估算部分，用于接收Z数据和来自频率估算部分的值，并利用从频率估算部分接收的所述值估算Z数据的相位偏移；
频率和相位校正部分，用于利用具有负1倍于相位估算的起始相位和负1倍于频率估算的旋转的单位向量对再取样数据去旋，其中，所述相位估算是由相位估算部分确定的，频率估算是由频率估算部分确定的。
5.根据权利要求4所述的解调器，其特征在于所述第一再取样部分包括多相滤波器。
6.根据权利要求5所述的解调器，其特征在于所述第一再取样部分和所述第二取样部分包括相同的多相滤波器。
7.一种解调数字取样的数据子帧的方法，包括：
利用多个定时假设对数字取样的数据子帧再取样；
通过针对每个定时假设对数据子帧的结果I2+Q2的累加确定每个定时假设的总能量；
根据定时假设的最高能量和它的至少一个邻近点确定定时估算；
根据该定时估算对数字取样的数据子帧再取样，以产生复数I/Q取样；
通过将复数I/Q取样移入相同象限，消除复数I/Q取样的调制，由此产生Z数据；
通过对Z数据进行线性调频Z变换来确定频率偏移，确定变换数据的数据集合的频谱功率并利用最高频谱功率进行内插来确定频率偏移估算；和
通过以负4倍于频率估算旋转的数据向量来对Z数据去旋确定该相位偏移。
8.根据权利要求7所述的方法，其特征在于通过多相滤波器利用多个定时假设对数字取样的数据子帧再取样。
9.根据权利要求7所述的方法，其特征在于利用多相滤波器根据定时估算对数字取样的数据子帧再取样。
10.一种解调数字取样的数据子帧的方法，包括：
利用多个定时假设对数字取样的数据子帧再取样；
通过计算每个定时假设的能量值，并内插与具有最高相关性的定时假设相关的能量值以及它的两个邻近点，确定定时估算；
根据该定时估算对数字取样的数据子帧再取样以获得再取样的数据；
通过将再取样数据变换到频域，确定变换数据的数据集合的频谱功率并利用最大频谱功率计算频率偏移估算来确定频率偏移估算；
通过利用由频率估算确定的数据向量对再取样数据去旋确定相位偏移估算；和
通过利用由确定的相位偏移和确定的频率偏移所确定的向量对再取样数据去旋来调节再取样数据的相位和频率。
11.一种解调四相相移键控数据的数字取样数据子帧的解调器，该解调器包括：
第一再取样装置，用于在多个定时假设对数字取样的数据子帧再取样；
用于通过针对每个定时假设对数据子帧的结果I2+Q2的累加确定每个定时假设的能量值的装置；
用于根据具有与数据子帧最大相关性的定时假设的能量值确定定时估算的装置；
第二再取样装置，用于根据该定时估算对数字取样的数据子帧再取样，以便产生复数I/Q取样；
用于通过将复数I/Q取样移入相同象限，消除复数I/Q取样的调制，由此产生Z数据的装置；
通过对Z数据进行线性调频Z变换确定频率偏移估算，确定变换的数据的数据集合的频谱功率，并使用最大的频谱功率执行内插，以便确定频率偏移估算的装置；以及
用于通过利用以负4倍于频率估算旋转的数据向量对Z数据去旋来确定相位偏移的装置。
12.根据权利要求11所述的解调器，其特征在于所述第一再取样装置包括多相滤波器。
13.根据权利要求11所述的解调器，其特征在于所述第一再取样装置和所述第二再取样装置包括相同的多相滤波器。
14.根据权利要求10所述的方法，其特征在于
对再取样数据的相位偏移估算的确定使用了以负4倍的频率估算旋转的数据向量。
15.根据权利要求10所述的方法，其特征在于
对再取样数据的相位和频率的调整使用了单位幅度的向量，所述的单位幅度具有负1倍的相位估算的起始相位和负1倍的频率估算的旋转。
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