天线越多路由就越好?
“天线越多覆盖越广,天线越多信号越强,总之天线越多路由就越好”——觉得很“常识”的朋友可以继续往下看正文了。
首先,大家也应该注意到了,老一代无线路由器的天线肯定不会超过一根,这里的“老一代”指的是802.11n协议以前的802.11a/b/g路由,老的54M产品就只有一根天线。这样的话,802.11n显然成了一条分水岭,也是从那时开始天线不再只有孤零零的一根(1t1r的150M是个例外),那到底是怎么一回事?这里我们就要提到一项11n协议之后才得到具体应用的多天线技术,也是无线通信领域一项非常重要的技术——MIMO(Multiple-Input Multiple-Output,多入多出)
电脑技术吧小编提示如果你不知道路由器的相关知识,建议先阅读路由器基础知识全接触
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接下来一页我们会进一步深入了解MIMO技术的神奇。
MIMO技术
搜各种百科资料IEEE802.11词条,我们可以读到,从802.11n开始,数据传输速率或者说承载的数据量有了很大的提升。首先,802.11n有了40MHz模式,然而按照之前的理论,它的发射范围应该因此降低一半才对,但事实上数据反而提升了一倍(70m),这又是怎么一回事?
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空时分组码(Space-Time Block Code,即STBC):在多天线上的不同时刻发送不同信息来提高数据可靠性。Alamouti码是空时分组码里最简单的一种。为了传输d1d2两个码,在两根天线1,2上分别发送d1,-d2*和d2,d1*。由于多径,我们假设两根天线的信道分别为h1h2,于是第一时刻接收端收到的信息r1=d1h1+d2h2,之后接收的信息r2=-d2*h1+d1*h2。接收到的这个2维方阵只要乘以信道,就可得到d1d2的信息了。看不懂没关系,总之呢就是Alamouti找到一组正交的码率为2×2矩阵,用这种方式在两根天线上发射可以互不影响;可以用一根天线接收,经过数学运算以后得到发射信息的方法。
其他的MIMO呢,在概念上可能比较好理解,比如2个发射天线t1t2分别对两个接收天线r1r2发射,那么相当于两拨人同时干活,速度提升2倍等等。但是实际实现起来一方面在硬件上需要多个接收天线,另一方面需要信道估计等通信算法,那都是非常复杂,并且耗时耗硬件的计算了。
讲上面两种技术实际上是MISO(Multiple-Input Single-Output)的方法,也是想从另外一个方面证明,天线多了不代表他们能一起干活。100年前人们就知道天线越多越好越大越好了,但是天才的Alamouti码1998年才被提出来多天线技术的802.11n协议2009年才开始应用。
20年前,人们用OFDM(Orthogonal Frequency Division Multiplexing,即正交频分复用,多载波调制的一种技术)对抗由于城市间或室内障碍太多造成的多径衰落,而如今我们已经开始利用多径来提高通信质量。这是技术上突飞猛进的发展,而不是简单的“想当然”就可以实现的。
写在最后
MIMO本身就是一个时变的、不平稳的多入多出系统。关于MIMO的研究,是一个世界性课题,留下的疑问还有很多,同样的问题学术上甚至也会出现不同的说法。不过,对于一般消费者大可不必深究,认清了开头我们讲的“误区”,知道路由天线是个“工具”,普通家庭双天线足以,选购时看清产品规格,不要被商家误导。
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这就要得益于MIMO技术了,刚才我们讨论的种种手段都是为了对抗恶劣的多径环境,但是多径有没有好的一面呢?事实上,MIMO也是基于多径的,我们称之为空间多样性。多天线的应用有很多种技术手段,这里简单介绍两种:波束成型(Beamforming)和时空分组码(主要介绍Alamouti'scode)。这两种技术的优点是不需要多个接收天线。尤其是Alamouti码,连信道信息都不用,只用数学运算就可以利用两根天线实现3dB的增益,很赞对吧。
而不需要多个接收天线的优点在于并不是所有设备都能装上多天线。为了避免旁瓣辐射(天线方向图上,最大辐射波束叫做主瓣,主瓣旁边的小波束叫做旁瓣),满足空间上的采样定理,一般以发送信号之一半波长作为实体的天线间距。无论是GSM信号1.8GHz,1.9GHz还是Wi-Fi信号的2.4GHz,我们暂取2GHz便于计算,半波长为7.5cm。所以,我们看到的路由器上天线的距离大多如此,也正是因此,我们很难在手机上安装多个天线。
波束成型(Beamforming):借由多根天线产生一个具有指向性的波束,将能量集中在想要传输的方向,增加信号传输品质,并减少与其他用户间的干扰。我们可以简单笼统地这样理解天线的指向性:假设全指向性天线功率为1,范围只有180度的指向性天线功率可以达到2。于是我们可以用4根90度的天线在理论上提高4倍的功率。波束成型的另外一种模式是通过信道估算接收端的方位,然后有指向性的针对该点发射,提高发射功率(类似于聚光的手电筒,范围越小,光越亮)。智能天线技术的前身就是波束成型。
空时分组码(Space-Time Block Code,即STBC):在多天线上的不同时刻发送不同信息来提高数据可靠性。Alamouti码是空时分组码里最简单的一种。为了传输d1d2两个码,在两根天线1,2上分别发送d1,-d2*和d2,d1*。由于多径,我们假设两根天线的信道分别为h1h2,于是第一时刻接收端收到的信息r1=d1h1+d2h2,之后接收的信息r2=-d2*h1+d1*h2。接收到的这个2维方阵只要乘以信道,就可得到d1d2的信息了。看不懂没关系,总之呢就是Alamouti找到一组正交的码率为2×2矩阵,用这种方式在两根天线上发射可以互不影响;可以用一根天线接收,经过数学运算以后得到发射信息的方法。
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其他的MIMO呢,在概念上可能比较好理解,比如2个发射天线t1t2分别对两个接收天线r1r2发射,那么相当于两拨人同时干活,速度提升2倍等等。但是实际实现起来一方面在硬件上需要多个接收天线,另一方面需要信道估计等通信算法,那都是非常复杂,并且耗时耗硬件的计算了。
讲上面两种技术实际上是MISO(Multiple-Input Single-Output)的方法,也是想从另外一个方面证明,天线多了不代表他们能一起干活。100年前人们就知道天线越多越好越大越好了,但是天才的Alamouti码1998年才被提出来多天线技术的802.11n协议2009年才开始应用。
20年前,人们用OFDM(Orthogonal Frequency Division Multiplexing,即正交频分复用,多载波调制的一种技术)对抗由于城市间或室内障碍太多造成的多径衰落,而如今我们已经开始利用多径来提高通信质量。这是技术上突飞猛进的发展,而不是简单的“想当然”就可以实现的。
写在最后
MIMO本身就是一个时变的、不平稳的多入多出系统。关于MIMO的研究,是一个世界性课题,留下的疑问还有很多,同样的问题学术上甚至也会出现不同的说法。不过,对于一般消费者大可不必深究,认清了开头我们讲的“误区”,知道路由天线是个“工具”,普通家庭双天线足以,选购时看清产品规格,不要被商家误导。
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先来看个例子,有人说,为什么我买了一个最新款的3天线支持802.11ac协议的无线路由器,结果信号强度、覆盖范围甚至连速度都没上去呢?天线不够?告诉你,300根也没用,检查一下你用的接受终端支不支持AC协议吧。比如你用的iPhone 3,这手机可只支持11a/b/g连11n都谈不上,那么即便是你给这它拆了加几根天线也没用。怎么解决?加装AC网卡或者换终端,总之加天线是没有用的。
为什么这样说?首先,Wi-Fi应用的环境是室内,我们常用的802.11系列协议也是针对这种条件来建立的。由于发射端到接收端之间存在各种各样的障碍物,收发时几乎不存在直射信号的可能。那怎么办?这个办法叫做多径传输,也叫多径效应。多径,从字面上也很好理解,就是把增加传输途径。
那么问题来了,既然是多径,传输的路程就有长有短,有的可能是从桌子反射过来的,有的可能是穿墙的,这些携带相同信息但是拥有不同相位的信号辗转最终一起汇集到接收端上。现代通信用的是存储转发的分组交换,也叫包交换,传输的是码(Symbol)。由于障碍产生不同的传输时延,就造成了码间干扰ISI(InterSymbolInterference)。为了避免ISI,通信的带宽就必须小于可容忍时延的倒数。
对于802.11a/b/g 20MHz的带宽,最大时延为50ns,多径条件下无ISI的传输半径为15m。在IEEE802.11协议中我们可以看到,这个值最大范围是35m,这是协议中还有误码重传等各种手段保证通信,并不是说有一点ISI就完全不能工作。这样的话你会发现,对于802.11a/b/g协议,即使加装再多的天线也没有任何意义。假设这些天线可以同时工作,反而会使多径效应更加恶劣。
总之,无线路由器的发射范围是这个IEEE802.11协议决定的,而非单纯的看天线。
小结
说了这么多,单天线路由、双天线路由、三线四线甚至更多究竟有没有区别?有,但对于实际使用过程中的影响并不大,这包括信号覆盖、信号强度,天线多速度快就更是无稽之谈了。抛开已经很少见的单天线,剩下的“多天线”都只是实现MIMO技术的“介质”或者说是“工具”,区别在于使用的架构不同而已:常见的双天线产品主要用1T2R或2T2R,三天线产品则用到的是2T3R或3T3R。
理论上,增加天线数量会减少信号覆盖盲点,但我们通过大量的评测证实,这种差异在普通家庭环境中完全可以忽略不计。而且,就像内置天线不输外置一样,三天线覆盖不如双天线的情况也绝非个例,说到底产品质量也是一个重要因素。至于信号强度和“穿墙”则取决于发射功率,这个东西工信部作过规定,不得高于20dBm(即100mW),“天线越多信号越强”也就不攻自破了。最后的结论就是,只要路由采用了有效的MIMO技术,无须在意天线数量。
接下来一页我们会进一步深入了解MIMO技术的神奇。
MIMO技术
搜各种百科资料IEEE802.11词条,我们可以读到,从802.11n开始,数据传输速率或者说承载的数据量有了很大的提升。首先,802.11n有了40MHz模式,然而按照之前的理论,它的发射范围应该因此降低一半才对,但事实上数据反而提升了一倍(70m),这又是怎么一回事?
这就要得益于MIMO技术了,刚才我们讨论的种种手段都是为了对抗恶劣的多径环境,但是多径有没有好的一面呢?事实上,MIMO也是基于多径的,我们称之为空间多样性。多天线的应用有很多种技术手段,这里简单介绍两种:波束成型(Beamforming)和时空分组码(主要介绍Alamouti'scode)。这两种技术的优点是不需要多个接收天线。尤其是Alamouti码,连信道信息都不用,只用数学运算就可以利用两根天线实现3dB的增益,很赞对吧。
而不需要多个接收天线的优点在于并不是所有设备都能装上多天线。为了避免旁瓣辐射(天线方向图上,最大辐射波束叫做主瓣,主瓣旁边的小波束叫做旁瓣),满足空间上的采样定理,一般以发送信号之一半波长作为实体的天线间距。无论是GSM信号1.8GHz,1.9GHz还是Wi-Fi信号的2.4GHz,我们暂取2GHz便于计算,半波长为7.5cm。所以,我们看到的路由器上天线的距离大多如此,也正是因此,我们很难在手机上安装多个天线。
波束成型(Beamforming):借由多根天线产生一个具有指向性的波束,将能量集中在想要传输的方向,增加信号传输品质,并减少与其他用户间的干扰。我们可以简单笼统地这样理解天线的指向性:假设全指向性天线功率为1,范围只有180度的指向性天线功率可以达到2。于是我们可以用4根90度的天线在理论上提高4倍的功率。波束成型的另外一种模式是通过信道估算接收端的方位,然后有指向性的针对该点发射,提高发射功率(类似于聚光的手电筒,范围越小,光越亮)。智能天线技术的前身就是波束成型。
空时分组码(Space-Time Block Code,即STBC):在多天线上的不同时刻发送不同信息来提高数据可靠性。Alamouti码是空时分组码里最简单的一种。为了传输d1d2两个码,在两根天线1,2上分别发送d1,-d2*和d2,d1*。由于多径,我们假设两根天线的信道分别为h1h2,于是第一时刻接收端收到的信息r1=d1h1+d2h2,之后接收的信息r2=-d2*h1+d1*h2。接收到的这个2维方阵只要乘以信道,就可得到d1d2的信息了。看不懂没关系,总之呢就是Alamouti找到一组正交的码率为2×2矩阵,用这种方式在两根天线上发射可以互不影响;可以用一根天线接收,经过数学运算以后得到发射信息的方法。
其他的MIMO呢,在概念上可能比较好理解,比如2个发射天线t1t2分别对两个接收天线r1r2发射,那么相当于两拨人同时干活,速度提升2倍等等。但是实际实现起来一方面在硬件上需要多个接收天线,另一方面需要信道估计等通信算法,那都是非常复杂,并且耗时耗硬件的计算了。
讲上面两种技术实际上是MISO(Multiple-Input Single-Output)的方法,也是想从另外一个方面证明,天线多了不代表他们能一起干活。100年前人们就知道天线越多越好越大越好了,但是天才的Alamouti码1998年才被提出来多天线技术的802.11n协议2009年才开始应用。
20年前,人们用OFDM(Orthogonal Frequency Division Multiplexing,即正交频分复用,多载波调制的一种技术)对抗由于城市间或室内障碍太多造成的多径衰落,而如今我们已经开始利用多径来提高通信质量。这是技术上突飞猛进的发展,而不是简单的“想当然”就可以实现的。
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写在最后
MIMO本身就是一个时变的、不平稳的多入多出系统。关于MIMO的研究,是一个世界性课题,留下的疑问还有很多,同样的问题学术上甚至也会出现不同的说法。不过,对于一般消费者大可不必深究,认清了开头我们讲的“误区”,知道路由天线是个“工具”,普通家庭双天线足以,选购时看清产品规格,不要被商家误导。