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December 14, 2009

上次说到,手机在通过基站覆盖区域的时候,基站之间要进行手机的交接,如果基站之间交接不好,就不能保证手机的通讯畅通,会发生掉线现象。

可是,现在的基站处理速度,应该是可以应付高速公路上面汽车的速度的。高速列车的速度,也不比汽车的速度快很多。那么,为什么会发生高速列车上面,手机经常掉线的事情呢?

一个原因,是基站的密度。新建的高速铁路,会有部分通过人烟稀少地区,那篇报道里面也提到,会经过隧道。隧道造价不菲,通常都是在高山峻岭里才不得不造,这种地方,基站的密度自然就比较小了。另外高山和隧道对信号的阻隔,也是一个因素。高速公路,特别是已经建好的高速公路周围,往往已经有很多基站,信号的覆盖就不再是问题。

还有一个原因,我相信也是不可忽略的,就是手机的密度。按照一般的安全驾驶规范,前后车距应该是4秒,也就是说,一个单向4车道的高速,每秒会有一辆车通过一个检查点。假设这个检查点,正好就是基站A和基站B交接手机的边界,并且每辆车里面有一个手机,那么每秒要交接一个手机,双向就是两个手机。

当然实际操作要远比这个复杂,因为交接不是瞬间完成的,所以交接不是在一个点完成,而是在一段路完成。不过我们这里就是估算一下比例。

假定火车速度和汽车一样快,都是200公里每小时,那么一秒通过检查点的火车长度,就是55.6米。和谐号的总长213.9米,共8节车厢,那么一秒就有两节车厢通过检查点。每个车厢座位大概是72人,如果满员的时候,在每个人都有至少一个手机的中国,一秒种通过检查点的,就是144个手机。

3 Comments »

  1. 分析得很对。

    还有一个重要因素是速度确实对无线接收机接收质量有影响。

    无线电波信号会发生反射和折射。所以手机收到的基站信号是多个通过多个路径传播信号的叠加。无线信道很复杂,这些信号的相对强弱随时都在变化。

    gsm系统中的编码设接收机的设计专门考虑了这个问题,才能够保证手机信号的接收。这个系统比较复杂,但是不变的一点是系统必须捕捉无线信道的主要特征并随时调整系统参数。

    高速移动的手机,无线信道的变化模式对于静止的手机有明显区别。所以系统处理起来就更困难。

    还有一个问题是多普勒频移:由于无线电波是“波”,接收信号的频率会随收发端的相对速度而变化。雷达用这个测距,但是放在手机这里,更增加了接收信号的变化复杂度。

    总之,高速度会增加无线信道的复杂程度,让接收机难于捕捉信道变化。

    这一点,加上博主指出的覆盖,交接,基站用户变化等问题,构成了移动手机系统的主要困难。新的3G和4G系统对这些东西都有考虑,移动接收也是系统测评的重要指标之一。但是对于一般消费者来说,服务商主要宣传的还是带宽。

    Comment by Song — December 15, 2009 @ 12:22 am

  2. 谢谢楼上的专业解答。的确,无线通信是非常复杂的,我没有想到的是,多普勒效应真的会影响到通信,毕竟因为手机移动而产生的频率变化(红移/紫移)是相当小的。

    去查了一下,载频的间隔是200kHz,的确很小。

    对于用户来说,是不太关心这些技术细节的,能够直接感受到的,一个是话音质量,另外就是带宽了。

    Comment by 美人她爹 — December 15, 2009 @ 7:22 am

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    Comment by coach outlet online — May 29, 2012 @ 12:54 am

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