Hi.This week, we will focus on the radio interface of L. T. E..We will see how information is transmitted by radio between the eNodeBs and the UEs.
嗨。本周，我们将重点关注L. T. E的无线电接口。我们将看到如何通过无线电在eNodeB和UE之间传输信息。
The physics of the transmission induces constraints that we have to handle.传输的物理特性引发了我们必须处理的约束。
We also need to share the band-width between users of thesame cell and to interface with the core network.我们还需要共享同一小区的用户之间的带宽以及与核心网络的接口。
To manage all that, the radio interface of L. T. E. is structured into several layers.为了管理所有这些，L.T.E.的无线电接口被构造成若干层。
Each video of this week will present one of these layers.本周的每个视频都将展示其中一个图层。
Let’s start with the first one, the “physical layer”.让我们从第一个开始，即“物理层”。
The goal of this layer is to transfer binary messagesbetween two points; namely an eNodeB and a UE.该层的目标是在两点之间传输二进制消息; 即eNodeB和UE。
For that, L. T. E. uses modulation anderror correction techniques.为此，L。T. E.使用调制和纠错技术。
We will explain these notions.我们将解释这些概念。
What is a modulation?As you know, radio relies on electromagnetic waves to transmit information.什么是调制？如你所知，无线电依靠电磁波传输信息。
Waves can travel a long distance.波浪可以长途跋涉。
But, they are subject to disturbance.但是，他们受到干扰。
For example, their amplitude can be modified or they canbe subject to delays or to noise.例如，它们的幅度可以被修改，或者它们可能受到延迟或噪声的影响。
But, as long as these disturbances remain limited, thereceiver can detect that something has been sent.但是，只要这些干扰仍然有限，接收器就可以检测到已发送的东西。
“Modulation” is a transmission technique that consists ofusing a high-frequency bearer to transmit a message.“调制”是一种传输技术，包括使用高频承载来传输消息。
To do so, we modify the bearer; we say we “modulate” it,according to the information contained in the message to transmit.为此，我们修改了承载;我们说根据要传输的消息中包含的信息“调制”它。
If the receiver is able to recognize these modificationsthen it can retrieve the message.如果接收者能够识别这些修改，那么它可以检索该消息。
L. T. E. can use several kinds of modulations.L. T. E.可以使用几种调制。
We will look at two of them to understand theirdifferences and when to use each one.我们将看看其中两个，以了解它们之间的差异以及何时使用它们。
In L. T. E., the simplest modulation is BinaryPhase Shift key-ing, or BPSK.在L. T. E.中，最简单的调制是二进制相移键控或BPSK。
As its name indicates, it operates on the phase of the bearer.正如其名称所示，它运行在承载的阶段。
To transmit a 0, we don’t modify the signal.要传输0，我们不会修改信号。
But to transmit a 1, we modify the phase by afactor of Pi, which, in this case, is the same as takingthe opposite of the signal.但是为了传输1，我们将相位修改为系数Pi，在这种情况下，它与采用相反的信号相同。
This plot represents the modulated signal transportingthe information: 0,1,1,0.该图表示传输信息的调制信号：0,1,1,0。
You see that the phase is modifiedevery time the information changes and that it remainsconstant if the information does not change.您会看到每次信息更改时都会修改阶段，如果信息没有更改，则会保持不变。
This graphic is a representation of the statesthat the modulated signal can take.该图形表示调制信号可以采用的状态。
On this circle, the modulus represents the amplitudeand the argument represents the phase.在该圆上，模数表示幅度，参数表示相位。
But we could have more than two phase states.但我们可以拥有两个以上的阶段状态。
This is what QPSK does.这就是QPSK所做的。
QPSK stands for Quaternary Phase Shift Key-ing.QPSK代表第四相移键控。
It enables four phase states.它可以实现四相状态。
So we can transmit 2 bits at a time.所以我们一次可以传输2位。
For example we can take the convention that a phase of Pi/2 is used to transmit the couple of bits (0,0).例如，我们可以采用Pi / 2的相位来传输比特（0,0）的约定。
A phase of 3 Pi/2 corresponds to (0,1).3 Pi / 2的相位对应于（0,1）。
And so on: 5 Pi/2 means (1,1) and 7 Pi/2 means (1,0).依此类推：5 Pi / 2表示（1,1），7 Pi / 2表示（1,0）。
This notion is important.这个概念很重要。
That’s why we gave it a name.这就是我们为它命名的原因。
We call a “symbol” the period of time during which theinformation transmitted remains constant.我们将“符号”称为传输信息保持不变的时间段。
As you can see, in QPSK we can transmit 2 bits per symbol.如您所见，在QPSK中，我们可以为每个符号传输2位。
Whereas, in BPSK, we only transmit a single bit per symbol.然而，在BPSK中，我们每个符号只传输一个比特。
So, QPSK can transmit twice as much information as BPSK.因此，QPSK可以传输两倍于BPSK的信息。
It means that the data rate of QPSK is twice the one of BPSK.这意味着QPSK的数据速率是BPSK的两倍。
But, there’s no such thing as a free meal.
但是，没有免费餐这样的东西。
So there is necessarily a tradeoff.所以必然需要权衡。
Looking at these graphics, we see that in QPSK,the states are closer to each other than in BPSK.看看这些图形，我们看到在QPSK中，状态比BPSK更接近彼此。
When disturbances occur, the states willdrift from their initial position.当发生干扰时，状态将偏离其初始位置。
And if the disturbances are really important, the stateswill overlap and the receiver will not retrieve the correct information.如果干扰真的很重要，状态将重叠，接收器将无法检索正确的信息。
At this point, the thing to remember is that, to use anefficient modulation, we need a good quality channel.在这一点上，要记住的是，要使用有效的调制，我们需要一个高质量的通道。
In other words: We always have to compromise betweentransmission speed and the error rate.换句话说：我们总是要在传输速度和错误率之间做出妥协。
But keep in mind that channel conditions are changing continuously.但请记住，渠道条件正在不断变化。
For that reason, L. T. E. has to adapt the modulation in real time.因此，L。T. E.必须实时调整调制。
Now, let’s talk about errors.现在，我们来谈谈错误。
Errors are quite frequent in radio.无线电中的错误非常频繁。
For that, modern systems likeL. T. E. make use of error correction.为此，像L. T. E.这样的现代系统利用纠错。
This is based on mathematical encoding which is outsidethe scope of this course.这是基于数学编码，这超出了本课程的范围。
Simply put, the idea is to addredundant information when sending, to increase thechances of reconstructing the message upon reception.简单地说，想法是在发送时添加冗余信息，以增加在接收时重建消息的机会。
For example, this message has to be well understood.例如，必须很好地理解这个消息。
But an error can occur.但是可能会发生错误。
And I don’t want that.我不希望这样。
So I will repeat it three times.所以我会重复三次。
So the receiver can identify where the error is and correct it.因此，接收器可以识别错误的位置并进行纠正。
It does not always work; it uses upband-width, but it’s still quite effective.它并不总是有效; 它使用了带宽，但它仍然非常有效。
We call the ratio of theuseful information over total transmitted information “coding rate”.我们称有用信息与总传输信息的比率为“编码率”。
In L. T. E., this can vary between one third, forextensive correction, to almost 1 for nearly no correction.在L. T. E.中，对于大范围校正，这可以在三分之一之间变化，对于几乎没有校正几乎为1。
Up to now, we have seen that, depending on the propagation conditions,L. T. E. can use different modulations and different coding rates.到目前为止，我们已经看到，根据传播条件，L。 T. E.可以使用不同的调制和不同的编码率。
The combination of a given modulation with a given codingrate is called Modulation Coding Scheme (or in short MCS).
给定调制与给定编码率的组合称为调制编码方案（或简称MCS）。
Since channel conditions are changing continuously andindependently for each mobile,L. T. E. has to adapt the MCS of each mobile in real time.由于每个移动台的信道条件连续且独立地变化，因此。 T. E.必须实时调整每个移动设备的MCS。
What are the important points of this lesson?First, to use efficient modulation, we need good propagation conditions.
本课的重点是什么？首先，要使用有效的调制，我们需要良好的传播条件。
Otherwise, we have to switch to a more robust modulation.否则，我们必须切换到更强大的调制。
But in this case, the throughput will be lower.但在这种情况下，吞吐量会降低。
We can also modify the coding rate.我们还可以修改编码率。
Secondly, L. T. E. continuously adapts these parametersfor each terminal in real time.其次，L。T. E.不断地为每个终端实时调整这些参数。
转载于:https://www.cnblogs.com/sec875/articles/9953065.html
