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信息时代: 空时编码 (MIMO)

The Information Age: Space-Time Codes (MIMO)

1886年 德国物理学家海因里希·赫兹有个惊人的发现
In 1886, German physicist Heinrich Hertz, made a startling discovery.
他注意到在房间一侧产生的火花
He noticed that a spark generated on one side of a room,
会同时导致房间另一侧的金属间隙
would cause another spark to appear between a metal gap
产生另一个火花
on the other side of the room, at the exact same time,
即使它们之间没有相连
even though they were not connected.
这是因为电火花会产生一种看不见的能量波
This is because an electric spark creates an invisible wave of energy
这种波以球面形式向外传播
that propagates outwards in a spherical pattern.
当这种波传过任何金属 如附近的导线
And when this wave passes through any piece of metal, such as a nearby wire,
就会产生电振荡
it causes it to vibrate electrically,
并在导线中产生电流 即所谓的电磁感应
and generates an electric current in the wire, known as electrical induction.
后来人们发现这些实际上是不可见的光波 也叫无线电波
It was later discovered that these were actually invisible light waves, known as radio waves.
基于这一发现 人们发明了无线电报
This discovery led to the radio telegraph.
无线电报发射机结构很简单 仅仅是一个大功率电池
The transmitter is simple, it’s just a powerful battery,
只要按下一个按键 它就会通过天线发射电脉冲
which sends electrical pulses through an antenna, whenever a key is pressed.
每个电流脉冲都将产生无线电波
Each pulse of electrical current generates a radio wave,
在遥远的接收端是一个同样的线圈或天线
and far away on the receiving end is an identical loop of wire or antenna,
它连着一个叫接收机的设备来检测电流
which is connected to a device which can measure electrical current, known as a receiver.
直到上世纪80年代 我们需要一种更快的方式来传输越来越多的数字数据
By the 1980s, we needed a faster way to transmit a growing amount of digital data,
靠手动按按键来实现是不现实的
doing it by hand wasn’t practical.
用无线电波快速发送数字数据要分为两步进行
To send digital data rapidly using radio waves is a two-step process.
首先 发射机发出连续的无线电波 我们称之为载波
First, the transmitter sends out a continuous radio wave, we call it carrier wave.
这个可以利用天线发送交流电来实现
This can be done by sending an alternating electrical current through an antenna.
第二步是在载波上叠加信息
The second step is to send a message on top of the carrier wave.
这个可以通过改变发射机的电脉冲强度来实现
This is done by changing the strength of the electrical pulses at the transmitter
强度或高或低 这个过程被称为幅度调制
between high and low, known as amplitude modulation.
在接收端 这些振幅或高或低的电波
On the receiving end, these low versus high amplitude waves
都被转换成电流脉冲
are converted into pulses of electrical current,
其中低振幅的转换成0
and the low waves are converted into zeros,
高振幅的转换成1
and the high waves are converted back into ones.
这就是数字无线通信的基础
And that’s the basis of digital wireless.
要记得 天线将信号像声音一样发往四面八方:
And remember antennas send out signals in all directions, and just like the sound:
就像警铃响起时 该区域的每个人都能听到
when a loud siren rings, everyone in the area can hear it.
所以只需用一个大功率的发射机
So with a single powerful transmitter,
我们可以将信号发送给一定区域内的多个设备
we can send signals to many devices in a general area,
即使设备在移动 信号也能到达
and the signal will reach each device even as it moves around.
这是因为无线电波能够穿透
This works because radio waves can travel through
大多数的非金属表面 如墙壁
most nonmetallic surfaces, such as walls.
唯一能阻挡它们的是金属含量很高的表面
The only thing that blocks them are surfaces with high metal content,
例如金属结构或者岩石
such as metallic structures or rocks.
然而 当电波碰到金属表面
However when they hit a metallic surface,
它们会像光一样发生反射
they bounce just like light does.
所以信号可以通过反射到达接收设备
So the signal can arrive thanks to reflections,
这些反射被视为无线电回波
think of these as radio echoes.
这使得信号能更容易地从发射机到达接收机的每个角落
And this makes it easier for signals to reach around corners from transmitter to receiver,
这也是为什么我们几乎在任何地方都能收到手机信号
and it’s why we are used to getting a cell phone signal almost anywhere.
当然 在某些地方
But of course in certain locations,
设备和基站之间的位置关系是这样的
the positioning between the device and base station
你可能会没有信号
is such that your signal can get lost,
这可能是这些无线电回波带来的不良后果造成的
and this can be caused by the unwanted consequences of these radio echoes.
它们之间会相互干扰
They can interfere with each other.
最常见的情况是两个或更多的回波到达接收机
Most often what happens is two or more copies reach the receiver.
如同在水中一样 这些电波相互重叠 相互影响
And just like in water, these overlapping waves interact with each other,
叠加在一起形成第三个电波
adding together to create a third wave.
第三个电波是前两个电波的叠加
The third wave represents the addition of both waves.
如果前两个电波同时到达
If the waves arrive at the same time,
那么第三个电波就会被放大 形成一个很强的信号
then the third wave is amplified, resulting in a very strong signal,
也就是所谓的相长干扰 它是一种有益的干扰
known as constructive interference, it’s the good kind of interference.
如果前两个电波不同时到达 我们将有麻烦了
We run into a problem though, when the waves arrive at different times.
这种情况发生在两个信号在收发机间的传播距离略有不同的时候
This happens when two signals travel slightly different distances from transmitter to receiver,
因为它们的传播路径不同
because they follow different paths.
这将导致两个波在时间上发生偏移
This results in one wave being shifted in time compared to the other,
我们称之为相位偏移
we call this a phase shift.
如果两个信号到达后 其中一个恰好偏移了半个波周期
If two signals arrive, and one is shifted exactly by half a wave cycle,
它们会相互抵消 导致信号丢失
they cancel each other out, resulting in a lost signal.
这就叫做相消干扰 它是有害的干扰
This is known as destructive interference, it’s the bad kind of interference.
还需要考虑一种情况:
And there is one other case to consider:
当到达的信号叠加后形成一个
this is when the arriving signals combine to give a signal
偏移半波周期的信号 导致波形翻转
that is shifted by half a wave cycle, causing the wave to flip.
这也是一种有益干扰
And this is also a good kind of interference,
因为接收机只需要检测电波的高低电平
since receiver just needs to detect high versus low waves.
因此 相长干扰和相消干扰之间的平衡
And so it’s the balance between constructive and destructive interference
决定了接收信号的质量
that determines the quality of the received signal.
我们用衰落特性来表征信号质量
We express this using something called a fading characteristic,
记作字母H
represented with the letter H.
例如 如果只有一个信号到达 没有其它干扰
So for example, if a single copy of a signal arrives with no other interference,
说明没有发生衰落 我们就让H等于1
then there is no fading, we express this as H equals 1.
通常 相长和相消干扰间存在不平衡因素
Usually what happens is there is some other imbalance of constructive and destructive interference.
所以关键问题是:
So the key problem becomes:
在H等于0的情况下 我们如何应对?
how can we combat the situation when H equals 0?
上世纪90年代中期 一些来自信息理论协会的研究人员
In the mid 1990s, several researchers from the Information Theory Society,
提出了“空时编码”的概念
introduced the concept of “Space-Time codes”.
这个概念的提出是基于以下发现:
It was based on the following insight:
我们可以通过多次观测来减少
we can use multiple observations to decrease the chances
因相消干扰而导致数据丢失的机会
that we will lose data, due to the destructive fading.
首先 要知道当信号不好时
First, realize that when you have a bad signal,
附近的某人可能信号很不错
someone nearby may have a good one.
这是因为所处的环境不同 回波形式与衰落特性也就不同
And that’s because different environments experience different echo patterns or fading characteristics.
这就是为什么通常挪动手机后信号会变好
This is why moving your phone around will often result in better signals.
假如我们把第二根天线装在手机上
So imagine we connected a second antenna to a mobile phone,
两根天线间保持一定距离
and separated them by some distance.
假设接收机1经历的衰减为H1
And let’s say receiver one experiences fading H1,
也许它是完全相消的 即H1=0
maybe it’s perfectly destructive, so H1 equals 0.
接收机2经历的衰减是H2
And receiver two experiences fading H2,
此例中 它没有受到干扰 所以H2=1
and in this case, it’s a clean signal, so H2 equals 1.
现在对于接收信号来说 我们有更好的选择
And now we have a better chance of receiving the signal,
因为两个信道只要一个是好的 我们就能解调出数据
because just one of the two possible channels needs to be good, in order to recover the data.
比如 如果我们发送一个两比特的信息:01
So for example, if we are sending a two bit message: 01,
每根接收天线检测到的是原始信号乘以衰落特性的结果
each receiving antenna will measure the original signal multiplied by the fading characteristic.
接收机1会收到00
Receiver one will measure 00,
接收机2会收到01
receiver two will measure 01.
因此 如果其中一个信道 具有非零衰落特性
And so if one of the channels has a non-zero fading characteristic,
如例中的接收机2 我们就能够解调出数据
and in this case it’s receiver two, then we can recover the data.
这种方法的问题是 对于小型设备来说
The problem with this method is it’s not practical to
装配多根天线是不现实的
rely on multiple antennas for small devices.
问题是我们能否在基站或发射端
The question is can we instead use multiple antennas
安装多根天线
at the base station, or the transmit side,
来模拟多根接收天线带来的优势呢?
to mimic the advantages of multiple receiver antennas?
例如 加入第二根发射天线
For example, using a second transmit antenna,
两根天线相距一定距离 发射相同的信号
separated by a distance, sending the same signal.
和之前一样 由于这些信号传播路径各不相同
As before, because these signals travel unique paths,
它们经历的衰落也不同 分别为H1和H2
they will fade differently: H1 and H2.
这使得我们能够在接收端只用一根天线
And this gives us two chances to receive the signal correctly
却有两次正确接收信号的机会
on the receiving end, with a single antenna.
然而 要使系统工作 还有个小问题要解决:
However, there is one subtle problem to address to make this system work:
这是因为发射的两个信号会在接收天线处叠加
it’s due to the fact that the transmitted signals will combine at the receiver antenna.
所以从接收机的角度来看
So from the receiver’s perspective,
就像是只接收了一个信号
it looks like a single signal was received,
这个信号经历新的衰落特性 我们称其为H3
with a new fading characteristic, let’s call it H3,
它是H1与H2的叠加
which is the addition of H1 and H2.
因此 如果H1和H2恰好完全不同相
And so if H1 and H2 happened to be perfectly out of phase,
它们叠加后会使得H3=0 即信号相互抵消
they will combine to form H3 equals 0, erasing the signal,
即使两个信道单独看起来都很好
even though both of the channels were good on their own.
为了解决这个问题 人们提出了空时编码
To address this problem, space-time codes were invented,
我们引用Alamouti的著名结构来描述它的思想
and we illustrate the idea with a well known construction by Alamouti.
假设信息长度为两个符号:A和B
Let’s say the message is two symbols long: A and B.
他在每个时隙都发射两个不同的符号
He sends two different symbols at each time step.
所以发射机1将发射AB 发射机2将发射BA
So transmitter one will send AB, and transmitter two will send BA.
和前面一样 发射机经历的衰减特性为+1和-1
And as before, the transmitters experience fading characteristic plus one and minus one.
现在让我们想想在接收端会发生什么
Now let’s think about what would happen on the receiving end.
第一个时隙 接收机会检测到两个符号的叠加信号
At the first time step, the receiver would measure a blend of both symbols,
在此例中 叠加信号为B-A
and in this case: B minus A.
这时 接收机并不知道A和B的值分别是多少
And at this point, the receiver doesn’t know those individual values of A and B.
例如 如果接收机检测出0 那A和B是多少?
For example, if it measures 0, what’s A and B?
A和B可以同时为0或者同时为1
A and B could both be 0 or both be 1.
我们不能确定A和B 因为我们没有足够的信息
We can’t say for sure yet, we don’t have enough information.
因此 接收机需要更多的信息 这便是下个时隙要做的事
So the receiver needs more information and that’s what the next time step is for.
发射机1将发射B而发射机2将发射A
Transmitter one will send B and transmitter two will send A.
这次接收端将会检测到A-B
And on the receiving end, it will measure: A minus B.
假设这次的检测结果还是0
And let’s say this measurement works out to 0,
那么我们知道 A-B=0
so we know A minus B equals 0.
现在我们有两条信息
Now we have two pieces of information.
但我们仍然不知道A和B是
But that still doesn’t help us because we don’t know if
同时为1还是同时为0
A and B both equal 1 or A and B both equal 0.
问题就在于我们的第二次检测
The problem here is that our second measurement:
A-B总是可以从第一次检测中获得
A minus B can always be found from our first measurement.
只要将第一个式子乘以-1 就能得到A-B
Multiplying the first equation by minus one, gives A minus B.
所以两次传输实际上在以不同形式发送同样的信息
So our two transmissions effectively send the same information in different forms.
Alamouti让系统工作起来的聪明之处在于
And Alamouti’s clever trick to make this work was to
他将其中一次检测结果变成A+B
make one of these measurements: A plus B.
其中一个发射机需要在某一时隙发送
One of the transmitter needs to send the negative value or amplitude
某个符号的负值 或者说负振幅
of one of the symbols at one of the time slots.
Alamouti选择令发射机2的第一个符号翻转 添上负号
Alamouti chooses the first symbol from transmitter two to flip, using a negative.
因此 发射机1发送A和B
So transmitter one sends A and then B,
发射机2发送-B和A
transmitter two sends negative B and then A.
如此一来 接收机将能检测到两个观测量
And if we do that, the receiver will measure two observations:
B-A和A+B
B minus A and A plus B.
我们假设:A+B的检测结果是2
And let’s say for example: A plus B was measured at two,
那么接收机可以确定A=1和B=1
now the receiver knows for sure that A equals 1 and B equals 1.
而对于A+B的检测结果为0的情况
And in the other case where A plus B worked out to 0,
我们也可以肯定 A和B都等于0
then we know for sure that both A and B equal 0.
这就是解调出单个符号和信息的方法
And that’s how the individual symbols and the message are recovered.
这就像是我们在用两根天线接收一个符号一样
It works just like we received each symbol from two different receiving antennas,
但实际上我们只用了一根接收天线
but using a single receiving antenna.
这是个激动人心的发现 因为它使用一个简单的编码和一根额外的发射天线
This is an exciting insight because it increases the reliability of all receiving devices,
就提高了所有接收设备的可靠性
using a simple code, and one extra transmit antenna.
提高可靠性仅仅是多天线
Increasing reliability is just one of the
带来的众多优势中的一个
many advantages offered by multiple antennas,
这类系统现在被称为多输入多输出 或多入多出
such systems are now known as multi input multi output, or MIMO.
信息理论协会的研究人员对多入多出的理论和实践
And the researchers from the Information Theory Society have developed
进行了深入的研究
the theory and practice of MIMO,
并提出了一类叫空时码的新型编码方式
resulting in a new class of codes called Space-Time codes.
过去10年 这些系统一直在被使用
These systems have been in use for the past 10 years,
目前研究人员正在探索用于未来无线通信系统的
and new approaches for using MIMO systems are being explored
多入多出系统的新方法
for future wireless communication systems.

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视频概述

本视频介绍了空时编码提出的背景及其工作原理,最后简述了多天线系统的优势

听录译者

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翻译译者

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审核员

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视频来源

https://www.youtube.com/watch?v=cbD4NsZQKYw

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