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原视频来自: https://www./watch?v=MwCemE5N8pc。视频英文字幕根据声音自动产生，有拼写错误情况，翻译过程中做了纠正。 文中顺序依次是视频、英文原文和中文译文。
英文（根据视频整理） And this is the basic structure and when we move beyond that, what we find is that in one of the numerologies, the number of subframe is equal to 1, that is what we have said and that matches with the case when u is equal to 0, and this is what is the baseline structure matches with whatever is present in the earlier generation system. So, in that case, number of symbols per subframe would be equal to 14, because you have only one slot in the subframe. So, if you see the change over the picture that we have drawn from the previous, this is the generic picture. Now, we change over there is only one slot, but since number of symbols per slot is constant to 14, so the entire OFDM symbols stretch to 14. So, for obvious reason, the duration of this OFDM symbol has to be longer, and what we see is that the duration of OFDM symbol is 66.66us, or 66.67us, and that corresponds to the 15 KHz subcarrier bandwidth. What we have given over here is the cyclic prefix length 4.7 us for a normal CP, right, and so that it fixes into the picture very well, and this is the frame structure for u is equal to 0. So, when we move further, what we find is that on the frequency axis, so this is the frequency axis, because remember we are talking about time axis horizontally. So, this is the time axis, this is the time or OFDM symbol index l, time index. One can say OFDM symbol index, and this axis is the f axis, or one can also think a ssubcarrier index k going in this direction. So, each of these are different subcarriers, whose detailed picture or zoomed picture we had seen earlier, right? So that means, in one subframe duration there would be several such OFDM symbols, right? Because here we are saying in one numerology there are 14 OFDM symbols, there is only one slot present. So, there will be several such OFDM symbols, and each OFDM symbol is consisting of several subcarriers depending upon the size of the bandwidth that is available. This is very very important to note. So, this entire resource grid, we called it the resource grid. So, this entire resource. So, this also called probably not visible, so let me write it here instead of. So, the entire resource grid is available for allocation or doing modulation, and coding scheme allocation towards having higher and higher throughput，and group of 12 subcarriers, this is also important to note, group of 12 subcarriers forms a resource block, this is important. So, this is the minimum unit, that is addressable. Each of these elements are called resource elements, which we have defined earlier. Alright, so each of these are resource element, each resource element is one subcarrier of one OFDM symbol for an antenna port. So, if there are multiple layer, transmission will be having them on stacked on top of each other. So, if there is a single antenna system, then you have only one layer, and then it is basically the smallest unit, which can carry information. Now, for all reasons, we have also discussed this thing that you can’t address every single resource element. Then the overhead will be so high, there is you will actually the system is going to break down, it is going to actually some benefit in terms of transmission of data. So, they are grouped together, 12 subcarriers are grouped together. And you can see that the slots keep on varying. So, the total number of OFDM symbols, that are part of or resource element that are part of the resource block, keep on varying, because of the numerology, right? As we move beyond that to the second numerology, that is u goes from 0 to 1, we will find that there are 2 slots per OFDM symbol(subframe?). So, that is what we get over here from 1 get 2. So, there was 1, and from this 1， this changes to 2, right? and we get 2 such slots, and hence in each of the slots we are going to get 14 OFDM symbols. So, that means 28 in all in case of normal cyclic prefix, but if there is extended cyclic prefix, the value would be different, and what we have going to find is that since there are more OFDM symbols in the same duration, right? That means, it is now 1 ms, but the total number of OFDM symbols has become 28 from 14, right? We had 14, that has become 28. So, the symbol duration of each OFDM symbol must decrease, right? So, earlier it was 66, it will become 33, because it has to be half of that, and cyclic prefix length also reduces, because that is a combination we have said. So, now because of which, because you have reduced the OFDM symbol duration, if you look at the transition that happens in this frequency domain effect, because of time domain, we know this thing when we shrink the pulse, the bandwidth of the system increases. So, that is what is depicted over here, for ease of understanding, the subcarrier bandwidth increases，the resource elements, which were longer, if you look at this resource element, which was longer and narrower in bandwidth, will become shorter in time and wider in bandwidth. The resource grid definition remains the same. So, this again, and 12 subcarriers forming are source block that remains the same. Only thing that changes in number of elements in this in a slot remains the same, there is not variation, but number of such resource blocks available changes, because if the total system bandwidth remains the same, and one chooses two different numerologies, the number of subcarriers supported would obviously to be different. So, number of resource blocks (RBs) would change as per u, and bandwidth allocation and these are said by higher layer parameters and they can accordingly be chosen based on the appropriate operation So, here again, this is the resource element, so, the resource element definition remains the same，resource block definition remains the same, resource grid is the total number of resource elements that are available for scheduling，and hence the entire structure keeps on modifying based on the condition. So, now what we have is that there are more number of symbols in the time domain, which we have already seen and depicted graphically in this picture, to give a proper view of things. So, then we move to the next numerology, we find that there are 4 such slots, that is how the picture is going to happen and hence since there are 4 such slots, the symbol duration OFDM symbol duration, which was stretched here,will now reduce much of further, right? If the OFDM symbol duration reduces much further, then as depicted in the picture below it, the subcarrier bandwidth increases further, right? Resource elements, which were longer will become shorter in duration and they will wider in bandwidth, and there will be more number of resource elements in the time duration to be allocated. So, again depending upon how one wants the use the particular setup, one has to, one will set parameters accordingly the structure changes. The definition of resourc eblock remains the same, 12 subcarriers, but now one can have multiple such slots within that time frame to allocate. So depending upon the different typesof services that are being addressed by 5G, one can do this different types of service by 5G, one can do this different types of service multiplexing or service multiple access through such a flexible mechanism. So, this provides a very beautiful flexible physical layer, which was much needed for a long time,and only now time has come that people have agreed to this common frame work,that this is being allowed. Now we will discuss in the next lecture that why this flexible structure is necessary and the how this all started and probably took longer time to come on to this consensus, but one can easily understand that if one has this flexibility, onecan do a lot of things in terms of providing support to various kind of services, not only that there are various different, other reasons also to have this basic thinking to the picture. So, if one goes to the next higher level,that is the third one the fourth one, one would get 8 slots. As we transition from the 4 to 8, we find that the number of OFDM symbols increase, that is point number 1. The symbols duration shrinks further. So, to help the picture being readable, we did not reduce the picture over here, but we are changed the notation or the values of it, and one can clearly see that the subcarrier bandwidth increases even further, symbol duration becomes shorter, but the subcarriers become wider and hence one can support more number of such symbols in the time duration. So, there are various advantages and disadvantages of each combination, has to appropriately choose. Now, one thing I would like to point out at this instant of time is that if we track back the sequence of events that we are we have been looking at and so, what we are trying to see here is that transition and is very important to look at one very important issue, we had said earlier that the subcarrier bandwidths indicated over here should be narrow enough, so that they experience nearly flat fading,right, if this is the channel gain, right. And then when we transition from one numerology to anther numerology, then the subcarrier bandwidth decreases. So, as we transition from one numerology to another numerology, and we keep focusing on this particular section, which is about the subcarrier bandwidth, what will find is that the bandwidth becoming larger and larger. So, if the bandwidth becomes larger and larger, this fluctuation in the channels strength would appear in some cases such that this subcarriers are no longer experience in flat fading. So, this is something one has to understand and carefully choose the numerology or choice. Further, we will also see later that the cyclic prefix length becoming smaller and smaller as one changes from one numerology to another. So, what we have started of discussing is the numerology, where the subcarrier spacing was 4.7us in this particular picture, right? we had 4.7 us and from that number, this number is changing to the level of 0.57 and even 0.29 even if you go one level further, it will be 0.29 instead of 0.57. So, in that case one will find that the channel impulse response might be long enough and one is not able to use that particular numerology. So, one has to see this different effects, and choose an appropriate numerology of choice, one actually implement this particular system. So, moving further, in this particular slide we have combined the information in a little bit abstract manner, where we have removed the subcarrier picture to see only grid structure. So, this is easier to lookat. So, what we have is, for u equals to 1, we have the subframe structure and what we have over here is for a u equals to 0, the subframe structure and the total number of OFDM symbols is on this axis and the total number of subcarriers is on this axis. So overall we see that in both cases, number of subcarriers per resource block number of subcarriers per resource block is 12 here again, we see that number of subcarrier per resource block will again remain as 12. And, here the resource element is 1 subcarrier, here again the resource element is 1 subcarrier. Along with these complex modulations will go in case of u equals to 0, your subcarrier spacing is 15 KHz, in case u equals to 1, your subcarrier spacing is 30 KHz. Here what you see is that more number of subcarrier available, here lesser number of subcarriers are available within the same duration of time, that here there are more symbols are available, right where as, sorry here there are lesser symbols available, here there are more symbols available,because u equals to 1, the symbol duration decreases. So, more number ofsymbols would fit in. So, this is how the structure is going to change and this indicates the number of OFDM symbols that are present in the entire structure. So, here again we simply show what would structure look like in a simplistic picture when the subcarrier spacing is a certain value and here the subcarriers spacing is of larger value. So, when it is larger value as you can see this width is equal to this width. Here the subcarriers spacing is half compare tothe subcarrier spacing over here, but here the symbol duration is half compare to the symbol duration over here. So, these are just different pictures to show you the complete story. Further, because this frame structures are flexible, it provides an opportunity to provide multiple different services within the same frame. They can all be put independing upon the requirements. So, one of them what we see is that URLLC, which is the Ultra Reliable Low latency Communication service. So, when we talk about a low latency, we note that the symbol duration should be small, so that one can have shorter duration of transmit time. If the shorter duration oftransit time is there, then the overall round trip time will be reduced, and one can address lower latency applications. So, we stop this particular lecture here. We will continue on this in the next lecture. 译文: 这是基本结构，除此以外，我们发现在其中一个参数集中，子帧的数量等于1，这与u等于0的情况匹配，与上一代系统（4GLTE）的帧结构一样。因此，在这种情况下，每个子帧的符号数等于14，因为子帧中只有一个时隙。因此，如果您看到我们从前一张图片中获得的变化，这就是通用图片。现在，我们切换到只有一个时隙，由于每个时隙的符号数恒定为14，因此一个子帧的OFDM 符号数是14，OFDM符号的持续时间为66.66us或66.67 us，对应15 KHz子载波带宽。我们在这里给出的是正常CP的循环前缀长度4.7 us，这就是u等于0的帧结构。 图中的横轴是时间轴，在时间轴上OFDM符号的索引用l表示。纵轴是频率轴，OFDM符号正在频率轴上用k表示，k对应子载波索引。因此，这些都是不同的子载波，我们之前已经看过它们的详细图片或放大图片。因此，这意味着，在一个子帧持续时间内，将有几个这样的OFDM符号，这里，我们说的是在一个参数集中有14个OFDM符号，因此仅存在一个时隙。一个OFDM符号包括的子载波的数量取决于可用带宽的大小， 这一点非常重要。因此，我们将整个资源网格称为资源网格。 （Resource Grid这几个字可能大家看不见，因此我在这里写一下。） 因此，整个资源网格都可分配给用户和进行调制，而编码方案也支持越来越高的吞吐量。12个子载波构成一个资源块，这一点很重要，这是在资源调度中可寻址的最小单位。这些元素中的每一个都称为资源单元，我们之前已经对其进行了定义。所以这些都是资源单元，每个资源单元对应一个天线端口的一个OFDM符号的一个子载波。因此，如果在空口使用多层传输，则传输中多层的数据要堆叠在一个资源单元上（一个资源单元用来发送多个层的数据）。如果只有一个天线系统，那么您只有一层，那么它基本上就是可以承载信息的最小单元。 由于种种原因，调度中不能寻址每个资源单元，如果那么做则开销将很高，系统也不能支持。因此，我们把12个子载波作为一组。我们可以看到，采用不同的参数集时，一个子帧中时隙的数量可以变化，一个子帧的OFDM符号的总数也一直在变化。当我们使用第二个参数集时，u从0变到1，我们将发现每个子帧有2个时隙。每个时隙都有14个OFDM符号。 因此，这意味着在使用正常循环前缀的情况下，一个子帧总共有28个符号。但是如果扩展了循环前缀，则该值将有所不同，并且我们将发现，由于在相同的持续时间内有更多的OFDM 符号，现在1ms对应的OFDM符号的总数已经从14个变为28个，对不对？之前是14个，现在变成了28个。因此，每个OFDM符号的持续时间必须减少，对吗？因此，之前是66，它将变成33，因为它必须是它的一半，并且循环前缀长度也减少了。由于减少了OFDM符号的持续时间，如果您查看在频域中发生的变化，由于在时域上每个脉冲信号的持续时间都减少了，则系统带宽就必须增加。 因此，当子载波带宽增加了，每个资源单元在频域上变长，并在时域上变短。资源网格定义保持不变，还是12个子载波组成了一个资源块。一个时隙内的资源单元数量不变，但是总的可用的资源块的数量变了。如果总系统带宽保持不变，而人们选择了两种不同的参数集，子载波的数量显然将不同。因此，资源块（RB）的数量将根据u改变， 而系统带宽的分配是由高层参数指示。 因此，资源单元的定义保持不变，资源块定义保持不变，资源网格是可用于调度的资源单元的总数。到目前为止，整体结构根据条件做修改。我们在图中看到，现在时域上有了更多的符号。接下来我们转到下一个参数集，我们发现有4个这样的时隙。由于有4个这样的时隙， OFDM符号持续时间现在减少很多，对吧？如果OFDM符号持续时间进一步减少，则如下图所示，子载波带宽进一步增加，对吗？资源单元在时域持续时间变得更短，而它的带宽将变得更宽，并且在时域上有了更多数量的资源单元。 因此，根据我们想要的特定设置，我们必须相应地设置参数。资源块的定义保持不变，即12个子载波，但是现在可以在1ms内分配多个这样的时隙。因此，根据不同类型的服务， 5G可以通过这种灵活的机制，来对这些不同类型的服务做多路复用，或支持多种接入方式。5G提供了漂亮而灵活的物理层，很长时间以来都有这种需要，但直到现在3GPP才支持这种通用框架。 我们将在下一次讲座中讨论为什么需要这种灵活的结构，这一切是如何开始的？以及为什么需要更长的时间才能达成共识？但是可以很容易地理解，如果具有这种灵活性，则可以对各种服务提供支持。如果进入第四个参数集，将获得8个时隙，我们发现OFDM符号的数量增加了，符号持续时间进一步缩小。为了使图片可读，我们没有在此处缩小图片，而是更改了符号或它的值，并且可以清楚地看到子载波带宽进一步增加，符号持续时间变短。子载波变得更宽，因此在时域上可以支持更多这样的符号。每种组合都有各种优缺点，必须适当选择。 如果子载波带宽很窄，信道会受到平坦衰落的影响。当子载波带宽足够大时，信道不再受平坦衰落影响。我们还将看到，循环前缀长度随着一种从一种参数集向另一种参数集而变得越来越小。 在这张图片中子载波间隔为4.7us，其他参数集可以使用0.29us和0.57us，在某些情况下，人们会发现信道脉冲响应可能足够长，导致无法使用某个特定的参数集。因此，必须看到这种差异，并选择合适的参数集。 因为该帧结构是灵活的，所以它提供了在同一帧内提供多种不同服务的可能，可以根据需求设置。我们看到的其中之一就是URLLC（超可靠的低延迟通信服务）。因此，当我们谈论低延迟时，我们注意到符号持续时间应较小，以便缩短传输时间。如果传输时间缩短，那么总的往返时间将减少，可以支持需要较低延迟的应用程序。我们将在下次讲座中继续介绍这一点。 |
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