From the series:Understanding the 5G NR Standard
Marc Barberis, MathWorks
在此视频中,您将了解5G新无线电(NR)中的同步信号块(SSB)。SSB由主同步信号(PSS和SSS)以及广播信道(BCH)组成,包括主信息块(MIB)。PSS启用帧同步,以及与SSS一起确定物理小区ID的确定。MIB包含试图进入网络的UE的必要信息,例如下一条信息的位置和格式:系统信息块类型1或SIB1。您将了解SSB的编码和调制链,以及适用的重要性。最后,该视频讨论了同步信号突发,这是一组多个波束形成的SSB和它们的周期性。该视频还显示了具有5G工具箱™的SSB生成的示例。
This is a new episode of our series5G Explained。在此视频中,我们在5G新无线电中讨论同步信号块或SSB。
We will look at its components: the synchronization signals and the broadcast channel, which carries the master information block. Finally, we will have a look at the block repetition pattern and organization as burst.
The synchronization block helps with initial synchronization. It consists of three components.
The first component of the SSB is the primary synchronization signal, which is one of three possible sequences. The PSS comes first and occupies the center 127 resource elements of the 240 block. The UE typically runs three correlators in parallel, one tuned to each one of the possible sequences and, when it detects one of them, it knows the timing of the SSB.
第二组分是次级同步信号,316个可能的序列中的一个。SSS稍后来自两个OFDM符号,并且还占据中心127资源元素。第二步标识已发送哪些SSS。PSS和SSS的组合产生了三次336或1008个可能的物理单元ID中的一个。
最后,将主信息块的广播频道或BCH具有以其基本的一组来开始启动,完成SSB。DMRS符号沿PBCH发送并以黄色显示。
Altogether, the synchronization signal block is always 4 OFDM symbols long and 240 subcarriers wide, irrespective of the subcarrier spacing. Note that, when using the highest subcarrier spacing of 240kHz, the SSB is close to 60MHz wide.
SSB通常不会仅发送一次,但在几乎重复SSB的突发中。我们稍后会在此视频中更详细地讨论它。SSB的主要用途是初始同步和小区搜索,以及连接时的相邻小区的小区搜索。它还提供了关于合适的波束成形或换句话说,基站和UE的相对位置的第一部分信息,因为我们稍后会看到。
While the concept of PSS and SSS and two-step cell search is identical to LTE, there are a few differences worth noting:
·序列更长(127而不是62)。
· There are more cell IDs because there are more possible SSS compared to the number of SSS pairs used in LTE.
· The scheduling is quite different, because the PSS, SSS and PBCH are always sent together as a block. The repetition interval for PSS is no longer 5ms. It can be longer. Also, only one SSS is sent, as opposed to a pair of 31 possible SSS in LTE.
· Finally, the SSB can now be beamformed as explained in another episode of this series5G解释说。
广播频道的目的是携带主信息块或MIB。广播信道被映射到物理广播信道,其作为同步信号块(SSB)的一部分发送。
主要的不同ce in the processing chain compared to LTE is the use of polar coding instead of tail-biting convolutional coding, as we saw in the “Downlink Control Information” episode of this5G Explainedseries. The MATLAB code below shows how this chain is implemented in 5G Toolbox.
We now want to have a look at the payload itself or MIB.
The information carried by the broadcast channel consists of two parts. One part, the MIB, which is constant over 80ms, and another part that changes over 80ms and, therefore, is not really part of the MIB.
MIB包括用于访问单元格的UE的基本参数和信息:
·细胞是否可访问?
· Information about where to find the next piece of information, the system information block 1 or SIB1
· The location of the common resource grid, which carries the SIB1
·系统帧号
· Other pieces of information, which vary over 80ms include:
·SS块索引。实际上,该信息仅存在于MMW的FR2,并且它仅包含识别SSB所需的6位。我们将在我们的初始收购程序中解释本集中内容的原因。
·其他信息,如系统帧号的4个LSB以及CRC
Once encoded, the broadcast channel content undergoes scrambling and QPSK modulation before being mapped to the grid. This chain is similar to the downlink control channel chain.
在一个半框架内,含有5ms的半帧,有许多SS块出现。注意,因为每个块的扰码取决于块索引,所以这些事件彼此重复。
The broadcast channel may appear in the first or second half of a frame. Its position is indicated by the half frame bit, which is part of the BCH content. Each one of these occurrences can be switched off, which means that the cell does not necessarily transmit all of them. One group of occurrences is called a synchronization signal burst, and it consists of one or several synchronization signal blocks or SSB.
We have already mentioned that the BCH content in different occurrences of an SSB is different. One additional important point to understand is that the DMRS in each occurrence is also different. It can be one of eight possible sequences. This will let the UE tell apart those occurrences, as will be explained in more detail in our episode about initial acquisition procedures.
The SSB can be transmitted with different subcarrier spacings, ranging from 15kHz to 240kHz. Note that this subcarrier spacing of 240kHz is available for the BCH but not for data or PDSCH, as explained in the 5G introduction episode of this5G Explainedseries. Also, 60kHz is never used for BCH.
请记住,无论子载波间距如何,SSB始终占用240个子载波。这意味着它的带宽随着子载波间隔的增加而增加,但同时,其持续时间缩小。
The table on this slide shows the maximum number of occurrences of an SSB in a synchronization signal burst. It is four or eight, depending on the carrier frequency, for FR1, but it can be as high as 64 for mmWave or FR2.
At those subcarrier spacings, the duration of the SSB is much shorter, and it is possible to transmit more of them in the same amount of time. This enables finer beamforming at mmWave frequencies.
This slide and the next show different configurations for BCH type A through E. For cases A, B, and C, two configurations are possible with a maximum number of occurrences of four or eight depending on the carrier frequency. In each case, all occurrences fit within one half frame.
对于病例D和E,一个同步信号突发中的最大出现次数始终为64,这里也是如此,突发适合在一个半帧或5ms内。
在LTE中,每5ms发送PSS,每10ms发送广播信道。在5G NR中,两者的时段都是相同的,但它可以将值低至5ms,高达160ms。
If you consider that the SSB is the only always-on signal in a cell, you can see that it is possible to have very low transmission power in some 5G cells with no or low traffic. This is quite different from LTE, with its always on cell-specific reference signals and frequent PSS/SSS and BCH.
As the standard says a UE can assume an SSB occurs every 20ms, there seems to be some conflicting data here. The reason is that, in ordinary cells, the SSB periodicity is likely to be 20 ms or less, but 5G NR allows for extra power savings in cells. Those cells may not be discovered by a UE, or at least not reliably, but they may be reserved for other purposes such as a secondary carrier component, which is not meant to be stand-alone.
Here, you can see how you can set up the SSB in MathWorks 5G Toolbox. Parameters include the block repetition pattern, as we just saw, the physical cell ID, which blocks get transmitted, where a 1 indicates a transmitted block, and the periodicity.
Then MIB content, such as information to access the SIB1, or whether the cell is accessible or barred. The last line generates the synchronization signal burst.
Here, you can see two examples of settings, with the resulting time-frequency content of the SS burst. These pictures should be familiar as we have used them all along in this video. You can see that you can select to only send the first four blocks, as would be necessary under 3GHz, by setting the last four bits of the SSBTransmitted bitmap to zero. On the other hand, you can send all eight blocks when operating over 3GHz. Note that the carrier frequency at which we operate is not explicitly stated, and you could choose to send only a few blocks even above 6GHz.
This concludes this episode of the5G Explainedseries on the synchronization signal block.
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