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原视频来自：https://www./watch?v=aW5nT_xW7qw。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、中文译文和英文原文。 这是“ 5G解释” 系列的新内容，本视频中将讨论5G NR的上行链路数据传输。 我们将研究上行链路共享信道的处理过程，包括LDPC编码；物理上行链路共享信道，包括可选的变换预编码；以及不同类型的PDSCH映射。 上行链路共享信道（UL-SCH）是承载用户数据的信道。它的结构与下行链路共享信道非常相似，包括CRC、码块分段、LDPC编码（NR与LTE的关键区别）以及速率匹配。上述处理的输出对应一个码字。与下行链路共享信道不同，它最多使用四层，这意味着上行链路传输始终使用单个码字。编码后的数据映射到物理上行链路共享信道（PUSCH）。 上图中你可以看到上行链路共享信道处理过程中的每个阶段是怎么映射到Mathworks 5G 工具箱的功能模块的，比如CRC编码、码块分段、LDPC编码和速率匹配。 同样，这个处理过程与下行链路类似，但有两个显著区别。第一个是存在变换预编码过程，DFT操作使用单载波OFDM（SC-OFDM）调制，有时又称为DFT-s-OFDM。变换预编码是可选的，这意味着上行链路同时支持常规OFDM和SC-OFDM。第二个区别是需要在上行链路指定预编码类型，我们将看到这里包括两种预编码类型。 我刚刚提到过，转换预编码是可选步骤。 SC-FDMA能提供较低的功率峰均比（PAPR），这意味着UE功率放大器的回退会较少。 这有助于当UE位于覆盖范围较大的小区中，或UE功率受限的场景。 OFDM调制方案与下行链路的调制方案相同，范围从QPSK到256QAM。启用转换预编码后，对于小区覆盖范围比较大并且SNR比较差和低功率UE的传输场景，可以使用Pi / 2-BPSK调制。 上行链路层映射机制与下行链路相同 (这一机制在“ 5G解释”系列的另一集中进行了讨论)。不同之处是上行链路最多映射到四层。因此上行链路总是使用单个码字。N个输入比特数据作为一个输入块映射到N层（N取值范围在1和4之间）。 预编码操作把层数据映射到天线端口。如前所述，有两种预编码模式，分别是基于码本的预编码和不基于码本的预编码。当不使用基于码本的预编码时，UE通过对下行链路信号（CSI-RS, 信道状态信息参考信号）执行测量来确定预编码矩阵。当使用基于码本的预编码时，UE使用由gNB指定的预编码矩阵。
在这里，我们重点研究一下不基于码本的预编码方案，UE在此模式下自行决定要使用的预编码矩阵。如刚刚提到的，UE通过测量CSI-RS信号来确定合适的预编码矩阵，然后把使用这个预编码矩阵用于发射SRS信号（探测参考信号）。之后，gNB通知UE使用波束赋形矢量的哪个子集。最后，UE可以将选择的波束赋形矢量用于PUSCH传输。总之，在该模式下，尽管UE推荐了一个预编码矩阵，但gNB仍然可以否决某些波束成形向量，并且可能减少传输使用的层数。
对于基于码本的预编码，gNB基于TDD模式的下行链路测量，或探测参考信号的上行链路测量来确定预编码矩阵。后，它在下行链路控制消息（包括上行链路调度信息）中将预编码信息发送给UE,UE通过搜索一系列预定义的表格数据得到预编码矩阵。不同的层数、是否使用基于码本的预编码，对应不同的表格。“5G解释”视频系列的另一集中详细介绍信道探测和预编码的整个过程。
PUSCH符号到资源网格的映射分为A和B两种类型，这与下行链路的映射类型相同。映射类型A用于从时隙起始位置开始分配的场景，DMRS (解调参考信号) 映射到符号2或3。映射类型B用于从时隙中间位置开始分配的场景，在这种情况下，DMRS映射到分配的第一个符号。有关上行链路数据传输的讨论到此结束。 英文： This is a new episode in ourseries '5G Explained.' In this video, we discuss uplink data transmission in 5G New Radio. We will look at the uplin shared channel chain, which includes LDPC coding; the physical uplink shared channel chain, including the optional transform precoding; and the different types of PDSCH mapping. The uplink shared channel, orUL-SCH, is the channel that carries user data. It has a structure very similar to the downlink shared channel, with CRC, code block segmentation, the use of LDPC, a key difference with LTE, and rate matching. The output of a coding chain is a codeword. Contrary to the downlink shared channel, it is limited to four layers, which means that uplink transmissions always use a single codeword. The coded data is then mapped to the physical uplink shared channel, or PUSCH. Here you can see how every stage of the uplink shared channel processing is mapped to functions in MathWorks 5G Toolbox. You can recognize CRC encoding, code block segmentation, LDPC coding,and rate matching. Here again, the chain is similarto the downlink chain, with two notable differences. The first one is the presence of transform precoding, a DFT operation which is what turns OFDM modulation into single carrier OFDM or SC-OFDM, sometimes known by another name, DFT spread OFDM, or DFT-S-OFDM. Transform precoding is optional, which means that the uplink supports both regular OFDM and SC-OFDM. The second difference is that precoding is specified on the uplink, and we will see it can be of two types. I just mentioned transform precoding is an optional step. SC-FDMA provides lower PAPR, which means that less backoff is needed on the power amplifier. This helps with UEs that are in larger cells or have limited power. Modulation schemes in the OFDM case are the same as for the downlink, ranging from QPSK to 256QAM. When transform precoding is enabled, an additional modulation scheme，Pi/2-BPSK is also available to help with very low SNR transmission in a large cell or with low power. Layer mapping on the uplink uses the same mechanism as on the downlink, a mechanism that is discussed in another episode of this '5G Explained' series, with one big difference. There can be a maximum of four layers. For that reason, the uplink always includes a single codeword. This single codeword is mapped to n layers, with n between 1 and 4, by mapping input blocks of n input bits to n layers, as shown here. Precoding is the operation mapping layers to antenna ports. As mentioned earlier, there are two precoding modes, codebook-based and non-codebook based precoding. For non-codebook based precoding, the UE determines the precoding matrix based on measurements itperforms on downlink signals, typically the channel state information reference signals. For codebook-based precoding,the UE simply applies the precoding matrix specified by the gNodeB. Here we have a closer look at the non-codebook based precoding scheme. This is the mode where the UE makes its own decision about which precoding matrix to use. As just mentioned, the UE determines a suitable precoding matrix from CSI-RS measurements. It then applies the proposed precoding matrix to the SRS, or sounding reference signal. The gNodeB then informs the UE of which subset of the beamforming vectors it can use. Finally, the UE can apply the selected beamforming vectors to PUSCH transmission. In summary, in that mode, while the UE does propose a precoding matrix, the gNodeB can still veto some of the beamforming vectors, which would reduce the number of layers for transmission. For codebook-based precoding,the gNodeB determines the precoding matrix, based either on downlink measurements for TDD, or an uplink measurement of sounding reference signals. It then sends the precoding information to the UE in the downlink control message for uplink. And the UE simply looks up the matrix in a series of tables. There are different tables for different number of layers, as well as for transmission with or without transform precoding. The whole process of channel sounding and precoding is described in detail in another episode of this '5G Explained' video series. PUSCH symbols are mapped to the resource grid using one of two mapping types, A or B. These are the same mapping types as for on the downlink. Mapping A is meant for allocations that start at the beginning of the slot. The demodulation reference signal is then mapped to symbol 2 or 3. Mapping type B is meant for allocations that start partway through the slot, in which case the DM-RS is mapped to the first symbol of the allocation.This concludes this episode of the '5G Explained' video series on uplink data transmission. |
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