be grant-based or grant-free. A grant-based transmission is a transmission signaled by the network using the corresponding DCI. A grant-free transmission is an uplink transmission made automatically by the device without any explicit prior grant being transmitted in the downlink. Grant-free transmissions are crucial for low latency. In a normal scenario, a device requests an uplink data transmission using a Scheduling Request (SR) and then waits for the gNB to deliver an uplink grant. With grant-free transmission, the device can send data in the uplink once it is available. There are two types of grant-free transmissions: Type 1 and Type 2. In both cases, the set of resources that the device can use for uplink transmissions is configured semi-statically via RRC. In Type 1, there is no Layer 1 (L1) signaling while in Type 2, additional L1 signaling is required to activate or deactivate the resources configured for this grant-free transmission. The PUSCH DMRS The PUSCH reference signal structure is similar to the downlink case. The DMRS is front-loaded and additional OFDM symbols can be configured for high-mobility cases. The only difference is that the DMRS sequence itself changes depending on the waveform. For CP-OFDM, it is the same as in the downlink, and for DFT-OFDM, it is a Zadoff-Chu-based sequence. 5G Air Interface and Physical Layer Procedures - Part 3 The PUSCH processing chain The following figure shows the PUSCH coding chain for the CP-OFDM waveform. It is very similar to the downlink case. The only difference, in this case, is that the precoder is included after the layer mapping. The precoder to be used in the uplink is explicitly included in the specification. One of the reasons is Multi-User Multiple Input Multiple Output (MU-MIMO). When the gNB configures MU-MIMO, it must coordinate the precoders between different devices SO that the data streams for each one can be correctly separated: codewords Scrambling Scrambling Modulation Modulation Mapper Mapper Layer Mapper layers Precoding antenna ports 