ficult for the device. Other alternatives for assigning resource for time-critical service are available. The simplest method is reserving a dedicated resource for such service. When the traffic load is high and a dedicated resource is not desirable, the signal intended for UE2 can be superposed with the signal for UE1, instead of preempting the latter. The signals for the two UEs can be separated at the receiving sides if the favorable beam directions of the 13.3 Low Latency, Flexible Timing, and Modem Control Flow Complexity two users are disjoint. When signal superposition is used, it is also possible to perform power boosting for the signal with a higher reliability demand. 13.3.1.6 Front-Loaded DMRS By locating the reference signals at the beginning of a PDSCH transmission, a UE can start performing channel estimation early on. Once the channel estimate is obtained, the receiver can process the received symbols immediately without having to buffer the received signal prior to data processing. This design can reserve some more processing time for the modem. Besides the front-loaded DMRS, NR supports additional DMRS in later OFDM sym- bols within the resource allocated to the PDSCH. The channel estimation at front-loaded and additional DMRS can be interpolated to improve the accuracy of channel estimates. When there is no additional DMRS for the PDSCH, the design for channel estimation becomes more complicated. In this case, extrapolation of channel estimation is needed. This either degrades the performance of channel estimation or requires a more sophis- ticated algorithm to maintain a comparable performance. 13.3.1.7 OFDM Symbol-Based PUCCH When low latency is required, the delay due to a scheduling request, UE feedback, retransmission, and SO on, should be taken into account. All these activities are relevant to the transmission of the PUCCH. Unlike in LTE where a PUCCH takes one subframe duration, the duration of a PUCCH in NR can be as short as one OFDM symbol. The above features provide the basic support