4G/LTE - Basic Procedures  




Uplink Data Transmission Scheduling - Non Persistent Scheduling

In Persistent Scheduling mode, UE can send the data to Network anytime since Network is sending UL Grant all the time. But what if Network does not send UL Grant all the time ? In this case, UE has ASK the network to send UL Grant (DCI 0). If network send UL Grant, then UE can send UL data as allowed by the UL Grant.

Overall procedure is as follows :

  • i) UE send SR (Scehduling Request) on PUCCH
  • ii) Network send UL Grant (DCI 0) on PDCCH
  • iii) UE decode DCI 0 (How UE can figure out a DCI 0 information in PDCCH is allocated for it ? Simply put, it performs the blind decoding for the whole PDCCH area (PDCCH Search Space) and check if there is any DCI 0 information that has CRC value encoded with C_RNTI allocated to it).
  • iv) UE Transmit PUSCH based on the RBs specified by DCI 0.
  • v) Network decode the PUSCH (How network figure out which part of Uplink subframe (UL RB) is for which UE ? It is simple. eNB knows exactly knows which UE send PUSCH at which RB because network specified this location in DCI 0 at step iii)
  • vi) Network send ACK/NACK on PHICH
  • vii) If Network send NACK, go to [Retransmission] Procedure ( For the details of [Retransmission] process, refer to HARQ Process page)

Following is the illustration of SR based PUSCH scheduling for FDD (Refer to refer to SR/DCI 0 Timing, DCI 0/PUSCH Timing  of TDD Overview for TDD scheduling)

Now let's look at a real life example, which might look more complicated and confusing but hopefully look more interesting :). This shows an example of what's happening during the initial process (RACH process) after you turn on your mobile phone.

Again, the log and background RB map is from Amarisoft LTE Network simulator. All the labels were put manually (If you roll over the mouse pointer onto each channel it shows some detailed information, but it would not show information on the exact contents. This is understandable.. because Physical channel by itself does not have any detailed knowledge on the contents).

In this example, you can see almost every cases that might happen in the real communication. You may ses SR and DCI 0 in response to SR and even PUSCH retransmission due to CRC failure.

How can I figure out all the details printed on each labels shown above ? It came from the text based log as shown below.

It took me almost an hour to pul all the lables shown above based on the log below. However, this can be a good practice if you are at learning phase of LTE protocol.. or you HAVE TO go through this tedious process when you are in troubleshooting situation.