In a previous post I’ve been looking at the concept of the 5G Control Area Set (CORESET). The essence of the post was that the CORESET is the areas in the 5G NR resource grid on the air interface in which scheduling information for downlink and uplink data is sent from the network to mobile devices. The two main parameters a CORESET describes is at which Resource Block (RB) in the frequency domain the control area starts and how many RBs it spans. In this post, I’ll take a closer look at how the information in a CORESET is organized, i.e. how the downlink and uplink scheduling messages are organized.
How Many Symbols are in a CORESET?
O.k. So in the simplest case, a CORESET has a length of exactly 1 OFDM symbol on the time axis. With QPSK modulation, a symbol can encode 2 bits. On the frequency axis lets say the CORESET is configured for 217 Resource Blocks (RB) over a complete 80 MHz channel. Each Resource block contains 12 symbols so in total there are 217*12*2 bits in a CORESET area = 5208 bits.
How Many Scheduling Messages
Now that we have an idea how large a CORESET is, let’s have a look how the information in it is organized. A Resource Block (12 symbols on the frequency axis) is referred to as a Resource Element Group (REG) and 6 of them form a Control Channel Element (CCE), which is the smallest unit a scheduling message can be put in. 6 x 12 (symbols) x 2 bits (QPSK modulation) gives us 144 bits for a scheduling message. It is not quite 144 bits, however, as 3 UE specific Demodulation Reference Symbols (DM-RS) per 12 symbols have to be subtracted. This reduces the number of available bits per message (i.e. a CCE) to 126 bits. To increase redundancy, several CCEs can be aggregated to spread a scheduling message over more bits. Typically, aggregation levels 4, 8 and 16 are used and the level can be changed based on channel quality.
Content of the DCI Scheduling Messages
So as established above, a CCE or several aggregated CCEs contain a scheduling message, which is referred to as Downlink Control Information (DCI). A DCI can have several formats, but let’s limit ourselves to Format 1_0 which is downlink scheduling information and Format 0_1 which is uplink scheduling information:
A Downlink DCI contains parameters like:
- The frequency domain resource assignment, i.e. how many symbols on the frequencxy axis are assigned to a user for receiving data. More about this in a follow up post.
- The time domain resource assignment. Again, more on this in a follow up post.
- The modulation and coding scheme of the data
- Downlink assignment index (which Bandwidth Part configuration to use)
- Whether it is new data or a redundancy version of previously sent data that was not received correctly (Hybrid Automatic Repeat Request, HARQ)
- Which HARQ process number the data belongs to. Up to 8 Ack/Nack HARQ queues are used on the 5G air interface.
- Transmit Power Control (TPC) to adjust a mobile device’s uplink power
- An indicator where to send the acknowledgement for the data (PUCCH resource indicator)
- A HARQ feedback indicator to let the device know when to send its acknowledgement for data received in the downlink direction.
An Uplink DCI on the other hand contains, among other things, the following parameters:
- A frequency domain resource assignment (see above)
- The time domain resource assignment (see above)
- Modulation and coding scheme
- HARQ information (see above)
For details see 3GPP TS 38.212, Chapter 7.3.1.
So in short, the CORESET contains the Downlink Control Channel which carries a number of control messages that are referred to as DCI messages. A message can contain an assignment of downlink resources, i.e. the location in the resource grid and its size where a mobile devices has to look for downlink data. Another DCI message format contains information for the mobile device where and when to send data in the uplink direction.