5G Physical Downlink Control Channel (PDCCH)

This is a new episode of our series, "5G Explained." In this video, we discussed the concept of CORESETs and how it applies to downlink control information. We will introduce CORESETs, look at the CORESET structure and characteristics, and how the PDCCH is mapped to it. We will discuss two different types of mapping, interleaved and noninterleaved, and explain how search spaces enable to further reduce the complexity of control information decoding at the UE.

A control resource set, or CORESET, is a set of time-frequency resources where a PDCCH may be transmitted. CORESETs are semi-statically configured by the network. There can be many CORESETs in a carrier, and they can occur anywhere in the slot and the frequency range of the carrier. But they are, at most, three OFDM symbols long. PDCCH is transmitted within the CORESET. The picture here shows the CORESET in green, and the PDCCH may occupy part of all of CORESET frequency location at defined time instance.

The basic unit for a CORESET is resource element groups. Resource element groups consist of 12 resource elements by one OFDM symbol, and they were introduced in another episode of this "5G Explained" series about downlink control information. A CORESET spans a multiple of possibly noncontiguous groups of six resource blocks in frequency, and between one and three contiguous OFDM symbols in time.

A CORESET represents locations where a given device may receive PDCCH. Importantly, there may be no control transmission at some locations in time and frequency. To simplify control information search in the UE, actual PDCCH locations may be further restricted by search spaces as explained later. Also, a CORESET may not span the whole bandwidth. This is particularly important because UEs in a cell may not support the whole bandwidth, which may be up to 400 megahertz, but they still need to be able to decode control information. As control regions are flexible, 5G New Radio supports frequency-domain interference coordination between cells. What this means is that neighboring cells can plan their CORESET locations to avoid intercellular interference affecting the control signal.

A PDCCH is mapped to a specific CORESET. I'd like to summarize that fact by saying that orange can only be on top of green on this picture. As seen in the episode on downlink control information, a PDCCH occupies 1, 2, 4, 8, or 16 control channel elements or CCEs. You may remember from that episode of the "5G Explained" series that one CCE corresponds to six resource element groups. One of PDCCH configuration parameters may specify a period for repeated occurrences.

We're now going to look in more detail at some of the parameters that are provided by 5G New Radio to specify a CORESET. The pictures on this and the next slides were generated with MathWorks 5G Toolbox. Here, we can see the CORESET in green. The time duration is specified to be three OFDM symbols. The frequency or occupancy is specified in terms of groups of six resource blocks that are occupied, namely groups 0, 1, and 3. The gap in frequency corresponds to group 2, which is not occupied by this CORESET. Symbol allocations 0 and 7 mean that the CORESET is present starting at OFDM symbols 0 and 7, and it is three OFDM symbols long each time as seen earlier. Finally, we can see the CORESET is allocated in slots 0 and 1.

On this view of the same frequency-time location, we also represent demodulation reference symbols or DMRS associated with the PDCCH in orange and the data channel or PDSCH in teal. DMRS are shown in yellow. Out of every set of 72 resource elements where a PDCCH is mapped, 54 are for PDCCH and 18 are for associated DMRS. We will look at DMRS in detail in another episode of this series, "5G Explained." On this final view of the CORESET, we can see the periodicity for the CORESET. Slots 0 and 1 are assigned, and the CORESET then repeats every five slots, as specified by the period.

Here, we have a more detailed look at the PDCCH parameters. The first line specifies which CORESET the PDCCH is mapped to. The period shows that the PDCCH is mapped every three CORESET occurrences, and the allocated search space parameters indicates that the allocation is in occurrence number 0. Finally, from a time and frequency point of view, the PDCCH starts at CCE number 1, which is the second CCE or group of six resource element groups. And it uses the granularity of four, meaning that it includes four CCEs or a total of 24 resource element groups. Note that CCE number 0 corresponds to the first CCE within the CORESET.

Let us have an interactive look at those parameters with a utility that uses MathWorks 5G Toolbox. Here, we can quickly vary the CORESET and PDCCH or DMRS definition and observe the effect on the resulting 5G OFDM grid. We are looking at one subframe with a 15-kilohertz subcarrier spacing. And we can see the CORESET in green, the PDCCH in orange, the data channel or PDSCH in teal, and all associated DMRS in yellow. Let us change the subcarrier spacing to 30, still showing one subframe. We now have two slots or 28 OFDM symbols in one subframe, and we can see further allocation of the PDCCH on the right.

Let us now reduce the aggregation level for the PDCCH from 4 to 2, which means that we use fewer CCEs to encode the DCI, and look at the PDCCH in orange. The size of a PDCCH has been halved. It is now two CCEs or two times six equals 12 resource element groups. As the CORESET is three symbols long, this means the PDCCH occupies three OFDM symbols in time by four resource blocks in frequency. If we change the CORESET duration in time from 3 to 2, we can see that the PDCCH, which still uses 12 resource element groups, is now shaped as two symbols by six resource blocks, which is narrower and taller. Also, the PDCCH now occupies a higher percentage of a CORESET as there is reduced control capacity.

Let's have a larger view and represent four subframes or eight slots. We can see the periodicity of a CORESET. It occupies symbols 0 and 7 in slot 0 and 1, and the period is five slots, which means that the same pattern occurs five slots later. If we change the period to six slots, we see the pattern move one slot to the right. There are many more options which you can explore in detail with MathWorks 5G Toolbox, but hopefully this short example helps making those parameters more concrete.

5G NR allows for interleaved CCE-to-REG mapping with different bundle sizes. What this means is that contiguous physical resources may not correspond to contiguous bits in the PDCCH. The interleaver defines a bundle size which introduces a similar concept as resource block bundling for PDSCH. As a reminder, resource block bundling in PDSCH enables to specify blocks of resources that are guaranteed to have the same precoding. Resource block bundling for PDSCH is further discussed in another episode of this "5G Explained" series about channel sounding.

As mentioned earlier in this section, monitoring all CORESETs for all possible aggregation levels is too expensive for UE to do. Further restrictions in the location and size of possible PDCCH are introduced via the concept of search spaces. A search space is a set of candidate control channels formed by CCEs. It specifies the aggregation level, which means the size, for each candidate. Search spaces apply to device-specific and common search spaces, common search spaces containing information that is relevant to several or all UEs. Blind search is considered to be successful if the CRC checks and the content of the decoded DCI makes sense.

Here is one example of possible search space. In this example, they are five search locations defined within this CORESET. The UE does not need to look for possible control information at all locations with all granularities. It only needs to look for a 2 CCE PDCCH at three locations, as well as 4 CCE PDCCH at another two locations. This saves a considerable amount of time and power compared to searching through all groups of 1, 2, 4, 8, and 16 CCEs. This concludes this episode of the "5G Explained" series on CORESETs.