Carrying on from the previous post on things that are different in 5G NR compared to LTE, the CORESET comes to mind. When I first heard the term I had to think of 1960s core memory, a core dump and other memory related things. But the 5G CORESET is something entirely different.
Like LTE, the 5G NR air interface uses a resource grid to transmit data. On the time axis, symbols are transmitted on a 15 or 30 kHz (sub-) carrier that encode several bits each. To make things faster, many symbols are transmitted simultaneously on different frequencies, i.e. sub-carriers. In an 80 MHz 5G NR channel that uses a sub-carrier spacing of 30 kHz, 2604 channels are used simultaneously. If you visualize that you end up with a grid of symbols on the time and frequency axis.
The question then arises how data is transmitted over this grid for individual devices. In LTE, one or more out of 14 symbols on the time axis and all of the symbols on the frequency axis are used once every millisecond to carry the Physical Dedicated Control Channel (PDCCH) that contains the information when and which device should receive or transmit its data in a part of the resource grid. So finding the PDCCH is pretty simple as it is broadcast every millisecond over the complete channel bandwidth.
In 5G NR this mechanism is very similar but had to be somewhat extended. This is because different parts of the overall channel could be used by different applications and do not even have to have the same sub-carrier spacing. For example: While most of the channel in the future could be used for fast Internet access, a part of the channel could be reserved for machine type communication that requires little speed but fast round trip delay times. For this purpose, the network operator might select a completely different sub-carrier spacing compared to the part of the carrier that is used for fast Internet access. While this is not yet done today, the air interface specification needs to allow for this already today. On of the implications of this is that a single PDCCH for resource assignments that spans over the complete carrier bandwidth is no longer viable.
Instead, the PDCCH is now contained in what is called control regions. A device can be assigned one or more control regions that have to be placed inside their Bandwidth Parts (see the previous post on BWP). Since there can theoretically be more than one control region, the concept is referred to as Control Region Set, or CORESET. So in other words, the CORESET is the areas inside the BWP of a device in which the PDCCH is placed.
In practice, mobile Internet access is the only application used on the 5G NR air interface today and the network assigns the complete carrier bandwidth to the BWP of each mobile device. In effect the PDCCH thus spans the complete carrier bandwidth, just like in LTE. But the flexibility is there for dividing the bandwidth of a channel in the future.
The next question then is what is contained in a CORESET, i.e. how do the downlink and uplink assignment messages look like? I will have a closer look at this in the 5G next post.