Network operators in a lot of countries around the world are getting ready at the moment to launch first 5G networks in the 3.x GHz spectrum (i.e. 3GPP band n78). One of the particularly interesting things about this band is that its made for TDD operation and not FDD as most operators are used to. This means the downlink and uplink are on the same channel and separated in time and not on the frequency axis. So how does the network inform the device about when to expect downlink and uplink transmission of the channel in general (not individual reception or transmission opportunities)?
There are a few options how to do that in the 3GPP specifications but the most straight forward one is to announce the UL-DL configuration in an RRCReconfiguration message when the 5G channel is added to connection in EN-DC (E-UTRAN-New Radio Dual Connectivity) mode. The nitty gritty details are described in 3GPP TS 38.213 chapter 11.1 but here’s a summary for a typical example: For band n78, the channel is usually configured for 30 kHz sub-carriers, 14 symbols per slot and a slot length of 0.5 ms. A slot is the smallest timeframe that a device can be scheduled to transmit or receive data (compare that to the 1 ms sub-frame that is the minimum scheduleable timeframe in LTE). According to 38.213 the uplink / downlink configuration of the channel is contained in the tdd-UL-DL-ConfigurationCommon information element. Here’s an example with values that might make sense in practice:
tdd-UL-DL-ConfigurationCommon
{
referenceSubcarrierSpacing kHz30,
pattern1
{
dl-UL-TransmissionPeriodicity ms2,
nrofDownlinkSlots 2,
nrofDownlinkSymbols 11,
nrofUplinkSlots 1,
nrofUplinkSymbols 0
}
}
The 5 parameters lay out that the periodicity is 2 milliseconds. As a slot in this configuration has a length of 0.5 ms, the period thus contains 4 slots, each with 14 symbols. Out of those 4 slots, 2 are downlink slots and 1 is an uplink slot. The fourth one is a ‘special’ slot, between the downlink and the uplink slots. Some of the symbols in this special slot can be used for downlink transmissions, some for uplink transmissions and some are left free, e.g. to allow the transceiver to switch direction. In this case the special slot has 11 downlink symbols and 0 uplink symbols, i.e. 3 symbols at the end are left free for switching the direction. In other words, most of the special slot is used for the downlink. That means that almost 3 slots are used for downlink data transmissions and 1 slot is used for the uplink, so the downlink/uplink ratio is 3:1. This configuration is sometimes also referred to as DDSU (Downlink – Downlink – Special – Uplink).
The specs contain a table of around 60 ‘slot configurations’ at the moment. That’s a bit confusing at first because that’s got nothing to do with the uplink downlink ratio and the slot configuration number is also not a value that is broadcast to devices anywhere. So I’m not quite sure why they have been defined, they do not seem to be necessary when the tdd-UL-DL-ConfigurationCommon information element is used. But that is just one way to announce the UL/DL configuration, there are other options as well, e.g. via individual DCI messages. Perhaps they are used there, but I didn’t go into further details here because doing things this way seems to be somewhat of an overkill for the main use of early 5G deployments, i.e. fast Internet connectivity.