TD-LTE – Number of Uplink Timeslots Observed in Practice

When I was recently in China, it was an ideal opportunity to have a look at the ratio of downlink and uplink timeslots in a real network. The standard is quite flexible in this regard and offers many options so I was not quite sure which option would actually be used in practice.

For the technical details of timeslot use of uplink and downlink have a look here and here. The up/downlink ratio of a TD-LTE carrier can be found in System Information Block 1 in the TDD-Config section. In a live network in China this section looked as follows:

TDD-Config
  subframeAssignment = 2
  specialSubframePatterns = 6

The values are decoded as follows:

Subframe assignment 2 is a configuration in which a 10 millisecond frame is split into 6 downlink subframes (1 ms each) and 2 uplink subframes (1 ms each). One subframe is used for both downlink and uplink transmission with a gap in between to allow for the uplink transmission delay (timing advance) and the time required to switch from receive to transmit mode. The exact timing is described by the SpecialSubframePatterns parameter. In this network, pattern 6 is used, which, according to the second link above, reserves a large portion of the 1 ms subframe for the downlink and only very little for the uplink.

In other words, only 20 per cent of the carrier is used for uplink transmissions. Under ideal conditions, this would result in a datarate of 15 Mbit/s in a 20 MHz TD-LTE channel. This compares to a theoretical maximum data rate of 75 Mbit/s in a 20 MHz FDD channel where all timeslots are used in the uplink direction.

15 Mbit/s perhaps doesn’t sound too bad at first but this is the theoretical maximum. In practice it will be far less and has to be split between all devices currently transmitting in the uplink direction in a cell.

Next time I’m in China, I have to run some uplink speed tests, but I would be surprised if I could get more than 5 Mbit/s out of the channel. Even carrier aggregation doesn’t help as in practice, CA is only applied in the downlink direction but not in the uplink.