Time Slot Allocation for Uplink and Downlink – TDD specific
The time division nature of LTE TDD will also require radio engineers to consider how time slots are shared between uplink and downlink based on both customer input as well as commercial users usage pattern between uplink and downlink in that country. This will have a direct impact on the EUTRAN capacity. There are 7 time sharing configuration between Uplink and Downlink in LTE TDD as defined by 3GPP. They are shown in the figure below and in summary, they are (DL : UL) – 1:3 or 2:2 or 3:1 or 2:1 or 7:2 or 8:1 or 3:5
Besides time sharing configuration, there is also a need to define how uplink and downlink pilots are configured based on the Guard band requirement. The guard band duration is also a direct result of propagation delay requirement due to the designated cell coverage radius. Inadequate guard band provisioning will result in direct interference between users within the same cell due to difference in signal delay arrival. 3GPP has defined 9 different guard period configuration schemes for Operator to choose. They are listed in the figure above.
Cyclical Prefix Allocation:
In order to accommodate extended propagation delay (e.g. due to large cell radius coverage), a lower order cyclic prefix value of 6 can be used instead of the common value of 7. This will result in the reduction of OFDM symbols that can be carried per time slot and therefore reducing the overall sector capacity.