What (Semi-Theoretical) Maximum Downlink Throughput values can you achieve in LTE TDD ? ( For Coding Rate = 1 )

By Azar

Last time I discussed absolute theoretical throughput you can achieve in case of TDD downlink when all resource blocks are allocated to PDSCH. However, even in case of computing

theoretical throughput. You need to specify the minimum and a maximum number of PDCCH symbols being allocated. For different PDCCH symbols, there will be different throughput

computations for TDD Downlink. So today, we are going to focus on TDD throughput in downlink which can be achieved :

  • For Different PDCCH Symbols ( Maximum and Minimum)
  • Different Uplink Downlink Subframe Configuration
  •  Different Modulation Schemes
  • Different Special Subframe Configurations for TDD

Well, the above bullet points are ambitious to analyze the throughput for all case extensively. Therefore we will do the analysis for selected cases only.

( Note: The reasons and details about why Subframe Configurations and Special Subframe Configurations are chosen for throughput description in the table below are already discussed in here  )

What if we remove the overheads caused by physical channels such as  :

  • Physical Control Format Indicator Channel (PCFICH)
  • Physical HARQ Indicator Channel (PHICH)
  • Physical Broadcast Channel (PBCH)
  • Primary and Secondary Synchronization Signals ( P-SS and S-SS)

In order to compute maximum physical layer throughput in the downlink while overheads for control channels are removed. We can say that Coding Rate = 1

TDD RADIO FRAME CONFIGURATIONS

 

The results are presented for only two UL-DL Subframe Configurations

  • Uplink-Downlink Subframe Configuration 0
  • Uplink-Downlink Subframe Configuration 5

 

 MAXIMUM PDCCH SYMBOLS

 

  •  Subframes 1 and 6: there are maximum 2  OFDM Symbols for all Channel Bandwidths
  • Other subframes: All other sub frames have a maximum of 3 OFDM symbols for all channel bandwidths except 1.4 MHz. There are maximum of 4 OFDM Symbols for 1.4 MHz

 

MINIMUM PDCCH SYMBOLS

 

  • Subframes 1 and 6: the minimum OFDM symbols is 1 expect for bandwidth 1.4 MHz. There are minimum of 2 OFDM symbols for 1.4 MHz
  • Other subframes: All other sub frames have a minimum of 1 OFDM symbols for all channel bandwidths except 1.4 MHz. There are minimum of 2 OFDM Symbols for 1.4 MHz

UPLINK-DOWNLINK SUBFRAME CONFIGURATION 0

 

 

LTE TDD Theoretical Throughput for coding rate = 1 for different PDCCH Symbols, TDD UL-DL Subframe Configuration 0 ( Credits: LTE in Bullets)

UPLINK-DOWNLINK SUBFRAME CONFIGURATION 5

 

 

LTE TDD Theoretical Throughput for coding rate = 1 for different PDCCH Symbols, TDD UL-DL Subframe Configuration 5 ( Credits: LTE in Bullets)

QUICK OBSERVATIONS

 

  • The throughput values in tables above are less than the values shown in (LINK TO TABLE).   For example, the maximum throughput associated with the 20 MHz channel bandwidth, uplink-downlink subframe configuration 5, special subframe configuration 4  and 4×4 MIMO decreases from 357 Mbps to 287 Mbps or 249 Mbps ( depending upon the number of PDCCH symbols allocated to control channels ). This demonstrates the impact of the overhead generated by physical channels and physical signals which do not transfer any application data.
  • There is an inverse relationship between maximum throughput achieved and associated overhead.   Maximum Throughput capacity will decrease as the traffic and associated overhead increases
  • The amount of overhead added is large when UE experiences poor channel conditions and small when UE experience good channel conditions
  • System Information Blocks (SIBs), paging messages and RRC signaling are transferred using PDSCH channel.  The overhead by these SIBs, Paging and RRC signaling depends upon the traffic loading of the cell.

 

CONCLUSION:

 

The tables above give you a comparison of throughput vs. overhead tradeoff which affects the overall physical throughput which can be achieved in case of LTE Downlink for TDD. It is shown only

for two TDD configurations.  For other configurations, it can be derived accordingly. If you have any quick observations, questions or want to speak your mind. Hit them in the comments below.