Uplink Throughput for LTE FDD, including PRACH overhead (Coding Rate = 1)

By Azar

If you stumbled upon this article out of nowhere and don't know the context. I recommend reading this one first. CLICK HERE TO READ PRE-REQUISITE

 

Now you have read the previous article. Previously for Uplink throughput computation we assumed that only affect of PUCCH is included, while Physical Random Access Channel (PRACH) and Sounding Reference Signal (SRS) overheads are not included. 

This time we are considering, how much throughput in the Uplink can be achieved if  we take into account PRACH overhead as well, while still ignoring Sounding Reference Signal  and other protocol stack overheads etc. 


Remember the overhead generated by the PRACH depends upon Random Access Preamble Configuration.

  • The minimum configuration allows 1 msec PRACH preamble every 2 radio frames. 
  • The maximum configuration allows ten 1 msec PRACH preambles every radio frame
  • PRACH configuration can also have  five 2 msec PRACH preambles every radio frame


The throughput in that case is shown below which assumes a single 2msec PRACH transmission per radio frame.


Normal Cyclic PREFIX + PRACH OVERHEAD INCLUDED

Normal Cyclic Prefix: Maximum FDD throughput for PUSCH including PRACH, while excluding protocol stack overheads, no Sounding Reference Signal Overhead. Credits: LTE in Bullets

Important Observations

  1. Overhead generated by 2 msec PRACH transmission per radio frame has a more significant impact upon the throughput associated with smaller bandwidths such as 1.4 MHz, whereas it has less impact on larger channel bandwidths --- You may think that is obvious 🙂  Duh 
  2. PUSCH uses a combination of rate 1/3 Turbo coding and rate matching to generate redundancy.  
  3. During poor channel conditions, the quantity of physical layer redundancy is relatively large when UE experience poor channel conditions and is very small when UE experience good channel conditions.


EXTENDED Cyclic PREFIX + PRACH OVERHEAD INCLUDED

Extended Cyclic Prefix: Maximum FDD throughput for PUSCH including PRACH, while excluding protocol stack overheads, no Sounding Reference Signal Overhead. Credits: LTE in Bullets


The maximum throughput shown in case of extended cyclic prefix is 69.7 Mbps.

How Throughput is reduced with overheads?

The throughput calculated above is only taking PRACH overhead into account. 

In reality, the actual throughput is affected by a number of factors such as:

  1. RRC signaling : Overhead for RRC signaling on PUSCH depends upon the quantity of traffic but it is usually small in kbps
  2. Re-transmissions reduce higher layer throughput as well. Hybrid Automatic Request (HARQ) retransmissions from the MAC layer reduce the throughput measured from above MAC layer
  3. Automatic Repeat Request (ARQ) retransmissions from Radio link control layer (RLC) layer reduce the throughput measured from above the RLC layer. 
  4. Similarly Transmission Control Protocol (TCP) reduce the throughput measured from above the TCP layer
  5. Protocol stack headers also reduce the higher layer throughput. MAC, RLC, PDCP, and IP layers add headers to application layer data.  
  6. Etc.

If you have questions or want to speak up your mind. Please do so in the comments below 🙂

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