What is the maximum DOWNLINK throughput that can be achieved in LTE FDD ?

In cellular technology.

The question always is and always will be about how much throughput a technology can achieve.

In order to answer, we need to specify the technology.

This discussion is focused on LTE.

For LTE. In Downlink throughput is  received through Physical Downlink shared channel (PDSCH) .

 

 

 

 

 

Factors for throughput in PDSCH ? 

The throughput achieved by the PDSCH depends upon:

 

• number of resource blocks allocated to the PDSCH
• modulation scheme applied to each Resource Element
• quantity of redundancy included by physical layer processing
• use of multiple antenna transmission schemes (MIMO)
• Use of carrier aggregation ( if applicable)

 

What factors PDSCH Resource Elements Depend upon ?

The number of PDSCH Resource Elements depends upon:

 

 

• the channel bandwidth
• the choice between the normal and extended cyclic prefix.
• Overhead generated by other physical channels and physical signals

 

Factors which Decide Higher Data Rate on PDSCH Channel 

Modulation

Higher data rate depends upon modulation scheme.

Modulation scheme and quantity of redundancy depend upon the RF channel conditions.

UEs experiencing good channel conditions are more likely to be allocated higher order modulation

schemes with less redundancy

MIMO

Multiple antenna transmission schemes (MIMO) increase the

throughput achieved by PDSCH.

2×2 MIMO using antenna port 0 and 1 approximately doubles the peak

throughput whereas 4×4 MIMO using antenna ports 0,1,2 and 3 approximately quadruples the peak throughput.

Cell specific reference signal overheads increase  when using MIMO with

antenna ports 0,1,2 and 3 so the throughput are less than double and quadruple than the single antenna case.

 

Shared Throughput 

PDSCH is a shared channel so its throughput capability is shared between all users.

Increasing the number of users reduces the throughput per user.

Users experiencing poor channel conditions reduce the overall total cell throughput .

Therefore as performance engineers and RF engineers you need to make sure users

experience minimal poor channel conditions.

Theoretical Throughput (non-MIMO)

The table below represents a set of theoretical absolute maximum physical layer throughput

which can be achieved if all Resource Elements are allocated to PDSCH channel and

the physical layer do not add any redundancy. ( This is not possible in practice , however

the table is provided as a reference from which you can have an understanding about

deriving the maximum expected throughput.

 

 

 

 

Non-MIMO Throughput

For non-MIMO throughputs values shown in the table A above.

These values have been generated by multiplying the modulation

symbol rate by the number of bits per symbol.

 

• As an example 20 MHz bandwidth has:
• 100 Resource Blocks in frequency domain
• 1200 subcarriers in frequency domain
• For normal cyclic prefix there are 14 OFDMA symbols during each 1 msec subframe

Computation of Bit Rate

Looking at the bullet points for 20 MHz bandwidth above.

The  Modulation symbol rate = 1200 * 14 / 0.001 = 16 Msps

The bit rate rate when using 64 QAM modulation scheme.

As there are 6 bits per symbol in case of 64 QAM modulation scheme

Therefore, 16.8 Msps * 6 bits per symbol = 100.8 Mbps

 

MIMO Throughput

The MIMO throughputs in the table above have been generated by multiplying

the 64 QAM througputs by the relevant MIMO rank, i.e.

the throughputs have been double for 2×2  MIMO and quadrupled for 4×4 MIMO

 

Near Realistic Throughput and Bit Rate Computation

In order to compute the maximum expected throughput is to remove the overheads generated by :

• Physical channels
• Physical signals i.e., PCFICH, PDCCH, PHICH and PBCH
• Primary and Secondary Synchronisation Signals (PSS an SSS)
• Cell Specific Reference Signal

 

The table below shows the maximum physical layer throughput with the overheads removed.

The results are computed with a coding rate of 1. Which means the physical layer has

not introduced any redundancy.

 

 

 

 

The table B above illustrates the significant impact of the number of OFDMA symbols allocated to PDCCH, PCFICH and PHICH.

These physical channels can be allocated

• 2, 3 or 4 symbols  for 1.4 MHz market channel bandwidth
• 1 , 2 or 3 symbols for other channel bandwidths i.e., 3 MHz, 5 MHz, 10 MHz , 15 MHz and 20 MHz

 

How to Decide how many OFDM Symbols allocated to PCFICH, PDCCH and PHICH ?

In industry practice, the number of OFDM symbols allocated to PCFICH, PDCCH and PHICH depends upon the quantity of traffic loading the cell.

 

Comparing Table A and Table B

The values in table B are significantly less than table A.

For example, the throughput associated with the following channel

20 MHz channel + Normal Cyclic Prefix + 4×4 MIMO

decreases as follows

403 Mbps to 325 Mbps, 306  Mbps  or 277 Mbps ( It all depends upon the number of symbols allocated to PDCCH, PCFICH and PHICH )  

 

This means the impact of overhead generated by physical channels and physicals

signals does affect the throughput and more overhead  reduces the application throughput.

As overheads do not transfer any application data.

 

Reduction in Application Throughput

The redundancy and overhead added to the physical layer reduces the throughput

measured at the top of the physical layer.

The PDSCH uses a combination of rate 1/3 Turbo coding and

rate matching to generated redundancy.

The redundancy is large: 

When UE experience poor channel conditions

The redundancy is small :

When UE experience good channel conditions

 

Coding Rate as a function of Channel Conditions

The figure below shows how physical layer define the physical layer coding as a

function of channel conditions and modulation scheme.

 

Coding rate reflects the quantity of redundancy added by the physical layer.

 

• A low coding rate indicates a large quantity of redundancy
• High Coding Rate reflects a small quantity of redundancy
• Coding rate of 1 indicates no redundancy

 

 

QPSK and a Low coding rate are associated with poor channel conditions.

Link adaption allocates larger transport block sizes as the channel conditions improve ,

but the modulation scheme is kept as QPSK.

On account of this, quantity of redundancy is decreased and coding rate is increased.

Therefore large quantities of data are transferred

without increasing the capacity of the physical channel.

 

Discussion on Picture

In the above picture of coding rate as a function of channel condition and modulation scheme.

Some things are visible

• Modulation is switched from QPSK to 16 QAM once the channel conditions have improved to allow coding rate to increase to 0.75
• Switching the modulation scheme increases the capacity of the physical channel such that the capacity of the physical channel so the
• quantity of redundancy can be increased.
• Link adaptation continues to allocate larger transport block sizes as channel conditions improve.
• The modulation scheme is switched from 16 QAM to 64 QAM once the channel conditions have improved enough to allow the coding rate
• to reach again 0.75

 

Once 64 QAM is allocated , link adaptation continues to allocate larger transport

block sizes as the channel conditions improve.

 

After 64 QAM there is no option to switch to a higher modulation scheme

once the coding rate reaches 0.75.  However, link adaptation

continues to allocate larger transport block sizes and the coding rate approaches 1.

 

What additional information comes on PDSCH ? 

Some additional information not associated with application data is also transferred on PDSCH.

This information includes:

• System Information Blocks ( SIB)
• Paging Messages
• RRC Signaling

 

The overhead generated by SIB, paging messages and RRC signaling depends

upon the quantity of traffic loading the cell but is likely to be relatively

small i.e, less than 100 kbps.

This additional information reduces the capacity of PDSCH available for application data.

 

 Retransmissions affects on PDSCH

Retransmissions reduce the higher layer throughputs.

Hybrid Automatic Repeat Request ( HARQ ) retransmissions from the MAC layer

reduce the throughputs measured from above the MAC layer.  A

utomatic Repeat Request ( ARQ  ) retransmissions from the RLC layer

reduce the throughputs measured from above the RLC layer.

Similarly TCP retransmissions reduce the throughput measured from above the TCP layer.

 

Protocol Headers

Protocol stack headers also reduce the higher layer throughputs.

The MAC , RLC , PDCP and IP layers add headers to the application data.

The PDCP layer provides header compression for IP data streams so is able

to reduce the impact of the IP header.

The TCP and UDP layers also add their own headers when using TCP or UDP applications

 

 

Conclusion:

In conclusion, the maximum throughput achieved on PDSCH is not a straight forward computation.

It all depends upon the scenario and assumptions for which you are computing the throughput available.

Also you read that based on the traffic in each Transmission Time Interval , the number of OFDM symbols can vary.

This can affect the overall throughput.

Therefore, in order to compute throughput on PDSCH in case of LTE FDD scenario. The most important point is to consider the context

in which you are computing the throughput and how much is the overhead in addition to actual application throughput.

 

In case if you are familiar with a scenario, please share it in comments below.