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In this paper, we discussed about LTE system throughput calculation for both TDD and FDD system.

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In this paper, we discussed about LTE system throughput calculation for both TDD and FDD system.

3GPP LTE technology support both TDD and FDD multiplexing. The paper describes all the factors which affect the throughput like Bandwidth, Modulation, UE category and mulplexing. It also describes how we get throughput 300Mbps in DL and 75Mbps in UL and what are assumptions taken to calculate the same.

Paper describes the steps and formulae to calculate the throughput for FDD system for TDD Config 1 and Config 2.

The throughput calculations shown in this paper is theoretical and limited by the assumptions taken to calculate for calculations

- December 2012 | | Preet Kanwar Rekhi, Mohit Luthra, Sukhvinder Malik, Rahul Atri White Paper Throughput Calculation for LTE TDD and FDD Systems 1. Introduction Contents Many of us might have heard about LTE’s peak throughput i.e. 1. Introduction 2. Overview of LTE 300Mbps, but how many of us know how we calculate that? This Physical Layer paper provides the information, how this number is calculated? And 3. Basic Terminology assumptions behind? 4. Maximum Throughput with Maximum In this paper, authors have explained the calculations of theoretical Bandwidth throughput for both the LTE FDD and TDD systems. 5. Use of 3GPP specification 36.213 for throughput calculation2. Overview of LTE Physical Layer 6. DL and UL throughput calculation for LTELTE Physical layer deals with parameters like frequency, bandwidth, FDDModulation, cyclic prefix, coding rate which plays importance in 7. LTE TDD and its frame structurecalculation of the throughput. 8. DL and UL throughput calculation for LTELTE system uses OFDMA as access technology in downlink to TDDincrease the spectral efficiency and SC-FDMA in uplink due to low 9. ConclusionPeak to Average Power ratio (PAPR) advantage. 10. ReferencesLTE supports both TDD and FDD duplexing, flexible bandwidthi.e.1.4, 3, 5,10,15,20 MHz and modulation schemes QPSK, 16 QAM,64 QAM.Later we will discuss the significance of each parameter. 1
- 3. LTE Basic Terminology There are some basic terminologies of LTE system that should be known to better understand the throughput calculation. These are explained below: Resource Element - The RE is the smallest unit of transmission resource in LTE, in both uplink and downlink. An RE consists of 1 subcarrier in the frequency domain for duration of 1, Orthogonal Frequency Division Multiplexing (OFDM) or Single Carrier- Frequency Division Multiplexing (SC-FDM), symbol in the time domain Subcarrier Spacing- It is the space between the individual sub- carriers, in LTE it is 15 KHz. There is no frequency guard band between these subcarrier frequencies, rather a guard Period called a Cyclic Prefix (CP) is used in the time domain to help prevent Multipath Inter-Symbol Interference (ISI) between subcarriers.LTE Basic Terminology Cyclic Prefix - A set of samples which are duplicated from the end of a transmitted symbol and appended cyclically to the beginning of the Resource Element (RE) symbol. This can form a type of guard interval to absorb Inter-Symbol is the basic unit in LTE Interference (ISI). The cyclic construction preserves orthogonality of which corresponds to a the subcarriers in an OFDM transmission. subcarrier in frequency . and a symbol in time Resource Block is the Time slot - 0.5 ms time period of LTE frame corresponding to 7 combination of 12 OFDM symbols (and 7 CPs) when Normal CP = 5 usec is used (the subcarriers in frequency standard case). And LTE 6 OFDM symbols (and 6 CPs) when the and 7 symbols time (0.5 Extended CP = 17 usec is used. msec). LTE frame is of 10 msec can be divided into 10 subframe or 20 timeslots A subframe is also called one TTI (Transmit Time Interval) Combining the above information we can now define a Resource Block. Resource Block - A unit of transmission resource consisting of 12 subcarriers in the frequency domain and 1 time slot (0.5 ms) in the time domain. So12 subcarriers x 7 symbols = 84 Resource Element (with Normal CP) makes a Resource Block. IF extended CP is used there are 72 Resource elements (RE). Since 12 OFDM subcarriers are used in a RB, the bandwidth of a Resource Block is 180 KHz. LTE Subframe or TTI- two slots i.e. 1 ms in time. 2 LTE Frame - 10 ms or 10 subframes or 20 slots.
- 3.1 Relation between Bandwidth and Resource Block:Bandwidth directly affects the throughput. Different BWs havedifferent number of RBs.Here is the calculation how to find out the numbers of subcarriersand Resource Blocks.10% of total bandwidth is assumed to be used for guard band.Though 10 % guard band assumption is not valid for 1.4 MHzbandwidth.Let’s take an example of 20MHz.10% of 20 MHz is 2 MHz, used as guard band, thus effectivebandwidth will be 18MHz.Number of subcarriers = 18 MHz/15KHz = 1200Number of Resource Blocks =18 MHz/180KHz = 100Same calculations can be done with other bandwidths to calculatethe number of subcarriers and Resource Blocks. Same is shownbelow:3.2 Multiplexing and Bandwidth:LTE supports both types of multiplexing FDD as well as TDD.FDD spectrum is also called paired spectrum, it means when we sayFDD 20 MHz, it has a pair of 20 MHz Bandwidth i.e. 20 MHz forDownlink and 20 MHz for Uplink.TDD spectrum is called Un-paired it means when we say TDD 20MHz, it has only 20 MHz which is used for both Downlink andUplink. 3
- This Multiplexing technique directly affects throughput as in FDD which has symmetric bandwidth so both Uplink and Downlink have same throughput, but in TDD the bandwidth is asymmetric and same bandwidth is shared by Uplink and Downlink on time sharing basis so the total throughput is also shared accordingly. Below figure shows the same. In coming example, we will show how FDD and TDD impact throughput. Choice of multiplexing depends on the band defined. The 700 MHz band used in US is FDD and 2300MHz band in India is TDD. 3.3 Modulation and Coding Rate: As per Release 8 (R8) LTE supports modulations like QPSK, 16 QAM and 64QAM in Downlink and QPSK, 16 QAM in Uplink. Each of Modulation has its bits carrying capacity per symbol. One QPSK symbol can carry 2bits, one 16QAM symbol can carry 4bits and 64 QAM symbol can carry 6 bits. This is shown below with constellation diagram:4
- Along modulation there is term called coding rate. Coding ratedescribes the efficiency of particular modulation scheme. Forexample, if we say 16 QAM with coding rate of 0.5, it means thismodulation has 50% of efficiency i.e. as 16QAM can carry 4 bitsbut with coding rate of 0.5, it can carry 2 information bits and restof the 2bits for redundancy of information.LTE uses different coding rate with QPSK, 16 QAM and 64QAM.The combination of Modulation and Coding rate is calledModulation Coding Scheme (MCS). Below figure shows MCSindex and Modulation Order which describes the type ofmodulation (2 for QPSK, 4 for 16QAM and 6 for 64 QAM).LTE supports 0 to 28 MCS in Downlink and 0to 22 MCS in Uplinkas per R8. LTE Access Network Modulation supported LTE supports QPSK,16-QAM and 64- QAM for data channel QPSK carries 2 bits per Symbol, 16 QAM carries 4 Bits per Symbol and 64 QAM carries 6 Bits per symbol. Each modulation is used with some coding which makes Modulation Coding Scheme (MCS). Each MCS have a corresponding TBS index value which is used for mapping TB size with Resource Block numbers to find the throughput. 5
- 3.4 UE Categories in LTE The category of UE specifies the ability of the Device in terms of DL/UL throughputs, Antenna Support in DL/UL, TBS size supported in DL/UL and Modulation supports. The below table shows the 8 categories of UE, existing UE categories 1 -5 are for release 8 and 9 and UE categories 6-8 are for release 10 LTE –Advance. Commercial UEs that we have today are mostly of Category 3 (Cat 3) which have 2 receive chains and 1 transmit chain. Cat 3 UE does not support 64 QAM in uplink. The Max TB size supported in DL is 75376 bits and in Uplink 51024 bits. This TB size limits the throughput at UE end while do not have such limitation at eNodeB side.Maximum Throughput withMaximum Bandwidth 4. Maximum Throughput with Maximum Bandwidth For any system throughput is calculated as symbols per second. There are different UE Further it is converted into bits per second depending on the how categories depending on many bits a symbol can carry. capabilities like TB size support, Number of In LTE for 20 MHz, there are 100 Resource Blocks and each antenna support and Resource block have 12x7x2=168 Symbols per ms in case of Normal Modulation support CP. Category 3 is available So there are 16800 Symbols per ms or 16800000 Symbols per second commercially. or16.8 Msps. If modulation used is 64 QAM (6 bits per symbol) then Maximum throughput throughput will be 16.8x6=100.8Mbps for a single chain. for LTE is 300 Mbps in Downlink and 75 Mbps For a LTE system with 4x4 MIMO (4T4R) the throughput will be in Uplink. Throughput is four times of single chain throughput. i.e. 403.2 Mbps. Many calculated as symbols simulations and studies show that there is 25% of overhead used for per second and further Controlling and signalling. So the effective throughput will be 300 converted in to bits per Mbps. second depending upon the Modulation used. The 300 Mbps number is for downlink and not valid for uplink. In uplink we have only one transmit chain at UE end. So with 20 MHz we can get Maximum of 100.8Mbps as calculation shown above. After considering 25% of overhead we get 75Mbps in uplink. This is the way how we get the number of throughput 300Mbps for 6 Downlink and 75Mbps for Uplink shown everywhere.
- 5. Use of 3GPP specification 36.213 for Throughput calculationIn 3GPP specification 36.213 “E-UTRA- Physical Layer”, table7.1.7.1-1 shows the mapping between MCS (Modulation andCoding Scheme) index and TBS (Transport Block Size) index. Thehighest MCS index 28 (64 QAM with the least coding), which ismapped to TBS index 26 as shown below. Specification Used from 36.213 Modulation and TBSTable 7.1.7.2.1-1 shows the transport block size. This table index table7.1.7.1-1 isindicates the number of bits that can be transmitted in a used for Modulationsubframe/TTI (Transmit Time Interval) w.r.t bandwidth (number of and TBS indexRBs).The Transport Block size given in this table is after mappingconsidering the controlling overhead. Resource block and TBS index mapping table 7.1.7.2.1-1 is used for TB sizeBy using these two tables the number of data bits can be calculated,with the combination of MCS Index and Number of ResourceBlocks.For example, with 100 RBs and MCS index of 28, the TBS is75376. Assume 4x4 MIMO, the peak data rate will be 75376 x 4 =301.5 Mbps.6. DL and UL Throughput calculation for LTEFDDThe FDD system has a paired spectrum, same bandwidth forDownlink as well as for Uplink. 20 MHz FDD system have 20MHz for Downlink and 20 MHz for Uplink.For throughput calculation, suppose:Bandwidth – 20MHzMultiplexing scheme - FDDUE category- Cat. 3Modulation supported- as per Cat 3 TBS index 26 for DL (75376 7for 100RBs) and 21 for UL (51024 for 100 RBs)
- So the throughput can be calculated by a simple formula: Throughput = Number of Chains x TB size So DL throughput = 2 x 75376 =150.752 Mbps UL throughput = 1 x51024 =51.024 Mbps As we have 2 receive chains and one transmits chain. 7. LTE TDD and its Frame structure Before starting throughput calculation, let’s become familiar with LTE- TDD. As stated earlier, TDD is unpaired spectrum. We have to use same bandwidth for DL and UL on time sharing basis. Suppose if we have 20 MHz spectrum, we have to use this 20 MHz bandwidth for both DL and UL. LTE TDD frame structure is shown below. The TD frame consists of Downlink subframe, Uplink and Special subframe. There are seven possible configurations for LTE TDD frame as shown below. Here D- is downlink, S- for Special subframe and U- for Uplink. As shown 5 ms periodicity frame have two “S” subframe and 10 mili sec frames have only one “S” subframe. Special subframe has 9 different configurations. A special subframe is divided into DwPTS, GP and UpPTS depending upon the number of symbols.8
- .Selection of TDD configuration is on operator‘s choice and servicemodel. If the service model is heavily downloading based, operatormay use TDD config 2 or TDD config 5 and if service model isheavily uploading based, operator may use TDD config 0 or TDDconfig 6.If service model is symmetric (50:50) or almost same for downloadand upload, operator may use TDD config 1 or TDD config 3.8. DL and UL Throughput calculations for LTETDDTDD system throughput calculations are somewhat complex ascompared to FDD system as same spectrum is used by uplink,downlink and for the guard period (Used for transition from downlinkto uplink).For throughput calculation, suppose:Bandwidth – 20MHzMultiplexing Scheme- TDDTDD Configuration- 2 (D-6, S-2 and U-2)Special Subframe configuration-7 (DwPTS-10, GP-2 and UpPTS-2)UE category- Cat. 3Modulation supported- as per Cat 3 TBS index 26 for DL (75376 for100RBs) and 21 for UL (51024 for 100 RBs)Throughput in TDD can be calculated by following formulaDL Throughput = Number of Chains x TB size x (Contributionby DL Subframe + Contribution by DwPTS in SSF)UL Throughput = Number of Chains x TB size x (Contributionby UL Subframe + Contribution by UpPTS in SSF)TB size for DL is 75376 and for UL it is 51024 for category 3 UE 9
- Let’s calculate throughput for the above assumptions: DL throughput = 2 x 75376 x [(0.6+0.2x (10/14)] Here 0.6 or 60% contribution is by 6 DL subframe and [0.2(10/14)] factor contribution by Special subframe comes twice whose 10 symbols out of 14 are for downlink. So DL throughput= 2 x 75376 x (0.742857) = 111.9872 Mbps ~ 112 Mbps. In same manner UL throughput will be UL throughput = 1 x51024 x [(0.2+0.2x (2/14)] Here 0.2 or 20% contribution is by 2 UL subframe and [0.2 x (2/14)] factor contribution by Special subframe comes twice whose 2 symbols out of 14 are for uplink. So UL throughput= 1 x51024 x (0.228571) = 11.66263 ~12 Mbps. Let’s do one more example TDD config 1 (D-4 S-2 and U-4) Special subframe configuration 7 (DwPTS-10, GP-2 and UpPTS-2) same UE category 3 DL throughput = 2 x 75376 x [(0.4+0.2x (10/14)] Here 0.4 or 40% contribution is by 4 DL subframe and [0.2(10/14)] factor contribution by Special subframe comes twice whose 10 symbols out of 14 are for downlink. So DL throughput= 2 x 75376 x (0.542857) = 81.8368 Mbps ~ 82 Mbps. In same manner UL throughput will be UL throughput = 1 x51024 x [(0.4+0.2x (2/14)] Here 0.4 or 40% contribution is by 4 UL subframe and [0.2 x (2/14)] factor contribution by Special subframe comes twice whose 2 symbols out of 14 are for uplink. So UL throughput= 1 x51024 x (0.428571) = 21.8674286~22 Mbps.10
- 8. ConclusionAuthors In this paper, we discussed about LTE system throughput calculation for both TDD and FDD system. 3GPP LTE technology support both TDD and FDD multiplexing. The paper describes all the factors which affect the throughput like Bandwidth, Modulation, UE category and mulplexing. It also describes how we get throughput 300Mbps in DL and 75Mbps in UL and what are assumptions taken to calculate the same. Paper describes the steps and formulae to calculate the throughput for FDD system for TDD Config 1 and Config 2.Mohit Luthra The throughput calculations shown in this paper is theoretical andLTE RF Design Engineer limited by the assumptions taken to calculate for calculation. 9. References 1. Wikipedia.com 2. www.3gpp.orgPreet Kanwar Singh Rekhi 3. 3GPP standard 36.211 , Evolved Universal TerrestrialLTE Testing Engineer Radio Access (E-UTRA)- Physical Channels and Modulation 4. 3GPP standard 36.212 , Evolved Universal Terrestrial Radio Access (E-UTRA)- Multiplexing and Channel Coding 5. 3GPP standard 36.213 , Evolved Universal Terrestrial Radio Access (E-UTRA)- Physical Layer Procedures 6. 3GPP standard 36.300 , Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description 7. LTE, The UMTS long Terms Evolution: From Theory toRahul Atri, LTERadio Access Network Engineer Practise Disclaimer: Authors state that this whitepaper has been compiled meticulously and to the best of theirSukhvinder Malik knowledge as of the date of publication. The information contained herein the whiteLTE Testing Engineer paper is for information purposes only and is intended only to transfer knowledge about the respective topic and not to earn any kind of profit. Every effort has been made to ensure the information in this paper is accurate. Authors 11 does not accept any responsibility or liability whatsoever for any error of fact, omission, interpretation or opinion that may be present, however it may have occurred