Factors affecting lte throughput and calculation methodology

LTE Throughput calculation and application in wireless rollout projects INTERNAL

Paper Title LTE Throughput calculation and application in wireless rollout projects

Author Name Ahmed Alaa Sarhan – A00315759

Huawei Technologies Co., Ltd.

LTE History:LTE started off as an idea in 2004 as NTT DoCoMo of Japan proposed LTE as the

international standard. While the first presentation of an LTE demonstrator offering HDTV streaming (>30 Mbit/s) and Mobile IP-based handover between the LTE radio test subject and the commercially available HSDPA RAT was shown during the ITU trade fair in Hong Kong in December 2006 by Siemens (today Nokia Siemens Networks).

In February 2007, Ericsson demonstrated for the first time in the world LTE with max throughput of 144 Mbit/s, later in September of the same year, NTT DoCoMo demonstrated LTE data rates of 200 Mbit/s with power level below 100 mW during tests. Eventually, LTE technology was introduced commercially in December 2009 by TeliaSonera in Norway and Sweden.

Figure (1): LTE commercial map around the globe, 2015


LTE Throughput:Upon the introduction of LTE, many have seen or heard about wild figures, mainly pushed by system vendors and consumed by operators, journalists and writers who like to wow the readers while promising 1 or 2 GBit/s throughput.

On the network operator side (customer), the capacity expectations has negative consequences as capacity mainly impacts the cost of the network both on the access side and the backhaul side. Exaggerated capacity figures would lead to under-dimensioning on the access side and over-dimensioning on the backhaul side. So, for example, if we think LTE cell will provide 100 Mbps of throughput while in reality it can only offer 50 Mbps, the operator will be short by 50% of capacity in the access network resulting in a "Below Expectations" user experience and will be 50% over the required capacity for backhaul in which case its investment in capacity that's sitting idle. This is why it is important for Vendors to match the capacity expectations right and the customer to be realistic about their demands.

While many have heard about the standard and realistic LTE's peak throughput, e.g. 300 Mbps, it's not a standard by any means and can vary based on a lot of dependencies starting from the hardware equipment all the way to the transmission medium conditions. In this paper, we will explain the calculations of theoretical throughput for both the LTE FDD and TDD systems and how to use this knowledge in the design and acceptance phases of wireless LTE rollout projects.

Overview of LTE Physical Layer:

The LTE physical layer deals with parameters like frequency, bandwidth, modulation scheme, cyclic prefix and coding rate which play important roles in the calculation of the peak throughput of LTE networks.

LTE system uses OFDMA as access technology in downlink to increase the spectra efficiency and SC-FDMA in uplink due to low peak-to-average power ratio advantage.

LTE supports both TDD and FDD duplexing, flexible bandwidth i.e. 1.4,3,5,10,15 and 20 MHz and modulation schemes QPSK, 16QAM, 64QAM. Below we will discuss the significance of each parameter.


(A) OFDM:- OFDM is used in LTE as users can be allocated to different resources in BOTH time

and frequency domains, unlike TDMA or FDMA where the user was mainly allocated to different resources in one of the two domains and fixed in the other.

OFDM main advantage is the spectral efficiency, as it eliminates the usage of guard bits in the frequency domain which allows the maximum and most efficient use and transmission of data.

Figure (2): Orthogonal Frequency Subcarriers eliminate the usage of guard bands

One of the other main advantages of using OFDM is resistance to multipath


interference effect, which is done through inserting a guard time slot in the time domain, usually referred to as "Cyclic prefix". In LTE we have 4 types of Cyclic prefixes:

Figure (3): Cyclic Prefix Types

Figure (4): Cyclic Prefix to eliminate the effect of multipath

(B) Resource Block:-


Combining the previous information about the OFDM, we can now understand the concept of a resource block. Resource Block - A unit of transmission resource consisting of 12 subcarriers in the frequency domain and 1 time slot (0.5 mSec) in the time domain.

Figure (5): Resource Block in Frequency and Time Domains

Frequency subcarriers: Each subcarrier is 15 KHz with no guard bands, each resource block contains 12 subcarriers. Number of RB in frequency channel = Channel Freq. in KHz / 12 * 15 KHz

Time Slot: 0.5 millisecond time period of LTE frame corresponding to 7 OFDM symbols (which is equal to 7 Cyclic Prefixes) when Normal - Type 1 Cyclic prefix is used and varies when other types of cyclic prefixes are used.

(C) LTE Frame:-


An LTE frame consists of 10 ms = 10 subframes = 20 time slots. While an LTE subframe or TTI, which is the least resource that carries data in LTE consists of two slots i.e. 1 millisecond in time.

Figure (6): LTE Time Frame / SubFrame System with CP in Time Domain

Figure (7): LTE Frame in Frequency Domain

Calculation and Factors:Before anything, think of LTE domain as 2-D plane. Where both frequency and time are


utilized to carry data. Like the following diagram putting together LTE frame, Frequency Channel & Resource Block.

Figure (8): LTE Frame format in Time and Frequency Domains

For Each channel frequency we'll have different number of resrouce blocks and higher througput, in this case, let's assume the 20 MHz channel, remove 10% as total guard bands (used to compensate the inter-bit loss).

Figure (9): Different LTE Channel Frequencies related to subcarriers after guard bands

So, our bandwidth now is 18MHz.


Number of subcarriers= Channel Bandwidth / Subcarrier Bandwidth = 18 MHz/15 KHz = 1200 Subcarrier

Number of Resrouce Blocks= Channel Bandwidth / (Subcarrier Bandwidth * Number of subcarreris in 1 RB)

= 18 MHz/ 15 KHz * 12 = 100 RB

LTE supports both types of Multiplexing FDD and TDD.

FDD spectrum is also called paired spectrum, which means DL uses 20 MHz and UL uses another 20 MHzTDD spectrum, however, is un-paired, which means DL and UL are using the same 20 MHz channel

So upcoming, we'll see how FDD and TDD impact the throughput.

In Release 8, it's defined that LTE supports modulation like QPSK, 16QAM and 64 QAM in DL and QPSK, 16 QAM in UL.

Each Modulation scheme has its bits carrying capacity per symbol. One QPSK symbol for example can carry 2bits. One 16QAM can carry 4 bits and 64QAM can carry 6 bits.

Figure (10): QPSK vs. 16QAM vs. 64QAM

While speaking of Modulation, we must also speak of a parameter called coding rate. Coding rate defines the efficiency of particular modulation scheme. for example, if we say 16 QAM with coding rate of 0.5, it means this modulation has 50% of efficiency i.e. as 16QAM can carry 4 bits


but with coding rate 0.5 it can carry 2 information bits and 2 redundancy bits for these information.This is called Modulation Coding Scheme (MCS). Below is the table for it. LTE supports 0 to 28 MCS in DL and 0 to 22 MCS in UL in R8.

Figure (11): MCS table for different modulation schemes

One more factor to calculate is the UE category, this is reported in the UE capability report in LTE signaling between UE and EUTRAN.


UE category 1-5 are for release 8 and 9 while UE category 6-8 are for release 10 (LTE-Advance).

Figure (12): UE category relevant to throughput bit rate

So back to calculation for the maximum throughput in LTE network.

Like we calculated before, in 20 MHz channel we have 100 RB.

Now, each RB contains a number of Resource Elements (RE), REs carry the modulated signal, so the more REs in RB the more data it can carry.

Number of REs = No. of Frequency subcarriers * No. of OFDM symbols= 12 * 7 = 84 RE in 1 RB

To get the number of REs in 1 time slot (1 millisecond) = number of REs in 1 RB * 2= 84 * 2 = 168 RE/millisecond

So if we have 100 RB, this means we have 168000 Resource elements in the 20 MHz frequency channel every 1 millisecond.

So that's 168000000 Resource elements in the 20 MHz freq. channel every 1 second.

Since we're using 64QAM, 1 RE will contain 6 bits. (Assuming no MCS)

Then in 1 second, we have 168000000 * 6 = 100.8 Mbit / Second

Then, assuming we use 4x4 MIMO system and that the UE can support this, then DL bandwidth = 100.8 * 4 = 403.2 Mbit/Sec.

In UL, however, no MIMO is used, so UL throughput is 100.8 Mbps


Many simulation studies show that 25% normally of the total channel bandwidth is used by overhead bits for controlling and signaling, calculated as follows:

- PDCCH channel can take 1 to 3 symbols out of 14 in a subframe. Assuming that on average it is 2.5 symbol the amount of overhead is 2.5/14 = 17.86%- Downlink RS signal uses 4 symbols in every third subcarrier resulting in 16/336= 4.76% overhead - Other channels (PSS, SSS, PBCH, PCFICH, PHICH) added together amount to approx. 2.6% of overhead.

Then added together will be almost 25.22%

So, roughly 25% of the 400 Mbps DL throughput, we have 300 Mbps DL and 75 Mbps UL when using:

20 MHz channel 64QAM Normal - Type 1 cyclic Prefix No MCS 4x4 MIMO

But be noted that this is shared throughput for an eNodeB, which means this peak throughput will be divided among users according the the QoS system of LTE

Figure (13): QoS for different LTE services

The QoS will be scheduled according to one of three algorithms:


Max-C/I Round Robin PF/EPF

With each one of the three giving various throughput to each of the users.

Now, when implementing an LTE FDD solution in rollout projects, there are several factors that you need to keep in mind when assuming or proposing the final throughput of the network.

1. Channel Frequency2. Modulation Technique3. Transmission Medium (Because if the medium is lossy, you cannot use high modulation

techniques otherwise interference will deliver very bad LTE signal or you will be forced to use MCS)

4. General Users Equipment category5. Cyclic prefix (Also to prevent multipath interference)6. MIMO capabilities (according to UE capability)

These factors are integrated into the eNodeB Cell information when creating the eNodeB 6.0 data configuration file as follows:Using ADD CELL command:

ULCYCLICPREFIX > Uplink Cyclic Prefix Length, you can choose between Normal and Extended

DLCYCLICPREFIX > Downlink Cyclic Prefix length, you can choose between Normal and Extended

ULBANDWIDTH > Uplink Bandwidth, CELL_BW_N6 (1.4 MHz) CELL_BW_N15 (3 MHz) CELL_BW_N25 (5 MHz) CELL_BW_N50 (10 MHz) CELL_BW_N75 (15 MHz) CELL_BW_N100 (20 MHz)

DLBANDWIDTH > Downlink Bandwidth, CELL_BW_N6 (1.4 MHz) CELL_BW_N15 (3 MHz) CELL_BW_N25 (5 MHz) CELL_BW_N50 (10 MHz) CELL_BW_N75 (15 MHz) CELL_BW_N100 (20 MHz)

TXRXMODE > MIMO Mode, 1T1R (No MIMO), 1T2R (No MIMO), 2T2R (throughput x2), 2T4R (throuput x2), 4T4R (throughput x4), 8T8R (througput x8)

Technology

Factors affecting lte throughput and calculation methodology