## Introduction

This article explains the calculation steps of Guard Band, for the different LTE channels’ Bandwidth.

“A guard band is a narrow frequency range that separates two ranges of wider frequency. This ensures that simultaneously used communication channels do not experience interference, which would result in decreased quality for both transmissions.

The guard band concept applies to wired and wireless communications. It also simplifies the process of signal filtering for hardware, software or both”

The Guard Band in LTE has defined to be 10% of available bandwidth. As known, LTE technology offers several available channels bandwidth:

- 1.4 MHz
- 3 MHz
- 5 MHz

- 10 MHz
- 15 MHz
- 20 MHz

This means that different channels bandwidth will have different Guard Band, as shown in the following Table.

Furthermore, it must to be clear in your mind that LTE protocol is defined to be an OFDMA System and channels are multiplexed in Resource Elements (RE) and Resource Block (RB).

*Table 1: Bandwidth, RB number, Useful Band and Guard Band for LTE*

From Table. 1 it should be clear that only in the case of 1,4 MHz bandwidth the guard band does not correspond to 10%. In fact, it is equal to **320 KHz** or** 22.85%**

Therefore, following it is possible to find an explanation about this difference between channel 1,4 MHz and the others.

### Guard Band calculation for 1.4 MHz bandwidth channel

As we described before, for an LTE channel we will consider a Guard Band equal to 20% of the available bandwidth, distributed to 10% on the lower limit and 10% on the upper limit.

Therefore, the Useful Band available is:

Since an RB has a spectral occupancy of 180 kHz (an RB is equal to 12 RE, each with 15 kHz) we can calculate the effective number of Resource Block assigned to this channel.

We have to remind the measure units in LTE are RE, RB and channel bandwidth. So, to calculate the right number of RB, we must to round down to the nearest number:

Now, we have an oddment of 0,22, that means:

Therefore, the guard band for to 1.4 MHz channel is equal to:

which is equal to 22,85% of 1,4 MHz.

Finally the effective Useful Band is:

### Guard Band calculation for all other bandwidth channel

Now it should be clear that using the following formula in all other bandwidth cases, the number of RB is an integer. It means that the bandwidth is a multiple of RB plus the Guard Band (10%).

Hi,

thanks for sharing this .Very good explanation.

Br

Ashok

Hi Ashok,

thanks for your post!

We are happy to share our articles and we are going to publish more.

Please share

Stay Tuned!

BR

Hi Michele Roselli,

Very Nice article indeed thanks for sharing

Calculation for same 5MHZ band giving me result of Bg=1039khz which is not right how it can be corrected.

Hi Shaan,

for calculating the guard band to 5MHz channel (and the other channels except that one to 1.4 MHz) you have only to consider the 10% of the available bandwdith, distributing to 5% on the lower limit and 5% on the upper limit.

So, the correct steps are showed as follow.

The Useful Band available is:

Bu = 5 MHz – (250 kHz * 2) = 4500 kHz

where

Bg = 500 kHz < => 250 kHz + 250 kHz

which is equal to 10% to 5MHz.

Therefore in this case the number of RB is:

# RB = Bu / 1RB = 4500 kHz / 180 kHz = 25

I hope I explained well this point and clarified your doubt.

Thanks for your comment!!

BR

thanks for this nice article, this answers some of the questions I had about LTE 1.4 MHz guard bands.

Let’s imagine that you have 1.4 MHz of spectrum available. Would this mean that you could populate it with 1.4 MHz LTE without any additional guard band?

Hi IOTC360,

generally speaking the answer is yes!

Anyway, the boundaries conditions could change the answer… I mean that it is a need to know all the system setup.

e.g. The additional guard band could be a need when there are multiple technologies in the same band. LTE and GSM systems (@1800 MHz), could need an adjustment.

Thanks for your comment,

BR

Thanks a lot Michele, again great post!

Share our posts and do not hesitate to ask us more article!!!

I think you forgot to deduct the DC subcarrier, i.e. the subcarrier located at the dead center of the carrier which doesn’t carry any modulation and is used by UE to locate the center of the carrier frequency. See figure 5.6.1 of 3GPP 36.101.

So, subtract 7.5KHz from the above calculation (assuming subcarrier spacing of 15KHz).

Hi Liu,

you are right, thank you very much for your contribution!

BR

Hi Michele,

Thanks for your efforts!

Is it possible to explain as to WHY the 10% is chosen for others and 20% chosen for 1.4 MHz? I see the calculation as to arrive at the number of RBs but I fail to see why they chose the specific % as guard band. Why not more or why not less?

Hi Arun,

considering the 1.4MHz total band and dividing it for the occupation of a RB (180 kHz), it gets:

1400/180 = 7.77777… -> That is 7 RB with a surplus of 0.77777…

But 0.7 * 180 = 126 kHz which is less than 140 kHz (corresponding to 10% of the total bandwidth).

At this point, to increase the Guard Band and bring it to at least 10%, the only way you can take is to reduce the number of RBs by a unit.

The result is then 1.77777… of surplus (in the previous case was 0.77777…) corresponding to 1.77777… * 180 = 320 kHz Guard Band.

This value is approximately 22.85% of the total band calculated also in the demonstration in the article.

Furthermore, if you apply the same flow to other channels, you will get the correct bandwidth results shown in the table.

Let us know!

Thanks,

MR

Hi Michele,

Thanks for sharing. It’s a great post.

In your latest reply,

“1400/180=7.77777… -> That is 7 RB with surplus of 0.77777…

But 0.7*180=126 kHz which is less than 140 kHz (corresponding to 10% of the total bandwidth)”

I found that, 0.7*180=126 kHz is correct.

However, 0.77777…*180=140 kHz (equal to 10% of 1.4MHz)

That is, if we choose full RB as 7 RBs for 1.4MHz channel bandwidth.

Bu=7*180=1260 kHz

Bg=140 kHz => 70 kHz + 70 kHz

which is equal to 10% of 1.4MHz.

In this case, the 10% rule of guard bands would be able to apply for all channel bandwidth (1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz)

But still, 3GPP defines full RB=6RBs for 1.4MHz.

Is it possible to explain why 6RB is chosen instead of 7RB?

Thanks a lot.

Eileen

The values of the LTE channel bandwidths are a compromise between in- and out-of-channel distortions and were extensively studied in 3GPP. When LTE1800 is located between GSM1800 frequency bands and the LTE1800 bandwidth is 1.4 MHz or 3 MHz, a minimum guard band of 200 kHz is needed. With the 1.4 MHz channel bandwidth a 10% guard band would not be enough, therefore the LTE physical layer specifies to assume the configuration with 6 RBs and a larger guard band (https://technicaltweak.wordpress.com/2017/07/09/lte-collocation-with-gsmwimaxumts-guard-band-and-usable-bandwidth/).

Hi Michele Roselli,

This is a great post, and cleared my doubt about the minimum bandwidth of LTE a little bit.

This is a top-down calculation, assuming 1.4M is “legit”.

My question is, since 1.08 is the maximum occupied bandwidth, why not define the minimum bandwidth as 1.08*1.1=1.2M? So that the guard band will be 50K each? Would it be even more logical?

Cyrus

Hi Cyrus,

the 1.4 MHz and 3 MHz channel bandwidth options have been chosen to facilitate a multitude of CDMA2000 and GSM migration scenarios.

Furthermore, the values of the LTE channel bandwidths are a compromise between in- and out-of-channel distortions and were extensively studied in 3GPP.

In fact, the basic OFDM spectrum comprises only slowly decaying sidelobes and efficient usage of the spectrum requires the use of filtering to effectively confine the out-of-band emissions. Such filters require a certain amount of transition bandwidth in order to be practical and to consume only a small amount of the cyclic prefix duration [1].

When LTE1800 is located between GSM1800 frequency bands and the LTE1800 bandwidth is 1.4 MHz or 3 MHz, a minimum guard band of 200 kHz is needed. With the 1.4 MHz channel bandwidth a 10% guard band would not be enough, therefore the LTE physical layer specifies to assume the configuration with 6 RBs and a larger guard band [2].

REFERENCES[1] H. Holma, A. Toskala – “LTE for UMTS – OFDMA and SC-FDMA Based Radio Access”, p. 286 (https://books.google.it/books?id=uhr3KwSww2kC&pg=PA286&lpg=PA286)

[2] https://technicaltweak.wordpress.com/2017/07/09/lte-collocation-with-gsmwimaxumts-guard-band-and-usable-bandwidth/

Hi! This is very helpful! Does this formula apply to both FDD and TDD LTE?

Yes, the possible configurations are shown in 3GPP TS 36.104 (http://www.qtc.jp/3GPP/Specs/36104-800.pdf)

Here you can find an overview: https://image.slidesharecdn.com/anritsulteguide-150714132105-lva1-app6891/95/anritsu-lte-guide-12-638.jpg