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Modulation and Signal Quality at LTE
July 28, 2017 | By Mohamed Abdel Monem @ Vodafone
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We are pleased to share with you all an interesting article contributed by Mohamed Abdel Monem. 


Mohamed Abdel Monem 

Radio Optimization Senior Engineer at Vodafone Egypt



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Each symbol represents bits depending on the modulation technique, Each data point represents a different bit pattern.  LTE devices use QPSK, 16QAM and 64QAM to modulate data and control information.  The eNodeB supports all of these modulation techniques for the Down Link direction. However, 64QAM is optional in the Uplink direction.


A modulation technique is selected based on the measured Signal to Noise Ratio (SNR). Subscribers located away from the eNodeB must use more robust modulation schemes (lower throughput), or they will experience unacceptable data loss rates. In addition, subscribers close to a sector boundary may experience inter-sector interference.


Each modulation scheme has a threshold SNR. For example, let’s assume

  • QPSK is associated with SNR 9 dB.
  • 16QAM with 15 dB.
  • 64QAM with 25 dB. 

If the SNR drops below 15 dB, the eNodeB will instruct the UE to use QPSK modulation.


Measuring Signal Quality

  • Signal quality measured using Reference Signals
  • eNodeB initially measures UE signal quality during network entry.
  • eNodeB controls the modulation selected for both DL and UL.

Both the eNodeB and the UE measure signal quality using Reference Signals.


The Reference Signals carry a known (pseudo-noise) bit pattern at a boosted power level.


The receiver performs channel equalization based on the received Reference Signals.


The eNodeB calculates the UE-transmitted signal quality by measuring UE Reference Signals, and by requesting Channel Quality Indication (CQI) feedback from the UE.


The eNodeB always controls and selects the modulation and coding scheme for both the Downlink and Uplink.

Amer hameed 2017-07-31 16:07:04

Good article , thanks

saoussane 2017-08-17 18:55:59


Thank you for sharing this interessting article.

I want to know if in theory we can estimate the CQI by estimating the SNR, can we do it just with the distance between the user and the eNodeB.

Best Regards.



Raj via LinkedIn 2017-08-25 09:58:42

Quite a refresher but I don't see the mention of 256 QAM.

James (VNPT Net Corporation) via LinkedI 2017-08-27 12:23:40


Akhmad (Baryon Hasta Persada) via Linked 2017-08-27 12:24:04

What's the likelihood 256QAM can be deployed?

MAGAYE (Ericsson) via LinkedIn 2017-08-27 12:25:39

@Mr. Raj, if we based description post from Mr Yoo, for 256QAM you need to add 10db for reach Max SNR related 256QAM, this means Dmax(256 QAM)= Dmax(64 QAM) /3 if I m not wrong. But if we have bad QoS on the link , system will switch you directly for an less modulation for adapte you on best QoS. For have 256QAM you need to best QoS at the right distance.


Oh excuse of course not forget the load, if you don't have x2 interface need Master SON for offload the maximum of user during Busy hour period for keep Max user on right distance with Max modulation.

Elias (Nokia) via LinkedIn 2017-08-28 10:31:54

Tq. Nice as a Refresher...

Ariel (Nokia) via LinkedIn 2017-08-31 09:21:47

You can find 256QAM in DL as additional options too:)

erayuyanik 2017-09-02 17:38:01

Too basic , 256 Qam already deployed in our network and not consider how Ues sending CQI information. PUCCH.. Thanks for the effort.



Elias (Nokia) via LinkedIn 2017-09-04 10:42:21

Tq. Nice as a Refresher...

Dharmawan (Nexwave Technologies) via Lin 2017-09-07 10:25:21


Mohammad Nabipour 2017-10-04 20:46:18

Thanks Very Good

San Bacho 2017-11-01 01:18:30

Thanks. We need to equate this also to category of UE being used whether can support the modulation technique.

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