Question

why QPSK modulation scheme is used for all control channels in LTE ?

Answer 1

Hi,

control channels are common in LTE, To decode control channel we need modulation type used by ENB but this is not conveyed by ENB ,so to make it common modulation type they come up with QPSK. Why QPSK is it is effective among other modulation type.

Comments

Is it possible to use 16QAM,  64QAM modulation for control channels,  Which modulation scheme is used for PBCH.

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PBCH is QPSK modulated. No if you change 16QAM or 64 QAM then UE side and ENB PHY side we need changes , it is not possible now. 16,64QAM is good modulation type if channel quality is better  but for all condition QPSK will be effective.

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Hi,
    If we are near to eNodeB , we use 64 QAM or 16 QAM in this situation PBCH will use QPSK or 16,64 QAM,

Answer 2

I think reasons to use qpsk modulation scheme for signalling in LTE could be...
1. Qpsk is most robust modulation scheme in LTE. So any ue cell edge or cell center, can decode the signalling.
2. To reduce complexity at eNb , only one qpsk modulation scheme used for signalling.