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In simplified terms, EPDCCH use certain number of PRBs (not necessarily consecutive) - from PDSCH portion - for transmission of downlink and uplink allocation information.
EPDCCH stand for "Enhanced Physical Downlink Control CHannel".
MPDCCH is based on EPDCCH.
MPDCCH stand for "MTC Physical Downlink Control CHannel".
Refer below sections [8] for more detailed information about EPDCCH and MPDCCH.
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36.211:6.2.4A
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Enhanced Resource-Element Groups (EREGs)
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36.211:6.8A.1
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EPDCCH formats
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36.211:6.8B.1
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MPDCCH formats
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36.213: 9.1.4.4
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PRB-pair indication for EPDCCH
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MPDCCH may also be repeated in consecutive subframes in same or different (frequency hopping) narrowband.
Same is the case with PDSCH.
There is always one subframe gap between last of the MPDCCH repetitions and first PDSCH.
MPDCCH repetition information is known beforehand whereas PDSCH repetition information is captured in DCI sent in MPDCCH.
MPDCCH carry control information related downlink and uplink allocation; three new DCI formats have been introduced for eMTC UEs.
Format 6-1 is meant for DL allocation, format 6.0 for UL allocation, and format 6.2 for Paging.
Refer 36.212:5.3.3.1 [10] for more details.
eMTC UE monitor MPDCCH candidates depending on its state and procedure.
Let us take an example of RACH.
eMTC SIB2 PRACH configuration provide details about MPDCCH like narrowband, repetition, and so on.
After sending RACH preamble, eMTC UE monitor the MPDCCH narrowband for RAR(MSG2).
eMTC SIB2 also provide information of RACH preambles - this is how eMTC UEs are distinguished from LTE UEs.
Let us look at MPDCCH in bit more detail from monitoring point of view.
36.36.213:9.1.5 [9] "MPDCCH assignment procedure" define four types of search spaces.
A search space is set of resources that UE attempt to receive and decode.
Resource is virtual representation of physical resources.
In case of MPDCCH, resource is an ECCE.
An ECCE is consists of multiple enhanced resource element groups (EREGs).
Physical resource block pair contain 16 EREGs.
A physical resource block pair is basically two slots in one subframe.
To simplify it for understanding, you may visualise one narrowband as pool of ECCEs.
A MPDCCH candidate is certain number of ECCEs.
So a MPDCCH search space shall contain one or more MPDCCH candidates - depending on aggregation levels - in a narrowband.
Higher aggregation levels take up more resources and result in lesser number of candidates.
Depending on type of MPDCCH search space, eMTC UE (RF function) receive ECCEs and (DSP function) attempt to decode MPDCCH candidates for all possible aggregation levels.
EPDCCH has concept of EPDCCH-PRB-set. An EPDCCH-PRB-set contain 2, 4, or 8 PRBs (not necessarily consecutive).
MPDCCH support 2, 4, or 2+4 PRBs in a set from 6 PRBs of a narrowband.
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