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EPDCCH DM-RS resource element indices

`ind = lteEPDCCHDMRSIndices(enb,chs)`

`ind = lteEPDCCHDMRSIndices(enb,chs,opts)`

returns
indices of the Enhanced Physical Downlink Control Channel Demodulation
Reference Signal (EPDCCH DM-RS) resource elements (RE) associated
with an EPDCCH transmission candidate in a subframe. By default, `ind`

= lteEPDCCHDMRSIndices(`enb`

,`chs`

)`ind`

is
a column vector of indices in one-based linear indexing form. Use
this form to directly index the EPDCCH DM-RS REs of an *N*-by-*M*-by-4
array that represents the subframe resource grid across the four possible
EPDCCH antenna ports (*p* = 107...110). You can also
generate alternative index representations. The order of the indices
is the same as required for the complex EPDCCH DM-RS symbols mapping. `lteEPDCCHDMRS`

generates these symbols.

The indices are parameterized in terms of a configured PRB pair set which defines:

the overall set of possible EPDCCH candidates and

the aggregation of one or more consecutive enhanced control channel elements (ECCE). This aggregation identifies the specific EPDCCH instance that the DM-RS are associated with.

The DM-RS indices are created only for the specific PRB pairs and antenna ports that the corresponding EPDCCH is mapped to. They do not account for any external precoding operations.

For a localized EPDCCH transmission, the EPDCCH is associated
with a single antenna port from *p* = 107...110,
dependent on the `RNTI`

and ECCEs selected. Thus,
the DM-RS antenna port indices (1...4 respectively, if one-based)
are output for that single port.

For a distributed transmission, the EPDCCH is mapped to two
antenna ports in an alternating fashion. Therefore, the DM-RS indices
are generated for the PRBs in both ports: *p* = 107,109
for normal cyclic prefix and *p* = 107,108 for extended
cyclic prefix. The output is ordered so that the symbols for the lowest
antenna index plane come first. These indices are suitable for indexing
an *N*-by-*M*-by-4 array representing
the subframe resource grid across the four possible EPDCCH antenna
ports (*p* = 107...110).

This syntax returns an NRE length column vector of one-based
linear indices for the DM-RS resource elements associated with a particular
EPDCCH candidate. The function is initialized according to the cell-wide
settings, `enb`

, and the EPDCCH transmission configuration, `chs`

.