REG is one of resource allocation units which is made up of 4 resource elements. (Refer to Resource Allocation Management Unit page for other type of units and for the relationshipt between REG and other units (e.g, RE or CCE)

Following is the visualization of the description on 36.211 6.2.4 Resource Element Groups.

It may look pretty completed because it has two make a group of 4 REs except the REs allocated for Reference Signal. The location of Reference Signal varies depending on number of antenna configuration and other factors (e.g, Physical Cell ID, Antenna port/DMRS etc)

The REG exists for the control channels. PCFICH, PHICH and PDCCH are all mapped as quadruplets of four modulation symbols, and each quadruplet fills one REG. The REG therefore has to hold exactly four usable resource elements, whatever the CRS takes from the symbol. Let's see how 36.211 forms the REGs for one, two and four CRS ports, and then which channels use them.

• How are the REGs formed in each control symbol ?
• Which channels are built from REGs ?
• Reference

How are the REGs formed in each control symbol ?

A REG always lies inside one OFDM symbol and one resource block. When the symbol carries no CRS, a REG is simply four consecutive subcarriers. When the symbol carries CRS, a REG spans six subcarriers, and two of them are reserved for the reference signal. So the number of CRS ports decides how many REGs each symbol holds, and the three cases below differ only in that.

Case 1

Case 1 is a cell with one CRS antenna port, port 0. The first OFDM symbol needs care here. For the REG definition, the UE assumes CRS on ports 0 and 1, so the resource elements of port 1 are reserved even though nothing is sent on them.

i) In the first OFDM symbol of the first slot in a subframe, the two resource-element groups in physical resource block

ii) In the second OFDM symbol of the first slot in a subframe in case of one reference signals configured, the three resource-element groups in physical resource block

iii) In the third OFDM symbol of the first slot in a subframe, the three resource-element groups in physical resource block

iv) In the forth OFDM symbol of the first slot in a subframe in case of normal cyclic prefix, the three resource-element groups in physical resource block

• Symbol l = 0 holds two REGs, k0+0 to k0+5 and k0+6 to k0+11. Each spans six resource elements, and four of them carry control data.
• The red resource elements at k0+0 and k0+6 carry CRS port 0. The yellow ones at k0+3 and k0+9 carry nothing, but they are treated as port 1 CRS, as the note in the picture says.
• Symbols l = 1, 2 and 3 carry no CRS, so each holds three REGs of four consecutive resource elements. The pink and green shading marks the groups.
• The red resource elements at l = 4 are CRS port 0 in the fifth symbol, which lies outside the control region.

Case 2

Case 2 adds a second CRS port. The REG layout of the control region does not change, because Case 1 already reserved the port 1 positions. Only the meaning of those resource elements changes, since they now carry real port 1 CRS.

i) In the first OFDM symbol of the first slot in a subframe, the two resource-element groups in physical resource block

ii) In the second OFDM symbol of the first slot in a subframe in case of two reference signals configured, the three resource-element groups in physical resource block

iii) In the third OFDM symbol of the first slot in a subframe, the three resource-element groups in physical resource block

iv) In the forth OFDM symbol of the first slot in a subframe in case of normal cyclic prefix, the three resource-element groups in physical resource block

• The grey resource elements at k0+3 and k0+9 in symbol l = 0 are now real port 1 CRS. The two REGs of the first symbol are the same as in Case 1.
• The numbers 0 and 1 in the l = 4 column give the port of each CRS in the fifth symbol.
• Symbols l = 1, 2 and 3 still hold three REGs each.

Case 3

Case 3 uses four CRS ports. Ports 2 and 3 put CRS into the second OFDM symbol, so l = 1 now loses four resource elements to reference signals. The second symbol then holds two REGs of six resource elements, the same shape as the first symbol.

i) In the first OFDM symbol of the first slot in a subframe, the two resource-element groups in physical resource block

ii) In the second OFDM symbol of the first slot in a subframe in case of four reference signals configured, the two resource-element groups in physical resource block

iii) In the third OFDM symbol of the first slot in a subframe, the three resource-element groups in physical resource block

iv) In the forth OFDM symbol of the first slot in a subframe in case of normal cyclic prefix, the three resource-element groups in physical resource block

• The grey resource elements at k0+0, k0+3, k0+6 and k0+9 in symbol l = 1 are CRS ports 2 and 3.
• The shading in l = 1 therefore shows two groups, k0+0 to k0+5 and k0+6 to k0+11, and not three.
• Symbols l = 2 and 3 are unchanged, with three REGs each.

The table below collects the three cases. It also adds the fourth symbol with extended cyclic prefix, which 36.211 v19.3.0 clause 6.2.4 lists separately. The fourth symbol belongs to the control region only when the bandwidth is 10 resource blocks or less.

< REGs per resource block and OFDM symbol, based on 36.211 v19.3.0 clause 6.2.4 >

• A REG always carries four data resource elements : it spans four subcarriers in a symbol without CRS and six in a symbol with CRS.
• One CRS port costs the same as two : for the REG definition the UE assumes ports 0 and 1 whenever only one port is configured.
• Four ports change only the second symbol : l = 1 drops from three REGs to two, and the other symbols stay the same.

Which channels are built from REGs ?

PDSCH does not use REGs. The REG is the unit that PCFICH, PHICH and PDCCH share inside the control region. So the question is how the REGs of a subframe are divided between the three channels, and how many are left for PDCCH.

PCFICH comes first. It always takes four REGs in the first OFDM symbol, spread over the band. PHICH comes next, and with normal cyclic prefix each PHICH group takes three REGs. PDCCH gets every REG that is not assigned to PCFICH or PHICH. It groups them into CCEs of 9 REGs each, so the number of CCEs is the number of remaining REGs divided by 9 and rounded down.

Let's count them for a 10 MHz cell with 50 resource blocks, a control region of three symbols and PHICH N_g = 1. With normal cyclic prefix, the number of PHICH groups is ⌈N_g(N_RB^DL/8)⌉ = ⌈6.25⌉ = 7. The table below does the rest for one or two CRS ports and for four CRS ports.

< CCE count for 50 resource blocks, CFI = 3 and N_g = 1, normal cyclic prefix >

The four port cell loses 50 REGs in the second symbol, and that costs it five CCEs. The PDCCH does not fill its CCEs in plain frequency order either. 36.211 clause 6.8.5 interleaves the quadruplets, shifts them cyclically by the physical cell identity, and then maps them to REGs time first. For each frequency position, the mapping visits every control symbol before it moves to the next frequency.

• PCFICH and PHICH come out first : PCFICH takes 4 REGs, and each PHICH group takes 3 REGs with normal cyclic prefix.
• A CCE is 9 REGs : the CCE count is the number of remaining REGs divided by 9, rounded down.
• CRS ports reduce PDCCH capacity : in the 10 MHz example, four ports leave 36 CCEs instead of 41.

Reference

• 36.211 : 3GPP - E-UTRA Physical channels and modulation, v19.3.0. Clauses 6.2.4, 6.7.4, 6.8.1, 6.8.5 and 6.9.