LTE PDCCH-Physical Downlink Control Channel

 LTE PDCCH channel or Physical Downlink Control Channel with respect to LTE system. It is used to carry DCI (Downlink Control Information). We get the information about number of OFDMA symbols used by PDCCH after decoding PCFICH. The symbols are always at the start of each subframe.

The REs(Resource Elements) allocated to PDCCH are grouped into group of 4 REs referred as quadruplets. RE quadruplets are grouped into CCE ( Control Channel Elements). There are 9 quadruplets in one CCE. Hence 36 REs per CCE. PDCCH uses QPSK which provides CCE capacity of about 72 bits.

There are total 4 PDCCH formats as described in document 3GPP TS 36.211. The same is mentioned in table-1 below.

PDCCH Format	Number Of CCE	Number Of RE Quadruplets	Number Of Bits
0	1	2	4
1	2	4	8
9	18	36	72
72	144	288	576

Table-1 LTE PDCCH channel formats

The PDCCH format is used as per required size of DCI. The DCI bits have 16 bit CRC attached prior to rate-1/3 channel coding & rate matching modules. The table-2 below mentions coding rate for each DCI and PDCCH format. coding rate = number of DCI bits after CRC attachment/Capacity of PDCCH.

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			Coding Rate
DCI Format	Channel Bandwidth	DCI Bits after CRC	PDCCH Format 0	PDCCH Format 1	PDCCH Format 2	PDCCH Format 3
DCI-0,1A,3,3A	5	41	0.57	0.28	0.14	0.07
	10	43	0.60	0.30	0.15	0.07
	20	44	0.61	0.31	0.15	0.08
DCI-1	5	43	0.60	0.30	0.15	0.07
	10	47	0.65	0.33	0.16	0.08
	20	55	0.76	0.38	0.19	0.10
DCI-1B	5	43	0.60	0.30	0.15	0.07
	10	44	0.61	0.31	0.15	0.08
	20	46	0.64	0.32	0.16	0.08
DCI-1C	5	28	0.39	0.19	0.10	0.05
	10	29	0.40	0.20	0.10	0.05
	20	31	0.43	0.22	0.11	0.05
DCI-1D	5	43	0.60	0.30	0.15	0.07
	10	44	0.61	0.31	0.15	0.08
	20	46	0.64	0.32	0.16	0.08
DCI-2	5	55	0.76	0.38	0.19	0.10
	10	59	0.82	0.41	0.20	0.10
	20	67	0.93	0.47	0.23	0.12
DCI-2A	5	52	0.72	0.36	0.18	0.09
	10	57	0.79	0.40	0.20	0.10
	20	64	0.89	0.44	0.22	0.11

Table-2: Coding rates for each DCI and LTE PDCCH format(BW-5,10,15MHz)

The number of CCE depends on channel BW and number of OFDMA symbols allocated for PDCCH. The table represents the same assuming no quadruplets have been allocated to PHICH and 4x4 MIMO is not used.

	Channel BW
OFDMA Symbolsfor PDCCH	1.4 MHz	3 MHz	5 MHz	10 MHz	15 MHz	20 MHz
1	-	2	5	10	16	21
2	2	7	13	27	41	55
3	4	12	21	44	66	88
4	6	-	-	-	-	-

Table-3 Maximum no. of CCE (Assuming 1 or 2 transmit antenna ports)

In Summary:
•  This channel is used to inform the UE about the resource allocation of PCH and DL-SCH

•  indicating the modulation, coding and hybrid-ARQ information related to DL-SCH

•  Generally, a maximum of three or four OFDM symbols can be used for PDCCH

•  The information carried on PDCCH is referred to as downlink control information (DCI)

•  Uses QPSK modulation

LTE PDCCH Channel REFERENCE: 3GPP TS 36.211