It is DCT which carries those detailed information like which resource block carries your data and what kind of demodulation scheme you have to use it to decode data and some other additional information. It means you (the reciever) first have to decode DCI and based on the informat you got from the DCI you can decode the real data. It means without DCI, decoding the data delivered to you is impossible.Not only in LTE, but also in most of wireless communication the reciever has the same problem(the same question). In WCDMA R99, Slot format and TFCI carries those information and in HSDPA HS-SCCH carried those information and in HSUPA E-TFCI carried it.
In terms of protocol implementation with respect to carrying these information, R99 seems to be the most complicated one. You had to define all the possible combination of resource allocation in the form of TFCS (a kind of look-up table for TFCI) and you have to convey those information through L3 message (e.g, Radio Bearer Setup message and RRC Connection Setup message) and the transmitter also have to configure itself according to the table. A lot of error meaning headache came from the mismatches between the TFCS information you configured in L3 message and the configuration the transmitter applied to itself (transmitter's lower layer configuration). It has been too much headache to me. HSDPA relieved the headache a lot since it carries these information directly on HS-SCCH and this job is done by MAC layer. The resource allocation information carried by HS-SCCH is called 'TFRI'. So I don't have to care much about L3 message.. but still I need to jump around the multiple different 3GPP document to define any meaningful TFRIs. And other complication was that even in HSDPA we still using R99 DPCH for power control and signaling purpose, so I cannot completely remove the headache of handling TFCS.Now in LTE, this information is carried by DCI as I explained above and we only have to care about just a couple of parameters like Number of RBs, the starting point of RBs and the modulation scheme and I don't have to care anything about configuring these things in RRC messages. This is a kind of blessing to me.
As one example showing how/when DCI is used, refer to http://jaekuryu.blogspot.com/2010/01/lte-signalinig-essentials.html section "Uplink Data Transmission Scheduling - Persistent Scheduling"
Types of DCIs
DCI carries the following information :
i) UL resource allocation (persistent and non-persistent)
ii) Descriptions about DL data transmitted to the UE.
L1 signaling is done by DCI and Up to 8 DCIs can be configured in the PDCCH. These DCIs can have 6 formats : 1 format for UL scheduling, 2 formats for Non-MIMO DL scheduling, 1 format for MIMO DL Scheduling and 2 formats for UL power control.
Format 0 : UL SIMO and UL Power Control. This functions as a Grant for UL transmission
Format 1, 1 A : DL SIMO and UL Power Control
Format 2 : DL MIMO and UL Power Control
Format 3 : UL Power Control Only (for multiple UEs)
Format 3A : UL Power Control Only (for multiple UEs)
DCI has various formats for the information sent to define resource allocations. The resource allocation information contains the following items in it.
i) number of resource blocks being used
ii) duration of allocation in TTI
iii) support for multiple antenna transmission
What determines a DCI Format for the specific situation ?
There are two major factors to determine a DCI format for a specific situation as follows :
i) RNTI Type
ii) Transmission Mode
This means that you cannot change only one of these parameters arbitrarily and you always have to think of the relationships among these when you change one of these parameters. Otherwise you will spend a long time for troubleshooting -:)
Those tables from 3GPP 36.213 shows the relationships between RNTI Type, Transmission Mode and DCI format.
Any relations between DCI format and Layer 3 signaling message ?
Yes, there is a relationship. You have to know which DCI format is required for which RRC message. Following tables from 3GPP 36.321 shows the relationship between RNTI and logical channel and you would know which RRC message is carried by which logical channel. So with two step induction, you will figure out the link between RRC message and it's corresponding DCI format.
For example, if you see the "Security Mode Command" message of section 6.2.2 of 36.331, it says
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: UE to E-UTRAN
If you see the table, you would see this message is using C-RNTI. and you will figure out the possible candiates from table 7.1-5 of 36.213 and if you would have detailed information of the transmission mode, you can pinpoint out exactly which DCI format you have to use for this message for a specific case. Assuming TM mode in this case is TM1 and scheduling is dynamic scheduling, if you see Table 7.1-2 you will figure out that this is using C-RNTI. With this RNTI Type and TM mode, if you see table 7.1-5, this case use DCI Format 1 or DCI Format 1A.
DCI Decoding Examples
Example 1 > DCIFormat 0, value = 0x2584A800
You can figure out the Start of RB and N_RB (Number of allocated RB) from RIV value.
How can I calcuate Start_RB and N_RB from RIV. The simple calcuation is as follows :
i) N_RB = Floor(RIV/MAX_N_RB) + 1= Floor(1200/50) + 1 = 25, where MAX_N_RB = 50 in this case since this is 10 Mhz System BW.
ii) Start_RB = RIV mod MAX_N_RB = 1200 mod 50 = 0
