LTE PDCCH Part 1 (DCI and RNTI)
Malaka Dahanayake
Telecommunication Engineering Professional | Expertise in 4G/5G Radio Access Network
This introductory article will lean towards theory, yet it's essential groundwork for delving deeper into the subject.
PDCCH stands for Physical Downlink Control Channel in LTE (Long Term Evolution), which is a standard for wireless broadband communication. It's a crucial component of LTE's physical layer and is responsible for carrying control information from the base station (eNodeB) to the user equipment (UE), also known as the mobile device.
The PDCCH carries essential control information such as resource allocation, scheduling assignments, power control commands, and other system-specific control messages. It operates in the downlink direction, meaning it's transmitted from the base station to the user equipment.
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Please go through LTE Channel Concept and Introduction to Air Interface Protocols if you are not familiar with LTE channel concept.
— Downlink scheduling assignments, including
????????????? ? Physical Downlink Shared Channel (PDSCH) resource indication
????????????? ? Transport format indication
????????????? ? Hybrid-ARQ information and transport block size
????????????? ? Control information related to Multiple Input Multiple Output (MIMO)
????????????? ? PUCCH power control commands if applicable
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— Uplink scheduling grants, including
????????????? ? Physical Uplink Shared Channel (PUSCH) resource indication
????????????? ? Transport format indication
? Hybrid-ARQ information
???????????? ? PUSCH power control commands
Before dig more on PDCCH let’s take a look into some of the LTE resource grid aspects.
One resource block in LTE is 12 subcarrier is frequency and 7 symbols(normal cycle prefix) in time domain.
?Frequency Domain :
Time Domain:
?When we put both Frequency and Time Domain together :
Resource Element :
?Resource element (RE) is the smallest unit of resource allocation in the time-frequency domain. RE consists of,
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Resource Element Group
?A Resource Element Group (REG) consists of four REs. In a REG, all REs are located on the same OFDM symbol within 12 consecutive subcarriers and grouped together with at most one RE (or hole) intervening.
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Control Channel Elements
The mapping of Physical Downlink Control Channel (PDCCH) to REs is subject to a certain structure. The structure is based on Control Channel Elements (CCE).
RE, REG and CCE mapping :
Based on what I've explained up to now, assumes that you now grasp the LTE frame structure and its constituent elements. Within the PDCCH, the basic unit employed is CCE.
As previously stated, the PDCCH transports critical control data including resource allocation, scheduling assignments, and power control commands, etc. These information will carried inside the PDCCH using predefine templates. These templates/Formats called DCIs (Downlink Control Indicator).
Downlink Control Indicator
The?DCI?format is nothing but a predefined format in which the downlink control information is packed/formed and transmitted in PDCCH. It is DCI which carries those detailed information like "which resource block carries your data ?" and "what kind of demodulation scheme you have to use?to decode data ?" and some other additional information.?
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DCI Formats
Below table summarize the DCI formats and their usage. Here you can see only 2 DCIs are for UL assignments, but DCI Format 0 will be the most widely used DCI for UL grant.
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Below 2 figures will provide the DCIs for one UL Grand and one DL Grant
This is based on 3GPP 36.212 - 5.3.3.1.1
DCI Format 0
?DCI Format 1
Lets discuss bit more about the fields we find in the DCI. Don’t worry if you didn’t understand all. Basic idea is enough at the beginning.
DCI in Action
Below picture will show on how DCI works for PDSCH and PUSCH scheduling. Here you can see DL data will be schedule in the same TTI as the DCI transmitted. And UL scheduling will happened in the forth subframe from the DCI subframe, which is a fixed value in LTE. In other words UL scheduling will happened after 4ms from the DCI transmitted. ?
As you observed during the reading of this article there are different types of DCI formats. 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 will be discussed in a different article.
Radio Network Temporary Identifier (RNTI)
RNTI stands for Radio Network Temporary Identifier. It's a unique identifier assigned to user equipment (UE) for communication with the eNodeB (LTE base station) within the radio access network (RAN).
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Here are some key points about RNTI in LTE:
3. Purpose: RNTIs are used by the eNodeB to address and control communication with individual UEs. They help in distinguishing between different UEs and managing their connections efficiently within the LTE network.
4. Dynamic Allocation: RNTIs can be dynamically allocated and deallocated as needed, depending on the UE's state and the ongoing communication requirements. For example, when a UE initiates a random access procedure, it is assigned an RA-RNTI temporarily for that specific procedure.
5. Security: RNTIs play a key role in LTE Air Interface security mechanisms.
All of these RNTI is used to scramble the CRC part of the radio channel messages. It implies that if UE does not know the exact RNTI values for each of the cases, it cannot decode the radio channel messages even though the message reaches the UE intact.
One of the most common questions that arise when discussing the RNTI is "There are a?lot of different types of RNTI, and I don't see any RNTI information on DCI or Higher layer signaling message. Then how can PHY layer know which RNTI it has to use to decode a data ?".
The answer is "MAC or Layer 1 controller would instruct PHY on which RNTI it has to be used".
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Then a next questions comes out. "How would MAC or Layer 1 controller know which RNTI to be used ?".
There is no explicit algorithm for this, MAC/L1 controller?needs to figure it out "based on context". For example, if it is at the subframe where SIB is transmitted, it would instruct PHY to use SI-RNTI. if UE is in connected mode, it may instruct to use C-RNTI, TPC RNTI etc.
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From the 3GPP specifications :
From 3GPP 36.213 shows the relationships between RNTI Type, Transmission Mode and DCI format. You would notice that same information (same RNTI type) can have multiple candidates of DCI format.
How network determine which DCI format it has to use at a specific moment ?
In certain instances, you may encounter a clearly defined criteria in the tables in 3GPP, while in others, the selection criteria might not be as transparent.
For example, What are the scenarios to use DCI format 1A and 2A conjunction with TM3,C-RNTI?
We can say, Use DCI format 2A in MIMO configuration and use 1 A in non-MIMO configuration.
But not all these kind of scenarios have clear answers.
If we put the learning till now into real world scenario,
Below is a UE logs which shows UE is identify the paging DCI in PDCCH which contain the paging message location is the PDSCH. This paging DCI is scrambled using P-RNTI(FFFE - 65534) and schedule within the paging occasion for that UE inside the PDCCH.
Hope now you have a better understanding to read the Layer1 UE logs with respect to PDCCH,DCU and RNTI.
Will discuss more on PDCCH in my second article for PDCCH.
References :
1) 3GPP 136.211
2) 3GPP 136.213
3) 3GPP 136.101