How DL Channel works in LTE?
The PDCCH, Physical Downlink Control Channel, allocates the DL and UL resources.There is one PDCCH per sub-frame.
It helps the UE retrieve the transport blocks from the PDSCH:
- Allocated RB
- Modulation and Coding Scheme (MCS)
- Multi-antenna transmission
PDCCH
- Shared between all UE of the cell
- Each TTI contains an aggregation of PDCCH allowing to address several users simultaneously (UL and/or DL)
- The CRC of each PDCCH is scrambled (XOR) with the UE RNTI
- Each UE tries to decode PDCCH and compares the resulting RNTI with its own identity (blind decoding)
PDCCH resource granularity = 1 CCE (Control Channel Element)1 CCE = 36 Resource Element = 72 bits (QPSK modulation only for PDCCH)Physical mapping is such that RE of a same CCE are scattered in time and frequency to get the best diversityPhysical mapping depends on the physical parameter ncell_id so that CCE of synchronized neigbouring cells do not collide The reception of the PDCCH is essential for the data transmission.Its size and location are not pre-defined since the amount of control data depends on the traffic.The Physical Control Format Indication Channel (PCFICH):
- allows the UE to decode the PDCCH.
- is carried on pre-defined time-frequency resources.
- indicates the number of symbols occupied by the PDCCH.
Understanding Downlink Channels in LTE
Downlink (DL) channels in LTE are used to send data from the network to your device. These channels work together to carry both control signals and user data, making sure you get everything from system messages to streaming content smoothly.
Main Downlink Channels
- PDSCH (Physical Downlink Shared Channel): This is where most of your user data like videos, files, or web pages are sent.
- PDCCH (Physical Downlink Control Channel): Tells your device when and where to receive data on PDSCH.
- PBCH (Physical Broadcast Channel): Sends system information your device needs when first connecting to the network.
- PCH (Paging Channel): Used to notify your device of incoming calls or messages while it’s idle.
How It Works Together
Your device first listens to PDCCH for instructions. Once it gets the timing and location, it goes to PDSCH to receive actual data. PBCH helps when you’re just joining the network, and PCH makes sure you don’t miss alerts even when not actively using your phone.