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Common Search Space(Type 1 PDCCH)

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The Common Search Space (Type 1 Physical Downlink Control Channel or Type 1 PDCCH) is a component of the physical layer in the LTE (Long-Term Evolution) and NR (New Radio) wireless communication standards. The PDCCH is responsible for carrying control information that is used by the user equipment (UE) to interpret and decode the physical downlink shared channel (PDSCH), which carries user data.

The Common Search Space refers to a set of resources within the LTE or NR system bandwidth that is reserved for transmitting control information that is common to multiple UEs. The Common Search Space is specifically designated for control channels that carry information relevant to multiple UEs, as opposed to the UE-specific search space, which is tailored for individual UEs.

Here's a technical breakdown of the Common Search Space (Type 1 PDCCH):

  1. Control Channel Structure:
    • The PDCCH is a physical channel that is transmitted in the downlink direction from the base station (eNodeB or gNodeB) to the UEs.
    • The control information carried by the PDCCH includes resource allocation, scheduling, and other control parameters.
  2. Search Spaces:
    • The PDCCH is divided into different search spaces to efficiently allocate control information. The Common Search Space is one of these search spaces.
    • The Common Search Space is defined as a set of resource blocks (RBs) and symbols within the system bandwidth.
  3. Aggregation Level:
    • The Common Search Space PDCCH can use different aggregation levels to group control information across multiple consecutive subframes.
    • Aggregation level determines how many control channel elements (CCEs) are bundled together to form a control channel candidate. CCE is the smallest allocation unit for PDCCH.
  4. Control Channel Element (CCE):
    • The PDCCH maps information to a sequence of CCEs in the frequency-time resource grid.
    • CCEs are then mapped onto resource elements within the Common Search Space.
  5. Blind Decoding:
    • UEs perform blind decoding in the Common Search Space to detect the PDCCH information relevant to them.
    • Blind decoding involves attempting to decode the PDCCH based on different hypotheses about the control channel format and location.
  6. Dynamic Allocation:
    • The resources within the Common Search Space can be dynamically allocated to different UEs based on their scheduling needs.
    • The dynamic allocation allows for efficient use of the shared resources among multiple UEs.

The Common Search Space (Type 1 PDCCH) in LTE and NR provides a dedicated area within the downlink channel for transmitting control information that is common to multiple UEs. This ensures efficient and reliable communication between the base station and a group of UEs within the wireless network.