5G-NR & LTE PDCCH Search Space: Difference and User Blocking Probability

To begin with, the topic let me tell you about what search space in PDCCH(Physical Downlink Control Channel is):

Search Space is defined as a group of Control Channel Elements (CCEs) that constitute the control region. There are two different kinds of search spaces, the UE-specific search spaces, and the common search spaces. The UE-specific search spaces are configured for each UE individually and in which PDCCHs carrying scheduling information for a single UE are allocated. The common search spaces are indicated to all UEs and in which common PDCCHs that schedule common messages are allocated. Here mostly I’ll be describing, the UE-specific search space for calculating the Blocking Probability of UE while scheduling. In particular, Search Space is the place where UE goes for Blind Decoding.

These CCEs are nothing but the allocated Aggregation Level to the individual UE(User Equipment) based on their location from the respective base station. To decode the DCI (Downlink Control Information) at the receiver we map the DCI to the corresponding Aggregation Level.

To calculate the CCE indices in a given NR-CORESET we use the NR-Hash function which is as follows:

Similarly, in the case of LTE we use the LTE Hasj function as follows:

After mapping the CCE indices to the search space we get the Search Space as follows:

The main problem in the search space allocation scheme in the LTE standard is that all the candidates are contiguous and the hashing function is only used to determine its starting point. This means that if the search spaces of 2 UEs collided in one of the candidates, it is highly probable that the next candidate of both UEs will collide as well, as shown in Fig 2 of LTE, and thus causes the low throughput problem stated earlier as one of the main problems of PDCCH. 

Whereas in NR because of the scattering of the candidate at the NR-Search Space, there is less probability of the next candidate of both UEs will collide and get blocked.

That’s why we are going for the complex NR-Hash function to reduce the User Blocking Probability.

The Plot of the User Blocking Probability of NR & LTE as Follows:

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This Article contributed by Anil Kr Shaw.

Anil Kr Shaw post graduated from Indian Institute of Technology (IIT) Hyderabad with specialization in Communication & Signal Processing..

You may reach him on Linkedin https://www.linkedin.com/in/anil-kr-shaw-wildmelophile/

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