ment at secondary cell addition, request for other system information (OSI), and beam failure recovery request, where some of the usages are supported using contention-free PRACH resources. 3.5.5 Downlink Control Channel Similar to LTE, NR uses the PDCCH to transmit control information in the downlink (3GPP TS 38.211). For PDCCH reception, a UE can be configured with a control resource set (CORESET), which is similar to the control channel region in LTE, but has more flexi- ble structure and position in time and frequency (see Figure 3.5.8). Such flexibility of the CORESET configuration allows addressing a wide range of new use cases and applications supported by NR systems. In particular, unlike LTE PDCCH, which always spans the entire system bandwidth, a CORESET can occupy certain subcarriers and OFDM symbols in a slot depending on higher layer configuration (e.g. RRC). To enable more efficient signal- ing of the CORESET configuration, the frequency domain resources for CORESET can be allocated with granularity of six REGs (Resource Element Groups) - each REG consists of 12 subcarriers over one OFDM symbol and a bundle of six REGs is termed a CCE (Control Channel Element). The CORESET position in time domain can be also flexible within a slot depending on the PDCCH search space set configuration. This is in contrast to LTE, where 3 5G System Overview Figure 3.5.8 CORESET transmission within a slot. symbol PDCCH can be only transmitted at the beginning of the downlink subframe. CORESET in NR can span one, two, or a maximum of three contiguous OFDM symbols within a slot to trade-off between low latency processing at the UE and PDCCH coverage, and to control the CORESET-induced overhead depending on the traffic load. When the CORESET spans multiple OFDM symbols, each control channel candidate is mapped to all symbols of the CORESET following time-first mapping. To improve the channel estimation performance for PDCCH, multiple adjacent REGs are grouped into a REG bundle in the frequency domain and transmitted using