Performance Enhancements of Control Channel in Non-Terrestrial Networks for 5G & Beyond

Non-terrestrial networks (NTN) are wireless networks consisting of satellites, unmanned aerial vehicles (UAVs), high altitude platforms (HAPs) etc. connected to ground using gateways. Low earth orbit (LEO) constellation of NTN is becoming popular due to high achievable data rates. However, the large distance, i.e, 600km, between the LEO satellite and user equipment (UE) makes the service link to experience significant pathloss, leading to an increase in the block error rate (BLER) resulting in a coverage gap. Multiple techniques such as physical downlink control channel (PDCCH) repetition, downlink control information(DCI) segmentation and DCI size reduction are proposed to reduce the coverage gap. The repetition of PDCCH increases the reliability, but also increases the energy consumption of the UE for decoding all the PDCCH repetitions. A novel method is proposed to overcome this issue, in which UE checks the CRC of each PDCCH repetition until a PDCCH repetition is passed resulting in the reduction of the energy consumption. To further reduce the coverage gap, DCI segmentation is considered, where the DCI bits are divided into M segments, such that each of the segment contain equal number of bits. This technique improves the BLER significantly but reduces spectral efficiency. Due to the specific characteristics of the NTN such as line of sight communication, large distance between the satellite and UE, and the mobility of the satellite, some of the information elements, e.g. TPC command in DCI etc, becomes insignificant or irrelevant. Therefore, proposed a DCI size reduction method by not sending such information in DCI. Proposed enhancements are analysed using evaluation methods followed in 3GPP. The simulation results indicate a considerable improvement in BLER when the PDCCH is repeated four times and the DCI size is reduced by 50% for different aggregation levels.