5G achieves higher reliability, particularly for Ultra-Reliable Low-Latency Communication (URLLC) services, through several key architectural enhancements.

Multi-connectivity (B), such as Dual Connectivity, provides redundancy by allowing a User Equipment (UE) to maintain simultaneous connections to multiple transmission points. If one link degrades or fails, the other can ensure uninterrupted service, directly increasing the reliability of the connection.

A lower Time Transmission Interval (TTI) (D), enabled by the flexible 5G NR frame structure (mini-slots), allows for much faster transmission and retransmission of data packets. This rapid feedback and error correction mechanism is critical for ensuring that data is successfully delivered within the strict latency and reliability budget (e.g., 99.999% success rate), thus driving higher overall reliability.

A. Higher spectral efficiency: This primarily increases network capacity and data throughput (bits/sec/Hz). It does not inherently make the radio link more robust or reliable against transmission errors.

C. Connectionless radio access: This feature is designed to reduce latency and signaling overhead for small, infrequent data transmissions (common in mMTC), not to provide the sustained, robust connection required for high-reliability services.

1. Nokia. (2020). 5G New Radio: Highlights of 3GPP Release 15 and 16. White Paper. Page 6, Section "Ultra-reliable low-latency communication". The paper states, "The key 5G NR enablers for URLLC are the new flexible frame structure with short TTI... and multi-connectivity for improved reliability."

2. 3GPP. (2017). TR 38.913 V14.3.0: Study on scenarios and requirements for next generation access technologies. Section 7.1, "URLLC service requirements". This document outlines the need for technologies that can meet 99.999% reliability, which underpins the design of features like short TTI and multi-connectivity.

3. Bennis, M., Letaief, K. B., et al. (2018). "Framing the 5G-URLLC Opportunity." 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC). DOI: 10.1109/PIMRC.2018.8580933. The paper discusses how "packet duplication over two distinct base stations" (multi-connectivity) and "short TTI" are fundamental enablers for achieving URLLC.