Early-detection schemes based on sequential probability tests for low-latency communications

Abstract

In the context of fifth-generation cellular network technology (5G) and beyond, a low-latency communication system requires a trade-off between reliability and delay. In particular, ultrareliable and low-latency communication (URLLC) targets critical applications such as remote surgery, tactile Internet, autonomous vehicles, and industrial automation, where transmitting short blocklength codewords with high reliability and minimal latency is essential. However, traditional techniques are designed for asymptotic regimes, that is, they rely on transmissions of long codewords. This thesis addresses the challenge of reducing communication latency by proposing an early-detection scheme (EDS) based on sequential probability-ratio test (SPRT), operating without feedback and using short codewords. In particular, the EDS assumes a finite blocklength (FBL) regime with probabilistic sequential detection. First, the minimum achievable latency for channels with additive white Gaussian noise (AWGN) and Rayleigh fading in both finite and infinite block length regimes is characterized. The results show that, for AWGN channels, increasing the signal-to-noise ratio (SNR) and bandwidth reduces latency. On the other hand, in Rayleigh fading channels, latency is influenced by the diversity, coherence interval, and channel estimation overhead. As a core contribution of this thesis, the design and analysis of a feedback-free EDS, which enables the receiver to make an early and reliable decision based on a sequential test, before the end of the received codeword, is presented. This decision is taken when the confidence in the selected hypothesis is high enough compared to a predefined threshold. Moreover, in order to make the sequential test feasible, it is combined with a list decoder, which reduces the number of candidate codewords. The results show that, with a blocklength of n = 500 and a coding rate of R = 0.5, the EDS reduces the average latency to 63% of the codeword time in AWGN channels for a block error rate (BLER) of 10−5. In contrast, in 2 × 2 multiple-input multiple-output (MIMO) Rayleigh block fading channels, a latency of 0.80T is achieved at 12 dB SNR and I = 1, stabilizing at 0.88T under high diversity. Overall, the thesis demonstrates that sequential schemes combined with FBL coding constitute an effective strategy for low-latency communications. Likewise, this thesis analyzes the practical integration of the proposed EDS into multicarrier communication schemes such as orthogonal frequency-division multiplexing (OFDM), as well as into multi-hop schemes. This demonstrates that the EDS maintains robustness in realistic environments for various channel conditions. The evaluation of trade-offs between reliability, latency, and complexity provides design guidelines in URLLC, particularly in mission-critical communications (MCC). It is noteworthy the use of unitary space-time modulation (USTM) in the case of the MIMO Rayleigh block-fading channel, which allows the evaluation of the proposed EDS in scenarios where the channel state information (CSI) is unknown, thus achieving a validated scheme for diverse and dynamic environments.

Date	3 Jul 2025
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Pascal Giard (Supervisor) & Ghyslain Gagnon (Co-supervisor)