Design Challenges and Solutions for Cellular Channel Access and Spatial Reuse in Shared Spectrum

This thesis addresses standalone (SA) 3GPP New Radio in unlicensed spectrum

(NR-U), where slot-synchronous NR transmissions must operate under contention-

based Listen-Before-Talk (LBT) Channel Access Procedures (CAPs). A key imple-

mentation challenge is the "synchronisation gap", where channel access completion

is inherently asynchronous, while NR-U transmissions must start on slot bound-

aries, thereby increasing sensing duration and latency.

This thesis makes three main contributions. First, Scheduled Type 1 CAP mea-

sures synchronisation gaps and schedules the CAP to minimise their impact, using

an Additional Sensing (AS) step immediately before scheduled transmissions to

reduce concurrent channel access with other devices and improve coexistence. Sec-

ond, Aligned Type 1 CAP exploits an interpretation of the 3GPP channel access

rules by introducing pause-and-resume logic for the exponential backoff timer,

thereby eliminating synchronisation gaps. Third, to mitigate conservative sensing

limitations that restrict spatial reuse, Null Space Projection (NSP) methods are

developed for both LBT sensing and digital precoding; directional sensing and

NSP are used to suppress interference towards and from neighbouring devices.

The proposed methods are evaluated using a 3GPP Release 18-compliant SA NR-

U simulation module developed in this work within the ns-3 network simulator and

the 5G-LENA framework, enabling comprehensive system-level analysis. Simula-

tion results show that Aligned Type 1 CAP reduces latency and sensing overhead

relative to conventional approaches, while the NSP -based methods improve spatial

reuse and area throughput under coexistence in the same frequency band. These

contributions provide a reproducible basis for analysing and improving standards-

compliant NR-U channel access and spatial coexistence mechanisms in shared

spectrum.