I. NTN Access: Random Access Channel (RACH) is a fundamental process for initial connection, uplink synchronization, and scheduling authorization between the terminal equipment (UE) and the network. While this is a mature and well-understood process in traditional terrestrial radio access networks (RANs), its implementation in Non-Terrestrial Networks (NTNs) presents a series of unique and more complex technical challenges.
In terrestrial RANs, radio frequency signals typically propagate over short and predictable distances, and the propagation environment is relatively stable; however, in NTN networks involving Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Orbit (GEO) satellites, radio frequency signals are affected by extremely long propagation distances, rapid satellite movement, dynamic coverage areas, and time-varying channel conditions. All these factors significantly impact the timing, frequency, and channel reliability that traditional RACH processes rely on.
II. NTN Characteristics: Due to extremely long transmission distances, rapid satellite movement, and time-varying coverage and channel conditions, NTN presents unique critical drawbacks (e.g., large propagation delay, long round-trip time, Doppler shift, beam mobility, and large contention domain) that severely challenge and impact the terminal's random access channel (RACH) behavior and performance. Furthermore, satellites are subject to strict limitations in terms of spectrum availability and power budget, making efficient and robust random access mechanisms particularly crucial.
III. Impacts and Solutions: To overcome the difficulties that NTN presents for terminal access, 3GPP has addressed some issues in its specifications, but the following aspects require attention:
3.1 TA (Timing Advance) Challenges
Impacts: In NTN networks, due to large cell areas, satellite movement, and varying distances between the UE and the satellite, timing advance estimation is far more complex than in terrestrial systems. Incorrect TA estimation can cause uplink transmissions to fall outside the satellite's reception window, resulting in collisions or complete reception failure.
Solution: Advanced TA estimation techniques are needed, such as utilizing satellite ephemeris data, GNSS assistance, or predictive algorithms, to dynamically adjust UE timing alignment and maintain uplink synchronization.
3.2 Doppler Shift Effects
Impacts: The relative motion between the satellite and the UE introduces significant Doppler shifts, especially in Low Earth Orbit (LEO) systems. These frequency shifts reduce preamble detection accuracy, impair frequency synchronization, and increase the likelihood of RACH attempt failures.
Solution: Robust Doppler pre-compensation and frequency tracking mechanisms are required on both the UE and network sides to maintain reliable RACH performance under high mobility conditions.
3.3 Channel Condition Variations:
Impact: NTN links are subject to atmospheric attenuation, shadowing, scintillation, and long-distance path loss. These factors increase the block error rate and may affect the UE's ability to correctly receive RAR messages after successfully transmitting the preamble.
Solution: Adaptive modulation and coding, power control, and robust physical layer design are needed to maintain reliable RACH detection and processing under various channel conditions.
3.4 Wide Coverage and High Terminal Density:
Impact: Satellite beams typically cover very large geographical areas, potentially serving thousands of UEs simultaneously. This significantly increases the level of RACH contention and the probability of collisions, especially in large-scale access scenarios.
Solution: Efficient RACH resource partitioning, load-aware access control, and intelligent contention management mechanisms are needed to scale random access performance.
3.5 Increased RTT (Latency and Round-Trip Time):
Impact:The large physical distance between the UE and the satellite introduces significant one-way propagation delay and longer RTT. For example, the round-trip time (RTT) for a geostationary orbit (GEO) satellite link can reach hundreds of milliseconds. These delays directly affect the timing of Random Access Response (RAR) message exchange, potentially leading to premature timer timeouts, increased access failure rates, and prolonged access delays.
Solution: RACH-related timers, such as the Random Access Response (RAR) window and collision resolution timers, must be designed based on NTN-specific RTT values. NTN-aware timer configuration is crucial to prevent unnecessary retransmissions and access failures.
3.6 Increased Collisions:
Impact: A large number of user equipment (UEs) contending for a limited number of RACH preambles increases the probability of preamble collisions, thereby reducing access efficiency and increasing latency.
Solution: Advanced collision resolution schemes, dynamic preamble allocation, and NTN-optimized access barring techniques are key to reducing collision probability.
3.7 Synchronization Challenges:
Impact: Initial synchronization in NTN is complicated by large timing uncertainties and frequency offsets. Failure to achieve accurate synchronization can prevent the user equipment (UE) from initiating the Random Access Channel (RACH) process altogether.
Solutions: Enhanced synchronization techniques, combining precise timing acquisition, Doppler compensation, and satellite position awareness, are needed for successful random access.
3.8 Power Control
Impact: UEs in NTN experience significant variations in path loss depending on their position relative to the satellite beam. Insufficient transmit power may lead to preamble detection failure, while excessive power can cause inter-UE interference.
Solution: Adaptive and location-aware power control mechanisms are crucial for balancing detection reliability and interference management.
3.9 Beam Management
Impact: NTN systems heavily rely on multi-beam architectures. UEs may need to perform beam acquisition or switching during the RACH process, which increases complexity and latency. Solution: Efficient beam discovery, beam tracking, and seamless beam switching mechanisms are essential for ensuring reliable RACH execution in beam-based NTN systems.
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