5G Architecture, D2D Communication, and RAN Fundamentals

Posted on May 27, 2026 in Computers

5G Frequency Domain Structure

  • Resource Element (RE): Smallest unit = 1 subcarrier × 1 OFDM symbol.
  • Resource Block (RB): 12 consecutive subcarriers in the frequency domain. Unlike LTE (2D), 5G RB is 1-dimensional (frequency only).
  • Scalable Subcarrier Spacing (Numerology): 15, 30, 60, 120, 240 kHz — selected based on frequency band and use case.
  • Bandwidth Parts (BWP): UE can be configured to use only a portion of the total carrier bandwidth, saving battery for IoT devices.
  • Maximum Single-Carrier Bandwidth: Up to 100 MHz in sub-6 GHz, up to 400 MHz in mmWave (FR2).

5G Time Domain Structure

Frame: 10 ms total.

Subframe: 10 subframes per frame = 1 ms each.

Slot: Each slot has exactly 14 OFDM symbols. Duration shrinks as spacing increases: 15 kHz→1 ms, 30 kHz→0.5 ms, 60 kHz→0.25 ms, 120 kHz→0.125 ms.

Mini-slot: 2, 4, or 7 symbols — used for ultra-low latency transmissions that cannot wait for a full slot.

Smaller slot duration = lower latency. 5G URLLC targets <1 ms end-to-end, achieved using 60 kHz or 120 kHz numerology.

Functions of AMF, SMF, AUSF, and UPF in 5G Core

  • AMF (Access & Mobility Mgmt Function): Control plane — handles connection setup, mobility tracking, paging, and access authentication. Equivalent to 4G MME.
  • SMF (Session Mgmt Function): Control plane — manages data sessions, assigns IP addresses, controls QoS, and instructs UPF on routing rules.
  • AUSF (Authentication Server Function): Control plane — verifies and authenticates users securely before granting network access.
  • UPF (User Plane Function): User plane (only user-plane element) — forwards actual data packets between RAN and internet, enforces QoS, and reports traffic usage.

5G RAN Entities and Functions

Diagram: UE ↔ (Uu) ↔ gNB [DU+CU] ↔ (NG) ↔ 5G Core; gNBs connect to each other via Xn.

  • gNB: Main 5G base station. Handles radio resource management, scheduling, QoS, MIMO, admission control, and handovers.
  • NG Interface: Connects gNB to 5G Core — NG-U to UPF (user data), NG-C to AMF (control signals).
  • Xn Interface: Connects neighboring gNBs for handover coordination and dual connectivity.
  • Uu Interface: Wireless air interface between gNB and UE — where the 5G NR radio transmission happens.
  • CU/DU Split: CU (cloud) handles higher layers; DU (near antenna) handles real-time radio processing.

Strengths and Weaknesses of 5G RAN Architecture

Strengths

  • Scalable OFDM numerology — works from sub-1 GHz to mmWave in a single architecture.
  • Native support for Massive MIMO and high-gain beamforming — extreme capacity and energy efficiency.
  • CU/DU split enables flexible cloud deployment and centralized multi-cell coordination.
  • Dynamic TDD maximizes spectrum efficiency by adapting UL/DL ratio to instant traffic.

Weaknesses

  • mmWave has very high path loss and poor penetration through walls — needs dense, expensive small-cell deployment.
  • Dynamic TDD in dense networks causes severe cross-link interference — complex and expensive to manage.

5G Support for Massive Machine Type Communication (mMTC)

  • Ultra-high density: Supports up to 1 million connected devices per km² — needed for smart cities and industrial IoT.
  • Long battery life: Devices designed for 10+ years on one battery — achieved via sleep modes and minimal signaling.
  • Deep coverage: Enhanced coverage reaches basements and remote areas that 4G cannot reliably serve.
  • Coded Random Access: Replaces heavy 4-step RACH with direct one-shot transmission. Devices send multiple coded replicas; base station uses SIC to decode even when many collide simultaneously.
  • NOMA (SCMA): Multiple IoT devices share the same time-frequency resource using unique codebooks — massively reduces signaling overhead per device.

Multi-Antenna Transmission in 5G

  • 5G uses Massive MIMO — base stations with 64, 128, or 256 antenna elements (vs 4–8 in LTE).
  • High-gain beamforming: Focuses energy precisely at each user to compensate for severe mmWave path loss and reduce interference for others.
  • Massive spatial multiplexing: Many more antenna elements → many more simultaneous spatial streams for multiple users (MU-MIMO).
  • Energy efficiency: Focused beams mean less wasted energy — 5G can serve more users per Watt than 4G.
  • Initial beam management is required to find the best beam pair.

Network Function Layer in Slicing Architecture

  • The Network Function Layer sits in the middle of the 3-layer slicing architecture — between the service layer (customer requirements) and the infrastructure layer (physical hardware).
  • It takes service requirements from above and creates the actual network slice by chaining together the right virtualized network functions (AMF, SMF, UPF, etc.).
  • Manages the life-cycle of these functions — decides to spin them up, scale them, or tear them down.
  • Key decision: Share a network function across multiple slices (efficient, complex) or dedicate one function per slice (simple, resource-heavy) — tradeoff managed by orchestrator.
  • Ensures the slice meets end-to-end SLA by monitoring function performance continuously.

Inter-Cell D2D Coordination

  • The Value: Inter-cell D2D extends coverage, improves throughput for cell-boundary users, and balances traffic across neighboring base stations.
  • The Burden: D2D users near cell boundaries create complex cross-cell interference.
  • Solution 1 (X2 coordination): Neighboring base stations exchange scheduling information over the X2 interface to avoid resource conflicts.
  • Solution 2 (Protocol-level ordering): Define transmission ordering rules between cells to prevent simultaneous conflicting transmissions.
  • Solution 3 (Disable inter-cell D2D): If coordination cost is too high, disable direct inter-cell D2D and fall back to infrastructure routing.

Basic RRM Toolbox for D2D

1. Mode Selection (MoS)

  • Decides whether a device pair communicates via direct D2D or cellular infrastructure.
  • Slow time-scale: Based on path loss and distance.
  • Fast time-scale: Based on instantaneous SINR and interference.

2. Resource Allocation (ReA)

  • Decides which Resource Blocks the D2D link reuses.
  • Cellular Protection Allocation: Ensures D2D users do not reuse RBs assigned to cellular users with poor channel conditions.
  • Minimum Interference Allocation: Assigns RBs that minimize mutual interference.

3. Power Control (PC)

  • Open-Loop PC: Compensates for path loss without feedback.
  • Utility-Maximization PC: Optimizes power to maximize system-wide throughput.
  • Binary PC: Simple on/off based on interference thresholds.

Research Challenges in D2D Communication

  • Interference management: Spectrum reuse creates cross-layer and co-layer interference.
  • Fast energy-efficient discovery: Balancing discovery speed with battery consumption.
  • Multi-hop RRM: Extending mode selection and resource allocation to multi-hop paths is NP-hard.
  • Mode selection with limited CSI: Gathering full CSI costs signaling overhead.
  • Cross-operator support: Requires standardization for different spectrums and protocols.
  • Security and privacy: Direct communication bypasses network security; requires strong authentication.
  • Mobility management: Rapid movement requires fast updates to avoid interference.

Features of 5G D2D RRM

  • Flexible UL/DL TDD: Based on MIMO-OFDMA, each scheduling slot can be dynamically assigned to UL, DL, or D2D.
  • Centralized vs Decentralized scheduling: Decentralized offers low overhead; centralized offers optimal interference management.
  • Advanced Mode Selection: Includes direct D2D, indirect (DID) routing, and fast per-slot selection.
  • Multi-hop support: Out-of-coverage UEs use in-coverage UEs as relays with orthogonal resource blocks.

D2D in National Security and Public Safety

  • Infrastructure independence: D2D sidelink works without base stations for ad-hoc mesh networking.
  • Broadband group communication: Supports high-data-rate multicast (video, maps, voice).
  • Physical layer broadcast/multicast: Designed for one-to-many transmission.
  • 3GPP ProSe: Standardized discovery and communication for public safety.
  • METIS contribution: Defined multi-hop relay scenarios for coverage extension.

Network-Assisted Multi-Hop D2D

  • Concept: An out-of-coverage UE uses one or more in-coverage UEs as relays to reach the base station.
  • Network role: The base station identifies devices, selects relays, and manages resource allocation.
  • Resource allocation: Each link in the hop chain must use orthogonal resource blocks to prevent self-interference.
  • Rate constraint: End-to-end data rate is limited by the slowest link.
  • Relay UE requirements: Must support simultaneous receiving and transmitting (or half-duplex relaying).