📡 5G Co-existence with Legacy Systems

NSA (Non-Standalone) is a 5G deployment architecture where the 5G New Radio (NR) is used in conjunction with the existing 4G LTE infrastructure. In NSA mode, the 4G LTE eNodeB acts as the Master Node (MN) while the 5G gNodeB acts as the Secondary Node (SN), connected via the X2 interface. This allows operators to deploy 5G services quickly using existing EPC (Evolved Packet Core) infrastructure without requiring a complete 5G Core (5GC) deployment [^36^].

In NSA (Non-Standalone) architecture, the X2 interface connects the LTE eNB (Master Node) with the 5G en-gNB (Secondary Node). The X2 interface was originally introduced in LTE for connecting two eNBs, but was enhanced in Release 15 to support eNB-to-en-gNB connections for EN-DC (E-UTRA-NR Dual Connectivity). The S1 interface connects eNB to EPC, NG interface connects gNB to 5GC (used in SA), and Xn interface is used between gNBs in 5G SA architecture [^36^][^42^].

Dynamic Spectrum Sharing (DSS) enables both LTE/4G and 5G NR systems to coexist in the same frequency band. DSS dynamically allocates spectrum resources between LTE and 5G with millisecond-level granularity (1ms, corresponding to LTE subframe duration) based on traffic demand. This allows operators to deploy 5G in existing LTE spectrum without requiring spectrum re-farming or new spectrum licenses, ensuring backward compatibility with legacy LTE devices [^39^][^41^].

In EN-DC (E-UTRA-NR Dual Connectivity), the LTE eNB acts as the Master Node (MN) and handles all control plane functions, including RRC (Radio Resource Control) signaling and NAS (Non-Access Stratum) message forwarding. The 5G gNB acts as the Secondary Node (SN) and primarily handles user plane data traffic. This architecture allows 5G NR to leverage the existing LTE control plane infrastructure while adding 5G data capacity, making it ideal for early 5G deployments using existing 4G cores [^34^][^42^].

3GPP defines several deployment options for 5G. Option 2 represents the pure Standalone (SA) architecture where the 5G NR connects directly to the 5G Core (5GC), enabling full 5G services including network slicing, ultra-reliable low-latency communication (URLLC), and massive machine-type communications (mMTC). Options 3, 4, and 7 are Non-Standalone (NSA) variants that rely on LTE infrastructure. Option 3 uses EPC (4G core), while Options 4 and 7 use 5GC but maintain dual connectivity with LTE [^29^][^34^].

Spectrum re-farming involves clearing a frequency band of legacy technology (like LTE) and dedicating it entirely to 5G. The main advantage is reduced inter-cell interference because neighboring cells don't transmit different waveforms (LTE CRS vs 5G NR) that can interfere with each other. However, re-farming is a slow process taking many years and is inefficient when subscribers haven't migrated to newer technology. DSS offers faster deployment and backward compatibility but may experience interference from neighboring LTE cells' Cell-Specific Reference Signals (CRS) [^41^].

The NSA Option 3 family consists of three variants that differ in where user plane traffic splitting occurs [^42^]:

Option 3: Traffic is split at the eNodeB (Master Node). The eNB receives traffic from the EPC and can forward part of it to the gNB via the X2 interface. The eNB maintains the S1-U connection to the EPC.

Option 3a: Traffic is split at the EPC (specifically at the S-GW). The S-GW maintains separate S1-U connections to both the eNB and gNB, allowing direct traffic delivery to both nodes without traversing the X2 interface between eNB and gNB.

Option 3x: Traffic is split at the gNodeB (Secondary Node). The gNB receives traffic from the EPC via S1-U and can forward part of it to the eNB via X2. This is often preferred when 5G capacity exceeds LTE capacity, as it allows the high-capacity 5G node to manage traffic distribution.

All three variants use the EPC (4G core) and maintain LTE as the control plane anchor, with the MME handling control signaling.

Tight Coupling: During LTE development, 3GPP adopted tight coupling between 2G/3G and LTE networks to ensure seamless service continuity. This required complex interworking between packet core entities (GPRS/2G/3G packet core and EPC), increasing core network complexity significantly [^40^].

Loose Coupling: 5G architecture aims to reduce tight coupling with legacy systems to avoid the complexity seen in 2G/3G-LTE interworking. The 5G packet core is designed to be access-agnostic, supporting mobility on-demand and common AAA mechanisms independent of the radio access technology.

Reasons for Reducing Tight Coupling:

• The 5G Core (5GC) introduces significant architectural changes including service-based architecture (SBA), network slicing, and CUPS (Control and User Plane Separation)
• Tight coupling with LTE RAN would introduce similar complexity as seen in 2G/3G-LTE interworking
• 5G aims to support diverse access technologies (Wi-Fi, fixed access, satellite) beyond just cellular
• Legacy interworking toward circuit-switched domains in 2G/3G can be reduced or eliminated
• Allows independent evolution and scaling of access and core networks

UDM Role in 5G: The Unified Data Management (UDM) is a 5G Core network function that manages subscriber data, authentication, and service authorization. It replaces the HSS from 4G and provides enhanced capabilities for 5G services including network slicing subscription data and support for 5G-specific authentication methods [^37^].

Interworking with HSS: During migration from 4G to 5G, operators must ensure seamless interworking between HSS (4G) and UDM (5G). 3GPP has defined several integration options in TR 23.732 and TR 23.973 [^37^]:

• Data Synchronization: Subscriber data must be synchronized between HSS and UDM to ensure service continuity when users move between 4G and 5G coverage
• Combined HSS/UDM: Some implementations use combined nodes that handle both 4G and 5G subscriber data
• Interworking Function: Dedicated interfaces allow HSS and UDM to communicate for authentication and subscription data retrieval
• IP Address Continuity: Combined SMF/PGW-C and UPF/PGW-U functions ensure IP address continuity during inter-RAT mobility

This interworking is essential because legacy 4G networks will operate alongside 5G for the foreseeable future, requiring consistent subscriber data across both systems.

NSA Option 3 Trade-offs:

• Pros: Faster deployment (reuses existing EPC and LTE infrastructure), lower initial CAPEX, immediate capacity boost through dual connectivity, backward compatibility
• Cons: Limited to eMBB services only, requires tight interworking with LTE, cannot support network slicing or URLLC, LTE anchor limits performance

SA Option 2 Trade-offs:

• Pros: Full 5G capabilities (eMBB, URLLC, mMTC), network slicing support, service-based architecture, cloud-native design, independent of LTE
• Cons: Longer deployment time, higher initial investment (new 5GC required), requires 5G coverage buildout before service availability

Recommendation for Limited Spectrum/Budget:

For an operator with limited spectrum and budget constraints, NSA Option 3 with Dynamic Spectrum Sharing (DSS) is the recommended approach [^39^][^41^]:

1. Enables rapid 5G deployment using existing LTE spectrum via DSS without new spectrum licenses
2. Software upgrade to existing LTE hardware minimizes infrastructure costs
3. Provides immediate 5G branding and enhanced data rates for early adopters
4. Allows gradual migration to SA as 5G coverage expands and budget permits
5. DSS ensures efficient spectrum utilization between LTE and 5G based on demand

However, the operator should architect the NSA deployment with a clear evolution path to SA to avoid stranded assets.

Interference Challenges in DSS:

When 5G NR shares spectrum with LTE via DSS, several interference mechanisms occur [^48^][^41^]:

1. Adjacent Channel Interference: LTE Cell-Specific Reference Signals (CRS) from neighboring cells can interfere with 5G NR data transmission in the serving cell, particularly when LTE CRS falls in the "notch" or whitespace of 5G waveforms like FBMC (Filter Bank Multi-Carrier)
2. RF Impairments: Transmitter impairments (phase noise, I/Q imbalance, non-linear distortion) degrade out-of-band emission characteristics, affecting the spectral containment of both LTE and 5G signals
3. Cross-technology Interference: Different waveform characteristics between LTE (OFDM) and 5G candidate waveforms (FBMC, UFMC) create unique coexistence scenarios
4. Synchronization Requirements: Both technologies must maintain tight time synchronization for efficient spectrum sharing; misalignment causes resource allocation conflicts

Mitigation Strategies:

• Guard Bands: Implementing appropriate guard bands between LTE and 5G allocations to reduce adjacent channel interference
• Coordinated Scheduling: Inter-cell interference coordination (eICIC/FeICIC) to align LTE CRS transmissions and minimize impact on 5G
• Advanced Waveforms: Using 5G candidate waveforms with better spectral containment (lower out-of-band emissions) like FBMC or UFMC instead of CP-OFDM
• Power Control: Dynamic power adjustment based on channel conditions and interference levels
• MBSFN Subframes: Using Multimedia Broadcast Single Frequency Network subframes in LTE to create transmission gaps for 5G
• CoMP (Coordinated Multi-Point): Coordinated transmission/reception across cells to manage interference [^40^]

Research using R&D testbeds with simulated RF impairments is essential to evaluate coexistence scenarios before field deployment [^48^].

Coexistence Challenges in Hospital Environment:

Hospital environments present unique electromagnetic coexistence challenges [^33^]:

1. Dense Multipath Environment: Hospital corridors, operating rooms, and patient wards create complex RF propagation with reflections and shadowing
2. Mission-Critical Systems: Life-critical devices (wireless infusion pumps, patient telemetry, robotic surgery equipment) operate under stringent EMC requirements (IEC 60601-1-2)
3. Multi-technology Interference: Coexistence of Wi-Fi 6/6E, Bluetooth Low Energy (BLE), legacy LTE, and new private 5G in shared or adjacent spectrum
4. Safety Thresholds: Must comply with ICNIRP, WHO, and IEEE RF exposure guidelines for patient and staff safety
5. Spectral Overlap: 3.5-3.8 GHz 5G may overlap with existing Wi-Fi 6E and other services

Measurement and Validation Strategies:

• 24-hour Spectrum Monitoring: Conduct high-resolution, continuous spectrum measurements across 0.4-6.1 GHz to characterize temporal and spatial occupancy patterns [^33^]
• RF Exposure Assessment: Measure received power densities at multiple locations (operating rooms, patient wards) and compare against ICNIRP/WHO safety thresholds
• Interference Testing: Controlled testing with medical devices to validate no measurable interference from 5G to critical equipment
• Spectral Isolation Verification: Confirm 5G network remains spectrally isolated with no interference to adjacent LTE or Wi-Fi channels
• Identify "Spectrum Islands": Locate interference-free spectral zones that can be allocated for safe medical data transmission
• 3GPP/ETSI Compliance: Verify emission requirements compliance for 5G equipment
• On-site RF Exposure Assessment: Conduct pre-deployment and post-deployment RF safety audits per ITU-T K.91 guidelines

Empirical studies have shown that well-engineered private 5G deployments can operate safely in hospital environments with received power levels of -16 dBm corresponding to power densities far below international safety thresholds, confirming stable coexistence with legacy wireless systems [^33^].