How Does DSS Work Between LTE and NR Systems?—Requirements, Techniques, and Lessons Learned

Abstract

Dynamic Spectrum Sharing (DSS) enables spectrum sharing between Long-Term Evolution (LTE) and New Radio (NR) systems, addressing spectrum scarcity in NR. To avoid interference when supporting NR traffic within LTE spectrum, key factors must be compatible. Effective DSS techniques are essential for coexistence. This paper discusses these issues in two parts. Part I covers LTE and NR coexistence using DSS, introducing resource grids, control signals, and channels, and explores DSS approaches for NR data traffic, including NR Synchronization Signal/Physical Broadcast Channels (SSB) transmission via LTE Multicast-Broadcast Single-Frequency Network (MBSFN) and non-MBSFN subframes with associated challenges and standardization efforts for DSS improvement. Part II presents a DSS technique using MBSFN subframes in a heterogeneous network with a macrocell and picocells running on LTE, and in-building small cells running on NR, sharing LTE spectrum via DSS. An optimization problem is formulated to manage traffic through MBSFN allocation, determining the optimal number of MBSFN subframes per LTE frame. System simulations indicate DSS improves Spectral and Energy Efficiency in small cells. The paper concludes with key lessons for LTE and NR coexistence.

1. Introduction

1.1. Background

Dynamic Spectrum Sharing (DSS) is an effective technology for addressing the growing spectrum scarcity faced by mobile Network Operators (MNOs). In the context of New Radio (NR), DSS plays a vital role by sharing Long-Term Evolution (LTE) spectrum with NR to accommodate NR traffic. Since the number of NR User Equipments (UEs) is much lower compared to LTE, serving NR within the LTE spectrum is essential during the early stages of NR deployment [1]. This approach helps MNOs avoid significant costs associated with NR spectrum licensing. Moreover, because NR frequency bands are typically much higher than LTE bands, NR encounters challenges in delivering extensive coverage at these high frequencies due to increased distance-dependent path loss and indoor penetration loss compared to LTE. Additionally, unlike spectrum refarming—which impacts LTE spectrum utilization because of its static allocation to NR regardless of LTE and NR UE densities—in this context, as the number of NR UEs increases relative to LTE UEs, a gradual and smooth migration of LTE spectrum to NR can be achieved.

In LTE, there are two types of subframes: normal or Non-Multicast-Broadcast Single Frequency Network (non-MBSFN) subframes and Multicast-Broadcast Single Frequency Network (MBSFN) subframes. DSS can use either type to transmit NR data within the LTE framework. However, supporting NR traffic within LTE spectrum requires several critical factors—including resource grids, subcarrier spacings, and control signals and channels for both systems—to be compatible; otherwise, interference may occur at the resource grid level. Specifically, each system has a certain number of reference signals, such as the Cell Reference Signal (CRS) in LTE and the Synchronization Signal/Physical Broadcast Channels (SSB) and Demodulation Reference Signals (DMRS) in NR. Proper operation of these signals is essential, as they are used to perform vital functions in their respective systems. For example, CRSs are used to synchronize LTE UEs with their serving base stations. Whereas NR uses SSBs to establish a connection with a UE on the NR network, and DMRSs to estimate channel conditions and demodulate NR data signals. Therefore, to support NR traffic in the LTE spectrum using DSS, proper alignment of these signals in the LTE resource grid is essential to prevent interference caused by collisions between signals from both systems, since they occupy the same LTE resource in time and frequency, particularly when LTE employs non-MBSFN subframes. Unlike non-MBSFN, the LTE MBSFN subframe’s resource grid depends on the fact that NR traffic is transmitted in LTE MBSFN regions where CRSs are completely muted; there is no collision between NR Physical Downlink Shared Channels (PDSCHs) and LTE CRSs during NR transmission with MBSFN subframes. This results in no interference from LTE CRSs on NR traffic, allowing efficient support of NR traffic through MBSFN subframes. Therefore, to enable DSS for the coexistence of LTE and NR without compromising LTE performance, a thorough understanding of the coexistence of these signals using DSS is essential.

Along with an understanding of how DSS works, another primary concern is how to implement DSS between LTE and NR systems. Although DSS can be implemented using both MBSFN and non-MBSFN subframes, the absence of LTE CRS interference on NR PDSCHs due to muting CRSs in the MBSFN region makes MBSFN one of the simplest and most effective solutions for implementing DSS. In MBSFN subframe-based DSS, MBSFN configuration parameters, including the number of MBSFN subframes per LTE frame and MBSFN Frame periodicity, have a significant impact on DSS performance. Moreover, while it is generally acknowledged that DSS depends on the traffic demands of LTE and NR, there is a lack of systematic treatment of MBSFN subframe-based DSS in the existing literature.

1.2. Related Work and Problem Statement

Studies explore DSS for LTE and NR coexistence, including [2,3,4,5,6,7,8]. For instance, ref. [4] introduces a dual bargaining game scheme; ref. [5] proposes a cross-band spectrum sharing method; ref. [3] suggests interference mitigation via buffer setting and rate matching; ref. [7] analyzes interference avoidance between Frequency Division Duplex (FDD) LTE and NR; ref. [8] discusses DSS concepts for sharing same frequencies; and finally, ref. [2] examines coexistence strategies for LTE and Fifth-Generation (5G) NR in shared scenarios. However, a detailed understanding of DSS coexistence, including resource grids, subcarrier spacings, and control signals, remains underexplored.

Few studies address MBSFN subframe-based DSS for coexisting radio technologies. In [9], an LTE and Wi-Fi scheme utilizes MBSFN to enable Wi-Fi by adaptively configuring muted subframes based on LTE and Wi-Fi spectrum data determined by the Technology Recognition and Traffic Characterization (TRTC) system, resulting in improved throughput over Almost Blank Subframes. Similarly, ref. [5] proposed a cross-band DSS using resource controllers for LTE and NR to assign muted MBSFN subframes and muted Multicast Broadcast Service (MBS) subframes on the LTE and NR bands, utilizing the TRTC system to enhance the throughput and spectrum utilization efficiency of the LTE and NR bands compared to a static band.

Because traffic demand varies over time, allocating the same number of MBSFN subframes at all times makes it inefficient. Hence, it is essential to develop a technique that adjusts MBSFN subframes per LTE frame based on the traffic demands of both LTE and NR UEs. With up to 80% of mobile traffic generated indoors [10], adequate in-building coverage and capacity are crucial, and small cells such as femtocells are key to meeting these demands. However, current research lacks solutions that optimize MBSFN subframe allocation without disrupting LTE traffic while accommodating NR traffic in small cell networks.

1.3. Contributions

To address the above issues, in Part I, we review various DSS techniques and discuss their limitations. In addition to NR PDSCH data transmissions, we provide details on NR control signals and channels, with a particular focus on NR SSB transmissions in the LTE spectrum. This includes the use of both MBSFN and non-MBSFN subframes. To utilize the LTE spectrum effectively, we elaborate on NR SSB transmission methods that leverage both MBSFN and non-MBSFN subframes, along with the associated challenges. Moreover, we also discuss standardization efforts for DSS improvement in Third Generation Partnership Project (3GPP) releases 16, 17, and 18. Likewise, in Part II, we propose a DSS technique based on MBSFN to enable the coexistence of LTE and NR in a heterogeneous network. This network consists of a macrocell and multiple picocells, as well as in-building small cells, all within the macrocell’s coverage area. Both the macrocell and the picocells utilize LTE technology, while the in-building small cells are equipped with NR technology. The small cells operate within the same LTE spectrum as the macrocell and picocells by employing the proposed DSS technique.

We formulate an optimization problem to ensure that MBSFN subframe allocation to NR balances the traffic demands of both LTE and NR. An analytical model is developed to address this optimization problem, determining the optimal number of MBSFN subframes per LTE frame for any fixed value of the MBSFN frame periodicity, which is updated at each renewal time. To evaluate the performance of the DSS technique, we derive metrics for average capacity, Spectral Efficiency (SE), and Energy Efficiency (EE). Using a system-level simulation, we assess the impact of NR UE density on the allocation of MBSFN subframes to both LTE and NR networks. Additionally, we examine the effects of employing DSS in NR small cells in terms of SE and EE while varying the number of MBSFN subframes from the minimum to the maximum. Finally, we summarize the key lessons learned in this paper to provide a clear overview of DSS between LTE and NR systems, including an example of a DSS technique.

1.4. Organization

We have organized this paper into Part I and Part II. Part I includes Section 2, Section 3, Section 4 and Section 5, whereas Part II includes Section 6, Section 7, Section 8 and Section 9. In Section 2, we discuss resource grids, subcarrier spacings, and control signals and channels for LTE and NR systems, with a focus on avoiding interference between LTE CRSs and NR SSBs. Section 3 examines various DSS approaches for transmitting NR PDSCH data using both LTE MBSFN and non-MBSFN subframes. In Section 4, we cover NR SSB transmissions that allow UE to access LTE spectrum using NR, incorporating both LTE MBSFN and non-MBSFN subframes. Section 5 discusses standardization efforts for enhancing DSS in 3GPP Releases 16, 17, and 18. In Section 6, we introduce the system model and then propose an MBSFN subframe-based DSS technique for the coexistence of LTE and NR technologies. In Section 7, we formulate an optimization problem for the proposed DSS method to manage LTE and NR traffic demands through MBSFN allocation and develop an analytical model to determine the optimal number of MBSFN subframes per LTE frame for LTE and NR networks at a fixed MBSFN frame periodicity. In Section 8, we derive SE and EE metrics and describe the relevant simulation parameters and assumptions and conduct a system-level simulation to examine the impact of DSS used in in-building small cells on the overall network of the MNO in terms of spectral and energy efficiencies in Section 9. Finally, Section 10 highlights key lessons learned throughout the paper, summarizing the insights on DSS between LTE and NR systems. We conclude the paper in Section 11.

2. Resource Grids and Control Signals and Channels

To understand how LTE and NR coexist using DSS, it’s important to know where control signals and channels are located in each system’s resource grid to prevent interference when serving NR PDSCH within LTE spectrum. Below, we detail the control signals and channels in the resource grids of LTE and NR, covering frame structures, the Physical Downlink Control Channel (PDCCH), LTE’s CRS, and NR’s SSB and DMRS. Both LTE and NR systems utilize a 10 ms radio frame structure for uplink and downlink transmissions, comprising 10 subframes, each 1 ms long and made up of two consecutive time slots. LTE employs a single Orthogonal Frequency Division Multiplexing (OFDM) numerology with a 15 kHz Subcarrier Spacing (SCS), whereas NR supports multiple numerologies with variable SCS values, including 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz. For simplicity in this analysis, NR also considers a 15 kHz SCS, matching LTE, to facilitate straightforward DSS.

It’s important to note that with an SCS of 15 kHz, each subframe contains two slots, and with a normal cyclic prefix, each slot has seven symbols, resulting in 14 symbols per subframe. Figure 1 illustrates the resource grid for one Resource Block (RB) over one symbol duration (for the normal cyclic prefix) for both NR and LTE, depicting k subcarriers and l symbols. The smallest unit, covering an SCS of 15 kHz and one symbol duration, is called a Resource Element (RE). Twelve REs form a Resource Element Group (REG), also known as an RB. Efficiently allocating RBs from LTE to NR without overlap determines the effectiveness of DSS between the two technologies.

2.1. LTE Resource Grids

In LTE, there are two kinds of subframes: normal or non-MBSFN and MBSFN subframes. DSS can utilize either to transmit NR data within the LTE framework. Each subframe type has a distinct resource grid structure. CRSs and PDCCHs primarily determine the LTE non-MBSFN subframes’ time-frequency grid (Figure 1). The number and placement of CRSs and PDCCHs depend on the number of antenna ports. Figure 1 illustrates LTE CRS mapping for different antenna port configurations in non-MBSFN subframes. The CRS appears at symbols 0, 4, 7, and 11 for antenna ports 1 and 2, while for antenna port 4, it appears at symbols 0, 1, 4, 7, 8, and 11.

LTE PDCCHs are mapped only to symbols 0 and 1 for both non-MBSFN and MBSFN subframes. At symbols 0 and 1 (for 4 antenna ports), PDCCHs are partially mapped to REs not occupied by CRSs. REs allocated to CRSs and the mentioned PDCCHs cannot be used for NR traffic to prevent collisions and interference. Unlike non-MBSFN, the LTE MBSFN subframe’s time-frequency resource grid mainly relies on the fact that, apart from the first two symbols 0 and 1 shown in Figure 1, the remaining 12 symbols in a normal cyclic prefix are allocated for NR traffic. LTE CRS and PDCCH signals are transmitted in the non-MBSFN region (defined by symbols 0 and 1 shown in Figure 1), while the remaining 12 symbols can be allocated for MBSFN transmission.

2.2. NR Resource Grid and SSB Structure

Figure 2a illustrates the resource grid layout of NR for a single RB over one symbol duration. In a typical scenario without NR-LTE DSS, either the first three symbols—0, 1, and 2—or all three may carry NR PDCCH. NR DMRSs are positioned at symbols 3 and 11, while the other nine symbols in the subframe are available for NR PDSCH or data traffic. For an NR UE to connect to the NR network, it must access an SSB within the NR frame. Similarly to LTE, NR uses two synchronization signals: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). An NR SSB is thus composed of PSSs, SSSs, and Physical Broadcast Channels (PBCHs).

The UE needs PSSs for downlink frame synchronization, SSSs for downlink subframe synchronization, and PBCHs for receiving additional system information. Additionally, the UE acquires cell access details by synchronizing with the cell in time and frequency and detecting the cell’s physical cell ID through SSs and PBCHs. An SSB consists of 20 consecutive RBs, which can start at any RB index within the resource grid and cover four contiguous symbols, as shown in Figure 2b. SSBs are transmitted periodically every 5 ms, 10 ms, 20 ms, 40 ms, 80 ms, and 160 ms.

3. DSS Deployment Approaches

DSS is primarily relevant for low-band FDD spectrum, typically ranging from 600 to 2600 MHz, where it can flexibly facilitate refarming from LTE to 5G. Depending on the LTE subframe types—MBSFN and non-MBSFN—DSS deployment between LTE and NR can occur in three different ways [14].

3.1. Option 1: Multicast-Broadcast Single-Frequency Network-Based

In LTE, certain subframes within a radio frame, excluding symbols 0 and 1, reserve the remaining 12 OFDM symbols, as shown in Figure 3, for services other than LTE data transmission, specifically for LTE broadcast services. These subframes are known as MBSFN subframes. Deploying DSS between LTE and NR can utilize MBSFN subframes so that, instead of transmitting LTE broadcast services, NR data can be sent during these 12 reserved OFDM symbols in each LTE MBSFN subframe. MBSFN subframes are transparent to LTE UEs; when a subframe is detected as MBSFN, LTE UEs skip it. LTE CRSs and PDCCHs are transmitted in the non-MBSFN region, which is defined by symbols 0 and 1 at the start, as shown in Figure 3.

3.2. Option 2: Mini-Slot Based

In option 2, NR data can be transmitted anywhere within an LTE slot except during symbols that occupy LTE CRSs. This results in mini-slots formed by symbols not occupied by CRSs, as shown in Figure 3 (bordered by black lines). Since no REs at CRS-occupied symbols are used for NR transmission in each subframe, this approach to DSS deployment results in poor resource utilization and is not suitable for Enhanced Mobile Broadband (eMBB) services.

3.3. Option 3: CRS Rate Matching Based

In this option, NR UEs puncture REs in any LTE non-MBSFN subframe to detect LTE CRSs, preventing the NR scheduler from assigning those REs to NR data transmission. This can be implemented at two levels: the RE level and the RB level. At the RE level, only REs occupied by LTE CRSs are avoided, while at the RB level, the entire RB (including both occupied and unoccupied LTE CRS REs) is excluded from NR scheduling, as shown in Figure 3 (bordered by dotted-red lines). RB-level scheduling is simpler but results in fewer REs available for NR compared to the RE level. Note that option 3 differs from option 2: in option 2, symbol index 1, reserved for LTE PDCCHs, can still be scheduled for NR traffic, whereas in option 3, it cannot.

Since every DSS deployment option has its own benefits and drawbacks, and each may be suitable for different scenarios, it is common for an effective DSS solution to involve combining two or more of the options discussed above. In

Table 1. Comparative analysis of different DSS deployment options.

Attributes	DSS Deployment Options
	Option 1	Option 2	Option 3
LTE subframes	MBSFN	Non-MBSFN	Non-MBSFN
Applications	This option works well for high-data-rate applications.	Option 2 is ideal for applications requiring extremely low latency.	Option 3 is suitable for high-data-rate applications and RE-level CRS rate matching.
Limitations	Overusing this option decreases LTE throughput.	This option results in high resource utilization in NR data scheduling.	This option has limited resources for RB-level CRS rate matching.
NR services	eMBB	Ultra-Reliable Low-Latency Communication (URLLC)	eMBB
Placement of LTE control signals and channels	Control signals and channels are present in the first two symbols of each LTE subframe.	Control signals and channels are present throughout the LTE slot	Control signals and channels are present throughout the LTE slot
NR traffic transmission in the LTE spectrum	NR traffic transmission takes place only within the MBSFN region without CRSs.	NR traffic transmission occurs anywhere in an LTE slot, except for symbol 2 and the symbols allocated for LTE CRSs.	NR traffic transmission occurs anywhere in an LTE slot, except for symbol 2 and the symbols allocated for LTE CRSs.

, we analyze these deployment options systematically to illustrate their relative performances.

4. NR SSB Transmissions

The primary factor enabling LTE and NR systems to coexist is sharing LTE’s spectrum with NR for data and control signals, without disrupting the current LTE operation. Both MBSFN and non-MBSFN LTE subframes can be utilized to implement DSS for the coexistence of NR and LTE. A key concern for an NR UE is its inability to connect to the LTE spectrum and perform DSS with LTE if the NR SSB control signals are not aligned on the LTE resource grid. For NR PDSCH data transmissions, while CRS rate matching is an effective method to deploy DSS using LTE non-MBSFN subframes—helping to avoid collisions between NR PDSCH and LTE CRSs—puncturing an NR SSB control channel is quite challenging because it affects downlink synchronization and measurements. This indicates that to align NR UEs with the LTE spectrum, a different slot mapping of NR SSBs (each consisting of a group of 20 contiguous RBs over 4 contiguous symbols) on the LTE resource grid is necessary when using DSS to transmit NR PDSCH, regardless of the presence or absence of NR SSBs on the LTE spectrum.

In the time domain, SSB locations are fixed in NR. For example, in the n41 Time-Division Duplex (TDD) frequency band, two SSBs are present: one covering symbols 2 to 5 and another covering symbols 8 to 11 within a 14-symbol subframe at 15 kHz SCS. With an SCS of 30 kHz, four SSBs can be transmitted in a subframe, spanning symbols {4, 5, 6, 7}, {8, 9, 10, 11}, {16, 17, 18, 19}, and {20, 21, 22, 13}. Depending on the employed SCS and the number of beams formed, an SS burst—comprising a set of SSBs—of length L is transmitted across different beams. A UE scans for SSB signals and locks onto the strongest beam to connect to the NR network. It is important to note that a UE requires at least one SSB out of the L SSBs to synchronize with the network. The necessity for four contiguous symbols to position SSBs within the resource grid presents a significant challenge for applying DSS on the LTE spectrum to serve NR PDSCH, especially in the absence of NR SSBs or with current configurations. Below, we examine this scenario—mapping NR SSBs onto LTE spectrum—and explore potential alternative solutions, highlighting their inherent limitations under both LTE non-MBSFN and MBSFN subframes.

4.1. NR PDSCH Without SSB Transmissions Using LTE Non-MBSFN Subframes

If DSS is used in the LTE spectrum with LTE non-MBSFN subframes (for 4 antenna ports), LTE’s CRSs and PDCCHs occupy symbols 0 and 1, while NR’s PDCCH takes symbol 2. NR’s DMRSs are placed in symbols 3 and 12 (considering the alternative DMRS location), and LTE’s CRSs partially occupy some REs within symbols 4, 7, 8, and 11. Figure 4 depicts how the resource grid structure of NR cells in Figure 2a is altered into NR-LTE cells with DSS within the LTE spectrum. Collisions between LTE CRSs and NR PDSCHs (at symbols 4, 7, 8, and 11) can be mitigated by applying CRS rate matching at either the RE-level or RB-level. This process subsequently decreases the number of REs available for NR PDSCHs, especially in this non-MBSFN region within a slot. For instance, when RE-level CRS rate matching is used, the REs accessible to NR PDSCHs are reduced by 16 in LTE symbols 4, 7, 8, and 11.

Remark 1 (NR DMRS). 

When deploying DSS between LTE and NR, one NR DMRS at symbol 3 is adequate. However, if a greater number of LTE REs per RB is allocated for NR traffic, an extra DMRS must be transmitted at symbol 11 of NR. The locations of NR DMRSs at symbols 3 and 11 for each RB per subframe are fixed within NR for a 15 kHz SCS. It’s important to note that LTE CRSs are present at symbol 11 in any LTE non-MBSFN subframe. Therefore, to prevent a collision with LTE CRSs, the additional NR DMRS cannot be placed at symbol 11 of an LTE non-MBSFN subframe when using DSS. Although not explicitly stated in the NR specification, a provision exists for relocating the additional NR DMRS from symbol 11 to symbol 12 in LTE when utilizing DSS to handle NR data on LTE spectrum, as illustrated in Figure 4.

4.2. NR PDSCH with SSB Transmissions Using LTE Non-MBSFN Subframes

Figure 5a illustrates NR SSB transmissions without DSS at 15 kHz SCS, demonstrating that two SSBs, each covering 4 symbols, occupy space in the NR resource grid. As previously noted, NR PDCCH can occupy any of the symbols 0, 1, 2, or all three symbols (i.e., 0, 1, and 2), while NR DMRSs are positioned on symbols 3 and 11 in NR at 15 kHz SCS. However, LTE CRSs are located on symbols 0, 1, 4, 7, 8, and 11 when considering 4 antenna ports at 15 kHz SCS (Figure 5c). Figure 5b depicts the collisions that arise between NR SSBs and LTE CRSs when utilizing DSS at 15 kHz SCS. Specifically, from Figure 5a,c, collisions happen between SSB0 and CRSs at symbol 4, as well as between SSB1 and CRSs at symbols 8 and 11. This indicates that NR SSBs cannot be positioned within the LTE spectrum. Consequently, NR UEs are unable to connect to the LTE spectrum, which creates a significant challenge in implementing DSS between LTE and NR systems.

The challenges associated with implementing DSS between LTE and NR can be addressed by utilizing the previously mentioned mixed numerology. Specifically, in contrast to LTE, which employs a single numerology, NR features a scalable or mixed numerology that allows the selection of different SCSs, symbol durations, and the number of symbols per slot. For instance, the baseline SCS of 15 kHz chosen for LTE can be scaled by a factor of 2^(x−1) where x is any non-negative integer such that $x\in \left\{1,2,3,4\right\}$. This scaling results in SCSs of 15 kHz, 30 kHz, 60 kHz, and 120 kHz. In this context, 3GPP suggested utilizing mixed numerology for frequency multiplexing to accommodate multiple services on the same carrier. Specifically, they proposed an SCS of 15 kHz for data traffic and an SCS of 30 kHz for control signals to enable DSS between LTE and NR systems, as shown in Figure 5b.

The effects of utilizing mixed numerology are illustrated in Figure 6. More specifically, with the SCS altered from 15 kHz to 30 kHz for SSBs, LTE CRSs, and LTE PDCCHs occupy symbols 0 and 1. At the same time, an NR SSB takes up symbols 2 and 3. Since LTE CRSs use symbol 4, NR PDCCHs must be moved to symbol 5, and NR PDSCHs have to be shifted to at least symbol 6. This shift results in a shortage of the necessary symbols for NR PDSCH Type A mapping, as NR PDSCH Type A mapping is required to begin at either symbol 0, 1, 2, or 3. Consequently, we are forced to transition to NR PDSCH Type B mapping, which limits our options (see Remark 2 for details). However, a significant issue with NR PDSCH Type B mapping is that the maximum number of symbols in Type B is restricted to either 2, 4, or 7. Taking into account the highest symbol count of 7 in Type B, if NR PDSCHs start from symbol 6, they could extend to symbol 12, leaving symbol 13 unoccupied in the subframe. This results in inefficient resource utilization. The constraints observed when applying DSS using LTE non-MBSFN subframes provide support for the use of LTE MBSFN subframes in NR SSB transmissions on the LTE spectrum, which we will explore next.

Remark 2 (NR PDSCH mapping for DSS). 

There are two categories of PDSCH scheduling in NR—Type A and Type B. Type A mapping involves slot-based PDSCH allocation, allowing PDSCHs to be assigned at the slot level starting from symbols 0, 1, 2, or 3, and can be scheduled with a minimum of 3 to a maximum of 14 symbols per NR subframe. It is important to note that Type A mapping must commence at either symbol 0, 1, 2, or 3, and the DMRS must occupy either symbol 2 or symbol 3 within a slot. To address the limitations of Type A mapping, Type B, or mini-slot mapping is employed. In Type B mapping, the transmission can be initiated at any symbol within a slot. To prevent delays, the duration of Type B is restricted to 2, 4, or 7 symbols. In Type B mapping, DMRS is assigned to the first symbol of the PDSCH allocation, regardless of its position in the slot, making it beneficial when the allocation begins midway through a slot.

4.3. NR PDSCH with/Without SSB Transmissions Using LTE MBSFN Subframes

In LTE, for broadcasting Television (TV) channels, a specific subframe can be designated as an MBSFN. In the MBSFN subframe, symbols 0 and 1 are allocated for LTE CRSs and PDCCHs. The remaining 12 symbols in the subframe are left vacant to transmit solely TV signals or broadcast services utilizing LTE networks. When an LTE UE detects an LTE subframe set as MBSFN, it disregards the corresponding subframe. This indicates that an LTE MBSFN subframe with completely unoccupied 144 REs (i.e., REs) can be utilized to convey NR traffic. Figure 7 illustrates NR SSB transmissions using LTE MBSFN subframes.

Since, apart from the first two symbols, MBSFN mutes all other 12 symbols for NR PDSCHs, NR SSBs can be sent without any interference with LTE CRSs and PDCCHs, unlike in non-MBSFN subframes. Therefore, in contrast to non-MBSFN subframes, NR PDSCHs and NR SSBs can be effectively accommodated using LTE MBSFN subframes. However, it should be noted that not every subframe within an LTE radio frame can be designated as an MBSFN. Subframes 0, 4, 5, and 9 within an LTE frame contain system-specific information and, as such, cannot be designated as MBSFNs. Specifically, subframes 0 and 5 carry PSS, SSS, and Broadcast Channel (BCH) in LTE FDD, while subframes 4 and 9 convey LTE paging details. This means that out of 10 subframes in an LTE frame, a maximum of six subframes, which include subframes 1, 2, 3, 6, 7, and 8, can be assigned as MBSFN subframes. Consequently, an operator can assign between 1 and 6 LTE subframes within an LTE frame as MBSFNs, based on the requirement for NR traffic. Allocating more LTE subframes as MBSFNs leads to reduced throughput for LTE users. As NR SSB can be transmitted efficiently using MBSFNs, an optimal DSS strategy would involve a combination of both MBSFN and non-MBSFN subframes.

Table 2. NR PDSCH with/without SSB transmissions using LTE MBSFN and non-MBSFN subframes.

	NR PDSCH with SSB Transmissions	NR PDSCH Without SSB Transmissions
LTE MBSFN subframes	❖ There is no collision between LTE CRS and PDCCH with NR SSBs. NR SSBs can be efficiently accommodated within MBSFN subframes. ❖ Aligning LTE subframes with NR SSB slots is important because SSB slots may overlap within potential MBSFN subframes in an LTE frame. ❖ Minimal impact on NR SSBs is observed. By using mixed numerology, it is possible to transmit all four SSBs (corresponding to four antenna ports) within a single LTE subframe.	❖ The simplest method to implement DSS between LTE and NR is through MBSFN, which can effectively include NR PDSCH. ❖ Not every LTE subframe can be configured for MBSFN; up to six out of ten subframes in an LTE radio frame may be assigned as MBSFNs for DSS. ❖ Aside from the first two symbols, the remaining 12 symbols are available for DSS. Typically, most operators initially utilize LTE MBSFNs for their DSS operations.
LTE non-MBSFN subframes	❖ Mixed numerology, featuring a 15 kHz SCS for PDSCH and a 30 kHz SCS for SSB, is beneficial for SSB transmission with LTE non-MBSFNs. ❖ It can only transmit one SSB per subframe, causing delays in SSB transmissions. ❖ Since Type A mapping must begin at symbols 0, 1, 2, or 3, only Type B mapping is feasible for PDSCH transmissions, which results in resource wastage as symbol 13 remains unused.	❖ The same numerology, such as 15 kHz SCS, can be used for both NR and LTE in DSS. ❖ Collisions between NR PDSCHs and LTE CRSs happen when serving NR PDSCHs within LTE spectrum. These can be prevented by using either RE- or RB-level CRS rate matching. ❖ Both Type A and Type B mapping methods are applicable for NR PDSCH transmissions.

summarizes NR PDSCH with and without SSB transmissions using LTE MBSFN and non-MBSFN subframes.

5. 3GPP Standardization Efforts for Further DSS Enhancements

In NR standardization, 3GPP details various functions and requirements across multiple releases, beginning with Release 15, to enable DSS for the simultaneous operation of NR and LTE within the same LTE spectrum. Release 15, in particular, specifies several functions to support LTE and NR coexistence in FDD bands. To boost performance and support new use cases, additional features approved in Release 16 and further developments proposed in Releases 17 and 18 focused on DSS are examined. This section highlights the standardization efforts for enhancing DSS in 3GPP Releases 16, 17, and 18.

5.1. 3GPP Releases 16 and 17 Enhancements

Following the initial launch of 5G in Release 15 [17], 3GPP has continued to develop 5G to enhance performance and support new use cases. This includes features approved in Release 16 and proposed enhancements in Release 17 for DSS [18], which are discussed below.

(1) Multiple CRS rate matching patterns: In Release 15, CRS matching is only set for NR and LTE Component Carriers (CCs) of the same bandwidth. However, if an NR carrier has a larger bandwidth than an LTE CC, multiple CRS rate matchings are required. Consequently, in Release 16, CRS rate matching patterns for LTE are expanded to cover a wider bandwidth, allowing an NR carrier to include multiple CCs using Carrier Aggregation (CA) [19] (Figure 8). Up to three CRS rate matching patterns were agreed upon to be configured within a single NR carrier.

(2) Improvement of PDSCH mapping type B: In Release 15, PDSCH mapping Type B is employed when the PDCCH is located in symbols beyond the initial three. However, only PDSCH lengths of 2, 4, and 7 symbols are considered, which leads to underutilization of the available resources since the remaining resources in a slot cannot be fully used. For instance, the fifth symbol remains unused when trying to fit a 7-symbol PDSCH, as the remaining 10 symbols cannot be allocated (see Figure 9). To address this, additional symbol lengths, such as a 10-symbol PDSCH [19,20], have been defined for PDSCH mapping type B to ensure full resource utilization in Figure 9.

(3) Cross-carrier scheduling: Due to the limited shared resource use of PDCCHs for NR and LTE, there might not be enough NR scheduling capacity in shared resources as the number of NR devices grows [21]. To solve this in Release 17, cross-carrier scheduling has been added [22]. This method helps overcome the PDCCH resource constraints of the NR primary cell by leveraging secondary cells (approach 1). Alternatively, PDSCH can be scheduled across multiple cells through the PDCCH of the Primary (Secondary) Cell/Secondary Cell using a single Downlink Control Information (DCI) (approach 2) as shown in Figure 10 [20].

(4) Employing DSS on mid-band TDD: In addition to deploying in LTE low-band, several network operators who deployed LTE in mid-band TDD would like to deploy DSS in mid-band TDD as well [23].

5.2. 3GPP Release 18—Flexible Spectrum Use

3GPP Release 18 introduces further enhancements to support more flexible and efficient spectrum use for 5G deployments in various scenarios with different spectrum allocations. The key enabling features are outlined below [24].

(1) NR for spectrum < 5 MHz: The current 5G NR specifications support a minimum channel bandwidth of 5 MHz. However, interest is increasing in deploying NR in dedicated spectrum with a bandwidth of less than 5 MHz. In Release 18, 3GPP will introduce updates to NR to enable deployments in spectrum allocations smaller than 5 MHz [24]. For example, Europe’s railway communication, currently based on the Global System For Mobile Communications-Railway (GSM-R), will transition to the Future Railway Mobile Communication System (FRMCS) using the harmonized 900 MHz spectrum block (2 × 5.6 MHz FDD) as shown in Figure 11. However, FRMCS must coexist with GSM-R by reserving part of the spectrum for it, which will limit the spectrum available for NR deployment in FRMCS to less than 5 MHz [24].

(2) NR DSS enhancement: In DSS, the capacity of the PDCCH is limited because NR PDCCH and LTE PDCCH share the first three OFDM symbols within a slot. Additionally, NR PDCCH cannot use symbols that overlap with LTE CRS. In Release 18, 3GPP will investigate the possibility of allowing NR PDCCH transmission in symbols overlapping with LTE CRS to improve PDCCH capacity in DSS (Figure 12) [24].

(3) NR MultiCarrier Enhancement: 3GPP Release 18 will introduce a feature that allows a single DCI to schedule multiple PDSCHs or Physical Uplink Shared Channels (PUSCHs) across carriers and study enhancements for multi-carrier uplink operation as shown in Figure 13 [24].

(4) NR Duplex Operation Evolution: To support more flexible spectrum usage, 3GPP Release 18 will explore the possibility of enabling simultaneous downlink and uplink operation within a standard TDD band (Figure 14). This effectively combines aspects of TDD and FDD in the duplex [24]. Practically, the study will focus on subband non-overlapping full duplex at the 5G Node B (gNB). Specifically, a traditional TDD band will be split into separate subbands for uplink and downlink, while UEs will continue to use half-duplex.

(5) Cross-link interference (CLI): Additionally, 3GPP Release 18 will investigate methods to handle CLI to improve support for dynamic TDD in commercial deployments [24].

6. System Model and Proposed DSS Technique

In Part II—ranging from Section 6, Section 7, Section 8 and Section 9, we present a DSS technique using LTE MBSFN subframes to enable the coexistence of LTE and NR following the principles and requirements described in Part I through Section 2, Section 3, Section 4 and Section 5 in a heterogeneous network with a macrocell, picocells, and in-building small cells.

6.1. System Model

The architecture of an MNO is illustrated in Figure 15. It consists of a Macrocell Base Station (MCBS) that includes several Picocell Base Stations (PCBSs) and in-building Small Cell Base Stations (SCBSs). SCBSs are installed explicitly in Three-Dimensional (3D) multistory buildings, with each one serving a single UE at a time, known as small cell UEs.

Most macrocell UEs, which are linked to the MCBS, are primarily served by the MCBS itself or offloaded to the PCBSs. A portion of these macrocell UEs is assumed to be located within multistory buildings equipped with small cells. The MCBS and PCBSs utilize LTE technology operating in the 2 GHz spectrum to provide extensive outdoor coverage. Meanwhile, each in-building SCBS is enabled with NR technology, which leverages the LTE spectrum of the MNO via DSS to manage indoor small cell NR traffic, as shown in Figure 15. It is important to highlight that while operating in the LTE spectrum using DSS, an SCBS can only support its UEs during muted MBSFN subframes of LTE.

DSS between LTE indoor macrocell UEs and NR in-building small cell UEs, when both types of UEs are located within the same building (Figure 15b), works as follows. At each renewal time, t, based on the estimation of LTE and NR UE densities, an optimal number of MBSFN subframes per LTE frame is defined for NR small cell UEs and LTE macrocell UEs. For a given MBSFN frame periodicity, NR small cell UEs are scheduled to LTE spectrum to serve their traffic only during their designated number of MBSFN subframes per LTE frame at every LTE MBSFN frame period over renewal time t. Macrocell UEs located indoors and outdoors, as well as those offloaded to picocells, are served during all other MBSFN subframes at renewal time t. The same process repeats at every renewal time t.

Due to the dynamic nature of LTE UEs and NR UEs, their UE densities change over time, leading to a corresponding adjustment in the total number of MBSFN subframes allocated to NR small cell UEs at each renewal time t. Since NR and LTE operate orthogonally in the time domain, both at the subframe level (within an MBSFN or a non-MBSFN frame) and at the frame level (within a renewal time t), no co-channel interference occurs between NR and LTE UEs when accessing the same LTE spectrum. In summary, NR small cells operate on the same LTE spectrum as macrocells and picocells, utilizing DSS during MBSFN subframes defined by macrocell and small cell UE densities at renewal time t.

Remark 3 (DSS in Small Cells). 

We are considering the use of DSS for small cells to tackle several key challenges mentioned earlier. Notably, in the initial phase of NR deployment, the demand for NR traffic is generally low. Therefore, it is advantageous to serve NR traffic from small cells using the LTE spectrum. Additionally, since a significant portion of data traffic occurs indoors, there may be instances where the indoor traffic demand exceeds the capacity of the NR spectrum. In such cases, MNOs can utilize DSS to manage the excess NR traffic within the LTE spectrum. Furthermore, the limitation of NR spectrum coverage can also be mitigated by operating small cells within the LTE spectrum. DSS can thus play a vital role in meeting the demands of indoor small cell NR traffic. This is especially relevant when the indoor NR traffic demand is too low to warrant the use of the NR spectrum or when there is a high demand that cannot be adequately served by NR alone. These considerations lead us to explore the application of DSS between LTE and NR networks, specifically for in-building SCBSs.

6.2. Proposed DSS Technique

The following features guide the proposed DSS technique between LTE and NR systems: (1) Efficiently supporting NR traffic through LTE spectrum; (2) NR cells operating indoors to serve indoor traffic; (3) LTE spectrum allocation based on the traffic demand of NR relative to LTE; (4) Providing a simple and effective solution.

To propose a DSS technique guided by the features mentioned above, we analyze and identify relevant factors that can address these features, based on the knowledge from Part I regarding the fundamentals of DSS between LTE and NR systems. Recall that, unlike non-MBSFN, LTE MBSFN subframes utilize a resource grid where NR traffic is transmitted in regions with muted CRSs, preventing collisions with LTE PDSCHs. This eliminates interference from LTE CRSs on NR traffic, enabling efficient support of NR via MBSFN subframes (i.e., Feature 1). Additionally, with up to 80% of mobile traffic occurring indoors [10], indoor coverage and capacity are essential. Small cells, such as femtocells, are crucial for fulfilling these indoor needs by operating in DSS mode, which does not require licensing additional spectrum, addressing Feature 2. Furthermore, since DSS depends on the traffic demands of LTE and NR, it is important to allocate LTE spectrum based on NR’s traffic demand relative to LTE, addressing Feature 3. Finally, muting CRSs in MBSFN prevents LTE CRS interference on NR PDSCHs, making MBSFN a simple and effective DSS solution, addressing Feature 4.

The above analysis indicates that the proposed DSS technique relies on LTE MBSFN subframes, indoor small cells, and the traffic demands of both LTE and NR systems. Therefore, we propose an LTE MBSFN subframe-based DSS technique for in-building NR small cells located alongside picocells within the coverage of a large macrocell, sharing the same LTE spectrum as the macrocell and picocells, based on the traffic demands of NR small cells relative to LTE macrocells at any given renewal time t.

More specifically, we consider operating in-building small cells in the same microwave spectrum as that of the macrocell, and present an LTE MBSFN subframe-based DSS for LTE and NR coexistence based on evaluating the optimal number of MBSFN subframes within each LTE frame, ranging from a minimum of 1 to a maximum of 6, while keeping the MBSFN frame periodicity fixed. Although MBSFN frame periodicity can vary from a minimum of 1 to a maximum of 32, a common practice is to use a higher periodicity during low NR traffic demand and a lower periodicity during high NR traffic demand. Figure 16 depicts the structure of LTE frames when implementing DSS between LTE and NR systems, featuring MBSFN subframes [25]. Within each muted MBSFN subframe, symbol indices 0 and 1 are designated for the CRS and PDCCH of LTE, commonly called the non-MBSFN region. The remaining symbols in these muted MBSFN subframes, which are known as the MBSFN region, consist of symbol indices 2 to 13 and can be utilized to transmit NR traffic. The proposed DSS technique is stated in the following:

NR small cells only operate during the designated MBSFN subframes per LTE frame. The number of MBSFN subframes allocated depends on NR UE traffic demand, which can also be considered in terms of UE density, assuming equal traffic per UE. Initially, at low NR UE density, the number of MBSFN subframes is set to a minimum of 1. As the density of NR UEs increases, the number of MBSFN subframes allocated to small cells also increases, reaching a maximum of 6 when NR traffic demand surpasses that of LTE significantly.

7. Optimization Problem Formulation

Few studies, including [9], which addresses the coexistence of different radio access technologies, and [5], which proposes a cross-band DSS scheme for uncoordinated LTE and NR systems, have examined MBSFN subframe-based DSS using the Technology Recognition and Traffic Characterization (TRTC) system. Unlike [5,9], our DSS method involves joint scheduling of LTE and NR by dynamically allocating shared LTE spectrum resources based on traffic load, utilizing MBSFN subframes. Since LTE and NR systems share the same spectrum, we focus on resource management via a coordination channel, a dedicated link for LTE and NR base stations to exchange key information on resource utilization and traffic conditions. However, despite the importance of determining optimal MBSFN subframe counts within a specific period for in-building small cell networks, to the best of the author’s knowledge, current research has made no contributions in this area.

To determine the optimal number of MBSFN subframes per LTE frame to serve NR traffic, one approach is to normalize the NR UE density relative to that of LTE, thereby representing the NR traffic demand on a scale ranging from 0 to 1, corresponding to the number of MBSFN subframes ranging from 1 to 6. As NR UE density gradually increases relative to LTE, the MBSFN subframe count per frame correspondingly increases. Note that since MBSFN subframes are allocated to LTE and NR UEs orthogonally, i.e., no MBSFN subframe is allocated simultaneously to both LTE and NR UEs, Co-Channel Interference (CCI) due to the coexistence of LTE and NR UEs in the same LTE spectrum is avoided.

To formulate the optimization problem, let $\epsilon$ denote a muted MBSFN frame such that $\epsilon \in \mathit{\epsilon }=\left\{1,2,3,\dots ,{\epsilon }_{\max }\right\}$ with a periodicity of $m_{p,t}^{\mathrm{NR}}$ such that $m_{p,t}^{\mathrm{NR}}\in {\mathit{M}}_{\mathit{p},\mathit{t}}^{\mathbf{NR}}=\left\{1,2,4,8,16,32\right\}$$=\left\{1,2,\dots ,M_{p,t}^{\mathrm{NR}}\right\}$. Let $m_{\epsilon ,t}^{\mathrm{NR}}$ denote the number of MBSFN subframes at any MBSFN frame $\epsilon$, which can be allocated to NR at renewal time $t$ such that $m_{\epsilon ,t}^{\mathrm{NR}}\in {\mathit{M}}_{\mathit{\epsilon },\mathit{t}}^{\mathbf{NR}}=\left\{1,2,\dots ,6\right\}=\left\{1,2,\dots ,M_{\epsilon ,t}^{\mathrm{NR}}\right\}$. Let $M_t^{\mathrm{NR}}$ denote the optimal value of the total number of MBSFN subframes to be allocated to NR UEs at renewal time $t$. The renewal time $t$ is defined by an MNO based on factors such as NR channel characteristics and traffic demands, and can be chosen as an integer multiple of the maximum MBSFN periodicity $M_{p,t}^{\mathrm{NR}}$ as follows.

$$t=k\times M_{p,t}^{\mathrm{NR}}$$

(1)

where $k$ is any positive integer excluding zero. For a good channel condition, the renewal time $t$ can be chosen long.

Let $M_{\epsilon ,t}^{\mathrm{TOT}}$ denote the total number of muted MBSFN at any $\epsilon$ and $t$ such that $M_{\epsilon ,t}^{\mathrm{TOT}}=M_{\epsilon ,t}^{\mathrm{NR}}+M_{\epsilon ,t}^{\mathrm{LTE}}=6$. Let $U_{\epsilon ,t}^{\mathrm{NR}}$ and $U_{\epsilon ,t}^{\mathrm{LTE}}$, respectively, denote the number of NR UEs and LTE UEs at any MBSFN frame $\epsilon$ and renewal time $t$ such that $u_{\epsilon ,t}^{\mathrm{NR}}\in {\mathit{U}}_{\mathit{\epsilon },\mathit{t}}^{\mathbf{NR}}=\left\{1,2,3,\dots ,U_{\epsilon ,t}^{\mathrm{NR}}\right\}$ and $u_{\epsilon ,t}^{\mathrm{LTE}}\in {\mathit{U}}_{\mathit{\epsilon },\mathit{t}}^{\mathbf{LTE}}=\left\{1,2,3,\dots ,U_{\epsilon ,t}^{\mathrm{LTE}}\right\}$. Let $e_{\epsilon ,t}^{\mathrm{NR}}$ and $e_{\epsilon ,t}^{\mathrm{LTE}}$, respectively, denote the average traffic per NR UE and per LTE UE at any $\epsilon$ and $t$. Then, NR, LTE, and total traffic demands at any $\epsilon$ and $t$ are given, respectively, by

$$A_{\epsilon ,t}^{\mathrm{NR}}=U_{\epsilon ,t}^{\mathrm{NR}}\times e_{\epsilon ,t}^{\mathrm{NR}}$$

(2)

$$A_{\epsilon ,t}^{\mathrm{LTE}}=U_{\epsilon ,t}^{\mathrm{LTE}}\times e_{\epsilon ,t}^{\mathrm{LTE}}$$

(3)

$$A_{\epsilon ,t}^{\mathrm{TOT}}=A_{\epsilon ,t}^{\mathrm{NR}}+A_{\epsilon ,t}^{\mathrm{LTE}}$$

(4)

Since we assume a fixed MBSFN frame periodicity $M_{p,t}^{\mathrm{NR}}$ at renewal time $t$, an optimal value of the total number of MBSFN subframes $M_t^{\mathrm{NR}}$ to be allocated to NR UEs at renewal time $t$ is solely defined by the optimal number of MBSFN subframes. Moreover, allocating more MBSFN subframes to NR UEs leads to increasing the capacity of NR UEs while decreasing that of LTE UEs, we consider a minimization problem to define an optimal number of MBSFN subframes at renewal time $t$ to ensure that LTE UEs are not affected by the DSS mode to share the LTE spectrum with NR UEs as follows.

$$\begin{array}{ll}\hfill \underset{m_{\epsilon ,t}^{\mathrm{NR}}\in {\mathit{M}}_{\mathit{\epsilon },\mathit{t}}^{\mathbf{NR}}}{{\displaystyle \mathbf{min}}}& M_{\epsilon ,t}^{\mathrm{NR}}\hfill \\ \hfill \mathrm{subject} \mathrm{to}& (\mathrm{a}) M_{\epsilon ,t}^{\mathrm{NR}}\le M_{\epsilon ,t}^{\mathrm{TOT}}\hfill \\ \hfill & (\mathrm{b}) M_{\epsilon ,t}^{\mathrm{LTE}}\le M_{\epsilon ,t}^{\mathrm{TOT}}\hfill \\ \hfill & (\mathrm{c}) M_{\epsilon ,t}^{\mathrm{NR}}+M_{\epsilon ,t}^{\mathrm{LTE}}=M_{\epsilon ,t}^{\mathrm{NR}}\hfill \\ \hfill & (\mathrm{d}) t=k\times M_{p,t}^{\mathrm{NR}}\hfill \end{array}$$

(5)

The solution to the above problem, when considering the same average traffic per LTE and NR UEs, is given by

$$M_{\epsilon ,t}^{{\mathrm{NR}}^{\ast }}=⌊\left({\displaystyle \frac{U_{\epsilon ,t}^{\mathrm{NR}}}{U_{\epsilon ,t}^{\mathrm{NR}}+U_{\epsilon ,t}^{\mathrm{LTE}}}}\right)\times \left(M_{\epsilon ,t}^{\mathrm{NR}}+M_{\epsilon ,t}^{\mathrm{LTE}}\right)⌋$$

(6)

Proof. 

see Appendix A. □

Therefore, the optimal number of muted MBSFN subframes for LTE UEs at any $\epsilon$ and $t$ is given by

$$M_{\epsilon ,t}^{{\mathrm{LTE}}^{\ast }}=\left(M_{\epsilon ,t}^{\mathrm{TOT}}-M_{\epsilon ,t}^{{\mathrm{NR}}^{\ast }}\right)$$

(7)

Hence, an optimal number of total MBSFN subframes to be allocated to NR UEs in renewal time t for a given MBSFN frame periodicity $M_{p,t}^{\mathrm{NR}}$ is given by

$$M_t^{{\mathrm{NR}}^{\ast }}=t\times \left({\displaystyle \frac{M_{\epsilon ,t}^{{\mathrm{NR}}^{\ast }}}{M_{p,t}^{\mathrm{NR}}}}\right)$$

(8)

The above solution signifies that the optimal number of muted MBSFN subframes to be allocated to serve NR UEs in renewal time t is affected proportionally by the number of MBSFN subframes in any LTE frame $\epsilon$ (i.e., $M_{\epsilon ,t}^{{\mathrm{NR}}^{\ast }}$), whereas inversely by the MBSFN periodicity (i.e., $M_{p,t}^{\mathrm{NR}}$) in renewal time t.

8. Performance Metrics Estimation

Let L denote the number of buildings in a macrocell coverage of the MNOs. Let S_F denote the maximum number of small cells per 3D building, where S_F is assumed to be the same for all buildings. Let S_M denote the number of macrocells, and S_P denote the number of picocells per macrocell. Also, let T denote simulation run time with the maximum time of Q (in time steps each lasting 1 ms) such that T = {1, 2, 3,…, Q}. Let $P_{\mathrm{MC}}$, $P_{\mathrm{PC}}$, and $P_{\mathrm{SC}}^{\mathrm{LTE}}$ denote, respectively, the transmission power of a macrocell, a picocell, and a small cell, respectively, all operating in the LTE spectrum of the MNO.

Using Shannon’s capacity formula, the link throughput at RB = i in the Transmission Time Interval (TTI) = $\mathrm{\tau }$ at any renewal time t in bps per Hz is given by [26],

$${\mathrm{\sigma }}_{\mathrm{\tau },i}^t\left({\mathrm{\rho }}_{\mathrm{\tau },i}^t\right)=\left\{\begin{array}{ll}0,\hfill & {\mathrm{\rho }}_{\mathrm{\tau },i}^t<-10 \mathrm{dB}\hfill \\ \beta {\log }_2\left(1+{10}^{\left({\mathrm{\rho }}_{\mathrm{\tau },i}^t\left(\mathrm{dB}\right)/10\right)}\right),\hfill & -10 \mathrm{dB}\le {\mathrm{\rho }}_{\mathrm{\tau },i}^t\le 22 \mathrm{dB}\hfill \\ 4.4,\hfill & {\mathrm{\rho }}_{\mathrm{\tau },i}^t>22 \mathrm{dB}\hfill \end{array}\right\}$$

(9)

where β denotes the implementation loss factor.

Let $N_{\mathrm{LTE}}$ denote the LTE spectrum in RBs. Then, the capacity of all macrocell UEs at renewal time t is given by

$${\mathrm{\sigma }}_{\mathrm{MCBS}}^t\left(\mathrm{\tau },i\right)={\displaystyle {\sum }_{\mathrm{\tau }=1}^Q{\displaystyle {\sum }_{i=1}^{N_{\mathrm{LTE}}}{\mathrm{\sigma }}_{\mathrm{\tau },i}^t\left({\mathrm{\rho }}_{\mathrm{\tau },i}^t\right)}}$$

(10)

where $\mathrm{\sigma }$ and $\rho$ are responses over $N_{\mathrm{LTE}}$ LTE RBs of all macro UEs in $\mathrm{\tau }\in \mathit{T}$ at renewal time t. Since an SCBS operates only in $M_t^{{\mathrm{NR}}^{\ast }}$ MBSFN frames by RBs of LTE spectrum $N_{\mathrm{LTE}}$, then, the aggregate capacity served by an SCBS in a building at renewal time t is given by

$${\mathrm{\sigma }}_s^t\left(\mathrm{\tau },i\right)={\displaystyle \sum _{\mathrm{\tau }=1}^{M_t^{{\mathrm{NR}}^{\ast }}}{\displaystyle {\sum }_{i=1}^{N_{\mathrm{LTE}}}{\mathrm{\sigma }}_{\mathrm{\tau },i}^t\left({\mathrm{\rho }}_{\mathrm{\tau },i}^t\right)}}$$

(11)

Likewise, the aggregate capacity served by all SCBSs per building at renewal time t is given by,

$${\mathrm{\sigma }}_{S_{\mathrm{F}}}^t\left(\mathrm{\tau },i\right)={\displaystyle {\sum }_{s=1}^{S_{\mathrm{F}}}{\mathrm{\sigma }}_s^t}\left(\mathrm{\tau },i\right)$$

(12)

We assume that the CCI between small cells of adjacent buildings is negligible, such that the same LTE spectrum (using DSS) can be reused in small cells within each building located over the macrocell coverage. The average capacity, SE, and EE for the total network employing DSS to small cells are given, respectively, by

$${\mathrm{\sigma }}_{\mathrm{cap},L}^{\mathrm{TOT},\mathrm{DSS},t}\left(\mathrm{\tau },i\right)={\mathrm{\sigma }}_{\mathrm{MBS}}^t\left(\mathrm{\tau },i\right)+\left(L\times {\mathrm{\sigma }}_{S_{\mathrm{F}}}^t\left(\mathrm{\tau },i\right)\right)$$

(13)

$${\mathrm{\sigma }}_{\mathrm{SE},L}^{\mathrm{TOT},\mathrm{DSS},t}\left(\mathrm{\tau },i\right)={\mathrm{\sigma }}_{\mathrm{cap},L}^{\mathrm{TOT},\mathrm{DSS},t}\left(\mathrm{\tau },i\right)/\left(N_{\mathrm{LTE}}\times \left|\mathit{T}\right|\right)$$

(14)

$${\mathrm{\sigma }}_{\mathrm{EE},L}^{\mathrm{TOT},\mathrm{DSS},t}\left(\mathrm{\tau },i\right)={\displaystyle \frac{\left(\left(L\times S_{\mathrm{F}}\times \left(\left(\left(M_t^{{\mathrm{NR}}^{\ast }}/\left|\mathit{T}\right|\right)\times P_{\mathrm{SC}}^{\mathrm{LTE}}\right)\right)\right)+\left(S_{\mathrm{P}}\times P_{\mathrm{PC}}\times \left|\mathit{T}\right|\right)+\left(S_{\mathrm{M}}\times P_{\mathrm{MC}}\times \left|\mathit{T}\right|\right)\right)}{{\mathrm{\sigma }}_{\mathrm{cap},L}^{\mathrm{TOT},\mathrm{DSS},t}\left(\mathrm{\tau },i\right)}}$$

(15)

9. Simulation Parameters, Assumptions, and Results

9.1. Simulation Parameters and Assumptions

The default assumptions, parameters, and models adhere to the guidelines established by standardization organizations such as 3GPP and the International Telecommunication Union—Radiocommunication Sector (ITU-R), as listed in Table 3, to facilitate numerous evaluations of the proposed DSS technique. Several multi-story buildings are located within the macrocell coverage area, and a set of femtocells, also known as small cells, are considered to be situated inside the apartments of each multi-story building, each equipped with LTE spectrum using DSS. The following section highlights notable assumptions, accompanied by relevant justification.

• The average traffic per UE for both LTE and NR systems is considered to be the same, remaining constant at any renewal time t.
• We assume each small cell can serve at most one UE at a time for simplicity in analysis and to obtain a closed-form solution.
• The 2 GHz microwave spectrum is designated for outdoor macrocells and picocells within the LTE spectrum to ensure broad coverage with fewer handoffs.
• The DSS of LTE spectrum is considered to serve NR UEs in in-building small cells.
• In indoor environments, we use the directional Line-of-Sight (LOS) 1m Close-In (CI) free space reference distance model as it is effective across a wide frequency range [27].
• The renewal or update period t is not fixed; it changes over time based on traffic demand and channel conditions. If necessary, t can be set to less than 32 LTE frames when the MBSFN frame periodicity needs adjustment.
• The Proportional Fair (PF) resource scheduler is used to achieve a balanced trade-off between throughput and fairness in radio resource allocations.
• The full buffer model is employed for simplicity, assuming resource schedulers can serve user traffic at any moment within the renewal time t.
• Performance results are generated by simulating all assumptions, parameters, and models listed in Table 3 using a MATLAB R2016b-based simulator on a personal computer.

Table 3. Default simulation parameters and assumptions.

Parameters and Assumptions			Value
E-UTRA simulation case 1			3GPP case 3
Cellular layout 2			Hexagonal grid, dense urban, and 3 sectors per macrocell
Transmit direction			downlink
Carrier frequency 2,3,10; System bandwidth			2 GHz Non-Line-of-Sight (NLOS); 10 MHz
MBSFN renewal time, t			32 frames or 320 ms
Number of cells			1 macrocell, 2 picocells, and 54 small cells per building
Transmission power (dBm); External wall penetration loss 1			46 for microcell 1,10, 37 for picocells 1,10, and 20 for small cells; 20 dB
Small-scale fading 1,6			Frequency-selective Rayleigh
Path loss	Macrocell and a UE 1	Outdoor UE	$PL\left(\mathrm{dB}\right)=15.3+37.6{\log }_{10}d$, d(m)
		Indoor UE	$PL\left(\mathrm{dB}\right)=15.3+37.6{\log }_{10}d+L_{\mathrm{ow}}$, d(m)
	Picocell and a UE 1		$PL\left(\mathrm{dB}\right)=140.7+36.7{\log }_{10}d$, d(km)
	Small cell and a UE 7,8	$P{L}^{\mathrm{CI}}=\mathrm{FSPL}\left(f_C,1 \mathrm{m}\right)+10n{\log }_{10}\left(d_{3\mathrm{D}}/1 \mathrm{m}\right)+X_{\sigma }$$\mathrm{and} \mathrm{FSPL}\left(f_C, 1 \mathrm{m}\right)=32.4+20{\log }_{10}\left(f_C/1 \mathrm{GHz}\right)$ where $d_{3\mathrm{D}}$$\mathrm{is} \mathrm{in} \mathrm{m}, f_C$$\mathrm{is} \mathrm{in} \mathrm{GHz}, \mathrm{and} X_{\sigma }$ is in dB
Lognormal shadowing standard deviation (dB)			8 for microcell 2, 10 for picocell 1, and 10 for small cell 5,9
Antenna configuration			SISO for all base stations and UEs
Antenna pattern (horizontal)			Directional (120 degrees) for microcell 1; omnidirectional for picocell 1 and small cell 1
Base station antenna gain			14 for macrocell (dBi) 2, 5 for picocell (dBi) 1, and 5 for small cell (dBi)
UE noise figure 2,4,6, speed 1, and antenna gain 2,3,4,6			9 dB, 3 km/h, 0 dBi
Receiver thermal noise 4			−121.44 dBm per RB
Macrocell UEs (total), small cell UEs (per building)			30, 54
Picocell coverage; offloaded macrocell UEs 1; in-building macrocell UEs 1			40 m (radius); 2/15; 35%
Scheduler; traffic model 2			Proportional fair; full buffer
Type of SCBSs			Closed Subscriber Group (CSG) femtocells
TTI 1; scheduler time constant			1 ms; 100 ms
Total simulation run time			320 ms

Taken ¹ from [28], ² from [29], ³ from [30], ⁴ from [31], ⁵ from [32], ⁶ from [33], ⁷ from [34], ⁸ from [35], ⁹ from [27], ¹⁰ from [36].

Table 3. Default simulation parameters and assumptions.

Parameters and Assumptions			Value
E-UTRA simulation case 1			3GPP case 3
Cellular layout 2			Hexagonal grid, dense urban, and 3 sectors per macrocell
Transmit direction			downlink
Carrier frequency 2,3,10; System bandwidth			2 GHz Non-Line-of-Sight (NLOS); 10 MHz
MBSFN renewal time, t			32 frames or 320 ms
Number of cells			1 macrocell, 2 picocells, and 54 small cells per building
Transmission power (dBm); External wall penetration loss 1			46 for microcell 1,10, 37 for picocells 1,10, and 20 for small cells; 20 dB
Small-scale fading 1,6			Frequency-selective Rayleigh
Path loss	Macrocell and a UE 1	Outdoor UE	$PL\left(\mathrm{dB}\right)=15.3+37.6{\log }_{10}d$, d(m)
		Indoor UE	$PL\left(\mathrm{dB}\right)=15.3+37.6{\log }_{10}d+L_{\mathrm{ow}}$, d(m)
	Picocell and a UE 1		$PL\left(\mathrm{dB}\right)=140.7+36.7{\log }_{10}d$, d(km)
	Small cell and a UE 7,8	$P{L}^{\mathrm{CI}}=\mathrm{FSPL}\left(f_C,1 \mathrm{m}\right)+10n{\log }_{10}\left(d_{3\mathrm{D}}/1 \mathrm{m}\right)+X_{\sigma }$$\mathrm{and} \mathrm{FSPL}\left(f_C, 1 \mathrm{m}\right)=32.4+20{\log }_{10}\left(f_C/1 \mathrm{GHz}\right)$ where $d_{3\mathrm{D}}$$\mathrm{is} \mathrm{in} \mathrm{m}, f_C$$\mathrm{is} \mathrm{in} \mathrm{GHz}, \mathrm{and} X_{\sigma }$ is in dB
Lognormal shadowing standard deviation (dB)			8 for microcell 2, 10 for picocell 1, and 10 for small cell 5,9
Antenna configuration			SISO for all base stations and UEs
Antenna pattern (horizontal)			Directional (120 degrees) for microcell 1; omnidirectional for picocell 1 and small cell 1
Base station antenna gain			14 for macrocell (dBi) 2, 5 for picocell (dBi) 1, and 5 for small cell (dBi)
UE noise figure 2,4,6, speed 1, and antenna gain 2,3,4,6			9 dB, 3 km/h, 0 dBi
Receiver thermal noise 4			−121.44 dBm per RB
Macrocell UEs (total), small cell UEs (per building)			30, 54
Picocell coverage; offloaded macrocell UEs 1; in-building macrocell UEs 1			40 m (radius); 2/15; 35%
Scheduler; traffic model 2			Proportional fair; full buffer
Type of SCBSs			Closed Subscriber Group (CSG) femtocells
TTI 1; scheduler time constant			1 ms; 100 ms
Total simulation run time			320 ms

Taken ¹ from [28], ² from [29], ³ from [30], ⁴ from [31], ⁵ from [32], ⁶ from [33], ⁷ from [34], ⁸ from [35], ⁹ from [27], ¹⁰ from [36].

9.2. Simulation Results

The proposed DSS technique leverages MBSFN subframes that are free from CRS interference because they do not include CRS in their MBSFN region (from symbol 2 to symbol 13 within an LTE frame). As a result, it is not appropriate to compare this method with other established CRS interference mitigation techniques, such as rate matching at the RE level. A more relevant comparison would be with techniques that also utilize MBSFN subframes for DSS between LTE and NR systems. However, the authors are unaware of any similar work. Therefore, we assess the performance of the proposed DSS by comparing its SE and EE with those of MBSFN subframes used without DSS.

(1) Optimal allocation of MBSFN subframes for LTE and NR: Figure 17 shows the impact of varying NR UE density on optimal allocation of MBSFN subframes for LTE and NR in renewal time t = 32. We vary the NR UE density, approximately 98–99% of the LTE UEs, limited to 30 (i.e., from 0 to 3000). Since the maximum number of MBSFN subframes per LTE frame is 6, these subframes are allocated between LTE and NR based on their respective UE densities, as given by $M_{\epsilon ,t}^{{\mathrm{LTE}}^{\ast }}$ and $M_{\epsilon ,t}^{{\mathrm{NR}}^{\ast }}$, respectively. As the number of NR UEs increases, the number of MBSFN allocations designated for NR also rises, which leads to a decrease in the MBSFN allocations available for LTE UEs, as shown in Figure 17.

(2) Spectral and Energy Efficiencies: When employing DSS for a specific value of MBSFN periodicity, an increase in the number of MBSFNs allocated to NR UEs per frame results in improvements in SE and EE. The enhancement is particularly significant when the MBSFN periodicity is at its minimum value of 1, as this allows for the allocation of MBSFN subframes to NR UEs in each LTE frame. Figure 18 illustrates the improvements in SE and EE when employing DSS to in-building small cells to serve NR small cells UEs for both maximum (i.e., 32) and minimum (i.e., 1) MBSFN frame periods.

Generally, as the number of MBSFNs increases, both SE and EE performance metrics improve. Notably, when the MBSFN period is at its maximum value of 32, there is only a slight improvement in SE and EE, ranging from nearly 0% to a maximum of around 0.3% for SE, and up to 2% for EE. In contrast, at the minimum MBSFN period of 1, both SE and EE show significant improvements even with the lowest number of MBSFN subframes. Specifically, with just one MBSFN, there is an increase of nearly 2% for SE, and about 10% for EE. With a maximum of 6 MBSFNs, these improvements reach approximately 10% for SE and 58% for EE. This suggests that EE is generally more affected by variations in the number of MBSFNs per frame than by SE. Further, the impact of DSS is influenced not only by the number of MBSFNs per frame but also by the MBSFN periodicity, which we confirm with the following case study.

(3) Case Study: Analyzing the Number and Periodicity of MBSFN: Let’s consider a renewal time of 32. If we fix the number of MBSFN subframes per frame to a maximum of 6 and vary the value in the MBSFN period from a minimum of 1 to a maximum of 32, the maximum of 6, the total number of MBSFN subframes allocated to NR in a renewal time of 32 changes from 6 (i.e., $6\times 1$) to 192 (i.e., $6\times 32$). The impact, therefore, is 3100% (i.e., $\left(192-6\right)/6$). Now, if we fix the MBSFN period to a minimum value of 1 such that maximum number of LTE frames of 32 can be allocated and vary the number of MBSFNs per frame from the minimum value of 1 to the maximum of 6, the total number of MBSFN subframes allocated to NR in a renewal time of 32 changes from 32 (i.e., $1\times 32$) to 192 (i.e., $6\times 32$). Consequently, the impact in this scenario is 500% (i.e., $\left(192-32\right)/32$). This analysis indicates that the MBSFN period has a 5.2 (i.e., $\left(3100-500\right)/500$) times greater impact on DSS performance compared to the number of MBSFNs per frame. Therefore, determining an optimal value for MBSFN periodicity is essential, and we consider this for future study.

10. Lessons Learned

The following are the key lessons learned throughout the paper for effective DSS between LTE and NR systems.

(1)

DSS Essential: Sharing LTE’s spectrum with NR enables coexistence, allowing NR data and control signals to transmit without disrupting LTE operations.

(2)

LTE and NR Resource Grid Structure: LTE uses a single 15 kHz SCS OFDM numerology, while NR has multiple with 15, 30, 60, 120, and 240 kHz. To keep DSS simple, NR also considers 15 kHz. For normal cyclic prefix, each slot has seven symbols, totaling 14 symbols per subframe for both LTE and NR at 15 kHz.

(3)

LTE Non-MBSFN: DSS for LTE-NR can use both non-MBSFN and MBSFN LTE subframes. With LTE non-MBSFN subframes, NR PDSCHs can be transmitted via CRS rate matching using a 15 kHz SCS at RE or RB level, but NR SSBs can’t be broadcast on LTE spectrum with a 15 kHz SCS due to interference with LTE CRSs. To resolve this, LTE-NR DSS employs a mixed numerology approach: 15 kHz SCS for PDSCHs and 30 kHz SCS for SSBs. This restriction doesn’t apply when LTE MBSFN subframes transmit NR SSBs.

(4)

LTE CRS and PDCCH: LTE CRSs and PDCCHs regulate the resource grid for LTE non-MBSFN subframes. Their number and placement depend on antenna ports. In non-MBSFN subframes, LTE CRSs are at symbols 0, 4, 7, and 11 for 1 and 2 ports, while 4 ports have them at symbols 0, 1, 4, 7, 8, and 11. PDCCHs appear at symbols 0 and 1 in both subframe types. No CRSs are in the LTE MBSFN region, which spans 12 symbols in MBSFN subframes. CRSs and PDCCHs occupy symbols 0 and 1 in the non-MBSFN region of MBSFN subframes.

(5)

NR SSB and PDCCH: Under normal conditions (without DSS involving NR and LTE), NR PDCCHs can occupy the first three symbols (0, 1, and 2) or all three combined. NR DMRSs are placed at symbols 3 and 11. The remaining 9 symbols of the NR subframe are for NR PDSCH or data. An SSB consists of 20 adjacent RBs, starting at any RB index, extending over four symbols. SSBs repeat at intervals of 5 ms, 10 ms, 20 ms, 40 ms, 80 ms, and 160 ms. A UE needs at least one SSB from L SSBs for synchronization, requiring four contiguous symbols for placement. An NR UE can connect to the NR network only if it can access the SSB. NR and LTE systems cannot perform DSS if NR SSBs can’t integrate into the LTE spectrum.

(6)

LTE MBSFN: Not every subframe within an LTE radio frame can be set up as an MBSFN. Specifically, subframes 0, 4, 5, and 9 contain system-specific data and cannot be configured as MBSFNs. Subframes 0 and 5 include PSS, SSS, and BCH in LTE FDD, while subframes 4 and 9 hold LTE paging info. Of the 10 subframes in an LTE frame, a maximum of six—subframes 1, 2, 3, 6, 7, and 8—can be MBSFN. An operator can configure 1 to 6 LTE subframes as MBSFNs based on NR traffic. MBSFN subframes are transparent to LTE UEs, which skip these when detected.

(7)

NR DMRS: When DSS is used between LTE and NR, one NR DMRS at symbol 3 is enough. If more LTE REs per RB are used for NR, a second DMRS is needed at symbol 11 of NR. NR DMRSs at symbols 3 and 11 per RB per subframe are fixed in NR for 15 kHz SCS. LTE CRSs are at symbol 11 in any LTE non-MBSFN subframe. To avoid interference, the additional NR DMRS cannot be placed at symbol 11 of LTE non-MBSFN subframes during DSS. Although NR doesn’t specify this, it’s possible to shift the extra NR DMRS from symbol 11 to 12 in LTE when transmitting NR over LTE spectrum.

(8)

NR PDSCH: PDSCH scheduling in NR includes Type A and Type B. Type A mapping is slot-dependent, starting at symbols 0–3, with 3–14 symbols per subframe, and requires DMRS on symbols 2 or 3. To address Type A limitations, Type B or mini-slot mapping is used, allowing transmission to start at any symbol. Type B is limited to 2, 4, or 7 symbols to reduce delay, with DMRS on the first symbol, even if the start isn’t at the beginning of a slot. For better LTE and NR DSS, Type B with shifted DMRS (release 16) is recommended.

(9)

NR Operational Mode: Initially, NR operates in Non-Standalone (NSA) mode, using LTE as an anchor while SSBs and Random Access Responses (RARs) are sent. In NR Standalone (SA) mode, system info like System Information Block 1 (SIB1), Other System Information (OSI), and paging channels are also transmitted with SSBs and RARs. During initial access, it’s hard to tell if the cell is an SA or a combined LTE-NR cell. To detect the cell accurately, NR SSBs must be transmitted without colliding with LTE. Since an SSB covers 20 RBs over four symbols in a subframe, it’s best to use LTE MBSFN subframes rather than non-MBSFN ones to avoid interference with LTE CRSs.

(10)

NR Reference Signal: NR reference signals include Channel State Information Reference Signals (CSI-RS) and Tracking Reference Signals (TRS). After decoding, CSI-RS is sent back to the base station via PUSCH and Physical Uplink Control Channel (PUCCH), often in the last symbol of an LTE MBSFN subframe. TRSs are transmitted regularly in consecutive slots, allowing UEs to track and adjust for time and frequency variations.

(11)

Number and Periodicity of MBSFN Subframes: As NR UEs increase, MBSFN subframe allocations for NR also rise, reducing LTE UEs’ allocations. Using DSS for a specific MBSFN periodicity, more MBSFNs for NR per frame improve SE and EE, especially at the minimum periodicity of 1, allowing MBSFN subframes for NR in each LTE frame. EE is more affected by the number of MBSFNs per frame than SE at a fixed periodicity. While both the number and period of MBSFN subframes influence DSS performance, the period has a greater effect. Properly selecting both is essential to meet LTE and NR traffic demands.

11. Conclusions

In this paper, we addressed the fundamental question of how LTE and NR coexist using DSS in Part I. Specifically, we highlighted and discussed compatibility issues for DSS, including resource grids, subcarrier spacings, and control signals and channels for both LTE and NR systems. We then reviewed various DSS techniques and their limitations. In addition to NR PDSCH data transmissions, we detailed NR control signals and channels, especially NR SSB transmissions in the LTE spectrum using both MBSFN and non-MBSFN subframes. We also covered standardization efforts for DSS improvement in 3GPP releases 16, 17, and 18.

In Part II, we introduced a DSS technique utilizing LTE MBSFN subframes to enable the coexistence of LTE and NR in a heterogeneous network with a macrocell, picocells, and in-building small cells. The macrocell and picocells use LTE, while NR is employed in the small cells, sharing the same spectrum through the DSS approach. We formulated an optimization problem to balance traffic needs by determining the optimal number of MBSFN subframes per LTE frame based on periodicity. Metrics for average capacity, SE, and EE were derived to evaluate the DSS method.

System-level simulations showed how NR UE density affects MBSFN subframe distribution and examined the impact of varying MBSFN subframes on SE and EE in NR small cells. Our results demonstrate that using DSS for in-building small cells serving NR small cell UEs significantly enhances SE and EE based on MBSFN frame periods. With a minimum of one MBSFN subframe per LTE frame and a maximum period of 32, SE improved only slightly by up to 0.3%, while EE increased by up to 2%. In contrast, with a minimum period of 1, SE improved by nearly 2%, and EE by about 10%. However, at a maximum of six MBSFNs per frame, improvements reached approximately 10% for SE and 58% for EE, indicating that EE is more sensitive to changes in the number of MBSFNs than SE.

Finally, we listed key lessons learned throughout this paper. As part of the study’s extension, in Part II, we assume a fixed MBSFN frame period. However, it is essential to find an optimal periodicity that dynamically adjusts to changes in LTE and NR UE densities during renewal periods. We plan to address this in future research.