Packet Duplication for URLLC in 5G Dual Connectivity Architecture Jaya Rao and Sophie Vrzic Huawei Technologies Canada Ottawa, Ontario, Canada K2K 3J1 E-mail:{jaya.rao,sophie.vrzic}@huawei.com Abstract – This paper addresses the problem of satisfying the extreme requirements related to Ultra-Reliable Low Latency Communications (URLLC) in 5G Radio Access Network (RAN). Complementary to the existing Physical (PHY) layer techniques, this paper focuses primarily on higher layer solutions, particularly, on Packet Duplication (PD) as a practical and low complexity technique for URLLC. The theoretic framework behind PD is investigated and the recent enhancements made in the 5G Dual Connectivity (DC) architecture for supporting PD are discussed. For improving the radio resource utilization and to dynamically control the activation of PD, an optimization problem subject to URLLC constraints is formulated and solved heuristically to give the resource configuration in terms of MCS and PRB allocation over multiple links. Following this, it is shown numerically that performing PD in various deployment scenarios results in better utilization of radio resources compared to using a single highly reliable link while effectively satisfying the URLLC requirements. Keywords- URLLC, Dual Connectivity Architecture, 5G NR I. INTRODUCTION Ultra-reliable low-latency communications (URLLC) are characterized by extreme requirements targeted for supporting use cases requiring high criticality, resilience and robustness. For URLLC, the reliability requirement is intertwined with latency and both performance metrics have to be jointly considered in system design. This is because transmitting packets with high reliability is consequential only if the packets are received within the sub- milisecond latency constraint. The corresponding use cases evaluated by 3GPP for URLLC include industry automation, mobile eHealth, interactive augmented reality, drone communications and connected vehicles [1]. The requirements for these use cases as defined by 3GPP are a maximum round trip time of 1ms on the user-plane (UP) and transmission reliability of 1-10-5 for a packet size of 32 bytes [2]. For more advanced URLLC use cases the latency and reliability requirements can range between 0.5ms to 10ms and 1-10-5 to 1-10-9, respectively for packet sizes of up to 300 bytes. The existing PHY layer techniques in LTE, designed primarily for providing high spectral efficiency, cannot be straightforwardly extended to support the typical URLLC requirements in 5G New Radio (NR). As an illustration, to achieve a block error rate (BLER) target of 1-10-5 on a single link, it is necessary to have highly favorable channel conditions at all times, use a low and robust modulation and coding scheme (MCS) index (e.g. QPSK with 1/3 coding rate) and a bandwidth allocation of at least 20MHz [3]. For higher reliability requirements, it is necessary to use either high diversity orders (up to 16) or significantly increase the signal-to- noise (SNR) and bandwidth allocation [4]. The implementation of other PHY layer solutions based on link adaptation [5] and link combining [6] reveal certain improvement in terms of reliability but not without trading off major increase in functional complexity. Also, such schemes can be applied only in certain highly restrictive scenarios (e.g. cell center, low load, very low mobility), thus limiting their usage to support only a moderate number URLLC- capable devices in most deployment scenarios. In legacy LTE RAN, the reliability requirements are conventionally satisfied at the Radio Link Control (RLC) and Medium Access Control (MAC) layers using the Automatic Repeat Request (ARQ) and Hybrid ARQ (HARQ) retransmission techniques. Although these techniques enable certain reliability level to be achieved, the resulting latency due to packet retransmissions exceed the sub-milisecond latency requirements for most of URLLC use cases. Moreover, incorporating enhancements to these techniques come at the expense of architectural modification, increase in resource usage and in standardization effort. As such, it is necessary to consider a fundamentally new technique that is not only effective for URLLC but can also be practically implemented with low complexity. Considering the different architectural options available in 5G, a promising solution to address the extreme requirements is packet duplication (PD). Particularly, the PD technique can be directly applied in the DC architecture [7] without excessively increasing the complexity in the RAN. The fundamental principle underlying PD involves generating multiple instances of a packet at higher layers and transmitting the packets simultaneously over different uncorrelated channels or transmission links [8]. At the receiver, the redundancy and diversity in the channel conditions is exploited such that higher transmission reliability is achieved. The PD technique has been recently adopted by 3GPP for satisfying the reliability and latency requirements in 5G. The corresponding standardization effort to incorporate PD in the NR RAN protocol stack is currently underway [9]. In light of the recent progress, this paper provides an overview of the URLLC related standardization activities in NR RAN. Particularly, more focus is given towards the solutions and the enhancements made at the higher layers (i.e. above PHY) and in the RAN architecture for supporting the PD technique. In this regard, the system model and the DC architecture enhancements for PD are discussed in Sections II and III. This is followed by techniques and system design considering fast triggering of PD via dynamic control in Section IV. Finally, the PD related performance evaluations as well as the conclusions are provided in Sections V and VI, respectively. II. SYSTEM MODEL FOR PACKET DUPLICATON At the fundamental level, the reliability of a wireless system can be increased by transmitting the same packet over multiple redundant links, each experiencing a different channel condition [10]. To 2018 IEEE Wireless Communications and Networking Conference (WCNC) 978-1-5386-1734-2/18/$31.00 2018 IEEE realize this in practical network implementations, consider a RAN architecture consisting of transmission links, each utilizing a non-overlapping frequency carrier for transmitting simultaneously to the UE (User Equipment). In this case, the overall system reliability R can be determined as: = 1 (1 ) (1) where is the success probability over link ∈ . Assuming is the overall latency, is the SNR achievable on link and Φ is the bandwidth allocation over link , then is defined as: = ( ≤ ) > | (Φ > ( )| ) (2) where i) is the latency requirement for URLLC, ii) = ( ) is the SNR threshold for achieving a BLER target on link when using MCS index ∈ (from the available MCS set M) and iii) is the transport block size of the URLLC packet when using MCS index . Note that includes the latency due to processing, propagation and transmission between the transmitting and receiving Packet Data Convergence Protocol (PDCP) entities in the RAN (i.e. UE and access node). From Eq. (1) and (2), clearly to increase the overall reliability it is necessary to increase either the reliability of each link or the number of links carrying the same packet. The straightforward approach to improve the robustness of each link against channel effects, and consequently , is to increase the transmit power and allocate more radio resources over each link. Both of these techniques, however, may adversely affect the spectral efficiency, power efficiency and interference, hence are not applicable for NR system design. On the other hand, to satisfy the latency requirements in URLLC, the packets transmitted in each link have to be received within the latency deadline of less than 1ms. To this end, the use of PD where the packets are proactively transmitted simultaneously, addresses both the latency and reliability requirements without having to rely on feedback and retransmissions as done in ARQ and HARQ in LTE. At first glance, duplication may imply potential loss in throughput and spectral efficiency. However, at closer inspection it becomes clear that exploiting the diversity from using multiple links provides the means to achieve high reliability on a statistical basis without actually expending more resources. More precisely, targeting a lower BLER value on a particular link and, correspondingly, using a higher MCS index on that link makes it possible to minimize the radio resources usage on that link. Consequently, when multiple links are configured to support PD, it is possible to minimize the total amount of resources required over all links to be less than that of using a single highly reliable link. Such techniques can be applied in practical systems in NR RAN to meet the URLLC requirements and to balance the reliability- resource usage trade off. III. ARCHITECTURE ENHAMCEMENTS IN 5G NR RAN A. NR RAN Protocol Stack Enhancements for PD The NR RAN protocol stack in the user plane (UP) is collectively responsible for ensuring reliable over-the-air transmission of protocol data units (PDUs) in both uplink (UL) and downlink (DL) directions [9]. The RRC entity, which is the primary control plane (CP) function in RAN, is responsible for configuring the protocol layers in both the network and the UE. The RRC is also responsible for establishing, maintaining and releasing of the radio bearers between the network and the UE. The radio bearers in NR are categorized into two types namely, the data radio bearers (DRBs) and signaling radio bearers (SRBs), both of which are used for transmission of UP and CP packets, respectively. The DRBs are generally configured to satisfy a set of Quality of Service (QoS) requirements which include a guaranteed bit rate and priority level. In comparison to DRBs, SRBs are characterized by less frequent transmissions, smaller PDU sizes and higher scheduling priority. In NR, a new Service Data Adaptation (SDAP) layer is introduced for performing the mapping between the QoS flows and DRBs. This is to ensure that the QoS flows, which originate and terminate in core network (CN) are handled appropriately in the RAN with the right priority treatment and resource provisioning. Next, sequence numbering, header compression and ciphering operations are performed in the NR PDCP to ensure in-order and secure delivery of both the UP and CP packets. To enhance transmission reliability, a new duplication function is incorporated in PDCP whose role is to make duplicates of the PDUs associated with a set of DRBs and SRBs configured by RRC. Also, each instance of the duplicate PDU carries the same PDCP sequence number in order to facilitate the receiving PDCP entity to detect and remove the duplicates. This is followed by the RLC layer which is responsible for PDU segmentation and handling of different transmission modes which include the Acknowledged Mode (AM) and Un-acknowledged Mode (UM). When PD is configured the original and duplicated PDUs are handled by two RLC entities, each correspondingly assigned to a unique logical channel. While both RLC transmission modes are supported with PD in NR, for URLLC the RLC operates in the UM mode where the reception status of the RLC PDUs does not require to be acknowledged to the transmitter to further minimize the latency. Subsequently, the MAC layer performs scheduling, multiplexing and mapping of the PDUs originating from different logical channels to transport channels. For ensuring reliable transmission, each transport channel is assigned to a separate HARQ process which enables transmission of ACK/NACK feedback messages and the retransmission of PDUs. Also, for URLLC a maximum of 1 HARQ retransmission can be accommodated. When PD is supported, the RRC configures mapping restrictions in the MAC to ensure that the PDUs in the two logical channels do not end up in the same transport channel and consequently, assigned to the same carrier in the PHY layer. At the PHY layer the transport channels are mapped to physical channels, after which the provisioning of the Physical Resource Block (PRBs) and MCS selection is done in an assigned carrier. In NR, the PHY layer is expected to support multiple numerologies, each configured with different subcarrier spacing ranging from 15kHz to 120kHz as well as shorter transmission time interval (TTI) of 0.125ms [11]. This is to enable greater flexibility in supporting diverse set of use cases with varied requirements. B. Enhancements in NR DC Architecture for PD Dual-connectivity (DC) architecture in NR is primarily intended to provide high throughput and high reliability by enabling the use of radio resources from two access nodes with distinct schedulers of the same or different radio access technologies (RATs) [7]. The access nodes in DC consist of the Master Node (MN), which hosts the full RAN protocol stack, and Secondary Node (SN) hosting the lower layers (i.e. RLC, MAC and PHY). Both the MN and SN are connected via a non-ideal backhaul over the Xn interface. While the Xn interface supports data forwarding and flow control functions, the fact that the packets may traverse through a non-ideal backhaul may result in high latency, hence restricting the ability to perform inter-node coordination. As such only semi-static coordination at the RRC level is supported in DC. On the other hand, both MN and 2018 IEEE Wireless Communications and Networking Conference (WCNC) SN have greater flexibility in independently scheduling resources for the UEs. The MN supports multiple carriers (i.e. frequency bands) which collectively form the Macro Cell Group (MCG). The SN, in turn, controls the Secondary Cell Group (SCG) consisting of its own set of carriers. Also, both MN and SN are capable of supporting direct bearers as well as split bearers. Specifically, in the case of direct bearers the packet flow is routed directly from the CN through either MN or SN while in split bearer case, the packet flow is split between MN and SN using a splitting ratio configured by RRC. The split bearer enables the UE to receive and send packets simultaneously from two access nodes to realize higher throughput performance. In 5G, both access nodes host the NR RAN protocol stack and are connected to the 5G Core Network (5GC) in standalone NR-NR DC architecture [7] as shown in Fig. 1. In this case, the MN and SN are referred to as Master Next-Gen Node B (MgNB) and Secondary Next-Gen Node B (SgNB), respectively. Here, the MgNB supports the MCG bearer and MCG split bearer while the SgNB supports SCG bearer and SCG split bearer. In comparison, the existing LTE DC does not support the SCG split bearer. Fig 1: NR-NR DC Architecture. MN and SN use the NR RAN protocol stack In the case when the MN and SN belong to different RATs in a non-standalone architecture, where one of the nodes uses the LTE RAN protocol stack while another node has NR protocol stack, the generalized DC is referred to as the Multi-RAT DC (MR-DC). Within the MR-DC architecture, there can be multiple options depending on the type of CN available and the RAT supported at the MN and SN. These options are listed as follows: i) EN-DC: MN uses LTE protocol stack (eNB) and SN uses NR protocol stack (gNB). MN is connected to (legacy) Evolved Packet Core (EPC) and SN is connected to MN via (legacy) X2 interface ii) NGEN-DC: MN uses LTE protocol stack (eNB) and SN uses NR protocol stack (gNB). MN and SN are connected to 5GC and SN is connected to MN via Xn interface iii) NE-DC: MN uses NR protocol stack (gNB) and SN uses LTE protocol stack (eNB). MN and SN are connected to 5GC and SN is connected to MN via Xn interface In the regards to PD, since the existing LTE PDCP is not enabled with the duplication function there will be certain restrictions on the variants of DC architectures that can be used for URLLC. Particularly, in the MCG split bearer case, PD can only be configured in certain MR-DC architectures that host the NR PDCP. Likewise, direct bearers can be configured for PD over multiple carriers from any node hosting the NR PDCP, similar to the carrier aggregation (CA) technique in LTE. While this can be applied to all direct bearer DRBs, there are certain limitations for performing PD in the SRB case. However, enhancements have been made in the LTE Release 14 to support NR PDCP at the LTE node and to remove the duplicates at the receiving LTE PDCP entity for the SRBs when PD is performed at the transmitting PDCP entity. In the MR-DC case, both the MN and SN have their own RRC entities which can communicate with the UE either via direct SRBs or split SRBs. For example the channel measurement reports intended for the RRC entity in SN can be sent directly to the SN using the direct SCG SRB. When configured for split SRBs, the RRC PDUs from either MN or SN are sent via the Xn interface. For supporting PD in the UL, the DC-capable UE uses a common PDCP and a pair of lower layer stacks consisting of RLC, MAC and PHY, mirroring that of the DC network. Additionally, at the hardware level, it is assumed that the DC-capable UE utilizes separate RF chains with different power amplifiers in order to transmit the duplicate packets simultaneously to the MN and SN using power control and beamforming techniques. IV. DYNAMIC CONTROL OF PACKET DUPLICATION As detailed in Section III, the RRC signaling is used for configuring the radio bearers for PD in the DC architecture. While the semi- static configuration using RRC may be adequate for duplication of SRBs, for DRBs, which carry the bulk of the traffic in the network, enabling PD at all times may be resource-wise wasteful. For this reason it is necessary to incorporate a faster mechanism linked to dynamic scheduling that allows more dynamic control of PD. In NR RAN, it is the responsibility of the scheduler in network to dynamically allocate physical resources in both UL and DL transmissions for all UEs. The resource allocation can be performed at the granularity of a TTI over a duration ranging from 1 slot to multiple slots. Once the resource allocation decision is made, the scheduler provides the resource assignments in the DL control information (DCI) message in order for the UE to appropriately decode and process the received packets. For supporting UL transmissions, the scheduler provides the UE with UL grants, indicating the PRBs and the MCS indices to be used in UL. While the scheduling related messages are generally handled at the PHY layer, certain control information related to dynamic scheduling can be exchanged using the MAC control element (MAC CE). In LTE, the MAC CEs sent in DL are used to carry information pertaining to activation/deactivation of carriers, while those in UL carry the Buffer Status Report (BSR) and Power Headroom Report (PHR). In NR a new packet duplication command (PDC) MAC CE (sent in DL) is introduced to facilitate dynamic activation and deactivation of PD [9]. The following describe the different triggering mechanisms for enabling dynamic control of PD. A. Network Triggered Packet Duplication For making the dynamic PD decision, it is necessary for the network to have the up-to-date channel and load (i.e. data in buffer) information over all configured transmission links. The MN and SN determine the channel quality in UL and DL based on the sounding reference signals (SRS) and channel quality information (CQI) reports transmitted by the UE. In UL, if the data buffers at PDCP and RLC are non-empty, the UE may also transmit the BSR in the UL MAC CE to each access nodes. Here, the BSR is triggered on the basis of a logical channel group (LCG), composed of a set of logical channels with similar QoS requirements. In the case of PD, both the original and duplicated PDUs are mapped to two logical channels which are associated with different LCGs to enable independent handling of BSR generation and UL grant allocation. Apart from BSR, the UE can also provide the PHR, indicating the difference between the maximum allowable transmit power level and the power currently used in UL to both MN and SN. This Xn MAC RLCRLCRLC PDCPPDCP SDAP SDAP PHYPHYPHY MAC RLCRLCRLC PDCPPDCP SDAP SDAP PHYPHYPHY MCG Bearer MCG Split Bearer SCG Split Bearer SCG Bearer MgNB SgNB 2018 IEEE Wireless Communications and Networking Conference (WCNC) information enables the scheduler to determine the transmit power and, consequently, the MCS index to be used in UL on the allocated PRBs. Based on the provided buffer and channel measurements, an optimization problem formulation, applicable at the scheduler in each TTI for determining the optimum resource allocation over the N available links, is given as follows: min{ , , } (3.0) . . ≤ , (3.1) ( , ) ≥ , (3.2) ( , ) ≤ 1 (3.3) where ∈ {0,1} determines the activation/deactivation of link and denotes the PRBs allocated on link from a total of available PRBs. The function ( , ) in Eq. (3.2) can be modeled as a linear function that maps from the MCS index and number of PRBs to a transport block size (TBS) [12]. The reliability constraint in Eq. (3.3) follows from Eq. (1) where the function ( , ) is determined from link-level LTE simulations with Rayleigh fading. The following algorithm provides a low complexity technique that can be implemented at the radio resource management (RRM) function in the network to solve the optimization formulation in Eq. (3) heuristically. This based on the greedy approach to determine the dynamic PD decision and the corresponding resource allocation (MCS and PRB) to be applied on the activated links. Algorithm 1: Dynamic Control of Packet Duplication Input: TBS of URLLC packet, over all links ∈ , overall reliability target Output: Number of activated links | |, MCS index on each link, PRB allocation on each link 1: Sort links with index in descending order with decreasing SNR 2: Sort MCS with index in descending order 3: Initialize: Assign maximum MCS ← to all links 4: do 5: Set ← 1 (Selection of best link with max SNR) 6: do 7: Selected link ← link and assign ← 8: Compute = ( , ) and R using Eq. (1) 9: if ( ≥ ) 10: Compute = ( , ) using Eq. (3.2) 11: Return | |, and , 12: else 13: = + 1 (Select next best link) 14: while ( ≤ ) 15: = + 1 (Select next highest MCS index) 16: while ( ≤ ) Note that Algorithm 1 determines the best transmission mode for the UE with URLLC requirements by identifying the number of links, | | to be used and the corresponding resource allocation. In this case, when | | > 1 PD is activated and alternatively, when | | = 1 PD is deactivated and a single best link is selected. In the DL, the decision to activate PD results in both MN and SN providing the resource assignments in the DCIs. It may be possible that the decision made is to deactivate PD and to fall back to the split bearer configuration using a traffic splitting ratio. In either case, the PD decision will be transparent to the UE. The UE will receive and process the received DL packets based on the DCIs using the existing procedure in LTE. In the UL, the decision for PD is made in the network and conveyed to the UE via the PDC MAC CE as shown in Fig. 2. Here, the PDC MAC CE contains a bitmap to identify the command for activation and deactivation for each DRB configured for PD. Note that in DC it is possible for the MN and SN to send the MAC CEs independently from the individual schedulers. In this case, the MN and SN are assumed to operate without coordination and each will send the PD command based on the observation of its own channel and loading conditions. Although, this may not result in the optimal decision for activating and deactivating PD due to the unavailability of complete information of the other links, it has the advantage of avoiding the latency over the backhaul, hence suitable for URLLC. The UE can either combine the MAC CEs received from both access nodes or act based on the last received MAC CE. Alternatively, in the coordinated case in DC, the node which hosts the PDCP will send one PDC MAC CE containing the joint decision. In both of these cases, the MN and SN should provide the UL grants in the DCI to the UE to carry out the UL transmissions. The MN may also activate semi-persistent scheduling (SPS) configuration via the DCI. Based on the received grants, the UE transmits the data on Physical UL shared channel (PUSCH) while continuing to report the channel conditions on all activated links. Fig 2: Signaling flow showing the dynamic control of Packet Duplication in NR DC Architecture for UL transmission B. UE Triggered Packet Duplication In the UE triggered case the MN initially configures a set of DRBs via RRC signaling for PD. However, in contrast, both the PD decision and enforcement are made in the UE based on the monitoring of the channel conditions on the configured links. Note that the UE triggered case applies only for the UL transmissions and used primarily to support URLLC. In addition, the UE triggered approach also applies to the grant- free or configured grant technique, introduced in NR as an enhancements from SPS, for enabling fast UL transmission without an UL grant. The grant-free technique relies on pre-allocated resources which are provided by the gNB on a per-UE basis for transmitting short URLLC packets up to K times without having to go through the conventional dynamic scheduling procedure. Alternatively, the gNB can configure the grant-free resources in a common pool accessible by multiple UEs on a contention basis. The network may also provide resource configuration (e.g. PRBs, range of potential MCS) on the grant-free resources via RRC signaling. In addition, the grant-free resources may contain a validity timer, indicating the duration in which the resources in each UE Measure UL Channel Measure UL Channel Enforce PD decision UL SRS UL SRS BSR + PHR PDC MAC CE URLLC Packets URLLC Packets MN SN PD Decision BSR + PHR UL Grant UL Grant Channel Info on Xn Forward Packets on Xn RRC Configuration for PD 2018 IEEE Wireless Communications and Networking Conference (WCNC) of the corresponding links are valid and reserved for the UEs. In regards to PD, the network may pre-allocate the resources on multiple configured links without explicitly activating PD. Once the PD decision is made at the UE, it would be possible to send the duplicate PDUs in UL on multiple grant-free resource pools. While this may increase the probability of collision, the UE can use higher MCS on a minimum number of resources in different pools without compromising on the overall achievable reliability. This approach will not only improve the grant-free resource utilization but also lowers the collision probability. C. Push and Pull Buffer Management Mechanisms In the split bearer case, the BSRs are used to notify the network on the data buffered in both the PDCP and RLC entities at UE for UL transmissions. The BSR is triggered when a certain data threshold level is crossed at the PDCP in UE, based on which the network allocates the resources and provides the UE with the UL grants. In the case of PD, the PDCP data threshold is ignored and two BSRs, containing both original and duplicated data, are sent by UE to the schedulers in MN and SN as shown in Fig. 3. The network may also consider other factors which include the overall traffic load of all UEs and channel conditions when providing the UL grants. Fig 3: Buffer Management Mechanism when performing PD In general, the buffer management with PD can be performed based on the Push and Pull mechanisms. The push mechanism refers to the case where once PD is activated the data associated with a DRB is pre-duplicated and pushed to each of the logical channels in the corresponding buffers in RLC. In subsequent steps, normal BSR triggering is used to request for UL grants in order to clear the data buffers in the RLC entities. Since the resource allocation is done individually for two BSRs by two schedulers, it is possible that one of the links has much faster rate in transmitting and clearing the buffer due to favorable channel and loading conditions at the gNB. If the rate of duplication at PDCP is governed by the rate at which the fast link clears the buffer then the duplicated PDUs will be pushed to each of the RLC entities. In the other slower link, the new and existing PDUs may result in buffer pile-up due to the unavailability in UL grants, leading to further delay. To address this issue, the feedback from lower layers (e.g. HARQ in MAC on the fast link) can be used to discard the un- transmitted packets in the slow link if the packets in the fast link are received successfully. Alternatively, the pull mechanism involves first buffering the data in PDCP followed by “pulling” into the RLC only if the UL grants have been provided by the gNBs. While the pull mechanism enables to address the slow link problem because it is not governed by the performance of either one of the RLC entities, it requires a sizeable buffer size in the PDCP for storing the PDUs. Also, another adaptation that can be implemented is by linking the duplication function at the PDCP with the availability of UL grants. Here, the BSRs are initially generated based on a virtual amount of data in PDCP prior to duplication and then the PDUs are sent to lower layers after duplication once sufficient UL grants are available. Although this approach may reduce the buffer size requirements in PDCP and RLC, it may result in higher latency since duplication is not performed until the UL grants are available. When PD is deactivated, implying only a single best link is used and duplication is not applied for new PDUs, there may still be certain un-transmitted PDUs in the RLC entities. If the UL grants are available for both links, then the existing packets in the RLC can be cleared regardless of the deactivation command. Alternatively, the UE may suspend the transmission on the deactivated link and discard the packets in the buffer. If the deactivation command need not be applied immediately, a timer can be used to clear the existing buffer after which the remaining packets can be discarded. V. PERFORMANCE ANALYSIS This section presents the numerical results obtained from evaluations performed to investigate the impact of packet size, transmission link SNR and number of links on the effectiveness of applying PD for URLLC. The evaluations are based on link-level simulations adapted to consider a multi-connectivity scenario in the RAN consisting of an MN node (primary link) and multiple SN (secondary links) nodes as described in [7][12]. In the simulations, the network nodes perform DL transmissions with variable packet sizes of 32, 100 and 200 bytes over a carrier frequency of 2GHz. Each link/carrier dedicated for the UE is assumed to be allocated with a bandwidth of 20MHz (100 PRBs per link). Additionally, the Extended Vehicular A (EVA-70Hz) channel model is applied and adapted to ensure that the channel conditions on different links are independent and uncorrelated. Fig 4: Number of PRBs vs. Packet Size for different number of links. PD is applied only when more than 1 link is used Fig. 4 shows the number of PRBs required for satisfying the reliability and latency requirements of 1-10-5 and 1
