Impact of eNB functional splits on the 5G fronthaul performance: an experimental evaluation.

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University of Pisa and Scuola Superiore Sant’Anna Department of Computer Science

Master Degree in Computer Science and Networking

Impact of eNB functional splits on the 5G fronthaul performance:

an experimental evaluation

Candidate Supervisor

Iuliia Dzevik prof. Luca Valcarenghi

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Abstract

Driven by the need to cope with exponentially growing mobile data traffic and to support new traffic types from massive numbers of machine-type devices, several innovative approaches and technologies are being considered as potential elements making up such a future fifth generation (5G) mobile systems, including cloud Radio Access Networks (RANs), application of SDN principles, exploiting new and unused portions of spectrum, utilization of massive MIMO, full-duplex communications and so on.

5G mobile network architectures will most likely be developed as an evolution of LTE technology. Hence, the introduced concepts need to be as transparent and compatible as possible with the 3rd Generation Partnership Project (3GPP) network architectures while satisfying operational and customer demands on performance. The 5G mobile network architecture will need to support the potentially dynamic flexible centralization of radio access network functionality. As promising paradigm for 5G wireless communication systems, cloud RAN has been shown to reduce both capital and operating expenditures as well as to provide high spectrum and energy efficiency. The fronthaul in such networks, defined as the transmission link between a baseband unit (BBU) and remote radio heads (RRHs), requires high capacity and low latency, but it is often constrained. Eight functional splitting schemes proposed by 3GPP introduce various bandwidth and latency requirements for a fronthaul interface. But are all these functional split options necessary? Can we reduce the number of options?

In this thesis, we propose an alternative approach for the fronhaul functional split. We design, study and analyze four different experiments that emulate 5G wireless communication networks. For research performing open source OpenAirInterface software by EURECOM was used as a realistic and flexible 4G/5G experimentation platform and complete 3GPP standards compliant software that offers to configure, build and run the 5G system, generate a traffic and monitor the behavior of the system using the Wireshark network protocol analyzer.

For each deployed scenario, we measured the control and user planes traffic between user equipment and a base station with the full protocol stack using Wireshark software. And based on the received single set of results for each experiment we have computed traffic capacity requirements for each proposed potential splitting point. For each deployed scenario, also we have evaluated one way fronthaul latency and calculate periodicity for each packet type.

The important research issue of the following experiments is to understand which proposed functional splits can optimize performance and meet the requirements for the future 5G fronthaul in terms of traffic capacity and low-latency.

We demonstrate exemplarily how different functional split options could contribute depending on the network characteristics and illustrate the effectiveness of our approach in terms of required bandwidth and latency.

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Glossary

ARQ Automatic Repeat Request BBU BaseBand Units

CU Central Unit C-RAN Cloud RANs

CAGR Compound Annual Growth Rate CoMP Coordinated Multipoint

CN Core Network

CP Cyclic Prefix DU Distributed Unit

eICIC Enhanced Inter-Cell Interference Coordination EPC Enhanced Packet Core

eNB Evolved Node B

E-UTRAN Evolved Universal Terrestrial Radio Access Network FFT Fast Fourier Transform

HARQ Hybrid Automated Retransmit Request HSS Home Subscriber Server

iFFT Invers Fast Fourier Transform

IEEE Institute of Electrical and Electronics Engineers IoT Internet of Things

ITU International Telecommunication Union ICIC Intercell Interference Coordination GPON Gigabit Passive Optical Networks LTE Long Term Evolution

LTE-A Long Term Evolution - Advanced

mMTC Massive Machine-Type Communication M2M Machine-to-Machine Communication MAC Medium Access Control

MME Mobile Management Entity MIMO Multiple Input Multiple Output NC Network Controller

NFV Network Functions Virtualization

NR New Radio

NG Next Generation

NGFI Next Generation Fronthaul Interface NGMN Next Generation Mobile Networks NAS Non-Access Stratum

OAI OpenAirInterface

OMG OpenAirInterface Mobility Generator OTG OpenAirInterface Traffic Generator

OFDM Orthogonal Frequency Division Multiplexing PaaS Platform as a Service

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PHY Physical Layer

PRB Physical Resource Block PHR Power Header Report PDU Protocol Data Unit QoS Quality of Service RAN Radio Access Network

RANaaS Radio Access Network Service RAP Radio Access Points

RAT Radio Access Technology RAU Radio Aggregation Unit RCC Radio Cloud Center RWALK Random Walk RWP Random Way Point RLC Radio Link Control RRU Radio Remote Unit RRC Radio Resource Control RRM Radio Resource Management RRH Remote Radio Head

RRS Remote Radio System S-GW Serving Gateway SAP Service Access Point SDU Service Data Unit SRB Signaling Radio Bearer

SCBR Small Packet Constant Bit Rate SDN Software-defined Networking

SCTP Stream Control Transmission Protocol TDD Time Division Duplex

3GPP Third Generation Partnership Project TTI Transmission Time Interval

TN Transport Node UE User Equipment

uMTC Critical Machine-Type communication V-RAN Virtualized RAN

VoLTE Voice over LTE xMBB Massive broadband

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Chapter 1 Introduction

As reported by Cisco in the Global Mobile Data Traffic Forecast [1] the global mobile data traffic grew 74 percent in 2015 and mobile video traffic accounted for more than half of all mobile data traffic. More than half a billion of mobile devices and connections were added to mobile networks. Smartphones accounted for most of that growth. Globally, the average mobile network connection speed was around 2 Mbit/s in 2015.

The Forecast [1] also provides the analysis whereby the mobile data traffic will reach the following milestones within the next 5 years:

- In general, mobile data traffic will grow at a CAGR of 53 percent reaching 30 exabytes per month and the number of mobile-connected devices per capita

will reach 1.5 by 2020 where smartphones will cross four-fifths of mobile data traffic.

- The average global mobile connection speed will surpass 3 Mbit/s by 2017. - 4G connections will have the highest share (40.5 percent) of total mobile

connections by 2020.

- Video traffic will take three-fourths (75 percent) of the world’s mobile data traffic

with the average speed of growth at a CAGR of 26 percent and will reach nearly 6.5 Mbit/s by 2020.

The major trends contributing to the growth of mobile data traffic are the following: 1. The increasing number of wireless devices that are accessing mobile networks worldwide. The most noticeable growth is going to occur in M2M connections, followed by tablets. The forecast projects that globally 92 percent of smartphones and tablets (5.5 billion) will be IPv6-capable by 2020. This estimation is based on IPv6 operation system support (primarily Android, iOS) and the accelerated move to higher-speed mobile networks (4G or higher) capable of enabling IPv6.

2. The mobile applications explosion and phenomenal adoption of mobile connectivity by end users on the one hand and the need for optimized bandwidth management and network monetization on the other hand is fueling the growth of global 4G deployments and adoption.

Service providers around the world are busy rolling out 4G networks to help them meet the growing end-user demand for more bandwidth, higher security, and faster connectivity on the move. The transition from 2G to 3G or 4G deployment is a global phenomenon. Although the growth in 4G with its higher bandwidth, lower latency, and increased security will help regions bridge the gap between their mobile and fixed network performance, deployment of Low-Power Wide-Area such as M2M networks will help enhance the reach of mobile providers in the M2M segment. This situation will lead to even higher adoption of mobile technologies by end users, making access to any content on any device from anywhere and IoT more sustainable.

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1.1 5G aspects and key requirements

From the existing wireless networks, functional and structural evolution perspectives current mobile networks are designed only for peak-provisioning and typical Internet traffic. In order to meet the enormous growth of mobile data traffic, traditional wireless macro-cell networks need to be transformed into architectures comprising large numbers of small cells complemented with macro cells for ubiquitous coverage. The introduction of ultra-dense, low-power, flexible and small-cell deployments with very high spatial reuse is a promising way to allow future data rate demands handling.

Whereas 4G has been driven by device proliferation and dynamic information access, next, the 5th_{generation of mobile networks will give possibility to reach totally}

new and globally different level in the network services quality, including high bandwidth, broader coverage, ultra-low latency, very dense deployments and centralized processing. 5G will be driven largely by IoT applications.

The 5G networks will be built around people and things and will natively meet the requirements of three groups of services [29]:

- xMBB that delivers gigabytes of bandwidth on demand. - mMTC that connects billions of sensors and machines.

- uMTC that allows immediate feedback with high reliability and enables, for example, remote control over robots and autonomous driving.

So, 5G era will not only continue to support existing network applications, but will also introduce many new user applications such as high-mobility applications at speed over 350 km/h, sensor networks and so on [32]. 5G networks will give possibility for the M2M technology realization (smart cars, health-critical applications, virtual reality, industry automation so on) – the devices that are located not far from each other will be able to interact directly without involving the 5G base stations or a core network. Only signaling traffic will pass the 5G core network. The benefit of this technology is a possibility to move the data transmission to the unlicensed spectrum band for the network load balancing. Moreover, such approach is highly efficient from power control standpoint and can also reduce interference in unlicensed frequency bands. Conventional cellular architecture does not allow direct communicate between UEs.

As most 3G and 4G mobile networks are based on the 3GPP standards, 3GPP as well as IEEE are responsible for the 5G standardization that is expected by 2020. But 3GPP is still at too early stage as opposed to the industry expectations. There might be some reasons with seemingly slow progress in 3GPP, but the demand in the industry got too high to wait until 3GPP is fully ready with the specification. As a result, some major players in industry [10], [12] decided to deploy a small scale 5G system with their own specification that some people call “Pretrial”.

The 5G networks requirements should be released by the ITU in 2017. The next-generation technology will eventually be defined in a standard that will be known as “5G.” According to the NGMN Alliance it is expected that:

- 5G will provide Internet connections at least 40 times faster and with at least four times more coverage worldwide than the current 4G standard with the network deployment in the places with the high speed Internet connection need providing data rate up to the 10 Gbit/s per user. That is essential for the mMTC including the

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monitoring and automation of buildings and infrastructure, smart agriculture, logistics etc.

- Low latency and high network capacity that as well as data integrity are important for the time-critical IoT. In this type of application, monitoring and control occur in real time, end-to-end latency requirements are very low (at millisecond levels), and the need for reliability is great [19].

- The user-centric 5G network model will be implemented with smart antennas that change the antenna’s diagram depends on the environment and customers need. Hence, flexibility and scalability become the fundamental requirements to allow the adaptation for the required network to the needs of the individual services.

5G will serve also many different purposes with respect to availability, user throughput, connection and traffic densities, reliability, energy efficiency and mobility.

Contrary to the previous generations of mobile networks evolution, 5G will require not only improved networking solutions but also a sophisticated integration of massive computing and storing infrastructure.

To address the imposed 5G requirements, a novel 5G mobile network architecture is foreseen which provides the means to support the expected service diversity, flexible deployments, and network slicing.

Furthermore, since the network slicing concept was initially proposed to be adopted by the 5G CN, NGMN uses the term “end-to-end network slicing” to refer to the overall system design concept, including both CN and RAN aspects [29]. In that context, network slices must fulfill a set of requirements such as the need for sharing and efficiently reusing resources (including radio spectrum, infrastructure, and transport network). The end-to-end network slicing support appears as one of the key requirements in 3GPP.

The 5G networks should support more sophisticated mechanisms for traffic differentiation than those of legacy systems, to fulfill diverse and more stringent end-to-end QoS requirements. 5G networks will have to provide the resources separation and prioritization on a common infrastructure for operational and security purposes. For example, a new radio access model that supports highly scalable video distribution or mMTC data uploading might require additional transport facilities – such as a scalable way to provide multicasting.

Not only the transport network will serve many radio sites, but each site will support massive traffic volumes, which might be highly burst due to the peak rate available in 5G. For example, a UE that is connected to several sites simultaneously, may also be connected to several different access technologies. The device may be connected to a macro-cell over LTE, and to a small cell using a new 5G radio access technology. Multi-site and multi-rate connectivity will provide greater flexibility in terms of how UEs connect to the network and how end-to-end services are set up across radio and transport.

5G networks have several advantages. One is a high degree of flexibility. The network will serve highly diverse types of communication – for example, between humans, machines, devices and sensors - with different performance attributes. 5G will also enforce the necessary degree of flexibility, where and when needed, regarding capability, capacity, security, elasticity and adaptability.

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1.2 5G design objectives

Availability of new wider high-frequency spectrum bands

In 5G networks, spectrum availability is one of the key challenges for supporting the enormous mobile traffic demand. Nowadays, the current spectrum is already crowded. Especially in very dense deployments it will be necessary to go higher in frequency and use larger portions of free spectrum bands. This means that 5G networks will operate in a wider spectrum range with a diverse range of characteristics, such as bandwidth and propagations conditions. Thus, appropriate mechanisms are needed that today do not exist in the current 4G systems. This implies that the new 5G architecture should allow spectrum to be managed more efficiently, by accurately monitoring spectrum usage and by enabling sharing strategies in mobile networks. Also, it would allow prioritization and allocation of traffic across heterogeneous access technologies in a dynamic way to diversified spectrum resources [29]. The new 5G technology is expected to use so-called “millimeter wave” and “macro wave” radio spectrum with the frequency above 4-6 GHz that increases the data rates up to the 10 Gbit/s per user.

Ultra-dense deployment

The higher frequencies carry significantly more data. But they are also more easily blocked by buildings, foliage, and even rain, making their use for mobile communications quite challenging. To decrease the requirements for the maximum transmission distance ultra-dense network solutions such as low-power pico- and femto-cells will be deployed. Such densification results in higher spectral efficiency (due to the reuse of time-frequency resources across multiple cells) can also reduce a mobile device power consumption due to its communication with nearby pico-cell. This solution significantly improves network coverage. Such small cell base stations can be deployed as low powered femto-cells typically used in enterprise/residential deployments or higher powered pico-cells for outdoor coverage improvement or macro-cells served fewer users. In a conventional small cell deployment, a considerable number of sites would consume energy and computational resources under such conditions. This opens the opportunity for more targeted provisioning of data rates, leading to more efficient use of spectral and energy resources [15].

Beamforming

The technology that steers signal transmission to a specific direction using multiple number of antenna elements. It can maximize communication capacity at the cell edge and can also be used as a major 5G technology to minimize interference between user equipment [12]. Antenna systems less in size can be used over the short distance waves in the millimeter diapason. Advanced multi-antenna technologies such as MIMO that uses few antennas on transmitter and receiver sides, give possibility to solve the problem of fully steerable radio communication efficiently. For example, the mechanism 2x2 is used for the LTE networks and data are transmitted using two independent channels that increase the transmission speed of data almost in twice.

Advanced radio coordination solutions, such as CoMP and eICIC, aim at improving the spectral efficiency in the radio network and in particular at the cell edges [33]. Different

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coordination levels (i.e., moderate, tight, and very tight) pose different constraints on the transport network. Moderate coordination techniques (e.g., eICIC) have no specific requirement in terms of transport performance. On the other hand, tight coordination techniques introduce strict latency constraints (i.e., between 1 and 10 ms), while they do not introduce very strict capacity constraints (i.e., lower than 20 Mbps). Finally, very tight coordination techniques introduce very demanding constraints in terms of both latency (lower than 0.5ms) and capacity (in the range of several Gbps using CPRI).

Abstraction and programmability

New paradigms and enablers such as SDN and NFV for network resources and functionality abstraction as well as managing services on-the-fly through programmatic APIs should be followed and supported. In such an ultra-flexible environment, it is necessary to consider innovative solutions, such as user and control planes separation, and possibly, the boundaries re-definition between the network domains (e.g. radio access network and core network) to reduce network complexity, and increase flexibility.

4G-5G Interworking

5G should provide an efficient interworking between 5G and an evolution of LTE as the latter could already meet the requirements for some of the uses cases discussed for 5G like the Narrow Band IoT. Interworking allows seamless handover between 4G and 5G networks for reliable connectivity and enhanced coverage. Control signal can be carried over 4G for reliability and data can be transmitted over 5G for ultra-high speed data connection.

1.3 5G logical architectures

Like previous generations, 5G will consist of a set of evolved network technologies. It will bring the evolved versions of existing RANs, cloud and core technologies together with some new complementary technologies, to cater for more traffic, more devices and more types of devices, with different operating requirements and thousands of different use cases. As most 3G and 4G mobile networks are based on 3GPP standards, we use the LTE technology as our baseline for both network architecture and radio access, and outline an evolutionary path from it.

5G architecture comprises both the RAN itself and its interconnection to the core network functions where these functions are deployed at distributed or centralized nodes in the fixed network. To deploy these functions, the fixed network encompasses several aggregation nodes that offer computing and storage capabilities. These capabilities can be used flexibly for the mobile network’s efficient operation. Depending on the use case, efficient operation can be achieved by centralized or distributed network functions together with the transport network resources virtualization.

In traditional RAN, the performance of wireless communication is decoupled with computational signal processing on its own, even in peak hours. And the information exchange demand between base stations, and between base stations and the core network elements is minimized since all signal redundancy is removed through local processing. Traditional RAN suffer several limitations including:

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limited scalability and flexibility;

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allows only point-to point logical topology;

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lack of modularity and limited density;

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increased management costs;

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inefficient energy management due to lack of resource sharing.

A distributed architecture is another solution for coordination performing which has potential of minimizing the infrastructure and signaling protocol cost associated with base station’s links and the central processing unit, so conventional systems need not undergo major changes. Furthermore, the radio feedback to several nodes could be achieved without additional overhead. Distributed standalone base stations with backhaul, remote radio heads at the antennas are connected to the radio site cabinet through an optical fiber. All processing is performed locally at base stations.

A major challenge in the 5G mobile radio access network is the additional layer’s of small cells efficient integration into the existing macro-cell network. Besides using the classical distributed RAN also for small cells, C-RAN and V-RAN are considered as innovative approaches.

1.3.1 C-RAN

As networks become denser, ultra-dense, small-cell deployment is bound to create problems involving frequent handover between cells and signal interfaces. These problems must be resolved thought the centralized control plane. Centralized processing permits to implement efficient RRM algorithms, which allow radio resource coordination across multiple cells. They also allow optimization of the radio access performance at the signal level, for example, through joint multi-cell processing and ICIC. RRM and ICIC algorithms improve RAN performance by avoiding, cancelling, or exploiting interference between adjacent cells. At the network level, centralized processing is required to orchestrate and optimize ultra-dense networks by adding spectrum resources and configuring the network to fine tune user data traffic delivery. Furthermore, central resource pools may allow the flexible software deployment.

Cloud technology has already received increasing attention for the mobile core network functionalities deployment. Operators investigate the possibility of commodity hardware implementations to exploit the benefits of cloud technology (e.g., by means of NFV and SDN [15].

Recent attention has shifted to meeting the RAN baseband-processing requirements on high-volume hardware. Aiming to address the classical RAN limitations and take advantage of pooling and coordination gains, C-RANs with the option of flexible processing splits, as well as V-RAN with performed resource virtualization have been proposed. In both cases, as illustrated in the Figure 1.1, small cells are deployed as separated RRHs connected to a centralized macro-cell BBUs via a fronthaul interface. In C-RAN, time-domain I/Q samples are aggregated from scattered antenna sites to a central office for UL direction processing or sent out in the opposite direction after DL processing [35]. Some or all signal processing is shifted from a RRH to the BBU in the central unit. The fronthaul in such networks, defined as the transmission link between a

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BBU and RRH, requires high capacity and low-latency, but is often constrained. It needs to meet the evolving demand for wireless network architecture by means of BBU-RRH function remodeling. As the fixed network comprises a heterogeneous set of technologies that is integrated by using Ethernet as a common transport platform, Ethernet is also an interesting possibility for the fronthaul transport. This might be an attractive choice as current fronthaul interfaces e.g., CPRI, ORI, or OBSAI – may encounter capacity bottlenecks when confronted with 5G scenarios.

Figure 1.1 – An example of the 5G centralized C-RAN architecture.

The centralized BBU deployment offers significant advantages, including network deployment acceleration, reduced operating and investment costs due to fewer number of sites, effective support for cooperative technologies, easy software upgrade, performance improvement with coordinated multi-cell signal processing, carrier aggregation, increased resource utilization efficiency, low energy consumption, and light interference and other key LTE-A technologies, as well as improved network performance.

Also, the RRHs have a much smaller footprint than a base station with on-site processing, allowing for simpler and cost-effective network densification. However, moving the data to a remote location will add to the communication some latencies and costs. Therefore, one of the central issues in C-RAN environment is determining which functionality is executed centrally at the data center and which remains local to the RRH and BBUs providing high bandwidt and low latency.

Different BBU-RRH functional splitting options appear to be an interesting option for architecture flexibility improvement to achieve low latency and high reliability. In the following chapters, we propose different functional splits. Which of theirs can optimize performance and meet the requirements for the future 5G fronthaul in terms of traffic capacity and low-latency will be study and analyze in detail in the following chapters of this thesis.

In consequence, such C-RANs need to be integrated efficiently into the classical distributed RAN architecture. The remote processing requirements for operational

RRHs RRHs RRHs Enhanced Packet Core Fronthaul Backhaul BBU pool Signal

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network purposes together with the need to support a wide variety of compute and storage end user services, introduce the need of high bandwidth transport connectivity, with stringent delay and synchronization requirements between the radio units and the remote compute and storage resources. In addition, elastic resource allocation in the transport network becomes critical to the realization of statistical multiplexing gains [29]. The backhaul deployment and the ability to use existing infrastructure as well as non-ideal backhaul technologies play a critical role in the economic analysis of cloud-RAN.

Neither distributed RAN nor C-RAN are always optimal. Whilst distributed RAN is nest for supporting low latency, C-RAN is more suitable for high spectral efficiency, high energy efficiency and reduced cost.

1.3.2 V-RAN

Virtual RAN extends flexibility through abstraction or virtualization of the execution environment and can be applied to various RAN aspects, through spectrum virtualization, hardware sharing, virtualization of RATs, and computing resources virtualization.

- Spectrum virtualization allows the available spectrum to be utilized more efficiently by permitting multiple network operators to share the same spectrum.

- Hardware and network sharing is of particular relevance for small cells in order to avoid massive over-provisioning.

- Virtualization of multiple RATs allows simplified management of different RATs, each dedicated to different services and offering a different QoS.

- Computing resources virtualization is a new option that builds upon the idea of co-locating the processing resources of multiple base stations at a central processing center.

While early implementations provided each physical base startion with its own dedicated computing resources, which resulted in an over-provisioning of computing resources, more advanced implementations permit a dynamic reassignment of processing resources to base stations [16].

In V-RAN, BBUs are based on virtual machines and can be activated in general purpose servers located anywhere in the RAN. However, how to transport both the control plane and the user plane data between RRH and BBU remains an open issue as well as which functional split optimizes performance [28].

RANaaS is a disruptive technology in many ways and imposes new challenges on the signal processing in 5G mobile networks. Most importantly, it will exploit GPPs to execute RAN functionality with no or little dependency on a dedicated hardware (e.g. DSP, FPGA, or ASIC). Thus, the flexibility offered by a pure software-defined radio systems improves service life-cycle and cross-platform portability at the cost of lower power and computational efficiency.

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Figure 1.2 - An example of the eNB functions virtualization in C-RANs.

As depicted in the Figure 1.2 the combination of RANaaS and one or several RAPs form a virtual base station eNB. The virtual eNB controller function located in the RANaaS platform is responsible for function placement, distributed functionalities coherent execution, management and configuration of virtual eNB components [15]. The standard 3GPP interfaces (S1, X2) are maintained toward the core network and other eNBs. The SDN-capable backhaul TN provides interfaces for exchange of information about backhaul capabilities and available bandwidth that can be used to choose an optimal degree of centralization. TNs area controlled by a NC for on-demand reconfiguration and path control of the backhaul network in cooperation with the virtual eNB controller function within the RANaaS platform.

RANaaS will give possibility to implement many new 5G applications. It offers the possibility of using signal processing software dedicated to a special purpose based on the actual service. It reflects the diversity of services, use cases, and deployments through flexibility and scalability of the signal processing platform. In addition, it may even consider the complexity and abilities of terminals during the signals processing. Finally, RANaaS avoids the typical vendor lock-in as in current deployments that follow a similar development observed in the mobile core network, which may be implemented on cloud-platforms.

1.4 5G physical architecture

The 5G mobile network architecture would include both physical and virtual network functions, as well as edge-cloud and central-cloud deployments. Further, it is clear that the 5G mobile network needs to integrate LTE-A evolution with novel 5G technologies on RAN level, whereby RAN level integration would go far beyond existing

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interworking between access technologies, fulfilling the vision of what NGMN calls a “5G RAT family”.

The 5G network EPC is expected to be multi technological and should support the compatible work of the different access technologies (GPRS, LTE, Evolved LTE, Wi-Fi etc.) for UL and DL traffic generated by the different equipment types. The network architecture should be modular, elastic with support of the network functions virtualization, with principles and protocols of the SDN networks, dynamic network resources orchestration and with separation of the control and data planes. By splitting control and data plane in the core network the enhancement of the small cells operations o can be reached. As illustrated in the Figure 1.3 the 5G RAN consists of 5G eNBs, providing the 5G radio access user plane and control plane protocol terminations towards the UE. The 5G eNBs are connected by means of the NG1 interface to the NG core network.

- NG-C interface supports interface management and UE context management, UE mobility management, paging etc.

- Xn Interface allows to interconnect two eNBs or one eNB and one eLTE eNB with each other. The Xn interface is also applicable for the connection between two eLTE eNBs.

- The Xn-C interface supports error indication, setting and resetting up Xn, updating the Xn configuration data, UE connected mode mobility management, UE context retrieval.

- The Xn-U interface supports the data forwarding.

Figure 1.3 – An example of the 5G network topology.

The main idea here is that control plane provides connectivity and mobility, whereas user plane provides the data transport. This results in the fact that UE is connected to multiple base-stations, viz. macro and small cell. Such a definition of new carrier type in 3GPP, results in improved spectral efficiency as data transport is handled

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by small cell. There is also significant gain in energy efficiency of the network infrastructure as small cells can be switched off in case of lightly loaded scenarios.

A logical CN - RAN split will exist, allowing for an independent evolution of both RAN and CN, and for cross-layer optimizations in some deployments when the functions are co-located.

RAN performs transfer of user data, radio channel ciphering and deciphering, integrity protection, header compression, mobility control functions, inter-cell interference coordination, connection setup and release, load balancing, synchronization, radio access network sharing, paging, positioning, interworking with non-3GPP systems

For RAN two kinds of architecture can be distinguished with respect to the way this information is made available at the different transmission points: centralized baseband and distributed RAN with non-ideal backhaul as mentioned previously.

Beside the radio access and core network, the transport network will play a key role in 5G to flexibly and dynamically address the requirements of future mobile networks. In order to support the required flexibility, a unique packet-based network is required. Three main types of interface are envisioned: packetized CPRI, next generation fronthaul interface and backhaul. To address these interfaces, traffic class concepts will be introduced.

The backhaul relies mostly on fiber optic physical medium. Optical fiber can provide a multi-Gbit/s throughput connectivity that can be achieved using GPON technologies. Although a fiber-based backhaul offers long-term support with respect to increasing capacity requirements, this comes at a relatively high CAPEX and costly deployment.

Network functions as part of the cloud domain in 5G will increasingly be deployed as virtualized software instances running in data centers. This deployment pattern, which has been characterized as cloud deployed SDN/NFV, simplifies scaling and management of network infrastructure and provides flexibility of the network.

Network applications such as EPC, VoLTE, and future 5G core network functions will be cloud enabled: that is, they will have the ability to execute in the SDN/NFV cloud environment. Consequently, the applications will have the advantage of being automatically scalable as well as flexible in terms of where in the network they can be deployed. New applications are increasingly designed to be cloud native. Instead of designing applications with integral aspects and functions, applications rather use services offered by the cloud PaaS.

The entities network management in 5G systems will be able to automate and orchestrate a range of lifecycle management processes, and will be capable of coordinating complex dynamic systems of applications, cloud, transport and access resources [19]. Network management in 5G systems requires additional work to include VNFs deployed in cloud data centers.

To serve such a diverse ecosystem, the telecommunication operator will have to deploy orchestrator functions that will allocate appropriate computing and networking resources to the services targeting diverse and dedicated business driven logical networks. These logical networks, so-called network slices, will contained specialized networking and computing functions that meet the desired KPIs of the service providers.

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Chapter 2 5G RAN logical architecture

3GPP Technical Specification Group for RAN and ITU have started the work to develop requirements and specifications for NR systems such as 5G. 3GPP works on the technology components identification and development that needed for successful NR system standardization timely satisfying both the urgent market needs and the more long-term requirements. Evolutions of the radio interface as well as radio network architecture are considered in the study item “New Radio Access Technology” [42] under Release 14. The detailed specification is shown in the Table 2.1.

Table 2.1- 3GPP Standards specification for 5G Technology.

2.1 Radio interface protocol aspects 2.1.1 Overall protocol structure

NR radio protocols and procedures should be designed to have as much as possible commonality between tight interworking LTE (Long Term Evolution) and

Specification

number Title

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Study on New Radio Access Technology: Radio Access Architecture and Interfaces

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TR 38.802

Study on New Radio Access Technology: Physical Layer Aspects

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RF and co-existence aspects

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TR 38.804

TR for Study on New Radio Access Technology:

Radio Interface Protocol Aspects

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TR 38.805

Study on New Radio Access Technology: 60 GHz Unlicensed Spectrum

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TR 38.900

Study on channel model for frequency spectrum

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TR 38.912 Study on New Radio (NR) Access Technology

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TR 38.913

Study on Scenarios and Requirements for Next Generation Access Technologies

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standalone operations and enables a new generation of compute-communication platform for 5G.

The Figure 2.1 illustrates the elements of the LTE data and control planes protocol stack. In UE side the data plane includes Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and Physical Layer (PHY) protocols. The control plane additionally includes Radio Resource Control (RRC) protocol. At user plane side, the application creates data packets that are processed by protocols such as TCP, UDP and IP, while in the control plane, RRC protocol writes the signaling messages that are exchanged between the enhanced eNB and the UE. Non-access Stratum (NAS) is used for mobility management and other purposes between the mobile device and the MME. NAS messages are tunneled through the radio network, and the eNB just forwards them transparently. NAS messages are always encapsulated in RRC messages over the air interface. The other purpose of RRC messages is to manage the air interface connection , for example, for handover or bearer modification signaling.

Figure 2.1 – LTE user plane and control planes protocol stack.

The S1 interface connects the eNB to the EPC as defined in Chapter 1. It is split into two interfaces, one for the control plane and the other one for the user plane. The protocol structure over S1 is based on a full IP transport stack with no dependency on legacy SS7 network configuration. S1 control plane is based on the SCTP/IP stack. The SCTP protocol is well known for its advanced features inherited from TCP that ensure the required reliable delivery of the signaling messages. In user plane a transport bearer is identified by the GTP tunnel endpoints and the IP address. The S-GW sends DL packets of a given bearer to the eNB IP address (received in S1-AP) associated to that particular

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bearer. Similarly, the eNB sends upstream packets of a given bearer to the EPC IP address (received in S1-AP) associated with that particular bearer.

LTE layer 2 (PDCP, RLC, MAC) and RRC functions are taken as a baseline for NR from the UE side. The Figure 2.2 shows the LTE radio interface protocol architecture. Each block in Figure 2.2 represents an instance of the respective protocol. SAPs for peer-to-peer communication between MAC and the physical layer provide the transport channels. The SAPs between RLC and the MAC sublayer provide the logical channels. The RLC layer provides three types of SAPs, one for each RLC operation mode (Acknowledgment Mode, Acknowledgment Mode, and Transparent Mode). PDCP is accessed by PDCP SAP. The service provided by layer 2 is referred to as the radio bearer. The Control plane radio bearers, which are provided by RLC to RRC, are denoted as signaling radio bearers.

Also, as shown in the Figure 2.2 there are connections between RRC and MAC as well as RRC and L1 providing local inter-layer control services. An equivalent control interface exists between RRC and the RLC sublayer, between RRC and the PDCP sublayer. These interfaces allow the RRC to control the configuration of the lower layers. For this purpose, separate Control SAPs are defined between RRC and each lower layer (PDCP, RLC, MAC, and L1).

Figure 2.2 - LTE radio Interface protocol architecture.

2.1.2 Physical layer aspects

The physical layer offers information transfer services to MAC and higher layers. It is based on OFDM with a CP in the DL and UL. Half duplex operation is supported

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using TDD. A single component carrier bandwidth of 100MHz is supported. A resource block spans 12 sub-carriers with a sub-carrier bandwidth of 75kHz over a duration of 0.1 ms [10]. The radio frame consists of 50 subframes and has a length of 10 ms. Each subframe has a length of 0.2ms and link direction (DL and UL) for data transmission can be dynamically switched on a subframe basis.

The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface. An adequate term for this is 'Transport Channel'.

A general classification of transport channels is divided into two groups:

- common transport channels: where there is a need for inband identification of the UEs when particular UEs are addressed;

- dedicated transport channels: where the UEs are identified by the physical channel, i.e. code and frequency for FDD and code, time slot and frequency for TDD.

Common transport channel types are the following:

- Random Access Channel (RACH): a contention based UL channel used for relatively small amounts of data transmission e.g. for initial access or non-real-time dedicated control or traffic data.

- Forward Access Channel (FACH): common DL channel without closed-loop power control used for relatively small amount of data transmission. In addition, FACH is used to carry broadcast and multicast data.

- Downlink Shared Channel (DSCH): a DL channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

- Uplink Shared Channel (USCH): an UL channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

- Broadcast Channel (BCH): a DL channel used for system information broadcasting into an entire cell.

- Paging Channel (PCH): a DL channel used for control information broadcasting into an entire cell allowing efficient UE sleep mode procedures.

- High Speed Downlink Shared Channel (HS-DSCH): a DL channel shared between UEs by allocation of individual codes, from a common pool of codes assigned for the channel.

Dedicated transport channel types are:

- Dedicated Channel (DCH): a channel dedicated to one UE used in UL or DL direction.

- Enhanced Dedicated Channel (E-DCH): a channel dedicated to one UE used in UL direction only. The E-DCH is subject to eNB controlled scheduling and HARQ. 5G supports paired and unpaired spectrum and strives to maximize commonality between the technical solutions, allowing FDD operation on a paired spectrum, different transmission directions in either part of a paired spectrum, TDD operation on an unpaired spectrum where the transmission direction of time resources is not dynamically changed,

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and TDD operation on an unpaired spectrum where the transmission direction of most time resources can be dynamically changing [43].

The modulation schemes supported are: QPSK, 16QAM, 64QAM.

5G defines PRB - in frequency-domain, a PRB (or multiple PRBs) is the resource unit size for DL control channel. In UL control channels - PRB (or multiple PRBs) is the minimum resource unit size. In the frequency-domain aspects, a UE monitors for DL control information in one or more “control subband” where a “control subband” is smaller than or equal to the carrier bandwidth (up to a certain limit) and consists of an integer number of RBs/PRBs in the frequency domain. This does not preclude that UE may receive additional control information elsewhere within or outside the control subband in the same or different OFDM symbol(s). UE-specific DL control information monitoring occasions at least in time domain can be configured.

From eNB perspective, DL control channel can be located at the first OFDM symbol(s) in a slot and/or mini-slot.

Physical layer procedures are the following: scheduling, HARQ related procedures, initial access and mobility, synchronization signal and DL broadcast signal/channel structure, timing control, power control, beam acquisition

2.1.3 MAC layer services and functions

MAC protocol performs the following services to upper layers:

- Data transfer. This service provides unacknowledged transfer of MAC SDUs between peer MAC entities. It also provides data segmentation on HS-DSCH but not for other transport channels. Therefore, segmentation/reassembly function should be achieved by upper layer when HS-DSCH is not used and optionally when HS-DSCH is used.

- Reallocation of radio resources and MAC parameters. This service performs on request of RRC execution of radio resource reallocation and change of MAC parameters, i.e. reconfiguration of MAC functions such as change of identity of UE, change of transport format (combination) sets, change of transport channel type. In TDD mode, in addition, the MAC can handle resource allocation autonomously. - Reporting of measurements. Local measurements such as traffic volume and

quality indication are reported to RRC.

The MAC layer provides data transfer services on logical channels. A set of logical channel types is defined for various kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred.

The configuration of logical channel types is depicted in the Figure 2.3.

The MAC operates on the channels defined above; the transport channels are described between MAC and Layer 1 the logical channels are described between MAC and RLC. There is mapping between logical channels and transport channels in DL and UL directions.

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Figure 2.3 - Logical channels structure.

The functions of MAC include:

- Mapping between logical channels and transport channels.

- Selection of appropriate Transport Format for each Transport Channel depending on instantaneous source rate. Given the Transport Format Combination Set assigned by RRC, MAC selects the appropriate transport format within an assigned transport format set for each active transport channel depending on source rate. The control of transport formats ensures efficient use of transport channels.

- Priority handling between data flows of one UE. When selecting between the Transport Format Combinations in the given Transport Format Combination Set, priorities of the data flows to be mapped onto the corresponding Transport Channels can be considered. The priority handling is achieved by selecting a Transport Format Combination for which high priority data is mapped onto L1 with a "high bit rate" Transport Format, at the same time letting lower priority data be mapped with a "low bit rate" (could be zero bit rate) Transport Format. Transport format selection may also take into account transmit power indication from Layer 1 [43].

- Priority handling between UEs by means of dynamic scheduling. In order to utilize the spectrum resources efficiently for burst transfer, a dynamic scheduling function may be applied. MAC realizes priority handling on common transport channels, shared transport channels and for the dedicated E-DCH transport channel.

Traffic Channel

Dedicated Traffic Channel (DTCH)

Common Traffic Channel (CTCH)

MBMS point-to-multipoint Traffic Channel (MTCH) Control Channel

Broadcast Control Channel (BCCH) Paging Control Channel (PCCH) Dedicated Control Channel (DCCH)

Common Control Channel (CCH) Shared Channel Control Channel (SCCH) MBMS point-to-multipoint Control Channel (MCCH) MBMS point-to-multipoint Scheduling Channel (MSCH)

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- Identification of UEs on common transport channels. When a particular UE is addressed on a common DL channel, or when a UE is using the RACH, there is a need for inband identification of the UE. Since the MAC layer handles the access to, and multiplexing onto, the transport channels, the identification functionality is naturally also placed in MAC.

- Multiplexing/demultiplexing of upper layer PDUs (Protocol Data Unit) into/from transport blocks delivered to/from the physical layer on common transport channels. MAC should support service multiplexing for common transport channels, since the physical layer does not support multiplexing of these channels. - Multiplexing/demultiplexing of upper layer PDUs into/from transport block sets delivered to/from the physical layer on dedicated transport channels. The MAC allows service multiplexing for dedicated transport channels. This function can be utilized when several upper layer services can be mapped efficiently on the same transport channel. In this case, the identification of imultiplexing is contained in the MAC protocol control information.

- Multiplexing/demultiplexing of upper layer PDUs into transport blocks delivered to/from the physical layer on HS-DSCH. The MAC allows service multiplexing for HS-DSCH. This function can be utilized to multiplex data from several upper layer services. In this case, the identification of multiplexing is contained in the MAC protocol control information.

- Traffic volume measurement. Measurement of traffic volume on logical channels and reporting to RRC. Based on the reported traffic volume information, RRC performs transport channel switching decisions.

- Transport Channel type switching. Execution of the switching between common and dedicated transport channels based on a switching decision derived by RRC. - Ciphering. This function prevents unauthorized acquisition of data. Ciphering is

performed in the MAC layer for transparent RLC mode.

- Access Service Class selection for RACH transmission. The RACH resources may be divided between different Access Service Classes in order to provide different priorities of RACH usage. Each access service class will also have a set of back-off parameters associated with it, some or all of which may be broadcast by the network. The MAC function applies the appropriate back-off and indicates to the PHY layer the RACH partition associated to a given MAC PDU transfer.

- HARQ functionality for HS-DSCH and E-DCH transmission. This functionality ensures delivery between peer entities by use of the ACK and NACK signaling between the peer entities.

- Data segmentation/re-assembly for HS-DSCH.

- In-sequence delivery and assembly/disassembly of higher layer PDUs on HS-DSCH.

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2.1.4. RLC services and functions

RLC protocol performs the following services to upper layers:

- Transparent data transfer. This service transmits upper layer PDUs without adding any protocol information, possibly including segmentation/reassembly functionality.

- Unacknowledged data transfer. This service transmits upper layer PDUs without guaranteeing delivery to the peer entity. The unacknowledged data transfer mode has the following character-ristics:

- Detection of erroneous data: The RLC sublayer shall deliver only those SDUs to the receiving upper layer that are free of transmission errors by using the sequence-number check function.

- Immediate delivery: The receiving RLC sublayer entity shall deliver a SDU to the upper layer receiving entity as soon as it arrives at the receiver. - Duplication avoidance and reordering: The RLC sublayer shall deliver SDUs

to the receiving upper layer entity in the same order as the transmitting upper layer entity submits them to the RLC sublayer without duplication. SDUs may be delayed by this procedure to ensure in-sequence delivery. - Out-of-sequence SDU delivery: Alternative to immediate delivery, the RLC

sublayer shall deliver SDUs to the upper layer receiving entity as soon as they can be recovered from PDUs, without waiting for earlier insequence SDUs to be recovered.

- Acknowledged data transfer. This service transmits upper layer PDUs and guarantees delivery to the peer entity. In case RLC is unable to deliver the data correctly, the user of RLC at the transmitting side is notified. For this service, both in-sequence and out-of-sequence delivery are supported. In many cases a upper layer protocol can restore the order of its PDUs. As long as the out-of-sequence properties of the lower layer are known and controlled allowing out-of-sequence delivery can save memory space in the receiving RLC. The acknowledged data transfer mode has the following characteristics:

- Error-free delivery: Error-free delivery is ensured by means of retrains-mission. The receiving RLC entity delivers only error-free SDUs to the upper layer.

- Unique delivery: The RLC sublayer shall deliver each SDU only once to the receiving upper layer using duplication detection function.

- In-sequence delivery: RLC sublayer shall provide support for in-order delivery of SDUs, i.e., RLC sublayer should deliver SDUs to the receiving upper layer entity in the same order as the transmitting upper layer entity submits them to the RLC sublayer.

- Out-of-sequence delivery: Alternatively, to in-sequence delivery, it shall also be possible to allow that the receiving RLC entity delivers SDUs to upper layer in different order than submitted to RLC sublayer at the transmitting side.

- QoS maintenance as defined by upper layers. The retransmission protocol shall be configurable by layer 3 to provide different levels of QoS.

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- Notification of unrecoverable errors. RLC notifies the upper layer of errors that cannot be resolved by RLC itself by normal exception handling procedures. RLC protocol performs the following functions:

- Segmentation and reassembly. This function performs segmentation/reassembly of variable-length upper layer PDUs into/from smaller RLC PDUs. The RLC PDU size is either fixed or flexible, depending on the configuration. Flexible RLC PDU size is only supported when mapped on HS-DSCH.

- Concatenation. If the contents of an RLC SDU cannot be carried by one RLC PDU, the first segment of the next RLC SDU may be put into the RLC PDU in concatenation with the last segment of the previous RLC SDU.

- Padding. When concatenation is not applicable and the remaining data to be transmitted does not fill an entire RLC PDU of given size, the remainder of the data field shall be filled with padding bits.

- Transfer of user data. This function is used for conveyance of data between users of RLC services. RLC supports acknowledged, unacknowledged and transparent data transfer. QoS setting controls transfer of user data.

- Error correction. This function provides error correction by retransmission (e.g. Selective Repeat, Go Back N, or a Stop-and-Wait ARQ) in acknowledged data transfer mode.

- In-sequence delivery of upper layer PDUs. This function preserves the order of upper layer PDUs that were submitted for transfer by RLC using the acknowledged data transfer service. If this function is not used, out of sequence delivery is provided.

- Duplicate Detection. This function detects duplicated received RLC PDUs and ensures that the resultant upper layer PDU is delivered only once to the upper layer.

- Flow control. This function allows an RLC receiver to control the rate at which the peer RLC transmitting entity may send information.

- Sequence number check. This function is used in unacknowledged mode and guarantees the integrity of reassembled PDUs and provides a mechanism for the detection of corrupted RLC SDUs through checking sequence number in RLC PDUs when they are reassembled into a RLC SDU. A corrupted RLC SDU will be discarded.

- Protocol error detection and recovery. This function detects and recovers from errors in the operation of the RLC protocol.

- Ciphering. This function prevents unauthorized acquisition of data. Ciphering is performed in RLC layer for non-transparent RLC mode.

- SDU discard. This function allows an RLC transmitter to discharge RLC SDU from the buffer.

2.1.5. PDCP services and functions

PDCP protocol performs the following services to upper layers:

- Header compression and decompression. Header compression and decompression of IP data streams (e.g., TCP/IP and RTP/UDP/IP headers) at the

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transmitting and receiving entity, respectively. The header compression method is specific to the particular network layer, transport layer or upper layer protocol combinations e.g. TCP/IP and RTP/UDP/IP.

- User data transfer. Transmission of user data means that PDCP receives PDCP SDU from the NAS and forwards it to the RLC layer and vice versa.

- Support for lossless SRNS relocation or lossless DL RLC PDU size change. Maintenance of PDCP sequence numbers for radio bearers that are configured to support lossless SRNS relocation or lossless DL RLC PDU size change.

2.1.6 Data flows through layer 2

Data flows through layer 2 are characterized by the applied data transfer modes on RLC (acknowledged, unacknowledged and transparent transmission) in combination with the data transfer type on MAC, i.e. whether a MAC header is required or not. The case where no MAC header is required is referred to as "transparent" MAC transmission.

According to the [46] a MAC PDU is a bit string that is byte aligned (i.e. multiple of 8 bits) in length. MAC SDUs are bit strings that are also byte aligned (i.e. multiple of 8 bits) in length. An SDU is included into a MAC PDU from the first bit onward. A MAC PDU header consists of one or more MAC PDU subheaders; each subheader corresponds to either a MAC SDU, a MAC control element or padding. The content and the size of the MAC header depends on the type of the logical channel, and in some cases none of the parameters in the MAC header are needed. Padding occurs at the end of the MAC PDU, except when single-byte or two-byte padding is required. Padding may have any value and the MAC entity shall ignore it. When padding is performed at the end of the MAC PDU, zero or more padding bytes are allowed.

Acknowledged and unacknowledged RLC transmissions both require a RLC header. In unacknowledged transmission, only one type of unacknowledged data PDU is exchanged between peer RLC entities. In acknowledged transmission, both (acknowledged) data PDUs and control PDUs are exchanged between peer RLC entities. The resulting data flow case is illustrated in the Figures 2.4. Differences between acknowledged and unacknowledged RLC transmission are not visible. Acknowledged and unacknowledged RLC transmission is shown as one case, referred to as non-transparent RLC. At the bottom are radio frames. A full frame is 10 ms but we normally think in terms of the 1-ms subframe, which is the entity that contains the transport block. Within the transport block is the MAC header and any extra space (padding). Within that there is the RLC header, then within the RLC header there can be a number of PDCPs that are concatenated. There is a somewhat arbitrary relationship between the IP packets coming in, which form the SDUs (service data units), and how the RLC PDUs are formed.

A transport block is a group of resource blocks with a common modulation/coding. The physical interface is the transport block, which corresponds to the data carried in a time period of the allocation for the particular UE. Each radio subframe is 1 ms long; each frame is 10 ms.

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Figure 2.4 - Data flow for PDCP, RLC and MAC protocols.

2.1.7 Layer 3 functions

The RRC layer handles the control plane signaling of Layer 3 between the UEs and RAN. The resulting data flow case is shown in the Figures 2.5.

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The RRC performs the following functions:

- Broadcast of information provided by the NAS to CN. The RRC layer performs system information broadcasting from the network to all UEs. The system information is normally repeated on a regular basis. The RRC layer performs the scheduling, segmentation and repetition. This function supports broadcast of higher layer (above RRC) information. This information may be cell specific or not. - Establishment, reestablishment, maintenance and release of an RRC connection

between the UE and RAN. The establishment of an RRC connection is initiated by a request from higher layers at the UE side to establish the first Signaling Connection for the UE. The establishment of an RRC connection includes an optional cell re-selection, an admission control, and a layer 2 signaling link establishment. The release of an RRC connection can be initiated by a request from higher layers to release the last Signaling Connection for the UE or by the RRC layer itself in case of RRC connection failure. In case of connection loss, the UE requests reestablishment of the RRC connection. In case of RRC connection failure, RRC releases resources associated with the RRC connection.

- Establishment, reconfiguration and release of Radio Bearers. The RRC layer can, on request from higher layers, perform the establishment, reconfiguration and release of Radio Bearers in the user plane. A number of Radio Bearers can be established to an UE at the same time. At establishment and reconfiguration, the RRC layer performs admission control and selects parameters describing the Radio Bearer processing in layer 2 and layer 1, based on information from higher layers.

- Assignment, reconfiguration and release of radio resources for the RRC connection. The RRC layer handles the assignment of radio resources (e.g. codes) needed for the RRC connection including needs from both the control and user plane. The RRC layer may reconfigure radio resources during an established RRC connection. This function includes coordination of the radio resource allocation between multiple radio bearers related to the same RRC connection. RRC controls the radio resources in the UL and DL such that UE and RAN can communicate using unbalanced radio resources (asymmetric UL and DL). RRC signals to the UE to indicate resource allocations for purposes of handover to GSM or other radio systems.

- RRC connection mobility functions. The RRC layer performs evaluation, decision and execution related to RRC connection mobility during an established RRC connection, such as hand-over, preparation of handover to GSM or other systems, cell re-selection and cell/paging area update procedures, based on e.g. measurements done by the UE.

- Paging/notification. The RRC layer can broadcast paging information from the network to selected UEs. Higher layers on the network side can request paging and notification. The RRC layer can also initiate paging during an established RRC connection.

- Routing of higher layer PDUs. This function performs at the UE side routing of higher layer PDUs to the correct higher layer entity, at the UTRAN side to the correct RANAP entity.

Impact of eNB functional splits on the 5G fronthaul performance: an experimental evaluation.