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An overview of the 5G mobile network architecture

This kind of tightly bound network functions would increase the Operations and Management costs. This would require 5G to be more flexible and customizable control network functions. The type of applications range from Traditional Voice/Data applications, 3D Video, Cloud-based applications, Smart Home and Smart Building applications, Smart City applications, Industrial Automation and mission-critical applications to self-driving vehicles.

Some of the key technical requirements are: (i) Bandwidth requirement of at least 100 MHz, (ii) Bandwidth up to 1 GHz for frequencies above 6 GHz; (iii) Peak downlink and uplink data rates of at least 20 Gbps and 10 Gbps respectively; (iv) low-latency of the order of milli-seconds; (v) support for up to 500 Kmph user mobility on high-speed trains; (vi) Downlink and peak spectral efficiency of at least 30 and 15 bit/s/Hz respectively; (vii) Target downlink and uplink “user experienced data rate” in Dense Urban scenarios of 100 and 50 Mbps respectively; and (viii) control plane latency under 20 ms, with 10 ms preferred.

In response, the 3GPP 5G standards (Release-15, planned in late 2018) defines two frequency bands: below 6 GHz and milli-metre wave (24-86 GHz). The important components and layers of the 5G Wireless network architecture, that will help realize the 5G network, include the Radio Access Network (RAN) and the Evolved Packet Core (EPC).

B. Radio Access Network (RAN)

The RAN component consists of the following layers:

1)  The Physical (PHY) Layer, that deals with communications over the wireless channel. This channel is used between the user equipment (UE) and the remote radio head (RRH) unit on the enodeB or gNodeB, i.e. the base station. The RRH unit is in turn connected to the Baseband unit (BBU) that processes the baseband signals with the help of a digital signal processor (DSP) for traffic to and from the UE. The PHY layer radio access technology network is referred to as New Radio (NR) in 5G networks.

The main functionalities include symbol transmission on the channel and related ones such as signaling, channel quality measurements and support for hybrid automatic repeat request (ARQ).

2) The medium access control layer (MAC) resides above the PHY layer and utilizes the latter’s services. The MAC layer’s primary functionalities include logical-to-transport channel mapping, multiplexing, de-multiplexing, and error control.

3) The radio link control (RLC) layer functionalities include radio resource allocation using scheduling algo-rithms and data transfer.

Essentially, the RAN takes care of sending the UE’s packets to and from the EPC. The EPC then forwards the packets to the external Internet and vice-versa. This paper does not deal with the RAN aspects, except for the Next Generation RAN (NG-RAN) aspects described in Section III. The reader is referred to [1] for additional details about the PHY/MAC/RLC layers.

C. 5G Evolved Packet Core

With current heterogeneity of radio technologies, multi dimensional services [4] and considering the current softwarization and data-plane innovations [5] a new core network was defined, which would support known and unknown use cases for 5G. This new core network is called Next Generation Core (NGC). The 3GPP Release-15 document specifies the service-based representation as shown in Figure 1. This is referred to as the Service-Based Architecture (SBA) and is to be adopted in 5G systems.

The interactions from front end to the NGC is represented in two ways: (i) Interactions among the control plane network functions within the core; (ii) Interactions between Radio Access Network (RAN) Network Functions and 5GC Network Functions. Network Functions within the 5G control plane (CP) will use service-based interfaces for their interactions. The User Plane (UP) functions and radio interactions shall continue to use the reference point interfaces.

The 5G standard also includes the concepts Control Plane/User Plane (CP/UP) separation (CUPS) and Network Slicing (NS). Network slicing discussed in Section V in more detail.

The functional architecture of 5G core is flexibly designed with intentions to adopt any implementation changes. The entities in the 5G core network within the control plane enables cross-domain interactions allowing other authorized network functions to access their services. This structure would allow custom based communications or leverage communication with HTTP based APIs, replacing protocols such as Diameter [6]. The 5G SBA would create an independence with NR, allowing standalone mode without dependency on legacy networks [7]. The slicing approach would form the custom QoE models for UE allowing different cost models also allow services like single UE simultaneously connecting to multiple services over multiple slices with optimized access and mobility signaling. The mobility management can be made custom to allow different range options to the users, also allowing a demand based mobility, which led a new service direction called Mobility Management as a Service (MMaaS) [8].

A service requirement in 5G network architecture is defined as a set of independent interacting functional units (that are referred to as Network Functions (NF)). Each NF can service several dimensions of service requirements if they share some functional part in common. This requires a separate demarcation at NF to distinguish different flows. For such management and operations entities are defined in 5G Core to make the operations plain sailing. This kind of abstraction ensure potential upgrades or addition of new NF would not effect the existing network services. This network services or NF can be virtualized which we call as Virtual Network Function (VNF) and managed by a service called Network Function Virtualization (NFV) [9], which would help implement the OAM of VNFs.

The 5G core is defined with several functional entities, each functional entity is a set of several NFs. The primary network functions are discussed in Section IV. The typical 5G architecture divides into three parts: Front-haul, Mid-Haul, and 5G-Core. The evolution of RAN discussed in Section III has created a mid-haul entity to the previous 4G mobile networks.

 

III. Evolution to next Generation RAN (NG-RAN)

 
This section describes the basic concepts of Cloud Radio Access Network (C-RAN) and the evolution to next generation RAN (NG-RAN).

A.  Cloud-RAN

In early LTE cellular systems, the baseband processing is performed at the radio front-end i.e., near the cellular site. In such architecture, there is difficulty in managing distributed sites and huge operational cost is involved in maintaining the baseband units(BBUs) at each cellular site.

Using software-defined radios, it is possible to implement BBU functions on programmable platforms. Thus, the frontend can be split by moving all the high-level services of baseband to a central pool unit called the Cloud-RAN (C-RAN) [2]. These software-based baseband units executed on a cloud system provide necessary services to the Remote Radio Heads (RRHs) that are deployed at the cell site as shown in [10], [11]. This has generated wide interest both from industry and academia [12]–[16].

However, C-RAN does have its drawbacks. High Fronthaul capacities are needed to address higher bandwidth requirements between BBUs (Base-Band Units) and RRHs. Optical links can be used to address this need, but leads to increased capital expenditures (CAPEX). Also, BBU cooperation is needed to share user data. This requires significant architectural changes to the existing. Further, virtualized interfaces are required to enable unified interactions among heterogeneous networks.

B. Evolution from Cloud-RAN to NG-RAN

A large number of operators are now evaluating Next-Generation RAN (NG-RAN) as a way to meet future service requirements. From the initial days of creating C-RAN, which was business oriented to save operational costs, the architecture has now evolved to meet the future requirement in NG-RAN.

Functional split was one of the topics studied. In FluidNet [17], re-configurable fronthaul that can flexibly support one-to-one and one-to-many logical mappings between Base Band Units (BBUs) and Radio Resource Heads (RRHs) was considered. An optimal functional split is discussed in [18]. Various options for the RAN and its interfaces to the core are described in [7].

eq17Figure 2: Split of C-RAN and NG-RAN

In the NG-RAN architecture, real-time (RT) functions are deployed near the antenna site to manage air interface resources. Also, non-real-time (NRT) control functions are hosted centrally to coordinate transmissions across the coverage area. This functional split concept is shown in Figure 2, that shows a Central Unit (CU) and Distributed Unit (DU). This functional architecture is now native to the 3GPP specification [19].

Implementation of the NGRAN architecture and the subsequent deployment in the network depends on the functional split between distributed radio and centralized control, these we call as DU and CU split. The DU will process low level radio protocol and real-time services while the CU will process non-real-time radio protocols. 3GPP has recognized eight different split options, discussed in [7], of these the most widely discussed two splits, still in debate is discussed in 3GPP status meeting [20].

The different splits is shown in Figure 2. The services of CU and DU can be virtualized and put in Commercial off-the-shelf (COTS) servers, these virtualized network nodes or Virtual Network Functions (VNFs) can be realized with a network architectural concept called Network Function Virtualization (NFV) [21]. NFV offers a new way to design, deploy and manage virtual network nodes. It also enables us to decouple suppliers hardware and software business models, opening new innovations and opportunities for SW integrators. The managing and operational aspects of NG-RAN with CU and DU splits would be plain sailing with NFV. There are several research papers which already attempted in virtualizing mobile network functions [5], [9], [22].

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