CC BY
0THE FRONTHAUL NETWORK IN 4G/5G SYSTEMS BASED ON OPTICAL COMMUNICATION SYSTEMS
Sultonova M.O.
Tashkent university of information technologies named after Muhammad al-Khwarizmi
https://doi.org/10.5281/zenodo.13956663
Abstract. This article discusses the key factors driving the increase in data transmission speeds in telecommunication networks, including modern mobile communication networks. It highlights the importance of utilizing the latest technologies for enhancing data transfer rates with the development of 4G/5G networks. Among the main issues addressed there are the organization of transport channels for radio access networks, meeting the growing traffic demands of users by providing competitive solutions, effectively replacing energy-intensive coaxial systems in cell towers with optical systems, and the superior scalability and construction speed of optical infrastructure compared to coaxial systems.
Keywords: optical communication system, subscriber, bandwidth, telecommunications, networks, radio transmission block, high frequency, interfaces.
The fourth generation of mobile communication networks (based on LTE technology) has led to a significant leap in data transmission capabilities, providing users with full functional internet connectivity while in motion. In LTE technology, the bandwidth is approximately 20 Mbps, while in LTE-Advanced technology, the theoretical data transmission speed can reach Gbps. To achieve such bandwidth, 256 QAM (Quadrature Amplitude Modulation) is used, along with a 10 spatial stream configuration featuring two levels of low-frequency carriers and two levels of high-frequency carriers in a 4x4 MIMO (Multiple Input Multiple Output) setup. Additionally, it includes the possibility of carrier aggregation or the combination of multiple 20 Mbps channels.
4G networks currently do not meet the modern demands of mobile communication subscribers for various services [1]. Moreover, 4G mobile communication networks lack sufficient flexibility, are relatively complex, and require significant operational costs. Of course, new fifth-generation (5G) networks are being built to compensate for these shortcomings (see Figure 1).
Specifically, the new 5G/IMT-2020 technologies, which are a continuation of the enhancement of mobile communication systems, are being implemented [2]. The International Telecommunication Union (ITU) considers the following indicators as key metrics for 5G networks:
- the network capacity for a subscriber should reach up to 10 Gbps in the downlink (DL) and 5 Gbps in the uplink (UL).
- the monthly data consumption per subscriber should be able to reach 500 GB.
- the number of active subscriber devices in a cell should reach 300,000, and up to 1 million per square kilometer.
- transmission delays in the network must not exceed 1 ms.
- the spectral efficiency of the radio interface should increase by three times.
- the operational lifetime of subscriber terminals without recharging should significantly increase, and energy consumption should be low.
- the operational costs and energy consumption of 5G networks should be reduced by a factor of ten compared to 4G networks.
Figure 1. Architecture of 4G/5G Networks MME/S-GW (Mobility Management Entity/ Serving Gateway) - mobility management block/serving gateway
eNB (eNodeB, EvolvedNodeB) - LTE network base station
AMF/UPF (Access and Mobility Management Function/ User Plane Function) -connection and mobility management function/user data transmission function gNB (gNodeB) - 5G network base station S1, X2 - interfaces
Today, increasing data transmission speeds is one of the key factors in the development of telecommunication networks, including modern mobile communication networks.
Table 1 presents the simplified architectures of the fourth and fifth generations. These architectures meet the ITU-R requirements listed in the table.
To achieve such performance indicators, it is necessary to ensure a channel bandwidth of at least 200 MHz with transmission speeds reaching dozens of gigabits per second.
Table 1
Mobile communication standard LTE-advanced 5G NR
Advantages Increased capacity, ip-based network, multimedia support, speeds up to hundreds of mbps Speeds over 1 gbps, improved energy efficiency
Data transmission speed 100 Mbps - 1 Gbps 1 Gbps; 6.5 Gbps
Base station operational radius From 0.5 to 3 km From a few hundred meters to 1-2 km
The ITU recommendations emphasize that with the development of 5G networks, the latest technologies are increasingly being utilized to enhance data transmission speeds. One of the main challenges will be establishing transport channels for radio access networks [3].
Mobile operators can meet the growing traffic demands of users by offering more new and competitive solutions. One solution to this problem is replacing the energy-intensive coaxial systems in mobile communication towers with efficient optical systems. Optical infrastructure provides better scalability and construction speed compared to coaxial systems [4].
Initially, mobile communication towers utilized long copper cables to connect the receiving/transmitting blocks located at the base of the tower to the antenna systems positioned at the top. This construction of base station architecture has several drawbacks, particularly the need for a sufficiently large room equipped with appropriate power and climate control systems.
The modern systems for constructing base stations differ fundamentally from their predecessors in that the radio transmission unit is located next to the antenna, with plans for future integration directly into the antenna itself. Additionally, this architecture allows for the Remote Radio Head (RRH) and Baseband Unit (BBU) to be separated by considerable distances, facilitating the implementation of cloud radio systems (cloud-RAN or C-RAN). In contemporary mobile communication towers, optical cables are used instead of coaxial cables, which allow for reduced noise during data transmission while also offering low energy consumption and high bandwidth [5].
One of the key features of 5G mobile communication systems is that the Base Band Unit (BBU) control block is located in the cloud or data center. This setup consists of multiple BBUs that have significant computing and storage capabilities. BBUs handle resource processing and dynamically allocate these resources to various RRUs based on the existing network needs.
The Remote Radio Head (RRH) is a wireless network component that connects wireless devices such as towers or access points in traditional cellular networks.
The fronthaul, or transport network, serves as the connection layer between the Baseband Units (BBUs) and the RRHs, providing high-capacity channels to meet the demands of multiple RRHs. Fronthaul can be implemented using various technologies such as optical fiber communication, wired connections, or millimeter-wave communication. Optical fiber is ideally suited for C-RAN architecture, as it meets the requirements for maximum bandwidth.
A transport channel is envisioned to establish the interaction between the RRH and the BBU, utilizing the continuously evolving and improving Common Public Radio Interface (CPRI) communication interface. According to specifications accepted for fifth-generation mobile communication networks, this interface is designated as eCPRI. The use of the eCPRI interface enhances the efficiency of base stations, complying with the requirements set forth in the 3GPP standards.
The eCPRI specification offers significant flexibility for functional separation of base station resources compared to the CPRI interface, although it does not provide the reverse flexibility of CPRI. The eCPRI architecture divides the functions of the base station into two primary blocks—eREC (eCPRI Radio Equipment Control) and eRE (eCPRI Radio Equipment)— which are interconnected with the operator's transport network. Physically, the eREC block can be installed next to the antenna, while the eRE block can be located in a telecommunications cabinet or data center. Regardless of the distribution of functions between the eREC and eRE blocks, the transport networks connecting 4G/5G base stations (eNB/gNB) are referred to as Fronthaul networks (FH) [6].
Mobile communication operators can utilize various solutions based on specific advanced technologies for organizing transport networks. The selection of such solutions is one of the key factors determining the reliability, quality, and economic efficiency of the mobile communication system being built.
In general, the transport networks of a communication operator serve to connect individual mobile communication nodes to central stations that have access to data processing centers where
the required content is located. These networks are designated as xHaul and consist of Fronthaul, Midhaul, and Backhaul segments. One of the main criteria for these networks is bandwidth, along with requirements for the distances between xHaul network nodes, as depicted in Figure 2.
Figure 2. Requirements for the xHaul Network Midhaul: The midhaul is an intermediate network that supports the interoperability between the central and distributed blocks of the 5G network base station.
Backhaul: The backhaul encompasses all resources that perform transport functions within telecommunications networks. It includes not only transmission systems but also relevant management tools, operational switching, redundancy, and control. In a cellular network, the backhaul consists of the network segment between the operator's core network and the base station.
The descriptions of the interfaces for fronthaul, midhaul, and backhaul networks are presented in Table 2. In 5G networks, the separation of data processing centers into local and remote centers is anticipated, which means that existing transport networks from 4G cannot be utilized without modernization to fully leverage the new network's performance [7].
xHaul transport networks can be built on specific technologies based on optical fiber, wireless, and wired communication systems. The selection of solutions is carried out according to the following parameters: Bandwidth Resilience to outages Recovery speed Scalability Transparency Agility
Manageability
Table 2
Descriptions of Fronthaul, Midhaul, and Backhaul Network Interfaces
Network Type Interface Description
Fronthaul Radio transmission block network and the interface between the distributed block.
Midhaul Logical interface between the central and distributed blocks
Backhaul Logical interface between the 5G base station and the 5G core network.
According to the working document approved by the ITU for 5G, the implementation of the fronthaul network can be achieved using various channels, including high-speed wireless channels.
In urban environments, constructing wired communication channels poses certain challenges and may sometimes be technically or otherwise infeasible. Therefore, 5G networks also utilize wireless communication lines in the optimal E-band (60 GHz to 90 GHz frequency range). However, the
maximum connection speed in wireless networks operating in the E-band does not exceed 6 Gbit/s [7]. Consequently, one of the options for the fronthaul transport network in the initial phase of implementing 5G networks is to use atmospheric optical communication systems. Currently, the transmission speed in atmospheric optical communication systems can reach up to 10 Gbit/s with sufficient reliability over distances of up to 5-6 km. Additionally, these systems can operate in frequency ranges above 400 GHz and do not require licenses for frequency usage, giving them advantages over other wireless systems.
The fronthaul transport network in 4G networks connects centralized blocks for generating modulation signals from distributed radio blocks, while the backhaul network is intended to establish a reverse channel for linking BBU blocks to the 4G Evolved Packet Core (EPC). In 5G networks, the receivers/transmitters (AAU, Active Antenna Unit) are directly connected to the BBU blocks, which can be divided into centralized (CU) and distributed (DU) blocks.
A new midhaul network with the standardized 3GPP F1 interface is envisioned to connect the CU block to the DU block. Initially, operators aimed to provide eMBB services through a 5G backhaul network that resembles the 4G network, but due to the high efficiency and capacity of the new 5G (AAU) receivers/transmitters, it offers significantly greater bandwidth [8].
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