Thursday 26 November 2020

NTT Docomo's 5G RAN Infrastructure

We looked at NTT Docomo's 5G Journey and 5G Network Deployment details recently here. In this post we will look at the 5G Infrastructure that Docomo is using in their network. It is detailed in their latest Technical Journal here. In this post we will look at the infrastructure part only.


The 5G network configuration is shown in Figure 4. With a view to 5G service development, NTT DOCOMO developed a Central Unit (CU) that consolidates the Base Band (BB) signal processing section supporting 5G, extended existing BB processing equipment known as high-density Base station Digital processing Equipment (BDE), and developed a 5G Radio Unit (RU) having signal transmit / receive functions. Furthermore, to have a single CU accommodate many RUs, NTT DOCOMO developed a 5G version of the FrontHaul Multiplexer (FHM) deployed in LTE. Each of these three types of equipment is described below.

1) CU
(a) Development concept: With the aim of achieving a smooth rollout of 5G services, NTT DOCOMO developed a CU that enables area construction without having to replace existing equipment while minimizing the construction period and facility investment. This was accomplished by making maximum use of the existing high-density BDE that performs BB signal processing, replacing some of the cards of the high-density BDE, and upgrading the software to support 5G.

(b) CU basic specifications: An external view of this CU is shown in Photo 1. This equipment has the features described below (Table 3). As described above, this equipment enables 5G-supporting functions by replacing some of the cards of the existing high-density BDE. In addition, future software upgrades will load both software supporting conventional 3G/LTE/LTE-Advanced and software supporting 5G. This will enable the construction of a network supporting three generations of mobile communications from 3G to 5G with a single CU.

The existing LTE-Advanced system employs advanced Centralized RAN (C-RAN) architecture proposed by NTT DOCOMO. This architecture is also supported in 5G with the connection between CU and RUs made via the fronthaul. Standardization of this fronthaul was promoted at the Open RAN (O-RAN) Alliance jointly established in February 2018 by five operators including NTT DOCOMO.  Since the launch of 5G services, the fronthaul in the NTT DOCOMO network was made to conform to these O-RAN fronthaul specifications that enable interoperability between different vendors, and any CU and RU that conform to these specifications can be interconnected regardless of vendor. The specifications for inter-connecting base-station equipment also con-form to these O-RAN specifications, which means that a multi-vendor connection can be made between a CU supporting 5G and a high-density BDE supporting LTE-Advanced. This enables NTT DOCOMO to deploy a CU regardless of the vendor of the existing high-density BDE and to quickly and flexibly roll out service areas where needed while making best use of existing assets. In addition, six or more fronthaul connections can be made per CU and the destination RU of each fronthaul connection can be se-lected. Since 5G supports wideband trans-mission beyond that of LTE-Advanced, the fronthaul transmission rate has been extend-ed from the existing peak rate of 9.8 Gbps to a peak rate of 25 Gbps while achieving a CU/RU optical distance equivalent to that of the existing high-density BDE.

2) RU
(a) Development concept: To facilitate flexible area construction right from the launch of 5G services, NTT DOCOMO developed the low-power Small Radio Unit (SRU) as the RU for small cells and developed, in particular, separate SRUs for each of the 3.7 GHz, 4.5 GHz, and 28 GHz frequency bands provided at the launch of the 5G pre-commercial service in September 2019. Furthermore, with an eye to early expansion of the 5G service area, NTT DOCOMO developed the Regular power Radio Unit (RRU) as the RU for macrocells to enable the efficient creation of service areas in suburbs and elsewhere.

A key 5G function is beamforming that aims to reduce interference with other cells and thereby improve the user’s quality of experience. To support this function, NTT DOCOMO developed a unit that integrates the antenna and 5G radio section (antenna-integrated RU). It also developed a unit that separates the antenna and 5G radio section (antenna-separated RU) to enable an RU to be placed alongside existing 3G/LTE/LTE-Advanced Radio Equipment (RE) and facilitate flexible installation even for locations with limited space or other constraints.

(b) SRU basic specifications: As described above, NTT DOCOMO developed the SRU to enable flexible construction of 5G service areas. It developed, in particular, antenna-integrated SRUs to support each of the 3.7 GHz, 4.5 GHz, and 28 GHz frequency bands provided at the launch of the 5G pre-commercial service and antenna-separated SRUs to support each of the 3.7 GHz and 4.5 GHz frequency bands (Photo 2). These two types of SRUs have the following features (Table 4).

The antenna-integrated RU is equipped with an antenna panel to implement the beamforming function. In the 3.7 GHz and 4.5 GHz bands, specifications call for a maximum of 8 beams, and in the 28 GHz band, for a maximum of 64 beams. An area may be formed with the number of transmit/receive beams tailored to the TDD Config used by NTT DOCOMO. In addition, the number of transmit/receive branches is 4 for the 3.7 GHz and 4.5 GHz bands and 2 for the 28 GHz band, and MIMO transmission/reception can be performed with a maximum of 4 layers for the former bands and a maximum of 2 layers for the latter band.

The antenna-separated SRU is configured with only the radio as in conventional RE to save space and facilitate installation. With this type of SRU, the antenna may be installed at a different location. Moreover, compared to the antenna-integrated SRU operating in the same frequency band, the antenna-separated SRU reduces equipment volume to 6.5ℓ or less. The antenna-separated SRU does not support the beamforming function, but features four transmit/receive branches the same as the antenna-integrated SRU for the same frequency band.

(c) RRU basic specifications: The RRU was developed in conjunction with the 5G service rollout as high-power equipment compared with the SRU with a view to early expansion of the 5G service area (Photo 3). This type of equipment has the following features (Table 5).


Compared with existing Remote Radio Equipment (RRE) for macrocells, the volume of RRU equipment tends to be larger to support 5G broadband, but in view of the latest electronic device trends, NTT DOCOMO took the lead in developing and deploying an antenna-separated RRU that could save space and reduce weight. Maximum transmission power is 36.3 W/100 MHz/branch taking the radius of a macrocell area into account. The RRU features four transmit/receive branches and achieves the same number of MIMO transmission/reception layers as the antenna-separated SRU.
NTT DOCOMO also plans to deploy an antenna-integrated RRU at a later date. The plan here is to construct 5G service areas in a flexible manner making best use of each of these models while taking installation location and other factors into account.

3) 5G FHM
The 5G FHM is equipment having a multiplexing function for splitting and combining a maximum of 12 radio signals on the fronthaul. It was developed in conjunction with the 5G service rollout the same as RRU (Photo 4).
If no 5G FHM is being used, each RU is accommodated as one cell, but when using a 5G FHM, a maximum of 12 RUs can be accommodated as one cell in a CU. At the launch of 5G services, this meant that more RUs could be accommodated in a single CU when forming a service area in a location having low required radio capacity (Figure 5). Additionally, since all RUs transmit and receive radio signals of the same cell, the 5G FHM can inhibit inter-RU interference and the occurrence of Hand-Over (HO) control between RUs as in the conventional FHM. Furthermore, the 5G FHM supports all of the 5G frequency bands, that is, the 3.7 GHz, 4.5 GHz, and 28 GHz bands, which means that service areas can be constructed in a flexible manner applying each of these frequency bands as needed.

All the fronthaul and other interfaces that Docomo used in their network was based on O-RAN alliance specifications. In a future post, we will look at some of the details.

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