Analysis on the development trend of indoor distribution system in the 5G era

With the development of mobile Internet communication technology and user needs, more and more mobile services are taking place indoors. As 5G commercialization approaches, it is particularly important to build a high-quality indoor distribution network.

In the traditional 2G/3G era, mobile communications work in low frequency bands. Using outdoor macro base station signals to cover indoor areas and building traditional indoor distributed antenna systems (DAS) are both effective solutions to indoor coverage. In the 4G era, traditional DAS is still the main means to solve indoor coverage. With the development of mobile Internet and Internet of Things technologies, some business-intensive scenarios (such as transportation hubs, large venues, etc.) have begun to introduce new digital indoor distributed systems. Compared with traditional DAS, the new digital indoor distributed system has the advantages of simple deployment, low construction coordination difficulty, flexible expansion, and visible operation and maintenance. It can greatly increase network capacity and improve network operation and maintenance efficiency.

In the 5G era, DAS can only be used in some low-frequency and low-capacity scenarios (such as tunnels, underground parking lots, elevators, etc.) because its passive devices and feeders do not support high frequency bands, cannot monitor devices, have high transformation costs, and cannot be expanded on a large scale. However, the new digital indoor distribution system is more suitable for a smooth transition to 5G due to its advantages such as simple deployment, visualized operation and maintenance, and support for large-scale MIMO, and will be the protagonist of the 5G indoor distribution system.

Analysis of the current situation of mobile communication indoor distribution system

At present, domestic operators still mainly use traditional DAS to solve indoor coverage problems. Traditional DAS mainly uses passive devices, has a mature industrial chain, and has the advantages of small investment, low failure rate, simple and effective system, and can be expanded by combining multiple systems in the future. However, with the changes in mobile services, traditional DAS faces huge challenges.

• First, the engineering construction is difficult and upgrading is difficult. The traditional indoor distributed system requires the deployment of a large number of passive components, which is difficult to construct, has many nodes, many potential faults, and is difficult to coordinate with property management. When the LTE indoor distributed system is dual-channel, many new nodes are built, and some areas may have no room for transformation. At the same time, due to different degrees of device aging and different construction processes, it is difficult to ensure the balance of LTE dual channels, let alone support large-scale MIMO.
• Second, troubleshooting is difficult. The installation and maintenance of indoor distribution systems require coordination with the property management. Traditional indoor distribution devices are numerous, and passive devices cannot be monitored. Problems are usually discovered through complaints or inspections. For large indoor distribution systems, it is difficult to inspect every antenna end during inspections, especially for concealed indoor distribution systems, where it is even more difficult to discover problems. At the same time, due to the excessive number of connection points, building renovations, and untimely updates of drawings, troubleshooting of fault points often requires more manpower and material resources, which directly increases network operation and maintenance costs.
• Third, current devices do not support high frequency bands. The maximum frequency band supported by current traditional DAS passive devices is mostly around 2.7GHz, and it is basically unusable for 3.5GHz and above. The transmission loss of coaxial cable increases significantly with the increase of frequency band. In the 3.5GHz and above frequency bands, the loss per 100 meters is basically unusable in engineering. See the table for specific losses.

Due to the shortcomings of traditional DAS, some manufacturers have launched M-DAS (fiber distribution system), which is conducive to visual operation and maintenance. However, the fiber distribution system still needs to connect to RRU as a signal source, and the nature of its repeater relay remains unchanged, which is not conducive to capacity expansion and smooth evolution of the system.

Introduction and development of new distributed room distribution system

In response to the huge bottlenecks faced by traditional DAS and M-DAS, various manufacturers have launched new digital indoor distribution systems, such as Huawei's Lampsite, ZTE's QCELL, Ericsson's Radio DOT, Nokia's FlexiZone, etc. Compared with traditional DAS, the new digital indoor distribution system has the advantages of simple engineering implementation, visual operation and maintenance, multi-channel MIMO, easy expansion and evolution, etc., and major operators have deployed them in large and medium-sized scenarios. Practice has proved that the unit area traffic of LTE new digital indoor distribution is 4 to 10 times that of traditional DAS.

With the rapid development of mobile Internet, more and more new mobile services such as high-definition video, indoor positioning, AR/VR, etc. have put forward rigid requirements for large bandwidth, high capacity, and low latency on the network. This requires that the indoor distributed system should have the characteristics of simple construction, easy coordination, smooth upgrade, visible operation and maintenance, and intelligent network management. In this regard, the new digital indoor distributed system has obvious advantages.

First, the project implementation is simple and can be smoothly upgraded. In the 5G era, high frequency bands such as 3.5GHz and 4.9GHz are mainly used. Affected by this, the number of indoor distributed system terminals will increase massively, which requires that 5G indoor distributed equipment should be small in size, light in weight, simple to install, and quickly deployed. The new digital indoor distributed system adopts a simple three-level architecture, with base station side (similar to BBU), Hub, and remote radio frequency transmission unit, with fewer connection points, reducing the hidden dangers of failure; network cables and optical fibers replace traditional coaxial cables. The network cables serve as transmission media while also powering the remote radio frequency unit. The network cables are light in weight and easier to construct and deploy. According to calculations, the construction period is shortened to 1/5~1/3 compared with traditional DAS. At the same time, it also reduces visual impact and makes it easier to communicate with property management. At present, major operators have deployed new digital indoor distributed systems on a large scale in some large and medium-sized indoor scenes (such as transportation hubs, shopping malls, hospitals, etc.), and the system is running well, and the user experience rate has increased significantly.

At the same time, in order to minimize the operators' repeated investment, 6A network cables were pre-buried when deploying the new 4G digital indoor distribution system, to ensure that "points remain unchanged and lines remain unchanged" when upgrading to 5G in the future, and 5G NR can be quickly superimposed, thereby minimizing the secondary entry cost while ensuring feasibility.

Second, visual operation and maintenance, simple system maintenance and troubleshooting. In the 5G era, dense network networking will become the norm, and the number of terminal RF transmission units will increase significantly. Therefore, real-time monitoring of network equipment and terminal transmission units is one of the basic functions of the network, and try not to go to the site for non-hardware reasons. The new digital indoor distributed system basically uses active devices, which can monitor the working status of all equipment in real time, quickly locate fault points, and realize visual operation and maintenance; at the same time, the new digital indoor distributed system can also automatically realize self-diagnosis, self-optimization, and self-healing according to the surrounding channel conditions and user density, minimize manual intervention, and effectively reduce maintenance costs.

Third, it is easy to upgrade and transform. The new digital indoor distributed system mainly uses optical fiber and network cable to transmit digital signals, supports high frequency bands, and is conducive to a smooth transition to 5G. At the same time, the new digital indoor distributed system was designed with MIMO in mind. Currently, all devices support 2T2R by default, and can be upgraded to 4T4R using software control. Massive MIMO is one of the key technologies of 5G networks. In the future, 5G indoor distributed systems should at least support 4˟4MIMO.

In summary, the new digital indoor distribution system is simple to deploy, has visible operation and maintenance, intelligent management, and convenient maintenance. It can follow the standards, support the business, capacity, user experience and other requirements of 4G, 4.5G, and 5G at each stage, and can smoothly evolve to protect the investment of operators to the greatest extent. It will be the main solution for indoor distribution systems in the 5G era. Traditional DAS will still have a place, mainly used in special scenarios that do not require high capacity and solve coverage problems (such as elevators, underground parking lots, tunnels, etc.).