PARTNER FEATURE: In 2020, China’s 5G network construction has entered the fast-forward mode. Large-scale construction and application of indoor and outdoor network not only brings new customers and new experience, also keeps asking for more competitive network performance and more elaborate networking solution.

In the existing 4G network, with inter-frequency networking used mainly, the interference problems between indoor and outdoor environment are not serious. In the initial stage of 5G network construction, only 100MHz bandwidth can be used for NR. To build 5G networks for both indoor and outdoor environments simultaneously, the co-frequency collaborative networking is the only way, thus co-frequency interference will be inevitable, including interference on the public control channels and on the service channels, therefore resulting to the deterioration of network coverage performance and user experience. Consequently, the co-frequency interference of 5G network and the macro and micro cells co-frequency collaborative networking between indoor and outdoor environment have become an urgent needs to be researched and figured out, which also is a pain point that telecom operators and equipment suppliers need to face together.

Multiple dimension and new attempts

In response to the topic of 5G indoor and outdoor co-frequency collaborative networking, Guangzhou China Mobile and ZTE have carried out in-depth cooperation to explore the interference formation mechanism and related solution from multiple perspectives, and finally ZTE launched field practice from three perspective of network planning and design evaluation, passive interference avoidance, and active interference coordination, to reduce indoor interference, improve indoor coverage, and enhance indoor performance for the co-frequency networking between indoor and outdoor coverage.

First, based on the feasibility of signal isolation being as a reference for network planning and design, we studied the impact of different signal isolation between indoor and outdoor cells on service performance as a guide for co-frequency networking. While the RSRP signal at indoor cell edge is higher than that at outdoor cell, in different threshold of -3 dB, 0 dB, 3 dB, 5 dB, 10 dB and 15 dB, the UE performance of the specific fixed points inside the room are analyzed, which can provide a reference for indoor coverage planning and optimization, and can be helpful to give guideline for field parameter configuration in typical scenarios.

Secondly, considering passive interference avoidance, the interfered side utilizes the beam management to avoid the interference of the public control channel on the service channel, including SSB and CSI-RS beam management. For example, the operators have generally adopted the 7 or 8 horizontal beams scanning broadcast in Massive MIMO macro cell. However, either the new digital intelligent indoor distribution system of QCell or the traditional DAS system, the existing indoor distribution systems all utilize single beam transmission scheme, and the non-alignment of macro and micro SSB beams will inevitably cause interference.Therefore, by mean of the configuration of 8 beams for QCell SSB broadcast, the alignment the broadcast beams of micro cell and macro cells, including the number of SSB beams and offset, consequently reduce the interference of the SSB signals between the macro cells and the micro cells, and the interference for micro cells service channel.

Finally, considering the interference coordination, the inter-cell PRB randomization is also proved to be an effective anti-interference method for service channels. The basic principle is to divide different starting positions of RB allocation for different cells, and then each cell selects a fixed RB allocation sequence according to the current cell type. When the RB usage of a cell is not high, the frequency domain resources of different types of adjacent cell can be staggered, so as to reduce interference and improve throughput. In addition, dynamic multiple beams coordination between outdoor and indoor cells is also an effective means for active interference mitigation.

Easy deployment and Excellent Experience

Based on the above guidance, China Mobile Guangzhou and ZTE have verified by field tests in Panyu and Pazhou, and obtained effective verification.

According to the result of the performance evaluation test under different indoor isolation levels, under the worst indoor and outdoor isolation condition, when the SINR of the test point of indoor cell (closer to the outdoor area) is -3 dB, the downlink data rate can reach 228 Mbps, and the uplink rate can reach 34 Mbps when the adjacent cell is under full load , which can meet normal user services requirement.

According to the test result of the 5G indoor and outdoor public channels, by mean of the SSB beam coordination configuration, RSRP in the indoor fixed-point with multiple beams configuration has a gain of about 6dB, comparing with the single beam configuration, and the downlink data rate is increased by 11% at middle point and far point. In the drive test, comparing with single beam configuration, RSRP with multiple beams configuration of indoor cell increased by about 3 dB and the downlink data rate increases by 4-5%.

According to the test result of the service channel interference avoidance scheme, the uplink spectrum efficiency of the test cell increase by 6.7%, 8.86% and 10.81% at the near, middle and far points respectively, and the downlink spectrum efficiency increase respectively by 8.81%, 11.4% and 13.84% at the near, middle and far point.

In short, without the need for additional investment, by mean of the combinations of above mentioned technologies, this solution can solve the problem of the co-frequency interference between indoor cell and outdoor cell at low and medium load conditions in the existing 5G network, and effectively improve the performance by about 10% for users at the junction area between indoor and outdoor coverage.

Low cost and easy to promote

In addition to significantly reducing the interference on the indoor coverage and services from outdoor macro cell to indoor cell and improving the indoor user experiences, the solution also has the characters of low cost and easy promotion.

On the one hand, this solution does not require additional investment and engineering. It only needs to carry out refined network planning, and performs the corresponding parameter optimization after commissioning. On the other hand, the solution can be directly deployed in existing networks, and is applicable to various scenarios where macro and micro cells overlap, especially when co-frequency interference is serous under low and medium load in the 5G existing network. The solution has positive significance for 5G large-scale commercialization,

technology innovation, industry application exploration and the development of the telecommunication industry.


The co-frequency networking solution between indoor micro cell and outdoor macro cell reduces the interference from outdoor macro cell to indoor micro cell through the combination of SSB alignment, CSI-RS alignment, PRB randomization and interference isolation, and improves the indoor user experiences. The solution has the features of low cost, practicability and easy promotion, and requires no new investment and additional engineering reconstruction. The existing network can directly use it and widely adapted it to 5G large-scale commercialization, technology innovation, industrial application exploration, and industry development.