In 2017, 5G standardization is accelerating and the 5G era is around the corner, as evidenced by various 5G-related products released by a number of vendors. With the identification of available 5G spectrum resources, an architecture of 4G and 5G inter-working networks is becoming increasingly clear, and 5G network testing and deployment are high on the agenda of mobile operators.
For the deployment of 5G network, the coverage became the most concerned issue, because 5G takes higher frequency bands to achieve more bandwidth. As a feature of wireless transmission, the higher the frequency is, the greater the transmission loss, and the weaker the base station coverage.
It has been widely accepted in the industry that compared with the 1.8 GHz frequency band in 4G, the main frequency band (3.5 GHz) of 5G has a 3dB transmission loss and a 6 dB penetration loss. In addition, as 5G in 3.5GHz use TDD mode, doesn't transmit continuously, this result in some additional loss. Total loss is around 14 dB. In a normal way, we need to deploy several times that of the existing base stations to bridge the gap of 14 dB and ensure continuous coverage. This poses serious problems of site location selection, resulting in higher cost. Millions of eNodeBs have already been deployed for 4G. In densely-populated urban areas, the distance between 4G eNodeBs is usually a few hundred meters. It will almost be a mission impossible for operators to deploy several times more base stations for 5G
5G can Provide Better Coverage than 4G
"Link budget" is the most popular way to evaluate the coverage. However, Dr. Xiang Jiying, ZTE's chief scientist, deems that the traditional method of link-budget calculation can no longer cater to the 5G era. This is because there are three unreasonable assumptions during the calculation, which cannot include the contribution of 5G technologies.
The first assumption of link-budget calculation is that there is only one user in the entire cell. However, this assumption is not reasonable. In fact, each cell accommodates a number of users. Thus, the actual coverage for a single user is far below the calculated link budget value. However, with SDMA (space division multiplex) technology, 5G multi-fold the capacity without decreasing coverage even multi-user is simultaneously transport. So the 5G experienced coverage is better than 4G.
The second assumption of link-budget calculation is that the only user in the cell is in exactly in the optimal position of the base station RF beam. Again, this assumption is unreasonable. Normally, the beam width of the base station antenna is only about 6 degrees. In most cases, the user is not in the best angle, and therefore the actual coverage is weaker than the link budget value. In 5G, however, since SDMA track the user dynamically, the user is always in the optimized position.
The third assumption of link-budget calculation is that there is only one cell in the entire region. Actually, numerous cells exist in the urban area, in some case, the users are not coverage-limited, but interference-limited. However for 5G, multi-antenna technology can significantly decrease the inter-site interference. Thus even under weaker calculated-coverage, the 5G experienced coverage is better than 4G.
In addition, the link budget is based on the PUSCH in most cases, but the coverage is often CCH/SRS limited. In 5G, it's possible to enhance these control channels to solve the problem.
As a summary, 5G technology directly improves the link-budget, for example, to bridge the gap of 14 dB by the multi-antenna technology of both base stations and UE. In addition, 5G improve the experienced coverage by many ways, this is out-of the scope of the link-budget calculation.
It believes that, with the use of a large number of new technologies, 5G networks can provide not only much higher capacity but also better coverage than 4G networks, even take 3.5G frequency.
Figure 1 Uplink Coverage Distances at a Cell-Edge Rate of 2 Mbps
Feasibility of Co-Located 5G and 4G Base Stations
The simulation indicates that, at a cell-edge rate of 2 Mbps, the coverage of a 3.5 GHz 5G base station (uplink time slots accounted for 20%) in densely-populated urban areas is very close to that of a 1.8 GHz FDD LTE (dual polarized antenna) base station, and better than those of 2.6 GHz FDD LTE and TDD LTE (8 antennas) base stations. If a higher cell-edge rate is available, the coverage of the 3.5 GHz 5G base station is still equal to that of the 1.8 GHz FDD LTE base station, but much better than those of 2.6 GHz FDD LTE and 1.9/2.6 GHz TDD LTE base stations.
Figure 2 Uplink Cell Throughput for 20 UEs
Figure 1 shows a comparison of base station coverage in case of single-user link budget calculation. If multiple base stations and multiple users are involved, 5G networks can provide even better performance than 4G networks. It can be easily found from the system simulation result in Figure 2 that, in a scenario involving 20 users per cell where the ISD is 300 meters, the uplink throughput of cell-edge users in the 5G network is three times that of the 1.8GHz FDD LTE network and 4–5 times that of the 1.9 GHz TDD LTE network.
"Based on the simulation, ZTE believes that co-location of 5G and 4G base stations is absolutely feasible, which will greatly save the location selection and deployment costs of 5G base stations.“ said Dr. Xiang Jiying.
The Space Division Multiple Access (SDMA) is a key 5G technology, for improving network coverage, as well as improving the coverage. The SDMA technology plays different roles for different 5G spectrum.
"Compared with higher frequency bands, the spectrum bandwidth resources available to operators at sub-6 GHz frequency bands are still very limited. Therefore, the SDMA technology is particularly important for 3.5GHz operators." said Dr. Xiang Jiying.
ZTE set to conduct research on the SDMA technology in 2013, and is the first company engaged in SDMA technology research and development. ZTE was the first to apply Massive MIMO, a typical 5G SDMA technology, to 4G networks, providing 4G users with 5G-like experience on LTE UEs. By virtue of its leading position in the SDMA technology, ZTE released TDD Massive MIMO and FDD Massive MIMO products. The TDD Massive MIMO products have been commercialized in Japan's SoftBank, efficiently enhancing the capacity in high-traffic areas. Through the SDMA technology, ZTE has accumulated quantities of data in real scenarios and extensive commercial experience in commercial 4G networks, laying a solid foundation for development and commercialization of 5G Massive MIMO products, and accelerating the sophistication and commercialization of 5G networks.
With the intensive analysis and innovation in the 5G NR technology, and the effective improvement of 5G coverage, ZTE deems that the 3.5 GHz 5G base station can be co-located with 4G base stations. The co-location of 5G and 4G base stations not only lowers the difficulties in 5G network deployment, but also saves network investment for operators. It paves the way for the evolution from 4G to 5G, thus accelerating the process of commercial 5G deployment.
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