Why do base stations need to go to the sky?

Over the past few decades, mobile communication technology has developed from 1G to 5G, with more and more base stations being built and network coverage getting better and better. Today, we can pick up our mobile phones to surf the Internet and watch videos almost anytime and anywhere.

But have you noticed a problem? No matter whether it is 3G, 4G or 5G, the service quality provided is unstable and fluctuates greatly, causing the mobile Internet speed to be sometimes fast and sometimes slow. Sometimes, even when the signal is full, the Internet speed is still very slow.

Specifically, the mobile communication network uses cellular networking, dividing the entire coverage area into multiple hexagonal cells, and each cell is equipped with a base station to provide services for mobile phones within the coverage area of ​​the cell. In this way, each base station can provide coverage nearby, and mobile phones can communicate with the nearest cell. However, within each cell, the quality of network service usually decreases as the distance between the mobile phone and the base station increases, resulting in a large difference in user experience. For example, the network speed at the near point, midpoint and edge of the cell varies greatly.

What is the cause of this? Is there any way to solve this problem?

Let us first go back to the era of 1G and 2G voice calls.

The 1G network uses analog technology to allocate bandwidth to different users through frequency division multiplexing. Not only does the network capacity have low confidentiality, but the network is also extremely susceptible to interference. When the mobile phone user is far away from the base station, the voice quality will decrease, resulting in a smaller effective coverage range for each cell.

Entering the 2G era, GSM adopted digital technology and introduced low-rate coding, more advanced error correction coding and other technologies, which improved system capacity, spectrum utilization, and system anti-interference capabilities, greatly improved user call quality, and increased the effective coverage of the cell.

As shown in the figure above, under the 1G network, the user's voice quality will continue to decline as the distance between the mobile phone and the base station increases; while under the 2G network, this downward trend is flatter, which means that 2G base stations can provide users with high-quality voice calls in a larger range.

At the same time, since GSM networking adopts frequency reuse, adjacent cells use different frequencies to avoid mutual interference. In the GSM system, even if the user is at the edge of the cell, the network can still guarantee good network quality, allowing users to enjoy a good voice call experience.

But in the 3G/4G/5G data era, users at the edge of the community are no longer so lucky.

In the 3G era, in order to support data services, support larger bandwidth, and ensure higher Internet access speeds for users, the WCDMA system adopted code division multiple access (CDMA) technology. All cells in the entire network use the same frequency, so there is no need for frequency reuse planning. Instead, scrambling code planning is used to distinguish different cells and avoid interference from neighboring cells.

This means that SINR (signal to interference and noise ratio) will decrease as the distance between the mobile phone and the base station increases. The SINR at the edge of the cell is lower, resulting in the network rate experienced by users at the edge of the cell being much lower than that at the near or midpoint of the cell.

Moreover, since the WCMDA network itself is a self-interference system, as the number of users in the cell increases, the network traffic increases, the network interference will increase, and the effective coverage of the cell will decrease. This is what we often call the cell breathing effect, that is, the cell coverage changes with the number of users. The cell breathing effect is like many people talking in a room. The more people talk at the same time, the louder the noise in the room, and you can only hear each other's voices when you get closer.

Frankly speaking, when 3G WCDMA was born, although the telecommunications industry often promoted CDMA as a new technology and new function, after the network construction was completed, the operators found that this new technology actually caused a lot of headaches in network optimization and service assurance, and it also became one of the main challenges faced by mobile communication technology in the development of data services.

In the 4G and 5G era, in order to cope with the growing demand for data services, the network requires a larger bandwidth. All cells still use the same frequency band. Although OFDMA orthogonal frequency division multiple access technology is adopted, the orthogonality between subcarriers in the cell can be used to avoid interference between users, and there is no longer a "breathing effect", the cell edge will still be affected by interference from adjacent cells. Therefore, when the user is at the edge of the cell, due to the interference from the adjacent cells and the distance from the base station, the SINR will still decrease, resulting in poor service quality and low rate for users at the cell edge.

In short, in the era of data services, from 3G to 5G, although cellular network technology has continued to evolve and solved the growing demand for network traffic, they have a common defect: the variability of network service quality is too large, and the gap between the cell edge rate and the cell center rate is too large, and it is impossible to guarantee a consistent experience for each user.

This problem is more prominent than in the 2G GSM voice era.

Let's talk about SINR. As mentioned above, SINR refers to the ratio of useful signal power to the sum of interference power and noise power, which directly reflects the quality of received signal. From the definition of SINR, we can see that there are two main factors that lead to its poor performance: one is large interference. Sometimes we find that the mobile phone signal is full and the signal reception level is very high, but the network speed is very low. This is caused by large interference; the second is low signal reception level, that is, poor network coverage. For example, at the edge of the cell, the distance between the mobile phone and the base station is far, the signal reception level is very low, and the SINR value is also very small. Usually, if the signal reception level is high, the SINR is not necessarily good, because there is large interference, which will also lead to a low SINR value; but if the signal reception level is very low, the SINR must be poor.

Why is the signal reception level so low when users are at the edge of a cell? This is mainly because the height of the base station tower built on the ground is only a few dozen meters. When the antenna on the tower transmits signals to the mobile phone, it will be blocked by various obstacles such as buildings and trees. The radio waves are reflected, scattered, diffracted and transmitted, and the signal attenuates greatly. As a result, when we are far away from the base station (for example, at the edge of a cell), the signal level will drop rapidly. Sometimes, even if we are close to the base station, the signal reception level is still very low.

Okay, after talking so much, let’s finally get back to the topic. If we move the base station from the ground to the air, can we solve the above problems?

Taking HAPS (High Altitude Platform System) currently being tested by many operators as an example, the base station equipment is installed on a flying platform in the stratosphere about 20km above the ground, providing connection services to mobile phones or IoT terminals on the ground in an air-to-ground manner.

In this way, wireless signals propagate in a line-of-sight (LOS) environment. Except for indoor coverage scenarios where wireless signals still need to penetrate buildings, the wireless propagation paths in other scenarios are almost unobstructed by obstacles, and the distance between each terminal and the aerial base station is almost the same, which can better solve the problem that in a ground base station environment, the farther the terminal is from the base station, the lower the signal level.

Of course, the problem is that although the aerial base station and the ground terminal are in a line-of-sight environment and the signal propagation loss is small, since the distance between the two is more than 20KM, the terminal's transmission power and uplink loss are the shortcomings that restrict the aerial base station from becoming a reality.

The problem can be solved. The current industry practice is to use large-diameter, high-gain active antennas to improve uplink capabilities by obtaining a receiving gain of about 30dB. This antenna is very large, and to obtain effective gain in the 2GHz frequency band, the diameter must be about 3 meters.

This large-scale active antenna also supports narrow beam transmission, which forms a continuous coverage area consisting of multiple cells on the ground by transmitting multiple narrow beams (up to hundreds) to the ground, with each beam corresponding to a cell. Usually, the radius of each cell is about 1-2KM.

Since these narrow beams have good interference suppression capabilities, and in the air-to-ground scenario, the distance between each terminal and the aerial base station is basically the same, this can better solve the problem of SINR decreasing rapidly as the distance between the terminal and the base station increases, allowing terminals at different locations on the ground to obtain good SINR, bringing a consistent network experience to all users.

Therefore, "air base stations" can not only improve the wide-area coverage of mobile networks and face the continuous growth of data traffic in the future, but also solve the long-standing challenge faced by 3/4/5G networks - poor experience at the edge of the cell, so that all users in the cell, no matter where they are, can get a good and consistent network experience. This is one of the main reasons why the industry is working hard to explore moving base stations "to the sky". ​