How to calculate 5G backhaul bandwidth?

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This article is reprinted from the WeChat public account "Wireless Deep Sea", which can be followed through the following QR code. To reprint this article, please contact the Wireless Deep Sea public account.

When we access the Internet through wireless networks, the signals sent by mobile phones need to pass through base stations and bearer networks to reach the core network. At the same time, the data sent by the core network also needs to pass through the bearer network and base stations to reach the mobile phone, thus achieving the great harmony of life.

So, how much bandwidth is needed between the base station serving multiple users and the core network? This involves the estimation of the backhaul bandwidth.

In fact, the original algorithm for backhaul bandwidth does not distinguish between 4G and 5G, but considering that the 5G architecture also includes fronthaul and midhaul, a general introduction is given in the first part.

1. What is a callback?

When it comes to 5G network architecture, the most popular one is the standard structure of AAU, DU, CU plus core network shown in the figure below.

The picture comes from the public account "Xianzao Classroom"

AAU: Active Antenna Unit, which is the combination of RRU and antenna;

DU: Distributed Unit, which is the part of the original 4G BBU that needs real-time processing and is usually deployed at the site together with the AAU;

CU: Centralized Unit, the non-real-time processing part of the original 4G BBU is split out. It is generally believed that it should be deployed in a centralized manner and can manage multiple DUs;

Despite this architecture, in actual 5G network deployment, there is no strong motivation to split network elements, and DU and CU are basically deployed together. In this case, DU plus CU is still called BBU.

Since CU and DU are deployed together as BBU, the transmission between them becomes the internal interface. Therefore, the transmission between BBU and core network is what everyone is most concerned about: backhaul.

The technologies related to backhaul network are already very mature. In general network planning, the wireless side must pass a key data to the bearer:

How much backhaul bandwidth does a 5G station need?

2. How to calculate the backhaul bandwidth?

Assuming a 5ms single cycle, the theoretical peak rate of a cell with 100M bandwidth can reach 7.2Gbps, and a station generally has 3 cells. Should the following information be conveyed to the bearer network: A 5G base station requires at least 21.6Gbps of bandwidth?

Of course, if the network is built according to this value, the network operation will definitely be fine, but it is still a bit wasteful: Can the cell under a base station reach the peak value at all times? In addition to the payload, there are also various packet headers. Have the operation and maintenance expenses been taken into account?

In fact, in the real network, the base station covers an area, some users are near and some are far away, some signals are good and some are bad, and not all users are working hard to download. Most of them are chatting on WeChat and browsing Weibo, so it is impossible to reach the peak rate of the cell, and the final result is an average rate.

Regarding the average rate of this cell, it is necessary to obtain the average spectrum efficiency through simulation, and then multiply it by the actual cell bandwidth to obtain the average rate of the cell.

For example, if we take the average spectrum efficiency of 5G as 10 bits per second per Hertz, at a bandwidth of 100M, the average rate of a cell is 1Gbps.

In summary, taking the average rate into account and adding 10% overhead, a three-cell site requires an average backhaul bandwidth of 3.3 Gbps.

Although the average rate can generally ensure smooth operation of the site, it may burst at some point and exceed this average value. Therefore, we still need to consider the peak value.

Therefore, the industry generally uses (1x cell peak value + 2x cell average value) x 110% as the peak value of the entire site, commonly known as "one peak plus two average values", where 110% takes into account 10% of the overhead.

Therefore, for a three-sector 100M bandwidth 5G site, the required peak backhaul bandwidth is (1x7.2Gbps + 2x1Gbps)x110% = 10.12Gbps.

In actual deployment, the backhaul bandwidth of the site must be greater than the average, and it is best to design the peak value according to the peak value.

3. What about C-RAN?

Logically, this question should end here, but due to the diversity and evolution of 5G network architecture, an architecture called C-RAN is often seen.

In fact, the full name of C-RAN is Centralized RAN, which means the centralized deployment of BBU (hence it is also called BBU Pool). Although this abbreviation can also be interpreted as Cloud RAN, there is still a long way to go before the cloudification of wireless networks. We still assume that BBU is centralized.

In this case, how much bandwidth backhaul resources do these centrally deployed BBUs need? Just add up the bandwidth of each BBU!

Similar to the previous idea, every four BBUs can be grouped together, with the backhaul of one BBU calculated based on the peak value and the backhaul of the other three BBUs calculated based on the average value. The backhaul bandwidth required by a C-RAN site can be calculated by adding up the backhaul bandwidth of multiple such groups.

In this way, on average, each BBU can be allocated a bandwidth slightly larger than its average bandwidth. What if there is a burst of traffic at some sites? Because all BBUs share the total transmission bandwidth, the traffic may be high or low, but it generally tends to the average.

In summary, building a network requires consideration of costs and benefits, and the backhaul bandwidth only needs to be sufficient.