5G access network is enough to read this article

Date: Oct 25, 2022

Related Tags:1. All About 5G: Understanding 5G based on Data and Technology
2. Leveraging Open RAN to Diversify the Telecommunications Supply Chain

Abstract: Access network, in our wireless communication, generally refers to the wireless access network, which is commonly referred to as RAN (Radio Access Network).

The object to be studied today is the 5G access network.

What is an access network? If you are a student who has been paying attention to the fresh jujube classroom for a long time, you will not be unfamiliar with this concept.

Move out this mobile communication architecture diagram that Xiaozaojun has used countless times:

Access network, in our wireless communication, generally refers to the wireless access network, also known as RAN (Radio Access Network).

To put it bluntly, the function of connecting all mobile terminals to the network is the wireless access network.

The well-known base station (BaseStation) belongs to the radio access network (RAN).

Wireless Base Station

Although we started from 1G, went through 2G, 3G, and went all the way to 4G, which is known as the rapid evolution of technology, the logical architecture of the entire communication network has always been: mobile phone → access network → bearer network → core network → bearer network → connection Internet access → mobile phone.

The essence of the communication process is encoding and decoding, modulation and demodulation, encryption and decryption.

There are so many things to do, and all kinds of equipment perform their duties to complete these things.

The replacement of communication standards is nothing more than changing the name of the device or moving the location, but the essence of the function has not changed.

The same is true for the base station system and even the entire wireless access network system.

A base station usually includes BBU (mainly responsible for signal modulation), RRU (mainly responsible for radio frequency processing), feeder (connecting RRU and antenna), and antenna (mainly responsible for the conversion between the guided traveling wave on the cable and the space wave in the air).

Components of a base station

In the earliest days, equipment such as BBU, RRU and power supply unit were packed and stuffed in a cabinet or a computer room.

Base station integration

Later, slowly began to change.

How does it change? The communication bricks split them up.

First of all, it is to split the RRU and BBU first.

The hardware is no longer put together, and the RRU is usually hung on the wall in the computer room.

The BBU is sometimes wall mounted, but most of the time it is in the cabinet.

BBU in the cabinet

Later, the RRU was no longer placed indoors, but was moved to the side of the antenna (the so-called "RRU remote").


In this way, our RAN becomes D-RAN, or Distributed RAN (Distributed Radio Access Network).

What's the benefit of doing this?

On the one hand, the length of the feeder between the RRU and the antenna is greatly shortened, which can reduce signal loss and also reduce the cost of the feeder.

On the other hand, it can make network planning more flexible. After all, the RRU plus antenna is relatively small, so you can put it as you want.

Speaking of which, please pay attention: the development and evolution of communication networks is nothing more than two driving forces, one is for higher performance, and the other is for lower costs.

Sometimes cost is more important than performance, and if a technology needs to spend a lot of money, but the return is less than the cost, it is difficult to achieve widespread adoption.

The evolution of RAN is, to a certain extent, the result of cost pressures.

Under the D-RAN architecture, operators still have to bear huge costs. Because in order to place the BBU and related supporting equipment (power supply, air conditioner, etc.), operators still need to lease and build a lot of indoor computer rooms or shelters.

Lots of computer rooms = lots of costs

As a result, operators came up with the C-RAN solution.

C-RAN, meaning Centralized RAN, centralized radio access network. This C, not only represents centralization, but also represents other meanings:

Compared with D-RAN, C-RAN does better.

In addition to the RRU, it has all the BBU locked up. Where is it? Central computer room (CO, Central Office).

This pile of BBUs becomes a BBU baseband pool.

In this way, C-RAN solves the cost problem mentioned above very effectively.

Do you know about the energy consumption of base stations in the entire mobile communication network?


Do you know about the energy consumption of air conditioners in the base station?


Power consumption analysis of traditional computer room

That is to say, most of the operator's money is spent on base stations, infrastructure, and electricity bills.

After C-RAN is adopted, the number of base station computer rooms can be greatly reduced and the energy consumption of ancillary equipment (especially air conditioners) can be reduced through a centralized approach.

Several small computer rooms have entered the large computer room

With fewer computer rooms, less rent, less maintenance costs, and less labor costs. This cost saving is simply a blessing in disguise for operators suffering from operating pressure.

In addition, the extended RRU can be installed with an antenna closer to the user. The closer the distance, the lower the transmit power.

Low transmit power means extended user terminal battery life and reduced radio access network power consumption. To put it bluntly, your phone will save more power, the standby time will be longer, and the operator will save more power and money!

More importantly, in addition to saving money for operators, the adoption of C-RAN will also bring great social benefits and reduce a lot of carbon emissions (CO2).

In addition, after the scattered BBU becomes the BBU baseband pool, it is more powerful, can be managed and scheduled in a unified way, and the resource allocation is more flexible!

Under C-RAN, the base station is actually "disappeared", and all physical base stations become virtual base stations.

All virtual base stations share the user's data transmission and reception, channel quality and other information in the BBU baseband pool. The enhanced cooperative relationship enables joint scheduling to be realized. Interference between cells becomes cooperation between cells (CoMP), which greatly improves spectrum usage efficiency and improves user perception.

Coordinated Multiple Points Transmission/Reception (CoMP) refers to a plurality of transmission points separated geographically and cooperatively participate in the transmission of data (PDSCH) for one terminal or the joint reception of data (PUSCH) sent by one terminal.

In addition, since the BBU baseband pools are all in the CO (central computer room), they can be virtualized!

Virtualization is network element function virtualization (NFV). To put it simply, BBU used to be a special hardware device, which was very expensive. Now, find an x86 server, install a virtual machine (VM, Virtual Machines), run software with BBU functions, and then you can use it as a BBU!

This time I saved a lot of money!

It is precisely because the centralized method of C-RAN will bring huge cost reduction that it is welcomed and sought after by operators (of course, equipment manufacturers will not be too happy).

Guess who came up with C-RAN? Not the equipment manufacturer, but China Mobile. . . China Mobile is the most active promoter of C-RAN. . . As the world's largest operator, China Mobile regards C-RAN as a treasure.

In the 5G era, the access network has undergone great changes.

In the 5G network, the access network is no longer composed of BBUs, RRUs, and antennas. Instead, it was refactored into the following 3 functional entities:

CU (Centralized Unit, centralized unit)

DU (Distribute Unit, distribution unit)

AAU (Active Antenna Unit)

CU: The non-real-time part of the original BBU will be split and redefined as CU, which is responsible for processing non-real-time protocols and services.

AAU: Some of the physical layer processing functions of the BBU are combined with the original RRU and passive antenna to form an AAU.

DU: The remaining functions of the BBU are redefined as DUs, which are responsible for handling physical layer protocols and real-time services.

In short, CU and DU are distinguished by the real-time nature of processing content.

In simple terms, AAU=RRU+antenna

I'll throw another picture for you, and you should be able to see it more clearly:

Note that in the figure, EPC (that is, 4G core network) is divided into two parts: New Core (5GC, 5G core network) and MEC (mobile network edge computing platform). The MEC moves together with the CU, which is the so-called "sink" (closer to the base station).

Some functions of the core network sink

The fundamental reason for the splitting of BBU functions and the sinking of the core network is to meet the needs of different 5G scenarios.

5G is a "one-size-fits-all" network, in addition to fast network speed, there are many features, such as low latency, support for massive connections, support for high-speed mobile phones, and so on.

In different scenarios, the characteristics of the network (network speed, delay, number of connections, energy consumption...) are actually different, and some are even contradictory.

For example, when you watch a live broadcast of a high-definition concert, what you care about is the quality of the image. In terms of timeliness, the overall delay is a few seconds or even ten seconds, and you don't feel it. When you drive remotely, what you care about is the delay. If the delay exceeds 10ms, it will seriously affect the safety.

Therefore, the disassembly and refinement of the network is to more flexibly respond to the needs of the scene.

Speaking of which, it is necessary to mention a key concept of 5G - "slicing".

Slicing, in simple terms, is to divide a physical network into N logical networks according to application scenarios. Different logical networks serve different scenarios.

Different slices for different scenarios

Network slicing can optimize network resource allocation, achieve maximum cost efficiency, and meet diversified requirements.

It can be understood that because of the diversification of requirements, the network needs to be diversified; because the network is diversified, it needs to be sliced; because of the need to slice, the network element must be able to move flexibly; because the network element moves flexibly, the connection between the network elements is also Be flexible.

Therefore, there are new architectures such as DU and CU.

According to the standard proposed by 5G, CU, DU, and AAU can be separated or co-located. Therefore, there will be various network deployment forms:

Backhaul, mid-pass, and pre-pass are the connections between different entities

The network deployment forms listed in the figure above are as follows:

① Consistent with the traditional 4G macro station, CU and DU share hardware deployment to form a BBU unit.

② DU is deployed in the 4G BBU equipment room, and CU is deployed centrally.

③ DU is deployed centrally, and CU is centrally deployed at a higher level.

④ CU and DU are co-located and deployed in a centralized manner, similar to the C-RAN method of 4G.

The selection of these deployment methods requires comprehensive consideration of multiple factors, including service transmission requirements (such as bandwidth, delay, etc.), construction cost investment, and maintenance difficulty.

For example, if the fronthaul network is ideal for transmission (with money, the fiber goes directly to the antenna), then CU and DU can be deployed in the same centralized point. If the fronthaul network is not ideal transmission (no money, not so many fibers), DU can be deployed in a distributed manner.

For another example, if it is a low-latency scenario such as the Internet of Vehicles, your DU must find a way to move forward (deployed close to the AAU), and your MEC and edge cloud will come in handy.

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