Shulou(Shulou.com)11/24 Report--

What is the most important thing in the 21st century? Of course it's the network signal!

But recently, many friends are complaining: now that we have entered the 5G era, why is it so slow to surf the Internet when we take the high-speed train?

Let's take a look at this: in order to improve the quality of communication, how does high-speed rail mobile communication technology evolve?

Evolution of Mobile Communication Technology for High Speed Railway

GSM-R at present, GSM-R is a special digital mobile system widely used in China's railways.

GSM-R is the extension of GSM technology in the railway system, following the basic functions of GSM.

The difference is that GSM-R serves the railway system, which means it is more targeted to the following questions:

The phenomenon of rapid signal attenuation and frequency shift caused by high-speed train

The problem of signal coverage of trains passing through tunnels, valleys and other special terrain conditions

System inefficiency caused by frequent handover between base station cells

However, GSM-R is a narrowband communication system, which only works with a narrow spectrum of 4 MHz. In this spectrum range, most of the spectrum is used to carry voice (phone calls) services, and a small part of the spectrum is used to carry data (Internet services). As a result, we often get stuttered when we go online on the high-speed rail.

With the rapid development of high-speed railway, LTE-R has higher and higher requirements for data services, GSM-R has been unable to meet the demand, the industrial chain has shrunk, railway communications have gradually transitioned to LTE-R.

LTE-R uses modulation technology to improve the speed and capacity of the network, and there are great innovations in both transmission technology and system architecture.

It not only inherits all the services of GSM-R, improves the safety and efficiency of railway operation, but also provides users with functions such as multimedia cluster scheduling, videophone, real-time video surveillance, passenger transport comprehensive information release and so on.

In addition, LTE-R 's voice solution can provide passengers with a richer data experience and more accurate regular value-added services.

5G-R later, China National Railway Group Co., Ltd. also launched the research work related to the railway next generation mobile communication system, aiming at the railway 5G dedicated mobile communication (5G-R), defined the research plan and construction objectives.

The main business types of 5G-R are as follows.

Driving applications: dispatching communications, traffic dispatching commands, CTCS-3-level train control systems and autopilot, etc.

Operation and maintenance applications: operation and maintenance voice, operation and maintenance data and operation and maintenance video.

The service carried by 5G-R is still dominated by driving applications, which requires high safety and reliability of equipment.

Three application scenarios of 5G-R in railway system in the future:

Railway main line wide area communication dispatching communication voice, dispatching communication multimedia, driving safety data, driving information data, on-board operator voice and so on. Hot areas such as railway station / hub cover on-board video surveillance data, station maintenance operation multimedia communication, marshalling yard communication and so on. Ground facilities along the railway line monitor ground infrastructure monitoring data transmission. The challenge of high-speed railway mobile communication technology the railway communication system is developing so rapidly, why do we still feel that the network speed is slow?

This is because the 5G signal covers the high-speed rail scene, although the prospect is broad, but it also faces many challenges.

01. Signal penetration loss and propagation loss most of the high-speed train body adopts a fully enclosed structure of stainless steel, alloy and other metal.

The traditional architecture of direct communication between in-vehicle users and out-of-vehicle base stations makes the signal penetration loss in the vehicle is larger, the drop rate increases, and the handover success rate and connection success rate decrease, resulting in a decline in network performance.

02. Doppler effect brings frequency offset Doppler effect: the wavelength of object radiation changes due to the relative motion of the wave source and the observer. In front of the moving wave source, the wave is compressed, the wavelength becomes shorter and the frequency becomes higher; behind the moving wave source, the opposite effect occurs. The wavelength becomes longer and the frequency becomes lower; the higher the velocity of the wave source, the greater the effect.

In mobile communication, when the mobile station moves to the base station, the frequency becomes higher, and when it is far away from the base station, the frequency becomes lower, so the Doppler effect should be fully considered in mobile communication.

The faster the speed of the high-speed railway is, the greater the frequency offset is, which will also degrade the performance of the base station receiving signals. The Doppler effect is instantaneous, and the demodulation performance of the receiver is degraded by high speed, which is a great challenge.

03, instantaneous network load when high-speed rail transit, the number of users in the base station area increases sharply, the network load is too high, and the user perception decreases.

04. Frequent handover impact perception due to the limited coverage of the base station along the high-speed railway, the time required for the train to cross the single station coverage is very short at high speed.

In order to maintain the continuity of communication between train and ground during train operation, it is necessary to convert the communication link from one base station cell channel to another base station cell channel.

Therefore, users will produce frequent cell handover and reselection when using the mobile network. When it can not meet the time cost of handover and reselection, it is very easy to have network problems such as slow handover, handover failure, disconnection and so on, which will affect users' perception.

Solution 01, how to solve the problem of large penetration loss and propagation loss of the signal? Mobile relay technology is adopted. In short, it is to set up "intermediate forwarding" devices, so that the whole communication process is divided into two parts-between the external base station and the relay device, and between the relay device and the user. Signals from base stations and users in the vehicle are forwarded by the relay device.

02. How to solve the problem of frequency deviation caused by Doppler effect? Set the base station site reasonably to reduce the influence of Doppler frequency shift. The frequency offset correction / problem is carried out in the RF unit, that is, the processing of the baseband signal.

03. How to solve the problem of instantaneous network load and frequent cross-zone handover? Cell merging: a base station processing unit (BBU) connects multiple radio frequency extension units (RRU) and logically sets multiple cells as the same cell, thus turning "cell handover" into "cell cooperation", improving network performance, increasing coverage distance, and reducing handover times.

Handover algorithm optimization: generally speaking, the handover parameters involved in the handover process are optimized to effectively prevent the ping-pong effect and the connection interruption of the wireless link, and improve the handover success rate.

Railway dedicated wireless communication is evolving to broadband network. It is believed that in the near future, the application of 5G key technologies will add bricks to the construction of railway wireless private network.

Before that, you might as well put down your phone and take a look at the scenery along the way.

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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