When it comes to LTE, what everyone immediately thinks of is 4G. When it comes to 4G, what comes to many people’s minds is TDD LTE, because the first 4G license in my country is TDD LTE, and the current one is also TDD LTE. , the operator aspect is mainly dominated by China Mobile. However, as the FDD LTE license is about to be released, China Unicom and China Telecom will adopt FDD LTE or TDD LTE+FDD?LTE hybrid networking, so more people are beginning to care about FDD LTE.

FDD LTE and TDD? LTE are two different system modes of LTE. Most of the basic technologies of the two are the same. The main difference is that FDD is frequency division duplex, while TD is time division duplex. The two are not compatible with each other.

1. TDD LTE is a single channel, suitable for hotspot area coverage

If a mobile phone wants to access the Internet, it must establish uplink and downlink channels. For example, when you click on WeChat, your phone will send a request through the uplink channel, and then the WeChat server will send your latest unread messages to your phone through the downlink channel.

TDD LTE transmitting and receiving signals are performed in different time slots of the same frequency channel, and are separated by a certain guaranteed time. Currently, 23 TDD LTE commercial networks have been launched in 18 countries, with a total number of users exceeding 5 million.

TDD? LTE uses one frequency, which is responsible for both uploading and downloading, and has high resource utilization. Many users may feel that a channel can easily cause traffic congestion. In fact, in daily life, most mobile phone users use more downlink channels and less uplink channels. Therefore, more often than not, "traffic jam" does not occur in a channel. Due to this way of working, TDD LTE is more suitable for hotspot area coverage. When people use mobile phones, they spend more time reading, watching and downloading, and less time is spent uploading. If the phone's wireless network is visible, you will find that the download channel is constantly flowing with data, but the upload channel is rarely used. Therefore, in densely populated areas where many mobile phones are used, frequency band resources are relatively tight, and it is more suitable to build TDD LTE base stations.

2. FDD LTE is a two-lane road, suitable for wide area coverage

Different from TDD LTE, FDD LTE uses two symmetrical frequency channels to transmit and receive signals respectively. There is a certain frequency band guard interval between the receiver and the receiving channel.

In other words, in order to establish upstream and downstream channels, FDD is divided by frequency. At two symmetrical frequencies, one tube downloads and one tube uploads. Just like a two-lane road, cars in both directions do not interfere with each other and flow smoothly.

The advantage of TDD? LTE is that it combines the upload and download channels into one, and then flexibly controls it through time. For example, the time allocated to download accounts for 70% and upload accounts for 30%. In this way, you will It is found that the traffic flow of the entire channel is always full, and the resource utilization rate is higher. However, although TDD LTE is particularly economical, it cannot be fully adopted. In hotspot areas with dense users, frequency band resources are very tight. At this time, FDD's "dual lane" seems wasteful, and TDD is more suitable. However, since TDD is limited in uplink, the base station coverage is smaller than FDD. Therefore, in non-hotspot wide coverage areas (suburban areas, towns and highways), TDD needs to build more base stations than FDD, and the cost is too high.

3. What are the differences between TDD and FDD

Below, we list the advantages and disadvantages of the two (using FDD to compare and illustrate)

( 1) When using TDD technology, as long as the uplink and downlink time intervals between the base station and the mobile station are not large and less than the channel coherence time, it is relatively simple to estimate the channel characteristics based on the other party's signal. For general FDD technology, the general uplink and downlink frequency interval is much larger than the channel coherence bandwidth, and it is almost impossible to use the uplink signal to estimate the downlink, and it is impossible to use the downlink signal to estimate the uplink. This feature makes the TDD mobile communication system in power control and There are clear advantages in the use of smart antenna technology. But also because of this, the coverage radius of the TDD system is smaller. Due to the uplink and downlink time intervals, the base station coverage radius is significantly smaller than the FDD base station. Otherwise, user signals at the cell edge will not be synchronized when they arrive at the base station.

(2) TDD technology can flexibly set uplink and downlink conversion times to achieve asymmetric uplink and downlink service bandwidth, which is conducive to realizing Internet services with obvious uplink and downlink asymmetry. However, the setting of this switching time must be coordinated with neighboring base stations.

(3) Compared with FDD, TDD can use fragmented frequency bands, because the uplink and downlink are distinguished by time, and there is no need to require frequency bands with symmetrical bandwidths.

(4)TDD technology does not require a transceiver isolator, only a switch.

(5) The moving speed of the mobile station is limited. When moving at high speed, the Doppler effect will cause fast fading. The higher the speed, the higher the fading transformation frequency and the deeper the fading depth. Therefore, the moving speed must not be too high.

For example, in the TD-SCDMA system using TDD, based on the current chip processing speed and algorithm, when the data rate is 144kb/s, the maximum moving speed of TDD can reach 250km/h. Compared with the FDD system, it is still faster. There is a certain gap. Generally, the moving speed of TDD mobile stations can only reach half or even lower than that of FDD mobile stations.

(6) Transmission power is limited. If TDD wants to send the same amount of data as FDD, but the transmission time is only about half of FDD, this requires TDD to have a higher transmission power. Of course, more complex network planning and optimization technologies are also required.

Hybrid networking is the best

Some experts said that TDD LTE and FDD LTE can be regarded as one system, but there are certain technical differences in service implementation. Therefore, there is a hybrid networking model in the world that combines TDD, LTE and FDD to give full play to their respective strengths. TDD LTE is used for hotspot area coverage, and FDD is used for wide area coverage.

Due to the same technical foundation, TDD? LTE and FDD have very good prospects in hybrid networking. This is also what the ITU expected to achieve when it formulated the 4G standards - that is, to minimize the differences in the physical layer between different standards, so that network standards will eventually converge.

China Mobile has adopted a hybrid networking method of TDD, LTE and FDD in Hong Kong’s 4G network, and has accumulated experience in this area. Judging from the current news, the Ministry of Industry and Information Technology is also about to organize operators to carry out verification of hybrid networking.