4G TDD-LTE and FDD-LTE two models, do you understand?

Source: Internet
Author: User

4G TDD-LTE and FDD-LTE two models, do you understand?

At present, there are two 4G standards based on LTE, namely lte fdd and lte tdd (China is used to call lte tdd TD-LTE, both standards are based on different branches of LTE, with a similarity of more than 90%. Next we will use traffic as an example to analyze the differences between the two:

I. TD-LTE saves resources and FDD is fast

Lte fdd (Frequency Division Duplexing, FDD) uses Frequency duplex, TD-LTE (Time Division Duplexing, TDD) is Time duplex. TDD has the following advantages:

Flexible frequency configuration and use of scattered frequencies that are not easy to use in the FDD system;

You can adjust the upper and lower time slot conversion points to increase the proportion of the downstream time slot, which can well support asymmetric services;

With upstream and downstream channel consistency, the receiving and sending of base stations can share some RF units, reducing equipment costs;

When receiving upstream and downstream data, you do not need to send and receive isolator. You only need one switch to reduce the complexity of the device;

With upstream and downstream channel interconnectivity, it can better adopt transmission preprocessing technology, such as pre-RAKE Technology, combined transmission (JT) technology, and smart antenna technology, it can effectively reduce the processing complexity of mobile terminals.

However, TDD duplex has obvious shortcomings compared with FDD:

Because the time resources of TDD are allocated to the uplink and downlink, the launch time of TDD is about half of that of FDD. If TDD needs to send the same amount of data as that of FDD, it is necessary to increase the TDD sending power;

The uplink of TDD Systems is limited, so the coverage of TDD base stations is significantly smaller than that of FDD base stations;

Communication channels in TDD Systems are in the same frequency. Interference isolation is not allowed, and interference exists between the system and the system;

To avoid interference with other wireless systems, TDD requires a large protection band, which affects the overall spectrum utilization efficiency.

As shown in, FDD receives and sends messages on two separated symmetric frequency channels, and isolates the receiving and sending channels using protective frequency bands. FDD must use paired frequencies to distinguish between upstream and downstream links. The resources in a single direction are consecutive in time. FDD can make full use of upstream and downstream spectrum when supporting symmetric businesses. However, when supporting asymmetric businesses, the spectrum utilization will be greatly reduced.

TDD uses time to separate the receiving and sending channels. In TDD-based mobile communication systems, different time slots of the same frequency carrier are used for receiving and sending as the channel's bearer, and their single-direction resources are not sequential in time, time resources are allocated in two directions. In a certain period of time, the base station sends signals to the mobile station. In addition, the mobile station sends signals to the base station. The coordination between the base station and the mobile station must be consistent in order to work smoothly.

Aside from these terms, Let's explain them in a simpler way:

First, to access the Internet through a mobile phone, you must establish an upstream and downstream channel. For example, if you click, the mobile phone will send a request through the Upstream Channel, and then the server will go through the downstream channel, upload your latest unread message to your mobile phone. In general, we usually spend a lot of time downloading data, but little time uploading data.

In order to establish the uplink and downlink channels, FDD is divided by frequency. On two symmetric frequencies, one is downloaded, and the other is uploaded. Like a two-lane car, two directions do not interfere with each other, smooth and unobstructed. Your mobile phone shows a fast speed.

TD-LTE uses another method. It uses only one frequency for both upload and download. The advantage is lower than FDD a frequency occupation, higher resource utilization (in fact, TD-LTE in order to avoid interference, need to reserve a large Protection Band, will also consume some resources); TDD shortcomings are also very obvious, because it is a "one-way street" running two-way "traffic", TD-LTE can only control traffic through time (time duplex), will let the download traffic through, will let the upload traffic through. It is displayed on the mobile phone end, which is slower than the FDD network speed.

At present, the theoretical downlink speed of lte fdd is 150 MBPs, and the theoretical downlink speed of TD-LTE is 100 Mbps.

2. TD-LTE is suitable for hotspot coverage and FDD is suitable for wide area coverage.

We have encountered this situation in our daily life: during peak hours, the traffic in the city is congested, while the traffic on the road out of the city is scarce. This is undoubtedly a waste of resources.

In the process of surfing the Internet through mobile phones, this phenomenon is more common: people use mobile phones, more reading, viewing, and downloading, and a little time for uploading. Therefore, if the wireless network of the mobile phone is visible, you will find that the data on the download channel is constantly flowing, but the upload channel is rarely used.

The advantage of TD-LTE is that it combines upload and download channels into one, and then flexible control through time, such as allocated to download time accounted for 70%, upload accounted for 30%, so that, you will find that the entire channel is always full, and the resource utilization rate is higher.

Since TD-LTE is so economical, it cannot be all used. In User-intensive hotspot areas, the frequency band resources are very tight. At this time, the "dual-lane" of FDD is very wasteful, and TDD is more suitable. However, TDD is limited in uplink, and the base station coverage is smaller than that of FDD. Therefore, in non-hotspot areas (suburban areas, towns and highways, TDD requires more base stations than FDD, and the cost is too high.

3. Can the TD-LTE and FDD work together?

The FDD and TD-LTE LTE Branch Standards have their own strengths, but the basic technology is very similar between the two. Some experts said that TD-LTE and lte fdd can be seen as a system, only in the business implementation of a certain technical difference. Therefore, the International has the TD-LTE and FDD hybrid network mode, give full play to their respective strengths, TD-LTE for hot area coverage, FDD for wide area coverage. Because of the common technical foundation, TD-LTE and FDD have a very good prospect in hybrid networking. This is also what ITU expected to achieve in the Development of 4G standards-that is, to minimize the physical layer differences between different standards, so that network standards can eventually be integrated.


TD-LTE and lte fdd share a set of standards in essence, there is a certain difference in the technology of business implementation. TD-LTE saves channel resources and is suitable for concentrated regional coverage of hot spots; FDD theory is the highest speed faster, the base station coverage is wider, suitable for suburban, highway and railway wide area coverage. Hybrid networking is a better choice.

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