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An in-depth look at 4G standards LTE FDD and LTE TDD

               

lit. deep and shallow (idiom); concise and easy to understand4G Standards: LTE FDD and LTE TDD

Currently based on LTE 4G standards have two, respectively, LTE FDD and LTE TDD (domestic customary LTE TDD called TD-LTE), look at the name you will have an intuitive impression, the two standards are based on different branches of LTE, similarity of more than 90%. Next, we use the traffic as an example, specifically analyze the differences between the two.

TD-LTE saves resources, FDD speed is faster

LTE FDD (Frequency Division Duplexing, FDD) uses Frequency Division Duplexing and TD-LTE (Time Division Duplexing, TDD) uses Time Division Duplexing.TDD has the following advantages:
  1. The ability to flexibly configure frequencies to utilize scattered frequency bands that are not easily accessible to FDD systems.
  2. By adjusting the uplink and downlink time slot transition points, the ratio of downlink time slots can be increased, which can well support asymmetric services.
  3. With upstream and downstream channel consistency, the receiving and transmitting of the base station can share part of the RF unit, which reduces the cost of equipment.
  4. When receiving upstream and downstream data, there is no need for a transceiver isolator, only a switch is needed, reducing the complexity of the equipment.
  5. With upstream and downstream channel reciprocity, it can better adopt transmission pre-processing technology, such as pre-RAKE technology, joint transmission (JT) technology, smart antenna technology, etc., which can effectively reduce the processing complexity of mobile terminals.
However, compared with FDD, TDD duplexing also has obvious shortcomings.
  1. Since the time resources of TDD are divided into uplink and downlink, the transmission time of TDD is only half of that of FDD, and if TDD wants to send the same amount of data as FDD, the transmission power of TDD has to be increased.
  2. TDD system uplink limitation, so the coverage area of TDD base station is obviously smaller than that of FDD base station.
  3. TDD system transceiver channels are of the same frequency and cannot be isolated from interference, and there is interference within and between systems.
  4. In order to avoid interference with other wireless systems, TDD needs to reserve a large protection band, which affects the overall efficiency of spectrum utilization.

As shown in the figure below, FDD receives and transmits on two separated symmetric frequency channels, and uses a protected frequency band to separate the receiving and transmitting channels.FDD must use pairs of frequencies, relying on the frequencies to differentiate between uplink and downlink, and its unidirectional resources are continuous in time.FDD can make full use of the uplink and downlink spectra when supporting symmetric services, but the spectrum utilization rate will be greatly reduced when supporting asymmetric services. However, when supporting asymmetric services, the spectrum utilization will be greatly reduced.
TDD uses time to separate the receiving and transmitting channels. In TDD mobile communication system, receiving and transmitting use different time slots of the same frequency carrier as the bearer of the channel, and its unidirectional resources are not continuous in time, and the time resources are allocated in two directions. The base station sends signals to the mobile station at one time and the mobile station sends signals to the base station at another time, and the base station and the mobile station must work together in order to work smoothly.


Let's put aside the jargon and explain it in a simpler way.
First of all, if you want to access the Internet, you have to establish an uplink and a downlink channel: for example, if you click on WeChat, your cell phone will send a request through the uplink channel, and then WeChat's server will send your latest unread messages to your cell phone through the downlink channel. In general, we use the downlink (download) more and the uplink (upload) less.
In order to create uplink and downlink channels, FDD is split by frequency, with two symmetrical frequencies, one for downloading and one for uploading. It's likedual carriageway"and the cars in both directions do not interfere with each other.unobstructed(SIGHS) On your cell phone, it's like it's fast.
TD-LTE takes a different approach. It only uses one frequency and is responsible for both uploading and downloading. The advantage is than FDD save a frequency occupation, resource utilization is higher (in fact, TD-LTE in order to avoid interference, need to reserve a larger protection band, will also consume some resources); TDD shortcomings are also very obvious, because it is a "one-way" on the run two-way "traffic ", TD-LTE can only control the traffic through time (time division duplex), a moment to let the download traffic through, a moment to let the upload traffic through. Performance in the cell phone, will be slower than the FDD network speed.
At present, the theoretical downlink speed of LTE FDD is 150MBPs, and the theoretical downlink speed of TD-LTE is 100Mbps.

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

We have encountered this situation in our lives: during rush hour, the traffic in the direction of the city is very congested, while the traffic on the road next to the direction of the city is sparse. This is a waste of resources.
This phenomenon is even more prevalent when it comes to cellular Internet access: people use their cell phones more for reading, viewing, and downloading, and spend very little time uploading. Therefore, if the cell phone's wireless network is visible, you'll see a steady stream of data on the download channel, while the upload channel is rarely used.
The advantage of TD-LTE is that it combines the upload and download channels into one, and then controls them flexibly by time, for example, 70% of the time is allocated to download and 30% to upload, so that you will find that the whole channel is always full of traffic, and the utilization of resources is higher.
Since TD-LTE is so economical, it can't be used all the time. In the user-intensive hotspot areas, the frequency band resources are very tight, at this time, FDD's "two lanes" seems to be a waste, TDD is more suitable. However, due to the limitation of TDD in uplink, the coverage area of base station is smaller than FDD, therefore, in the non-hot spot in the wide coverage area (suburbs, townships and highways), TDD needs to build more base stations than FDD, and the cost is too high.

III Can TD-LTE and FDD Mix Networks?

FDD and TD-LTE the two LTE branch standards have their own strengths, but the basic technology between the two is very similar. Some experts said, TD-LTE and LTE FDD can be completely regarded as a system, only in the business realization of certain technical differences. Therefore, the international TD-LTE and FDD mixed networking mode, play their respective strengths, TD-LTE for hot spot area coverage, FDD for wide area coverage. Due to the common technical basis, TD-LTE and FDD have very good prospects in mixed networking. This is also when the ITU in the development of 4G standards expected to achieve - that is, to minimize the differences between the physical layer of different standards, so that the network standards eventually towards convergence.

IV. Summary

TD-LTE and LTE FDD essentially share a set of standard basis, in the business realization of the technology has certain differences. TD-LTE saves channel resources, suitable for hotspot centralized area coverage; FDD's theoretical maximum speed is faster, the base station coverage is wider, suitable for suburbs, highway and railroads, and other wide-area coverage. Mixed network is a better choice.