Speaking of LTE, we immediately think of the 4G, and when it comes to 4G, many people in the mind is TDD LTE, because our first 4G license is TDD LTE, currently is TDD LTE, operators mainly for China Mobile's dominant. However, with the FDD LTE license soon to be released, China Unicom and the Chinese telecom will use FDD LTE or TDD LTE+FDD LTE hybrid network, so more people began to care about FDD LTE.
FDD LTE and TDD LTE are two different system modes of LTE respectively. Most of the basic technology is the same, the main difference is that FDD for frequency division duplex, and TD for Time Division duplex. The two are not compatible with each other.
TDD LTE is a single channel, suitable for hot area coverage
The mobile phone wants to surf the internet, must establish the uplink and the downlink channel. For example, if you click on a micro-letter, the hand sends a request through the uplink, and the micro-mail server passes through the downlink to upload your latest unread message to your phone.
TDD LTE transmit and receive signals are carried out at different time slots in the same frequency channel, and are separated by a certain guarantee time between each other. Currently, 18 countries have opened 23 TDD LTE commercial networks, with a total number of users exceeding 5 million.
TDD LTE uses a frequency that is both responsible for uploading and downloading, with 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 channel, and the uplink channel is less, so, in more times, a channel does not happen "traffic jam" situation. Because of this way of working, TDD LTE is more suitable for hotspot area coverage. When people use mobile phones, they are more likely to read, watch and download, with little time for uploading. If your mobile phone's wireless network is visible, you will find that there is a stream of data on the download channel, and the upload channel is rarely used. Therefore, in the use of mobile phone a lot of densely populated areas, the frequency of resources are more tense, more suitable for the construction of TDD LTE base station.
Second, FDD LTE is a two-way road, suitable for wide-area coverage
Unlike TDD LTE, FDD LTE uses two symmetrical frequency channels to transmit and receive signals separately, and there is a certain frequency band protection interval between the transmitting and receiving channels.
In other words, in order to establish uplink and downlink channel, FDD by frequency to split, at two symmetrical frequencies, a tube to download, a tube to upload. It's like a two-lane, two-direction car with no interference, unimpeded.
The advantage of TDD LTE is that it merges the upload and download channels into one and then controls it flexibly over time, such as 70% of the time allotted to the download, and 30% for uploads, so you'll find that the traffic is always full and resource utilization is higher. However, TDD LTE cannot be fully used, although it is particularly economical. In the user-intensive hotspot area, the frequency band resources are very tight, at this time, FDD's "two lanes" is very wasteful, TDD more suitable. However, because TDD is limited in the uplink and the base station coverage is less than FDD, TDD needs to build more base stations than FDD, which is too expensive for the wide coverage area (suburbs, towns and highways) of non hotspots.
Third, what is the difference between TDD and FDD
Below, we list the pros and cons of both (in FDD to compare the description)
(1) When using TDD technology, as long as the time interval between the base station and the mobile station is small and less than the channel coherent time, it can be simpler to estimate the channel characteristics according to the other's signal. But for the general FDD technology, the general frequency interval is much larger than the channel coherent bandwidth, can hardly use uplink signal estimation downlink, also can not use downlink signal estimation uplink; This feature makes the mobile communication system of TDD has obvious advantages in power control and the use of smart antenna technology. But also because of this, TDD system's coverage radius is small, because of the upper and lower row time interval, the base station coverage radius is obviously smaller than FDD base station. Otherwise, the cell edge of the user signal to the base station will not be synchronized.
(2) TDD technology can be flexibly set up and down conversion time, to achieve asymmetric uplink and downlink business bandwidth, is conducive to the implementation of the obvious Top-down asymmetric internet business. However, the setting of this conversion time must be coordinated with the neighboring base stations.
(3) compared with FDD, TDD can use a fragmented band, because the upper and lower rows are distinguished by time and do not require bandwidth-symmetric bands.
(4) TDD technology does not need to send and receive isolators, only need one switch.
(5) The moving speed of mobile station is restricted. When moving at high speed, the Doppler effect leads to fast fading, the higher the speed, the higher the fading frequency, the deeper the fading depth, and the higher the moving speed must be required. For example, in TD-SCDMA systems using TDD, based on the current chip processing speed and algorithm, when the data rate is 144kb/s, the maximum mobile speed of TDD can reach 250km/h, compared with FDD system, there is a certain gap. The general TDD Mobile can move only half as fast as the FDD mobile station or even lower.
(6) The transmitting power is limited. If TDD sends as much data as FDD, the launch time is only about half the FDD, which requires TDD to be sent in a larger power. Of course, it also requires more complex network planning and optimization techniques.
The Best of hybrid networking
Some experts said that TDD LTE and FDD LTE can be regarded as a system, only in the business implementation of a certain technical differences. As a result, there is a hybrid network of TDD LTE and FDD in the world to play their respective strengths, TDD LTE for hot area coverage, FDD for wide area coverage.
With a common technology base, TDD LTE and FDD have a very good prospect in hybrid networking. This is what ITU hoped to achieve when it developed the 4G standard-that is, to minimize the differences between the physical layers of the different standards, so that the network standards eventually converge.
Mobile in Hong Kong's 4G network is the use of TDD LTE and FDD hybrid network approach, accumulated experience in this area. From the current news, the Ministry of Information will also be the organization of operators to carry out a hybrid network verification.