Which of the following is the best 4G technology? Clarify the relationship between FDD and TDD (1)

Which of the following is the best 4G technology? Clarify the relationship between FDD and TDD (1)

1. Technical aspects: they are all born with a mother-in-law.

The technical differences between FDD and TDD are actually very small. The main difference lies in the adoption of different duplex modes. FDD and TDD are two different duplex modes.

FDD is used to receive and send messages on two separated symmetric frequency channels. It uses protective frequencies to separate the receiving and sending channels. 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.

Generally speaking, 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.

TDD adopts another method. It uses only one frequency for both upload and download. The advantage is that FDD saves a lot of time and consumes more resources (in fact, TDD needs to reserve a large protection band to avoid interference and consume some resources ). Because it is a "one-way street" that runs two-way "traffic flow", TDD can only control traffic through time (Time Division Duplex), so that the downloaded traffic will pass through and the uploaded traffic will pass.

Many people put TD-SCDMA and TDD together, in fact, there is no relationship between the two. Why should TDD be called a TD-LTE? You're just a brand name for TD-LTE.

As mentioned just now, the main difference between FDD and TDD is the difference between one-way and two-way streets. This difference leads to some network problems that everyone cares about, such as speed and coverage.

2. Speed | it doesn't make sense to be faster than anyone else

Theoretically, with the same bandwidth, for example, FDD allocates 10 M + 10 M, TDD allocates 20 M, and TDD delivers a lower rate than FDD, this is mainly because there is a frame called a special sub-frame in the frame structure of TDD. These frames will be wasted (such as the protective time slot) and will not transmit any data, FDD frames do not have such a waste.

Why is it meaningless to compare the speed theoretically?

To be honest, the speed difference caused by a few special sub-frames is not very large. This is not like the rate comparison between 3G and 4G, and it is not too high for users. Furthermore, there is always a gap between theory and reality. In reality, there are many factors that affect the speed, such as signal strength, interference, Channel configuration, UE performance, and number of users, this tests the comprehensive capabilities of a network.

In addition, with the continuous development of LTE technology, the application of carrier aggregation technology, and the future of LTE Networks, whether TDD or FDD, are not a problem.

3. High-Speed scenario | FDD wins TDD

The moving speed of the mobile device is limited. FDD is a continuous control system, and TDD is a time separation control system. During High-speed movement, the Doppler effect will lead to rapid decline. The higher the speed, the higher the frequency of the decline transformation, and the deeper the decline depth. Based on the current chip processing speed and algorithm, when the data rate is 144kb/s, the maximum movement speed of TDD can reach 250 km/h, which is different from that of FDD systems. Generally, TDD mobile stations can only move half or even lower of FDD mobile stations.

The reason for this problem is that the channel changes rapidly during high-speed motion, and both Channel Estimation and resource scheduling need to be fast. In FDD, the upstream and downstream operations are the same. If the mobile phone end finds that the quality of the receiving channel is poor, it can be quickly notified to the base station for adjustment through the uplink. However, in TDD, the channel information reported by the mobile phone may be delayed due to time-sharing. For example, after the channel change is detected in this reception, the upstream channel cannot be used to feedback the information immediately. If you have used a standalone walkie talkie, you may feel that when the other party speaks, you cannot speak at the same time, knowing that the person is yelling at you. You cannot respond immediately. When it is your turn to speak, you may only remember the last sentence that the other party scolded you for. You cannot remember the previous words, so it is too late to respond to them.

Therefore, FDD has an absolute advantage in high-speed rail coverage.

4. Coverage | coverage is the primary weakness of TDD.

Everyone said that TD-LTE is suitable for hotspot coverage, FDD is suitable for wide area coverage, why? The key is coverage.

According to a report from Qualcomm in the early years, FDD can provide better coverage than TDD under the conditions of the same frequency and the same power. TDD coverage is 80% lower than FDD (DL/UL =) /small 40% (DL/UL = 1:1 ). This is mainly because the TDD uplink transmission power (in a 10 ms frame) is shorter than the FDD time.

I used a netizen (unknown author) to analyze this issue.

The difference in coverage between the two is theoretically discussed. If TDD (up and down rows are 10 M in total) and FDD (up and down rows are 5 M + 5 M) if the bandwidth is the same, the power is the same, and the receiving mode is the same, the FDD and TDD Systems should not have the difference in coverage. However, we usually need to consider this coverage difference in LTE network planning.

Qualcomm once published a report saying that TDD only covers 80% of FDD and explains the reason that when the transmit power is the same, the TDD uplink has a transmit power time (within a 10 ms frame) it takes less time than FDD. In the FDD mode, the uplink and downlink frequencies are separated. Therefore, the uplink UE can continuously occupy 10 ms of transmit power, while in TDD mode, the same frequency can be reused at different times, therefore, the duration of the uplink UE transmitting power is less than 5 ms, which is related to the TDD frame structure. For example, when the configuration mode is 1, the ratio of available sub-frames in the upstream and downstream lines is 4: 4. In a 10 ms scenario, there are four sub-frames with transmit power in the uplink, that is, 4 ms, the uplink bandwidth is equivalent to 4/10 m. In the FDD mode, the uplink bandwidth is continuously occupied for 10 ms (5 M). Therefore, the uplink throughput of TDD is equivalent to 80% of that of FDD.