Some key LTE technologies in Vernacular

Some key LTE technologies in Vernacular

I have never been in touch with the actual LTE equipment because I am working on CDMA, but I have read a lot of theoretical materials, including the red books. I personally feel that Chinese books are not very good yet, it covers a wide range of topics, but there are very few details. If you want to study it in depth, unless you look at the agreement, but how tired the agreement looks, not like the books of previous motorcycles and Ericsson, the knowledge point is thorough. Some people say that BAI baoshu is good. I plan to buy a book online. I am also a learning process.

OFDM

This technology is very mysterious. In fact, it has been used in wimax and wifi for a long time. I have said before that OFDM is not higher than CDMA spectrum utilization, however, his advantage is that the support of large bandwidth is simpler and more reasonable, and the combination of mimo is better.

For example, CDMA is a class that speaks Chinese and English. If the volume is well controlled, although it is a frequency, it can not interfere with each other, therefore, the bandwidth of 1.25m can achieve a rate of 4.9m. OFDMA can be imagined as a 10-meter-wide Viaduct in Shanghai, with a 5-meter-wide elevated road built on it. In fact, the Road Traffic Area is 15 meters, in this way, although the horizontal road surface does not increase, the number of accessible vehicles increases. OFDM also uses this technology to import orthogonal sequences using Fourier fast transformation, which is equivalent to setting up N Viaduct in a limited bandwidth, currently, the signal of the first half of an ofdm signal and the signal of the previous one is reused, and the signal of the second half and the next one is reused.

The signal frequency overlaps how to distinguish, It is very simple, OFDM, O is the meaning of orthogonal, orthogonal is to ensure uniqueness, for example, A and B overlap, however, A * a + B * B, a and B are different orthogonal sequences. If I want to obtain only A from the same frequency, then through calculation, (A * a + B * B) * a = A * a + B * B * a = A + 0 = A (because of orthogonal, a * a = 1, a * B = 0 ). Therefore, OFDMA allows overlapping frequencies, and can even theoretically overlap to an infinite number. However, to increase the sensitivity of demodulation, LTE currently supports half of the overlapping wavelength of OFDM.

MIMO

In fact, one of the important reasons why LTE abandoned CDMA in the Early Days was that CDMA had poor support for MIMO, while the sub-carrier data used by OFDM was converting serial data into parallel data, parallel data can be well adapted to MIMO reception. (For the reason why CDMA is not used, you can take a good look at hongbao)

MIMO is the meaning of multi-input and multi-output, so that I can transmit multiple signals in space, which can be divided into several categories. I will briefly introduce them to you (I just saw MIMO, I can't find the proper information, but I can understand it)

A. single-ant. Single-antenna transmission (Basic Mode)

B. transmitting-diversity. It is suitable for edge coverage and transmits the same data on different antennas in different modes to improve transmission quality. It is like asking for directions in Hong Kong. One person speaks Cantonese and the other person speaks Mandarin. You can always understand one.

C. open/close loop sdma. It is suitable for coverage, and increases the rate through space division multiplexing. For example, if your ears receive two different contents at the same time, the same amount of information doubles.

D and mu-mimo are suitable for scenarios where two users are separated from each other, and different content can be transmitted to two users to increase the amount of information output.

E. close loop rank = 1. Suitable for covering edges, transmitting from one antenna, receiving from two antennas, similar to c, but receiving from the same content from two ears increases reliability

F. Also, a waveform is aligned with the user. The smart antenna can locate the user direction based on the beam (wave interference principle) and align the main valve with it to increase reliability.

To put it bluntly, mimo is a small protocol between the base station and the transmitting antenna. By judging the user's location and signal strength, the base station determines the method used to send data to the user, which is implemented by the antenna, there may be many other types of sub-categories. I have not read them all here, and the details are confused.

HARQ

In fact, when I first saw this knowledge point, I thought that there was also a HARQ in CDMA, but I did not read it carefully. Later, I learned a lot about it. In CDMA, HARQ was designed to stop early, for example, this package was scheduled to be successfully launched four times. I asked the other party to send a reply each time. If it was sent to the NAK, it indicates that the package was not demodulated, if ACK is given, it indicates that the other party is demodulated and can be terminated. Similar to the judgment statement in programming, if the other party sends the ACK message twice, it indicates that the ACK message is successfully sent in advance, saves resources.

Why does HARQ in LTE come out separately, because it not only has the early stop function, but also has the error correction function, which is equivalent to HARQ = FEC + ARQ. FEC is the tubo code of QPP, for example, if I send a package, the other party does not demodulated the package. The other party does not discard the package, but keeps the package, returns the package to NAK, and sends the package for the second time. The other party receives the package and compares it with the package re, if the two missing content are complementary, the packet can be restored and ACK is returned. This is equivalent to two steps. The packet can be corrected and detected before ACK is returned, ARQ cannot be corrected.

PAPR

Because the subcarriers in the frequency domain overlap each other, if the two subcarriers are just positive energy, the merger will be higher, which may easily lead to a peak-to-average ratio, if the peak-to-peak ratio is poor, the power requirement is high. The direct effect is that the power amplifier utilization is low, and the linear power amplifier is better. For example, if you live in a house, if you live with Yao Ming, it will take 3 meters for the layer to be high. If there is no Yao Ming, it will be enough for the layer to be 2.7 high. What should developers do if they do not want to increase costs, the simplest way is to fold Yao Ming and keep him sitting. The same is true for OFDM. This technology is called PAPR. The following two methods are used:

A. throttling means that the signal is limited before it passes through a non-linear component to reduce the peak signal. in mathematics, a target value is set, multiply the value above the target value by a coefficient to bring it down (there are mathematical formulas in the book), just like bending over Yao Ming, but the falling part will interfere with the original part, it is also an unfavorable aspect.

B. Compression expansion. In addition to limiting the bandwidth, the high-power signal can be compressed, while the small-power signal can be amplified to narrow the gap, thus reducing the average peak ratio. In fact, IFFT is used to calculate an average amplitude value, which plays a neutral role when loading the average amplitude value in the inverse transformation process.

Add CP cyclically

Each sub-carrier of OFDM must be an integer waveform, but if latency occurs, it may not be complete at the receiving end. For example, if you send 1234 messages (in fact there is a protection interval), but due to latency, some messages exceed the receiver's time window, and the receiver receives 234 messages ......, In this way, the subcarrier cycle is not a complete wave, and the original orthogonal characteristics are lost, resulting in inter-Carrier Interference. Therefore, a concept CP is introduced.

Many people know CP, but may not know how to add it. In fact, the second half of the waveform is copied to the first half to form a protective band. The following example is also used, after cp is added, the value is 341234, of which 34 is cp, and the receiving end receives the value 41234. In this way, I can ensure that 1234 can be received, at the same time, 4 is part of my waveform (4123 itself is a complete wave, that is, the phase has changed but it can still be demodulated), without disrupting the orthogonal. After the receiving end extracts the CP, it restores the original data, which can solve the latency problem. The size of the CP is determined by your environment. Too many resources are occupied during the CP conference, affecting the speed. Too small may cause interference, which is also sensitive.

We have talked about key technologies such as OFDM, MIMO, adaptive coding, and HARQ. There is still one cell to eliminate interference. Now I have time to write it.

First, let's talk about the cause of Inter-cell interference. If you are in a single cell, you and other users are differentiated by OFDMA, that is, orthogonal, there will be no interference. But what if you are between two communities? That's different. You are in the same frequency between zone A and Zone B, and there is A unified OFDMA, so for this user, you use, B is interference. For example, if your student ID in your class is No. 1 and you answer questions in your class, you must be right if your student ID is No. 1, however, if your class is mixed with other classes, the teacher said, answer questions on the first day, then the first day does not necessarily represent you. This is called inter-zone interference.

How can we overcome this problem? Here are several methods.

1. Disturbing Method

In CDMA, there is also a disturbance. The function is to avoid full 0 or full 1, increase the demodulation reliability, and use LTE. The same is true for increasing the randomness of cell edge signals, I think it should be UEid and other things based on your original signal. In this way, the probability of the same number will decrease (in fact, the correlation will decrease, just understand it ), for example, if you are on the first day and the number is changed to 101 after the disturbance is added, the number 1 of another class may be changed to 201 after the disturbance is added. Then, the number is also on the first day, the probability of interference decreases.

2. Frequency Hopping Method

This is very simple. Anyone who has studied GSM knows how to stagger the frequency and cannot interfere with it. For example, if two classes are merged and one class only needs a ticket number, in another shift, as long as there is a dual number, each number will not conflict. At present, LTE has a sub-frame Internal Frequency Hopping and sub-frame frequency hopping, which means the frequency hopping range is different.

3. Transmitting Side Beam Shaping

This technology comes from the TD-SCDMA, is through the resonance to determine the direction of the user to be disturbed, in this direction of power down, to reduce the interference to it, listen to it is more mysterious, at present, it has proved that the use of td is very unsuccessful. LTE is only optional and has not been used by that company yet.

4. irc

Another abbreviation is actually not difficult, that is, to use the weighting between multiple antennas to overcome interference, which seems to be rarely used at present. I don't quite understand either. My understanding is similar to that where an antenna is x and y. I can solve the interference through different weights of the two antennas.

5. Inter-cell interference coordination

This is the most nb-effective, and I think it is also the most available. The main design idea is to separate the boundary users through different frequencies. This is different from the Hop Frequency, frequency Hopping increases randomness through frequency changes, while inter-cell Interference isolates two cells. The main implementation method is frequency-based coordination and power-based coordination.

Frequency Coordination is simple, that is, dividing the frequency into three equal points. users in the center of the residential area use all the frequency bands, and users at the edge use 1/3 of the frequencies. In this way, the edge user is equal to the frequency score, so there will be no interference. For example, the red, blue, and yellow colors are used by users in the center of the residential area to ensure the speed. The edge of the Residential Area A is red, the edge of the Residential Area B is blue, and the edge of the Residential Area C is yellow, in this way, the edge users will not be mixed, and there will be no interference.

This involves an important question. How can we differentiate border users? In fact, the UE will measure the quality of the residential area and report an RSRP. The residential area is to determine whether the edge is covered Based on RSRP, so as to decide whether to provide the full band or part of the band.

The other is power coordination. The function is the same as above, but the implementation method is different. Each of the three residential areas has a high frequency band power to ensure that the edges are occupied. For example, the yellow power of Area A is high, the blue power of Area B is high, and the red power of Area C is high. Although I did not distinguish edge users, however, from the perspective of coverage, the border is also divided into red, yellow, and blue.

Of course, it can also be divided into static and semi-static. All the above is static, and semi-static is more complex. We can decide which PRB is assigned to reduce interference.

I would like to talk about signaling, but considering that many people have practical experience and I don't even have access to the device, that's a lot of information. Let's talk about qos. Like cdma, the qos of LTE is based on bearer. That is to say, it is based on connection. If your connection is set, the qos is fixed. For example, if you repair a high speed, the qos is 120 km. If you repair a National Highway, the qos is 80. No matter what car is running above, the pdcp below, rlc, mac, and physical layers are more about maintaining the priority of this link than the package. At present, the minimum granularity of qos is this. LTE is classified into nine qos types, which are identified as qci. I hope you can understand that no qos granularity has reached the packet level, what if a person downloads a video and another video? Let's just say, create two hosts.