What are the typical differences between 5G wireless communication and 4G? What new technologies are used?(二)

TIME:2021-02-19      VIEWS:3

So is there a way to solve the above problem? The answer is still: yes, please see Beam Management.
Beam Management. The principle of this function is simple: The base station sends specific things like reference signals in all directions, and the terminal detects and gives a feedback to the base station, so that the base station knows the direction of the terminal.
LDPC coding. 5G abandons the turbo code used in 4G and replaces it with LDPC. Why to change it? How is LDPC code better than turbo? There are two main reasons: 1)The most important reason is that Qualcomm is awesome. 2) Because Turbo coding introduces operations such as interleaving, when the code length is longer, the complexity increases and the time delay becomes very large. However, as we said at the beginning, low latency is one of the three requirements of 5G, so Turbo seems a little weak. LDPC is different. Due to the sparseness of its check matrix, its decoding algorithm has a shorter delay, which has obvious advantages over Turbo when it comes to long codes.Therefore, 5G abandons Turbo and uses LDPC which sounds right.
Third, what are the things that have been applied in 4G, and 5G has been modified and used.
UL Waveform. 
We know that in 4G systems, OFDMA is used in the downlink and SC-FDMA is used in the uplink.This is due to the high peak-to-average ratio of OFDM and stricter requirements on equipment hardware.In order to reduce the cost of mobile phones, after discussion, everyone decided to use SC-FDMA instead of OFDM for the uplink transmission of the 4G system. So in the 5G era, what kind of multiple access is used?After the R15 agreement came out this month, I found no major changes in this area.A major change is that the uplink supports OFDM and DFT-S-OFDM.In addition, the sub-carrier spacing in 4G is fixed at 15Khz,However, because 5G is at high frequencies, the usable bandwidth is very large, so a new term is introduced: numerology.

In 5G, the sub-carrier spacing is not fixed at 15Khz in the 4G era, but variable, but one RB still has 12 sub-carriers, and this has not changed.


Subframe Structure. We know that in 4G, a radio frame is 10ms, a subframe is 1ms, and a slot is 0.5ms. In 5G, the length of the radio frame and subframe has not changed, and it is still 10ms and 1ms.But the slot length has become configurable, and its value depends on two parameters: μ and slot configuration. See below picture:


When μ is 0 and slot configuration is 0, 1 radio frame contains 10 subframes, 1 subframe contains 1 slot, and 1 slot contains 14 symbols.




When μ is 1, the slot configuration is 0, it becomes the following situation: 1 subframe contains 2 slots, and each slot has 14 symbols, That is to say, 1 subframe contains 28 symbols.




By analogy, when μ is set to 5 and slot configuration is set to 0, a subframe can have up to 32*14=448 symbols, which is ten times more than symbols. This brings the possibility of exponentially increasing the speed.
HARQ. 5G introduces the concept of self-contained subframes, that is, the HARQ cycle is from the minimum 4ms in the 4G era. Shorten it to within 1ms. This provides some help for the ultra-low latency.





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