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Xianzao Classroom
", author: Teacher Tongxun ROCK, thank you.
In 2020, the Year of the Rat, China’s fight against the epidemic has attracted worldwide attention. This year, the first year of 5G, China’s communications also performed well.
According to the latest statistics, China has completed the construction of more than 700,000 5G base stations and the number of 5G terminal connections has exceeded 200 million. There is no doubt that Chinese communications people have delivered a satisfactory answer, and the domestic 5G market is booming.
On the other hand, there are frequent discussions about the high power consumption of 5G base stations. People are worried that the electricity costs of 5G base stations will overwhelm operators.
In today’s article, the author will analyze in detail the energy consumption of 5G and the energy-saving aspects of product design.
5G is actually more power-efficient
Do 5G devices really consume more power than 4G devices?
Unfortunately, the answer is yes. 5G consumes a lot of power for two main reasons:
4G base station RRU uses 8 antennas, the antenna matrix implements 2D MIMO, and the full power output is 160W RF signal; 5G base station AAU uses 64 antennas, the antenna matrix implements 3D MIMO, and the full power output is 320W RF signal. If the efficiency is the same, the energy consumption of 5G AAU is twice that of 4G RRU.
In fact, thanks to more efficient PA (Power Amplifier) chips and better DPD (Digital Pre-Distortion) algorithms, 5G equipment is more efficient than 4G. In other words, the energy consumption of AAU is less than twice that of RRU.
Due to the requirements of high bandwidth and Massive MIMO, 5G uses higher frequency bands. In the SUB-6GHz spectrum, China Mobile uses the 2.6GHz band (the same as 4G); China Telecom and China Unicom use the 3.5GHz band (the 4G TDD LTE band of the two operators is the 2.6GHz band).
According to the free space transmission loss formula of wireless signal:
Loss (dB) = 32.44 +20lg( d(km)) +20lg( f(MHz))
China Mobile's 5G coverage is comparable to 4G, while China Telecom and China Unicom's 5G coverage distance is about 0.75 times that of 4G, and the number of sites is theoretically increased by 1.8 times. To achieve the same coverage effect as 4G, the number of 5G sites is theoretically 1.2~1.4 times that of 4G.
Combining the above two factors, the energy consumption of the entire 5G network will be 2.4 to 2.8 times that of the entire 4G network.
It seems that the energy consumption of 5G cannot be underestimated. So, what is the reason why 5G chooses Massive MIMO and high frequency bands?
The advantages of Massive MIMO are obvious:
-
By utilizing the vertical and horizontal latitude antenna freedom, the utilization rate of time and frequency resources is improved;
-
Reduced interference between users;
-
Improve cell throughput;
-
Improve user experience at the cell edge.
High-frequency spectrum helps realize 5G Massive MIMO.
The size of the antenna is related to the frequency. The higher the frequency, the shorter the wavelength of the RF signal, and the smaller the corresponding antenna size.
5G's AAU currently uses a 64-antenna array, and will have 128-antenna arrays and 256-antenna arrays in the future. Large-scale antenna arrays have prompted 5G to choose high-frequency spectrum resources.
5G does consume more energy than 4G, but we cannot look at this issue in a one-sided way. Although 5G, the "thousand-mile horse", consumes more energy, it pulls a bigger car - the capacity of the 5G system is more than 20 times that of 4G.
In fact, in terms of power consumption per bit of information transmitted, 5G is more energy-efficient.
The energy consumed per bit of data in 5G is about 1/10 of that in 4G.
Energy conservation and emission reduction are always on the way
Let’s look at another set of data - the electricity bills of the three major operators in 2018: China Mobile 24.5 billion yuan, China Telecom 14 billion yuan, and China Unicom 12 billion yuan.
According to previous estimates, 5G energy consumption is 2.2 to 2.4 times that of 4G. It is estimated that after the full 5G network is built,
the annual electricity bill of the entire network will reach 120 to 140 billion yuan
. Although 5G will bring profit returns to operators, energy conservation and emission reduction of base station equipment is an issue that operators must pay attention to.
Chairman Mao proposed in "On Contradiction" that we should grasp the main contradiction of the problem. The main contradiction of base station energy consumption is AAU (RRU), and the main contradiction of AAU energy consumption is the
RF power amplifier of the transmitter
.
When talking about improving the efficiency of RF power amplifiers in AAU, we must mention a set of "golden efficiency improvement solutions":
Doherty+CFR+DPD
.
Let’s first look at the background of the generation of this efficient solution.
In fact, this high-efficiency solution has been used since the 4G era and is proposed for the OFDM system.
Since OFDM symbols are composed of multiple independently modulated signals, the synthesized signal may generate a relatively large peak power. The more carriers there are, the greater the peak signal power.
When analyzing this type of signal, the concept of Peak-to-Average Ratio (PAR) is proposed. PAR is the ratio of the peak power of the symbol to the average power:
PAR (dB) = Ppeak (dbm) - Pmean (dbm)
OFDM time-frequency signal
The characteristics of OFDM system signals are that the time domain is a non-constant envelope state and the peak power appears randomly. The appearance of large peak-to-average ratio signals will reduce the efficiency of the RF front-end power amplifier.
With the emergence of high peak-to-average ratio signals,
peak clipping technology (CFR)
was born.
CFR is a technology that reduces the peak-to-average ratio of signals. Peak clipping of signals will cause certain distortion, and excessive peak clipping will affect the bit error rate of the receiver.
In 4G and 5G mobile communications, the original peak-to-average ratio of the signal is more than ten dB. After peak clipping, the peak-to-average ratio of the signal sent to the transmitting unit is generally 6~7dB.
In order to meet the linearity index of the transmitter, engineers may choose a power back-off solution when designing the amplifier. Power back-off means that the power amplifier is allowed to output a power signal much lower than its saturation power to ensure the linearity index of the output signal.
The solution is easy to implement and has a simple structure.
For example, to output a 10W signal, engineers will choose a power amplifier with a saturation power greater than 10W as a solution.
However, the emergence of high peak-to-average ratio signals requires the power to be backed off to above the peak power. For example, if the output average power is 10W and the peak-to-average ratio is 6dB, engineers need to choose a power amplifier tube with a saturation power greater than 40W to make a solution.
However, the output power and efficiency of the power amplifier are directly proportional. In order to meet the linear indicators of instantaneous large signals, the use of a back-off solution in a high peak-to-average ratio system will lead to low efficiency. If the AAU transmitter amplifier uses a pure back-off solution, the efficiency of the final power amplifier will be less than 15%, and the efficiency of the entire unit will be less than 10%.
The Doherty amplifier solves the problem of low efficiency in high peak-to-average ratio systems.
Its structure is shown in the figure. Doherty amplifiers usually use two identical amplifier tubes to amplify the signal. Due to space limitations, its working principle cannot be explained in detail. Its biggest feature is that the efficiency of the amplifier when the output power is 6db lower than the saturation power is equivalent to the efficiency when the output power is saturated.
Doherty Structure
That is, in 5G systems, Doherty amplifiers have saturated power efficiency when outputting average power. For example, if the saturated power of a Doherty amplifier is 100W and the peak-to-average ratio of the input signal is 6db, when the output power is 25W, its efficiency is the same as when the amplifier outputs 100W.
Doherty efficiency curve
At present, the efficiency of the final Doherty amplifier of AAU of several major domestic manufacturers has reached more than 50%, and the efficiency of the whole AAU exceeds 40%. Compared with the pure fallback solution, the energy consumption of the whole Doherty solution is 1/4 of that.
But there are gains and losses. The Doherty amplifier is not omnipotent. It achieves the efficiency of output saturation power at 6db back-off, but this is at the expense of linear performance. In order to ensure the linear performance of the system, DPD comes into play.
First, let’s understand
the pre-distortion technology (PD)
. It is to artificially add a system with characteristics that are exactly opposite to the nonlinear distortion of the system. The two nonlinearities compensate each other and eventually eliminate the nonlinear component, as shown in the figure. DPD is a digital pre-distortion technology, and the pre-distortion signal is generated in the digital domain.
Linear compensation of predistortion
DPD technology collects and analyzes the nonlinear characteristics of the Doherty amplifier output signal, and compensates the original baseband signal in the digital domain, so that the signal output by the transmitter meets the linear performance index (ACPR) required by the protocol.
The efficiency of AAUs from major manufacturers now largely depends on the ability of their DPD algorithms to correct nonlinear power amplifiers. The stronger the correction capability, the higher the efficiency of the transmitter.
Let’s sort out the workflow of this high-efficiency transmitter solution:
-
CFR technology reduces the peak-to-average ratio of the signal to 6-7dB.
-
The Doherty amplifier amplifies the clipped signal and has the same efficiency when outputting average power as when outputting saturated power, but its linearity index (ACPR) is poor.
-
DPD technology corrects the nonlinearity of the amplifier so that the transmitter meets the linearity requirements of the protocol.
In addition to using Doherty+CFR+DPD, 5G has also implemented other efficiency-enhancing initiatives:
-
Choose a more efficient GaN power amplifier to replace the LDMOS power amplifier;
-
Apply clean energy such as solar energy and wind energy to the energy supply of 5G base stations;
-
"AI+Big Data" intelligently monitors and controls AAU channel switches, making the load of 5G base stations more efficient;
-
The separation of CU and DU and the centralized management of CU also improve efficiency to a certain extent and reduce 5G energy consumption.
Chinese communications professionals are constantly innovating and working hard to optimize AAU RF solutions and energy solutions to achieve higher 5G efficiency. Although 5G consumes more energy than 4G, its direct and indirect impacts are extraordinary.
The China Academy of Information and Communications Technology predicted in its White Paper on the Economic and Social Impact of 5G that “by 2030, in terms of direct contribution, 5G will drive total output, economic added value, and employment opportunities of 6.3 trillion yuan, 2.9 trillion yuan, and 8 million yuan, respectively; in terms of indirect contribution, 5G will drive total output, economic added value, and employment opportunities of 10.6 trillion yuan, 3.6 trillion yuan, and 11.5 million yuan, respectively.”
4G changed our lives, and 5G will change society. Let us wait and see!
*Disclaimer: This article is originally written by the author. The content of the article is the author's personal opinion. Semiconductor Industry Observer reprints it only to convey a different point of view. It does not mean that Semiconductor Industry Observer agrees or supports this point of view. If you have any objections, please contact Semiconductor Industry Observer.
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