Lei Feng network (search "Lei Feng network" public concern) : Author Li Yilei, UCLA doctoral student, Qualcomm (RF group) trainee engineer, said (micro signal: silicon_talks) master pen.
The 5th generation mobile networks (5G) are getting closer to commercial use (2020). These days, Huawei, Samsung and other major manufacturers have also released their own solutions. Remarkable."
A key indicator of 5G is the transmission rate: According to the expectations of the communications industry, 5G should achieve a transmission rate that is more than ten times faster than 4G, that is, 5G transmission rate can achieve 1Gb/s. This means that it takes only 10 seconds to transfer a 1GB HD movie with 5G! In addition, such a high transfer speed will also bring some other applications, such as cloud games (games executed in the cloud server, directly to the implementation of the screen image back to the phone, so that the phone configuration is not high can play large games), virtual reality (same Put the operation into the cloud, the mobile phone is only responsible for outputting the picture) and so on.
How does 5G achieve such a high transmission rate?There are basically two methods to increase the transmission rate of wireless transmission. One is to increase the spectrum utilization, and the other is to increase the spectrum bandwidth. In wireless transmission, data is transmitted in the form of symbols. When the symbol transmission rate (code rate) is constant, the wireless bandwidth occupied by the signal does not change, and the amount of information data transmitted by each symbol is determined by the modulation method.
Modulation means how to use signal to transmit information.
In ancient times, people used beacon towers to transmit information. When there was a situation, they ignited the bonfire and extinguished the bonfire in each case. From modern communication theory, we have modulated the bonfire. Since the average bonfire has only two states (ignition and extinguishment), the beacon can transmit only one bit of information at a time (0 = extinguished = no enemies, 1 = ignited = enemies). Can the Beacon Tower improve its ability to pass more information at a time? We can achieve this by introducing more states. For example, in the improved Beacon Tower, we can control the fire of a camp fire and divide the fire into four states of extinguishment, minor fire, medium fire, and big fire so that we can pass two bits of information at once (00 = extinguished = no enemies, 01 = Small fire = there are enemies and far away from us, 10 = medium fire = there are enemies and not far from us, 11 = fire = there are enemies and soldiers have already approached the city).
However, not the best of both worlds is that introducing more states will also increase the possibility of error in information transmission . If the weather is bad, it may be seen as a small fire, so that the transmission of information is wrong. In contrast, if there are only two states (extinguish and ignite), the chance of an error is small.
This is also the case with modulation in wireless communication, which can produce different states of the carrier by manipulating the amplitude and phase of the radio waves. When the modulation method changes from simple to complex, the number of carrier states increases, and the amount of information (bit number) represented by one symbol also increases.
However, on the other hand, the spacing between each symbol state also becomes smaller, and therefore it is susceptible to noise interference so that the symbol deviates from the original position and causes decoding errors. Therefore, the complex modulation has relatively high requirements on the channel. Using a complex modulation in the case of a large channel noise will result in a high data transmission error rate, and the circuit required for decoding will also be very complicated, resulting in large power consumption.
State diagrams from simple (left) to complex (right) modulation
Compared to improving spectrum utilization, the method of increasing the spectrum bandwidth is simpler and more straightforward. With the same spectrum utilization, double the available bandwidth and double the data transfer rate that can be achieved. The problem is that the frequency bands currently used below 5GHz are already very crowded. Where can we find new spectrum resources? The method that most major manufacturers invariably think of is to use millimeter wave technology.
What is millimeter wave? Millimeter wave characteristics?Millimeter-wave refers to electromagnetic waves with wavelengths on the order of millimeters, and its frequency is approximately between 30 GHz and 300 GHz.
According to the communication principle, the maximum signal bandwidth of wireless communication is approximately 5% of the carrier frequency, so the higher the carrier frequency, the greater the signal bandwidth that can be achieved. In the millimeter wave band, the 28 GHz band and the 60 GHz band are the two bands that are most promising to use in 5G. The available spectrum bandwidth of the 28GHz band can reach 1GHz, while the usable signal bandwidth of each channel of the 60GHz band reaches 2GHz (the entire 9GHz available spectrum is divided into four channels).
In contrast, the highest frequency carrier in the 4G-LTE frequency band is up and down 2GHz, while the available spectrum bandwidth is only 100MHz. Therefore, if you use the millimeter wave band, the spectrum bandwidth can easily be turned 10 times, and the transmission rate can also be greatly improved. In the 5G era, we can use the millimeter-wave band to easily watch Blu-ray quality movies on the phone with 5G online, as long as you are not afraid of running out of traffic!
Available spectrum bandwidth comparisons in various frequency bands
Another characteristic of the millimeter wave band is that the attenuation in the air is large and the diffraction ability is weak . In other words, using a millimeter wave to achieve signal through the wall is basically impossible. However, the attenuation of millimeter waves in the air can also be exploited by us. It's not a bug, it's a feature!: The attenuation of the millimeter wave signal used by your mobile phone is indeed larger, but similarly emitted by other terminals. Millimeter-wave signals (which are interference signals for you) also attenuate so much that millimeter-wave systems are designed without special consideration of how to deal with interfering signals, as long as different terminals do not get too close together. The 60 GHz is chosen to take this point to the extreme, because 60 GHz is exactly the resonant frequency of oxygen, so the electromagnetic wave signal of 60 GHz attenuates very fast in the air, thus can completely avoid the interference between different terminals.
Of course, the fact that the millimeter wave attenuates very much in the air is also doomed to the fact that the millimeter wave technology is not very suitable for applications where the distance between the outdoor mobile terminal and the base station is far away. The major manufacturers' plans for using the 5G band are to use the more traditional 6GHz frequency band in outdoor open areas to ensure signal coverage, and use indoor microcells and millimeter wave technology to achieve ultra-high speed data transmission.
Millimeter wave must be used with a micro base station (or access point)
Another characteristic of millimeter waves compared to the conventional 6GHz frequency band is that the physical size of the antenna can be relatively small. This is because the physical size of the antenna is proportional to the wavelength of the waveband, and the wavelength of the millimeter wave band is much smaller than the frequency band below the traditional 6GHz, and the corresponding antenna size is also relatively small. Therefore, we can easily equip a millimeter-wave antenna array on a mobile device so as to realize various MIMO (Multiple-Input Multiple-Output), which means that multiple transmit and receive antennas are used at the transmitting end and the receiving end, respectively, so that the signal passes through the transmission. End-to-receive multiple antennas transmit and receive to improve communication quality) technologies, including beamforming (for beamforming, we'll talk more about this in the next article).
Millimeter wave transceiver chip how to achieve
Millimeter Wave Transceiver Architecture Diagram Developed by NICT
Commercial millimeter-wave transceiver chips use CMOS (complementary metal-oxide-semiconductor technology, which refers to a semiconductor-oxide-metal stack process to form a semiconductor device, and is the most commonly used integrated circuit manufacturing process) . Can integrate with digital modules, on the other hand to save costs.
The structure of the millimeter wave transceiver chip is similar to the traditional band transceiver, but the millimeter wave transceiver has a unique design challenge.
One is how to control power consumption. Millimeter-wave transceivers require CMOS devices to operate in the millimeter wave band, so CMOS devices are required to have very high signal sensitivity. We can refer to the tap in daily life to illustrate this issue.
Everyone must always have the experience of switching faucets. When many faucets are closed, they need to be screwed a lot and they will come out a little bit of water. Then, as the water flow gets larger, as long as the water flow is a little bit more, it will become much larger. Here, the action of the hand-tight tap is the excitation signal, and the corresponding flow change is the output response. CMOS devices are essentially like faucets. They adjust the output flow through the control port (ie, the CMOS gate) (water flow to the faucet and output current to the CMOS).
Therefore, if a CMOS device is required to respond quickly to a weak millimeter-wave signal, its DC current must be adjusted to be very large (equivalent to setting the faucet to a very large current). As a result, CMOS circuits require large amounts of power to process millimeter-wave signals.
Another problem that must be considered in millimeter wave chips is the transmission line effect.
I believe we still remember the force analysis in high school physics. (The left side of the figure below) It is very simple to analyze the force condition of a static rope (static analysis). The elastic force of the rope is equal to the pulling force of the person on the rope, and every point All the same, this problem belongs to the delivery question in the high school physics test. But if you do not pull the rope statically, but use your hand to swing the rope (right picture below), then a mechanical wave is generated on the rope. The force of each point is different, and the change of force depends not only on the hand. The force applied to swing the rope hand also depends on the material of the rope (determining the wavelength). It is difficult to analyze the force at this time, and it belongs to the level of high school physics competition.
Millimeter wave circuit design will also face similar challenges. We can analogize the wires in a circuit to a rope and analogize the source in the circuit to someone who applies force to the rope. When the frequency of the signal change is very slow, it is approximately equal to the static analysis. At this time, the signal at each point on the wire is approximately equal to the signal of the signal source. When the signal changes quickly, because the wavelength of the signal is close to or less than the length of the wire, we must carefully consider the condition of each point on the wire, and the nature of the wire (characteristic impedance) will greatly affect the propagation of the signal.
This effect is called "transmission line effect" in electromagnetism. When designing a millimeter-wave chip, it is necessary to carefully consider the transmission line effect to ensure the normal operation of the chip.
However, despite the design challenges, the large-scale commercialization of millimeter-wave chips has now dawned. Broadcom has introduced a 60GHz transceiver chip (BCM20138), which targets the WiFi standard (802.11.ad) in the 60GHz band and can also be seen as a solution to 5G millimeter-wave chip solutions. Qualcomm also did not delay to acquire Wilocity, which focuses on millimeter wave technology, two years ago. At the same time, Samsung, Huawei Hass and other heavyweights are also stepping up research and development of millimeter-wave chips. It is believed that in the near future we will see the millimeter wave RF chip market become very lively.
Wilocity's 60GHz chip
ConclusionMillimeter wave technology can realize ultra-high-speed wireless data transmission by increasing the spectrum bandwidth, and thus become one of the key technologies in 5G communication technology. Millimeter wave chip design must overcome the two major difficulties of power consumption and electromagnetic design, when these two problems are solved, large-scale commercial use is only a matter of time.
5G Communication Technology Series Articles:
"Interpretation of 5G Communication Technology | Massive Antenna Array Technology"
"Interpretation of 5G communication technology | How does beamforming add wings to 5G? 》
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