Expressways can improve traffic capacity through multiple factors such as multi-layer traffic, multiple lanes, lane direction, vehicle capacity, cargo packaging, and drivers.
We compare 5G to a highway, so how does 5G improve its traffic capacity? How fast can 5G millimeter wave be?
Today, let's do the math——
multi-level traffic
Modern highways are often elevated and interchanged, layer by layer, which greatly improves the traffic capacity and efficiency.
This kind of multi-layer traffic, in the 5G network, is actually the ability of mobile phones and base stations to use the same resources to simultaneously send and receive multiple channels of data, also known as MIMO (Multiple Input Multiple Output).
As can be seen from the figure below, the capabilities of mobile phones are different under different frequency bands. On the mainstream frequency band of 5G in China, 3.5GHz or 2.6GHz, the mobile phone can support 4 channels of reception and 2 channels of transmission; the millimeter wave frequency band is next, can support 2 channels of reception and 2 channels of transmission; low frequency like 700M has good coverage, But the mobile phone only supports 2 ways of receiving and 1 way of transmitting.
vehicle capacity
To increase the capacity of vehicles on the road, in 5G, is to increase the "modulation order". The higher the modulation order is, the larger the compartment is, and the more bits are carried at the same time.
5G adopts QAM (Quadrature Amplitude) modulation, and uses different phases and amplitudes of signals to represent different data. The figure below is a diagram of 16QAM. It can be seen that each point can represent different 4 bits according to the difference in amplitude and phase. data.
In practical applications, 64QAM or 256QAM is mostly used. Under 64QAM, the modulation order is 6, and 6 bits of data can be sent at the same time, a total of 64 (2 to the 6th power) combinations of 0 and 1; similarly, 256QAM can send 8 bits of data at the same time, a total of 256 (2 to the 8th power) a combination of 0 and 1.
Multi-lane (lane direction)
The allocation of lane directions can also affect the carrying efficiency of the road. For example, sometimes the traffic flow in one direction is dense, while the other direction is empty, which is equivalent to only half of the road utilization, and tidal lanes need to be introduced to optimize.
As can be seen from the figure above, the tidal lanes travel in different directions in different time periods to adapt to changes in traffic flow in different directions.
Similarly, 5G mainly adopts the TDD (Time Division Duplex) method. According to business needs, different time lengths are assigned to upload and download, so that resource utilization is better.
Below we use three typical frame structures of mmWave to illustrate TDD's flexible allocation of uplink and downlink resources. In the frame structure in the figure below, 0.625 milliseconds is a cycle, which contains multiple downlink time slots (D) and uplink time slots (U), and a special time slot (S) for uplink and downlink conversion.
Generally speaking, when people surf the Internet, whether they browse Weibo or watch movies, they mainly download content, and rarely upload content. This corresponds to frame structure option 1, which is the most conventional frame structure: the downlink time accounts for 77%, and the uplink time accounts for about 23%.
However, for applications such as high-definition video surveillance that mainly upload videos, frame structure option 1 is obviously inappropriate, so option 2 is needed: the downlink time accounts for 35%, and the uplink time accounts for about 65%.
Similarly, for applications such as remote video conferencing that have both download and upload, and the bandwidth requirements of the two are similar, it is necessary to balance the distribution of uplink and downlink time, which requires the use of frame format option 3: downlink time Accounting for 56%, uplink accounted for about 44%.
Highways generally have multiple lanes, and different vehicles can run side by side on different lanes. 5G is no exception. It divides its frequency bandwidth into multiple small units: subcarriers.
The smallest unit consisting of a subcarrier in the frequency domain and a symbol in the time domain is called a resource unit. The product of the frequency interval of the resource unit and the length of the time slot is a constant value, so the smaller the interval between subcarriers, the longer the length of the time slot; the larger the interval between subcarriers, the smaller the length of the time slot.
5G low frequency generally uses 15KHz subcarrier spacing, and the length of each time slot is 1 millisecond; intermediate frequency generally uses 30KHz subcarrier spacing, and the length of each time slot is 0.5 milliseconds; millimeter wave uses 120KHz subcarrier spacing, and the length of each time slot is only 0.125 milliseconds. Therefore, mmWave can support lower air interface delay.
The unit of subcarrier is too small, so 5G packs 12 subcarriers together, called a resource block (Resource Block, RB for short).
As can be seen from the table below, the maximum system bandwidth of 5G intermediate frequency is 100M, including 273 resource blocks; the maximum system bandwidth of millimeter wave is 400M, including 264 resource blocks.
Although the resource block of the millimeter wave is slightly smaller than the intermediate frequency, its time slot length is much shorter, only a quarter of the intermediate frequency, so the efficiency of transmitting data in the same time is much higher, and the upload and download rates are still limited. Great improvement.
cargo packaging
In road transportation, it is necessary to add packaging to the goods, protect the foam, etc. to prevent the goods from being bumped and damaged. Therefore, even if the carriages are fully filled, some parts are always "useless".
5G is no exception. Channel coding needs to add some redundancy to the data for error detection and correction. The highest encoding rate supported by the current 5G protocol is 0.92578, which means that up to 92.578% of the transmitted data is useful.
driver
There must be a driver to drive, and the space occupied by the driver cannot be used to pull goods. This part of the cost must be paid.
For 5G, some resource units are also used as control channels and cannot be used to send data. The resources used by these system controls are called "overhead".
The theoretical overhead of 5G low frequency and intermediate frequency is 14% for downlink and 8% for uplink; the downlink overhead of millimeter wave is 18% and 10% for uplink.
mmWave computing (example)
With the above information, we can calculate the 5G peak rate that the mobile phone can achieve.
We assume that millimeter waves with 400M bandwidth are used, and the frame structure option 1 is used to focus on the downlink. It can be calculated that the download rate is 2.98Gbps, and the upload rate is 0.75Gbps!
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