How is 5G modulation achieved?

Latest update time：2020-07-10

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When it comes to modulation, I think many students will immediately think of these keywords: BPSK, QPSK, amplitude modulation, phase modulation, QAM, constellation diagram...

As we all know, modulation and demodulation are important components in the basic business process of communication. Without them, our mobile communication would not be possible at all.

So, what exactly is modulation? Why is modulation needed? And how is 5G modulated?

Next, we will introduce them one by one.

What is modulation used for?

Let's look at an example from real life:

We travel every day. When we travel, we will choose the appropriate means of transportation according to our itinerary.

The speed of travel will vary depending on the mode of transportation you take.

The whole process is probably a model like this:

In fact, the communication system is similar to this model. The travel model above is to transport people from the starting point to the destination. The communication system is to transmit data signals from the sender to the receiver.

We perform the following conversion:

We can draw a simple communication model by analogy:

Do you see it? Modulation is like finding a vehicle for the signal to carry the information through the channel to its destination.

We know that in wireless channels, signals are transmitted in the form of electromagnetic waves .

So, how do electromagnetic waves transmit information?

Let’s first take an example of “ using fruit to convey information ”.

For example, if we want to pass 0 and 1, we can let apple represent 0 and banana represent 1.

We send it to the receiving end. When the receiver sees that it is an apple, he knows that the one sent is 0. When he receives the banana, he knows that the one sent is 1.

To put it another way, what if only apples could be used to transmit information?

We agree that red apples represent 0 and green apples represent 1.

When the receiver sees a red apple, he knows that the one sent is 0. When he receives a green apple, he knows that the one sent is 1.

Let’s look at it another way. If there are only red apples, how can we convey the message?

We can use a big red apple to represent 0 and a small red apple to represent 1. When we see a big red apple, we know that 0 is sent. When we receive a small red apple, we know that 1 is sent.

In this process, we actually use the three characteristics of fruit type, color, and size to convey information.

Similarly, electromagnetic waves can be described by sine waves. A sine wave also has three major characteristics: amplitude, phase, and frequency . We can use these three major characteristics of electromagnetic waves to transmit information.

The following formula (1) describes a sine wave signal:

The so-called amplitude modulation, frequency modulation, and phase modulation are as shown in the figure below:

Did you see it? 0 and 1 are “tuned” into different electromagnetic wave waveforms.

5G is so fast, how is it modulated?

In the 3GPP protocol (TS 38.201) , the modulation methods supported by 5G are defined as follows:

According to the characteristics of the carrier used, the modulation methods used by 5G can be divided into two categories:

The phase of the carrier changes, but the amplitude does not change: π/2-BPSK, QPSK. This is the PSK (Phase-Shift keying) mentioned earlier.
The phase and amplitude of the carrier wave are changed: 16QAM, 64QAM, 256QAM. This kind of technical term is called QAM (Quadrature Amplitude Modulation) .

Constellation

The differences between various modulation methods are still not easy to understand.

Think about it, why can we easily distinguish the differences between various fruits? ( What is an apple, what is a banana, what is a red apple, what is a big apple .)

This is because we have seen the real thing and have seen the fruit in different states.

So, can we also represent the modulation method using a graph?

Can.

In order to intuitively represent various modulation methods, we introduce a tool called constellation diagram . The points in the constellation diagram can indicate the possible states of the amplitude and phase of the modulated signal.

BPSK defines two phases, representing 0 and 1 respectively, so BPSK can modulate 1 bit of information on each carrier.

π/2-BPSK is a BPSK modulation signal with a phase shift of π/2 when the sequence has odd bits, and the phase of the even bits of the sequence is the same as the BPSK modulation signal. That is, π/2-BPSK defines four phases to represent 0 and 1.

The full name of QPSK is Quadrature Phase Shift Keying, which defines 4 different phases, representing 00, 01, 10, and 11 respectively. Therefore, QPSK can modulate 2 bits of information on each carrier.

16QAM: One symbol represents 4 bits.

64QAM: One symbol represents 6 bits.

256QAM: One symbol represents 8 bits.

Here is a dynamic diagram to help you understand:

QAM diagram ( from Cisco )

From the constellation diagram, we can see that the amplitude of the PSK modulated signal remains unchanged, but the phase changes. The amplitude and phase of the QAM modulated signal change.

正是因为每个符号能代表的bit数不断提升，使得携带的信息量提升，最终让这个“ 交通工具 ”能显著提升速率。

Maybe everyone thinks that 5G is not that difficult. Since we already have the two magic weapons of communication model and constellation diagram, can we build a next-generation communication system ourselves?

Hoho, do you think 256QAM is so easy to come up with?

Picture above!

Mapping relationship of 5G modulation methods defined in 3GPP 38.211 protocol

I'm confused! Are you?

In the end, communications is all about mathematics!

Modulation and demodulation principles

Let’s briefly talk about the principles of modulation and demodulation.

All modulation methods of 5G can be implemented using IQ modulation and demodulation .

We start from formula 1 and perform various magical formula conversions.

Draw Equation 2 as a block diagram, this is IQ modulation:

Demodulation is the process of extracting the received modulated signal. The modulated signal is converted into the original signal after demodulation. The demodulation process can be explained by the following formula.

From formula 3, we can see that after the received signal is multiplied by the carrier of the corresponding phase and integrated, the original signal can be obtained. Drawing formula 3 into a block diagram is IQ demodulation.

Combining the two block diagrams, we give the block diagram of IQ modulation and demodulation below.

IQ modulation can be understood in complex terms.

The modulation formula is described as follows:

The demodulation formula is described as follows:

Correspondingly, we give the block diagram in plural form.

This block diagram, combined with the 5G modulation mapping relationship in the previous 3GPP protocol, is a relatively complete 5G modulation and demodulation process.

Are you completely confused? Modem, from getting started to giving up!

Source: ZTE Documents

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