A Comprehensive Guide to 5G Modulation: Understanding the Basics

Wireless communication has long been the primary means of communication in our work and daily lives. Whether it is the popular 5G, WiFi 6, Beidou, or the Bluetooth BLE supported by the Funpack’s fourth phase main character board, as well as several fiercely competitive standards in the Internet of Things, all are based on wireless communication. In the foreseeable future, wireless will occupy an increasingly important position.

A few weeks ago, Teacher Su gave a spoiler that the main character of the Funpack event in the first phase of the new year is the ADALM-PLUTO from ADI, a quite powerful SDR (Software-Defined Radio) pocket device.

So, before the official launch of the event, we prepared some interesting small knowledge related to wireless. Old hands can review, and new friends can understand the concepts, to have more fun in the upcoming events.

Today, let’s combine the popular topic of 5G and discuss an important component in the basic communication process: modulation.

Source: ZTE Document

When it comes to modulation, friends in the communication industry may quickly think of: BPSK, QPSK, amplitude modulation, phase modulation, QAM, constellation diagrams…

So, what exactly is modulation? Why is it necessary? How does 5G modulate? Let’s introduce them one by one.

What is the purpose of modulation?

Let’s look at an example from daily life: we travel every day. When traveling, we choose the appropriate mode of transport based on the itinerary.

The speed of travel varies with different modes of transport. The entire process can be modeled like this:

In fact, the communication system is similar to this model. The travel model above transports people from the starting point to the destination. The communication system transmits data signals from the sender to the receiver. We can make the following transformation:

This can be compared to a simple communication model:

Do you see it? “Modulation” is like finding a mode of transport for signals, allowing it to carry information across the channel to its destination.

We know that in the wireless channel, signals are transmitted in the form of electromagnetic waves. So, how do electromagnetic waves transmit information?

Let’s start with an example of “using fruits to transmit information.”

For example, to transmit 0 and 1, we can let an apple represent 0 and a banana represent 1.

When we send it to the receiver, if the receiver sees an apple, they know it is a 0; if they receive a banana, they know it is a 1.

In another way, if we can only use apples to transmit information, we can agree that a red apple represents 0 and a green apple represents 1.

If the receiver sees a red apple, they know it is a 0. If they receive a green apple, they know it is a 1.

In yet another way, if we only have red apples, how do we transmit information? We can use a large red apple to represent 0 and a small red apple to represent 1. If it is a large red apple, it indicates a 0. If it is a small red apple, it indicates a 1.

In this process, we are actually using the type, color, and size of the fruit to transmit information.

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

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

Amplitude modulation, frequency modulation, and phase modulation are represented as shown in the picture below:

Can you see it? 0 and 1 are “modulated” into different electromagnetic waveforms.

How does 5G achieve such fast speeds in modulation?

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

Depending on the characteristics of the carrier used, the modulation methods used by 5G can be divided into two main categories:

Carrier phase changes without amplitude changes: π/2-BPSK, QPSK. This is what was mentioned earlier as PSK (Phase-Shift Keying).

Carrier phase and amplitude both change: 16QAM, 64QAM, 256QAM. This category of professional terminology is called QAM (Quadrature Amplitude Modulation).

Constellation Diagram

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

Think about it, why can we easily distinguish between different fruits? (What is an apple, what is a banana, what is a red apple, what is a large apple?)

This is because we have seen the physical objects and the different states of the fruits.

So, can we also represent modulation methods using diagrams?

Yes. To visually represent various modulation methods, we introduce a tool called a 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, so BPSK can modulate 1 bit of information on each carrier.

π/2-BPSK is BPSK with a phase shift of π/2 for the odd positions of the sequence, while the even positions remain the same as the BPSK modulated signal, meaning π/2-BPSK defines four phases to represent 0 and 1.

QPSK, or Quadrature Phase Shift Keying, defines four different phases representing 00, 01, 10, and 11, allowing QPSK to 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’s an animated image to help understand:

QAM Illustration (from Cisco)

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

It is precisely because of the increasing number of bits represented by each symbol that the amount of information carried increases, ultimately allowing this “mode of transport” to significantly enhance speed.

You might think that 5G doesn’t seem so difficult. Since we already have the communication model and the constellation diagram, can we create our own next-generation communication system?

Hoho, do you think that 256QAM is that simple to create? Look at the picture above!

Mapping relationships defined in the 3GPP 38.211 protocol for 5G modulation methods

Feeling confused? Communication ultimately comes down to mathematics! (But it’s okay, you feel confused at first because you just saw it. The experts who do research in these communication theories have immersed themselves for many years. If you dedicate a few years like them, mastering the mapping relationships above will also be a piece of cake. Of course, mathematics must be learned well. — Editor of Hard Grain)

Principles of Modulation and Demodulation

Let’s briefly discuss the principles of modulation and demodulation.

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

Starting from Formula 1, we perform various magical formula transformations.

Drawing Formula 2 as a block diagram, this is IQ modulation:

Demodulation is the process of extracting the modulated signal received; the modulated signal is converted back to the original signal through demodulation. The demodulation process can be explained by the following formula.

From Formula 3, we can see that the received signal is multiplied by the corresponding phase carrier and integrated to obtain the original signal. The block diagram of Formula 3 represents IQ demodulation.

By combining the two block diagrams, we present the block diagram of IQ modulation and demodulation.

IQ modulation can be understood in terms of complex numbers. The modulation formula description:

Demodulation formula description:

We provide the corresponding block diagram in complex number form.

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

The modulation and demodulation process is quite complex. However, it’s okay if you don’t fully understand it now; there will always be time and opportunities to delve deeper. Nowadays, experimental instruments are powerful and readily available. As long as you take the initiative to try it out, starting from simple ASK, FSK, and PSK, you will gradually get the hang of frequency, phase, and amplitude.

Technical Materials and Communication Group

For friends interested in wireless technology or SDR, Pluto, we have prepared a communication group and technical materials. Please send “SDR” in the backend of the “Hard Grain Academy” public account to obtain the download link and group QR code for group communication (recently, there have been too many advertisements in the group, so we adopted this method and maintain the group in a timely manner, striving to provide everyone with a clean technical communication environment).

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