Source: https://www.telecomhall.net/t/radio-...esources/11945

Introduction

Introduction

Modern communication networks such as LTE and 5G NR are the result of a continuous evolution in search for more efficient communication: in a nutshell transmit (and receive) the biggest amount of information in the shortest possible time.

However, the concepts needed for a complete understanding are many, and are somewhat complex.

Let’s try to understand how everything works from a simpler point of view, looking to do analogies with real-life examples, and try to assimilate such concepts in a natural way.

The understanding of these basic concepts, instead of understanding calculations only, brings us several advantages and built a solid foundation that will certainly help in the future.

So, let’s go and try to understand today, in an easy way, some of the main concepts of LTE and 5G NR networks?

Communication

The most basic and important concept to be understood when you want to learn a new communication technology is how data is transported from a point to other.

We can understand communication as the transport of information from one place to another. Given a certain information (INFO), how does it go from point A to point B.

We can represent the same above figure, but now with the main elements of a radio access network: a user equipment (UE) and a base station (BS). And of course, our data (1’s and 0’s or ABC) already represented.

For the above communication to happen, several transformations are necessary to convert information from the digital to analog world and vice versa, as well as several “standards” need to be followed - and both parties involved need to know about.

In other words, for both parties to “talk”, they need to speak the same language – or more specifically, same protocol.

The information will travel through a physical medium that is always analog (the real world is analog), but will also be logically controlled. Therefore, in all wireless communication we have the definition of physical channels and logical channels.

But let’s not worry now about understanding these concepts, as well as all the complicated modulations and techniques used to obtain better use of the signal (Spectral Efficiency).

Whatever the radio access technology used to communication (2G GSM, 3G UMTS, 4G LTE, 5G NR…) there is a very basic concept and common to all: the physical channel.

We can understand as a physical channel the means by which information will be transformed, in order to be transported.

Transporting information

Going back to our example, we could use a pipe, and directly transport the information from point A to point B.

But this analogy would be more related to transport by circuit (the pipe or tube). In our case, in modern networks the communication is made with packets. The information is broken into small pieces, just like small packets.

We can make an analogy of transporting packages with transport made by wagons. And in our example, if we broke the information into three parts, we’d need three wagons.

Our information must be broken into pieces according to existing wagons, which follow standard rules and formats.

And the size of the wagon that will carry our information follows a series of standards, in order to have the “best possible size” so that transport can also be done in the best way – for example without delays.

And to understand the “best possible size” of the wagon we need to consider the domains in which the information is represented.

Time and Frequency Domains

In more advanced networks such as LTE and 5G NR information is not only represented in the Time domain but is also represented in the Frequency domain. (And also, the Amplitude domain, but we can ignore this component for now).

The following figure helps us visualize this concept - the time-frequency plan.

We won’t go into detail for now of why and how this is done, but let’s just consider that the information uses all these domains.

And because it is also represented in the Frequency domain, it follows the rules of a very famous algorithm - Fast Fourier Transform - that allows you to convert a signal from its original domain to a representation in the frequency domain and vice versa. Although it is not simple (we will talk about it in another opportunity) the Fast Fourier Transform greatly facilitates the processing of the signal, which allows us to extract information from it and make it more appropriate for specific applications.

For now, let’s just “accept” that Fast Fourier Transform (or FFT) allows that, instead of working with all the bandwidth being transmitted, we work only with smaller parts of it.

Bandwidth

The 3G/WCDMA communication system is a broadband system - it works with the entire band. But as we saw above, this is not recommended, and that is exactly why it has evolved (remember LTE = Long Term “Evolution” ) and started to divide the band into smaller parts. Broadband communication systems like LTE and 5G NR don’t work with the entire band, but they divide the bandwidth into smaller parts.

For this, they use for example OFDMA, which allows the distribution of broadband into many (tens to thousands) narrow carriers that will carry the signals, thus obtaining a better spectral efficiency, and a robust resistance to signal degradation.

We see in the above example that in case of problems of degradation of the signal (i.e., due to fading), the transmission via several smaller multi carriers is better (only 1/4 of the signal was lost) compared with the single carrier transmission.

This is exactly what LTE does: it divides a broadband into several narrow carriers, with width of 15 kHz. It is very important to make clear however that this has nothing to do with modulation - it is only one way in which we divide a higher frequency into smaller orthogonal carriers, called subcarriers.

Now that we’ve briefly reviewed some concepts, let’s move on.

Just remembering that signals like LTE and 5G NR involve management of the Frequency, Time, and Code domains.

This is very complex – especially for those who are not very familiar with OFDM/OFDMA (the multiplexing by division of orthogonal frequencies) and that only name already scares.

So, let’s return to follow our analogy. Then we go back to the technical terms.

Wagon in Time

Going back to our analogy and trying to understand how our information behaves in time, we again have the figure of the wagon moving in time.

In our association, the wagon holds a certain amount of information. For example, we may have a seat, and one person can travel in it.

But the wagons can be different – having for example 6 or 7 seats, depending on the “spacing” between the seats.

Wagon in Time and Frequency

Until now in our analogy we used a wagon, in one domain only – Time domain. But we need to proceed with our analogy for the time-frequency plan, where for every moment (in time) we can have several associated frequencies.

So now in our analogy we continue with a wagon that moves in time - but also has seats.

Our wagon can be for two people sitting side by side. Or we could even have a wagon for 12 people, next to each other!

In our example, considering a wagon with 7 seats with 12 people sitting side by side, we would have a wagon moving as below, with 84 people!

Information in Time and Frequency

In our wagon example, we can consider that each person moving is able to carry information, for example a letter.

Let’s assume that every person who enter the wagon receives a letter and sit in a certain place.

The amount of information in each letter will depend on the conditions of the environment at the exact moment they were written.

It may be for example that some letters have been written in a well-lit place and using a good pencil. In this case, the letters will contain plenty of written words.

But it may be that some of them have been written with a bad (broken) pencil. And in this case, without much precision, the letters had to be written a little bigger, causing the number of words to be smaller.

And it may be that some letters have been written in a place with terrible conditions: in a place without enough light and writing with a broken pencil. In this scenario, the letter will contain very few words.

We can consider the “symbols” as the “person” in each place of the wagon.

LTE and 5G NR signals consist of many (tens to thousands) of narrow carriers carrying symbols – just like the wagons can carry people!

If we understand how information flows in the example we’ve seen so far, then we can understand how this applies to network communication systems like LTE and 5G NR!

And therefore, we can start to introduce some concepts and terminology specific to LTE and 5G NR.

Resources in Time and Frequency

Radio Access Technologies such as LTE and 5G NR have a complex three-dimensional arrangement of resource allocation – time domain, frequency domain and spatial domain. And precisely because of this, many people end up not fully understanding how they work, and simply give up.

But if we understand how allocation happens in the time-frequency plan the whole arrangement ends up being understood with ease.

So, let’s do it.

Radio Frame

In the Time domain, transmissions are organized in FRAMEs (which in LTE for example is 10 ms long).

Subframe

Each frame consists of 10 SUBFRAME lasting 1 ms.

Slot

Each subframe is made of two SLOTs, each one lasting 0.5 ms.

Symbols

And finally, each slot is made of 7 or 6 OFDM symbols, depending on whether a Short (Normal) or Long (Extended) Cyclic Prefix (CP) was used. (The useful duration of the symbol is fixed, as well as the duration of the slot - for this reason, depending on the size of the CP we will have 6 or 7 symbols).

The CP is created by prefixing each symbol as a copy of the end of the symbol (more details in our tutorial here[COLOR=var(--primary-medium)]5[/COLOR]). Longer CP is useful in environments with large propagation delays.

In the following figure we have also a “somewhat” proportional representation of frame, subframe, and slots sizes.

And we can also make the same representation without the 3D effect, which is what we find most in the documentations.

Right. So far, we’ve talked about the Time domain of time. Now let’s see the terminology in the frequency domain.

Resource Element (RE)

Let’s go back to our example and consider our wagon in the smallest possible configuration - with only one seat.

In the time domain we have the time slots and symbols - the wagon moves in the symbol time!

Now, in the frequency domain we have identified that each wagon is associated with a single small carrier (for LTE 15 kHz in LTE).

And the person sitting on it can carry information. The information (bits) is stored as a symbol through modulation[COLOR=var(--primary-medium)]3[/COLOR](QPSK, 16QAM, 64QAM, 256QAM) depending on the quality of the channel being used in that moment (QPSK = 2 bits, 16QAM = 4 bits, 64QAM = 6 bits,256QAM = 8 bits).

Just like our above example, where the letters were written and the final content was according to the the environment conditions.

So far we have seen the representation of a wagon with only one place. But as we’ve already seen, the wagon may have more seats - for example 12 seats next to each other (each seat associated with an unique and orthogonal subcarrier - of 15 kHz for LTE). And they move in time, for example during for example 0.5 in a slot with 7 symbols.

For the above considerations, the representation would be as below.

In the time-frequency plane above, where we represented the time slots, symbols and carriers that make up the signal, the smallest amount of resources we can identify is what we call the Resource Element (RE).

The Resource Element (RE) is the smallest amount of data (modulated by a certain amount of bits) that we can identify, and is formed by a subcarrier and an OFDM symbol.

Resource Block (RB)

Although the RE is the “smallest part of the signal” (a modulation symbol associated with a subcarrier), the smallest “usable" part of the signal is the Resource Block (RB) - consisting of 12 subcarriers (15 kHz in LTE) and 7 (or 6) symbols.

We say usable (the RB) because it is the smallest unit of resources that can be allocated to a user.

Resource Block (RB): a group of 12 subcarriers (12 x 15 = 180 kHz) in a time interval of 1 slot (0.5 ms).

[FONT=Arial]Thus, an RB has 12