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What are software defined radios?

Updated: Jul 29, 2022

With the advancement of silicon technologies and printed circuit manufacturing in recent decades, there has been exponential growth in the use of electromagnetic waves for wireless communication. Radio-Frequency (RF) and Microwave (MM) bands that were once used exclusively by military systems, are now used by any consumers, like you and me, for communication between cell phones, televisions and even refrigerators. However, with the increasing use of the RF/MM spectrum, some problems arise. In a conversation between two people, if both try to talk at the same time, neither is able to transmit and receive information correctly. Likewise, if two devices try to use the same portion of the RF/MM spectrum at the same time, neither is able to transmit or receive data correctly.


With the above example in mind, we can understand the need for telecom engineers to visualize and understand how the dialogue between wireless devices takes place. For this purpose, equipment was created that could “listen” to this conversation in different ways. In cases of localized conversations, which take place in a small range of frequencies, it is enough that we have a receiver tuned to that frequency to hear the “dialogue”. However, on many occasions, it is necessary to understand the “dialogues” that occur in broad bands of the RF/MM spectrum.



Figure 1 - Comparison between Reception in an SDR system and a common RF Receiver

One of the solutions found by engineers is the use of Software Defined Radios (SDR), systems which allow use in broad bands of the RF/MM spectrum without the need for specialized equipment in each one of them. Through SDR systems we can check from low frequency bands (Low Frequency - LF) to very high frequency bands (Ultra High Frequency - UHF). In addition to being able to hear “dialogues” over large bands of the RF/MM spectrum, SDR systems have the ability to receive different modulations, equivalent to different “languages” in our previous analogy. As a result, SDR becomes a powerful tool for exploring and testing wireless systems, capable of assisting in the development of low-cost projects.


Figure 2 - Reception of signals of different frequencies and modulations by an SDR


History

The begining:


The idea of controlling radio systems via software was first mentioned in 1984. In this year the term “Software Controlled Radio” was coined by the company E-Systems, now known as Raytheon. The term was used to describe a new prototype developed by the company for military and espionage purposes, capable of selecting specific RF bands to be “listened to” and performing complex operations such as interference cancellation and signal demodulation through individual switching of several radio processors.



Figure 3 - Example of a software-controlled system

Despite having produced a system that, at first, meets the expectations of the introduction of this post, the system developed by E-Systems is considered a “Software Controlled Radio” and not a “Software Defined Radio”. As with writing the two definitions, the difference between the two lies in the details. It is then worth mentioning the definition of SDR, ratified by IEEE-P1900.1 in 2008:


"Radio where some or all physical layer functionality is software defined"

Unlike the E-Systems product, in which the physical layer functionalities were implemented in different hardware which were controlled and switched via software, the first SDR system built was the SPEAKeasy project, a military program developed in the USA by the Advanced Research Agency. Department of Defense (DARPA). With the success of the SPEAKeasy project, DARPA researchers were able to prove that many of the functionality of a radio, previously implemented through electrical systems, could be replaced by mathematical calculations performed by computers. From that starting point, other projects such as JTRS in the US and ESSOR in European states spread the use of SDR systems in the military/governmental environment.



Figure 4 - Example of a software-defined system

Recent Developments

In 2004, the FCC approved the development of SDR systems for commercial purposes, and from that moment onwards the emergence of several companies, such as Vanu Inc., Picochip and Lime Microsystems, committed to the production of increasingly capable SDR systems.

As at the beginning of the century, the progress of the most current SDR systems is directly linked to the interaction of the industry with the military environment and its needs. Faster Analog to Digital Conversion chips allowed for much higher bandwidth SDR systems, while the development of GPS/GNSS allowed the self-calibration of radio systems without the need for equipment downtime.

Due to increasing governmental and military demands for SDR systems, industries were encouraged to refine chip production techniques which led to reduced production costs as well as the size needed for SDR systems. The biggest example of cost and size reduction was the production of the RTL283U, a chip produced by the company Realtek with the ability to perform A/D acquisition of up to 24MHz together with a DVB-T COFDM RF demodulator and a USB2.0 communication interface. This chip features the ability to acquire RF signals in the range of 100kHz ~ 1.7GHz, with a cost of less than R$100.00.

Figure 5 - Example of SDR that uses the RTL283U chip

Software Defined Radio Gears


For those looking to undertake projects that include SDRs or even build their own SDR platform, it is important that a good understanding of how these systems work is achieved. For this we will use the diagram in Figure 5:



Figure 6 - Functional Block Diagram of an SDR system

Although the figure above shows the functional diagram of an SDR receiver, the blocks used are almost identical to those of an SDR transceiver. With this panorama in mind, we can separate an SDR system into 5 items: Antenna, RF Frontend, AD Converter, Symbol Demodulator, Decoder.


Antenna:


The first component encountered by an electromagnetic signal in your system will be the Antenna. For many, this component can be considered the most important part of a radio system. In a simplified way, in addition to defining more obvious parameters, such as the radio operating frequency, the antenna will also be responsible for factors such as directivity and the noise temperature of the system. All three factors are closely linked to antenna geometry and the type of antenna that is used.

The Antenna Operating Frequency defines which frequencies are received without significant signal attenuation. Antennas designed to be operated at 3GHz can receive signals at a higher frequency, such as 5GHz, or lower, such as 1GHz, however, the amplitude of the received signal is very low, which often makes its use unfeasible.

The directivity of an antenna concerns the attenuation of a received signal according to its location in relation to the antenna. Quasi-omnidirectional antennas, such as a dipole antenna, allow the reception of signals from almost all angles, in contrast, directional antennas, such as a quadrifiliate antenna, can only receive signals from specific directions.

Finally, we can mention the Noise Temperature of the antenna as the last factor to be considered in a simpler analysis. The noise temperature refers to the noise produced/captured by the antenna in relation to the environment in which it is located. This factor is difficult to measure and is therefore generally given by manufacturers. Antennas with higher noise temperature lead to the reception of signals with more noise and in very sensitive systems, such as astrometry and satellite receivers, can be decisive.

As examples of a choice that takes into account the above parameters, suppose we need to receive a WiFi signal 1km away from a common residential router. How would you choose an antenna for our SDR system? As we want to perform reception from only one point (the router in this case) we would need an antenna with high directivity and whose operating frequency is consistent with the WiFi range. Therefore, a good choice would be a satellite dish for 2.4 GHz frequencies (IEEE 802.11b/g/n/ax)


Figure 7 - Satellite Dish for 2.4GHz band1, and corresponding directivity graph

RF Frontend:


After reception with the antenna, the signal must be amplified, filtered and modulated by the Frontend RF. As several antennas can be used for the same SDR, in general we consider the Frontend RF the section of an SDR that defines the limits of its operation.

Industrial use systems make use of RF circuits with multiple stages of amplification and filtering so that the least amount of distortion and noise is introduced.

As RF circuits present great complexity, they will not be analyzed in this post. However, just understand that in general, the higher the range of frequencies we want to “listen to”, the more complex this portion of the SDR will be.


Figure 8 - USRP B2103 Printed Circuit Board

AD Converter:


After the amplification and filtering in the Frontend, the signal must be directed to the AD Converter (ADC) of the system. From this point on, analog signals are converted into binary numbers that can be read by a computer. For an ADC, three factors are initially important to be analyzed: Reference Voltage, Resolution and Sampling Frequency.

As in a conversation between people, we can only distinguish whether an individual “whispers” or “screams” based on some reference. Likewise, an ADC needs a reference so that it can assess the voltage level at its input. This factor is of great importance because if our reference is much larger than the signal to be acquired, we will not be able to “hear” with our SDR. In general, this parameter is chosen in conjunction with Frontend RF and its automatic gain module (ACG).

The other two parameters, Resolution and Sampling Frequency, define our ability to “hear” a signal. This topic is difficult to understand, but it is enough to know that the Resolution affects the maximum excursion of signals that we can “hear” (the term “Dynamic Range” is used for this characteristic), while the Sampling Frequency affects the maximum excursion of frequencies that we can “hear” at the same time (“Bandwidth”).

In a practical way, we are generally faced with a choice between being able to “listen” to signals of different amplitudes at the same time (suitable for analyzing Mesh systems for example), or signals of different frequencies (suitable for analyzing Multichannel systems for example).


IC

Resolution

Bandwidth

Realtek RTL2832U

8 bit I/Q

2.4 MHz

SiGe SE4110L

4 bit I/Q

8.0 MHz

Rafael Micro R820T2

12 bit I/Q

6.0 MHz

Maxim MAX2837

8 bit I/Q

20 MHz


Tabela 1 - Comparação entre ADCs4


Symbol Demodulator and Decoder:


After converting the analog domain to the digital domain, we can use computer algorithms to perform the next operations. In any electronic data transmission system, a message that is created by the transmitter must be encoded, that is, transformed into a sequence of binary numbers, and modulated, converted into a waveform that will be transmitted by the antenna. Likewise, in a radio receiver, the received signal must be demodulated into symbols and decoded (Fig. 9).



Figure 9 - Example of the Demodulation/Decoding process in FM signals

Taking figure 9 above as a reference, we can understand the process of encoding a signal as the language we choose to communicate with another person, while modulation can be understood as the means by which we communicate with him. An example of encoding would be the Portuguese language, and of modulation a written text, like this one. The reader has the task of being able to identify the letters/symbols and associate them in words and phrases (demodulation) so that they can understand the message that this text is intended to convey (decoding). In an SDR system both processes are carried out, in general, via software on a computer.


Having made this brief introduction to radio systems, in particular SDRs, we can more easily understand the benefits of using an SDR in your project and how we can make the choice of an SDR system, as well as the engineering challenges to be overcome.


SDR topologies


In engineering, problems of great complexity are usually dealt with where abstractions are often necessary so that we can communicate effectively. One way to communicate such complex concepts is through topologies. Instead of saying that an individual lives in a high-density residential complex with weight training and water aerobics facilities, we can say that someone lives in a building with a gym. In the same way, we can say that we will use SDR topologies.


GPP topology


GPP Topology: This SDR topology is the one we saw earlier. The signal is captured by the antenna and the Frontend, the acquisition process is performed by the ADC and the demodulation and decoding steps are performed by the CPU of a personal computer. While this is sufficient in most use cases, there are situations where there is not enough processing power available from the CPU.



Figure 10 - GPP5 Topology

GPP+GPU/FPGA topology


Topologia GPP+GPU: Nesta ocasião, utilizamos o paralelismo disponível da GPU de um computador para nos auxiliar nas etapas de demodulação e decodificação.



Figure 11 - GPP+GPU/FPGA5 Topology

Note that different topologies present different points of view of the problem to be optimized and thus have their advantages and disadvantages. The GPP configuration is simple and easy to build, while the GPP+GPU/FPGA configuration introduces the need for additional data transmission programs between the GPU/FPGA and the CPU. Therefore, the GPP+GPU/FPGA topology is optimized for performance, while the GPP topology is optimized for versatility.







References:

4. The Continued Evolution of Software-Defined Radio for GNSS, December 2017, James T. Curran, Carles Fernández-Prades, Aiden Morrison, Michele Bavaro

5. https://www.onesdr.com/2020/02/24/a-primer-on-rf-filters-for-software-defined-radio/

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