The range of frequencies used in radio frequency communication is called radio spectrum and includes frequencies between 3 kHz to 300 GHz.
High frequencies like microwaves ranging from 300 MHz to 300 GHz have line of sight limitations in the radio spectrum. Radio frequencies in lower ranges with higher wavelengths do not have a line of sight limitation. They can easily cover obstacles in their path and reach their destination due to the large wavelength of the signals.
The radio spectrum is further divided into several frequency bands in the following manner –
|3–30 Hz||105–104 km||Extremely low frequency|
|30–300 Hz||104–103 km||Super low frequency|
|300–3000 Hz||103–100 km||Ultra low frequency|
|3–30 kHz||100–10 km||Very low frequency|
|30–300 kHz||10–1 km||Low frequency|
|300 kHz – 3 MHz||1 km – 100 m||Medium frequency|
|3–30 MHz||100–10 m||High frequency|
|30–300 MHz||10–1 m||Very high frequency|
|300 MHz – 3 GHz||1 m – 10 cm||Ultra high frequency|
|3–30 GHz||10–1 cm||Super high frequency|
|30–300 GHz||1 cm – 1 mm||Extremely high frequency|
|300 GHz – 3 THz||1 mm – 0.1 mm||Tremendously high frequency|
Nowadays, radio communication is used in multiple applications and several fields, including military and commercial use of the radio spectrum. That is why the radio spectrum is regulated by every country. The Federal Communications Commission (FCC) in the United States, the European Communications Office in the European Union, and the Telecommunication Regulatory Authority of India (TRAI) in India are all examples of official bodies that regulate radio spectrum in the respective regions.
The high-frequency bands like 30 MHz up to 300 GHz are reserved for the telecommunication purposes like microwave communication, radar communication, radio, and television. Some radio bands are reserved internationally for industrial, scientific, and medical purposes other than telecommunications. These radio bands are called ISM (Industrial Scientific and Medical) radio bands. The ISM band is a part of the radio spectrum common worldwide for any application other than telecommunication in the Ultra high-frequency range. The frequency for the devices that operate in the ISM band normally ranges from few megahertz to 3 GHz. These frequencies are not reserved for any specific applications, and most of the frequencies in this band are free of license.
Not all frequencies within the ISM band are license-free. The license allocation and other restrictions like transmitting power limit for the ISM band vary worldwide, but there is a frequency band that is license-free worldwide. Actually, the ISM band is shared among non-ISM communication applications, which operate in 915 MHz, 2.450 GHz, and 5.8 GHz bands and are kept license-free. Of these, 2.4 GHz is the most commonly used license-free frequency. It is the same frequency used by Wi-Fi, Bluetooth, Zigbee, Cordless phones, and similar wireless communication technologies.
However, a large amount of transmission power is required for radio communication at 2.4 GHz. For radio communication over the same range as the 300 MHz frequency, a 2.4 GHz signal would require several times transmission power at the transmitter side than 300 MHz frequency. For license-free use, other options available are 433 MHz or 868 MHz frequencies. These frequencies are also available for license-free use in most regions of the world. There are also many wireless modules readily available which operate in these frequency bands.
After narrowing the selection to 433 MHz and 868 MHz, the question arises about what frequency is better for radio communication. The 433 MHz is license-free but has transmission power limitations. All the modules available on 433 MHz are of very low power, and due to this, they cannot communicate over a distance of more than 30 or 50 meters. With an antenna and maximum transmission power, a 433 MHz RF module can transmit data to a maximum distance of 300 meters but not beyond that. The 868 MHz RF modules are available with higher transmitting power and can have a range over a distance of more than 2 Km. Secondly, the 433 MHz bands can get highly congested because of the large number of devices operating in this frequency with a limited operational range. The antenna height for a 433 MHz RF module needs to be more than double the size required for an 868 MHz RF module. It would be around 16 cm which is undesirable in small-size devices. In the case of 868 MHz, the operational range will be more than 2 km, a small antenna will be required, and more transmission power can be supplied to the RF module. Also, the frequency band will not get easily congested due to fewer devices operating on this frequency and the availability of higher operational distance. So, 868 MHz is the frequency that should be selected for license-free radio communication.
That was all about selecting the frequency band; now let’s learn about the radio communication system or the radio electronics required for the communication project. A radio communication system includes a radio transmitter, a communication channel, usually air, and a radio receiver. A radio transmitter consists of a high-frequency carrier wave generator, intelligence or data circuit, modulator, and an amplifier. A radio receiver consists of an amplifier, demodulator, and output transducer.
At the transmitter, the information signal is synthesized from a source called an information signal or intelligence. This signal usually has a low frequency. The information can be digital data or audio waves, or any kind of media. A high-frequency carrier wave is generated at the transmitter on which the information signal is superimposed. Superimposing means that one of the characteristics like frequency, amplitude, or phase of the carrier wave is modified according to the information contained in the intelligence signal. This process of modifying carrier waves according to the information signal is called modulation. The modulated wave is amplified and transmitted using an antenna. The carrier wave travels through air and is detected by an antenna at the receiver. The receiver amplifies the detected wave as the wave losses considerable strength while propagating through the air over a large distance. A demodulator demodulates the amplified wave. The process of detecting information signals from the high-frequency carrier wave is called demodulation. The information signal is again amplified and passed to an output transducer for regenerating to the original form. For example, if the information signal was audio, the information signal will be passed to a speaker for regenerating the audio.
As already mentioned, one or more characteristics of the carrier wave are modified according to the information signal for transferring data over the high-frequency carrier wave. The characteristic altered can be frequency, amplitude, or phase of the carrier wave. Based on the feature of the carrier wave modified, modulation techniques are broadly classified into the following categories:
Amplitude Modulation (AM): In amplitude modulation, the amplitude of the carrier wave is modified according to the amplitude of the information signal. This is the first kind of modulation used in RF communication and is used in Longwave and Short wave radios.
Frequency Modulation (FM): In frequency modulation, the frequency of the RF wave is modified according to the amplitude of the information signal. Because the amplitude of the RF wave always remains constant, it gives the modulation technique several advantages over the Amplitude Modulation. This technique is widely used by FM radio stations, television, etc.
With the advancement in digital technology and electronics, most of the information and media also got digitized. It has now become less common that the information signal is in analog form. Even if information from a source is in analog form is first digitized for storage and manipulation by computers. So before transmission, the information is usually received from a computer and is in the form of a digital pulse. The same AM and FM modulation schemes, when implemented for digital information, are called digital modulation. The digital modulation techniques commonly used are ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), and PSK (Phase Shift Keying).
Amplitude Shift Keying (ASK): In ASK, the amplitude of the radio wave is shifted between two levels according to whether a logical 1 or logical 0 needs to be transmitted. In this modulation technique, the frequency remains constant, as in the case of AM.
Frequency Shift Keying (FSK): In Frequency Shift Keying, the frequency of the radio wave is shifted into two different values according to the digital information; logical 1 or logical 0. As in the case of FM, the amplitude remains constant. That is why this technique has several advantages over ASK.
Phase Shift Keying (PSK): In PSK, the phase of the radio wave is modified according to the digital information; logical 1 or logical 0. This phase modulation is a technique that is specifically designed for transmitting digital information.
The different digital modulation techniques have different advantages and disadvantages. Knowing these advantages and disadvantages helps in selecting a modulation technique for the radio communication project as per its requirements.
Amplitude Shift Keying (ASK) – It is easier to construct ASK transmitters and receivers. An ASK transmitter consumes less power compared to an FSK transmitter. Because, in amplitude shift keying, the amplitude is modified and frequency remains constant, an ASK signal requires small bandwidth. The major disadvantage of ASK, like the amplitude modulation, is noise.
Frequency Shift Keying – Frequency Shift keying also involves simple electronics, and its transmitter and receivers can be easily constructed. An FSK transmitter requires more power as the carrier wave frequency is altered according to the information signal. For the same reason, it requires more bandwidth. However, FSK signals are more immune to noise than ASK signals which is why they are better for carrying information despite slight cost increment and bandwidth requirement.
Phase Shift Keying – In phase-shift keying, both frequency and amplitude remain constant, and phase is modified. That is why PSK is more robust to noise and also requires less bandwidth, even in comparison to ASK. However, the electronics involved in building PSK transmitter and receiver are quite complex, which is the only disadvantage of this modulation technique.
When any radio communication system is designed, it is judged by certain performance parameters. The major concerns of the designers of radio communication systems are as follows:
Range: The maximum distance at which the radio communication system can successfully transfer information. It is the maximum distance at which the receiver can detect carrier waves and successfully extract information signals from them without loss.
Bandwidth: It is the range in which the modulated radio wave’s frequency varies. The communication devices and communication channels should be capable of handling frequencies in that range.
Data rate: The data rate is simply the speed at which the information (digital data) can be transferred in a radio communication system. It is usually expressed in bits per second or bytes per second.
The performance parameters mentioned above depend upon several design factors like transmitter power, receiver sensitivity, deviation, bit error rate, channel spacing, communication techniques like Phase Locked Loop, Frequency hopping, etc.
In the next tutorial of this series, we will design a radio communication system using a wireless microcontroller board and set it up for use. The series will continue with the examination of various performance parameters by altering the design factors programmatically.
Original: Engineers Garage