A Radio signal by itself is like a mail truck without letters. A radio signal alone, without superimposed information, is called a carrier wave. An unmodulated radio signal conveys only the information that there was once a source for the signal picked up by the receiver. Adding information to a carrier signal is a process called modulation. To modulate a radio carrier means that it is changed in some way to correspond to the speech, music, or data it is to carry.
The simplest modulation method is also the first used to transmit messages. The signal is turned on and off to transmit the characters of an agreed code. Text messages can be carried by the signal modulated in this way. Unique patterns stand for letters of the alphabet, numerals, and punctuation marks.
The least complicated modulation method capable of transmitting speech or music varies the carrier signal's instantaneous power. The result is called amplitude modulation, or AM. Another common system varies the signal's instantaneous frequency at an informational rate. The result is frequency modulation, FM.
If radio is to transmit speech and music, information must be carried that mimics the pattern of changing air pressure the ear would experience hearing the original sound. To transmit sounds these air-pressure changes are converted into electrical signals, amplified electronically, then used to modulate the carrier.
Amplitude modulation was the first process to have the capability of transmitting speech and varied the radio signal's instantaneous power at a rate that matched the original sound vibrations in the air. A better modulation technology followed that varied the instantaneous frequency of the radio signal but not the amplitude. Frequency modulation, or FM, has advantages compared to AM but both AM and FM are still in use.
Sound can be converted to digital data, transmitted, then used to reconstruct the original waveform in the receiver. It seems likely that a form of digital modulation will eventually supplant both FM and AM.
Amplitude Modulation (AM) is a system where the frequency of a carrier wave is held constant while the amplitude is varied in sympathy with the voltage of the modulating signal.
How far an AM station's signal travels depends on such things as the station's frequency (channel), the power of the transmitter in watts, the nature of the transmitting antenna, how conductive the soil is around the antenna (damp soil is good; sand and rocks aren't), and, a thing called ionospheric refraction. The ionosphere (see illustration below) is a layer of heavily charged ion molecules above the earth's atmosphere.
Ionospheric refraction is a big issue, because AM radio waves can end up hundreds and even thousands of miles away from where they started, and in the process interfere with all other stations on the same frequency.
But, as we'll see in a later module on international shortwave, ionospheric refraction can be good, because it makes possible long-distance communication.
Here's how that works.
Note that for AM radio stations the ground wave (in light blue above) doesn't go very far. This means numerous stations can be put on the same frequency without interfering with each other — assuming they are far enough apart. (Keep in mind that this drawing can't be anywhere near close to scale and show these things.)
The problem arises — if you want to see it as a problem — is the sky wave can end up in other states, provinces, or even in other countries.
The ionosphere is much more effective in reflecting these radio waves at night. (Incidentally, technically, it's refracting, not reflecting, but the effect is somewhat the same.)
That's why at sunset most AM radio stations in the U.S. and Canada have to:
- reduce power
- directionalize their signal (send it more in some directions than others), or
- go off the air (sign off until sunrise the next day)
This may explain why your favorite AM radio station goes off the air at sunset, or becomes much harder to hear (because of reduced power).
Frequency Modulation (FM) is a system where the amplitude of a carrier wave is held constant while the frequency is varied in sympathy with the voltage of the modulating signal.
To keep from interfering with each other FM stations must be 200KHz apart within the same geographic area. However, since the signals of FM stations cover only limited distances, the same frequencies can be used in different geographic areas of the country.
For the most part, FM and TV signals are line-of-sight. Although this means that FM stations don't interfere with each other, this characteristic creates a couple of other problems.
First, these waves go in a straight line and don't bend around the earth as AM ground waves do. Thus, they can quickly disappear into space.
So, the farther away from the FM or TV station you are, the higher you have to have an antenna to receive the FM or TV signal. Note that the earth is round — we hope this doesn't come as a shock to anyone — and, therefore, these signals will literally leave the earth after 75 miles or so.
And, there's another problem. Since FM and TV signals are line-of-sight, they can be stopped or reflected by things like mountains and buildings. In the case of solid objects like buildings, reflections create ghost images in TV pictures and that "swishing sound" when you listen to FM radio while driving around tall structures.
Basic Differences Between AM and FM
We need to mention a couple of other things before we leave the discussion of how radio works. We've talked about AM and FM radio, but we haven't explained the real difference.
In fact, there is a lot of difference — and not just a difference in the station numbers on your radio dial.
The first type of radio service — the one we've been talking about in the last couple of modules — was AM (amplitude modulation) radio.
The term modulation refers to how sound is encoded on a radio wave called a carrier wave; or, more accurately, how the sound affects the carrier wave so that the original sound can later be detected by a radio receiver.
In the top-left of this drawing the RF energy (carrier wave) is not modulated by any sound. There would be silence on your radio receiver.
In the broadcast process sound is made to affect (modulate) AM carrier wave by changing the amplitude (height) of the wave, as shown on the left.
Unfortunately, this type of modulation is subject to static interference from such things as household appliances — and especially from lightening.
AM also limits the loud-to-soft range of sounds that can be reproduced (called dynamic range) and the high-to-low sound frequency range (called frequency response, to be explained below).
FM radio, which came along in the 1930s, uses a different approach than AM. It's virtually immune to any type of external interference, it has a greater dynamic range, and it can handle sounds of higher and lower frequencies. This is why music, with its much greater frequency range than the human voice, sounds better on FM radio.
Note on the left that when the carrier wave of FM radio is modulated with sound that the distance between the waves, or the frequency of the carrier wave, changes.
Thus, AM radio works by changing the amplitude of the carrier wave and FM radio works by changing the frequency of the carrier wave.
Digital modulation methods
In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).
A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems.
According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission.
Fundamental digital modulation methods
The most fundamental digital modulation techniques are based on keying:
- In the case of PSK (phase-shift keying), a finite number of phases are used.
- In the case of FSK (frequency-shift keying), a finite number of frequencies are used.
- In the case of ASK (amplitude-shift keying), a finite number of amplitudes are used.
- In the case of QAM (quadrature amplitude modulation), a finite number of at least two phases, and at least two amplitudes are used.
In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.
In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase, frequency or amplitude.
If the alphabet consists of alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is symbols/second (or baud), the data rate is bit/second.
For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.
In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol.
Modulator and detector principles of operation
PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I+jQ (where j is the imaginary unit). The resulting so called equivalent lowpass signal or equivalent baseband signal is a complex-valued representation of the real-valued modulated physical signal (the so-called passband signal or RF signal).
These are the general steps used by the modulator to transmit data:
- Group the incoming data bits into codewords, one for each symbol that will be transmitted.
- Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values.
- Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing.
- Perform digital to analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP).
- Generate a high frequency sine carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine waveform with the I and Q signals, resulting in the equivalent low pass signal being frequency shifted to the modulated passband signal or RF signal. Sometimes this is achieved using DSP technology, for example direct digital synthesis using a waveform table, instead of analog signal processing. In that case the above DAC step should be done after this step.
- Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum
At the receiver side, the demodulator typically performs:
- Bandpass filtering.
- Automatic gain control, AGC (to compensate for attenuation, for example fading).
- Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sinewave and cosine wave frequency (see the superheterodyne receiver principle).
- Sampling and analog-to-digital conversion (ADC) (Sometimes before or instead of the above point, for example by means of undersampling).
- Equalization filtering, for example a matched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid intersymbol interference and symbol distortion.
- Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal.
- Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.
- Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).
- Parallel-to-serial conversion of the codewords into a bit stream.
- Pass the resultant bit stream on for further processing such as removal of any error-correcting codes.
As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.
Non-coherent modulation methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier wave. In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite is coherent modulation.
List of common digital modulation techniques
The most common digital modulation techniques are:
- Phase-shift keying (PSK):
- Binary PSK (BPSK), using M=2 symbols
- Quadrature PSK (QPSK), using M=4 symbols
- 8PSK, using M=8 symbols
- 16PSK, using M=16 symbols
- Differential PSK (DPSK)
- Differential QPSK (DQPSK)
- Offset QPSK (OQPSK)
- Frequency-shift keying (FSK):
- Amplitude-shift keying (ASK)
- On-off keying (OOK), the most common ASK form
- Quadrature amplitude modulation (QAM) - a combination of PSK and ASK:
- Polar modulation like QAM a combination of PSK and ASK.
- Continuous phase modulation (CPM) methods:
- Orthogonal frequency-division multiplexing (OFDM) modulation:
- discrete multitone (DMT) - including adaptive modulation and bit-loading.
- Wavelet modulation
- Trellis coded modulation (TCM), also known as trellis modulation
- Spread-spectrum techniques:
MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling).
OFDM is based on the idea of frequency-division multiplexing (FDM), but the multiplexed streams are all parts of a single original stream. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated sub-carriers are summed to form an OFDM signal. This dividing and recombining helps with handling channel impairments. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the orthogonal frequency-division multiple access (OFDMA) and multi-carrier code division multiple access (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users.
Of the two kinds of RF power amplifier, switching amplifiers (Class D amplifiers) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA, but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers.