Radio Modulation
4.2 The Basic Methods of Modulation
The modulation of the radio carrier signal may follow either an analogue or a digital regime. Analogue modulation has three basic forms: Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM, also known as quadrature modulation).
Digital modulation can be either be by `on/off' carrier signal modulation (i.e. by switching the carrier on and off), or by other methods such as Frequency Shift Keying (FSK), Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). These are really just variants of the analogue modulation methods. We discuss the various modulation methods in turn, commencing with the analogue methods.
Amplitude Modulation
Modems employing amplitude modulation alter the amplitude of the carrier signal, so that the trace outline of the carrier signal amplitude (Figure 4.3a) matches the original analogue signal. For amplitude modulation to workcorrectly, the frequency of the carrier signal must be much higher than the highest frequency of the information signal. (This ensures that the
`peaks and troughs' of the carrier signal of Figure 4.3a are more frequent than the `peaks and troughs' of the information signal (Figure 4.3b), so enabling the carrier signal to `track' and record even the fastest amplitude changes in the information signal.)
Amplitude modulation is carried out simply by using the information (end-user) signal to control the power of a carrier signal ampli®er. Demodulation can be carried out by ®ltering the carrier signal with a baseband ®lter.
In the digital version of amplitude modulation, the amplitude of the carrier frequency is varied between a given amplitude and zero. These two amplitude states correspond
The Basic Methods of Modulation 59
effectively to `on' and `off', respectively to values `1' and `0' of the modulating digital bit stream. Alternatively, two different, non-zero values of amplitude may be used to represent
`1' and `0'. A digital amplitude modulation scheme is illustrated in Figure 4.3c.
Frequency Modulation
In Frequency Modulation (Figure 4.4), it is the frequency of the carrier signal that is altered to carry the signal content of the modulating bit stream. The amplitude and phase of the carrier signal are left largely unaffected by the modulation process.
Frequency modulation is achieved simply by mixing the information and carrier signals.
The interference of the two signals during mixing leads to inter-modulation, creating sidebands (products of inter-modulation) near to the carrier frequency. A given frequency f in the original information signal creates intermodulation products at frequencies fc7f and fc+f, where fcis the carrier frequency. An original information signal occupying the band from frequency f1 to f2 is thus mapped into two sidebands fc7f2 to fc7f1 (lower sideband) and fc+f1 to fc+f2 (upper sideband). The sidebands thus mirror the signal spectrum of the original information signal. Figure 4.5 shows an example of frequency modulation of a baseband voice telephone channel (in the baseband 300 Hz to 3400 Hz) onto a carrier of 8000 Hz. This is the basis of Frequency Division Multiplexing (FDM) which we will discuss later in the chapter.
It is normal to ®lter the signal following modulation to remove both the baseband and one of the sideband signals (single sideband mode), and in some cases, the carrier signal is also removed prior to transmission (suppressed carrier mode). This leaves just a single sideband (or a single sideband and the carrier signal). This signal is ampli®ed for radio transmission. The bene®t of the suppressed carrier mode is that the transmitter power is not
`wasted' simply sending the carrier signal. Also, sending just one sideband is suf®cient, since it contains all the necessary information to recreate the baseband signal.
At the receiver, demodulation of an FM signal is achieved by once again mixing the carrier frequency with the received sideband signal. This again produces two `sideband-like' intermodulation products. One of these products is a reconstruction of the original
60 Radio Modulation
Figure 4.3 Amplitude modulation
baseband signal. The other is a high frequency intermodulation product which is removed using a ®lter.
One of the disadvantages of the suppressed carrier mode of operation is that an accurate oscillator and signal generator is required at the receiver in order to create the carrier signal necessary for demodulation. Thus, particularly in one-way broadcast radio systems, where the aim is often to keep the cost of receivers as low as possible, the suppressed carrier mode is unusual. Conversely, where an oscillator and carrier signal generator is available at both
The Basic Methods of Modulation 61
Figure 4.4 Frequency modulation
Figure 4.5 Spectrum content of a Frequency Modulated (FM) signal (narrowband frequency division multiplexing)
ends of the radio link(e.g. in a duplex public networklinkÐ where the carrier signal is needed for transmission anyway), the suppressed carrier mode is feasible.
Modems using frequency modulation for encoding of digital data (i.e. digital radio modems) are either OOK (on-off keying) or FSK (frequency shift keying) modems.
The simplest form of digital frequency modulation is OOK (on-off keying). An OOK radio simply modulates the radio carrier signal by switching it `on' and `off' according to the value `1' or `0' of the data `bit' to be carried (Figure 4.6a). Morse Code is a type of OOK.Frequency Shift Keying (FSK) is a slight advance on OOK, in which radio carrier signals of two or more different frequencies (both located within the `bandwidth' of the channel) are used to represent different `1' or `0' bit values of a digital bitstream. The simplest form of FSK is 2FSK in which two different frequencies are used to represent the bit values `0' and `1' (Figure 4.6b). The advantage of FSK over OOK is that a signal is always being sent, so the receiver can always tell that the transmission is active.
More complicated versions of FSK include 4FSK (Figure 4.6c). 4FSK was commonly used in previous generation digital radio equipment before the emergence of QAM (quadrature amplitude modulation, which we discuss later in this chapter). In 4FSK, four different frequencies are used to represent two digital bits at a time, one frequency each to represent the consecutive bit combinations 00, 01, 10 or 11. By coding two bits at a time, the baud rate of the transmitted radio signal may be reduced. The baud rate or symbol rate is the rate at which the radio receiver must be able to distinguish the symbols of the incoming signal. The higher the baud rate of the signal, the greater must be the agility of the radio receiver and transmitter. The signi®cance of the baud rate is that the maximum
62 Radio Modulation
Figure 4.6 Different forms of Frequency Shift Keying (FSK)
baud rate which may be carried is roughly equal to the bandwidth of the radio channel (carrier) being used.
Phase Modulation
In phase modulation, the carrier signal is advanced or retarded in its phase cycle by the modulating signal (end-user provided information). Phase modulation has appeared with the advent of digital radio systems, and Figure 4.7 illustrates the coding of a radio carrier signal to carrier digital information.
Under digital phase modulation, the radio carrier signal (at the beginning of each new bit) will either be allowed to retain its phase or will be changed in phase. Thus, in the example shown the initial signal phase represents value `1'. The change of phase by 1808 represents next bit `0'. In the third bit period the phase does not change, so the value transmitted is `1'.
Phase modulation (often called phase shift keying, or PSK) is conducted by comparing the signal phase in one time period to that in the previous period, thus it is not the absolute value of the signal phase that is important, rather the phase change that occurs at the beginning of in each time period.
The advantage of phase modulation is that radio systems using it are relatively less prone to the interference of noise and other disturbing signals. As a result, it is possible to get away with a weaker signal arriving at the receiver, and yet still recover the original digital signal without errors. Radio systems using phase modulation are thus less prone to link outage (unavailability). For a similar length of link, such radios exhibit a higher availability. Or seen another way . . . the same availability of linkcan be achieved over greater link range.
Quadrature Amplitude Modulation
The most modern digital radio systems employ Quadrature Amplitude Modulation (QAM). In QAM, the frequency of the radio carrier signal is left unchanged during
The Basic Methods of Modulation 63
Figure 4.7 Phase modulation of a digital signal
modulation, but the various bit patterns within the digital stream are instead coding onto the carrier by means of signal amplitude and phase (or quadrature) changes. Very sensitive QAM radios may be built. This is the advantage of QAM. Just like FSK, there are also different variants of QAM, allowing higher modulation such as 4-QAM, 8-QAM, 16-QAM, 64-16-QAM, etc. The value 4, 16, 64 or whatever represents the number of different quadrature/amplitude combinations used in the modulation scheme. Thus, in 16-QAM modulation, 16 combinations are available, allowing 4 bits (equivalent to binary value 16) to be carried for each symbol. 64-QAM carries 5 bits per symbol, and so on.
The signi®cant advantage of higher modulation schemes is the increased number of bits per symbol which may be transmitted. Thus, while 4-QAM is able to carry about 1 bit per Hertz of radio bandwidth, 16-QAM achieves about 2 bits per Hertz and 64-QAM 4 bits per Hertz. However, there is a penalty for higher modulation. This is that a more sensitive radio receiver is required, so either the equipment is more expensive or the system range is restricted. We consider the subject of higher modulation (also called multilevel transmission) next.