Page 56 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes The aim of digital baseband modulation methods, also known as line coding, is to transfer a
digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial
bus or a wired local area network.
The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a
phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over
another digital transmission system.
In music synthesizers, modulation may be used to synthesise waveforms with an extensive
overtone spectrum using a small number of oscillators. In this case the carrier frequency is
typically in the same order or much lower than the modulating waveform.

3.4.1 Frequency Shift-Keying (FSK)

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is
transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary
FSK (BFSK). BFSK literally implies using a pair of discrete frequencies to transmit binary (0s and
1s) information. With this scheme, the “1” is called the mark frequency and the “0” is called the
space frequency. The time domain of an FSK modulated carrier is illustrated in the figures to the
right.

3.4.2 Phase Shift-Keying (PSK)

Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or
modulating, the phase of a reference signal (the carrier wave).

Any digital modulation scheme uses a finite number of distinct signals to represent digital data.
PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each
phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented
by the particular phase. The demodulator, which is designed specifically for the symbol-set used
by the modulator, determines the phase of the received signal and maps it back to the symbol
it represents, thus recovering the original data. This requires the receiver to be able to compare
the phase of the received signal to a reference signal — such a system is termed coherent (and
referred to as CPSK).
Alternatively, instead of operating with respect to a constant reference wave, the broadcast can
operate with respect to itself. Changes in phase of a single broadcast waveform can be considered
the significant items. In this system, the demodulator determines the changes in the phase of
the received signal rather than the phase (relative to a reference wave) itself. Since this scheme
depends on the difference between successive phases, it is termed differential phase-shift keying
(DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no
need for the demodulator to have a copy of the reference signal to determine the exact phase
of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous
demodulation.

3.4.3 Quadrature Amplitude Modulation (QAM)

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme.
It conveys two analog message signals, or two digital bit streams, by changing (modulating) the
amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation
scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves,
usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers
or quadrature components — hence the name of the scheme. The modulated waves are summed,
and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-
shift keying (ASK), or (in the analog case) of phase modulation (PM) and amplitude modulation.
In the digital QAM case, a finite number of at least two phases and at least two amplitudes are

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