Wavecom W61PC V7.5.0 - Manual (Page 112)

102  Fundamentals of Radio Data Transmission WAVECOM Decoder W61PC/LAN Manual V7.5
demodulator for the FM or AM carrier, while the decoder demodulator processes the sub-carrier modula-
tion.
ASK (Amplitude Shift Keying)
The simplest way to impress the digital information on a carrier is to key the carrier on and off represent-
ing the two levels of binary information. This was the earliest keying method and is still used for Morse
links. As most noise encountered at radio frequencies is amplitude noise, this modulation method is very
susceptible to noise.
FSK (Frequency Shift Keying)
As electronic equipment became more refined (with improved frequency stability) and the demands for er-
ror free communication increased, frequency shift keying (FSK) was introduced. In this mode the trans-
mitter is continuously on, but transmits alternately on two different frequencies, one representing Mark
level and the other one Space level. As amplitude noise is additive, it can be eliminated by clipping all
spikes above the carrier level. The difference between the two frequencies (the frequency deviation) is
called the shift, and may be for example 170, 425 or 850 Hz.
For many years FSK was the mainstay of digital radio communication.
PSK (Phase Shift Keying)
PSK manipulates the carrier phase information in conformance with the digital information to be transmit-
ted, by advancing or delaying the phase of the carrier.
As equipment sophistication increased, phase keying became more common and is now in use on modern
high-speed HF radio links, e.g. in MIL or STANAG based systems. In satellite communication PSK is the
main modulation method; for instance Inmarsat C uses BPSK (2PSK), and other Inmarsat systems use
QPSK (4PSK).
M-ary-FSK (MFSK), M-ary-PSK (MPSK)
Modes on the HF bands utilize frequency or phase modulation of a sub-carrier, which then in turn modu-
lates a carrier using suppressed carrier SSB keying, or by directly modulating a carrier. Decoding can be
done from the receiver AF or IF output (USB, LSB, CW or FAX demodulator). Examples of this type of
modulation are ordinary two-frequency FSK, four-frequency FSK (Twinplex) and the STANAG PSK modes.
M-ary designates the number of modulation levels. FSK designates common two-frequency FSK.
MFSK systems deviate from the classical binary transmission of '0' (Mark) and '1' (Space), as a single tone
carries more information. This is the reason for a higher element period in MFSK, compared with binary
transmissions having the same bit rate. This produces a substantial increase in the insensitivity to multi-
path propagation and noise. Examples of MFSK systems are PICCOLO, COQUELET and CIS-36.
Due to intensive research and the availability of highly efficient hardware and software, the waveforms
originally developed within the STANAG specifications of NATO have gained considerable influence in radio
data communications. This has resulted in very efficient modems utilizing multi-phase modulated single-
tones, combined with the use of adaptive equalization and sophisticated coding and demodulation and de-
coding.
Satellite TV broadcasting utilizes QPSK or even 8PSK.
A special instance of m-ary modulation is QAM (Quadrature Amplitude Modulation) in which phase and
amplitude modulation are combined. An example of QAM is digital broadcasting in the MF and HF bands
(DRM).
OFDM (Orthogonal Frequency Division Modulation)
OFDM is a combination of multiplexing and modulation. The signal to be transmitted is first split into a
number of separate channels modulated by the data and then re-multiplexed to create an OFDM carrier.
HF modem research is ongoing, to investigate whether this type of modulation is well-suited to combat se-
lective fading, which is a major problem in HF data communication. OFDM is used for Wireless LANs and
ADSL. When OFDM is combined with coding it is known as Coded OFDM (COFDM), which is used in DAB-
broadcasting to combat multi-path.