To make it ‘real’, we will compare two actual modulation
schemes: Linear Simulcast Modulation (LSM) – a type of
PSK, and Continuous Four Frequency Modulation (C4FM)
– a type of FSK. We will describe them, cover their relative
advantages and disadvantages, then use an example
simulcast system to show a practical comparison.
Modulator Bit Stream 1,0 Bit to Symbol + 3, + 1, - 1, - 3
Q IQ DIAGRAM
Eye open 100% of the time
Eye open 80% of the time
Before filtering After filtering
What is LSM?
LSM is a type of differential QPSK,
meaning the symbols are separated
by differences in phase between one
symbol and the next. There are four
symbols (+ 3, + 1, - 1, and - 3) so each
symbol conveys two bits of data.
Symbol rate is 4800 symbols/sec,
giving a data rate of 9600bits/sec.
Transitions between consecutive
symbols involve changes in
amplitude and phase, so the resulting
signal is non-constant amplitude.
The LSM diagrams on the right
show IQ, eye diagrams and spectrum
before and after filtering. At the
output of the IQ modulator (before
filtering) the transitions between
symbols appear as straight lines and
the eye is 100% open, but spectrally,
there would be many sidebands.
The signal cannot be transmitted
in this form, so filtering of the
I and Q outputs is necessary to
remove unwanted spectral content.
After filtering, the transitions now
appear curved and the ‘eye’ has
closed slightly, to about 80% open.
Spectrally, we have a much cleaner
signal that can now be transmitted.
What is C4FM?
Continuous Four Frequency
Modulation (C4FM) is a type of
FSK, so symbols are separated in
frequency. The four symbols (again
named + 3, + 1, - 1, and - 3) each
conveys two bits of data. Symbol rate
is 4800 symbols/sec, giving a data
rate of 9600bits/sec. Transitions
between symbols involve changes in
frequency, so the resulting signal is
WHAT IS IQ MODULATION?
IQ modulation is an efficient way to transfer digital information.
“I” represents the in-phase of the wave form: “Q” represents the