Digital CW modulation
techniques
By
Mrs. Leena Mehta
Why Do We Need Digital-to-Analog
Conversion?
•The medium/channel is band pass, and/or
•Multiple users need to share the medium.
Digital-to -Analog Modulation
• Process of changing one of the characteristic
of an analog signal based on the information
in a digital signal
• Sine wave is defined by 3 characteristics
(amplitude, frequency, and phase) ⇒ digital
data (binary 0 & 1) can be represented by
varying any of the three parameters.
• application: transmission of digital data over
telephone wire (modem)
Types of Digital Types of Digital -to -
Analog Modulation Modulation
• Amplitude shift keying
• Frequency shift keying
• Phase shift keying
ASK
• ASK – Amplitude of carrier signal is varied to
represent binary 1 or 0
• both frequency & phase remain constant
while amplitude changes
• commonly, one of the amplitudes is zero
• S(t)=d(t)sin(2∏fct)
ASK
• ASK transmitter:
• Waveforms:
ASK spectrum
•Bandwidth of ASK = 2fb
•Disadvantages of ASK: It demonstrates poor
performance, as it is heavily affected by
noise and interference.
Frequency Shift Keying (FSK)
Baseband
Data
FSK modulated
signal
f1 f0 f0 f1
• FSK can be expanded to a M-ary scheme,
employing multiple frequencies as different
states.
FSK Transmitter and receiver
Fig: BFSK transmitter
Fig: BFSK receiver
Phase Shift Keying (PSK)
Baseband
Data
Binary PSK modulated
signal
s1 s0 s0 s1
where s0 = -A cos wct and s1 = A cos wct
Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and
FSK.
PSK can be expanded to a M-ary scheme, employing multiple phases and
amplitudes as different states.
Filtering can be employed to avoid spectral spreading.
Modulation - QPSK
(-1,1) Q (1,1)
Odd Data Q-Channel
(NRZ)
0
90
QPSK I
Cos Wc t
Even Data
(NRZ) I-Channel
(-1,-1) (1,-1)
Wc = Carrier Frequency, I = In phase channel, Q = Quadrature channel
Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q),
and therefore exhibits the same performance but twice the bandwidth efficiency.
Quadrature Phase Shift Keying can be filtered using raised cosine filters to achieve
excellent out of band suppression.
Large envelope variations occur during phase transitions, thus requiring linear
amplification.
Multi-level (M-ary) Phase and Amplitude Modulation
Amplitude and phase shift keying can be combined to transmit several bits per
symbol (in this case M=4). These modulation schemes are often refered to as linear,
as they require linear amplification.
16QAM has the largest distance between points, but requires very linear
amplification. 16PSK has less stringent linearity requirements, but has less spacing
between constellation points, and is therefore more affected by noise.
M-ary schemes are more bandwidth efficient, but more susceptible to noise.
16 PSK 16 QAM 16 APSK
Coherent Reception
An estimate of the channel phase and attenuation is recovered. It is then possible to
reproduce the transmitted signal, and demodulate. It is necessary to have an
accurate version of the carrier, otherwise errors are introduced. Carrier recovery
methods include:
Pilot Tone (such as Transparent Tone in Band)
• Less power in information bearing signal
• High peak-to-mean power ratio
Pilot Symbol Assisted Modulation
• Less power in information bearing signal
Carrier Recovery (such as Costas loop)
• The carrier is recovered from the information signal
Differential Reception
In the transmitter, each symbol is modulated relative to the previous symbol, for
example in differential BPSK:
• 0 = no change 1 = +180o
In the receiver, the current symbol is demodulated using the previous symbol as a
reference. The previous symbol acts as an estimate of the channel.
Differential reception is theoretical 3dB poorer than coherent. This is because the
differential system has two sources of error: a corrupted symbol, and a corrupted
reference (the previous symbol).
Non-coherent reception is often easier to implement.
Modulation Summary
Phase Shift Keying is often used, as it provides a highly bandwidth
efficient modulation scheme.
QPSK, modulation is very robust, but requires some form of linear
amplification. Alternatives (e.g. Offset QPSK and p/4-QPSK) can be
implemented, and reduce the envelope variations of the signal.
High level M-ary schemes (such as 64-QAM) are very bandwidth-
efficient, but more susceptible to noise and require linear amplification.
Constant envelope schemes (such as GMSK) can be employed since an
efficient, non-linear amplifier can be used.
Coherent reception provides better performance than differential, but
requires a more complex receiver.
