More on modulation, spectrum sharing, spread spectrum,
managing spectrum to combat fading (OFDM),
decision/detection theory (receiver processing aspects)
RF Systems Engineering
EE 5331 (EE 4328-005)
Fall 2025
Diplexer versus duplexer – generally think of using filters to separate of
combine signals for diplexer (courtesy Temwell Corporation)
Modulation performance (communications)
• Modulation
• In communications modulation selection depends substantially (but not
solely) on a trade-off between spectral efficiency and bit error rate
performance. Figures from
https://www.electronicdesign.com/technologies/communications/article/217
98737/understanding-modern-digital-modulation-techniques
• Notice that higher levels of CNR are generally not practical for a system such
as a cellular system due to channel effects (although they may be used in an
adaptive system when channel conditions permit).
Modulation performance (spectral efficiency)
Modulation performance (CNR requirements)
Modulation performance (other considerations)
• Most efficient use of available transmitter power (constant envelope
signals such as simple QPSK) is desirable.
• Consideration at the receiver such as what type of demodulation is
needed.
• Is carrier needed as reference or can it be retrieved from the signal –
coherent demodulation.
• Is differential detection an option (comparing carrier of current received
symbol with that of the previous received symbol).
• Or is noncoherent detection an option? (examples re envelope detector for
AM and discriminator for FM)
Modulation performance (considering channel effects)
• As data transmission rate increases greater bandwidth is used, but
the channel may have frequency dependent characteristics, leading
to distortion of the transmitted symbols. (Image next page from
https://cnx.org/contents/OFF8N0Z_@2/Signal-Time-Spreading)
• The resulting symbol distortion generally leads to increased errors as
it increases difficulty in identifying the particular symbol transmitted.
Modulation performance (considering channel effects)
Modulation performance (time domain distortion) – courtesy
SpringerLink
Modulation performance (introducing OFDM)
• Rather than transmit short symbols in this type of channel environment a
commonly used scheme is to transmit multiple lower data-rate signals over
multiple “orthogonal” carriers. OFDM – orthogonal frequency division multiplex
(Image from
https://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/
wlan-ofdm/content/ofdm_basicprinciplesoverview.htm)
Modulation schemes for radar
• For longer range radars a pulse modulation scheme is common.
• Pulses are commonly transmitted at a particular rate referred to as the pulse
repetition frequency (PRF), although this rate is often varied over time.
• Note that a fixed PRF can lead to range ambiguities in returns, so PRF
staggering is not uncommon and it can help with clutter suppression. Also,
there is a trade-off in range and frequency (doppler) sampling for pulsedoppler radars.
• But simple pulses can lead to high peak power transmit requirements, so may
not be the most appropriate choice.
• The same range resolution can be achieved with much lower peak power
transmit by “chirping” the pulse. (Idea is to have the same energy per pulse)
Modulation schemes for radar (pulse chirping)
• Simple pulse “chirping” (Image is from
https://www.sciencedirect.com/topics/engineering/dopplerresolution)
Modulation schemes for radar (orthogonal sequences)
• In modern systems there are other ways to achieve a pulse “chirp”
(actually the preferred term is pulse compression in these cases)
• Use of Barker codes is an example: (image from
http://www.ipcsit.com/vol28/017-CoimbatoreConferencesS00021.pdf)
Modulation schemes for radar - comment
• Although these examples appear to spread the signal over spectrum
note that what is actually happening is energy is being spread over
time and the amount of spectrum used for the chirp and Barker code
sequences is about the same as what would be used if a short pulse
pulse with the same bandwidth were to be transmitted.
• So, what are benefits of spreading (over time) in this case?
Modulation schemes short range radar
• FMCW radar is a common approach for short range radar, such as may be used in
automotive applications. (Image from https://hforsten.com/third-version-ofhomemade-6-ghz-fmcw-radar.html)
A couple of comments on noise and interference
• Noise and interference
• Note that noise and interference can also be related to characteristics of the
channel – there are several ways this is dealt with; we have introduced one
way via use of OFDM. For noises such as due to blackbody radiation the noise
is considered to be additive, and in most cases Gaussian. For noise due to the
channel it is actually due to a multiplicative effect (operating directly on the
signal in the channel). Terms such as Rayleigh fading and Rician fading are
used to describe this effect.
• For radar systems noise and interference can also be associated with signal
returns from objects other the intended objects (also referred as targets). A
commonly used term in this case is “clutter.”
Important radar performance parameters
• Target detection (probability of detection and related parameters of
probably of missed detection and probability of false alarm)
• Target range, position, and extent (for distributed targets)
• Speed (velocity) based on doppler shift
• Range resolution
• Then there is retrieval of information; e.g., for radars such as weather
radars information such as related to drop size and rain rate, weather
system dynamics. These are tied to return amplitudes and doppler
shifts, as well as to antenna system parameters.
• Many parameters tied to antenna system – more on this later.
Communication example – 64 QAM
(courtesy of Questtel Broadcase Systems and Signal Processing Stack
Exchange – high CNR right, low CNR left)
Concept of decision space - if the actual signal is too noisy decision
boundaries will be crossed (courtesy of ResearchGate)
Symbol timing recovery (courtesy Wireless Pi)
Quick review
• What are key parameters in being able to effectively retrieve “digital”
data at the output of a RF receiver?
Statistics in decision making (overlaps with detection theory) – example
with simple noise and signal + noise models (courtesy ResearchGate)
Possible outcomes of our thresholding decision in terms of probabilities
• Successful detection (threshold exceeded) and
target present
• Target is there but we missed it.
• Target detected but no target is present.