07-05-2014, 10:19 AM
Modulation Techniques for Mobile Radio
Introduction
Modulation: Encoding information in a baseband signal and then translating (shifting) signal to much higher frequency prior to transmission
Message signal is detected by observing baseband to the amplitude, frequency, or phase of the signal.
Our focus is modulation for mobile radio.
The primary goal is to transport information through the MRC with the best quality (low BER), lowest power & least amount of frequency spectrum
Must make tradeoffs between these objectives.
rapid non-linear, ∆f 3 improvement in output signal quality (SNRout) for increases in ∆f
“capture effect” : FM Rx rejects the weaker of the two FM signals (one with smaller SNRin) in the same RF BW → ∴ resistant to CCI
Increased ∆f requires increasing the bandwidth and spectral occupancy of the signal
must exceed the threshold of the FM detector, which means that typically SNRin≥ 10 dB (called the capture threshold)
Digital Modulation
Better performance and more cost effective than analog modulation methods (AM, FM, etc.)
Used in modern cellular systems
Advancements in VLSI, DSP, etc. have made digital solutions practical and affordable
Geometric Representation of Modulation Signal
Geometric Representation of Modulation Signals - Constellation Diagrams
Graphical representation of complex ( A & θ) digital modulation types
Provide insight into modulation performance
Modulation set, S, with M possible signals
Binary modulation → M = 2 → each signal = 1 bit of information
M-ary modulation → M > 2 → each signal > 1 bit of information
DPSK
DPSK → Differential Phase Shift Keying
Non-coherent Rx can be used
easy & cheap to build
no need for coherent reference signal from Tx
Bit information determined by transition between two phase states
incoming bit = 1 → signal phase stays the same as previous bit
incoming bit = 0 → phase switches state
OQPSK
Offset QPSK
The occasional phase shift of π radians can cause the signal envelope to pass through zero for just in instant.
Any kind of hard limiting or nonlinear amplification of the zero-crossings brings back the filtered sidelobes
since the fidelity of the signal at small voltage levels is lost in transmission.
OQPSK ensures there are fewer baseband signal transitions applied to the RF amplifier,
helps eliminate spectrum regrowth after amplification.