MSK PSD Quadrature MSK baseband waveforms are Bandpass MSK signal (IQ representation) IQ waveforms are orthogonal (independent) PSD of complex envelope g(t) is and since x(t) & y(t) have same basic shape m(t) = ±1 ECE 4710: Lecture #30

MSK PSD MSK PSD is Pulse shape is truncated cosine (or sine) over 2Tb FT of truncated cosine is ECE 4710: Lecture #30

MSK Two types of MSK PSDs for both MSK types (I & II) are the same Type I Pulse shape on x(t) and y(t) alternates between positive and negative half cosinusoid Differential encoding  Fast Frequency Shift Keying (FFSK) Yields one to one relationship between ±1 m(t) data and fH / fL Type II Pulse shape on x(t) and y(t) is always a positive half cosinusoid No one to one relationship between m(t) data (±1) and fH / fL fH / fL determined by m(t) data and encoding PSDs for both MSK types (I & II) are the same ECE 4710: Lecture #30

Note that MSK is Binary M = 2 MSK PSD Complex envelope PSD Observations FNBW MSK  0.75 R QPSK  0.5 R 50% smaller!! 1st sidelobe QPSK   13.4 dB MSK  23 dB!! MSK QPSK or OQPSK Note that MSK is Binary M = 2 ECE 4710: Lecture #30

MSK vs. QPSK BW MSK signal has 50% larger FNBW relative to QPSK Need wider channel BW for RF signal Spectral efficiency is not as good MSK sidelobes are much smaller than unfiltered QPSK Truncated cosine pulse shape for MSK Rectangular pulse shape for unfiltered QPSK Adjacent Channel Interference (ACI) from MSK is very good compared to unfiltered QPSK ECE 4710: Lecture #30

GMSK MSK baseband waveforms can be filtered to further reduce sidelobe levels Raised cosine (RC) filter for QPSK Eliminate all sidelobes Satisfies Nyquist criterion  No ISI @ proper sampling point within symbol period RC filter cannot be used for filtering MSK envelope MSK is constant envelope  enables non-linear Class C PA RC filtered MSK would have sidelobes regenerated by non-linear Class C PAs ECE 4710: Lecture #30

GMSK Gaussian MSK = GMSK  use Gaussian shaped filter to further improve spectral efficiency of MSK Filter rectangular m(t) waveforms prior to generating IQ baseband waveforms x(t) & y(t) Before data are frequency modulated on carrier Cannot filter x(t) & y(t) as that would make MSK not have a constant envelope Gaussian filter transfer function ECE 4710: Lecture #30

GMSK Gaussian Filter ECE 4710: Lecture #30 Significantly reduces spectral sidelobes Affect on FNBW is very minor for reasonable filter BW Does NOT satisfy Nyquist criterion Will cause unwanted ISI if BG is too narrow How do we quantify reasonableness or narrowness? Bandwidth-Bit Duration Product  BG Tb MSK FNBW is 0.75 R = 0.75 / Tb BG Tb provides normalized measure of filter BW wrt signal BW Tradeoff  narrow filter BW vs. increasing ISI ECE 4710: Lecture #30

GMSK ECE 4710: Lecture #30 Gaussian Filter 0.3 GMSK MSK QPSK or OQPSK Reasonable BG Tb 0.3 to 0.5 Greater than 0.5? sidelobe levels not reduced enough Less than 0.3? ISI becomes too large GMSK normally specified by amount of filtering Example: 0.3 GMSK is GMSK with BG Tb = 0.3 MSK QPSK or OQPSK 0.3 GMSK ECE 4710: Lecture #30

GMSK Even though m(t) data is shaped by Gaussian filter response GMSK is still a constant envelope modulation method just like MSK Gaussian filtered data used to frequency modulate carrier Carrier envelope remains constant Non-linear Class C amps used Excellent DC to RF efficiencies (80-90%) Enables long operation of mobile devices relying upon battery power supply Constant envelope also means GMSK is less susceptible to signal fading and interference which occurs during transmission thru channel ECE 4710: Lecture #30

GSM Global System for Mobile (GSM) ECE 4710: Lecture #30 First digital standard developed for cellular telephone Developed in Europe in late 1980’s and widely deployed in early 1990’s Long before digital PCS cell phones in U.S. in late 1990’s Significant use in non-European markets (Asia, South America, etc.) Most widely used 2G cell phone standard in the world Leads market share of all other technologies by factor of 3-4 T-Mobile and AT&T Mobile in U.S. used GSM after 2002 Modulation method for GSM is 0.3 GMSK ECE 4710: Lecture #30

MSK Generation Type 1 MSK with differential encoding Very simple use of FM Tx with differentially encoded m(t) Doesn’t require separate IQ waveforms, x(t) & y(t), generated from m(t) No need for two IQ oscillators Simple = Fast  Fast Frequency Shift Keying (FFSK) ECE 4710: Lecture #30

MSK Generation Type I MSK with no differential encoding IQ waveforms must be generated !! Parallel method of generation ECE 4710: Lecture #30

MSK Generation Type II MSK ECE 4710: Lecture #30 MSK is specific case of BPSK Serial generation filters BPSK RF (bandpass) signal with off-center BPF ECE 4710: Lecture #30

MSK Spectral Efficiency MSK is special case of BFSK with minimum DF NNBW spectral efficiency is better than BPSK but worse than QPSK 30-dB BW spectral efficiency is 4  better than QPSK Low sidelobe levels  low ACI Also better than 64 QAM! Note that BPSK with RCF & r = 0.5 has h = 0.667  same as MSK ECE 4710: Lecture #30

MSK & GMSK MSK & GMSK for mobile radio applications Low sidelobe level and low ACI Large number users spaced very close together in frequency domain with minimal interference between channels Support large number of users Non-coherent Rx for demodulation No carrier synchronization Simple & inexpensive Rx Constant envelope Non-linear Class C PA with high DC to RF efficiency Long battery life for mobile units ECE 4710: Lecture #30