413x0g2 - CCSDS

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CCSDS REPORT CONCERNING BANDWIDTH-EFFICIENT MODULATIONSsignals. The baseband filtered OQPSK/PM signal is formed by mapping the I and Q NRZdata to a four-state phase signal, filtering the phase signal and then phase modulating acarrier with the filtered phase signal.As with all modulations, the actual BER performance of filtered OQPSK is dependent on thereceiver detection method used. An integrate-and-dump receiver with symbol-by-symboldetection is mathematically optimal for unfiltered OQPSK waveforms only. For filteredOQPSK, without the use of a complex maximum-likelihood receiver, the optimal receiverdesign would employ a receive filter precisely matched to the transmitter filter and anequalizer to remove intersymbol interference. For a modulation technique in which multiplealternative filter types have been recommended, this may seem to be a problem. Fortunately,in practice an integrate-and-dump filter receiver will provide good performance for mosttypes of filtered OQPSK including those recommended.Subsection 3.2.2 describes in detail the I/Q and linear phase modulator implementations ofbaseband filtered OQPSK. Subsection 3.3.3 presents various filtering options, includingsimulated power spectral density plots for three filter types.3.2.2 MATHEMATICAL DEFINITION AND IMPLEMENTATIONTwo modulator forms are commonly used for implementation of baseband filtered OQPSK(reference [5]). The linear phase modulator implementation is described in 3.2.2.1 and theI/Q modulator implementation is described in 3.2.2.2. 43.2.2.1 Baseband Filtered OQPSK Linear Phase ModulatorOQPSK/PM uses a linear phase modulator to map a filtered phase signal onto the carrier.This configuration is shown in figure 3-19.I ChannelNRZ DataDataSourceQ ChannelNRZ DataI/Q datato PhaseMappingφ (t )Pulse ShapingFilter, h(t)PhaseModulators(t)Half ChannelSymbol DelayFigure 3-19: Filtered OQPSK with Linear Phase Modulator (OQPSK/PM)4 See 2.4 for bit/symbol terminology definitions used in this Green Book.CCSDS 413.0-G-2 Page 3-13 October 2009
CCSDS REPORT CONCERNING BANDWIDTH-EFFICIENT MODULATIONSThe input to the modulator is the in-phase (I) and Quadrature (Q) NRZ data streams. The Q-channel data stream is delayed by ½ symbol to create offset QPSK. The I and Q data ismapped to a phase signal which then goes through a pulse shaping filter. 5 This is then usedas an input to a linear phase modulator to produce the modulated output signal. The outputof the modulator can be expressed as:whereP = carrier powerf c = carrier frequencys( t)= 2Pcos(2π fct+ φ(t) * h(t))φ(t)is the phase output from the I-Q to phase mappingh(t) is the impulse response of the pulse shaping filter* denotes convolutionThe phase modulator implementation of baseband filtered OQPSK produces a constantenvelope signal, as can be seen in the phasor diagrams in figure 3-20. The constant envelopeproperty of the signal is important because this will reduce the impact of the nonlinearAM/AM and AM/PM distortion of the saturated transmitter power amplifier.Butterworth Filter, BT s =0.5SRRC Filter, Alpha = 0.51.51.5110.50.500-1.5 -1 -0.5 0 0.5 1 1.5 -1.5 -1 -0.5 0 0.5 1 1.5-0.5-0.5-1-1-1.5-1.5Figure 3-20: Baseband Filtered OQPSK/PM Implementation Phasor Diagrams5 Discrete spectral lines may be avoided if phase mapper uses the minimum phase rotation during transitionsfrom one phase to another, e.g. +90° instead of -270°.CCSDS 413.0-G-2 Page 3-14 October 2009
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