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pi/4 QPSK have 8 states in constellation. Its phase variation is limited in pi/4~3*pi/4, that's better than pi/2~pi of QPSK, so the amplitude variation caused by filtering is less than QPSK. And pi/4 QPSK can be demoded by non-coherent, it simplies the receiver designing.
[1]Progri, I.F..A MC-CDMA indoor geolocation system[A].Personal, Indoor and Mobile Radio Communications, 2005. PIMRC 2005. IEEE 16th International Symposium on vol.4[C].2005.
A direct sequence spread spectrum (DSSS), orthogonal frequency division multiple access (OFDM), code division multiple access (CDMA) and frequency division multiple access (FDMA) (or MC-CDMA) indoor geolocation system is presented and discussed in this paper. The data sequence on the transmitter is spread in time using two spreading sequences and then is modulated employing a quadrature phase shift keying (QPSK) scheme. The channel is modeled as a slowly varying frequency-selective, Rayleigh fading channel typical of an indoor office building or factory. The receiver utilizes a non-coherent data demodulator to minimize the sensitivity of the receiver to the Rayleigh fading channel. The bit-error-probability (BEP) performance of this system is compared to that of a DSSS-CDMA-FDMA (or C-CDMA) system when operating in a Rayleigh fading channel with additive white Gaussian noise, range and Doppler shift. Performance results indicate superior performance of the coded MC-CDMA systems of roughly equal transmitter and post RF receiver complexity.
I am doing Power amplifier design.
In many PA papers, QPSK is considered as a non-constant envelope signal.
But from its waveform, it seems it's constant-envelope.
Some books say the filtered QPSK is non-constant envelope.
Even in that case, how is the signal demodulated given that the demodulator simply times the singal with sine and cosine, without any use of amplitude information?
Hi
the main purpose of modulation is carry the signal to far distances , as well as , other advantages . in modulation process you make changes in frequency , amplitude , phase , etc.... , due to the variation of that factor in the information signal so you don't make any changes in the information signal , thus in demodulation you eliminate the carrier by timing it in sin or cos and filter it by LPF , then you have the signal , so you don't need any previous information about the amplitude .
i hope that help , and I'm ready for any further questions
regards
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