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We use OQPSK because of envelope of signal but my question is

"In Phase Shift Keying(or QPSK) the amplitude of the transmitted signal is constant then how the envelope will change in OQPSK"??

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  • $\begingroup$ also, i have asked the guys at the EE Stack Exchange about OQPSK and while no one made a detailed answer, one person pointed me to some lit on the practice of OQPSK. Now the acronyms being tossed about is "Shaped" OQPSK. there is SOQPSK-TG and SOQPSK-MIL. google those terms and see what you can find about OQPSK. $\endgroup$ – robert bristow-johnson Apr 14 at 7:12
  • $\begingroup$ i've said this before, but i have never been paid a dime to work on OQPSK, and i hadn't even known of it until this decade (and i hadn't done anything with QPSK or QAM or whatever since grad school in the 1980s) but i am intrigued with it. there is an elegance and simplicity that, if you can get your frames lined up well, it seems quite optimal. $\endgroup$ – robert bristow-johnson Apr 14 at 7:18
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Note that a QPSK signal using a band-limited pulse has an envelope that passes through zero every time there is a phase transition of $\pi$. So its envelope is not constant if all symbol transitions are allowed.

Offset QPSK (OQPSK) doesn't have phase transitions of $\pi$. By staggering the $I$ and $Q$ signals by half a symbol interval, the maximum phase transition is $\pi/2$, leading to an approximately constant envelope. Slight droops in the envelope (occurring at phase transitions of $\pi/2$) can be eliminated by hard-limiting.

Note that with an appropriate choice of the transmit pulse, phase transitions in OQPSK can be completely avoided. This results in a modulation scheme with an exactly constant envelope. One example of such a continuous-phase modulation is minimum-shift keying (MSK).

For more information on OQPSK take a look at this question and its answers. Also browse this site for other questions on OQPSK.

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  • $\begingroup$ To improve this good answer slightly but an important clarification: If I am not mistaken OQPSK could be a constant envelope waveform only if specifically pulse shaped to do so, but OQPSK on it's own is not necessarily constant envelope. If this is accurate it could be more generally stated "leading to a reduced peak to average power ratio". $\endgroup$ – Dan Boschen Apr 16 at 0:25
  • $\begingroup$ @DanBoschen: Thanks for your remark. You're right that I was oversimplifying, and I've changed the answer accordingly. $\endgroup$ – Matt L. Apr 16 at 8:13
  • $\begingroup$ i wonder if, referring to this rambling of mine, if the pulse shape was $$ p[n] = \operatorname{sinc}\left( \tfrac{n}{2} \right) w[n] $$ and the window $w[n]$ is good and wide so it's very nearly a sinc function of bandwidth that is $\frac14$ of the bit rate, i wonder if that would be a constant envelope? $\endgroup$ – robert bristow-johnson Apr 16 at 9:24
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    $\begingroup$ @robertbristow-johnson To be constant envelope and if rotating at a constant rate the pulse shape would be a cosine, thus shifting half a pulse one pulse is a cosine and one is a sine and thus it is clear we stay on the unit circle. This is MSK as Matt stated. The rotating at a constant rate is not a requirement for constant envelope and results in the abrupt transitions when we change direction of rotation, resulting in a higher spectrum than if those direction changes were slowed (such a GMSK). Note that a Sinc function does closely approximate a cosine function..... $\endgroup$ – Dan Boschen Apr 16 at 10:52
  • $\begingroup$ at least over half of the main lobe. What I didn't proceed to check but would answer your wondering specifically regarding constant envelope condition is if the summation of all p[n] for n odd together with all j p[n] for n even is equal to 1. $\endgroup$ – Dan Boschen Apr 16 at 10:57

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