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Offset or Staggered QPSK: Definition: A Quadrature Phase Shift Keying (QPSK) modulation scheme in which a delay is introduced into one of the quadrature processing elements equal to one-half the duration of the binary symbols or chips being modulated.
Note: The introduction of the delay in one of the quadrature processing elements makes it impossible for the phase in both processing elements to change at the same time. Hence, it is not possible for the output quadrature phase to change by 180°; only ±90° or zero degree phase shifts occur. Since a 180° phase shift corresponds to the signal passing through zero amplitude, avoiding 180° phase shifts avoids amplifier linearity problems caused when its input varies with amplitudes all the way down to zero.
Offset or Staggered QPSK:
Application: The mobile-subscriber unit uses Offset QPSK (OQPSK) which is sometimes also called Staggered QPSK (SQPSK). The advantages in amplifier design when SQPSK is used include tighter control of out-of-band emissions and better efficiency in converting prime DC power to transmitted RF power. These advantages are particularly important when the mobile is a handset powered by batteries. In IS-95, SQPSK is used as the PN spreading modulation on the reverse links. Hence, the delay introduced is one-half of a chip duration. At the PN chipping rate of 1.2288 Mchip/sec, a chip duration equals 813.80 nsec requiring a delay of 406.90 nsec. In IS-95 this delay is introduced into the Q or "quadrature" element as opposed to the I or "in-phase" element. The fact that offsetting chip timing in SQPSK makes it impossible to have 180° transitions is illustrated in the example below.
Offset or Staggered QPSK:
Example: The "in-phase" chips are denoted by A1, A2, etc. and the "quadrature" chips are denoted by Q0, Q1, etc. The offset chip streams are shown in the figure on the following page. The in-phase chip stream is depicted at the top of the figure and the delayed quadrature stream is shown just below it. At the bottom of the figure, time is shown in units equal to one-half of a chip duration. Note that the quadrature chip stream is delayed one-half of a chip relative to the in-phase stream. During T1 and T2 the in-phase chip remains at A1. The quadrature stream changes chip values beginning at T2, but A1 cannot change at this time. Hence, only the value of one of the chips in the two streams can ever change at the same time. When the in-phase chip remains unchanged, the phase must lie in either the first or fourth quadrants (in-phase positive) or in the second or third quadrants (in-phase negative).
A change in the quadrature chip value can at most change the phase by one quadrant or 90°. When the quadrature chip remains unchanged, the phase must lie in either the first or second quadrants (quadrature positive) or in the third or fourth quadrants (in-phase negative). A change in the in-phase chip value can at most change the phase by one quadrant or 90°.
The phase values during each time unit are as shown in the figure. In this example the phase advances by 90° in each successive time unit. This phase pattern corresponds to the in-phase and quadrature chip patterns equaling each other for a short time.
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