Quadrature PN Spreading and QPSK
Definition:The Pseudo-random-Noise (PN) spread spectrum modulation technique in which two independent PN generators are used to produce two chipping streams which when multiplied by cos2pfst and sin2pfst functions are added to form a Quadrature Phase Shift Keying (QPSK) signal.
Note: See Pilot PN Codes and PN Offsets for a block diagram of quadrature PN spreading circuitry and see Offset or Staggered QPSK for a discussion of Offset QPSK.
Quadrature PN Spreading and QPSK
Application:
The forward link in IS-95 uses quadrature PN spreading resulting in each channel being a QPSK signal. However, the power of each traffic channel is individually controlled. Individual channel control leads to implementation options which either scale each in-phase and quadrature signal component separately and then combine to form an aggregate QPSK signal or form the individual QPSK signals, amplify each QPSK signal, and then combine them to form the aggregate QPSK signal. In either case, the transmitted signal is a QPSK signal with varying amplitude.
Quadrature PN Spreading and QPSK
Example:
Let the in-phase PN sequence be denoted by PNi(t) and let the quadrature PN sequence be denoted by PNq(t). The quadrature PN QPSK spreader forms PNi(t)cos2pfst + PNq(t)sin2pfst, which by trigonometric identities can be written in the form v(2)cos[ß(t)-2pfst]. The tangent of the phase angle ß(t) equals PNq(t)/PNi(t). For example, when PNq(t) and PNi(t) both equal +1, ß(t) equals 45°. PNq(t) equal to +1 and PNi(t) equal to -1 produces a ß(t) of 135°. The other combinations of PNq(t) and PNi(t) yield ß(t) values of 225° and 315°. The ß(t) values can be easily determined from the values of PNq(t)
and PNi(t) using the sketch below.
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