Lecture Notes: QPSK Modulation

Overview

• QPSK: Quadrature Phase Shift Keying
  • 4-array PSK modulation
  • Requires taking two bits together

Input Bits for QPSK

• Example input: 00, 01, 11, 11
• Convert input bits into odd and even bits
• Odd Bits: 1, 3, 5, 7
• Even Bits: 0, 2, 4, 6

Voltage Representation

• Odd and even bits represented by voltages
  • +1 for 1 and -1 for 0

Signal Multiplication

• Even bits multiplied by sin(ω₀t)
  • If bit = -1, inverted sine wave
  • If bit = +1, same sine wave
• Odd bits multiplied by cos(ω₀t)
  • If bit = -1, negative cosine wave
  • If bit = +1, positive cosine wave
• Products from even and odd bits added to generate the QPSK waveform

QPSK Waveform

• Initial combination: 00
  • Starts from -√Ps
• For combination 01
  • Starts from +√Ps
• For combination 11
  • Starts from +√Ps
• Potential phase change every 2-bit interval
  • Phase change example: 00 to 01, a change of -90 degrees

QPSK Generation

• Block Diagram Input:
  • Serial input, parallel output with clock period 2Tᴠ s
• Duration of each bit: 2Tᴠ s
• Amplitudes: +1 or -1 volt
• QPSK output = sin(ω₀t) * even bits + cos(ω₀t) * odd bits

Bandwidth Requirement

• Sin(ω₀t) and cos(ω₀t) have the same spectrum, hence only one main lobe
• Bit duration: 2Tᴠ s, resulting in bandwidth of Fᴮ / 2
• Main lobe width: Fᴮ Hz
• Bandwidth: Fᴮ Hz
• Spectral efficiency: bit rate / bandwidth
  • Result: 1 bit/second/Hz

QPSK Detection

• Shows QPSK detection block data