**3.6 PSK Variations**

PSK can be set up to transmit multiple bits at a time, eg: 2 bits or dibits. A 4PSK system transmits double bits by using 4 different phases to represent the bit pairs, eg: 45^{o} indicates 00.

Dibit |
Phase |
---|---|

00 |
0 |

01 |
90 |

10 |
180 |

11 |
270 |

4PSK can be extended from using 90° shifts to using 45° shifts. This gives 8 different shifts that can be used to transmit **tribits** at each time interval.

Tribit |
Phase |
---|---|

000 |
0 |

001 |
45 |

010 |
90 |

011 |
135 |

100 |
180 |

101 |
225 |

110 |
270 |

111 |
315 |

PSK avoids the limitations of ASK and FSK and using 4PSK or 8PSK the bit rate will be greater than the baud rate because of the use of dibits, tribits, etc. However, an even more efficient and reliable system combines ASK and PSK. This is known as **Quadrature Amplitude Modulation (QAM)**.

**Quadrature Amplitude Modulation (QAM)**

QAM is a more complicated type of modulation that can be used to increase the amount of data sent without needing to increase the bandwidth. QAM works by combining ASK and PSK to give maximum contrast between bits, dibits, tribits, quadbits, etc.

The ITU-T recommends the use of 16QAM, which combines 3 amplitudes and 12 phases. This is the most efficient in terms of noise reduction as it has the greatest ratio of phase shifts to amplitude. The bit rate will be three times the baud rate which makes it extremely efficient.

The concept of bit packing is the key to advanced modulation techniques, such as QAM because rather than working with data one bit at a time, multiple bits can be coded as one particular state of the carrier. This is often referred to as group coding.

Quiz

Next: 3.7 Modems