# Communication Systems/M-ary Modulation Schemes

Now what if try to cram more information into a single bittime? If we
take 2 bits at a time, and arrange them together, we can assign each set
of 2 bits to a different symbol, and then we can transmit the different
symbols.

## Pronunciation

`"M-ary" is pronounced like "em airy".`

## Example: Q-ASK

Let us use the following scheme:

-   \"00\" = +5V
-   \"01\" = +1.66V
-   \"10\" = -1.66V
-   \"11\" = -5V

we can see now that we can transmit data *twice as fast* using this
scheme, although we need to have a more complicated receiver, that can
decide between 4 different pulses (instead of 2 different pulses, like
we have been using).

## Bits Per Symbol

All popular communication systems transmit an integer number of bits per
symbol. We can relate the number of bits (\"k\") and the number of
different symbols (\"m\") with the following equation:

$$m = 2^k$$

This causes the number of symbols to be a power of two.

With M-ary modulation techniques, the \"symbols per second\" rate can be
much slower than the \"bits per second\" data rate.

## QPSK

Quadrature phase shift keying (aka 4-PSK) is PSK modulation that has
four points in the constellation. !QPSK
Modulator !QPSK
Demodulator There are several
variations on this technique:

-   simple QPSK
-   DQPSK (differential QPSK)
-   OQPSK (offset QPSK)
-   SOPSK (shaped offset QPSK)
-   π/4 QPSK (shifted constellation QPSK)

## CPFSK (MSK)

\[MSK\]minimum shift keying

## DPSK

## For further reading

-   Wikipedia:Constellation
    diagram
-   Wikipedia:Quadrature amplitude
    modulation
