Amplitude Shift Keying
Amplitude Shift Keying (ASK) varies the amplitude or height of the signal to represent the binary
data. ASK is a current state technique, where one level of amplitude can represent a 0 bit and
another level of amplitude can represent a 1 bit. Figure 1.3 shows how a wave can modulate
an ASCII letter K using Amplitude Shift Keying. The larger amplitude wave is interpreted as a
binary 1, and the smaller amplitude wave is interpreted as a binary 0.
This shifting of amplitude determines the data that is being transmitted. The way the
receiving station performs this task is to first divide the signal being received into periods of
time known as symbol periods. The receiving station then samples or examines the wave
during this symbol period to determine the amplitude of the wave. Depending upon the
value of the amplitude of the wave, the receiving station can determine the binary value.
As you will learn later in this book, wireless signals can be unpredictable and also subject
to interference from many sources. When noise or interference occurs, it usually affects the
amplitude of a signal. Since a change in amplitude due to noise could cause the receiving station to misinterpret the value of the data, ASK has to be used cautiously.