Learning The Fundamental Digital Modulation Methods Part1

The  most  fundamental  digital  modulation  techniques  are  based  on  keying in the case of  PSK  (phase-shift keying), a finite number of phases  are used. in  the  case  of  FSK  (frequency-shift  keying),  a  finite  number  of  frequencies are used. in  the  case  of  ASK  (amplitude-shift  keying),  a  finite  number  of
amplitudes are used. in the case of  QAM  (quadrature amplitude
modulation), a finite number  of  at  least  two  phases,  and  at  least  two  amplitudes  are used.In  QAM,  an  in-phase  signal  (the  I  signal,  for
 example  a  cosine waveform) and a quadrature phase signal (the Q signal, for example a  sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK.  The  resulting  signal  is  equivalent  to  a  combination  of  PSK  and
ASK.In  all  of  the  above  methods,  each  of  these  phases,  frequencies  or amplitudes  are  assigned  a  unique  pattern  of  binary  bits.  Usually,  each phase,  frequency  or  amplitude  encodes  an  equal  number  of  bits.  This number of bits comprises the symbol that is represented by the particular phase, frequency or amplitude. If  the  alphabet  consists  of  M  =  2N alternative  symbols,  each  symbol  represents a message consisting of N bits. If the symbol rate (also known  as  the  baud  rate)  is  fS symbols/second  (or  baud),  the  data  rate  is  NfS bit/second. For example, with an alphabet consisting of 16 alternative symbols, each
symbol represents  4  bits. Thus, the data rate is four times the baud rate. In  the  case  of  PSK,  ASK  or  QAM,  where  the  carrier  frequency  of  the modulated  signal  is  constant,  the  modulation  alphabet  is  often conveniently  represented  on  a  constellation  diagram,  showing  the amplitude of the I  signal at the x-axis, and the amplitude of the Q  signal at the y-axis, for each symbol.
Modulator and Detector Principles of Operation; PSK  and  ASK,  and  sometimes  also  FSK,  are  often  generated  and detected  using  the  principle  of  QAM.  The  I  and  Q  signals  can  be combined into a  complex-valued  signal  I+jQ  (where  j  is the  imaginary unit).  The  resulting  so  called  equivalent  low-pass  signal  or  equivalent baseband  signal  is  a  complex-valued  representation  of  the  real-valued
modulated physical signal (the so called pass band signal or RF signal). These are the general steps used by the modulator to transmit data.