question about qpsk and 8psk

why the transmitted data (the kind of data we deal with) in qpsk is complex data while the one we deal with in 8psk is real data??

[mary]

if they are like i said .. if what i said is wrong .. what are the kind of transmitted data in each one of them?

Answer 1

[Walter Roberson]

Neither qpsk nor 8psk transmit complex data. Both can, though, be represented using complex data, with qpsk being more natural to represent that way.

qpsk can be interpreted as two independent psk channels offset by 90 degrees, so if one wanted to describe the state of the system at any point, one could represent one of the channels as the real portion of a complex number, and represent the other channel as the imaginary portion of the complex number. But that does not mean that data is actually complex: it is just a notation. One could use a notation such as (1,0) or (1,-1) or 10 instead.

8psk can be interpreted as two independent qpsk channels offset by 45 degrees. That gives three components per state, and representation as complex becomes messier but is still possible.

See http://en.wikipedia.org/wiki/Phase-shift_keying#.CF.80_.2F4.E2.80.93QPSK (Pi/4 PSK is another name for 8psk) and http://www.digitalhome.ca/forum/showthread.php?t=67973

[mary]

thank you so much for your help Mr.Walter

[mary]

may i know the bit rate and bandwidth of bpsk , qpsk and 8psk ??

[Walter Roberson]

bpsk -- 2 bits per cycle

qpsk -- 4 bits per cycle

8psk -- 8 bits per cycle

bandwidth -- I do not know.

[mary]

but as far as i know in qpsk Four different phases are used to represent two binary values. so the bit rate will be 2 bits per cycle!!!

[Walter Roberson]

Ah, yes, that sounds right. So it would be 1, 2, and 3 bits per cycle, respectively.

[mary]

okay thank you very much