# Transmission Signal HELP

1. Sep 5, 2010

### jeekeshen

Am getting actually puzzled about some facts from telecommunication. When signal are transmitted from two mobile phones, the signal which is converted at the sending end from analogue by the speaker into digital on the mobile phone, does the transmitted signal actually wirelessly i mean is in DIGITAL FORM.. Is that possible transmitted wave in digital form ??

2. Sep 5, 2010

### schip666!

Define "digital" in this context.... The signal itself has no choice but to be continuous or analog -- well, down to the Planck level anyway. However the information in it is encoded in some digital format. I don't know the details of various cell-phone formats, but it could be frequency-shift, phase-shift, or even good old morse-code on/offs (not likely these days though).

3. Sep 5, 2010

### Born2bwire

To be a bit more detailed, what we do is modulate the information into the carrier frequency. This can be done by mixing in a time-dependent frequency shift, adjusting the amplitude, and/or adjusting the phase of the signal. A common type of encoding is phase-shift keying which is used in such methods like wifi. What we do is look at the carrier signal in its phase space. In phase space the signal has an amplitude and phase and we can use these two to encode a bitstream. For example, we will have a two bit stream consisting of either 00, 01, 10, or 11. In phase space, a signal of amplitude 1 and phase 45 degrees will be 00, phase 135 degrees 01, phase 225 degrees 10 ad phase 315 degrees 11. Thus, the phase and amplitude of the signal can represent a two bit word. We can keep expanding the encoding by adding more points that depend on amplitude and phase and we can encode longer bit words into a single "sample" of the signal.