question about gsm
GSM baseband -> mixer -> Tx-VCO -> PA
GSM baseband singal goes into mixer is constant envelope like FM.
GSM singal went out from Tx-VCO is constant envelope also.
When the baseband GSM signal mixed with LO in the transceiver,
it would become AM signal.
My question is , GSM RF signal is consant envelope after it passed Tx-VCO.
What about the signal type after mixer ? It looks like an AM signal.
And there must be some processing before the signal goes into Tx-VCO.
Where can I find the document to explain this in time domain and freq domain ?
Please advise and thanks.
GSM standards contain a lot of interfaces, systems and sub-systems, also various
Types of signals (voice, packet data, and control)...
Let me assume that you are asking about the:
? GSM/EDGE 900 MHz standards.
? Air-interface (up/down link)
? Circuit switching voice signal (~ 13 kbps)
At this case, the RF modulation is BPSK with a carrier frequency at the 900MHz??
The signal will remain BPSK after the mixer, but with shifted spectrum (the new spectrum is determined by the allocated traffic channel)?
Some of the processing befor TX-VCO may contains:
1- PCM ENCODING.
2- CHANNEL CODING.
3- ENCRYPTION.
4- GMSK.
I HOPE THAT THE PROVIDED INFORMATION IS BENEFICIAL
Hi:
Your transmission chain is incorrect. Many GSM transmitters use a translation loop modulation scheme where the Modulation in introduced into the PLL Feedback loop. By doing this, you guarentee that there is no AM modulation in the signal, as the VCO in amplitude limiting. The PA is run into compression, which is why the GSM efficiency of the PA is good. Any amplitude variation is for transmit power control.
Edge is a different beast, and does introduce some AMplitude variation.
WCDMA needs a full IQ modulation approach.
Dave
Please visit www.3GPP.org and refre to the GSM specifications.