Hellofor CDMA (800 or 850 whatever you call), the total frequency

[sajib333]

Hello

for CDMA (800 or 850 whatever you call), the total frequency range is:
824–849 MHz/869–894 MHz (uplink/downlink)

Consider either up-link or down-link, the total band is 25 MHz. We know CDMA use 1.25 MHz band, hence the number of total channel/band for either link should be (25/1.25) MHz= 20; This is logical, just like GSM 25 MHz band has 124 channel, each with 200 KHz width + guard bands.
However I hardly could find any information in the Internet regarding the total number of channel in CDMA. Thus, I understand that there might be some other facts that need to be considered in such a band.
SO, please clarify me if the channel number 20 is right?. Please be precise while answering and do answer only if you are well aware of this matter.

Thanks in advance.

 

[jsgruszynski]

This line of thinking won't answer your question because it's CDMA, not FDMA or TDMA. It's code spread spectrum so channel division isn't by frequency or time but by code instead. So all channels are sitting on the same spectrum at the same time. Only the symbol distance and the needed SNR as results from the intersymbol interference of the spread code defines how many channels you can have.

 

[sajib333]

If it is the case then the users are suppose to use total 25 MHz bandwidth allocated for the CDMA system (either uplink /downlink). However, it is known that the user use 1.25 MHz band. Why it is it?

 

[jsgruszynski]

If it is the case then the users are suppose to use total 25 MHz bandwidth allocated for the CDMA system (either uplink /downlink). However, it is known that the user use 1.25 MHz band. Why it is it?

Just a trade-off of SNR to distance to channel count most likely. I'm not a hardcore comms guy - it just know enough to be dangerous thanks to Ham radio and working at HP T&M for 10 years.