How to create channel in RF

[rameshrai]

Hi guys,

does anybody know how RF channels are created ? Like GSM uses 200khz channels and there are 124 of them in each up link and down link? is this a baseband problem or **broken link removed**problem?

thanks

 

[MikeMl]

Mostly a political/regulatory problem.

 

[unclejed613]

it's far easier to generate multiple subcarriers as baseband, then the signal is fed to a single modulator stage to upconvert to RF. otherwise, it would need to be done the more difficult way of having each signal with it's own modulator (with a transmission mode such as MT63, this would require 63 modulators) and then combining all of the RF signals. the modulator used is an I/Q modulator, whose output is a SSB signal at the frequency of interest.

 

[RCinFLA]

The argument for evolution is to improve bandwidth utilization. Downside is equipment population obsolescence and upgrade.

Cellular companies have a bit more latitude on how they chop up the spectrum they own but they have to consider cost of upgrades. I think the average cellphone change out lifecycle is less then 3 years on average, at least in U.S.

OFDM / LTE, Digital television spectrum looks more like brick walls indicating good utilization. Old analog TV spectrum utilization was about as bad as you can get.

 

[rameshrai]

that means that the RF channels are processed and divided digitally in baseband system? If you have a circuit please share..

thanks

 

[unclejed613]

that really depends on the application and operating mode. a single channel system of let's say
ssb voice or audio bandwidth data operates within a 3khz slice of spectrum, so the baseband is 0-3khz. within that 3khz slice, voice communication could use up the whole 3khz, but with software that uses the computer sound card as a modem, you could fit 90 or so 31hz data channels. there is some ham radio software that allows one to decode many PSK signals within this 3khz slice. there is also software (which i mentioned as an example above) that uses a whole 1 or 2khz portion, using OFDM to transmit data using 63 redundant channels within that 1 or 2khz space. it's an extremely efficient and robust (it works well even with interference and selective fading) method of sending text or data. this OFDM signal is similar (except for bandwidth and data rate) to the way Wi-Fi works. all of the "magic" is done in the baseband (wi-fi's baseband signal is 20Mhz wide instead of 3khz) and the resulting signal is fed to a modulator, where it is mixed with an RF carrier to create the RF signal that goes out to the antenna. the channel width in the wifi spectrum is 20Mhz, so to switch to another channel, the local source of RF for the modulator is switched to a different frequency. the baseband signal remains the same.

 

[rameshrai]

thanks unclejed,

for example when a computer sends signal to the wireless WiFi modem it sends out 1s and 0s to the modem, but how is this 1s and 0s signal represented in WiFi 20MHz baseband?

 

[4pyros]

thanks unclejed,

for example when a computer sends signal to the wireless WiFi modem it sends out 1s and 0s to the modem, but how is this 1s and 0s signal represented in WiFi 20MHz baseband?

google is your friend;
https://www.tutorialspoint.com/wi-fi/wifi_radio_modulation.htm

 

[unclejed613]

the 1's and 0's go to a DSP chip. certain combinations of 1's and 0's get converted into pairs of numbers (from a look-up table) that get sent to two DACs, one of which generates the In-phase (I) and the other generates the Quadrature (Q) analog signals. these signals are fed to two balanced modulators (analog 4-quadrant multipliers). the RF signals being fed into the other input ports of the multipliers are 90 degrees out of phase (which is the reason one signal is called in-phase and the other quadrature). each multiplier generates different RF signals, and then the resulting signals are combined, and then they go to the antenna. in the receiver the same process is reversed.

 

[anny0498]

Hi,

To understand this better, I will suggest you to refer this book. You will have a better understanding. The survival guide 802.11 n by o really's.

 

[rameshrai]

thanks anny