#REACH DOWNLINK SMS LIMITATION SOFTWARE#
#REACH DOWNLINK SMS LIMITATION CODE#

After a burst there is always 8.25us long guard period when signal amplitude decreases to 0.

Each GSM burst transmitted inside of time slot has constant amplitude. The GSM signal's amplitude is periodic with the period of 156.25us - equal to the duration of one GSM TDMA time slot. I've coded the test so it first started with recording a signal on a GSM C0 (BCCH) frequency, then it switched to another central frequency where only noise was present and in the end it switched back to the same GSM C0 frequency. I will show it again here with a bit more of the background information.

#REACH DOWNLINK SMS LIMITATION CODE#

At some point (beginning of December 2015) I decided that some effort from my side might be required in order to speed up this process a bit.īefore I decided to code time-synchronization myself I have already shown that RTL-SDR receivers keep time-synchronization across re-tunings to different carrier frequencies. So time passed but somehow no ready-made solution materialized. This way it wasn't possible to capture for example GSM uplink and downlink synchronously as they are on different frequencies with large (at 45MHz for GSM900) separation. It was done off-line - on files with the captured signal stored to disk - and no way to synchronize channels that work on different frequencies was demonstrated. YO3IIU presented synchronization of the receivers working on the same frequency with use of cross-correlation of the signals coming from many receivers. When you start multiple RTL-SDR dongles at the roughly the same moment there always will be random delays (changing from one program start to another) between the sample streams coming from different receiver. What was missing was time-synchronization between RTL-SDR dongles. There was already some important work done on this subject, starting with Juha Vierinen's idea to synchronize clocks of multiple RTL-SDR receivers, so they were sampling with exactly the same rate. I had good reasons to think that it will eventually happen.

#REACH DOWNLINK SMS LIMITATION SOFTWARE#

I thought that it was a matter of a short time before someone demonstrate a working solution transforming cheap RTL-SDR dongles (digital television receivers re-purposed to act as a software radio receivers) into a multi-channel receiver. I was reluctant to implement a cheap receiver with multiple RF channels myself. Total price of such solution is about 4 thousand dollars - too big amount for someone who starts journey with software defined radio and who at the same time might be interested in gr-gsm. To achieve this with USRPs N200 two devices synchronized with a special cable are required.

For example: with two channel receiver it would be possible to receive uplink of a GSM channel in addition to downlink. The reason is that it would be possible to do more with my software for all users who are interested in giving it a try and not only those who can afford a multichannel device like an USRP. Multi-RTL Making multiple channels receiver from cheap RTL-SDR donglesĪs a developer of gr-gsm project - a GNU Radio based software for receiving GSM signal - I always thought that it would be great to have a multiple channels receiver that would be available for everyone.

Reach downlink sms limitation