Cellular Network Infrastructure

h1. GSM in 70cm HAM band

This page exists to collect some information around the topic of operating a GSM/GPRS network within the 70cm (~430 MHz) HAM radio band. This is something being done in cooperation between "DARC":https://darc.de/ under and "ARDC grant":https://www.ardc.net/apply/grants/2021-grants/grant-paving-the-way-to-use-gsm-gprs-technology-on-amateur-radio-bands/

h2. Duplexers

The BTS side requires a duplexer as it is transmitting and receiving at full-duplex at all time.  On the MS side, the traditional approach is to have a MEMS antenna switch to switch the antenna to either Rx or Tx.  This is possible at least for GSM and most GPRS multi-slot classes.  Until antenna switching via a GPIO is available from the Osmocom MS-SDR, a duplexer can also be used on the MS side

Traditional cellular duplexers for commercial cellular bands are  designed as two band-passes with one common (antenna) part.  This means they are multiple devices in one:

* Rx band filter to block out-of-band signals from entering the receiver

* Tx band filter to block out-of-band signals from being transmitted (including harmonics)

* isolator to prevent strong Tx signals from entering the Rx chain

h3. Existing 70cm duplexers

It appears that exsting duplexers in the 70cm band are not an arrangement of two band-passes (like commercial cellular) but rather two notch-filters.

This means they really only serve the purpose of preventing Tx leakage into Rx, but don't really provide the Rx/Tx band-filter functionality.

h4. SGQ-450

Available from a number of sellers on Amazon, ebay, Aliexpress, ... 

* consist of 6 cavities

* insertion loss around 1.5dB

* 228 x 145 x 35mm

* 1.2 kg

* N-type connectors 

* >= 75 dB isolation between Rx and Tx