New CI RAN subsystem plus clock distribution and RF duplexer boards.

The latest addition to the LibreCellular continuous integration (CI) hardware platform is a new modular radio access network (RAN) subsystem, together with reference clock distribution and LTE band 3 RF duplexer boards. This will allow automated testing of a new cost-optimised base station design, based upon a LimeSDR Mini 2.0 and UP 4000 single board computer (SBC).

Modular RAN subsystem

The original CI RAN subsystem, CIRAN1, was based on an original LimeSDR board plus an Intel NUC and two RF cavity duplexers. However, the new CIRAN2 subsystem takes a more modular approach and can accommodate two base stations on removable sleds. The idea being that these sleds can be swapped out over time, in order to permit testing of new base station designs.

In the above annotated image we can see the two RAN sled positions highlighted in green and in this case only one sled is fitted, in position 1. Located in between these we have a new custom board, CSAD10, which provides reference clock distribution within the chassis. Two CONIO-B boards are fitted to the front panel and provide remote console access, plus power and reset control.

The first sled is designed to accommodate an UP 4000 SBC and a LimeSDR Mini 2.0, plus a new LTE Band 3 Low Power Duplexer (B3LPD) board.

For further details, see the CIRAN2 documentation.

Clock shaping and distribution

A new clock shaping and distribution board with ten outputs (CSAD10) has been developed for use with CIRAN2, and could also be put to use in the CICLKD subsystem in place of the TADD-1 board. Indeed, this design could be put to use in many other applications also.

Some SDRs, such as LimeSDR Mini, require a square wave external clock reference with fast rise and fall times (steep sides). CSAD10 utilises an LTC6957IMS-3 low phase noise buffer/driver at its input, which is able to translate a sine wave signal to logic levels. This is followed by an LMK00101 ultra-low jitter fanout buffer, which provides ten CMOS level 50 ohm outputs.

For further details, see the CSAD10 documentation.

LTE Band 3 Low Power Duplexer

The cavity duplexers used in CIRAN1 are physically large and overkill for applications such as this with very low RF power levels. A new LTE Band 3 Low Power Duplexer board design (B3LPD) has been developed, which utilises a low cost Qorvo QPQ1297 Bulk Acoustic Wave (BAW) Duplexer which is rated at 30 dBm (1W) downlink, plus a TDK HHM22139A2 directional coupler. The latter provides a -24 dB coupled output to permit base station signal/power monitoring.

For further details, see the B3LPD documentation.