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Introduction

LibreCellular will use commodity compute hardware together with software-defined radio to create a highly flexible LTE base station, where the cellular core network may optionally run alongside this for a fully self-contained solution. A reference hardware platform will be provided, together with validated software versions and configuration to enable repeatable deployment.

The main use is envisaged as low power small cells, configured for typical bandwidths of 1.4MHz and 3MHz, operating in Ofcom Shared Access and CBRS spectrum. These networks will initially support a data service only, but support for native voice dialling via VoLTE — aka “HD voice” — is planned, along with potentially circuit-switched fallback (CSFB) also at some point in the future.

Roadmap

The project will be split at a high level into four phases, with the first being concerned with building out the CI hardware platform. This will be followed by CI configuration and test, support for basic LTE data service, and then support for native voice calling over LTE.

Continous Integration (CI) Hardware

The CI platform will integrate one or more test base stations with LTE modem banks via a cabled RF network, with reference clock distribution, control, and RF measurement.

This is the current development focus and see CI Hardware Platform for details.

CI Platform Configuration and Test

Automated testing will be made possible via use of the OsmoGSMTester software. Test coverage will be extended over time as the project develops.

Basic Service

Upon completion of this phase it will be possible to provision subscribers, connect user equipment (handsets or other terminals) to the network and use this to transfer data.

Voice over LTE (VoLTE) Service

An IP Multimedia Subsystem (IMS) will be integrated to enable native voice calling using the network.

Software Stack

The key software components in the provisional stack are summarised below, but this is by no means a comprehensive list and is subject to change.

eNodeB

The 4G base station component will be provided by srsRAN.

Evolved Packet Core (EPC)

It is planned to use Open5Gs to provide the core network.

IP Multimedia Subsystem

It is intended to use Kamailio for the IMS.

Reference Hardware Platform

Given that ease of use is a key aim, having a validated reference hardware platform is important and should save a lot of time getting new users up and running. In addition to which, this will make it much easier to rule out hardware problems when issues do arise.

The initial reference hardware is specified with a reasonable degree of headroom in terms of performance and flexibility, which clearly has cost implications and a future cost-optimised version is also anticipated.

Baseband Processing

An Intel NUC7i7DNBE SBC will be used for baseband processing and running the upper layers of the cellular stack.

SDR Hardware

The original LimeSDR-USB board has been selected for the SDR hardware.

RF Front End

LimeRFE will be used to provide a multi-band RF front-end complete with LNAs, PAs and duplexers. This provides a modulated output of 24dBm on cellular bands 1, 2, 3, 4 and 5.

Reference Clock

The LimeSDR will utilise a Leo Bodnar Mini Precision GPS Reference Clock reference for frequency stability and accuracy.