The following minimum parameters are required when configuring an LTE network:
Mobile Country Code (MCC)
Mobile Network Code (MNC)
RF channel number (EARFCN)
There are plenty of other configuration options, some of which are specific to the hardware configuration, while others define network behaviour. Examples of the former include the SDR type, receive and transmit gains, and whether to configure for SISO or MIMO operation. While examples of the latter include timers, resource scheduling and neighbouring cell information.
However, for simple networks at least we can mostly use defaults and just need to know what we want our network to be called, the MCC/MNC tuple to use, the radio channel (which we have licensed), and the APN which user equipment (handsets and modems) should connect to.
It’s best if the network name is something short.
It’s generally not possible to obtain a unique MNC, as these are very much in short supply, so instead we must use parameters which we know will not be used by any public mobile network. One option and the most appropriate for testing at least is an MCC/MNC of 001/01, which is designated for Test Networks. ITU-T Recommendation E.212 Amendment 1 (07/2018) specifies MCC of 999 for internal use within a private network and suggests that any MNC — which can be two or three digit — may be allocated under this.
A mobile network code of 99 is not the same as 099! Therefore the same MNC length must be used across all configuration.
The are various online tools which let you look up the EARFCN for a radio frequency allocation. What we need to know is the EARFCN number which corresponds to the centre frequency of our downlink allocation. With the Ofcom Shared Access 3.3 MHz allocation in LTE Band 3 (1800 MHz), this happens to 1878.4 MHz, which corresponds to EARFCN 1934.
The bandwidth must be configured such that we do not transmit outside of our allocation. In LTE this is typically expressed in terms of the number of resource blocks (PRBs), where:
1.4MHz = 6 PRBs
3MHz = 15 PRBs
5MHz = 25 PRBs
10MHz = 50 PRBs
15 MHz = 75 PRBs
20 MHz = 100 PRBs
Using the same Ofcom Shared Access example, we would need to configure our eNodeB to use 15 PRBs. Alternatively, we could configure it for just 6 PRBs and move the centre frequency (downlink EARFCN), so as to leave more free contigous spectrum. Then use the remaining spectrum to operate one or more GSM BTS and perhaps even configure the two networks to interoperate. Which would be possible under an Ofcom Shared Access low power license, which permits operation of as many base stations as required within a 50m radius. However, this would obviously be a much more complex configuration.
Not all LTE bands support all possible channel bandwidths.
An APN is usually configured with either a name of default or internet and we just need to ensure that user equipment has the same APN configured.
LTE Band 3
An example is provided for a 3 MHz channel in LTE Band 3, with SISO configuration. This corresponds to the Ofcom Shared Access 1800 MHz allocation.
Note that it will be neccessary to update the srsENB config if you need to use another channel.
The most commonly used parameters are described below and for further details, please see the srsRAN documentation.
The eNodeB is configured via
/etc/srsran/enb.conf and the main parameters of interest are:
mcc. As described above.
mnc. As described above.
n_prb. As described above.
dl_earfcn. As described above.
tx_gain. A value of 66 seems to work well.
rx_gain. A value of 47 seems to work well.
The tx_gain value probably shouldn’t be increased, as overdriving may compromise performance. rx_gain may be reduced if overloading is suspected, e.g. due to very close operation or perhaps use of an external LNA.
If an external GPS reference clock is not available, the device_args line should be trimmed to remove refclk=10e6, so that the SDR on-board reference clock is used instead. If configuring for an uplink (receive) frequency <1.5 GHz, the rxant parameter should be changed to LNAL, while being sure to also connect the receive antenna or duplexer port to the associated RF port on the SDR.
The value for the time_adv_nsamples parameter is specific to particular SDR hardware and corrects for the delay that this introduces. A value of 73 for the parameter appears to be optimal for LimeSDR-USB. Synchronisation is important in cellular networks and there is no harm in experimenting with this parameter in an attempt to futher improve performance.
The EPC is configured via
/etc/srsran/epc.conf and the main parameters of interest are:
mcc. As described above.
mnc. As described above.
full_net_name & short_net_name. Set both to the same, as described above.
apn. As described above.
dns_addr. Set to configure the DNS server for user equipment.
Note that subscribers must also be provisioned in the UE database and for details, see Subscriber Provisioning.