TETRA: Can you increase the capacity of a MTS1?

The MTS1 is a self-contained; small form factor, rugged TETRA base station that can be installed indoors or outdoors (IP66); on a pole or wall. It supports end-to-end encryption and authentication,  even when in local site trunking mode.

It contains a single carrier Base Radio (BR); power management and site controller. A single carrier means that the MTS1 can support one main control channel (MCCH) and up to three trunked traffic (TCH) channels - sometimes called 3+1.

Dual MTS1 is a configuration that allows two MTS1 base stations to work together as if they were one RF site.
There are two possible configurations: a two BR setup or a 1:1 standby setup.
With the former, the Site Controller of the one MTS1 controls the BR in both MTS1s, thus providing 7+1 operation (7 traffic channels + 1 control channnel).
The latter allows the two MTS1 to work in redundant mode so that if one MTS1 fails, the other MTS1 will take over. In this case, only 3+1 capacity is supported.

1+1 redundant MTS1 operation requires a software license whereas dual MTS1 (7+1) does not. Also, Dual redundant MTS1 is not currently supported in Dimetra Express.

The MTS1 has dual diversity as standard. When you connect two together they transmit on their own antenna and receive on both using an expansion output (Exp) on each station that connects to the RX2 input on the other.
The two duplexers are connected with two RF cables (blue) which are used to send the RX signal to the other duplexer for diversity use. TX-wise, air combining is used, therefore two TX/RX antennas are needed. My understanding is that it is possible to combine the two TX outputs to feed one antenna but this may not be practical as a combiner is needed, with its own weatherproof cabinet.

The two site controllers in each of the stations, are connected with two Ethernet cables (green) so they can talk to each other, though only the LAN switch functionality of the second site controller is used when not in redundant configuration.

Each station will need its own GPS antenna. The optional ground box would be connected to L1 on the station configured as BR1 - or on both of the stations, if if a redundant site link is used.