What would be the minimum set of hardware and software needed for ... ?

[Q] For a security guard platform (offgrid), what would be the minimum set of hardware and software needed to build a (small?) but authenticated/secure (tetra?) radio platform?

[A] This is not an easy question to answer in a blog post. I would need to really understand your operation; how you work and what (if any) challenges you face or forsee before even asking questions about what you need.

Authenticated/secure

If authentication and security are a must, my suggestion would be to use TETRA together with one of the more secure algorithms (i.e. not TEA1). Dimetra Express offers both of these with the least amount of hardware and software.

If authentication and security are important but so too is cost. My alternative suggestion would be MOTOTRBO. It offers AES-256 end to end encryption. Depending on the size of the network, Capacity Max will also do radio authentication but this solution is better suited for larger deployments. For smaller (non-Capacity Max) systems there is RAS - although not real authentication, it can prevent unauthorised access to the radio system.

Offgrid

Both technologies are able to operate off grid as long as there is sufficient power. If power is a problem for you, one option would be to go really, really small:

The smallest possible configuration for MOTOTRBO is just to use radios on simplex.

The same applies to TETRA but the range is much less due to transmitter power.

To extend the range of a DMR system, you can use a repeater.

For TETRA, you can use a MTM5500 with DMO repeater enabled and configured but this does not offer much security and capacity but can run on very little power.
In both of these cases, the repeater should be mounted in a good location such as on top of a building or hill and in some cases with a mast etc.

Pro and contra

TETRA has a lot of security-related features but uses reduced RF power and typically is only allowed in the UHF frequency band - the latter means that the coverage offered from a single base station is less than what DMR could offer. TETRA base stations also have receiver diversity which does help improve coverage. Because of these security features, it is used and trusted by public safety organisations in many countries,

Also, there is a huge amount of interoperability between radios and networks of different vendors - way more than DMR. This means that you could use any TETRA radio on a Dimtera Express system.

DMR also offers some security features but not as many as what TETRA (and P25) would offer. For example, DMR does not support mutual authentication or air interface encryption. 

Compared to TETRA, the level of interoperability between different systems and radios is somewhat limited. This means that if you want to use a MOTOTRBO radio on another vendors system, many features will work but some will not depending on the vendor/system.

In short

Without a proper understanding of what you do and what you would like to achieve or solve, it's difficult for me to give you a sufficient answer. If you want to get more details or would like to discuss, please get in touch with my colleagues via this link.