Frankly, it’s an oxymoron. This article http://www.automationworld.com/drastic-improvements-wireless-reliability describes one vendor’s approach – basically, two parallel wireless paths, and intelligence at each end to (a) duplicate packets; and (b) removes duplicates at the receiving end.
Not a bad idea. However, it has two drawbacks:
You need to use two RF channels, and in most wireless environments, there are already far too many devices competing for far too few channels.
Interference is a primary cause of packet loss. There are other causes, notably Fresnel zone effects, but interference from other wireless devices, and from various machinery, is the most common cause. The dual-link idea does address Fresnel issues, but does not do a lot to help the interference. To be fair, it will help some, because the two links are on different channels, and possibly even different bands, but if the interference is from radar, industrial machinery, or other broad-spectrum sources, y you’re still going to lose the link.
The best bet is to find a way to wire it. It’s hard to beat cat-5 for reliability…
If you really really can’t have a wired connected, and you really really need five nines of connectivity, you might want to rethink your design. Because reliable and wireless don’t belong in the same sentence.