The only way to do this on such a large scale is to involve the cellular carriers. They will end up building and owning this. The government will pay for it (and then some) and the public safety agencies will pay dearly for access to the system.
The carriers will cry foul because the won't make enough money off of it to cover their artificially inflated costs. They will then apply (and be granted) the rights to sell access to the "public safety network" to non-public safety users. This is already being discussed as the only way to make this work.
It's all very disappointing. That $7 billion could do so much better if this boondoggle was just cancelled.
Meantime, there are several manufacturers that are producing multi band radios that can easily do what is needed at a fraction of the cost without having to build all new infrastructure. At $4500 or so each, that would work out to about 1.5 Million multi band radios. Considering that the costs could be dropped considerably by volume discounts, that could likely be stretched to 2 million radios put in the hands of public safety users. Considering that many public safety agencies are having to replace/upgrade equipment anyway due to narrow banding, P25 upgrades, etc, they are already having to spend money.
But of course every first responder needs the ability to stream live video, surf the net, upload and download documents on the fly. Not. This part could easily be achieved by other means, including the existing cellular network using a priority data service. We already have the Wireless Priority Service for voice, they could prioritize packets just as easily.
Sinking 7 Billion into one network/platform/solution is foolish. Diversify your communications and save money at the same time. But I digress, the government is involved, and we just can't do that.