When AT&T was awarded the contract to build the FirstNet public safety network, it seemed that the US had finally, after 15 years of disputes and setbacks, achieved its goal of a national, unified system for emergency response. However, individual states do not have to sign up for FirstNet, and some are considering an alternative which is being proposed by Rivada Networks, a safety systems expert, backed by Macquarie Capital. The two companies said they will cooperate to pursue opt-out states, with Colorado looking likely to be their first sign-up. The state said last week that it had selected Rivada and Macquarie to build and maintain an LTE network specifically for emergency response, should state governor John Hickenlooper decide to…