Lately I have come across articles about Congressional measures to improve public safety communication interoperability in the event of a disaster like the Sept 11 attacks. The narrative goes that the 9/11 Commission recommended public safety radio equipment be made interoperable so members of different police and fire departments could communicate with one another, and this plan is simply implementing the recommendations.
The legislation would auction two 5 MHz segments of spectrum in the 700MHz range (formerly used by UHF TV channels 62 and 67) to non-emergency users with the understanding they would not use those frequencies in an emergency. The auction proceeds would be used to pay for an interoperable radio system that would support dozens of duplex pairs of voice channels, data transfer, and streaming video.
Some articles frame this issue as allocating 10MHz of spectrum to the cause of interoperability. Supporters were hoping to pass the bills before the tenth anniversary of the September 11 to capitalize on an increased focus on disaster preparedness.
How will 10MHz of spectrum be used for interoperability?
It won’t. It will allow more streaming data and more users on a single repeater. The new spectrum is located adjacent to two 5 MHz segments (formerly used by UHF channels 63 and 68) currently allocated to public safety. This would allow single transmissions with a bandwidth greater than 5 MHz.
When you dig into supporters’ arguments, they say public safety needs the additional bandwidth to support higher data throughput, not for interoperability.
How much spectrum bandwidth does it take to get a given amount of data throughput?
Bandwidth and data throughput are related so closely that the term “bandwidth” is sometimes used colloquially to refer to data throughput. The theoretical throughput capacity of a channel is defined by Shannon’s Law: C = B log[2] (1+S/N), where C is throughput capacity and B is the channel bandwidth. If you have enough signal to noise ratio, you can get any throughput you want even on a small amount of bandwidth. In a practical system, limitations of the dynamic range of the receiver, interference from other radio systems, interference from reflections off various objections in the environment, and the Gaussian noise in the receiver’s components all limit the maximum signal to noise ratio. It works out that in a practical system deployed in the field, you can get a maximum of 1 Mbps per every 1 MHz of spectrum space. In marginal channel conditions, you may get a tenth of that. This means you can fit one streaming video channel into 1 MHz of spectrum.
How should interoperability be accomplished?
Emergency repeaters could be set up that have one input frequency and one output frequency in each band allocated to public safety. All inputs would be re-transmitted on all outputs.
The reason for multiple inputs would be to allow each agency to program that frequency into their existing radios without buying new equipment. There should also be a simplex frequency so in the event of failure or loss of contact with any centralized repeater, responders using the same band could communicate. This approach is almost free. It also has the advantage of being simple and therefor less prone to failure. Some local police and fire departments police and fire departments have already worked out interoperability plans like this.
The proposed system is the opposite of a quick-and-dirty approach. It appears to be a way to find $6 to $10 billion for public safety agencies to buy some very nice equipment from companies like Motorola. I am not knowledgeable about how useful high-end communications equipment is to responders in an emergency situation. If it’s reasonably useful, we should spend the money instead of trying to find a cheap solution.
It is unfortunate that the Sept 11 attacks have been invoked so often to prop up dubious arguments. It usually requires aggrandizing the perpetrators of the crimes and belittling the power of democratic societies operating under the rule of law.
Supporters of the proposed interoperable broadband system need to make separate cases for why they need more bandwidth and more equipment. I like advanced radio equipment, so I would love to hear the amazing things it can do. I condemn evoking Sept 11 as the reason for new equipment and more bandwidth, but I am open to cogent arguments on the merits vs. costs.
Does public safety need $10 billion and/or 10MHz more to communicate in a disaster?
