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While it’s becoming common knowledge that the performance of many (perhaps most) digital trunked radio systems is deficient, it is also becoming widely known that P25 digital radio technology has been surpassed by newer alternatives. P25 is quickly on its way to becoming obsolete. City, county and state governments that are purchasing complex digital trunked radio systems will not be able to afford to maintain or replace them in the absence of continued Federal grant dollars. Less expensive and less problematic technology will prevail, and it will be available soon.

FCC rules for radio systems place strict limitations on the amount of radio spectrum that each radio channel can occupy. Many of the performance problems with P25 are related to the problem of converting speech to and from a highly compressed bit stream that can be sent over a tiny amount of radio spectrum authorized by the FCC. Digital radio would not be nearly as challenging if there was no need to compress speech to the degree that is currently required by the FCC. Emerging radio technology will solve this problem.

One of the most startling deficiencies in the P25 common air interface is that it does not include a workable provision to negotiate the selection of vocoder software. The vocoder converts speech to and from a compressed digital bit stream for transmission over the air. The IMBE vocoder that was selected for use with P25 was developed by a company called Digital Voice Systems, Inc. (DVSI) more than 12 years ago. As we know, it does not perform well when background noise is present. DVSI has made improvements in a new product called AMBE, however development of new and innovative vocoder technology by other firms is limited because of the need for compatibility with P25’s proprietary IMBE vocoder. Well-designed communications protocols would include a feature to support different types of vocoders in the same radio and have the radios negotiate the best vocoder to use, but this feature was not included in the P25 specification. This alone has already made P25 mostly obsolete. Digital vocoder technology will improve over time, but there is no way to assure that it will interoperate with existing P25 radios. Vocoder negotiation will be an essential design element for the digital radios that replace P25 products.

P25 is handicapped by its own standard because it is not easily adaptable to change. Its replacement will be based on truly open standards that are extensible. There will be an open-source reference implementation that will allow peer review and the ability for competitors to ethically collaborate.

Open standards tend to thrive while closed, proprietary standards die rather quickly. Do you remember IBM’s SNA and DEC’s DECNet? Both died at a relatively young age, mostly because deployment and innovation were hampered by proprietary licensing constraints. Contrast with TCP/IP which flourished because of its open architecture and appeal to innovators who could fully participate with almost no barrier to entry. P25 will die quickly, just like SNA and DECNet, and for many of the same reasons.

The ideal technology that will replace P25 will not be constrained by the FCC’s ultra low bit rate requirements. Most of the vocoder issues will be mitigated if the need to highly compress speech is eliminated. The technology exists for this to occur, while still being extremely spectrum efficient.

The prerequisite to ubiquitous interoperability is internetworking, and internetworking can be accomplished only with a packet-switched network like the Internet. P25 digital trunking is circuit-switched technology. Circuit-switched trunked radio systems are fragile because of the centralized computers that handle channel assignment while packet switched networks can avoid this vulnerability. Spectral efficiency by way of extremely low bit-rate channels will be surpassed by packet-switched networks where the spectral efficiency comes from dynamically sharing a composite bit stream. The packet-switched network needs to extend all the way to the subscriber radio. Broadband will prevail.

Cognitive radio has become a reality with recent advancements in technology. This concept is a paradigm for wireless communication in which a network of radio equipment changes its transmission and reception parameters to communicate efficiently on an as-needed basis. This alternation of parameters is based on the active monitoring of several factors in the external and internal radio environment, such as the radio frequency spectrum, user behavior and the network state. The radio equipment learns and adapts to its environment and is capable of sharing radio spectrum on a non-interfering basis with other radio systems, such as television broadcasting frequencies. It will be possible to efficiently allocate larger bit streams using cognitive radio technology so that it won’t be necessary for vocoders to attempt unreliably high compression.

I believe that some flavor of a wireless metropolitan area network based on the IEEE 802.16 (WiMax) or LTE standards combined with cognitive radio technology will define the future of public-safety communications.

Think twice before buying digital trunked radio technology. Remember that problems can be prevented if they can be predicted, and there is overwhelming evidence of problems with P25. A better future is ahead.


Related articles:

Urgent Communications – Intelligent radios almost ready for prime time (10/2008)

Urgent Comm – DARPA Cognitive radio project (10/2008)