Motorola drops 800 MHz bomb
Motorola Inc. dropped a bomb on the debate to resolve interference to public safety operations in the 800 MHz band.
In a May 6 letter to the Federal Communi-cations Commission, Motorola officials wrote that it is possible to alleviate a majority of the interference being experienced by public safety communications in the band through best practices and new technical solutions without impact on public safety systems that are not experiencing interference.
Nextel Communications Inc., working with several associations that represent public safety and private interests, has developed a plan (dubbed “the consensus plan”) to reduce the interference caused by the company’s push-to-talk service in the 800 MHz band by consolidating public safety use and eliminating the interleaving of CMRS channels with public safety in a major realignment of spectrum users in the band.
On May 16, Nextel filed a response to the Motorola plan saying that realignment is essential to fix the interference problem and to opening new spectrum to public safety. While Motorola’s comments are useful and should be part of the solution after realignment, Nextel said, Motorola makes assumptions that “do not withstand scrutiny.” A technical solution only perpetuates “an outdated band plan that permits inherently incompatible system architectures to operate on mixed, interleaved, and adjacent channels.”
All in the family
This is not really much of a fight because it is all in the family. Or if it is, the family members prefer privacy. Players from both companies were reluctant to be interviewed. In 1995, Motorola exchanged its SMR licenses in the 800 MHz band for a 20 percent ownership interest in Nextel. And it should be noted that Motorola was responding to an FCC request when it filed its plan. The FCC’s Edmond Thomas, chief of the Office of Engineering and Technology, requested that Motorola look into a technical solution to 800 MHz interference.
Motorola’s proposed solution already has ignited reverberations with stakeholders realigning on both sides. The board of directors of the Cellular Telecommunications & Internet Association voted June 4, for example, to join the coalition supporting Motorola’s technical solutions. Tim Donahue, president and chief executive officer of Nextel Communications Inc., stepped down as chairman of the CTIA board at that meeting.
The Consensus Plan would split the 800 MHz band into two contiguous blocks of spectrum: a non-cellularized block and a cellularized block. Public safety, business, industrial and land transportation entities, and traditional airtime service SMR licensees would remain in a 20 MHz non-cellularized block, and Nextel would relocate to a 16 MHz cellularized block. In the non-cellularized block, a 2 × 2 MHz guard band would be created at 814-816/859-861 MHz for low-power systems and other business licensees. Nextel would contribute $850 million to the relocation process and receive 10 megahertz of spectrum in the 1.9 GHz band.
The proposal also would provide a five-year period during which public safety entities could acquire the remaining channels vacated by Nextel in the non-cellularized block. Nextel officials insist this would provide additional channel capacity for state and local public safety agencies.
“Motorola has continued to work to overcome these challenges and, as the record in this proceeding has developed, it appears that there are new opportunities for successfully resolving many of the instances of interference through advances in receiver technology and increased signal strength for public safety systems,” the company said in the May 6 letter signed by Steve B. Sharkey, Motorola’s director of Spectrum and Standards Strategy.
Motorola officials say that they believe that technical advances in receiver design are commercially viable, will have limited impact on the cost of portable public safety equipment and provide an opportunity for alleviating interference to public safety.
The company said it continues to test to fully understand the degree to which these advances will mitigate interference to public safety. Company officials add that “testing and analysis to date is very promising, and Motorola plans to deploy the receiver technology advancements by the end of 2003.”
No one for Motorola or Nextel would comment verbally by press time.
In an e-mail, Norman Sandler, director of Strategic Global Issues for Motorola, said the company did not submit a specific proposal to the FCC. Rather, the company sent a technical response to issues raised by the FCC Office of Engineering and Technology. The office asked Motorola to comment on technology advancements available or being incorporated into public safety radio products to mitigate interference caused from Commercial Mobile Radio Systems (CMRS).
“This data collection is a normal part of a larger process that continued (May 29) with a meeting convened at FCC to review proposed technical solutions and related issues,” he said. “Motorola has always advocated two things: (1) sufficient spectrum to satisfy the needs of all of our customers, and (2) an open, participative and fact-driven regulatory process that considers which services or specific users may benefit from additional spectrum.”
Scope of the problem
Motorola and Nextel disagree on the scope of the problem.
Motorola says that while interference is serious when it occurs, the record indicates that it is occurring in a relatively small number of systems compared to the total number of public safety systems. A review of the APCO database on interference shows 24 unique customer issues in 2000, 7 in 2001, 23 in 2002, and 5 in the first quarter of 2003. This is compared to a total of 2,139 public safety systems deployed at 800 MHz Accordingly, focusing on these areas makes sense, provided that long-term mitigation is possible.
Nextel said in its response, “Motorola understates the extent to public safety interference experienced and reported during the past few years.”
The cases involve multiple interference problems incidents through out a coverage area. Nextel said that Motorola assumes that less than 3 percent of public safety systems are experiencing interference.
Nextel also said that Motorola assumes the realignment represents an agreement by the public safety community to strengthen on-street public safety signal strength. Nextel said that the public safety community already rejected the suggestion of improved public safety receivers and increased signal strength because of the cost and the technical and operational obstacles.
Boosting receivers and signal strength
Motorola says that carriers have focused on the intermodulation rejection performance of receivers and on the filter characteristics of the receivers. Motorola has demonstrated that public safety receivers provide from 1 – 14 dB greater intermodulation rejection than radios deployed by CMRS systems, the company said. Motorola also described the negative impact of further improving intermodulation performance — increased power drain and unacceptable battery life for portable units. With regard to use of narrower filters as proposed by some CMRS carriers, Motorola said it provided information showing the increased insertion loss for narrower filters compared to filters currently employed. An across-the-board increase in insertion loss reduces the sensitivity of the radio. That reduces public safety coverage in weak signal areas.
Motorola said it is confident that with increased signal strength in weak signal areas, some of the technical solutions that Motorola has pursued are feasible and will mitigate the interference.
This focuses resources and disruption in the limited number of areas where interference occurs rather than disrupting public safety operations nationwide, Motorola said. At the same time, this approach offers a long-term increase in the overall reliability of public safety systems.
While simply increasing signal strength will address interference in many cases, Motorola notes that it is often not a straightforward proposition to do so. In some cases adding additional base stations can be difficult because of cost, tower construction and site leasing needs. In some areas it also will not be possible just to add an additional base station because no additional frequencies are available. In such cases, users will have to convert their systems to support simulcast operation. Adding simulcast sites creates additional intrasystem interference concerns.
Nextel takes issue with assuming increased signal strength and improved receivers in those areas with problems will alleviate interference.
Even with increased signal strength, receiver advances are preliminary and potentially solve only half the problem, according to the Nextel response. Receiver advances are designed to reduce intermodulation interference; they do not reduce interference caused by CMRS out of band emissions (OOBE). Nextel estimates that this type of interference causes 50 percent of the incidents and can be addressed only by realigning the 800 MHz band to separate public safety and cellular operations.
Nextel officials add that the “fatal flaw” in Motorola’s reliance on improved receiver performance is cost. Nextel also takes issue with the idea that public safety and CMRS licensees can predict areas that are likely to experience interference and can deploy technical measures to resolve the problem.
Switchable attenuators
One method of addressing interference is by using a switchable attenuator to reduce the strength of signals entering the receiver that would otherwise result in non-linear operation of the low noise amplifier and mixer, creating intermodulation interference, according to Motorola. Activating the attenuator, however, desensitizes the radio and would result in loss of coverage in weak signal areas.
“Motorola has made substantial progress in addressing the challenges of implementing a switchable attenuator in a way that reduces interference in areas of sufficient desired signal strength while ensuring that the attenuator does not degrade the reliability of public safety communications by activating when the desired signal is too low,” the company said. It views this as a commercially viable feature when the public safety signal strength is strong.
Motorola believes that a combination of switchable attenuators and increased public safety signal strength in weak signal areas will mitigate the interference being experienced to a large extent.
Nextel says that using switchable attenuators requires a fundamental change in public safety communications design because public safety uses noise-limited systems that are not sufficiently strong enough to over come CMRS interference.
Implementing a switchable attenuator solution would require replacing or retrofitting radios in areas where interference can’t be resolved by other best practices. Motorola believes that a field upgrade kit could be developed for some radios. Radios with switchable attenuators also would be implemented through the normal purchasing cycle for users, raising the future overall immunity of systems to interference. Motorola expects to have radios with switchable attenuators commercially available by the end of 2003. The addition of a switchable attenuator will have only a minimal impact on the cost of receivers, the company says.
Nextel says that the idea requires an expensive system change, including new base stations in some cases.
Tunable varactors
Another solution that Motorola has pursued is to tune dynamically the varactor filter on its radios that operate in both the 700 MHz and 800 MHz bands. Filter performance for public safety radios has been a significant focus of some CMRS carriers, which claim that filters on newer public safety radios are unnecessarily wide and allow excessive energy from out-of-band sources into the front end of the radio. Motorola believes that the use of narrower filters is not feasible. Filters that are in radios covering both the 700 MHz and 800 MHz bands, the XTS 2500 and XTS 5000 models, are equipped with tunable filters, Motorola said.
Motorola has been testing new software that retunes the filter based on received signal strength. Motorola has found that, with the varactor filter tuned away from the cellular radio frequencies, increasing the in-band attenuation and providing the same effect as the switchable attenuator, interference has been significantly mitigated.
Nextel says that tunable varactors are an improvement only for Motorola’s 700/800 MHz dual-band radios, a small number of the public safety radios in service. There is no commitment to equip or fix millions of other radios in public safety, Nextel said.
Best practices
Motorola says best practices calls for a number of steps to be taken, including retuning CMRS channels away from public safety operators channels; modifying CMRS power levels, antenna height and antenna characteristics; assuring proper operation of base station equipment; and improving the local signal strength of the public safety communications system. With the availability of advanced receiver designs, Motorola now recommends that the best practices guide be modified to include deployment of enhanced receivers in areas where the public safety signal strength is sufficient to support their use. Where interference cannot be resolved, the public safety signal strength would need to be evaluated and deployment of new receivers would be included in conjunction with a plan to improve the public safety signal strength, if necessary.
According to the Nextel response, Motorola relies too heavily on a revised best practices regime that attempts to resolve interference at specific sites rather than eliminating its causes and is too risky for first responders. Interference is too unpredictable given the dynamic nature of CMRS systems. The operating restriction in best practices impose large financial and operational costs.
