Giving rebanding a head start

In July 2004, the FCC decided to reallocate spectrum within the 800 MHz band because of unacceptable levels of interference experienced by public-safety radio systems. When the staged reconfiguration project is complete, it should virtually eliminate interference threats to first responder communications. However, reconfiguration already is well behind schedule. What should public-safety agencies do in the meantime? Advanced filtering technologies can be deployed to minimize interference.

The root cause of the interference problem was the close proximity of public-safety spectrum with that assigned to commercial mobile radio services — most notably high-power iDEN networks. Specifically, commercial mobile radio channels, dispersed across frequencies from 851-866 MHz, were interleaved with public-safety channels in that same spectrum. (See Figure 1.) In addition, the National Public Safety Planning Advisory Committee (NPSPAC) band was sandwiched between the high-traffic Enhanced Specialized Mobile Radio (ESMR) band and the cellular band, which starts at 869 MHz. This unfortunate mix of disparate radio technologies generated the significant interference problems.

Two main sources of interference affect the receiver sensitivity of public-safety portable radios. These are spurious, or out-of-band, emissions from commercial mobile radio transmitters, and blocking, or receiver intermodulation.

Spurious emissions are caused by unwanted transmitter effects such as harmonics, intermodulation products and wideband signals that fall outside the transmit band. In the case of iDEN transmitters, the high-power, highly modulated iDEN signal spills outside its allocated 25 kHz channel and — in the absence of any guard band — into the adjacent channels on either side. The result is desensitization of the receiver for the adjacent channels, resulting in ineffective communications.

Receiver intermodulation is the result of the transmitted interfering signal itself. Put simply, the raw power of the iDEN signal overpowers the portable radio receiver, generating intermodulation products that can lead to interference, again degrading receiver sensitivity.

While waiting for the reconfiguration, system engineers can use several tactics to minimize the effects of spurious emissions and receiver intermodulation, led by RF filtering. But there are challenges regarding RF filtering in this band because of the interleaved spectrum. Public-safety receiver bandpass filters must pass all frequencies from 851-869 MHz, including those of high-power commercial carriers operating in the same block of spectrum. The same is true for transmitter filtering: All potential channels within a certain bandwidth must be passed. It is impossible to limit the out-of-band emissions of high-power signals on a channel-by-channel basis in this interleaved scenario.

It is possible, however, to apply RF filtering to groups of contiguous channels as permitted by the spectrum allocations of a particular site. Most sites use RF combining technology so multiple channels can be broadcast from a single antenna. In practice, each channel enters a separate resonating chamber, or cavity, tuned to that channel; on exiting the cavity the channel is coupled to the signals of other channels. The resulting wideband signal emitted from the combiner typically comprises six or more channels; this signal is then directed to the antenna for transmission.

As technology has advanced, these combiners have grown more sophisticated and now also incorporate advanced filtering technology within the cavity. In particular, autotune combiners have been deployed at many commercial mobile radio transmission sites. Although the autotune combiner’s primary function is to continuously monitor and adjust tuning to accommodate changes in carrier frequency and environment, the RF filtering aspect has become increasingly important.

For example, the four-channel radio used by many iDEN carriers can provide a combined output of four contiguous 25 kHz channels; this 100 kHz bandwidth corresponds to the bandwidth of a typical autotune combiner filtering cavity. Consequently, each group of four contiguous 25 kHz channels is filtered during the combining process to minimize out-of-band emissions, and hence interference with public-safety channels. The high filtering performance achieved by autotune combiners is the result of special dielectric-loaded filter cavities that provide a high quality-of-cavity response. The frequency spacing between autotune combiner cavities is typically 150 kHz.

Another victim of interference in the 800 MHz band prior to reconfiguration was the commercial Cellular B receive band, which ends at 849 MHz. Base station receivers could be affected on the uplink in much the same manner as the receivers of portable radios. In other words, they were subject to receiver desensitization due to spurious emissions or blocking from commercial mobile radio signals close to 851 MHz.

In such cases, installation of a bandpass filter in the interfering downlink to filter out-of-band emissions below 851 MHz could reduce by more than 50 dB the magnitude of wideband noise received by the affected base station at 849 MHz or below. Similarly, installation of a bandpass filter in the uplink of the affected base station could mitigate the real power of the interferer falling just outside the affected receive band. Depending on the transmitting power of the interfering base station, such uplink filters would need to achieve a minimum selectivity of up to 50 dB, particularly in co-location scenarios.

Once the reconfiguration of 800 MHz spectrum is completed, the interference situation will be entirely different — and more easily controlled. Most significantly, public-safety spectrum will be completely separate from that allocated to commercial mobile radio services, virtually eliminating interference between the two.

In the case of public-safety communications, the only remaining interference issue would be the somewhat unpredictable impact of transmitter intermodulation. This occurs when a non-linear combination of two high-power transmitted signals — such as an iDEN signal combined with a cellular CDMA signal — generate out-of-band emissions that can fall anywhere in the spectrum. Because of the high transmission powers involved, intermodulation products of a relatively high order may have sufficient power to interfere with public-safety channels. This is most likely to occur where iDEN and cellular CDMA services are co-located; however, the overall situation will still be dramatically improved after reconfiguration.

Any potential interference to other services can be eliminated more easily after rebanding using advanced filtering technology. The most noteworthy of these is the protection of the guard band between 861 and 862 MHz. As part of the reconfiguration process, commercial mobile radio carriers must pledge to provide at least 50 dB rejection at frequencies below 861 MHz.

The challenge here is the narrowness of the 1 MHz guard band, which requires an RF filter exhibiting sharp attenuation behavior if the entire ESMR spectrum is to be used. Such filters will be highly sophisticated, requiring multiple resonant chambers (or poles) to achieve the sharp attenuation behavior. In addition, cross-couplings, where dissipated RF energy is coupled back into the transmit path, will be required to compensate for the increased insertion loss that results from expanding the number of poles.

Where filters are to be used in iDEN networks, the profile and distribution of the resonators also will need to be carefully designed to accommodate the high peak power ratings typical of the iDEN transmitted signal. (Transmitted power typically ranges from 75-100 W, with instantaneous peaks of up to 12 kW not uncommon.)

To protect the non-cellular SMR; public-safety; and business, industrial and land transportation (B/ILT) services operating below 861 MHz, such high-performance filters will be required at all commercial mobile radio transmission sites. This requires a systematic overhaul of base station infrastructure and adherence to best installation practices. Ensuring that all accessories, such as connectors and jumpers, are correctly fitted and installed also minimizes the potential for passive intermodulation products.

The decision to reconfigure the 800 MHz spectrum undoubtedly was not made lightly by the FCC. Despite the planning, negotiation and deployment challenges, most industry experts agree that — ultimately — the plan will significantly improve the reliability, efficacy and timeliness of public-safety communications. Short-term interference abatement strategies can help alleviate the problem, but they are not all encompassing.

The long-term 800 MHz spectrum reconfiguration not only will eliminate the bulk of the interference threat to public-safety radio, but it also will facilitate the filtering of out-of-band emissions from commercial mobile radio carriers. Instead of high-traffic collisions caused by heterogeneity, the stratified spectrum will streamline all forms of 800 MHz radio communications. As the past years have shown, the road to this interference-free zone will not be easy, but it will be worth it in the long term.

Rodrigo Oliveira is the area product manager for wireless infrastructure solutions at Radio Frequency Systems.