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Rebanding and the RF filtering challenge Interference problems in the US public safety communications sector have led to a massive 800-MHz spectrum rebanding project. Yet interim interference abatement and long-term RF filtering strategies will also prove integral to the final solution. <strong>Radio</strong> spectrum allocation in the USA has evolved over the decades in response to changing needs. As new wireless communications technologies have emerged, new licenses have been granted, and each frequency band has been divided into thinner and thinner slices of spectrum. Although technologies to eke greater capacity out of this limited resource have become more sophisticated, it was perhaps inevitable that the heterogeneous allocation of some high-traffic bands would result in headlong collision. A case in point The root cause of the interference problem is the close proximity of public safety spectrum with that assigned to commercial mobile radio services— most notably high-power integrated digital enhanced network (iDEN) systems. The current configuration of the US 800-MHz band is illustrated in Figure 1. The ‘general category’ (transmitting 851 to 854.75 MHz) and ‘interleaved’ (transmitting 854.75 to 861 MHz) blocks of spectrum are licensed to various specialized mobile radio (SMR) services, business interleaved with public safety channels in that same spectrum. In addition, the NPSPAC public safety band is sandwiched between the high-traffic ESMR band and the Cellular transmission band, which starts at 869 MHz. This unfortunate mix of disparate radio technologies has generated the significant interference problems experienced today. 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 Figure 1. Current configuration of US 800-MHz spectrum. RF CONDITIONING 13 absence of any guard band—into the adjacent channels 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 is the US 800-MHz band—the latest to be and industrial land transportation (B/ILT) can lead to interference, again degrading targeted by the Federal Communications systems, some public safety communica- receiver sensitivity. Commission (FCC) in a high-profile tions and commercial cellular-like services. Recognizing that action needed to be rebanding project across the nation. The ‘Enhanced SMR’ (ESMR) band taken in order to meet the critical need The symptom leading to the FCC’s decision (transmitting 861 to 866 MHz) is licensed for robust and highly reliable emergency to reallocate spectrum within this band is primarily to commercial cellular-like communications systems, the FCC has unacceptable interference problems being services; while the ‘NPSPAC public safety adopted a spectrum reallocation plan that experienced by public safety radio systems. channels’ are currently transmitting in the will minimize the impact of commercial If emergency communications go down, 866 to 869-MHz band. 800-MHz services on public safety radio. any delays that occur during a search for alternate means of communication—such Unfortunate mix Implementation of the ‘Consensus Plan’, widely supported by public safety groups as police UHF frequencies, fixed line, or This boils down to the fact that commercial across the USA, will achieve the spectral even radio channel switching—can mobile radio channels, dispersed across separation of generally incompatible have tragic consequences. frequencies from 851 to 866 MHz, are technologies (Figure 2). This means the
NPSPAC public safety band will be relocated to the lower end of the band (transmitting 851 to 854 MHz), with all other public safety channels, non-cellular SMR and B/ILT services relocated (if necessary) to 854 to 860 MHz. All commercial cellular-like mobile radio services will be assigned contiguous spectrum between 862 and 869 MHz, with the spectrum between 860 and 862 MHz reserved as a 1-MHz expansion band and 1-MHz guard band. Short-term strategies By the time the staged rebanding project is completed sometime in 2008, it should have virtually eliminated the interference threat to public safety communications. However, there are advanced filtering technologies that can be deployed in the meantime—and in some cases have already been deployed—to minimize interference in the short-term. The ongoing challenge with RF filtering 14 RF CONDITIONING in this band is due to the interleaved spectrum. Public safety receiver bandpass filters must pass all frequencies between 851 to 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 Figure 2. Configuration of US 800-MHz spectrum after rebanding. permitted by the spectrum allocations of a particular site. Most sites utilize RF combining technology in order for multiple channels to be broadcast from a single antenna. In practice, each channel enters a separate resonating chamber (or cavity) tuned to that channel, and on exiting the cavity 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 ‘Quad’ radios 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 ‘Q’ or quality of cavity response. The frequency spacing between autotune combiner cavities is typically 150 kHz. Another victim of interference in the current 800-MHz configuration is the commercial Cellular B’ receive band, which ends at 849 MHz. Base station receivers can be affected on the uplink in much the same manner as the receivers of portable radios—in other words, they are 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 can 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 mitigates the real power of the interferer falling just outside the affected receive band. Depending on the transmitting power of the interfering base station, these uplink filters need to achieve a minimum selectivity of up to 50 dB—particularly in co-location scenarios. Safety spectrum separated The interference situation in the USA after the rebanding of 800-MHz spectrum 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 (operating in the ESMR band— Figure 2), virtually eliminating interference between the two. In the case of public safety communications, the only remaining interference issue will 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 transmitted powers involved, intermodulation products of a relatively high order may have sufficient power to interfere with public
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