Capital Currents - The SDARS–WCS Mess
The FCC created a mess back in 1997 when it allocated spectrum in the 2305-2360 MHz band, partly for the Satellite Digital Audio Radio Service (SDARS) and partly for the Wireless Communications Service (WCS). The SDARS is known commercially as Sirius XM, and the WCS is known commercially as … well, it isn’t known as anything because there is no commercial service.
But the FCC’s National Broadband Plan is the agency’s goal above all other goals. As a result, the headline announcing a new FCC decision trumpets: “FCC unleashes 25 MHz of spectrum for mobile broadband use – provides greater certainty for satellite radio and mobile broadband licensees.” I’m skeptical that the new rules will provide enough confidence to encourage investment in this particular 25 MHz of spectrum.
The basic problem is that the WCS allocation is adjacent to the 2320-2345 MHz SDARS frequency band. But SDARS satellite receivers must be very sensitive because they receive very low-power signals from satellites, and thus they are susceptible to interference from signals on adjacent bands. To protect them from interference, the FCC in 1997 imposed very tight technical rules on the WCS, particularly out-of-band emission limits.
Licenses to provide SDARS within the U.S. were awarded by auction in early April 1997. The two winners of the auction – XM and Sirius – were each assigned 12.5 megahertz of spectrum for their exclusive use on a primary basis. XM and Sirius launched their satellites and began commercial operations in 2001 and 2002, respectively. On Aug. 5, 2008, the FCC approved the merger of XM and Sirius. And as of March 31, Sirius XM had nearly 19 million subscribers in the contiguous United States.
The FCC auctioned WCS licenses in April 1997. Although the Commission permitted WCS licensees to provide both fixed and mobile services, it adopted different power limits for these two classes of service. For WCS fixed operations in the 2305-2320 and 2345-2360 MHz bands, the Commission adopted a peak power limit of 2 kW EIRP. For WCS mobile stations, the Commission adopted a peak power limit of 20 W EIRP. In addition, very stringent out-ofband emission limits were imposed in portions of the WCS band closest to the SDARS band – in fact, so stringent that they made mobile WCS devices impractical.
According to the FCC, Horizon Wi-Com, AT&T, Comcast, nTelos and NextWave Broadband collectively hold virtually all of the 2305-2320/2345-2360 MHz WCS licenses within the U.S.
Both Sirius XM and the WCS companies did a separate series of interference tests during 2009, some of which were observed by FCC staff. Differences in test setups included the power levels, duty factor or duty cycle of the WCS signal, WCS signal strength as a result of propagation losses, WCS antenna heights and positions (outside and inside the test vehicle), etc. Not surprisingly, Sirius XM and the WCS companies reached differing conclusions about power limits and out-of-band emission limits. Nonetheless, based on staff observations, the FCC felt that it could adopt new technical rules based on the test results.
The FCC has now adopted a set of complex new rules for WCS and SDARS. Most important are the new rules for WCS mobile and portable devices, which might be located in close proximity to those sensitive SDARS receivers. The WCS mobile devices will not be permitted to operate in the 2.5 MHz portions of the WCS band closest to the SDARS band, and in other portions they must operate with a reduced power limit of 250 mW average EIRP. In addition, the FCC imposed duty cycle limits and required the use of automatic transmitter power control. But it also relaxed the very tight out-of-band emission limits. In addition, a variety of complex technical rule changes were adopted for fixed WCS stations. New technical rules were also adopted for SDARS terrestrial repeaters, including power limits. In addition, blanket licensing of repeaters will be permitted, instead of a separate license for each transmitter site, but Sirius XM must notify affected WCS operators in advance to coordinate site locations.
The FCC has recognized that strict rules that eliminate all interference to SDARS will kill WCS as a viable service, so its goal is more modest – to eliminate interference that “repeatedly disrupts or seriously degrades service.” The FCC believes the modified WCS mobile and portable devices’ operating power and out-of-band limits will prevent interference to SDARS operations, except in rare instances (e.g., WCS mobile device in close proximity to SDARS receiver, high degree of mutual coupling between WCS and SDARS antennas, lack of obstructions between WCS transmitter and SDARS receiver, WCS mobile device transmitting channel immediately adjacent to SDARS receiving channel). But if more than occasional intermittent interference is suffered by SDARS listeners, the WCS operators are required to cure the problem.
Does that compromise give you the confidence to invest in WCS? “Unleashing” this 25 MHz of WCS spectrum lets the FCC put a check mark against one of the specific goals of its broadband plan, but the mark next to “Turn WCS into a viable service” remains a question mark.