Skip to main content

Qualcomm Examines Ch. 51/56 Interference on Wireless Systems

Last week Qualcomm submitted very detailed technical comments in response to the FCC Wireless Telecommunications Bureau Docket WT 12-69 NPRM - Promoting interoperability in the 700 MHz Commercial Spectrum

Qualcomm's comments focus on the problems with interference in the 700 MHz band that make it effectively impossible to provide the interoperability proposed in the NPRM without significantly degrading service. 

To support this claim, Qualcomm showed that the “Band 12” filters required to support service across the A, B, and C lower 700 MHz blocks (former TV channels 52, 53, 54 and 57, 58, 59) allow much more adjacent channel interference from Block E licensees (former Channel 56 spectrum), which are allowed to operate with effective radiated power of up to 50 kW, and high power TV on Channel 51 into the adjacent wireless bands. 

While it appears that Block B, which does not have base station or consumer device frequencies adjacent to Channels 51 and 56, would not be affected, Qualcomm shows, as Charlie Rhodes has pointed out in several of his TV Technology columns, that strong signals from Channel 51 and Channel 56 will create intermodulation interference in Block B. 

Qualcomm also points out that the studies presented in the NPRM that are based on testing in Atlanta greatly underestimate the amount of interference. As evidence, Qualcomm presented the results of drive tests showing the signal strength from Channel 55 MediaFLO transmitters in Dallas and Phoenix, as well as calculated receive power levels from Channel 51 TV stations in Chicago, and Montclair, N.J. These calculations were done with Communications Research Centre Canada’s (CRC) propagation software, and are based on a handheld cell phone device 1.5 meters above ground. Due to the high Channel 51 signal strength from WPWR-TV in the Chicago market, Qualcomm found that on Blocks B and C, “the majority of Chicago's Loop and surrounding areas is predicted to suffer a coverage gap, and that much of the city, its principal interstate and highway arteries, and surrounding suburbs, would suffer degraded service or no service at all.” 

This study, in addition to making a good case for not requiring wireless devices to provide interoperability over three transmit and receive channels, highlights the problem wireless device manufacturers and their customers are likely to face in rolling out service in the UHF spectrum obtained in the FCC's UHF TV spectrum auctions, unless the spectrum is contiguous and there’s a guard band (for white space devices?) between the wireless spectrum and high-power TV channels. 

It also shows that allowing medium power, wider spaced, wireless broadband “multicast” transmitters, such as those deployed by MediaFLO, will have a negative impact on the use of surrounding spectrum for two-way wireless broadband. This would indicate keeping broadcasting and wireless broadband separate is a good thing that will allow for the most efficient use of spectrum.

Doug Lung

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.