A group of broadcasters, wireless carriers and high-tech manufacturing companies has advised the FCC that guard bands larger than 12MHz provide no meaningful benefit from an engineering perspective and would unnecessarily decrease the amount of licensed spectrum available for auction.
In a letter dated May 3, the group provided the FCC with guidance on the optimal size of guard bands, including the required duplex gap necessary for a Frequency Division Duplex (FDD) band plan.
The duplex gap in an FDD band plan acts as a guard band between the transmit and receive channels. The separation must be sufficient to permit paired transmit and receive signals on a common antenna with minimum degradation of the RF signals, consistent with known duplexer and filter technologies.
The group said it believes that the size of the duplex gap appropriate for a paired spectrum FDD approach above TV 37 is between 10MHz and 12MHz.
“Guard bands, including the duplex gap, are technically reasonable if they are designed to both maximize the amount of licensed spectrum that is suitable for mobile wireless service while providing sufficient frequency separation needed between adjacent services to prevent interference and to ensure that the blocks adjacent to any guard band, including the duplex gap, are essentially fungible with all other 5MHz blocks,” the letter said.
“In our expert view, a separation larger than 12MHz provides no meaningful benefit from an engineering perspective and would unnecessarily decrease the amount of licensed spectrum available for auction. There is no sound engineering justification for the extremely large (18MHz to 28MHz) duplex gaps proposed by some people in this proceeding. Indeed, such a large duplex gap would decrease antenna efficiency and would unnecessarily consume as much as 15MHz (i.e. three 5MHz blocks) that could otherwise be auctioned,” the letter said.
A wider duplex gap, the group said, increases the amount of spectrum that the Commission would need to recover in the reverse auction in order to achieve a minimum sellable bandwidth in every geographic market in the country for the forward auction.
“With respect to the guard band between mobile downlink and TV broadcasters, according to our engineering analysis, no more than 10MHz of separation is needed between the highest remaining full power TV station and the adjacent downlink block to avoid saturation and blocking of the mobile device receiver or interference to TV broadcast services,” the letter said.