Inefficient Spectrum Usage
The UHF band of 400 MHz to 700 MHz is often termed the “beach front property” of spectrum due to its superior range and penetration compared to higher frequency bands. Globally, this band is typically licensed to TV broadcasters, which can be considered as primary transmitters (or primary users, PU) because they have the highest priority to access the spectrum as protected incumbents. When a geographical region has no primary broadcaster on a particular channel, that channel is said to be TV white space (TVWS), which is available for transmission by secondary users (SU) under today’s regulatory frameworks. Unfortunately, the large number of over-the-air TV broadcasters in many populated areas yields extremely limited white space availability. Nonetheless, in practice, the number of viewers watching TV via UHF is dwarfed by those watching via satellite or cable. For example, in the U.S., only 7% to 10% of all TV households rely on over-the-air UHF broadcast for TV programming. Moreover, most of the time these TVs are off.
Novel Spectrum Sharing with Smart Primary Receivers
As analyzed above, TVWS models calculate exclusion zones (areas where secondary transmissions are not allowed/transmit power is set to zero) based on transmitting TV channels and their corresponding tower locations (sub-figure (a) in the following). In contrast, we propose a dynamically computed exclusion zone characterized as the union of locations where secondary user transmit power must be reduced in order to protect active TV receivers (sub-figure (b) in the following). By exploiting that the receiver-based exclusion zone has a much smaller footprint than the transmitter-based exclusion zone, WATCH (WiFi in Active TV CHannels) enables vastly increasing secondary spectrum re-use.
As far as we know, WATCH is the first system to enable secondary WiFi transmission even in the presence of kilowatt-scale TV transmitters, while simultaneously protecting TV receivers when they are active. We have implemented the key components of WATCH (as discussed below) and experimentally evaluate their performance with FCC permission (FCC experimental license call sign WH9XHJ and file number 0121-EX-ST-2014).
Smart-Primary-Receiver-Aided Dynamic Central Controller
To protect active TV receivers from secondary transmissions, we introduce two mechanisms to dynamically control the exclusion zone: (i) By generalizing the functionality of the spectrum database controller in standards such as IEEE 802.11af, we design the WATCH spectrum database controller to collect information of active TV receivers and accordingly coordinate secondary transmissions. Namely, with active TV receiver channel usage and location information, the WATCH controller dynamically determines the maximum transmit power for SU’s. (ii) We design a WATCH TV receiver that can inform the controller of TV viewing. We introduce two complimentary feedback mechanisms to allow use with legacy TV systems: first, we propose a smart remote control coupled with a legacy TV, e.g., via a smartphone. Upon switching the TV channel via infrared, the enhanced remote also informs the WATCH controller of the new selection. Second, we propose a smart TV coupled with a legacy remote, in which the Internet-connected TV informs the WATCH controller of the new selection.
Novel Secondary Transmit-Receive Architecture to Cancel TV Interference
We design a novel secondary transmit-receive architecture that enables secondary WiFi transmission even when the kilowatt-scale TV transmitters are broadcasting. For secondary reception, we design an interference cancellation (IC) technique, WATCH-IC, which exploits the fact that TV signals are always being broadcasted, unlike IC in non-streaming-broadcast systems such as cellular or WiFi. In particular, our design cancels TV signals without requiring their preambles to be known a priori such that WATCH is compatible with any broadcast technology and is not specific to a regional TV coding scheme. For secondary transmission, we design CAT, a Constructive Addition Transmission scheme for secondary WiFi transmitters. CAT precodes transmissions and computes beam weights of the secondary transmitting antenna array to ensure that secondary signals add constructively after WATCH-IC. It addresses the problem of inadvertent cancellation of secondary signals without coordination with legacy TV systems. Moreover, we employ selective feedback to reduce CAT’s overhead. Compared to transmit beamforming in IEEE 802.11n, CAT adapts to continuous and strong interference.
X. Zhang and E. Knightly, “WATCH: WiFi in Active TV Channels,” in Proceedings of ACM MobiHoc 2015, Hangzhou, China, June 2015. Best Paper Award.