TVWS, short for TV White Space, is a form of wireless connection that makes use of television channels left unused in a given region. This page explains where these free channels come from, why they matter to anyone who wants to connect places that are far apart and covered by vegetation. It also tells what TECER learned by testing them.
Where do white spaces come from?
The electromagnetic spectrum is the set of frequencies along which wireless signals travel. Each country divides the spectrum into bands and defines what each one is used for: radio, television, cellular telephony, Wi-Fi and other services.
Part of the spectrum is reserved for free-to-air television and divided into channels. No city uses all the channels at the same time: local broadcasters occupy some channels. The rest are left empty. Because the empty channels vary from one place to another, a channel that is free in one region may be in use in the neighboring region. These empty channels are the white spaces.
TVWS transmits data over these channels on one condition: it must not interfere with the broadcasters that are on the air. To achieve this, the equipment consults databases that indicate which channels are free at each location, or observes the spectrum before transmitting. This mechanism is called dynamic spectrum management. The rules for using these bands vary from country to country.
Why do these frequencies pass through vegetation better?
The TV channels used by TVWS are below 1 GHz. The equipment tested in TECER operates between 470 and 698 MHz. Wi-Fi uses 2.4 GHz and 5 GHz.
The difference becomes clearer when you look at the size of the wave. A 470 MHz wave is about 64 cm long, roughly the width of a door. A Wi-Fi wave at 2.4 GHz is about 12 cm, the size of a cell phone, and at 5 GHz, about 6 cm. Larger waves are absorbed less by leaves and branches and get around obstacles more easily, a phenomenon called diffraction.
That is why TVWS can work in NLOS (non line of sight) conditions, when there are trees or terrain between the antennas. This does not mean that vegetation stops mattering: the signal still loses strength as it passes through it. The actual range depends on the power, the antennas, the installation height and the terrain.
The price of these advantages is capacity. A TV channel is relatively narrow. Less data per second passes through it than through a modern Wi-Fi channel. TVWS is suited to connecting a distant point that then distributes the connection locally. It is less suited to serving many people directly.
What does a TVWS network look like?
A TVWS network has two types of equipment:
- The base station, a radio connected to one or more antennas, installed at a high, central point, where the signal starts out.
- The CPEs, short for customer premises equipment: smaller radios with an antenna that receive the signal where the connection will be used.
In the architecture tested by TECER, TVWS works as backhaul: it connects the city to a point in the community, from where the connection can be distributed over Wi-Fi or a mesh network.
Is there a technical standard?
Yes. The IEEE 802.22 standard, published in 2011, was the first definition of a radio interface based on cognitive radio, a radio able to observe the spectrum and choose where to transmit, for opportunistic use of the TV bands without interfering with broadcasting. It was designed to offer broadband in areas of low population density. Not all commercial equipment follows this standard; many devices use each manufacturer’s own solutions.
What did TECER learn?
TVWS was the central technology of the project’s Phase 1. The tests showed that:
- in Mosqueiro, the range was far below what was expected, around 400 meters;
- in the UFPA/LASSE laboratory, part of the problem was attributed to signal loss in the cables and to oxidation on the connections, not to the technology itself;
- in Tururi, three of five points connected through the forest, without line of sight.
The analysis of the causes of the reduced range is in Why a radio link has a shorter range than predicted.
Sources
- IEEE 802.22 Working Group. Official page of the working group (now inactive). grouper.ieee.org/groups/802/22
- IEEE 802.22, overview of the standard (Wikipedia).
- Radio used in the project: Carlson Wireless RuralConnect.