In a typical broadcast transmission, radio signals bounce off objects in the environment, reaching the receiver over multiple paths. Distortion from ‘multi-path’ signals can produce fading, resulting in temporary failure of reception. Most of us have experienced this, moving a mobile phone around a room to get the strongest signal.
Modern broadband wireless technologies like wimax, DAB for audio, and DVB-T and DVB-H for video, use a modulation scheme called Orthogonal Frequency-Division Multiplexing (OFDM).
An OFDM transmission is spread across thousands of different sub-carriers, each carefully organised at slightly different frequencies within the channel. Spreading the transmission across a high number of sub-carriers increases the probability of maintaining error-free transmission.
Researchers have demonstrated that splitting the transmit power between multiple antennas can provide substantially more effective coverage than using a single antenna. Signal simulations carried out for project Pluto show a gain of up to 5 decibels could be achieved. The Physical Layer DVB Transmission Optimisation (PLUTO) project comprises a consortium of academics, equipment manufacturers, propagation experts and broadcasters from Finland, France, Germany and the UK, co-funded by the European Commission.Best in worst environments
“This technique does not require revision of any WiMAX, DVB or DAB standards,” says Maurice Bard of UK company Broadreach Systems, the technical leader of PLUTO. “The great advantage is that it can be exploited by existing in-service receivers without modification. All you need is an additional box that can split the signal.”
“The transmit antennas need to be spatially separated by between 10 and 20 wavelengths and a delay applied to one antenna to achieve effective de-correlation. The amount of separation and delay depends on the type of environment to be covered,” says Bard.
Coverage can be further improved if there are two or more antennas at the reception end, he explains: “For receive diversity, you need to separate the receive antennas by at least half a wavelength which is approximately 25 centimetres at UHF frequencies. If this can be achieved, then transmit and receive diversity can work together to deliver even greater benefits. The benefits of receive diversity, however, can only be exploited in terms of network design if all receivers in the network have diversity implemented.”Filling in black holes
A pseudo-random sequence is buried deep in the re-transmitted DVB-H signal,” explains Cosmas. “The sequence acts as a signature, allowing the repeater to differentiate the unwanted echoes from the wanted original signal and remove them from the re-transmission.”
“The method can work for repeaters of any OFDM based network.”
Broadreach Systems has provided equipment to process signals at the transmitter and monitoring stations that intercept and measure transmitted DVB signals. The monitor stations are networked to a control centre, developed by Brunel, enabling the effects of diversity to be evaluated in real time.”
There are still some hurdles to be overcome before PLUTO’s transmit diversity solution is suitable for all types of broadcast networks. Transmit diversity actually results in a degradation in reception where the receiver is in clear line-of-sight with the transmitter and the signals from each antenna are received at exactly the same power level.
The line-of-sight reception loss may not prove to be a problem for many networks. In a mobile TV network, all receivers will be in a non- or near- line-of-sight situation, very few will have rooftop antennas. But ‘good enough’ is not a position that the PLUTO consortium is prepared to stop at if they are to change traditional thinking.
“We need to show that the performance we saw in the lab can be achieved in all real situations, rain, snow, cities …” says Cosmas.
“And, we have to convince the broadcasters who designed traditional analogue networks, where multi-paths had to be avoided, that multi-paths are good.”
Christian Nielsen | alfa
Accelerating quantum technologies with materials processing at the atomic scale
15.05.2019 | University of Oxford
A step towards probabilistic computing
15.05.2019 | University of Konstanz
Engineers at the University of Tokyo continually pioneer new ways to improve battery technology. Professor Atsuo Yamada and his team recently developed a...
With a quantum coprocessor in the cloud, physicists from Innsbruck, Austria, open the door to the simulation of previously unsolvable problems in chemistry, materials research or high-energy physics. The research groups led by Rainer Blatt and Peter Zoller report in the journal Nature how they simulated particle physics phenomena on 20 quantum bits and how the quantum simulator self-verified the result for the first time.
Many scientists are currently working on investigating how quantum advantage can be exploited on hardware already available today. Three years ago, physicists...
'Quantum technologies' utilise the unique phenomena of quantum superposition and entanglement to encode and process information, with potentially profound benefits to a wide range of information technologies from communications to sensing and computing.
However a major challenge in developing these technologies is that the quantum phenomena are very fragile, and only a handful of physical systems have been...
Working group led by physicist Professor Ulrich Nowak at the University of Konstanz, in collaboration with a team of physicists from Johannes Gutenberg University Mainz, demonstrates how skyrmions can be used for the computer concepts of the future
When it comes to performing a calculation destined to arrive at an exact result, humans are hopelessly inferior to the computer. In other areas, humans are...
Scientists develop a molecular recording tool that enables in vivo lineage tracing of embryonic cells
The beginning of new life starts with a fascinating process: A single cell gives rise to progenitor cells that eventually differentiate into the three germ...
29.04.2019 | Event News
17.04.2019 | Event News
15.04.2019 | Event News
17.05.2019 | Materials Sciences
17.05.2019 | Physics and Astronomy
17.05.2019 | Materials Sciences