Take a look here too:
http://cordis.europa.eu/ictresults/i...tures/ID/89327

Double antennas deliver double the signal

The wireless shadows of tall buildings are a challenge for transmission.
PhotoDisc
Digital TV transmission techniques that deliver most benefit in the worst
reception environments have been developed by a consortium of European
researchers. The technologies promise to reduce the network infrastructure
needed for mobile TV, while minimising the power demands and complexity of
mobile TV receivers of the future.

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

The transmission-splitting technique under development by PLUTO, known
as ‘transmit diversity’, benefits the worst environments most. Reception
can be spectacularly improved indoors, or whilst walking or driving in
cities. Fewer transmitters and less power are required to achieve
economical coverage.

“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

Even with the reception improvements promised by transmit diversity, there
will be reception black spots, particularly indoors, where on-channel
repeaters will be needed. An on-channel repeater receives a signal from a
distant transmitter and re-transmits it at the same frequency. The
repeaters are prone to instability caused by the feedback of echoes from
the transmitter to the receiver. Here, the academics at Brunel University,
led by PLUTO project coordinator Professor John Cosmas, have developed an
innovative method to remove these echoes.

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.”




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