Double antennas double the signal

In a typical broadcast transmission, radio signals bounce off objects in the environment, reaching the receiver over multiple paths. 'Multi-path' signals can produce fading and distortion resulting in temporary failure of reception.

Technology developed by a consortium of European researchers claims to reduce the network infrastructure needed for mobile TV, while minimising the power demands and complexity of mobile TV receivers of the future.

Modern broadband wireless systems, such as WiMAX, DAB, and DVB-T and DVB-H for video, use a modulation scheme called orthogonal frequency-division multiplexing (OFDM).

These transmissions spread across thousands of different sub-carriers, each organised at slightly different frequencies within the channel. Spreading the transmission across a number of sub-carriers increases the probability of maintaining error-free transmission.

Researchers demonstrated that splitting the transmit power between multiple antennas can provide better coverage than using a single antenna.

Signal simulations carried out for the Physical Layer DVB Transmission Optimisation (PLUTO) project show a gain of up to 5 db could be achieved. The PLUTO project comprises a consortium of academics, equipment manufacturers, propagation experts and broadcasters from Finland, France, Germany and Britain, co-funded by the European Commission.

The transmission-splitting technique under development by PLUTO, known as 'transmit diversity', benefits the worst environments most.

Reception can be improved indoors, or while walking or driving in cities. Fewer transmitters and lower power are required to achieve economical coverage.

"This technique does not require revision of any WiMAX, DVB or DAB standards," said Maurice Bard of British 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 depend on the type of environment to be covered."

Coverage can be further improved if there are two or more antennas at the reception end, Bard explains: "For receive diversity, you need to separate the receive antennas by at least half a wavelength, which is approximately 25 cm 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."

Even with the reception improvements promised by transmit diversity, there will be reception black spots, particularly indoors, where on-channel repeaters will be needed.

The repeaters are prone to instability caused by echo feedback from the transmitter to the receiver. The academics at Brunel University, led by PLUTO project coordinator Prof John Cosmas, have developed a method to remove these echoes.

"A pseudo-random sequence is buried deep in the re-transmitted DVB-H signal," said 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.

"We need to show that the performance we saw in the lab can be achieved in all real situations, rain, snow, cities ..." said Cosmas.

"And, we have to convince the broadcasters who designed traditional analog networks, where multi-paths had to be avoided, that multi-paths are good."