Special Report
RF solution to
crew. There are many practical difficulties that
have to be overcome. The 13.629 km track
extends over a huge area which must be fully
covered with RF receivers. It is not possible to
fly a helicopter by night, so this dictated the
Cellular Diversity approach to the production,
ultimate test
which meant that the signals from the mobile
cameras could be relayed back through fixed
antennas placed around the track.
The most interesting challenge for the RF
of endurance
team was to find equipment that would
overcome the fading and Doppler effects
caused by transmitters in the cars travelling at
up to 350 km/h. DVB-T was designed for
transmissions from high power fixed
transmitters to fixed receivers. The received
signal gets distorted by reflections from
buildings and attenuation by trees etc. DVB-T
receivers compensate for this by measuring
pilot tones sent within the COFDM signal.
Typically the compensation to be applied is
calculated over a period of 12 symbols which
is about 3ms. For a car travelling at 300km/h
this is about 2 wavelengths of a 2.5GHz
signal. In a fading environment, multiple
fades will have moved into and out of the
COFDM spectrum during this period making
any channel compensation ineffective. Link’s
own modulation scheme LMS-T, has an
equivalent channel compensation period of
250us or 0.2 wavelengths enabling variations
in the received signal to be removed with
compensation.
Technology that provided the answers
The solution chosen for Le Mans 2008
Race used a series of individual receive sites
JOHN MULCAHY, managing director of Link Research, reports linked together with fibre optics to form a
on the challenges and recent successes of transmitting the
Cellular Diversity network. This allowed the
cars fitted with COFDM transmitters running
Le Mans 24 Hour Race.
LMS-T to move seamlessly around the circuit
he Le Mans 24 Hour Race is shown cover the track and the pit lanes. It is within the combined coverage area of all the
T
in most countries around the World. probably the longest race course to be receive sites.
The race is a test of endurance. covered with wireless cameras so far. The transmitters were the new Link XPu
Traditionally it is also a proving Visual TV used a new solution with in-car model from Link Research. The Link XPu is
ground for new technology - and 2008 was cameras to provide full night-time coverage designed for vehicle on-board applications. It
no exception. using Cellular Diversity. Also, with corporate is a pocket sized device, which can be used
Until this year, the Le Mans 24 Hour Race entertaining becoming a major commercial with an external amplifier placed close to an
had been covered using an aircraft as a relay activity at the race, more people want to see antenna to minimise cable loss or it can be
to pick up signals from the cars and beam the drivers’ views of the race from the used with an active car mount antenna. It
them back to base. For 2008, the wireless hospitality areas, so there were live feeds to takes in composite or SDI video and transmits
cameras and the in-car cameras relied some of the teams’ areas as well. at 1.95-2.7 GHz. It can also multiplex in an
exclusively on a Cellular Diversity network additional ASI stream from a second encoder age 80.
and fibre optic cables. This meant that the Why Le Mans is the production company’s to give simultaneous pictures from two
plane, which only flies in daylight hours, was greatest challenge cameras over one RF link. Visual TV used the
only required to provide aerial coverage. active car mount antennas.
The course at Le Mans is 13.629 km long The long duration of the race makes it Link is using a COFDM-based modulation
and needs more than 30 fixed cameras to exhausting for both drivers and television system, Link Modulation System (LMS-T),
Continued on P
78 IBE SEPTEMBER/OCTOBER 2008
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84