Information Technology Reference
In-Depth Information
On reception, the de-randomizer must contain the identical ring counter which must also be set to the starting
condition to bit accuracy. Its output is then added to the data stream from the demodulator. The randomizing will
effectively then have been added twice to the data in modulo-2, and as a result is cancelled out leaving the original
serial data.
The way that radio signals interact with obstacles is a function of the relative magnitude of the wavelength and the
size of the object. AM sound radio transmissions with a wavelength of several hundred metres can easily diffract
around large objects. The shorter the wavelength of a transmission, the larger objects in the environment appear to
it and these objects can then become reflectors. Reflecting objects produce a delayed signal at the receiver in
addition to the direct signal. In analog television transmissions this causes the familiar ghosting. In digital
transmissions, the symbol rate may be so high that the reflected signal may be one or more symbols behind the
direct signal, causing intersymbol interference. As the reflection may be continuous, the result may be that almost
every symbol is corrupted. No error-correction system can handle this. Raising the transmitter power is no help at
all as it simply raises the power of the reflection in proportion.
The only solution is to change the characteristics of the RF channel in some way to either prevent the multipath
reception or to prevent it being a problem. The RF channel includes the modulator, transmitter, antennae, receiver
and demodulator.
As with analog UHF TV transmissions, a directional antenna is useful with digital transmission as it can reject
reflections. However, directional antennae tend to be large and they require a skilled permanent installation. Mobile
use on a vehicle or vessel is simply impractical.
Another possibility is to incorporate a ghost canceller in the receiver. The transmitter periodically sends a
standardized known waveform known as a training sequence. The receiver knows what this waveform looks like
and compares it with the received signal. In theory it is possible for the receiver to compute the delay and relative
level of a reflection and so insert an opposing one. In practice if the reflection is strong it may prevent the receiver
finding the training sequence.
The most elegant approach is to use a system in which multipath reception conditions cause only a small increase
in error rate which the error-correction system can manage. This approach is used in DVB.
Figure 7.10
(a) shows
that when using one carrier with a high bit rate, reflections can easily be delayed by one or more bit periods,
causing interference between the bits.
Figure 7.10
(b) shows that instead, OFDM sends many carriers each having
a low bit rate. When a low bit rate is used, the energy in the reflection will arrive during the same bit period as the
direct signal. Not only is the system immune to multipath reflections, but the energy in the reflections can actually
be used. This characteristic can be enhanced by using guard intervals shown in
Figure 7.10
(
c). These reduce
multipath bit overlap even more.
Figure 7.10:
(a) High bit rate transmissions are prone to corruption due to reflections. (b) If the bit rate is reduced
the effect of reflections is eliminated, in fact reflected energy can be used. (c) Guard intervals may be inserted
between symbols.
Note that OFDM is not a modulation scheme, and each of the carriers used in a OFDM system still needs to be
modulated using any of the digital coding schemes described above. What OFDM does is to provide an efficient
way of packing many carriers close together without mutual interference.
