Murata Electronics North America DNT500FP Manual Do Utilizador
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DNT900 - 11/05/08
One disadvantage of direct sequence systems is that due to spectrum constraints and the design difficul-
ties of broadband receivers, they generally employ only a minimal amount of spreading, typically no more
than the minimum required by the regulating agencies. For this reason, the ability of DSSS systems to
overcome fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of
probing the entire band as necessary to find a channel free of interference. This means that a FHSS
system will degrade gracefully as the channel gets noisier, while a DSSS system may exhibit uneven
coverage or work well until a certain point and then give out completely.
ties of broadband receivers, they generally employ only a minimal amount of spreading, typically no more
than the minimum required by the regulating agencies. For this reason, the ability of DSSS systems to
overcome fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of
probing the entire band as necessary to find a channel free of interference. This means that a FHSS
system will degrade gracefully as the channel gets noisier, while a DSSS system may exhibit uneven
coverage or work well until a certain point and then give out completely.
Because it offers greater immunity to interfering signals, FHSS is often the preferred choice for co-located
systems. Since direct sequence signals are very wide, they tend to offer few non-overlapping channels,
whereas multiple hoppers may interleave with less interference. Frequency hopping does carry some
disadvantage in that as the transmitter cycles through the hopping pattern it is nearly certain to visit a few
blocked channels where no data can be sent. If these channels are the same from trip to trip, they can be
memorized and avoided. Unfortunately, this is generally not the case, as it may take several seconds to
completely cover the hop sequence during which time the multipath delay profile may have changed
substantially. To ensure seamless operation throughout these outages, a hopping radio must be capable
of buffering its data until a clear channel can be found. A second consideration of frequency hopping
systems is that they require an initial acquisition period during which the receiver must lock on to the
moving carrier of the transmitter before any data can be sent, which typically takes several seconds. In
summary, frequency hopping systems generally feature greater coverage and channel utilization than
comparable direct sequence systems. Of course, other implementation factors such as size, cost, power
consumption and ease of implementation must also be considered before a final radio design choice can
be made.
systems. Since direct sequence signals are very wide, they tend to offer few non-overlapping channels,
whereas multiple hoppers may interleave with less interference. Frequency hopping does carry some
disadvantage in that as the transmitter cycles through the hopping pattern it is nearly certain to visit a few
blocked channels where no data can be sent. If these channels are the same from trip to trip, they can be
memorized and avoided. Unfortunately, this is generally not the case, as it may take several seconds to
completely cover the hop sequence during which time the multipath delay profile may have changed
substantially. To ensure seamless operation throughout these outages, a hopping radio must be capable
of buffering its data until a clear channel can be found. A second consideration of frequency hopping
systems is that they require an initial acquisition period during which the receiver must lock on to the
moving carrier of the transmitter before any data can be sent, which typically takes several seconds. In
summary, frequency hopping systems generally feature greater coverage and channel utilization than
comparable direct sequence systems. Of course, other implementation factors such as size, cost, power
consumption and ease of implementation must also be considered before a final radio design choice can
be made.
DNT500 series modules achieve regulatory certification under FHSS rules at air data rates of 38.4, 115.2
and 200 kb/s. At 500 kb/s, the DNT500 series modules achieve regulatory certification under “digital
modulation” or DTS rules. At 500 kb/s DNT500 series modules still employ frequency hopping to mitigate
the effects of interference and multipath fading, but hop on fewer, more widely spaced frequencies than at
lower data rates.
and 200 kb/s. At 500 kb/s, the DNT500 series modules achieve regulatory certification under “digital
modulation” or DTS rules. At 500 kb/s DNT500 series modules still employ frequency hopping to mitigate
the effects of interference and multipath fading, but hop on fewer, more widely spaced frequencies than at
lower data rates.
2.0 DNT500 Radio Operation
2.1 Network Synchronization and Registration
As discussed above, frequency hopping radios such as the DNT500 periodically change the frequency at
which they transmit. In order for the other radios in the network to receive the transmission, they must be
listening to the frequency on which the current transmission is being sent. To do this, all the radios in the
network must be synchronized to the same hopping pattern.
which they transmit. In order for the other radios in the network to receive the transmission, they must be
listening to the frequency on which the current transmission is being sent. To do this, all the radios in the
network must be synchronized to the same hopping pattern.
In point-to-point or point-to-multipoint networks, one radio module is designated as the base station. All
other radios are designated as remotes. One of the responsibilities of the base station is to transmit a
synchronization signal to the remotes to allow them to synchronize with the base station. Since the re-
motes know the hopping pattern, once they are synchronized with the base station, they know which
frequency to hop to and when. Every time the base station hops to a different frequency, it immediately
transmits a synchronizing signal.
other radios are designated as remotes. One of the responsibilities of the base station is to transmit a
synchronization signal to the remotes to allow them to synchronize with the base station. Since the re-
motes know the hopping pattern, once they are synchronized with the base station, they know which
frequency to hop to and when. Every time the base station hops to a different frequency, it immediately
transmits a synchronizing signal.
When a remote is powered on, it rapidly scans the frequency band for the synchronizing signal. Since the
base station is transmitting on up to 50 frequencies and the remote is scanning up to 50 frequencies, it
can take several seconds for a remote to synchronize with the base station.
base station is transmitting on up to 50 frequencies and the remote is scanning up to 50 frequencies, it
can take several seconds for a remote to synchronize with the base station.