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DNT900 - 08/16/10
One disadvantage of direct sequence systems is that due to design issues related to broadband transmit-
ters and receivers, they generally employ only a minimal amount of spreading, often 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 hopping
throughout the entire band, statistically reducing the chances that a transmission will be affected by
fading or interference. This means that a FHSS system will degrade gracefully as the band 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 can offer only a few non-overlapping chan-
nels, whereas multiple hoppers can interleave, minimizing interference. Frequency hopping systems do
carry some disadvantages, in 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 utiliza-
tion 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.
DNT900 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 DNT900 series modules achieve regulatory certification under “digital
modulation” or DTS (DSSS) rules. At 500 kb/s DNT900 series modules still employ frequency hopping to
mitigate the effects of interference and multipath fading, but hop on fewer, more widely spaced frequen-
cies than at lower data rates.
2.0 DNT900 Radio Operation
2.1 Network Synchronization and Registration
As discussed above, frequency hopping spread spectrum radios such as the DNT900 periodically change
their transmit frequency. 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 all DNT900 networks, one radio is designated as the base. All other radios are designated as remotes
or routers. The base transmits a beacon each time it hops to a different frequency, which allows the other
radios in its network to synchronize with it. Since all radios in the network know the hopping pattern, once
they are synchronized with the base, they know which frequency to hop to and when.
When a remote or router is powered on, it rapidly scans the frequency band for the synchronizing signal.
Since the base is transmitting on up to 50 frequencies and the remotes and routers are scanning up to 50
frequencies, it can take several seconds to synchronize with the base.