DNT500
2008 by RF Monolithics,
Inc.
3
M-0500-0000 Rev D
One disadvantage of direct sequence systems is that due to spectrum constraints and
the design difficulties 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 if necessary to find a channel free of interference. Essentially, 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 fre-
quency 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 hop-
ping systems generally feature greater coverage and channel utilization than compa-
rable 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 in-
terference and multipath fading, but hop on fewer, more widely spaced frequencies
than at lower data rates.
