AN9614 Datasheet, PDF (3/3 Pages) Intersil Corporation – Using the PRISM® Chip Set for Low Data Rate Applications

Application Note 9614
HIGH RATE BURST TRANSMISSIONS WITH LOW
AVERAGE RATE
Generally, the incentive to use lower data rates is to achieve a
given range with the minimum amount of power. We can show
that this is also achievable by using the radio in its high data
rate design configuration. The PRISM is a packet radio
communications device and, as such, can send the data in a
short burst with open environment ranges up to 5 miles. This
has significant potential for power savings and reduction in
interference. In the high data rate configuration the design
considerations mentioned above are no longer of concern.
The system approach is to accept the 1 MBPS data rate of the
radio as long as the achievable range is acceptable, and use it
in a short burst mode which is consistent with its’ packet
nature. With a low power watch crystal, the controller can
keep adequate time to operate either a polled or a time
allocated scheme. In these modes, the radio is powered off
most of the time and only awakens when communications is
expected. This station would be awakened periodically to
listen for a beacon transmission. The beacon serves to reset
the timing and to alert the radio to traffic. If traffic is waiting,
the radio is instructed when to listen and for how long. In a
polled scheme, the remote radio can respond to the poll with
its traffic if it has any. With these techniques, the average
power consumption of the radio can be reduced by more than
an order of magnitude while meeting all data transfer
objectives.
Even using the 802.11 network protocols, the low data rate
can allow low average power operation. The Media Access
Controller (MAC) or network processor can operate the radio
in the sleep mode except for the times it needs to receive the
beacon signals.
The short, fast transmission is good for several reasons. First,
if the signal is corrupted for any reason, a retransmission will
occur without noticeable delay. Secondly, interference to other
spectrum users is of brief duration. Third, and most important,
the burst can be sent into small time gaps in the medium,
which makes it more effective against certain type of
interference in the ISM band. For example, if an 802.11 FH
network is operating in the vicinity, it could cause interference
with this network. The FH network has, however, a brief guard
time when it is hopping and none of its stations are on the air.
This time can be used to transmit the burst communications
packets. Additionally, the microwave oven has been identified
as an interference source of concern within the 2.4GHz ISM
band. The oven is a pulsed source with about a 50% duty
cycle. The gaps allow messages of about 1000 bytes through
at the 1 MBPS rate.
In addition, the system can be set at its sleep mode most of
the time to achieve low power consumption. It only needs to
operate at full power consumption during the transmission of
a packet or during the expected window for received packets.
The communications range achievable depends on the
nature of the environment. A line of sight (LOS) path allows
the best range. With 1W and 6dB gain in the antennas, you
can readily achieve a 5 mile LOS range. The propagation
loss at S-band is less than 0.5dB per mile in heavy rain, so
weather is not usually of great concern. Antennas with 6dB
gain are for fixed installations with one on one links. Mobile
and network installations use omnidirectional antennas with
around 0dB gain. Indoors, the range is much reduced by
extra losses due to walls and other obstructions. The power
is also usually reduced to 100mW for interference and safety
concerns. These reduce the available range, but most
applications will achieve sufficient range (300 ft.).
Antenna diversity is also used in the PRISM design to
combat multipath interference. Since the PRISM waveform
is wideband by being spread at the chip rate, the 1 MBPS
data rate is not a contributor to multipath problems and a
lower data rate is of no benefit.
So, in general, unless it is required to use low instantaneous
data rates to achieve some other purpose, the packet
capabilities of PRISM will serve well for these applications in
its normal high data rate design configuration.
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