HSP3824
defeated by listening to one of the scrambling spectral lines
Clear Channel Assessment (CCA) and
Energy Detect (ED) Description
since the unintentional receiver in this case is too narrow band
to recover the data modulation. This assumes though that
each user can set up different scrambling patterns There are
9 maximal length codes that can be utilized with a generator
of length 7. The different codes can be used to implement a
basic privacy scheme. It needs to be clear though that this
scrambling code length and the actual properties of such
codes are not a major challenge for a sophisticated intentional
interceptor to be listening in. This is why we refer to this
scrambling advantage as a communications privacy feature
as opposed to a secure communications feature.
The clear channel assessment (CCA) circuit implements the
carrier sense portion of a carrier sense multiple access
(CSMA) networking scheme. The Clear Channel Assess-
ment (CCA) monitors the environment to determine when it
is feasible to transmit. The result of the CCA algorithm is
available in real time through output pin 32 of the device. The
CCA state machine in the HSP3824 can be programmed as
a function of RSSI, energy detected on the channel, carrier
detection, and a number of on board watchdog timers to
time-out under certain conditions. The CCA can be also
completely by-passed allowing transmissions independent of
any channel conditions. The programmable CCA in combi-
nation with the visibility of the various internal parameters
(i.e. Energy Detection measurement results), can assist an
external processor in executing algorithms that can adapt to
the environment. These algorithms can increase network
throughput by minimizing collisions and reducing transmis-
sions liable to errors.
Scrambling is done by a polynomial division using a pre-
scribed polynomial. A shift register holds the last quotient
and the output is the exclusive-or of the data and the sum
of taps in the shift register. The taps and seed are program-
mable. The transmit scrambler seed is programmed by CR
15 and the taps are set with CR 16. Setting the seed is
optional, since the scrambler is self-synchronizing and it
will eventually synchronize with the incoming data after
flashing the 7 bits stored from the previous transmission.
There are two measures that are used in the CCA assess-
ment. The receive signal strength (RSSI) which measures
the energy at the antenna and the carrier sense (CS), which
is triggered upon valid PN correlation of the baseband pro-
cessor (HSP3824). Both indicators are used since interfer-
ence can trigger the signal strength indication, but it will not
trigger the carrier sense. The carrier sense, however, is
slower to respond than the signal strength and it becomes
active only when a spread signal with identical PN code has
been detected, so it is not adequate in itself. Note that the
CS is also vulnerable to false alarms. The CCA looks for
changes in these measurements and decides its state based
on these measures and the time that has elapsed since the
Modulator Description
The modulator is designed to support both DBPSK and
DQPSK signals. The modulator is capable of automatically
switching its rate in the case where the preamble and header
are DBPSK modulated, and the data is DQPSK modulated.
The modulator can support date rates up to 4 MBPS. The pro-
gramming details of the modulator are given at the introduc-
tory paragraph of this section. The HSP3824 can support data
rates of up to 4 MBPS (DQPSK) with power supply voltages
between 3.3V and 5.0V and data rates of up to 3 MBPS with
supply voltages between 2.7V and 5.5V.
CONTINUOUS
CCA TO MAC
WATCHDOG TIMER
RESET ON CCA = CLEAR
RESET ON M ms TIMEOUT
RESET
WAIT FOR CHANGE
IN ED OR CS STATUS
OR TIMER TIMEOUT
ED
CS
ED >THRESH
CS < THRESH
ED >THRESH
ED < THRESH
CS > THRESH
ED > THRESH
CS< THRESH
ED < THRESH
CNT = CNT + 1
CLEAR
CS > THRESH
ED < THRESH
CCA
LATCH
HOLDS
LAST
RESET CNT
BUSY
DECISION
BUSY
BUSY
CNT < N
CNT = N
CLEAR
RESET CNT
FIGURE 13. CCA FUNCTIONAL FLOW DIAGRAM
18
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