HSP3824
These two thresholds, bit sync amplitude CR (22 and 23)
Data Demodulation and Tracking
Description
and phase error CR (30 and 31) are used to determine if the
desired signal is present. If the thresholds are set too “low”,
there is the probability of missing a high signal to noise
detection due to processing a false alarm. If they are set too
“high”, there is the probability of missing a low signal to noise
detection. For the bit sync amplitude, “high” actually means
high amplitude while for phase noise “high” means high SNR
or low noise.
The signal is demodulated from the correlation peaks
tracked by the symbol timing loop (bit sync). The frequency
and phase of the signal is corrected from the NCO that is
driven by the phase locked loop. Demodulation of the DPSK
data in the early stages of acquisition is done by delay and
subtraction of the phase samples. Once phase locked loop
tracking of the carrier is established, coherent demodulation
is enabled for better performance. Averaging the phase
errors over 16 symbols gives the necessary frequency infor-
mation for proper NCO operation. The signal quality is taken
as the variance in this estimate.
A recommended procedure is to set these thresholds individu-
ally optimizing each one of them to the same false alarm rate
with no desired signal present. Only the background environ-
ment should be present, usually additive gaussian white noise
(AGWN). When programming each threshold, the other
threshold is set so that it always indicates that the signal is
present. Set register CR22 to 00h while trying to determine
the value of the phase error signal quality threshold for regis-
ters CR 30 and 31. Set register CR30 to FFh while trying to
determine the value of the Bit sync. amplitude signal quality
threshold for registers 22 and 23. Monitor the Carrier Sense
(CRS) output (TEST 7, pin 46) and adjust the threshold to pro-
duce the desired rate of false detections. CRS indicates valid
initial PN acquisition. After both thresholds are programmed in
the device the CRS rate is a logic “and” of both signal qualities
rate of occurrence over their respective thresholds and will
therefore be much lower than either.
There are two signal quality measurements that are per-
formed in real time by the device and they set the demodula-
tor performance. The thresholds for these signal quality
measurements are user programmable. The same two sig-
nal quality measures, phase error and bit sync. amplitude,
that are used in acquisition are also used for the data drop
lock decision. The data thresholds, though, are programmed
independently from the acquisition thresholds. If the radio
uses the network processor to determine when to drop the
signal, the thresholds for these decisions should be set to
their limits allowing data demodulation even with poor signal
reception. Under this configuration the HSP3824 data moni-
tor mechanism is essentially bypassed and data monitoring
becomes the responsibility of the network processor.
PN Correlator Description
These signal quality measurements are integrated over 128
symbols as opposed to 16 symbol intervals for acquisition, so
the minimum time to drop lock based with these thresholds is
128 symbols or 128ms at 1 MSPS. Note that other than the
data thresholds, non-detection of the SFD can cause the
HSP3824 to drop lock and return its acquisition mode.
The PN correlator is designed to handle BPSK spreading
with carrier offsets up to ±50ppm and 11,13,15 or 16 chips
per symbol. Since the spreading is BPSK, the correlator is
implemented with two real correlators, one for the I and one
for the Q channel.The same sequence is always used for
both I an Q correlators. The TX sequence can be pro-
grammed as a different sequence from the RX sequence.
This allows a full duplex link with different spreading parame-
ters for each direction.
Configuration Register 41 sets the search timer for the SFD.
This register sets this time-out length in symbols for the
receiver. If the time out is reached, and no SFD is found, the
receiver resets to the acquisition mode. The suggested value
is preamble symbols + 16 symbols. If several transmit pream-
ble lengths are used by various transmitters in a network, the
longest value should be used for the receiver settings.
The correlators are time invariant matched filters otherwise
known as parallel correlators. They use two samples per
chip. The correlator despreads the samples from the chip
rate back to the original data rate giving 10.4dB processing
gain for 11 chips per bit. While despreading the desired sig-
nal, the correlator spreads the energy of any non correlating
interfering signal.
Procedure to Set Signal Quality Registers
CR 26, 27, 34, AND 35 (RX_SQX_IN_DATA) are pro-
grammed to hold the threshold values that are used to drop
lock if the signal quality drops below their values. These can
be set to their limit values if the external network processor
is used for drop lock decisions instead of the HSP3824
demodulator. The signal quality values are averaged over
128 symbols and if the bit sync amplitude value drops below
its threshold or the phase noise rises over its threshold, the
link is dropped and the receiver returnes to the acquisition
mode. These values should typically be different for BPSK
and QPSK since the operating point in SNR differs by 3dB. If
the receiver is intended to receive both BPSK and QPSK
modulations, a compromise value must be used or the net-
work processor can control them as appropriate.
Based on the fact that correlator output pulse is used for bit
timing, the HSP3824 can not be used for any non spread
applications.
In programming the correlator functions, there are two sets
of configuration registers that are used to program the
spread sequences of the transmitter and the receiver. They
are CR 13 and 14 for transmitter and CR 20 and 21 for the
receiver. In addition, CR2 and CR3 define the sequence
length or chips per symbol for the receiver and transmitter
respectively. These are carried in bits 6 and 7 of CR2 and
bits 5 and 6 of CR3. More programming details are given in
the Control Registers section of this document.
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HARRIS [ HARRIS CORPORATION ]
