HSP3824
spectrum to be concentrated at the discrete lines defined by
PN Generator Description
the spreading code and potentially cause interference with
other narrow band users at these frequencies. Additionally,
the DS system itself would be moderately more susceptible
to interference at these frequencies. With scrambling, the
spectrum is more uniform and these negative effects are
reduced, in proportion with the scrambling code length.
The spread function for this radio uses short sequences. The
same sequence is applied to every bit. All transmitted symbols,
preamble/header and data are always spread by the PN
sequence at the chip rate. The PN sequence sets the Process-
ing Gain (PG) of the Direct Sequence receiver. The HSP3824
can be programmed to utilize 11,13,15 and 16 bit sequences.
Given the length of these programmable sequences the PG
range of the HSP3824 is:
Figure 11 illustrates an example of a non scrambled trans-
mission using an 11-bit code with DBPSK modulation with
alternate 1's and 0's as data. The data rate is 2 MBPS while
the spread rate or chip rate is at 11 MCPS. The 11 spectral
lines resulting from the PN code can be clearly seen in Fig-
ure 11. In Figure 12, the same signal is transmitted but with
the scrambler being on. In this case the spectral lines have
been smeared.
From 10.41dB (10 LOG(11)) to 12.04dB (10 LOG(16))
The transmitter and receiver PN sequences can are pro-
grammed independently. This provides additional flexibility to
the network designer.
The TX sequence is set through CR 13 and CR 14 while the
RX PN sequence is set through CR 20 and CR 21. A maximum
of 16 bits can be programmed between the pairs of these con-
figuration registers. For TX Registers CR13 and CR14 contain
the high and low bytes of the sequence for the transmitter. In
addition Bits 5 and 6 of CR 4 define the sequence length in
chips per bit. CR 13, CR 14 and CR 4 must all be programmed
for proper functionality of the PN generator. The sequence is
transmitted MSB first. When fewer than 16 bits are in the
sequence, the MSBs are truncated.
REF -24dBm
ATTEN 10dB
Scrambler and Data Encoder Description
The data coder the implements the desired DQPSK coding as
shown in the DQPSK Data Encoder table. This coding scheme
results from differential coding of the dibits. When used in the
DBPSK modes, only the 00 and 11 dibits are used. Vector rota-
tion is counterclockwise.
CENTER 280MHz
RES BW 300kHz
SPAN 50MHz
SWP 20ms
VBW 100kHz
FIGURE 11. UNSCRAMBLED DBPSK DATA OF ALTERNATE
1’s/0’s SPREAD WITH AN 11-BIT SEQUENCE
TABLE 8. DQPSK DATA ENCODER
REF -25dBm
ATTEN 10dB
PHASE SHIFT
DIBITS
00
0
+90
+180
-90
01
11
10
The data scrambler is a self synchronizing circuit. It consist
of a 7-bit shift register with feedback from specified taps of
the register, as programmed through CR 16. Both transmitter
and receiver use the same scrambling algorithm. All of the
bits transmitted are scrambled, including data header and
preamble. The scrambler can be disabled.
CENTER 280MHz
RES BW 300kHz
SPAN 50MHz
SWP 20ms
VBW 100kHz
Scrambling provides additional spreading to each of the
spectral lines of the spread DS signal. The additional
spreading due to the scrambling will have the same null to
null bandwidth, but it will further smear the discrete spectral
lines from the PN code sequence. Scrambling might be nec-
essary for certain allocated frequencies to meet transmis-
sion waveform requirements as defined by various
regulatory agencies.
FIGURE 12. SCRAMBLED DBPSK DATA OF ALTERNATE
1’s/0’s SPREAD WITH AN 11-BIT SEQUENCE
Another reason to scramble is to gain a small measure of pri-
vacy. The DS nature of the signal is easily demodulated with a
correlating receiver. Indeed, the data modulation can be
recovered from one of the discrete spectral lines with a narrow
band receiver (with a 10dB loss in sensitivity). This means
that the signal gets little security from the DS spreading code
alone. Scrambling adds a privacy feature to the waveform that
would require the listener to know the scrambling parameters
in order to listen in. When the data is scrambled it cannot be
In the absence of scrambling, the data patterns could con-
tain long strings of ones or zeros. This is definitely the case
with the a DS preamble which has a stream of up to 256
continuous ones. The continuous ones would cause the
17
HARRIS [ HARRIS CORPORATION ]
