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HSP3824

Direct Sequence Spread Spectrum Baseband Processor

Harris Semiconductor 

HSP3824

PN Generator Description

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:

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.

spectrum to be concentrated at the discrete lines defined by

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.

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.

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.

TABLE 8. DQPSK DATA ENCODER

PHASE SHIFT

0

+90

+180

-90

DIBITS

00

01

11

10

CENTER 280MHz

RES BW 300kHz

VBW 100kHz

SPAN 50MHz

SWP 20ms

FIGURE 11. UNSCRAMBLED DBPSK DATA OF ALTERNATE

1’s/0’s SPREAD WITH AN 11-BIT SEQUENCE

REF -25dBm

ATTEN 10dB

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.

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.

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

CENTER 280MHz

RES BW 300kHz

VBW 100kHz

SPAN 50MHz

SWP 20ms

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

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