HSP3824 datasheet(21/41 Pages) HARRIS

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HSP3824

acquisition decision. This worst case time line example

assumes that the signal is present on antenna A1 only (A2 is

blocked). It further assumes that the signal arrives part way

into the ﬁrst A1 dwell such as to just barely miss detection.

The signal and the scanning process are asynchronous and

the signal could start anywhere. In this timeline, it is assumed

that all 16 symbols are present, but they were missed due to

power ampliﬁer ramp up. Since A2 has insufﬁcient signal, the

ﬁrst A2 dwell after the start of the preamble also fails detec-

tion. The second A1 dwell after signal start is successful and a

symbol timing measurement is achieved.

Meanwhile signal quality and signal frequency measure-

ments are made simultaneous with symbol timing measure-

ments. When the bit sync level, SQ1, and Phase variance

SQ2 are above their user programmable thresholds, the sig-

nal is declared present for the antenna with the best signal.

More details on the Signal Quality estimates and their pro-

grammability are given in the Acquisition Signal Quality

Parameters section of this document.

At the end of each dwell, a decision is made based on the rel-

ative values of the signal qualities of the signals on the two

antennas. In the example, antenna A1 is the one selected, so

the recorded symbol timing and carrier frequency for A1 are

used thereafter for the symbol timing and the PLL of the NCO

to begin carrier de-rotation and demodulation.

Prior to initial acquisition the NCO was inactive and DPSK

demodulation processing was used. Carrier phase measure-

ment are done on a symbol by symbol basis afterward and

coherent DPSK demodulation is in effect. After a brief setup

time as illustrated on the timeline of Figure 15, the signal

begins to emerge from the demodulator.

If the descrambler is used it takes 7 more symbols to seed

the descrambler before valid data is available. This occurs in

time for the SFD to be received. At this time the demodulator

is tracking and in the coherent PSK demodulation mode it

will no longer scan antennas.

One Antenna Acquisition

When only one antenna is being used, the user can delete

the antenna switch and shorten the acquisition sequence.

Figure 16 shows the single antenna acquisition timeline. It

uses a 78 symbol sequence with 2 more for power ramping

of the RF front of the radio. This scheme deletes the second

antenna dwells but performs the same otherwise. It veriﬁes

the signal after initial detection for lower false alarm proba-

bility.

Acquisition Signal Quality Parameters

Two measures of signal quality are used to determine acqui-

sition and drop lock decisions. The ﬁrst method of determin-

ing signal presence is to measure the correlator output (or

bit sync) amplitude. This measure, however, ﬂattens out in

the range of high BER and is sensitive to signal amplitude.

The second measure is phase noise and in most BER sce-

narios it is a better indication of good signals plus it is insen-

sitive to signal amplitude. The bit sync amplitude and phase

noise are integrated over each block of 16 symbols used in

acquisition or over blocks of 128 symbols in the data demod-

ulation mode. The bit sync amplitude measurement repre-

sents the peak of the correlation out of the PN correlator.

Figure 17 shows the correlation process. The signal is sam-

pled at twice the chip rate (i.e. 22 MSPS). The one sample

that falls closest to the peak is used for a bit sync amplitude

sample for each symbol. This sample is called the on-time

sample. High bit sync amplitude means a good signal. The

early and late samples are the two adjacent samples and

are used for tracking.

The other signal quality measurement is based on phase

noise and that is taken by sampling the correlator output at the

correlator peaks. The phase changes due to scrambling are

removed by differential demodulation during initial acquisition.

Then the phase, the phase rate and the phase variance are

measured and integrated for 16 symbols. The phase variance

is used for the phase noise signal quality measure. Low phase

noise means a stronger received signal.

Procedure to Set Acq. Signal Quality

Parameters (Example)

There are four registers that set the acquisition signal quality

thresholds,

they

are:

22,

23,

30,

and

(RX_SQX_IN_ACQ). Each threshold consists of two bytes,

high and low that hold a 16-bit number.

FIGURE 17. CORRELATION PROCESS

T0 + 1

µs

CORRELATOR

CORRELATION

PEAK

T0 + 2

µs

CORRELATOR OUTPUT IS

THE RESULT OF CORRELATING

THE PN SEQUENCE WITH THE

RECEIVED SIGNAL

SAMPLES

AT 2X CHIP

RATE

EARLY

ON-TIME

LATE

OUTPUT

REPEATS

CORRELATION TIME

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