DSSS Modulation

LIST OF EXPERIMENTS:

To study and selection of various PN Sequences.
A. Gold sequence.
B. Maximum length sequence.
C. Barker code.

To study and observe the Modulation (Spreading) of DSSS signal using various modulation techniques.
A. BPSK.
B. QPSK.
C. OQPSK.

To study and observe the Demodulation (De-spreading) of DSSS signal using various modulation techniques.
A. BPSK.
B. QPSK.

To study the Bit Error Rate (BER) Measurement of known PRBS-11 data bits and observes the error in the recovered Data on COMSCOPE.

FEATURES:

Direct sequence spread-spectrum (DS-SS) modulator and demodulator
Programmable chip rates up to 10 Mchip/s
Spreading codes:
o Gold sequences (up to 2^23 -1 chips)
o Maximal length sequences, (max length 2^23 -1 chips)
o Barker codes (length 11, 13)
Code modulation: BPSK/QPSK/OQPSK
Internal generation of pseudo-random bit stream and unmodulated carrier for testing
Built-in channel impairments generation:
o Additive white Gaussian noise
o Frequency offset (Doppler)
Sequential code search
4-bit soft-quantized demodulated bits
Monitoring: Receiver lock and Carrier frequency error
Measures actual bit error while known PRBS-11 pseudo-random test sequence is being transmitted
Accurate BER measurement down to 10^-6
Fast automatic synchronization
Cycle slips detection
32-bit cumulative BER counter for long duration measurement
1-bit serial or 2-bit parallel input selection (I and Q bit)
Spreading-despreading with onboard NRZ binary data

SOFTWARE:

Key internal signals can be captured in real-time and displayed on host computer itself.
BER measurement is done by counting actual errors in the received bit stream.
Received bit stream is compared with locally generated replica of PRBS-11 sequence.
The reference sequence is a periodic 2047-bit long maximum length sequence generated by a 11-tap linear feedback shift register.
Spreading and despreading using extensive monitoring at the receiver for code lock, carrier lock, carrier frequency offset and code acquisition.
Monitors effect of synchronization, sequential code search in despreading.

CDMA-01 trainer provides a basic understanding of the concepts behind code division multiple access (CDMA), and various parameters that need to be considered in the design of a DSSS system. These include generation of various pseudo random (PN) codes like Gold, MLS and Barker with programmable tapping, variable chip rate, and digital modulations BPSK, QPSK and digital AWGN noise with programmable FIR low pass filter. Bit error rate (BER) measurement with known data sequence, overall data rate dependency parameters, spreading and despreading with DSSS, SNR control, offset control and so on can be performed.

DSSS Modulation/Demodulation Trainer System