"SHIVA: Correlator/Demodulator Chip for Direct-Sequence Spread-Spectrum 
RAKE-Receiver"

R. Zimmermann and M. Neeracher

Extended abstract
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Direct-Sequence Spread-Spectrum Systems

Microcellular wireless communications systems using direct-sequence
spread-spectrum (DSSS) techniques are becoming widely used e.g. for wireless
local area networks (wireless LAN). In a DSSS system, transmission bandwidth is
spread by multiplication of the data bits with a pseudonoise (PN) code
sequence, which e.g. allows the use of code division multiple access (CDMA).
Typical indoor application problems like frequency-selective fading effects and
multipath propagation are handled by a RAKE-receiver that permits the
simultaneous reception of multiple propagation paths by correlation with
different synchronized versions of the spreading code and by multipath
diversity combining.


SHIVA Operation

SHIVA integrates the code correlation and data demodulation operations of a
spread-spectrum RAKE-receiver on one chip. Correlation of the received signal
with the synchronized PN code is performed within six correlator arms, which
are set to different signal delays caused by the multipath propagation. All
arms can initially be used for code acquisition. In reception mode, two arms
are dedicated for code tracking while the remaining four arms perform data
correlation. The demodulation part performs phase estimation and correction and
a weighted path recombination of the correlated data bits. For code frequency
tracking, a frequency error detection signal is generated. The chip handles
BPSK and QPSK modulated data with differential or direct encoding as well as
BPSK and QPSK spreading codes.

SHIVA accepts PN code lengths of 15-1023 chips with chip rates up to fc = 16.4
MHz and using four samples per chip. This allows typical data transfer rates of
16 kbit/s - 2.2 Mbit/s with processing gains of 12 - 27 dB. The chip can be
controlled and data be read out via a 16-bit processor interface.


SHIVA Application

In addition to the SHIVA correlator/demodulator chip, the spread-spectrum
receiver requires a quadrature demodulator, two A/D converters and two chip
matched filters at the input side. Moreover, a PN code generator provides the
spreading codes and the entire system is controlled by a digital signal
processor (DSP).

SHIVA is a prototype chip which is used for verification of the system
architecture and the data processing algorithms. A final VLSI is planned for
higher data rates and for integration of even more system components on one
chip.