Much of the signal processing performed in modern wireless
communications systems—such as digital modulator/demodulator
applications—takes place in the digital domain and requires high
throughput. Dedicated hardware can provide the processing capability to
meet this requirement. The parallel processing capability of Altera®
programmable logic devices (PLDs) makes them ideal for
baseband/intermediate frequency (IF) digital signal processing
applications.
The direct sequence spread spectrum (DSSS) digital modem reference
design is a hardware design that has been optimized for the Altera APEX™
DSP development board (starter version), which features an APEX
EP20K200E device. The reference design is a spread spectrum
modulator/demodulator subsystem that can be used as a starting point
for a complete 3G or fixed wireless modem. The design highlights the ease
of use, performance, and efficiency of implementing a design using Altera
devices and DSP intellectual property (IP) cores.
The design uses the correlator, FIR compiler, and NCO compiler
MegaCore® functions, and uses 7,183 logic cells and achieves 100-MHz
performance. A parameterizable UART and a Windows application
enables communication between the APEX DSP development board and
a PC via the board’s RS-232 interface, yielding a true development
platform for intermediate frequency (IF) modem designs. The reference
design includes VHDL source code (except for the IP cores), and a
simulation library and test bench for simulation in ModelSim simulators.
Allocating channel resources using spread spectrum techniques are a
favorable alternative to allocating channels using frequency division
multiple access (FDMA) or time division multiple access (TDMA)
schemes. By spreading the spectrum of the data to be transmitted with
orthogonal codes, the receiver can decode each user’s data uniquely, even
though many users share the same spectral and temporal channel
resources.
Direct Sequence Spread
Spectrum (DSSS) Modem
Reference Design
A-FS-14-1.0
2 Altera Corporation
FS 14: Direct Sequence Spread Spectrum (DSSS) Modem Reference Design
The two primary methods used to spread the baseband data spectrum are:
■ Frequency hopping spread spectrum (FHSS)—The frequency of the
modulating carrier varies in a pseudo-random manner unique to
each user.
■ Direct sequence spread spectrum (DSSS)—The data is spread by
multiplication with a channelization code prior to up-conversion to
an intermediate frequency.
Each method has its advantages and disadvantages. For example, in a
DSSS systems, the designer can reduce narrowband interference in the
same channel by increasing the processing gain, but this technique cannot
be used in FHSS schemes. However, FHSS schemes are less susceptible to
jamming and the receiver is easier to implement, usually requiring only a
simple analog limiter/discriminator.
This functional specification discusses the DSSS digital modem reference
design. The design modulates direct sequence spread data onto an IF
carrier. The modulated data is then input to a channel model and is passed
to a digital receiver, which demodulates and recovers the data from the
received IF signal

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http://alteraliterature/fs/fs14_dsss.pdf