24-11-2012, 12:56 PM
Theory of Root-Raised Cosine Filter
Theory of Root-Raised.pdf (Size: 138.83 KB / Downloads: 44)
Abstract
The raised cosine filter is used in wireless transmission (e.g. 3GPP) to pulse-shape
the chip stream output before it is modulated to the RF. The spectrum is bandwidth limited in
order to avoid interferences with neighbour symbols.
INTRODUCTION
The amplitude steps in a digital chip stream are the
cause for high-frequency spectral components. Since the
signal is transmitted on a bandwidth-limited channel,
smearing of adjacent symbols may happen, known as
inter symbol interference (ISI). In order to avoid such
interference, the signal is low-pass filtered.
The raised-cosine filter satisfies the Nyquist criterion of
suppressing the spectral distortion at integral multiples
of the sampling rate. To improve noise cancellation, the
filter is usually split into two parts, the root-raised-cosine
filter, one at the sender side and the other at the receiver
side.
CONCLUSIONS
A digital FIR filter can be used to reduce the band
width of the step-affected chip stream. The filter has to
use oversampling (osf ≥ 2), as no frequencies above half
the filter’s sampling frequency can be handled (Nyquist),
while the upper frequency point !2 is at more than half
the chip rate.