Digital Signal Processing Reference
In-Depth Information
Figure 17.12. Square root raised
cosine impulse response.
Since the signal passes through both filters, the net frequency
response is the raised cosine filter. After passing through the
receive pulse shaping (also called matched) filter, the signal is
sampled. Using the sampled I and Q value, the receiver will
choose the constellation point in the I-Q plane closest to the
sampled value. The bits corresponding to that symbol are
recovered, and if all went well, the receiver has chosen the same
symbol point selected by the transmitter. This is very much
simplified, but
is the essence of a digital communications
process.
It can be seen why it will be easier to have errors when
transmitting 64-QAM as compared to QPSK. The receiver has
64 closely spaced symbols to select from in the case of 64-
QAM, whereas in QPSK there are only four widely spaced
symbols to select from. This makes 64-QAM systems much
more susceptible to ISI, noise or interference. An option is to
just transmit the 64-QAM signal with higher power, to spread
the symbols further apart. This is an effective, but very
expensive, way to mitigate the noise and interference which
prevents correct detection of the symbol at the receiver. Also,
the transmit power is often limited by the regulatory agencies,
or the transmitter may be battery powered or have other
constraints.
