Digital Signal Processing Reference
In-Depth Information
The frequency bandwidth is primarily determined by the
symbol rate. A QPSK signal at 1 MSPS will require about the same
bandwidth as a 16-QAM signal at 1 MSPS. Notice the 16-QAM
modulator is able to send twice the data within this bandwidth,
compared to the QPSK modulator. There is a trade-off however:
as the number of symbols increases, it becomes more and more
difficult for the receiver to detect which symbol was sent. If the
receiver needs to choose from 16 possible symbols which could
have been transmitted, rather than choose from from four
possibilities, it is more likely to make errors.
The level of errors will depend upon the noise and interference
present in the signal, the strength of the signal and how many
possible symbols the receiver must select from. In non-line-of-
sight systems (cellular phones), there are often high levels of
reflected and weak signals due to buildings or other objects
blocking the transmission path. In this situation, it is often pref-
erable to use a simple constellation, such as QPSK. Even with
a weak signal, the receiver can usually make the correct choice of
four possible symbols. Line-of-sight systems, such as satellite
systems, have directional receive and transmit antennas facing
each other. Because of this, the interfering noise level is usually
very low, and complex constellations such as 64-QAM or 256-
QAM can be used.
Assuming the receiver is able to make the correct choice
from among 64 symbols, three times more bits can be encoded
into each symbol, resulting in a 3
higher data rate. Some
adaptive communication systems allow the transmitter to
dynamically switch between constellation types depending on
the quality of the connection between the transmitter and
receiver.
17.2 Modulated Signal Bandwidth
We will examine an example of the QPSK constellation, with
a transmission rate of 1 MSPS. The baseband signal is two
dimensional, so must be represented with two orthogonal
components, which are by convention denoted I and Q.
Consider a sequence of five QPSK symbols, at time t
1, 2, 3, 4
and 5 respectively. The sequence in the two-dimensional
complex-constellation plane will appear as a signal trajectory
moving from one constellation point to another over time.
The individual I and Q signals are plotted against time in
Figure 17.4
. This is a two-dimensional signal, with each compo-
nent plotted separately. The I and Q baseband signals are
ΒΌ
