DSSS Frequency Modulation Function

DSSS Frequency Modulation Function
A balanced modulator modulates the spread PPDU by combining it with a carrier set at the
transmit frequency. The DSSS PMD transmits the initial PPDU at 1Mbps or 2Mbps using different
modulation types, depending on which data rate is chosen. For 1Mbps (basic access
rate), the PMD uses differential binary phase shift keying (DBPSK) modulation.
Phase shift keying varies the phase of the carrier frequency to represent different binary symbols.
Thus, changes in phase maintain the information content of the signal. Noise usually
affects the amplitude of the signal, not the phase. As a result, the use of phase shift key modulation
reduces potential interference.
The input to the DBPSK modulator is either a 0 or 1 coming from the PLCP. The modulator
transmits the binary data by shifting the carrier signal’s phase, as shown conceptually in
Figure 5.10.  For 2Mbps transmission (enhanced access rate), the PMD uses differential quadrature phase
shift keying (DQPSK) modulation to send data at 2Mbps. Figure 5.11 illustrates this concept. In
this case, the input to the modulator is combinations of two bits (00, 01, 10, or 11) coming
from the PLCP. Each of these two-bit symbols is sent at 1Mbps, resulting in a binary data rate
of 2Mbps. Thus, the four-level modulation technique doubles the data rate while maintaining
the same baud rate as a 1Mbps signal. This makes effective use of the wireless medium.
The transmit power levels for DSSS is
• 1,000 milliwatts for U.S. (according to FCC 15.247)
• 100 milliwatts for Europe (according to ETS 300-328)
• 10 milliwatts for Japan (according to MPT ordinance for Regulating Radio Equipment,
Article 49-20)
The effective power will be higher, though, using antennas that offer higher directivity (that is,
gain). Wireless LAN suppliers have optional antennas that provide a variety of radiation patterns.
The 802.11 specification also says that all PMDs must support at least 1 milliwatt transmit
power. Most access points and radio cards enable you to select multiple transmit power
levels via initialization parameters. In fact, the standard calls for power level controls for radio
that can transmit greater than 100 milliwatts. Higher-power radio must be able to switch back
to 100 milliwatt operation. DSSS wireless LAN devices are capable of operating at relatively high data rates, supporting
applications that require more range and bandwidth within a single cell. Be certain, however,
to consider frequency hopping spread spectrum and infrared Physical layers before making a
decision on which one to implement.