Early development Direct Sequence Spread Spectrum (DSSS) technology was in
the frequency range of 900 MHz
. At that time
there was not a standard modulation scheme in place. The basic concept of this
scheme was use all of the channel to produce one fast channel of 860 Kbps.
Otherwise, the channel was broken into smaller sections to produce more
channels, but those channels performed at slower speeds. For example, three
channels at 215 Kbps or two channels at 344 Kbps may have been produced.
Now that the 802.11 standards are in place, an RF engineer has to follow the
rules to make the hardware 802.11 compliant. The practice of using more of the
channel could no longer be used to achieve higher data rates. The new scheme
for 802.11 is to use very advanced modulation techniques to achieve higher data
rates.
Figure
shows a
block diagram of 802.11b DSSS. DSSS defines a channel as a contiguous band of
frequencies, 22 MHz wide. In the US, each channel operates from one of 11
defined center frequencies and extends 11 MHz in each direction
. For example,
Channel 1 operates from 2.401 GHz to 2.423 GHz, which is 2.412 GHz plus or
minus 11 MHz. Channel 2 uses 2.417 plus or minus 11 MHz, and so on.
There is significant overlap between adjacent channels. Center frequencies
are only 5 MHz apart, yet each channel uses 22 MHz of analog bandwidth. In
fact, channels should be co-located only if the channel numbers are at least
five apart. Channels 1 and 6 do not overlap, Channels 2 and 7 do not overlap,
and so on. There is a maximum of three co-located DSSS systems possible.
Channels 1, 6, and 11 are non-overlapping channels, as shown in Figure
. Note the 3-MHz
guard bands between each of these channels. In Europe, ETSI has defined a total
of 14 channels, which allows for four different sets of three non-overlapping
channels.
Whereas FHSS uses each frequency for a short period of time in a repeating
pattern, DSSS uses a wide frequency range of 22 MHz all of the time. The signal
is spread out across the different frequencies. Each data bit becomes a
chipping sequence, or a string of chips that are transmitted in parallel,
across the frequency range. This is sometimes referred to as the chipping code.
Regulating agencies set a minimum chipping rate for the different supported
speeds. IEEE 802.11 uses 11 chips. For example, the minimum chip rate for
802.11 DSSS, per the FCC, is ten chips for 1 and 2 Mbps (BPSK/QPSK) and eight
chips for 11 Mbps (CCK). Figure
shows an example
of a chipping sequence or code. If the bits in the chipping code for zero and
for one are examined closely, it can be determined that more than five data
bits out of 11 would have to be inverted in error, before the value would
change from a zero to a one, or from a one to a zero. This means that over half
of the signal can be lost, and the original message will still be recoverable.
802.11b uses three different types of modulation, depending upon the
data rate used:
-
Binary phase shift keyed (BPSK) – BPSK uses one phase to represent a
binary 1 and another to represent a binary 0, for a total of one bit of binary
data. This is utilized to transmit data at 1 Mbps.
-
Quadrature phase shift keying (QPSK) – With QPSK, the carrier
undergoes four changes in phase and can thus represent two binary bits of data.
This is utilized to transmit data at 2 Mbps.
-
Complementary code keying (CCK) – CCK uses a complex set of
functions known as complementary codes to send more data. One of the advantages
of CCK over similar modulation techniques is that it suffers less from
multipath distortion. Multipath distortion will be discussed later. CCK is
utilized to transmit data at 5.5 Mbps and 11 Mbps.