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.