Wireless Physical Layer Technologies
4.3 Direct Sequence Spread Spectrum (DSSS)
The IEEE 802.11 as a physical layer transport method defi nes direct sequence spread spectrum (DSSS). With the proliferation of 802.11b, some changes were made to DSSS to match the 5.5- and 11-MB data rates
written into the new standard. This is sometimes referred to as high-rate wireless. The 802.11 standard has both FHSS and DSSS listed as a choice for physical layer transport. The IEEE had foreseen that FHSS would be a much more difficult protocol to adapt to a higher bandwidth scenario. With DSSS being an easier protocol to adapt to a higher bandwidth scenario, it was chosen.
The reasoning behind originally putting FHSS in the 802.11 standard was for bandwidth and battery life. On an FH network, running multiple access points in the same location to provide bandwidth over the 2-MB standard was a big plus. Also noted by the IEEE was the increased battery life on FH devices compared to DS devices. These reasons are what keep FH in the 802.11 standard, unlike the 802.11b standard, which only chose a single spread spectrum. When the IEEE made 802.11b, it needed to also make it backwards capable or the existing market would not be so forthcoming to adopt this new technology. With higher data rates and backwards capability, 802.11b became a big hit and with it so did the underlying physical transport DSSS. This has made direct sequence (DS) one of the most widely used wireless LAN spread spectrum techniques today.
Direct sequence uses two of the modulation techniques we looked at in chapter one, BPSK and QPSK. Direct sequence is a method of transmitting data across the air on a 22-MHz-wide frequency range. This is done to prevent narrowband noise or interference. This is performed by sending the signal across a large bandwidth of frequency and eliminating any small narrowband noise. When this narrowband interference appears, which is often in short small spikes, it is easily distinguished from a slow-rising or sloping wave like the ones in DSSS. To have these larger, less interference- prone waves, a large frequency bandwidth is needed. This large bandwidth eats up the small amount of frequency allocated in the ISM spectrum.
Governmental bodies regulate allowed channels that can exist within this 2.4-GHz range. Looking at direct sequence channels, one notes that there are 14 available channels. Table 4.2 illustrates that each of these
Table 4.1 Frequency Hopping World Channel Allocation
Country Channel Range (GHz) Hop Size
United States 2 to 79 2.402–2.479 26 Canada 2 to 79 2.402–2.479 26 Britain 2 to 79 2.402–2.479 26 France 48 to 82 2.448–2.482 27 Spain 47 to 73 2.473–2.495 35 Japan 73 to 95 2.473–2.495 23
Wireless Physical Layer Technologies 55
channels starts 5 MHz from each other and each has a bandwidth of 22 MHz. This makes these channels often interfere with each other. The proper designing of wireless networks must take into account this inherent overlap and assign proper channel selection. Looking at the channels available in the United States, one notes that there are three nonoverlap- ping channels: (1) channel 1, which operates at 2.401 to 2.423 GHz; (2) channel 6, which operates at 2.426 to 2.448 GHz, and (3) channel 11, which operates between 2.451 and 2.473 GHz. Placing all these channels on a linear path gives one a better feel for their relation and inherent interference between the selectable channels. A full list of all the channels and their frequencies for North America is listed in Table 4.2.
Looking at Table 4.2, channels 1, 6, and 11 do not overlap each other. The exclusive uses of these channels are recommended in a best practice design. Understanding what causes overlapping channels is a big part of being a successful RF engineer. To understand channel overlap, take a look at the following example. Case 1: you have stuck with the design guidelines and have implemented on channels 1, 6, and 11. Now that you are done, there are some parts of the facility where you have channel 6 overlapping with itself. This creates a large, layer two domain, such as
Table 4.2 Direct Sequence Spread Spectrum World Channel Allocation
Frequency Channel Americas Middle East Asia Europe Japan Israel
2.412 1 X X X X X 2.417 2 X X X X X 2.422 3 X X X X X X 2.427 4 X X X X X X 2.432 5 X X X X X X 2.437 6 X X X X X X 2.442 7 X X X X X X 2.447 8 X X X X X X 2.452 9 X X X X X X 2.457 10 X X X X X 2.462 11 X X X X X 2.467 12 X X X X 2.472 13 X X X X 2.477 14
the same broadcast domain. This is difficult to avoid and appears multiple times. To overcome this, one can try moving to a four-channel design. This may increase the network throughput, although it is a trade-off between large broadcast domains and slight channel interference. Per- forming throughput testing using both methods would allow an engineer to adapt the best method to deal with the limited nonoverlapping space. To recap, if one is in a situation where one might need to use a four- channel design, perform some throughput testing to see if channel inter- face or a large broadcast domain produces better throughput in the particular situation.
Various countries limit the use of the available channels. Looking at Table 4.2, one can see each of the countries and their allowed wireless DSSS channels. In the United States, only the use of channels 1 through 11 are allowed. The United Kingdom can use channels 1 through 13. Finally, Japan uses all 14 channels. Be aware this can complicate matters when designing international wireless local area networks (LANs). This can also be seen when you deal with end devices that travel between these countries. When this happens, one can take two approaches: (1) use 802.11d to dynamically adjust client devices when connecting to an access point in different countries or (2) just design setting the common channels across all access points globally. This can be done by never using channels 12 through 14 in Europe or Japan.
Figure 4.4 Direct sequence spread spectrum channel overlap detail.
Frequency 2.400 2.405 2.410 2.415 2.420 2.425 2.430 2.435 2.440 2.445 2.450 2.455 2.460 2.465 2.470 2.475 2.480 Channel 1 2 3 4 5 6 7 8 9 10 11 Center Frequency 2.412GHz 2.417GHz 2.422GHz 2.427GHz 2.432GHz 2.437GHz 2.442GHz 2.447GHz 2.452GHz 2.457GHz 2.462GHz Copyright Aaron Earle 1/20/2005
Wireless Physical Layer Technologies 57