802.11 Wireless LANs

The original 802.11 standard defined two WLAN PHY methods:

• 2.4 GHz frequency hopping spread spectrum (FHSS)

• 2.4 GHz direct sequence spread spectrum (DSSS)

Frequency Hopping WLANs

FHSS WLANs support 1 Mbps and 2 Mbps data rates. As the name implies, a FHSS device changes or "hops" frequencies with a predetermined hopping pattern and a set rate, as depicted in Figure 3-8. FHSS devices split the available spectrum into 79 nonoverlapping channels (for North America and most of Europe) across the 2.402 to 2.480 GHz frequency range. Each of the 79 channels is 1 MHz wide, so FHSS WLANs use a relatively fast 1 MHz symbol rate and hop among the 79 channels at a much slower rate.


The hopping sequence must hop at a minimum rate of 2.5 times per second and must contain a minimum of six channels (6 MHz). To minimize the collisions between overlapping coverage areas, the possible hopping sequences can be broken down into three sets of length, 26 for use in North America and most of Europe. Tables 3-1 through 3-4 show the minimum overlap hopping patterns for different countries, including the U.S., Japan, Spain, and France.



In essence, the hopping patterns provide a slow path through the possible channels in such a way that each hop covers at least 6 MHz and, when considering a multicell deployment, minimizes the probability of a collision. The reduced set length for countries such as Japan, Spain, and France results from the smaller ISM band frequency allocation at 2.4 GHz.


FHSS PLCP

After the MAC layer passes a MAC frame, also known as a PLCP service data unit (PSDU) in FHSS WLANs, to the PLCP sublayer, the PLCP adds two fields to the beginning of the frame to form a PPDU frame. Figure 3-9 shows the FHSS PLCP frame format.


Direct Sequence Spread Spectrum WLANs

DSSS is another physical layer for the 802.11 specifications. As defined in the 1997 802.11 standard, DSSS supports data rates of 1 and 2 Mbps. In 1999, the 802.11 Working Group ratified the 802.11b standard to support data rates of 5.5 and 11 Mbps. The 802.11b DSSS physical layer is compatible with existing 802.11 DSSS WLANs. The PLCP for 802.11b DSSS is the same as that for 802.11 DSSS, with the addition of an optional short preamble and short header.



802.11 DSSS

Similar to the PLCP sublayer for FHSS, the PLCP for 802.11 DSSS adds two fields to the MAC frame to form the PPDU: the PLCP preamble and PLCP header. The frame format appears in Figure 3-14.


DSSS Basics

Spread-spectrum techniques take a modulation approach that uses a much higher than necessary spectrum bandwidth to communicate information at a much lower rate. Each bit is replaced or spread by a wideband spreading code. Much like coding, because the information is spread into many more information bits, it has the ability to operate in low signal-to-noise ratio (SNR) conditions, either because of interference or low transmitter power. With DSSS, the transmitted signal is directly multiplied by a spreading sequence, shared by the transmitter and receiver.

Physical Layer Building Blocks

To understand the different PMDs that each 802.11 PHY provides, you must first understand the following basic PHY concepts and building blocks:

• Scrambling

• Coding

• Interleaving

• Symbol mapping and modulation

Scrambling

One of the foundations of modern transmitter design that enables the transfer of data at high speeds is the assumption that the data you provide appears to be random from the transmitter's perspective. Without this assumption, many of the gains made from the other building blocks would not be realized. However, it is conceivable and actually common for you to receive data that is not at all random and might, in fact, contain repeatable patterns or long sequences of 1s or 0s. Scrambling is a method for making the data you receive look more random by performing a mapping between bit sequences, from structured to seemingly random sequences. It is also referred to as whitening the data stream. The receiver descrambler then remaps these random sequences into their original structured sequence. Most scrambling methods are self-synchronizing, meaning that the descrambler is able to sync itself to the state of the scrambler.


Coding

Although scrambling is an important tool that has allowed engineers to develop communications systems with higher spectral efficiency, coding is the mechanism that has enabled the high-speed transmission of data over noisy channels. All transmission channels are noisy, which introduces errors in the form of corrupted or modified bits. Coding allows you to maximize the amount of data that you send over a noisy communication medium. You can do so by replacing sequences of bits with longer sequences that allow you to recognize and correct a corrupted bit. For example, as shown in Figure 3-3, if you want to communicate the sequence 01101 over the telephone to your friend, you might instead agree with your friend that you will repeat each bit three times, resulting in the sequence 000111111000111. Even if your friend mistook some of the bits at his end—resulting in the sequence 100111111000101, with the second to last bit being corrupted—he would recognize that the original sequence was 01101 via a majority voting scheme. Although this coder is rather simple and not efficient, you now understand the concept behind coding.


The most common type of coding in communications systems today is the convolutional coder because it can be implemented rather easily in hardware with delays and adders. In contrast to the preceding code, which is a memory-less block code, the convolutional code is a finite memory code, meaning that the output is a function not just of the current input, but also of several of the past inputs. The constraint length of a code indicates how long it takes in output units for an input to fall out of the system. Codes are often described through their rate. You might see a rate 1/2 convolutional coder. This rate indicates that for every one input bit, two output bits are produced. When comparing coders, note that although higher rate codes support communication at higher data rates, they are also correspondingly more sensitive to noise.


Interleaving

One of the base assumptions of coding is that errors introduced in the transmission of information are independent events. This assumption is the case in the earlier example where you were communicating a sequence of bits over the phone to your friend and bits 1 and 9 were corrupted. However, you might often find that bit errors are not independent and that they occur in batches. In the previous example, suppose a dump truck drove by during the first part of your conversation, thereby interfering with your friend's ability to hear you correctly. The sequence your friend received might look like 011001111000111, as shown in Figure 3-4. He would erroneously conclude that the original sequence was 10101.


For this reason, interleavers were introduced to spread out the bits in block errors that might occur, thus making them look more independent. An interleaver can be either a software or hardware construct; regardless, its main purpose is to spread out adjacent bits by placing nonadjacent bits between them. Working with the same example, instead of just reading the 16-bit sequence to your friend, you might enter the bits five at a time into the rows of a matrix and then read them out as columns three bits at a time, as shown in Figure 3-5. Your friend would then write them into a matrix in columns three bits at a time, read them out in rows five bits at a time, and apply the coding rule to retrieve the original sequence.


Symbol Mapping and Modulation

The modulation process applies the bit stream to a carrier at the operating frequency band. Think of the carrier as a simple sine wave; the modulation process can be applied to the amplitude, the frequency, or the phase. Figure 3-6 provides an example of each of these techniques.