802.11a WLANs

At the same time that the 802.11b 1999 draft introduced HR-DSSS PHY, the 802.11a-1999 draft introduced the Orthogonal Frequency Division Multiplexing (OFDM) PHY for the 5 GHz band. It provided mandatory data rates up to 24 Mbps and optional rates up to 54 Mbps in the Unlicensed National Information Infrastructure (U-NII) bands of 5.15 to 5.25 GHz, 5.25 to 5.35 GHz, and 5.725 to 5.825 GHz. 802.11a utilizes 20 MHz channels and defines four channels in each of the three U-NII bands. This section provides you with the details to understand how to support OFDM.


802.11j

The IEEE 802.11j draft amendment for LAN/metropolitan-area networks (MAN) requirements provides for 802.11a type operation in the 4.9 GHz band allocated in Japan and in the U.S. for public safety applications as well as in the 5.03 to 5.091 GHz Japanese allocation. A channel numbering scheme uses channels 240 to 255 to cover these frequencies in 5 MHz channel increments.


OFDM Basics

Consider the simple QPSK symbol first introduced in the section, "Physical Layer Building Blocks," and then consider the transmission of two consecutive symbols. As these symbols travel through the transmission medium from the transmitter to the receiver, they experience distortions, and various parts of the signal can be delayed. If these delays are long enough, the first symbol might overlap in time with the second symbol. This overlapping is ISI. The time delay from the reception of the first instance of the signal until the last instance is referred to as the delay spread of the channel. You can also think of it as the amount of time that the first symbol spreads into the second. Traditionally, designers address ISI in one of two ways: employing symbols that are long enough to be decoded correctly in the presence of ISI or by equalizing to remove the distortion caused by the ISI. The former method limits the symbol rate to something less than the bandwidth of the channel, which is inversely proportional to the delay spread. As the bandwidth of the channel increases, you can increase the symbol rate, thereby achieving a higher end data rate. The latter method, often used in conjunction with the former, requires the use of ever more complicated and expensive methods to implement channel-equalization schemes to maximize the usable bandwidth of the channel.

Multichannel modulation schemes take a completely different approach. As a multichannel modulation designer, you break up the channel into small, independent, parallel or orthogonal transmission channels upon which narrowband signals, with a low symbol rate, are modulated, usually in the frequency domain, onto individual subcarriers. Similar to how you can modulate FHSS signal onto the appropriate carrier, you break the channel into N independent channels. For a given channel bandwidth, the larger the N that you choose, the longer the symbol period and the narrower the subchannel, so you can see that as the number of subchannels goes to infinity, the ISI goes to zero.

To build these independent symbols, a useful tool is the Fast Fourier Transform (FFT), which is an efficient implementation of a Discrete Fourier transform (DFT) and can convert a time domain signal to the frequency domain and vice versa. In the frequency domain, you generate N 4-QAM (Quadrature Amplitude Modulation) symbols, which are then converted to the time domain using an inverse FFT (IFFT). You should also know that making the size of the FFT a power of two allows for simple and efficient implementations. For that reason, OFDM systems usually pick N such that it is a power of two.

Without going into the intricacies of mathematics that are beyond the scope of this book, it simplifies the processing greatly if everything is done in the frequency domain using FFTs. To enable this processing at the receiver, however, the received signal must be a circular convolution of the input with the channel, as opposed to just a convolution. Convolution is a mathematical mechanism for passing a signal through a channel and determining the output. To ensure this property, you must take the time domain representation of an OFTM symbol and create a cyclic prefix by repeating the final n samples at the beginning. Figure 3-22 shows this process, where n is the length of the cyclic prefix and N is the size of the FFT in use.


Unlike some other multichannel modulation techniques, OFDM places an equal number of bits in all subchannels. In nonwireless applications such as asynchronous digital subscriber line (ADSL), where the channel is not as time varying, the transmitter uses knowledge of the channel and transmits more bits, or information, on those subcarriers that are less distorted or attenuated.