The primary purpose of the OFDM PHY is to transmit Media Access Control (MAC) protocol data units (MPDUs) as directed by the 802.11 MAC layer. The OFDM PHY is divided into two elements: the physical layer convergence protocol (PLCP) and the physical medium dependent (PMD) sublayers.

The MAC layer communicates with the PLCP via specific primitives through a PHY service access point. When the MAC layer instructs, the PLCP prepares MPDUs for transmission. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP sublayer minimizes the dependence of the MAC layer on the PMD sublayer by mapping MPDUs into a frame format suitable for transmission by the PMD.

Under the direction of the PLCP, the PMD provides actual transmission and reception of PHY entities between two stations through the wireless medium. To provide this service, the PMD interfaces directly with the air medium and provides modulation and demodulation of the frame transmissions. The PLCP and PMD communicate using service primitives to govern the transmission and reception functions.

Figure 1 illustrates the frame format for an 802.11a frame. The PLCP preamble field is present for the receiver to acquire an incoming OFDM signal and synchronize the demodulator. The preamble consists of 12 symbols. Ten of the symbols are short for establishing Automatic Gain Control (AGC) and the coarse frequency estimate of the carrier signal. The receiver uses the long symbols for fine-tuning. With this preamble, it takes 16 microseconds to train the receiver after first receiving the frame.

The signal field consists of 24 bits, defining data rate and frame length. The 802.11a version of OFDM uses a combination of binary phase shift keying (BPSK), quadrature PSK (QPSK), and quadrature amplitude modulation (QAM), depending on the chosen data rate as it is shown in Table 1. The length field identifies the number of octets in the frame. The PLCP preamble and signal field are convolutionally encoded and sent at 6 Mbps using BPSK no matter what data rate the signal field indicates, The convolutional encoding rate depends on the chosen data rate.

With 802.11a OFDM modulation, the binary serial signal is divided into groups (symbols) of one, two, four, or six bits, depending on the data rate chosen, and converted into complex numbers representing applicable constellation points. If a data rate of 24 Mbps is chosen, for example, then the PLCP maps the data bits to a 16QAM constellation.

After mapping, the PLCP normalizes the complex numbers to achieve the same average power for all mappings. The PLCP assigns each symbol, having a duration of 4 microseconds, to a particular subcarrier. An Inverse Fast Fourier transform (IFFT) combines the subcarriers before transmission.

As with other 802.11 based PHYs, the PLCP implements a clear channel assessment protocol by reporting a medium busy or clear to the MAC layer via a primitive through the service access point. The MAC layer uses this information to determine whether to issue instructions to actually transmit an MDSU.

Operating frequencies for the 802.11a OFDM layer fall into the following three 100 MHz unlicensed national information structure (U-NII) bands: 5.15 to 5.25 GHz, 5.25 to 5.35 GHz, and 5.725 to 5.825 GHz. Table 2 shows that there are twelve 20 MHz channels, and each band has different output power limits. In the United States, the Code of Federal Regulations, Title 47, Section 15.407, regulates these frequencies.

The 802.11a standard requires receivers to have a minimum sensitivity ranging from -82 to -65 dBm, depending on the chosen data rate.

IEEE ratified the 802.11a and 802.11b wireless networking communications standards in 1999, to create a standard technology that could span multiple physical encoding types, frequencies and applications in the same way the 802.3 Ethernet standard has been successfully applied to 10, 100 and 1,000 Mbps technology over fiber and various kinds of copper.

802.11b standard was designed to operate in the 2.4 GHz band using Direct Sequence Spread Spectrum (DSSS) technology. The 802.11a standard, on the other hand, was designed to operate in the 5 GHz UNII (Unlicensed National Information Infrastructure) band. the 802.11a standard uses Orthogonal Frequency Division Multiplexing scheme.

The 802.11a standard, which supports data rates of up to 54 Mbps, is the Fast Ethernet analog to 802.11b, which supports data rates of up to 11 Mbps. Like Ethernet and Fast Ethernet, 802.11b and 802.11a use an identical MAC (Media Access Control). However, while Fast Ethernet uses the same physical-layer encoding scheme as Ethernet (only faster), 802.11a uses an entirely different encoding scheme, called OFDM (orthogonal frequency division multiplexing).

The FCC has allocated 300 MHz of spectrum for unlicensed operation in the 5 GHz block, 200 MHz of which is at 5.15 MHz to 5.35 MHz, with the other 100 MHz at 5.725 MHz to 5.825 MHz. The spectrum is split into three working "domains." The first 100 MHz in the lower section is restricted to a maximum power output of 50 mW (milliwatts). The second 100 MHz has a more generous 250-mW power budget, while the top 100 MHz is delegated for outdoor applications, with a maximum of 1-watt power output. In contrast, 802.11b cards can radiate as much as 1 watt in the United States. However, most modern cards radiate only a fraction (30 mW) of the maximum available power for reasons of battery conservation and heat dissipation.

The 802.11a standard gains some of its performance from the higher frequencies at which it operates. Moving up to the 5 GHz spectrum from 2.4 GHz will lead to shorter distances. The 802.11a technology overcomes some of the distance loss by increasing the EIRP to the maximum 50 mW.

OFDM was developed specifically for indoor wireless use and offers performance much superior to that of spread-spectrum solutions. OFDM works by breaking one high speed data carrier into several lower speed subcarriers, which are then transmitted in parallel in a DMT way, the same tha is used by the ADSL modems. Each high speed carrier is 20 MHz wide and is broken up into 52 subchannels, each approximately 300 KHz wide. OFDM uses 48 of these subchannels for data.

Each subchannel in the OFDM implementation is about 300 KHz wide. At the low end of the speed gradient, BPSK (binary phase shift keying) is used to encode 125 Kbps of data per channel, resulting in a 6,000 kbps, or 6 Mbps, data rate. Using quadrature phase shift keying, you can double the amount of data encoded to 250 Kbps per channel, yielding a 12 Mbps data rate. And by using 16 level quadrature amplitude modulation (4QAM) encoding 4 bits per hertz, you can achieve a data rate of 24 Mbps. The 802.11a standard specifies that all 802.11a compliant products must support these basic data rates.

VOCAL Technologies, Ltd. modem software libraries include a complete range of ETSI / ITU / IEEE compliant modulations, optimized for execution on ANSI C and leading DSP architectures (ADI, AMD, ARM, CEVA, LSI Logic ZSP, MIPS and TI). This software is modular and can be executed as a single task under a variety of operating systems or it can execute standalone with its own kernel.

VOCAL is an active contributor to the International Telecommunications Union (ITU), Telecommunication Industry Association (TIA), ANSI T1E1.4, the Institute of Electrical and Electronics Engineers (IEEE) and the European Telecommunication Standards Institute (ETSI).