IEEE 802.11a Supplement to 802.11 Standards

IEEE 802.11a Supplement to 802.11 Standards
The IEEE’s 802.11a supplement to the original 802.11 standards defines a new PHY for transmissions
of up to 54 Mbps in the 5-GHz band using COFDM. The “Coded” in COFDM refers to error-control codes. The MAC sublayer is unchanged from the original 802.11 version [454]. The 5-GHz band is
also known as the unlicensed national information infrastructure (U-NII) band [452]. The European
Telecommunications Standards Institute (ETSI) HIPERLAN-2 WLANs also employ the OFDM, as
it is the most popular modulation technique for high-speed indoor WLANs [454].
The COFDM protocol is defined in the 802.11a standards. Using higher frequencies than the
HR/DSSS (the 802.11b protocol) and several modulation schemes (see following text), COFDM
delivers data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The 802.11a devices attempt to
communicate at the highest rate and drop to the next-highest rate if they encounter too many transmission
errors, dropping further still if necessary. The closer the devices are to one another, the faster
they will be able to communicate, owing to higher signal strength [453]. The shorter data rates can
cover distances of up to 100 m [454].
The COFDM’s advantages over the HR/DSSS (802.11b) include higher transmission rates, roughly
four times as many available channels – giving nearly eight times the transmission capacity, less risk
of interference from Bluetooth devices and portable phones operating in the same ISM band, and up
to five times the throughput in an office setting. An 802.11a administrator can achieve the throughput
gain by deploying APs at the same cost as an 802.11b network, or can keep the throughput at the
802.11b level with lower AP deployment expenses [453].
OFDM is a variant of frequency division multiplexing (FDM). Both split bandwidth into smaller
“subcarriers” and use the subcarriers as data transmission channels. FDM was used in first-generation
mobile phones but wasted bandwidth by leaving an unused channel between used subcarriers to guard
against interference from one phone to the next. In contrast, OFDM selects channels that can overlap
without interfering with each other, conserving bandwidth [471]4.
OFDM encodes a single transmission into multiple subcarriers, unlike another emerging encoding
technique, the code division multiple access (CDMA), which uses mathematical constructs more
complicated than the OFDM’s to send multiple transmissions on one carrier [471]. The advantage of
the OFDM’s less complicated mathematics is a savings on algorithm processing when transmissions
are decoded at the receiver [454]. Under the OFDM, a wide frequency channel is divided into
subchannels that each carry data and the subchannels are multiplexed into a single, fast channel for
transmission [471].
Table 4.7 802.11g Data rates, transmission types, and modulation
schemes [479]
Data rate (Mbps) Transmission type Modulation scheme
54 OFDM 64 QAM
48 OFDM 64 QAM
36 OFDM 16 QAM
24 OFDM 16 QAM
18 OFDM QPSK1
12 OFDM QPSK
11 DSSS CCK2
9 OFDM BPSK3
6 OFDM BPSK
5.5 DSSS CCK
2 DSSS QPSK
1 DSSS BPSK
OFDM achieves a gain in throughput over FDM by exploiting mathematical orthogonality. In
essence, overlapping subcarriers peak in the frequency domain when their neighboring subcarriers
have zero amplitude (see Figure 4.7). OFDM takes the coded signal for each subchannel and uses
the inverse fast Fourier transform (IFFT) to create a composite waveform from the strength of each
subchannel. OFDM receivers can then apply the FFT to a received waveform to extract the amplitude
of each component subcarrier [471].
According to the 802.11a standards, the OFDM PHY consists of two protocol functions:
• A PHY convergence function, which adapts the capabilities of the PMD system to the PHY
service. This function is supported by the PLCP, which defines a method of mapping the IEEE
802.11 PHY sublayer service data units (PSDUs) into a framing format suitable for sending
and receiving user data and management information between two or more stations using the
associated PMD system.
• A PMD system whose function defines the characteristics and method of transmitting and
receiving data through a wireless medium between two or more stations, each using the OFDM
system [452].
These two protocol functions are very much like the original 802.11 standards, but with changes
to support the use of OFDM and the faster 5-GHz band.
The modulation schemes used by the 802.11a change when the supported transmission speeds
rise. At the 6- and 9-Mbps rates, binary phase-shift keying (BPSK) modulation is employed, while
the 12- and 18-Mbps rates use quadrature phase-shift keying (QPSK). The 24- and 36-Mbps rates use
quadrature amplitude modulation (16-QAM), and the 48- and 54-Mbps rates employ 24-QAM [452].