730
21 Wireless Local Area Networks
The hopping sequences for IEEE 802.11 are grouped in hopping sets for
worldwide operation:
Set 1, Set 2, and Set 3. The sequences are selected when
FHSS basic service set is confi gured for a WLAN. The
hopping sets are designed
to minimize interference between neighboring FHSS radios in a set. The following
hop sets are valid IEEE 802.11 hopping sequences in North America and most of
Europe (except Spain and France).
Set 1
: (0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54,
57, 60, 63, 66, 69, 72, 75)
Set 2
: (1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55,
58, 61, 64, 67, 70, 73, 76)
Set 3
: (2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 32, 35, 38, 41, 44, 47, 50, 53, 56,
59, 62, 65, 68, 71, 74, 77)
802.11a — Orthogonal Frequency Division Multiplexing (OFDM)
The OFDM PHY provides the capability to transmit
PSDU frames at multiple
data rates up to 54 Mbps for a WLAN where the transmission of multimedia
content is a consideration. The OFDM PHY defi ned for IEEE 802.11a is similar
to the OFDM PHY specifi cation of ETSI-HIPERLAN 2.
The PPDU is unique to the OFDM PHY. The PPDU
frame consists of a PLCP
preamble and signal and data fi elds (see Figure 21.12). The receiver uses the PLCP
preamble to acquire the incoming OFDM signal and synchronize the demodulator.
The PLCP header contains information about the PSDU from the sending OFDM
PHY. The PLCP preamble and the signal fi elds are always transmitted at 6 Mbps,
BPSK-OFDM modulated using a
convolutional encoding rate
R
1
/
2
.
PLCP preamble fi eld
is used to acquire the incoming signal and train and
synchronize the receiver. The PLCP preamble consists of 12 symbols, 10 of which
are
short symbols, and 2 long symbols. The short symbols are used to train the
receiver’s automatic gain control (AGC) and obtain a coarse estimate of the carrier
frequency and the channel. The long symbols are used to fi ne-tune
the frequency
and channel estimates. Twelve subcarriers are used for the short symbols and
53 for the long. The training of an OFDM is accomplished in 16
s. The PLCP
preamble is BPSK-OFDM modulated at 6 Mbps.
The
signal
is a 24-bit fi eld that contains information
about the rate and length
of the PSDU. The signal fi eld is convolutional encoded rate
R
1
/
2
, BPSK-OFDM
modulated. Four bits
are used to encode the rate, eleven bits are used to defi ne the
length, one reserved bit, a parity bit, and six 0 tail bits (see Table 21.7).
The
length fi eld
is an unsigned 12-bit integer to indicate
the number of octets
in the PSDU.
The
data fi eld
contains the service fi eld, PSDU, tail bits, and pad bits.
A total of six tail bits containing 0s are appended to the PPDU to ensure that the
convolutional encoder is brought back to the zero state.
