Introduction
IEEE 802.11 (a/b/g)
13
Operating Manual 1171.5283.12 ─ 18
Data from the source (usually the next higher protocol layer, here MAC) must first be
scrambled, i.e. multiplied with a PN sequence. A 127-bit code generated by the follow-
ing generator polynomial is stipulated:
S(x) = x
7
+ x
4
+ 1
A feedback shift register generates the scrambling sequence. The start value of the
register for the data section should be randomly selected.
A subsequent convolutional coder adds redundancies to the bits thus scrambled (factor
of 2). The coder has 64 possible states (k = 7) and is described by the polynomials
g
0
=133
8
and g
1
=171
8
. To obtain the data rates of 6 to 54 Mbps defined by the standard,
different channel code rates are required. Bits generated by the convolutional coder
are therefore punctured (i.e. omitted) depending on the setting so that 1/2, 2/3 or 3/4
code rates are attained. Increasing the redundancy by channel coding is generally
mandatory in case of OFDM modulations since complete subcarriers may be elimina-
ted by frequency selective fading so that the loss of bits on the transmission path is in
many cases unavoidable.
To increase the performance of the convolutional coder, the coded data are interleaved
in the next step. Two interleaver stages ensure that the adjacent bits of the convolu-
tional coder are first distributed to different subcarriers and then to higher- or lower-sig-
nificant bits of the constellation used for subcarrier modulation. Long sequences of
defective bits can thus be avoided which significantly improves the faculties of the
Viterbi decoder in the receiver for a correction.
The next stage performs the actual modulation of the individual OFDM carriers.
Depending on the set data rate, the useful carriers are subjected to a uniform BPSK,
QPSK, 16QAM or 64QAM modulation. This is done by first calculating the I and Q
coefficients of each carrier. Gray coding is used to distribute the data bits to constella-
tion points. All carriers from 26 to +26, except carriers -21, -7, 0, 7 and 21, are used for
the transmission of user data. Carrier number 0 (directly at the center frequency later
on) is not used and is always 0. The remaining 4 are BPSK-modulated pilots. The pilot
carriers change their phase with each symbol. The phase variation is determined by
the 127-bit PN sequence already defined as scrambling sequence.
The actual OFDM modulation is performed by inverse discrete Fourier transform (IFFT)
in the next step. A 64-point IFFT is carried out with the I and Q coefficients of the sub-
carriers obtained before. To ensure sufficient spacing of aliasing products, only 52 of
the 64 possible carriers are used. The result is a discrete complex time signal in the
baseband with modulated OFDM carriers. A guard field which corresponds to a peri-
odic continuation of the same symbol is then appended before each OFDM symbol.
Multipath propagation can thus be easily compensated in the receiver.
Aliasing products are suppressed by oversampling, converting the discrete digital sig-
nal to an analog signal and subsequent filtering. In the last step, the baseband signal is
modulated onto the selected RF carrier and the complete signal is sent to the receiver
via the air interface.
Physical Layer OFDM
