11n WLAN 194
spatial channels [3]. In a MIMO OFDM system with NT transmit antennas and NR receive
antennas, the wideband channel can be characterized at discrete frequencies l??†f, l1 ?‰¤ l ?‰¤ l2
by a set of NR ?— NT channel matrices, H(l). Here we address the 802.11n 20 MHz baseband
channel that is divided into 64 subcarriers, with ??†f = 312.5 kHz and ??’32 ?‰¤ l ?‰¤ 31. Up to NM
= min{NT, NR} parallel channels may be synthesized in the MIMO system, with the number
of parallel spatial streams transmitted, NS, upper-bounded by NM.
Figure 9.1 shows a simplified system diagram for the 802.11n PHY, including
transmitter and receiver blocks. Scrambled data bits are encoded using the rate-1/2, 64-state
convolutional encoder that was originally introduced to the 802.11 standard by the 802.11a
amendment, and later also employed by 802.11g [4]. The coded bits are punctured to
achieve the desired code rate, which must belong to the set {1/2, 2/3, 3/4, 5/6}. A stream
parser distributes the coded and punctured bits to the NS spatial streams in a round robin
fashion (see [1] for details), and each stream is independently interleaved and mapped to a
complex constellation.
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