The DL frame is further divided into smaller slots,
each with its own burst profile. The first two frames are transmitted with frame management
information that includes both UL-MAP and DL-MAP. In a similar fashion
as SC-PHY, the UL subframe is divided into several slots with their own burst profile,
some of which are used for contention-based UL communication, while most of which
are assigned to a specific SS for UL communication by UL-MAP.
The multiple access method in OFDMA is significantly more complex. OFDMA supports
both TDD and FDD, whereby the downlink and uplink are divided into different
timeframes or frequencies, respectively. The mapping for both UL and DL happens in
two steps. First, the data gets mapped to one or more timeslots on one or more logical
channels. Second,, data in logical subchannels in each timeslot gets mapped to one more
physical subchannel. The end result is a complex mapping of data transmission into a
complex framework of timeslot and channel. Within this complexity, OFDMA also allows
for various burst profiles accommodating adaptive transmission techniques.
Forward Error Correction The OFDM PHY specified three different forward error
correction schemes, including block turbo codes and convolution turbo codes, which
provide 2??“3 dB of additional gain.
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