Support
for QPSK, 16-QAM, and 64-QAM are mandatory in the downlink
with mobile WiMAX. In the uplink 64-QAM is optional. Both convolutional
code and turbo code with variable code rate and repetition coding are supported.
The combinations of various modulation and code rates provide a fi ne resolution
of data rates. The frame duration is 5 ms. Each frame has 48
OFDM symbols with
44 OFDM symbols available for data transmission.
The base station (BS) scheduler determines the appropriate data rate for
each burst allocation based on the buffer size, channel
propagation conditions
at the receiver, etc. A channel quality indicator (CQI) channel is used to provide
channel state information from the user terminals to the BS scheduler.
WiMAX provides signaling to allow fully asynchronous operation. The
asynchronous operation allows variable delay between retransmissions which
gives more fl exibility to the scheduler at the cost of additional
overhead for each
retransmission. HARQ combined with CQICH and AMC provides robust link
adoption in the mobile environment at vehicular speeds in excess of 120 km/h.
