The PHY supports four different types of FEC techniques, each providing performance
suitable for different applications. The parameter of each type of FEC can be
further adjusted for additional optimization.
Modulation To make an efficient use of the bandwidth, the PHY makes use of
a multilevel modulation scheme, which includes QPSK, 16-QAM, and 64-QAM. For
the same average transmit power and channel bandwidth, the different modulation
schemes provide a tradeoff between channel throughput and robustness. For example,
64-QAM can transmit three times as much data as QPSK can, however, at the cost of
immunity from noise.
The modulation scheme can be selected on per-subscriber basis based on the quality
of the RF channel. For example, for a distant SS that would require more link margin, it
may be desirable to use QPSK. However, for a nearby SS with plenty of link margin, it
may be desirable to use 64-QAM to maximize the utilization of the spectrum. The modulation
scheme can thus be adjusted both for UL and DL channels for each burst profile.
Channel Bandwidth The PHY also defined default RF channel bandwidths in order
to ensure the products??™ interoperability over the RF interface.
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