Both TDD and FDD framing
structures are very similar to allow for support for both without incurring significant
system complexity. In either case, the frame used for the purpose of uplink traffic is
referred to as the uplink channel and the frame used for the purpose of downlink traffic
is referred to as the downlink channel.
Multiple Access For DL communication, the PHY uses the Time-Division Multiplexing
(TDM) scheme whereby data is transmitted by the BS as a single stream with data for
each SS multiplexed onto the stream and received by all SSs within the sector. All
the SSs using the same channel attempt to receive all portions of the DL, but retain
only the portion of the data they are meant to retain (identified by the CID for unicast,
multicast, and broadcast. Encryption also ensures that an unintended SS cannot
make meaningful use of the data). Though transmitted as a single stream of data, the
transmission may change its transmission characteristics (burst profile) multiple times
within the DL frame to tradeoff between channel robustness and throughput. The properties
and the boundary of each burst profile within the DL frame are indicated at the
beginning of the DL frame (called DL-MAP) for all the SSs to understand.
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