With each channel
transmitting data at a much slower rate, the ISI due to multipath is minimized, allowing
for recovery of transmitted data.
Multiple Access One of the key differences between OFDM and OFDMA is the multiple
access methods used by the two schemes. In the case of OFDM, all the subcarriers
are addressed to a single receiver at any given time and multiple access is provided
by means of TDMA. However, in the case of OFDMA, a subset of subcarrier can be addressed
to any one of the SSs at any given time. In OFDMA, multiple access is provided
by a combination of time and subcarriers. This combination raises the complexity of
OFDMA significantly.
As in the case of OFDMA, OFDM also uses a group of subcarriers (known as a subchannel)
to address a specific receiver, a technique referred to as subchannelization.
However, a key difference is that, in the case of OFDM, each subchannel can be thought
of as a separate independent channel, and the mapping of data to the channel is solely
based on TDMA, very similar to the TDMA scheme in the case of SC-PHY discussed in
the previous section. The channel is divided into frames, which is divided further into
DL subframes and UL subframes.
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