11 demodulated and detected. This works only when the wireless channel does not change for the preamble and the data blocks; hence the frequency with which the preamble is transmitted will depend on how fast the wireless channel is changing. This approach is called decision directed channel estimation (DDCE) and is discussed in detail by  and . DDCE is useful for systems operating in somewhat static channels. In particular, it is suitable for wireless cellular systems that we discussed in class; this is because such systems use a slot-based transmission mode and hence are inherently compatible with DDCE. The biggest advantage of this approach is that it enhances spectrum efficiency by using detected data as pilots. Unfortunately, DDCE causes error propagation to occur in fast fading wireless channels. However, this can be fixed by periodically using training-blocks  . 2.3 Mitigation of Frequency-Varying Effects Frequency-varying channel impairments are caused by the delay spread (due to multipath as
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