mohatta2015.pdf
signal processing from power amplifier operation control point of view
signal processing from power amplifier operation control point of view
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
THE LITERATURE 111<br />
both directions and then selecting the direction with smaller errors was proposed in<br />
[Hig89]. This approach can be refined to select (arbitrate) the direction separately<br />
for each symbol using a local Euclidean distance metric [Nel05]. (Replacing the local<br />
metric with MLSD [Bot08] leads to a form of assisted MLSD, which is discussed in<br />
the next chapter.) Decoder feedback can improve the arbitration process [Oh07].<br />
Another solution is to apply linear pre-filtering to convert the channel to minimum<br />
phase. As the DFE is usually designed assuming white noise, the pre-filter<br />
should be an all-pass filter so as not to color the noise. Early work on pre-filtering<br />
for DFE can be found in [Mar73]. More references on pre-filtering are given in the<br />
next chapter.<br />
We have assumed sufficient taps in the FF. If the number of taps is limited, then<br />
various tap selection approaches can be used. These are discussed in the previous<br />
chapter.<br />
A number of solutions have been proposed to address error propagation. One<br />
approach is to improve the feedback using the following.<br />
1. Erase unreliable decisions [Chi98, Fan99].<br />
2. Use a soft MMSE symbol estimate [GerOO, ArsOla]. This can be related to<br />
neural network processing [GerOO].<br />
3. Use multiple hard symbol values when the decision variable is small in magnitude<br />
[Dah88]. This can be done by having multiple DFEs, each feeding back<br />
a different detected value. An accumulated error can be used to select which<br />
DFE to use for the final detected value and further decisions. This can be<br />
interpreted as a form of arbitration in which the two DFEs process data in<br />
the same direction.<br />
4. In a coded system, error propagation can be reduced by using decoder feedback<br />
to improve the decisions [Koh86, San96, Ari98].<br />
In the MIMO/cochannel scenario, LE with multiple receive antennas combined<br />
with SIC has been proposed [AriOOb]. Ordering of detection is addressed in [Kim06].<br />
In CDMA, there are a number of DFE forms employed in multiuser detection.<br />
The classic structure of feedforward and feedback filtering can be found in [Abd94,<br />
Due95]. Using tentative decisions for future symbol periods as well is discussed<br />
in [Xie90a]. A subset of the users can be suppressed linearly rather than being<br />
subtracted [Woo02].<br />
A related approach found in the multiuser detection literature is successive interference<br />
cancellation (SIC), in which symbols are detected one at a time. Early<br />
work can be found in [Den93]. Repeating the process, so that the first symbol benefits<br />
from ISI removal as well, gives rise to multistage SIC. If we detect all symbols<br />
without ISI removal in the first stage, this gives rise to multistage parallel interference<br />
cancellation (PIC) [Div98]. Early work can be found in [Mas88]. Use of<br />
different nonlinearities in SIC and PIC is explored in [TanOla, Zha03]. A mixture<br />
of SIC and PIC can also be used [Kou98]. Note: If we remove the decision nonlinearity<br />
and use decision variables for subtraction, we end up with a form of block<br />
linear equalization in which the matrix equation of final decision variables is being