mohatta2015.pdf

182 PRACTICAL CONSIDERATIONS
When the clusters are far apart, it makes sense to estimate the channel at the
clusters and then interpolate those estimates to obtain channel estimates where the
traffic symbols are. Standard interpolation approaches include linear interpolation
and Wiener interpolation.
Another approach is to use both pilot and traffic symbols. For example, with
recursive channel tracking, the channel is estimated at time mT, and then symbol
s(m) is detected and treated as a known symbol for purposes of updating the
channel coefficient. This is referred to as decision-directed tracking.
8.3.4 Per-survivor processing
Instead of detected symbol values, we can use hypothesized symbol values for channel
estimation. With MLSD and per-survivor processing (PSP), we keep different
channel models for each state in the Viterbi algorithm. Each state corresponds to
a different hypothetical symbol sequence. We can use the hypothetical symbol values
to track the channel coefficients. Strictly speaking, the Viterbi algorithm is no
longer equivalent to a tree search. Thus, we can generalize PSP to include keeping
a channel model for each hypothetical symbol sequence kept in a tree search. Also,
PSP can be used to estimate other parameters besides channel coefficients.
8.4 MORE PRACTICAL ASPECTS
In this section, we consider additional practical aspects.
8.4.1 Acquisition
When the receiver begins processing the received signal, it must determine roughly
if a signal is present and where it is located in time and other dimensions. This
is often done by correlating the received signal to a known transmitted symbol
pattern at different relative delays. If one were to plot the magnitude square of the
correlation as a function of delay, one would obtain the power-delay profile (PDP).
The peak value in the PDP can be thresholded to see if a signal is present. The
delay at which the peak occurs gives an initial estimate of signal timing (see next
subsection). Sometimes the frequency reference of the receiver is not very accurate,
so that the receiver must search for the signal in both time and frequency.
8.4.2 Timing
Another practical aspect is timing. In addition to packet or frame timing (knowing
which symbol is which), there is symbol timing (a special case of sample timing).
Consider the case of a single-tap channel and a single-tap, linear equalizer. In this
case, we would like to filter the received signal with a filter matched to the symbol
waveform and sample at the point at which the received symbol waveform and
matching waveform are aligned. In practice, we have to estimate that timing.
Timing and channel estimation are somewhat intertwined. For a single-path
channel, estimating the absolute path delay of the first path is equivalent to estimating
the arrival of the first symbol (which includes both frame and symbol