Digital Signal Processing Reference
In-Depth Information
added to the main signal path in the
interleaved
issr decoder, which has been
discussed in Section 5.12. Since the duration of a transmitted pulse is shorter
than the length of the receive window and multiple pulses can arrive within
a single window, each of the parallel monitored receive slots experiences in-
dependent fading. It is thanks to the combination of signal power from several
independent channels
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that the reliability of the wireless link will significantly
improve.
Of course, providing backup information is
not the only task of the auxiliary receiver
bank. An auxiliary unit also plays an impor-
tant role in channel monitoring. During the
low-power sleep mode of the receiver, only
one of the auxiliary units remains awake
to monitor the channel for incoming mes-
sages. Since there is a reasonable chance
that the wakeup call from the transmitter ar-
rives during the off-state of the receive win-
dow from the only active receive unit, it can take some time before the receiver
becomes alerted. When the transmitter has been able to draw the attention of
the receiver, the latter one shall respond with an acknowledgement. At the
same time, the receiver also returns to a full power state. The receiver is not
synchronized to the transmitter at this moment and it also does not know which
receive slots offer the best link conditions. Therefore, all receive units are be-
ing assigned to slots that are uniformly spaced over the pulse period. A typical
indoor channel will provide a sufficient amount of delay spread (up to a few
100 ns) so that enough multipath power of the synchronization sequence al-
ways ends up in at least one activated receive slot.
Immediately thereafter, the receiver reallocates the main unit to the receive slot
with the highest signal power. Of course, it is not necessarily the case that all
other auxiliary units are being positioned at the best possible time slots. For this
reason, the auxiliary unit with the lowest signal-to-noise ratio is disconnected
from the active receive segment and is assigned to the slot with the oldest entry
in the history table. The auxiliary unit is reactivated when a better snr value
is detected in one of the current inactive receive slots. After this, the whole
process is repeats from the beginning, until the end of the transmission burst
has been reached.
The algorithm described above allows the system to track average shifts in the
power delay profile of the channel, without control track in fast changing envi-
ronments. Remark that the algorithm is also able to track slow displacements
time
AUX.
slot 4
AUX.
slot 3
slot 2
MAIN
slot 1
AUX.
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Each receive slot can be regarded as a separate transmission channel.
