Lecture Notes in Computer Science 4917

Implementation of an UWB Impulse-Radio Acquisition and Despreading Algorithm 85
The DBB is responsible for the following tasks, which are described in further detail
in Sect. 3:
– Demodulation of the payload data bits,
– Control of the timing circuit (programming the delay of the delay line),
– Synchronization of the spreading code with the received pulses.
3 Reference Application and Processor
First the algorithm, or application code, of the digital baseband is described. Then the
architecture of the reference processor is given and finally the results of the application
code running on the processor are given.
3.1 Application Code
The process of demodulating an UWB burst is done by the DBB. The most important
modes of the DBB are called: ACQ1, ACQ2, EOP and DATA for respectively the first
part of acquisition, the second part of acquisition, end of preamble and data mode.
ACQ1 Mode. In ACQ1 mode the DBB tests whether a signal is present on the channel
for a given clock delay. If the DBB finds a signal it will switch to ACQ2 mode, otherwise
it will remain in ACQ1 mode.
The test starts by retrieving Ncpb DVs. These DVs are correlated with Ncpb different
rotations of the spreading code. A higher correlation result means that the DVs are more
similar to the spreading code. The process of retrieving Ncpb DVs and correlating Ncpb
times is repeated Nb times. Nb is the number of tries on each clock delay, in this paper
we use Nb =3.TheNcpb correlations are accumulated over Nb tries, resulting in Ncpb
accumulated correlations. If one of these accumulated correlations is above a threshold
there is signal on the channel for this clock delay. If not the AFE is switched to a
different clock delay and the DBB tests for a signal on the channel again. The threshold
is SNRpulse dependent and is used to segregate noise from useful signal.
ACQ2 Mode. ACQ2 mode is quite similar to ACQ1 mode, however in this mode
the DBB searches the optimal clock delay and spreading code phase. The process of
retrieving Ncpb DVs and correlating Ncpb times is repeated for Nb times. Again the
results are accumulated, but now the highest accumulated correlation, the corresponding
spreading code phase and clock delay are stored. The spreading code phase can be
easily computed from the spreading code used in the correlation. This process is repeated
for every possible clock delay. When every clock delay is processed, the DBB
determines whether the highest stored correlation is above a threshold. If not there was
a false positive in ACQ1 mode and the DBB will go back to ACQ1 mode. If it is above
the threshold the DBB switches the AFE to the stored clock delay corresponding to the
highest stored correlation, depending on the corresponding spreading code phase the
DBB will ignore a number of DVs and the DBB switches to EOP mode.