Reconfigurable Baseband Processing for Wireless Communications - Adaptive Signal Processing in Wireless Communications - page 466

Bit allocation vector

1 0 0 4 0

Signaling channel

CP
Add

CP
Rev

De-
Mod

Mod IFFT FFT

Data channel

TX RX

FIgure 15.16 OFDM for cognitive radio.

structured way using techniques known from the design of array processors. In the sec-
ond step, the implementation of the tasks onto the Montium is analyzed. It is shown that
calculating a 127 × 127 discrete spectral correlation function requires approximately
140 µs on a system-on-chip (SoC) with 4 Montium cores. For energy detection, spec-
trum sensing can be done in different frequency resolutions with which the trade-offs
between performance and computational power can be made. This can be achieved by a
size-reconfigurable FFT on the Montium.
OFDM baseband processing for cognitive radio is a parameterizable OFDM process-
ing chain that is configured by a configuration manager (see Figure 15.17). By applying
different parameter settings in each task, the OFDM system is adaptive to various chan-
nel conditions and provides various data rates.
Relevant parameters are shown in Table 15.9 , but are not limited by this table and can
be extended to add more flexibility to the system. The number of OFDM symbols per
frame ( N sym ) is limited by the channel coherence time, the time during which the chan-
nel characteristics are constant. The number of guard samples ( N g ) is chosen to deal with
different channel delay spreads. Generally not all data are used to carry user informa-
tion. A part of the data (e.g., pilots) is used to guarantee reliable transmissions. Different
pilots are used for different purposes, such as channel estimation or phase offset estima-
tion. They can be placed in the preamble section prior to each frame or embedded in the

Configuration manager

Control and configuration
information

Parameterizable OFDM processing chain

FIgure 15.17 OFDM for cognitive radio.