<s>
One	O
of	O
the	O
key	O
challenges	O
of	O
the	O
cognitive	B-Protocol
radio	I-Protocol
based	O
wireless	B-General_Concept
networks	I-General_Concept
,	O
such	O
as	O
IEEE	O
802.22	O
wireless	O
regional	O
area	O
networks	O
(	O
WRAN	O
)	O
,	O
is	O
to	O
address	O
two	O
apparently	O
conflicting	O
requirements	O
:	O
assuring	O
Quality	O
of	O
Services	O
(	O
QoS	O
)	O
satisfaction	O
for	O
services	O
provided	O
by	O
the	O
cognitive	B-Protocol
radio	I-Protocol
networks	I-Protocol
,	O
while	O
providing	O
reliable	O
spectrum	O
sensing	O
for	O
guaranteeing	O
licensed	O
user	O
protection	O
.	O
</s>
<s>
Such	O
periodic	O
interruption	O
of	O
data	O
transmission	O
could	O
impair	O
the	O
QoS	O
of	O
cognitive	B-Protocol
radio	I-Protocol
systems	O
.	O
</s>
<s>
This	O
issue	O
is	O
addressed	O
by	O
an	O
alternative	O
operation	O
mode	O
proposed	O
in	O
IEEE	O
802.22	O
called	O
Dynamic	B-General_Concept
Frequency	I-General_Concept
Hopping	I-General_Concept
(	O
DFH	O
)	O
where	O
data	O
transmission	O
of	O
the	O
WRAN	O
systems	O
are	O
performed	O
in	O
parallel	O
with	O
spectrum	O
sensing	O
without	O
any	O
interruption	O
.	O
</s>
