The current 802.22 draft MAC employs the superframe structure as shown below.
At the beginning of every superframe, the BS sends special preamble and SCH
(superframe control header) through each and every TV channel (up to 3 contiguous)
that can be used for communication and that is guaranteed to meet the incumbent
protection requirements. CPEs tuned to any of these channels and who synchronizes
and receives the SCH, are able to obtain all the information it needs to associate
with the BS. During the lifetime of a superframe, multiple MAC frames are transmitted
which may span multiple channels and hence can provide better system capacity, range,
multipath diversity, and data rate. Note, however, that for flexibility purposes the MAC
supports CPEs which are capable of operating on a single or multiple channels. During
each MAC frame the BS has the responsibility to manage the upstream and
downstream direction, which may include ordinary data communication, measurement
activities, coexistence procedures, and so on.
The MAC frame structure is shown below. As we can see, a frame is comprised
of two parts: a downstream (DS) subframe and an upstream (US) subframe. The
boundary between these two segments is adaptive, and so the control of the
downstream and upstream capacity can be easily done. The downstream subframe
consists of only one downstream PHY PDU with possible contention intervals for
coexistence purposes. An upstream subframe consists of contention intervals
scheduled for initialization (e.g., initial ranging), bandwidth request, UCS (Urgent
Coexistence Situation) notification, and possibly coexistence purposes and one
or multiple upstream PHYPDUs, each transmitted from different CPEs.
At the beginning of every superframe, the BS sends special preamble and SCH
(superframe control header) through each and every TV channel (up to 3 contiguous)
that can be used for communication and that is guaranteed to meet the incumbent
protection requirements. CPEs tuned to any of these channels and who synchronizes
and receives the SCH, are able to obtain all the information it needs to associate
with the BS. During the lifetime of a superframe, multiple MAC frames are transmitted
which may span multiple channels and hence can provide better system capacity, range,
multipath diversity, and data rate. Note, however, that for flexibility purposes the MAC
supports CPEs which are capable of operating on a single or multiple channels. During
each MAC frame the BS has the responsibility to manage the upstream and
downstream direction, which may include ordinary data communication, measurement
activities, coexistence procedures, and so on.
 |
| Generalized Superframe |
of two parts: a downstream (DS) subframe and an upstream (US) subframe. The
boundary between these two segments is adaptive, and so the control of the
downstream and upstream capacity can be easily done. The downstream subframe
consists of only one downstream PHY PDU with possible contention intervals for
coexistence purposes. An upstream subframe consists of contention intervals
scheduled for initialization (e.g., initial ranging), bandwidth request, UCS (Urgent
Coexistence Situation) notification, and possibly coexistence purposes and one
or multiple upstream PHYPDUs, each transmitted from different CPEs.
 |
| Time/frequency structure of a MAC frame |
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