802.11n receiver operation

Hi...
I don't know if anyone here knows how 802.11n operates in the frequency
bands...

As you may know, a transmitter can send 20 MHz signals or 40 MHz
signals. In the 20 MHz case, the actual frequency band used can be with
offset 0, +10 MHz or -10 MHz (offset with respect to carrier frequency).
The offsets for 20 MHz are thus NONE, UPPER or LOWER:


Code:
--------------------

20MHz, no offset
___________
| |
-----|-----|-----|-----|-----|----------
-20 0 +20
___________
| |
----|-----------|--UPPER----|----------
-20 0 +20

__________
| |
----|--LOWER---|----------|----------
-20 0 +20

--------------------

In the case of 40 MHz the full band is used (with duplicated UPPER and
LOWER possible).

My question, is this: how can the receiver detect which band is used,
when serveral AP's may be operating in adjacent channels? The only
information transmitted from the TX to the RX concerning the BW is a
20/40MHz BW bit in the HT-SIG field.

Must the RX scan the frequency axis over a BW of 20 MHz every 5 MHz?


Code:
--------------------

___________
| | | start of scan at '0' MHz (relative)
-----|-----|-----|--------------------------------------------------
0
___________
| | | second scan at +5 MHz
--------|-----|-----|--------------------------------------------------
5
___________
| | | third scan at +10 MHz
-----------|-----|-----|--------------------------------------------------
10

--------------------

Assuming that it finds a valid 20MHz signal, I guess the SIG field must
be decoded first to see if it is a 20MHz signal or 40MHz signal; in the
latter case
it has to process the following 20 MHz too (the previous 20MHz was
already determined to have no valid signal)...

Is this correct, or what do you guys think?

Niko


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