11 Sep
2020
11 Sep
'20
12:50 p.m.
Hi Thomas,
it's been some time, hope you're doing fine!
Today on IRC we were collectively wondering about one detail in the osmo-trx
mcbts implementation that has been there from day one:
Your're creating the Channelizer and Synthesis class for four channels of 800kHz
within the 3.2Mbps wideband-channel. So far so good.
But why does the code ever only use up to three of them?
And why is there a specific re-ordering, see radioInterfaceMulti.cpp in
getLogicalChan() ?
It would be great if you could share your thoughts on that.
To me, it looks like the constraint to 3 channels is arbitrary and we should just
as well be able to use all four?
Thanks,
Harald
--
- Harald Welte <laforge(a)osmocom.org> http://laforge.gnumonks.org/
============================================================================
"Privacy in residential applications is a desirable marketing option."
(ETSI EN 300 175-7 Ch. A6)
11 Sep
11 Sep
10:35 p.m.
Hi Tom,
sorry for the noise.
On Fri, Sep 11, 2020 at 12:50:02PM +0200, Harald Welte wrote:
But why does the code ever only use up to three of
them?
tnt has meanwhile solved that mystery (due to the inherent property of
a polyphase filter bank)
And why is there a specific re-ordering, see
radioInterfaceMulti.cpp in
getLogicalChan() ?
We suspect it is to have a monotonically increasing frequency from logical
transceivers 0, 1, 2.
Regards,
Harald
--
- Harald Welte <laforge(a)osmocom.org> http://laforge.gnumonks.org/
============================================================================
"Privacy in residential applications is a desirable marketing option."
(ETSI EN 300 175-7 Ch. A6)
12 Sep
12 Sep
4:23 a.m.
Hi Harald,
Indeed, it's been awhile since I've looked at that code!
Correct. The mapping itself is arbitrary. Consecutive numbering was
one simple approach.
I'll also add that one of the most difficult aspects of the
multi-carrier implementation had little to do with software. B210 was
the target device when the code was developed; the on board AD9361
RFIC was never recommended by ADI for multi-carrier operation. The DC
and aliasing effects were quite visible when using shifted carriers in
addition to the reduction in dynamic range. Conformance tests for
spectrum mask were challenging.
-Tom
On Fri, Sep 11, 2020 at 12:50:02PM +0200, Harald Welte
wrote:
Yes, among the physical channels there will be one centered at
baseband and one aliased across the Nyquist boundary - the latter is
unusable. Wider bandwidth / higher channel count was possible though
limited by practicality (configuration was difficult) and RF effects
(see below).
But why does the code ever only use up to three
of them?
tnt has meanwhile solved that mystery (due to the inherent property of
a polyphase filter bank) And why is
there a specific re-ordering, see radioInterfaceMulti.cpp in
getLogicalChan() ?
We suspect it is to have a monotonically increasing frequency from logical
transceivers 0, 1, 2.
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2 comments
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participants (2)
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Harald Welte -
Tom Tsou