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[Discuss-gnuradio] MIMO Trigger (was Re: USRP2 questions)
From: |
Robey, Frank |
Subject: |
[Discuss-gnuradio] MIMO Trigger (was Re: USRP2 questions) |
Date: |
Tue, 25 Nov 2008 15:34:49 -0500 |
> Date: Fri, 14 Nov 2008 11:42:43 -0800
> From: Matt Ettus <address@hidden>
> Subject: Re: [Discuss-gnuradio] Re: USRP2 questions
>
> Matt Ettus wrote:
> I think the confusion here comes from the fact that there are multiple
> ways to get MIMO set up.
>
> There are 3 things necessary to get MIMO working.
> 1 - All systems need to be phase-locked to a common clock (10 MHz in
> this case)
> 2 - All systems need to have a common time reference (often a 1 PPS
> signal) so that they know which sample in one system corresponds to
> which sample in the other
> 3 - All the data needs to come from / go to the same place
(etc.)
Matt,
I would like to suggest that what you have described is necessary but not
sufficient for some (most?) MIMO or multi-channel applications and would like
to describe the other pieces needed for a more complete solution.
For many applications differential phase shift and time delay between channels
that is not observable without known external signals for calibration and that
changes every time you reset or power cycle is unacceptable. To ensure the
phase shift and time delay are stable and repeatable the following additional
steps are necessary. These steps are ordered in increasing channel-to-channel
precision.
A. Output buffers either need to be flushed to start filling when the trigger
occurs, or the next sample after the trigger needs to be marked in some fashion
so that it is identified in the data stream(s).
B. The state of the decimation counters for each USRP2 needs to be either reset
or read when triggered. An alternative is to count the number of 100MHz clocks
before the next internal sample is output to the ethernet buffer. Not
compensating for this could lead to a large time delay variation betweeen
channels depending on the decimation factor. This is translated to a phase
shift at the center frequency. For example, we like to run narrow-band outputs
with a sample rate of 50kHz. This is a decimation of 2,000 relative to the
100MHz clock. With a 30MHz center frequency input each sample is 30/100, or
3/10 of a cycle so a decimation difference of 2,000 clocks (20 microseconds) is
as much as 600 cycles of the RF signals.
C. The phase of the internal numerical oscillator needs to be synchronized
between USRP2 systems. This would normally be through a common, synchronized
reset, but could also be done by reading the phase register for the numerical
oscillators when the trigger/1pps occurs. This could lead to as much as a 360
degree phase variation if not compensated.
D. The 1PPS/trigger input needs to be resynchronized internal to the USRP2 so
that it is stable with respect to 100MHz clock edges. This implies meeting
setup and hold times for the 100MHz clock edges. Since the 100MHz clock edges
are not observable (or are they?) external to the USRP2 the 1pps needs to be
synchronized to the 10MHz reference clock and then internal to the USRP2
resynchronized to the 100MHz such that it is stable with respect to the 10MHz.
It needs to meet 100MHz setup and hold times internal to the FPGA. This could
lead to a channel-to-channel phase variation of 3/10 of a cycle at 30MHz,
sufficient that beams created from the multiple channels are not formed
properly with potentially significant loss in gain and high sidelobes.
A through C could be straightforward through a master synchronized reset or by
including metadata with the data block.
D is not straightforward since it requires an analysis of the timing internal
to the USRP2. The ability to implement either item D or the master
synchronized reset will not be clear until the schematic is published. With a
10MHz external clock input I expect the best we will be able to do for
channel-to-channel coherence is to align triggers to the 10MHz clock edge for
resolution of 100nsec.
Without these synchronizations the apparent phase or time delay will wander
even for a single channel of normally synchronous data from one trigger to
another.
When you are able to make the schematic available I would be happy to help
develop the multi-channel synchronization approach. My preference is your
scenario 3 since it is the most scaleable.
Thanks,
Frank
robey@ll.mit.edu.vcf
Description: robey@ll.mit.edu.vcf
- [Discuss-gnuradio] MIMO Trigger (was Re: USRP2 questions),
Robey, Frank <=