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diff --git a/README-SFN b/README-SFN new file mode 100644 index 0000000..de4ef7c --- /dev/null +++ b/README-SFN @@ -0,0 +1,127 @@ +README for the SFN patch +======================== + +Concept +------- + +Each modulator receives ETI frames with absolute timestamps. These timestamps +are composed of two parts: +- The TIST field as defined in the ETI standard, giving an offset after the + pulse per second signal; +- A time information transmitted using the MNSC, representing the precise time + when the frame must be transmitted, with one-second resolution. + +When CRC-DABMUX is called with -s, the TIST is defined in each frame. The +Time is always encoded in the MNSC. + +In a SFN setup, it is not possible to rely on a modulator to slow down the +Ensemble multiplexer. CRC-DABMUX takes an additional parameter -r to output one +ETI frame every 24ms. This output parameter only adds a simul:// output. + +The ETI stream must then be made available to several transmission sites. For +this, the eti_tcp server can be used. +The eti_tcp server listens on the port specified on the command line. +An example invocation is: +./mux-throttled.sh -s -m 1 | ./eti_tcp/eti_tcp.py 54001 + +Each modulator then receives the ETI stream through a TCP connection. Each frame +contains the complete timestamp, to which an per-modulator offset is added. +The sum is then given to the USRP. The offset can be specified in two ways: + +using -o delay: +Adds a constant delay specified on the command line (see 1st example). + +using -O delayfile +Adds the delay specified in the given file. The file must contains a single +line containing a floating point number. This file is read each 50 frames, and +the modulator delay is updated. + +In both cases, the units are seconds. + +Example modulator invocation (for a B100 USRP supporting 2048Msps and with a +fixed delay of 2.3 seconds) +nc localhost 54001 | ./moduhd-2048.sh -o 2.3 + +Example modulator invocation (for an USRP requiring resampling and with a delay +specified in a separate file "moddelay") +nc localhost 54001 | ./moduhd-resample.sh -O moddelay -G 5 + +It is also possible to set daugherboard gain using -G. Furthermore, the transmission +frequency is set on the command line, as seen in the examples in moduhd-XXX.sh. + +CRC-DABMOD uses the UHD library to output modulated samples to the USRP device. +When started, it defines the USRP time using the local time and the PPS signal. +It is therefore important to synchronise computer time using NTPd. + +When a frame arrives with a timestamp that is in the past, the frame is dropped. +If the timestamp is too far in the future, the output module waits a short +delay. + +Synchonisation can be verified by using an oscilloscope and a receiver. It is +very easy to see if the null symbols align. + + +Hardware requirements +--------------------- + +The following hardware is required to test the SFN patch to the CRC mmbTools: +- Two USRPs ; +- One or two computers with the mmbTools installed ; +- A network connection between the two computers ; +- A 10MHz refclk source ; +- A 1PPS source synchronised to the 10MHz ; +- An oscilloscope to check synchronisation. + +The lab setup used in the experiments : +http://pic.mpb.li/IMAG0411.jpg + +In this example, one USRP B100 and one USRP2 are used. Modulation is done on two +different computers, but it also works with a single computer. In that case, the +device string for the -u option for CRC-DABMOD must specify the device. (e.g. +-u "type=b100", -u "type=usrp2" or -u "serial=ABC123") + +The HP33120A generator at the bottom is used to generate the 10MHz REFCLK (sine, +2Vpp). It's sync output is used as 10MHz IN for the Agilent 33250A generator. +The Agilent is set to a square wave, 1Hz. Its sync output is then wired to the +PPS input of the USRPs. + +List of changes made +-------------------- +To CRC-DABMUX: +- Throttled output option instanciates a simul output +- Set TIST and use MNSC to transmit time +- Command line arguments handling + +Files: +Modified Eti.h and DabMux.cpp + +To CRC-DABMOD: +- Added multithreaded UHD Output module +- Added normalisation for UHD (we must divide output samples by + 32768.0) +- Command line arguments handling +- Changed the flowgraph to be able to chose between outputs +- Added decoding of time in MNSC +- Added delay control for the modulator using -o and -O +- Initially set the timezone to UTC, because the MNSC is in UTC + +Files: +Added OutputUHDThreaded.{cpp,h}, TimestampDecoder.{cpp,h}; Modified DabMod.cpp, +DabModulator.h, Eti.h, EtiReader.{cpp,h}; Changed some .h files to include +<stdio.h> because of compilation issues; Corrected OutputMemory char* name(). + + +Future elements to be changed +----------------------------- +- Communicate delay and buffer information between multiplexer and modulators, + to have a better control about delays. +- Handle UHD asynchronous messages in another thread, and use this information + for monitoring + +Repository: +http://hg.mpb.li/mmbtools-sfn/ + +2012-07-11 +Matthias P. Braendli +matthias@mpb.li + |