Hi Thomas.
Do you use TXLPF and which if yes? And also, please let us know other registers dump of TX part, especially TXVGA's. Btw, unfortunatelly, I do not know how to switch LPF otherwise than by direct registers access. Now I work with power consumption and forced to write to registers to see can LPF to suppress spurs (which occur without LDO regulators) or not.
Best regards, Andrey Sviyazov.
2012/10/25 Thomas Tsou ttsou@vt.edu
Hi all,
I spent some time yesterday testing UmTRX with MCBTS. Aside from some trivial fixes (inverted sample rate), the code ran fine with no issues. The default bandwidth of the LPF was insufficient, so that was widened. I tested with 9 handsets between 3 and 7 carriers on a Core2 Duo (P7570) laptop.
git://github.com/ttsou/openbts-multi-arfcn.git umtrx
Configuration is through sqlite database, which is unchanged from mainline; just set the number of ARFCN (7 max) and C1 channels. Because of the bandwidth involved, be sure to increase the maximum buffer sizes for UHD. The uhd_usrp_probe utility will warn if the target size cannot be set.
http://files.ettus.com/uhd_docs/manual/html/transport.html#linux-specific-no...
Design
There are three resampling stages.
- Outer resampler converts from the device rate to a multiple of the
400 kHz channel spacing. 2. Channelizer demultiplexes input stream to M channels 3. Inner resampler converts from 400 kHz to a multiple of the GSM symbol rate.
UmTRX rates:
Channelizer Rate Device Rate Decimation
1.6 Msps 1.625 Msps 8 3.2 Msps 3.250 Msps 4
Options
To broadcast dummy bursts on all carriers for spectrum testing, uncomment the following preprocessor declaration in radioParams.h.
#define ENABLE_ALL_CHANS
With dummy bursts on all carriers:
http://filebox.vt.edu/users/ttsou/http/7carrier_umtrx.PNG
Calibration
Calibration remains a concern. With single carrier, the carrier leakage and and IQ imbalance appear as in-band distortion. For MCBTS, OpenBTS offsets C0 on to the lowest carrier. This makes the carrier and image quite visible, as shown in the following uncalibrated capture. I checked with a swept tone to verify that this issue was not a result of baseband DSP. The USRP also shows similar effects.
http://filebox.vt.edu/users/ttsou/http/carrier_image.PNG
Any idea how much carrier and image suppression we should expect with calibration on UmTRXv2?
Thomas