Multicarrier OpenBTS on UmTRX

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-n… Design ===== There are three resampling stages. 1. 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