Desktop Testing of OpenBTS
Here's the hardware you will need for a desktop testing kit:
- A USRP. To our knowledge, OpenBTS has not been used with URSP2.
- A computer running Linux or Mac OS X with a USB-2.0 port and
libusrp installed. We recommend against Dell laptops because their
hardware tends to couple a lot of noise into the USRP via the USB
cables, at least in some models. We have had our best results with
Apple PowerBooks and MacBooks. We have not tested a lot of desktop
machines, but generally have had better success with them than with
desktops. Virtual machines are not likely to work, nor is Cygwin. We
would be interested in reports of what systems tend to work best.
- Two RFX900 daughter cards for GSM 850/900 operation.
The ISM band filter on the RFX900 will need to be removed. A stock USRP
with a 64 MHz crystal oscillator will not function correctly in the
1800/1900 bands, so there is no use for an RFX1800. You can convert
your RFX1800 into an RFX900 with the command
burn-db-eeprom -A --force -t rfx900
- You need two RFX boards to minimize crosstalk between the receive
and transmit sections. When installing the daughter cards in the USRP,
there will be an "A" board (installed on "TXA" and "RXA") and a "B"
board (installed on "TXB" and "RXB").
OpenBTS will transmit on the "A" board, on the
"TX/RX" connector.
OpenBTS will receive on the "B" board, on the
"RX2" connector.
- Antennas with SMA connectors to fit the USRP and matched to
whatever cellular band you intend to use. The VERT900 antenna, also
available from Ettus works fine. Or just do a Google search for
"cellular antenna SMA" and see what pops out. (You actually only need
one antenna for desktop development work.)
- At least two GSM phones. Our preferred testing phone is a DCT-3 series Nokia. We say at least two phones,
but for full-load testing of a single-ARFCN system you need seven.
- SIMs. Every phone needs a SIM. The requirements for the SIMs depend on the
band in which you chose to operate and are discussed below.
- A SIP phone. We've used a PAP2
adapter and we've used the Zoiper softphone applciation.
Need a visual of that? Here's a laptop, USRP and a couple of DCT-3
test phones:
The recommended testing configuration is to operate in a cellular
band not used in your area and use test phones that do
not support your local cellular bands. This arrangement forces your
test phone to camp to your basestation. It also prevents you from
inadvertently attracting phones from the local licensed carriers. For
example, to test in North America, you might use the EGSM900 band or the
DCS1800 band and use a phone that does not support GSM850 or PCS1900.
If you do operate in a band that is also used by
cellular carriers in your area, you will need to use SIMs from a carrier
that does not offer service in you area and has no roaming agreements
with carriers in your area. You can then use network parameters matched
to the SIM and, hopefully, your test phones will ignore the local
network and phones from your local network will ignore you.
If you do not operate in a band that is used by
cellular carriers in your area, you can use any SIM you want.
If you want any flexibility at all in your choice of SIMs, you will
need to use unlocked phones.
Be aware of power levels and local regulations. Here are some
technical guidelines:
- Operate in a cellular band not used by your local
carriers. This will reduce your likelihood of interfering with their
service and will make your own test more controllable.
- Do not use a transmit antenna. Plenty of power
leaks out of the USRP for close-range testing. In fact, using a transmit
antenna causes a lot of transmit power to couple into the receiver,
reducing receiver performance and limiting uplink range. (I realize
that photo above shows two antennas, but you will get better results
without the transmit antenna.)
- Do use a receive antenna. Remember that GSM uses
active uplink power control. The more sensitive your receiver is, the
less power the BTS will need to request from the phone.
- Keep the phones close to the USRP. Again, keeping the distance
short (less than 2m) will allow the BTS to control the phone down to
minimum power.
- Be aware of local radio applications and avoid their spectrum.
This is especially important for air navigation and air traffic control
radar systems. Perform a spectrum survey to know what frequencies are in
use in your area. Check you regulator's licensing database to know who
holds licenses in your area.
- Test indoors to limit propagation.
- Use a test phone with an RSSI display. You should not need
received power levels above -80 dBm for desktop protocol testing.
However, these are only technical pointers to help
you comply with your local regulations. They are not legal advice on
the regulations themselves. That is entirely your responsibility.
How to camp to OpenBTS with arbitrary SIM
To make your phone choose OpenBTS even if other networks try to steal your
connection you should:
- Change your OpenBTS settings to show identity of some NON
local operator, preferably some non-existent one, and start your OpenBTS.
- In your phone's menu find "Network selection" item somewhere in
"Settings" menu. You sill see that it's set to "Automatic".
- Change item value to "Manual". Phone will start searching for
available networks. It will take some time, be patient.
- When a window with available network operators appear, choose your
OpenBTS network. It may hide under some strange
name like "MCC 123 MNC 89" or "USA MNC 234" or with the name which is
different from you set in OpenBTS.config, because phones use internal DB of
MCC/MNC to operator name pairs and at this point just can't sense the
name you set for OpenBTS in its config.
- If you don't see any strange network in the networks list, your OpenBTS setup is sompletely wrong or you just
forgot to turn OpenBTS on. ;)
- From now on this phone will try to camp to OpenBTS ONLY until you change this in network
settings to other manual network or to "Automatic".
PS If you change MCC&MNC in your OpenBTS setup, don't forget to update network
settings in your test phones accordingly.
