cell phone harmonics in lbw while xmiting from inside the dome.

    People have been asking how cell phones transmissions affect observations done on the telescope. To get an unambiguous signal,  cell phone transmissions were done from inside the dome (on the stairwell leading into the turret floor).

Setup:

• The telescope was parked at az=279, za=10.
• lband wide was set to linear polarization (no hybrid).
• Three 50 Mhz bands centered at 1625, 1675, and 1725 Mhz were sampled once a second using the interim correlator. 2048 channels gave 48Khz resolution (after hanning smoothing).
• The first attempt at connecting from inside the turret room (with the doors closed) failed because of no signal. Stepping out onto the stairwell worked ok.
• The cell phone used was from cingular.
• Data was taken for 440 seconds. During this time there were two separate transmission cycles (turn on, connect, talk, turn off).

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Data processing:

• The median of the first 90 seconds was computed
• corinterprfi() was used to interpolate across rfi in the median bandpass.
• Each 1 second spectra was divided by this band pass correction.
• Channels 1200 to 1300 (which were free of rfi) were averaged giving 440 samples. These values were subtracted from the normalized data. This was done to remove 2 continuum sources that drifted through the beam.

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Plotting the data:

• dynamic spectra of 3 50 MHz bands (.gif)
• This shows the 3 50 Mhz bands: (1600=1650 Top), (1650-1700 middle), and (1700-1750 bottom). The cell phone transmission rfi occurs in the center image.  You can also see glonass at 1609, and iridium at 1620 to 1628 Mhz.
• dynamic spectra of the 1650-1700 Mhz band (.gif)
• The cell phone  transmissions are from (110 to 185 seconds) and (290 to 390 seconds).
• There are two frequency bands:
• 1653.4 to 1667.4  containing  35 400 Khz channels
• 1690.3 to 1692.6  containing   6 400 Khz channels.
• Individual spectra plotted (.ps) (.pdf):
• Fig 1. These are the average band passes during the cell phone transmissions. The top is transmit1 while the bottom is transmit 2. Black is pol A, red is pol B.
• Fig 2. The average band passes have been normalized. The y axis units are now tsys. You can see the frequency bins  of the transmitted signals.
• Fig 3. The 1 second spectra are over plotted for the 1653 to 1667 band of transmit 1. Color differentiates each 1 second sample. The bottom over plots 10 of these 1 second spectra with offsets for plotting. You can see the time variation of the transmissions.

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Discussion:

• 824-849 uplink mobile to base station
• 869-894 downlink base station to mobile
• 1850-1900 uplink
• 1930-1990 downlink

• 826.7 to 833.7
• 845.15 to 846.3

    The frequency bin width of 400 Khz at the second harmonic becomes 200 kHz at the fundamental. This is the channel width of the GSM format. The range 826 to 833 must be the  frequency channels allocated to the tower that the phone was talking to. The 6 channels at 845 to 846 look like the 5 common control channels that are used for control.  Gsm does a frequency hop as well as tdma. Some  of the time variation in a channel is from that.  The output power is also regulated depending on  how much talking is being done.

    The individual 1 second spectra show max levels of 1.5 times Tsys (about 25K*1.5). The peak value is larger than this since the device does not transmit continuously in 1 freq channel.

    These harmonics lie on top of the 1665/1667 OH line. Next thing is to see if we can see any  harmonics when transmitting from the visitor center.

    You can see more gsm info here.