Spectrum Management

Problem Statement

X-COM Systems Solutions for Spectrum Management

Either the X-COM Systems IQC-2110 or the WARP capture and playback systems would be utilized by either the satellite or terrestrial provider. The choice would primarily depend on the desired capture bandwidth. The IQC-2110 utilizes a signal analyzer as the RF front end and down converter and continuously records spectrum up to 110MHz wide, or as wide as the signal analyzer's span if that is the limiting factor. The capture span is centered on the signal analyzer's center frequency and this can be set remotely through the IQC control software. Two version of the WARP product are available and they can directly record (no signal analyzer required) either 1.5GHz or 6GHz of bandwidth. If a suitable down converter is available, the 1.5GHz version of WARP could easily capture an entire Ku Band transponder as long as the kTB noise power level was suitably below the down converted signals.

The fidelity of the captured signal when utilizing the IQC-2110 is only limited by the performance specifications of the RF front end in the signal analyzer used to down convert the span bandwidth prior to quantization. Since the IQC-2110 captures digital I & Q streams produced by the signal analyzer, users of the IQC-2110 will be able to take advantage of the signal analyzer's superior noise floor levels, SFDR, amplitude and phase flatness over the span bandwidth. The benefit is that users can capture and store the resultant I & Q streams with the same fidelity as the signal analyzer.

The time period over which the capture bandwidth is recorded is only dependent on the size of the disk array utilized by either the IQC-2110 or WARP products. In the case of the IQC-2110, with 12TB of memory, 60 hours of capture time is possible at a 10MHz bandwidth and over 5 hours at the full 110 MHz capture bandwidth.

In the transponder monitoring scenario, the X-COM equipment would be located at the network control center. If the WARP product was used, it would be connected directly to one of the station wide-band down converters. In the case of the IQC-2110, it would be connected to the digital I&Q outputs of the signal analyzer. The IQC-2110 is plug compatible with the PXA, MXA and EXA series from Agilent, the RSA 6000A or RSA5000A series from Tektronix and the FSV Series from Rohde & Schwarz. Captured files would be transferred to an X-COM SigAnalyst work station with a comparable amount of RAID5 disk storage for post processing.

Fig. 1 is a typical example of what a satellite operator might see using the X-COM Spectro-X software program to visualize the captured data file. Playback speed through the file can be set by the operator. Here, it has been paused and shows a time slice through the file. The top left plot is phase vs. time. The bottom left is magnitude vs frequency. This particular file was of a portion of a transponder shared by QPSK modulated carriers utilizing an FDMA access scheme. The carriers are only present for the duration of a phone call or data transfer and are randomly assigned frequency slots as they become available. Of interest are the two right hand images. The top one is a spectrogram that is color coded to indicate power vs frequency, scrolling vertically in time. Note the carrier 2 MHz to the right of the 12GHz center frequency. Its occupied BW seems much larger than the two carriers adjacent and higher in frequency even though all carriers are supposed to comply with the same spectral mask. The bottom plot is even more telling. It is a persistence spectrum plot so a ghosting of all carriers that access the transponder, independent of when in time they do so, is visually present. It is clear that one station has not properly band-limited its transmissions and is in violation of the spectral mask. Since in this case, carriers are assigned to the next available frequency slot as they request access, in turn driven by the random nature of telephone calls being placed in time, it would only be a matter of luck to see such an errant carrier without being able to capture long time views of the transponder accesses.

Now that the satellite operator is aware of an errant transmitting station, Spectro-X provides additional tool to investigate further. To specifically identify which earth station is violating the spectral mask, the carrier search feature of Spectro-X was used. Enlarging the lower left plot and using markers, the bandwidth of a "legal" carrier at a power level of -40dBm was measured to be 347 KHz. The bandwidth of the one at 12.002 GHz, at the same power level, was measured to occupy 730 KHz. So, an automated search was set up to look through the entire capture file, which might represent a time span of tens of hours, at the -40dBm power level for all carriers with a bandwidth near or exceeding 500 KHz. This was accomplished by setting the RBW of the search to that bandwidth. (See Fig. 2)

The results show a single occurrence of the "illegal" carrier and several other occurrences at 12.004 GHz. However, by clicking on one of those search results at, for example, 4.1098 MHz, Spectro-X displays a spectrogram (See Fig. 3) exactly at that time point in the captured file. Note the blue coloring just at the 3 MHz relative frequency point. This indicates a point in the frequency domain with power levels down around -50 to -70dBm and so one can conclude that this was just two adjacent legal carriers close enough that the search routine thought they might be a single carrier. The spectrogram provides the operator a quick visual verification that this search result can be discarded.

So, the errant carrier has been captured with an exact start of transmission time of 1.465 mSec from the start time of the capture and with duration of 2.41 mSec. Since the beginning of IQC-2110 capture files are time stamped (UTC), a search of the call request records (part of the network control system logs) at the network control center could identify which earth station requested, and accessed the transponder at that time.

Returning to the case of the terrestrial service provider mapping power levels from a transmission tower, the X-COM equipment would be on a mobile platform, either van or aircraft. In addition to the IQC-2110 or WARP system, the test configuration would include a GPS receiver to provide IRIG-B time stamps and NMEA Lat./Long. geo-tags for each sample in the file. Given the sample rates used (150MS/s for 110MHz of capture bandwidth), this setup would provide a continuous mapping of power and its precision only dependent on where the measurement platform was driven or flown. There would be no time gaps in the geo-referenced data and therefore no possibility of driving through a dead zone without recording data showing drops in signal level as the vehicle or plane entered, traveled through and departed the area of interest.