Column Control DTX

Identifying and Solving GSM Radio Interference

Case Studies

Goals to Resolve Interference

The first step in the testing process was to come up with a list of potential reasons why the interference is occurring. This is a crucial first step to resolving the problem. We came up with the following list of potential sources:

- Wireless Operator is overpowering (saturating) the input to the public safety’s radios.

- Adjacent Wireless Operator channels in the A”-band are interfering with public safety’s control channel at 868.7625 MHz.

- Wireless Operator channels are mixing in the County’s radios to produce intermodulation products that are on top of public safety’s channels.

- Public safety’s coverage and signal-to-noise (S/N) levels are borderline andare therefore impacted by any RF from nearby wireless operator sites.

In order to fully understand the impact of these potential sources of interference, we will need to add one additional test. The goal of this fifth test is to define a relationship between the composite power level of a Wireless Operator GSM signal and a public safety control channel when both signals are transmitted at the same center frequency.

This information is necessary to understand which intermodulation products pose a problem with public safety. As an example, it may be possible to have a 5th order intermodulation product at 878.7625 MHz (inside the radio) if the composite power is adequately below the composite power level of the county’s control channel which is at the same frequency.

Things get a little trickier when there is an offset between the center frequency of Wireless Operator’s signal and public safety’s signal. In these situations, we need to look at where on the GSM the Wireless Operator overlaps with public safety’s narrow 10 kHz signal.

As a final step we will need to verify our results in the field on actual problems for two different scenarios. First we will need to look at an area where public safety’s power level is high, such as –70 dBm and where it is borderline at around –110 dBm.

Identifying Root Cause(s) of Interference

Public safety allowed the testing to be performed at their facility. The location had strong over-the-air control signals from one of public safety’s towers in the area.

A 3-way splitter was used to inject up to three signals into a Motorola XTS 5000 radio. The Motorola XTS 5000 is a common radio used by the county and it provided a tone when it could not see a control channel from public safety’s RF network.

Two of the three ports of the splitter were connected to the signal generators. These signal generators were used to transmit signals at various center frequencies and amplitudes to mimic the interference from Wireless Operator sites. Signal generator #1 was used to transmit GSM signals whereas signal generator #2 generated constant wave (CW) signals.

The last port of the splitter was dedicated to injecting public safety’s control channel into the radio via the omni antenna from the Motorola radio. As indicated earlier, the isolator was used to prevent over-the-air retransmission of the signals from the signal generators.

A Keysight Handheld FieldFox is shown on top of the signal generators. This handheld test unit was used for various spectrum analysis and return loss measurements.

FieldFox has the capability to make vector network analysis, power meter and spectrum analysis measurements, and is designed for installation and maintenance use. In addition, FieldFox can make network analysis measurements without using a calibration kit, this is very powerful feature for field work.

In general, the front end of a radio is comprised of a duplexer, amplifier, mixer and VCO (voltage controlled oscillator). The duplexer plays the role of splitting both the transmit and receive signals, as well as providing

pre-selection. Transmitting signals are up-converted from an IF frequency to carrier frequency. On the other hand, receive signals are filtered through the duplexer and LNA, down convert to IF frequency via the mixer, then demodulated to get an audio signal.

The channel selection is done in the IF band, so the front end is relatively wide. It is easier to provide channel selectivity on IF frequencies than to design a tunable RF filter in the front end and a wider front end creates its own set of issues.

Return loss measurement

Before beginning the formal testing, we wanted to evaluate the front end of the Motorola XTS 5000 to see what kind of pre-selection it has. We performed a return loss measurement on the antenna port of the radio under test. Figure 4 shows the return loss of the radio we used for the testing.

The measurement shows that the front end of the radio (up to the internal amplifiers) is wide open from 850 to 950 for a return loss of 6 dB. This makes input saturation a real possibility for interference between public safety and carriers with low antenna sites.

Given the close proximity of public safety’s frequencies to carriers utilizing the A” band, the potential for adjacent channel interference is high regardless of the selectivity of the radio’s front end.

This is especially true for carriers transmitting GSM at high power levels. Therefore, adjacent channel interference, receiver intermodulation, and blocking are all potential problems that might arise between public safety and low Wireless Operator antenna sites.

Interference vs. public safety control channel power levels

Before testing for input saturation, adjacent channel power interference, and intermodulation, we needed to understand the relationship between the power levels for interference and public safety channels. In order to perform this test, we utilized the test setup shown in Figure 1 using only one signal generator for this test.

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