White Paper Synopsis
For signal boosters to maintain network transparency, at a minimum, two key elements are essential; 1) maintain stability, and 2) maintain signal fidelity.
In the industry, there have been concerns with the stability of signal boosters both from a design and installation standpoint. It is important to eliminate sources of instability in the design, and have detection capability and algorithms in place to squelch any oscillations that could occur from poor installation. CelLynx designs meet both these requirements. CelLynx is conservative in design, and in continuous monitoring.
The main concerns for design related stability issues are inherent gain stage stability and controlling bi-directional loop gain.
In addition to basic in-band matching, the paper design of each gain stage contains a stability analysis over a wide operating frequency range ensuring the device is stable up to the point where its gain is negligible. This is done for each stage in the amplifier. During prototype testing, these stability results are confirmed. In addition to room temperature, testing includes temperature extremes. This combination of analysis and prototype testing provides great confidence that the inherent amplifier stages are stable.
The loop gain of the amplifier is a concern since the amplifier design is bi-directional, sharing ports on the inputs and outputs. The operation frequencies are separated using duplexers within the signal booster. These duplexers have very high rejection within the operational band of the duplex frequencies. For example, for the Up-link amplifier chain, the duplexer provides very high rejection in the full Down-link band. Due to the characteristics of these duplexers, the operational bands and frequencies above and below these bands are well controlled. The frequency that is most problematic is the 'cross-over' frequency, that is approximately half way between the two operational bands. As shown in the graphic below, this is the area where the combined duplexer filters have the least rejection.
The cross-over performance of a duplexer is characterized in the design phase, and multiple duplexers and band pass (or high and low pass) filters are used to provide the required rejection. During prototyping, the loop gain can be directly measured by removing one component (a blocking capacitor for example) and inserting two RF cables to measure the loop gain directly. This is not practical in production, as it would require additional connectors (cost), and disrupt the standard manufacturing flow. Due to the technological nature of the duplexers, there is manufacturing variation in the performance of the cross-over region, and the design must contain significant margin to ensure high yields in the factory. This is determined from working with the filter suppliers to evaluate electrical variations of multiple manufacturing lots, and distributions within lots. Then, statistical analysis can be performed to determine the optimal number of filters within a design.
Manufacturing provides a controlled environment where external isolation issues (such as improper installation) can be eliminated. This presents an ideal condition to confirm the performance of the cross-over loop gain. A simple test is performed to determine whether the amplifier is capable of full open loop gain in both directions while remaining stable. If any instability is found, gain is reduced, and the firmware is then able to limit the operational gain of this unit to eliminate any chance of a positive loop gain during field operation. If the gain reduction is severe, this unit would be considered a failure, and need to be re-worked or scrapped. This maximum gain parameter will be monitored in production to pro-actively monitor suppliers' filter performance.
In the installation of a booster amplifier, it is important to maximize the isolation between antennas. Isolation can be reduced due to positioning of antennas or due to close proximity objects causing reflections and antenna pattern changes. A poorly designed booster may oscillate under such degradations in external isolation. CelLynx products limit the usable gain to ensure the devices remain stable.
Since the maximum gain of the product occurs in the desired bands of operation, any antenna isolation issue will cause stability problems within these same bands of operation. In this case, the internal detectors will be able to detect oscillations, just as they are able to measure desired signals.
By Continuous Control and Monitoring
When the signal booster begins operation after a power cycle, the gain of the unit is ramped
until a large signal is found, or maximum gain is reached. If a large signal is found, it is
assumed to be an oscillation. This is a very conservative approach, since if it is a real signal,
the full gain potential of the unit will not be reached. From this gain level found (either max
gain, or gain where a power threshold is reached), the operational gain is reduced by a
specified value to ensure stability. This ramp process is performed for each of the
operational bands in the unit.
During normal operation, the output power of each band is continually monitored. A high
value detector reading is possible due to real traffic signals, or from a change in antenna
isolation causing a stability problem. There are two main methods to squelch an oscillation,
absolute power limiting and oscillation detection limiting.
Absolute Power Limiting
If the output power measured is above a predetermined threshold, the gain is continually
reduced until this power level falls below the threshold. Once the threshold is crossed, an
additional gain reduction is introduced for margin. If the output power was due to an
oscillation, this additional gain reduction will squelch the undesired signal.
This method lowers the gain sufficiently to ensure that no oscillation is possible. The
drawback is there is no distinction between a desired traffic signal and an oscillation. To that
end, the gain and output power for a real traffic signal is reduced more than necessary, and
full boost is not available. It would be more desirable to keep the output power close to the
linearity capability of the amplifier chain. With this method, the power level of a real signal is
reduced and the range of booster performance is limited.
- Oscillation Detection
In a similar manner to the above algorithm, if the threshold power is exceeded, the gain of the amplifier is reduced until the threshold is crossed. The difference here is that near threshold, the statistics of the signal are monitored to determine if the signal is a real traffic signal or whether from a potential oscillation.
A real traffic signal will follow the change in gain, thus a 1 dB change in gain should roughly
equal a 1 dB change in power.
If the signal power is due to oscillation, a 1 dB change in gain will result in a large change in
power. If a unit is deep into oscillation (gain exceeds isolation by more than a couple dB),
the output power is near saturation, and a small gain change does not change power significantly. But if the unit is barely in oscillation, the output power is in the linear region (close to the threshold value) and a small change in gain will likely result in no power detected to a power above threshold. For a lightly oscillating unit, at minimum, the change in output power is significantly larger than a 1:1 scale as for a real traffic signal. Once the signal type (real traffic or undesired oscillation) is determined, the unit can apply the appropriate action; automatic power control around the threshold for a real traffic signal, and a gain step reduction to squelch an oscillation.
This algorithm improves the boost level for high power level traffic signals as no additional
gain reduction is required.
In addition to these algorithms, the unit will shut down in the event that the gain can not be
reduced sufficiently to squelch an oscillation.
In the CelLynz 5BARz Road Warrior, both methods of oscillation prevention are utilized.
Power for real signals is limited to ensure oscillations can be detected properly, and if/when
an oscillation is detected, further gain reduction is induced for margin.
- Signal Fidelity
The signal fidelity concept is much more straight forward than stability. Amplifiers, just like phones and base stations, must meet emissions requirements set forth by FCC. In order to maintain output power control, each up and down link channel incorporates a peak power detector to measure power after the last active device. This power level is maintained in the linear region (lower than P-1 dB over temperature). Although the unit is certified by FCC to operate at higher power levels, CelLynx maintains these lower levels for operating margin, as well as ensuring ability to detect oscillations as described above. As an example, CelLynx tested our current Road Warrior amplifier against a competitor's product. The test signal used is a Down-link waveform, 3GPP Test Model 1, with PAR=11 dB. The test was performed on an up-link Cellular channel. This PAR is extreme for Up-link, but not unexpected when two or more phones are amplified by the booster.
The CelLynx booster maintains this power level as input power is increased, and the peak
power level is just below 1 dB compression. The competitor's output power continues to
increase output power by another 2 dB with 5 dB higher input power. The peaks of the
waveform are clipped by at least 5 dB.
The CelLynx booster signal fidelity is excellent, and the output spectrum is well within the
CelLynx designed 5BARz technology to guarantee stability, then added proper firmware to eliminate set-up and environmental problems. CelLynx products substantiate that a cell signal booster can be installed in real-world field applications and remain stable with linear operation. CelLynx products are network compatible by design.