GSM base station clock frequency modulation and testing

In the "Wireless Subsystem Synchronization" of GSM 05.10 clause, it is stipulated that: "The base station should use a single frequency source with an absolute accuracy better than 0.05 ppm (5×10-8) as the time reference and RF generation source, and this frequency is also used for all carrier sources of the base station."
The impact of GSM base station clock frequency on communication
The base station system clock signal itself is generated by a high-precision quartz oscillator. Usually, this signal is synchronized with the master clock of the central system. When the master clock synchronization fails, the clock of the base station system itself must also be able to maintain network synchronization. However, even the most stable quartz oscillator will age and decay after a certain period of time or due to other environmental factors (such as temperature, humidity, etc.), until it exceeds the GSM reference frequency accuracy index requirement (5×10-8). Here we give an example, such as the reference frequency of the NOKIA base station clock board is 13MHz. According to the standard, the center frequency deviation of the NOKIA base station clock board shall not exceed ±0.65Hz.
　　
If the center frequency deviation exceeds ±0.65 Hz, it will affect the operation of other parts of the base station, especially the carrier frequency deviation, which can easily cause the user's call quality to deteriorate, or even cause single-talk, dropped calls and other faults. Therefore, after the base station has been working for a period of time, the reference clock of the clock board must be tested, and if there is any deviation, it must be adjusted in time. The GSM maintenance regulations stipulate that the base station clock frequency needs to be calibrated once every six months. Many mobile operators attach great importance to network optimization, but they all focus on the distribution of base stations and the adjustment of antenna pitch angles, while ignoring the calibration of base station clocks or ignoring the performance requirements of the frequency meter used to calibrate the base station clock. This is an important reason for the unsatisfactory network optimization.
The difference between quartz crystal time base and rubidium time base technology
To determine whether the reference clock of the clock board is offset, a frequency meter must be used for testing. There are many frequency meters on the market, but not many can meet the requirements of base station frequency testing. First, let's calculate the accuracy requirements of the frequency meter used in GSM base station clock calibration:
　　
1. According to ESTI regulations, the maximum allowable deviation of the GSM reference clock is 0.05 ppm (5×10-8);
　　
2. The influence of time and other environmental factors should allow for 3 to 6 times the basic accuracy, and the maximum adjustment deviation after calibration will be 5×10-8 (3…6 = 1.5…0.75×10-8) (i.e. the allowable deviation between two calibrations).
　　
3. According to the measurement standard: TUR>4 times, that is, the accuracy of the calibration instrument must be 4 times higher than the accuracy of the measured device, which means that the maximum error of the frequency meter should be 1.5…0.75×10-8 (4 = 4…2×10-9). In order to achieve the accuracy requirement of 4×10-9,
　　
the ordinary quartz crystal oscillator time base should be calibrated once every 10 days and the internal battery should be used to maintain continuous operation for a long time. Under normal circumstances, it can only achieve an accuracy of 10-8. Even such accuracy requirements need to be calibrated once a month and warmed up for more than 30 minutes. The actual test time for the GSM base station clock is less than 1 minute. In the past, because there was no more convenient and accurate clock frequency meter, ordinary quartz crystal oscillator frequency meter had to be used, which actually could not meet the requirements of on-site testing. Because for most engineers, after arriving at the site and turning on the power of the instrument, it takes half an hour or more to start testing, and the instrument needs to be sent for inspection every month, which is very inconvenient.
Advantages of Rubidium Time-Based Timing and Frequency Counters
The fastest and most accurate frequency meter currently uses HJ5414 Rubidium Atomic Frequency Standard as the time base, and can be used as a frequency meter for GSM base station reference clock frequency testing with CNT-90. It does not require battery maintenance or frequent calibration, so it can meet the calibration requirements of GSM reference in a wide range.
  
The CNT-90 time/frequency counter/analyzer newly launched by Swedish Pendulum Company has triggered a technological revolution:
*** Faster measurement speed and higher resolution
*** Multi-parameter and histogram, trend chart and modulation graphic display
*** Intelligent operation, easy to learn and use
　
CNT-90 provides users with unimaginable technical features at the price of ordinary counters. It has become an indispensable and powerful tool for R/D laboratories, calibration laboratories and industrial sites in the fields of frequency, time interval, phase measurement, calibration and modulation, domain analysis, etc.
Main functions and features:
1. Ultra-high resolution: frequency measurement--12 bits/second; time interval measurement--100ps; phase--0.001 degrees.
2. Ultra-fast measurement rate: 250,000 times/second----internal memory 2000 times/second----GPIB interface
3. Ultra-long digit display: 14 bits
4. Ultra-wide frequency range: standard configuration: 0.001Hz--300MHz; optional expansion to: 3, 8, 14GHz
5. Super multi-function display: multi-parameter digital display and histogram, trend chart, modulation graphic display
6. Super friendly interface: full-function GPIB and USB interface
7. When using the rubidium clock internal time base option, the annual aging rate is less than 2×10-10
8. When using the GPIB interface, the coding is fully compatible with the Agilent 53131 series
9. Added 20 GHz extended frequency option.
HJ5414 rubidium atomic frequency standard is a portable, economical, high-precision rubidium atomic frequency standard product. It adopts imported high-quality rubidium oscillator and precision frequency control circuit design, with the advantages of fast locking and rich output signals, and can provide high-precision frequency standard signals for measurement, communication, national defense and other departments. If the GPS receiver option is added, the HJ5414 rubidium oscillator can be automatically tamed on the GPS system, and the accuracy and long-term stability of the output frequency signal are greatly improved. HJ5414 supports external 1pps signal synchronization to trigger the 1pps frequency division signal of this clock, with excellent synchronization accuracy and timekeeping performance. HJ5414B rubidium atomic frequency standard outputs high-precision square wave signal, which is a cost-effective reference signal source to improve the test accuracy of instruments such as frequency counters.
Application field: secondary clock, communication clock
Features     : economical, portable, 1pps external synchronization