Field test case and data analysis of CDMA interference to GSM network

The operating frequency band of the A-band IS95A-CDMA is: uplink: 825-835MHz; downlink: 860-880MHz. The operating frequency band of GSM is: uplink: 890-908MHz (mobile), 909-915MHz (unicom); downlink: 935-960MHz.

From the operating frequency band, the downlink frequency band of CDMA is close to the uplink frequency band of GSM. If it is not properly done in the site selection and network planning, it will inevitably cause interference to GSM, resulting in a decline in the receiving performance of the GSM system (interference is mutual However, since the GSM transmission band is far from the CDMA receiving band, and CDMA is a self-spreading communication system, the anti-interference performance is good, so the interference caused by GSM to the CDMA system can be ignored). At its root, the most direct cause of CDMA's external interference is that the filter characteristics of the filter in the hardware are not ideal, resulting in a certain amount of out-of-band radiation. This paper will analyze the interference of CDMA to GSM network and give engineering reference data.

The main interference of CDMA on the GSM system is:

(1) In-band interference: The CDMA transmission signal acts indirectly as an in-band noise on the GSM receiver directly or through intermodulation, resulting in a decrease in sensitivity of the GSM receiver. This type of interference is further divided into transmitting spurious interference and intermodulation interference.

(2) Out-of-band interference: When the out-of-band interference is strong to a certain extent, the receiver will be saturated and blocked, which will affect the receiving performance of the GSM system. This type of interference is also called blocking interference.

• Spurious interference: Due to the roll-off characteristics of the transmit filter (any filter cannot be an ideal step), there is always some out-of-band radiation in the CDMA system, which is what we usually call the transmit spur. The transmit spur near 890M falls into the receiving band of the GSM system, which is bound to introduce another kind of interference for the GSM system. When the level of the interference signal exceeds the receiving sensitivity of the GSM system, it will cause its reception. The sensitivity decreases, resulting in a decrease in the signal-to-noise ratio of the GSM system and a decrease in the QoS index.

● Blocking interference: Any receiver has a certain receiving dynamic range. When the receiving power exceeds the maximum power level allowed by the receiving dynamics, the receiver will be saturated. Blocking can cause the receiver to malfunction, and long-term blocking can also cause permanent performance degradation of the receiver.

Intermodulation interference: When multiple signals of different frequencies are applied to the nonlinear device, the nonlinear transformation will generate many combined frequency signals, some of which may fall into the receiver band and become interference to the wanted signal. Intermodulation interference.

The A-band CDMA system has a transmission band of 870-880 MHz, and the third-order intermodulation product of the CDMA signal is located between 860-890 MHz (2f1-f2~2f2-f1, f1=870 MHz, f2=880 MHz). Outside the GSM receive band, high order intermodulation signals can be ignored due to their greater attenuation. Therefore, it is not considered in the following quantitative analysis of interference.

Among the three kinds of interference, spurious interference is the most important, and the impact is also the biggest. The following mainly analyzes the spurious interference.

(1) Stray interference calculation

The quantitative calculation model is shown in Figure 1. Assuming that the CDMA transmitter output power is Pcdma and the effective interference power level received at the GSM receiver is Pgsm, then: Pgsm-Pcdma=Gantcdma-Acdmafilter-Aair+Gantgsm-Afeed+Aconv.

Figure 1 Quantitative calculation model

Where, Gantcdma: CDMA antenna gain, 17.1 dBi; Acdmafilter: CDAM transmit filter attenuation; Aair: free space attenuation; Gangtsm: GSM antenna gain, 15.5 dBi; Afeed: feeder attenuation, GSM and CDMA feeder attenuation are no more than 3 dB A total of 6dB; Aconv: bandwidth conversion factor, CDMA carrier bandwidth of 1.25MHz, and GSM carrier bandwidth of 200kHz, CDMA transmit power only partially falls into the GSM band, bandwidth conversion factor is 10lg (WinVterfered/Winterfering) = 10lg (200k/1.25M ) = -8dB.

The following is a special analysis of the CDAM transmit filter attenuation: CDMA single carrier transmission according to the protocol requires that the spurious suppression should be greater than 60 dB when the off-center frequency is greater than 2 MHz.

The filtering characteristics of the external bandpass filter for multi-carrier are not specified in the protocol. Refer to the typical product in Figure 2 (at 890MHz, the bandpass filter is attenuated to 40dB). We can get the CDMA transmit filter. The total attenuation is 100dB, so: Pgsm-Pcdma=38.6-Aair; if GSM reception performance is to be protected from CDMA transmission spurs, Pgsm is required to meet the interference tolerance limit (ie Pgsm<GSM reception sensitivity -9dB) The interference is endured by redundancy =, GSM receiving sensitivity should be better than -105dB, CDMA base station maximum transmitting power is 43dBm, so: Aair>75.6dB; when calculating spatial isolation distance requirement: VERTICAL: I=28+40lg(Dv/λ) ;HORIZONTAL: I=22+20lg(Dh/λ)-(GCDMA+GGSM). Dv/Dh: antenna vertical/horizontal spatial isolation distance requirement; λ: wavelength (m); GCDMA: CDMA antenna gain; GGSM: GSM antenna Gain. It should be noted that the isolation in this formula considers the antenna gain and is the result of free space, so: I=Aair-(GCDMA+GGSM)>43dB; thus, it can be calculated: Dv=1m; Dh =162m. When CDMA and GSM sites reach the distance Said requirements, interference requirements may be met.

Figure 2 Typical product schematic

(2) Blocking interference

The blocking interference is related to the GSM receiver's out-of-band rejection capability. It involves the carrier transmission power of CDMA, the receiver filter characteristics, etc., and the receiver of the GSM system will be affected due to saturation and cannot work. The receiver's anti-band blocking indicator is specified in the GSM specification: -100kHz<f<860MHz<8dBm.

The maximum transmit power of the CDMA transmitter is 43dBm, but it should be noted that since the frequency is located in the CDMA band and the GSM band is out, the Acdmafilter=0, the GSM receive filter suppression should be considered (the GSM reception suppression at 860MHz is 40dB[# ]), namely: Pgsm-Pcdma=Gantcdma-Aair+Gantgsm-Agsmfilter-Afeed-Aconv; where: Pcdma=43dBm, Agsmfilter=40dB, Pgsm requirement is less than 8dBm, other parameters refer to the previous description, so Aair>15.6dB. When the anti-stray interference requirements are met, the anti-blocking interference requirements are met.

(1) Test instrument

CDMA interference was tested with Agilent's E7475A digital spectrum analyzer. This spectrum analyzer has a good noise floor and test speed, and has good portability, so it can easily find all kinds of uplink interference.

(2) Test method

The most straightforward way to test interference is to connect the receiving antenna of the GSM base station to the E7475A digital spectrum analyzer. Set the spectrum analyzer's test range to 876-915M and the resolution filter to 10k.

(3) Actual case

The author and the Shanxi Mobile Communication Company test group conducted a CDMA interference test on some GSM base stations in Changzhi City. Now, the third sector of the Shift base station is taken as an example for analysis.

As can be seen from the results of the test, the carrier frequency of Unicom CDMA is between -80dbm and 90dbm, and the power level outside the carrier frequency band (about 880MHz) has been attenuated to below -120dbm, and is relatively stable. . The burst on the right is the GSM handset signal. From the test results, CDMA has not been found to cause interference to the mobile company's GSM system.

Figure 3 actual case

Through the above analysis and actual test cases, the following suggestions are proposed for reference:

(1) It is necessary to find out the specific causes of interference through a large number of wireless measurements and technical tests, and take targeted measures;

(2) The external band-pass filter with good waveform roll-off must be installed at the transmitting end of Unicom CDMA base station to ensure better out-of-band radiation interference;

(3) Unicom CDMA transmitting antenna and GSM receiving antenna ensure sufficient spatial isolation (horizontal or vertical) to prevent in-band interference, and the specific data is available from the above calculation;

(4) reducing the transmission power of the Unicom CDMA base station;

(5) Adjusting the tilt or horizontal direction angle of the Unicom CDMA transmitting antenna, and the transmitting antenna cannot be facing the receiving antenna of the nearby GSM base station;

(6) A band rejection filter is added to the GSM receiver to reduce blocking interference of the GSM receiver.