Analysis of main interference types when WCDMA and GSM systems are co-sited

Firstly, the main interference types of WCDMA and GSM system co-sites are analyzed. The mathematical calculation models of various interferences are given. Then the interference between WCDMA and GSM systems is elaborated, and WCDMA and GSM are obtained. The isolation and antenna isolation requirements required for the site, and the solution in the project

With the opening of China's telecom market, the date of 3G license issuance is gradually approaching. For GSM network operators, WCDMA network construction is a systematic project. The project involves a wide range, long cycle and large investment. Operating costs, start the market as soon as possible, and the base station should carry out joint site construction if the conditions are met. This will inevitably increase the chance of interference between the WCDMA system and the co-located or adjacent GSM system. The electromagnetic environment compatibility between the WCDMA system and the GSM system will be exposed. This paper will separately analyze the mechanism and isolation calculation of interference generated by the common station, and propose a solution to eliminate interference in the project.

There are three performance losses to be considered for the victim system: reduced receiver sensitivity, IMP interference (ie, intermodulation interference), and receiver overload. The spurious emission signal received from the interfering station will cause the receiver sensitivity to decrease, and the synthesis of all carrier frequencies received from the co-located station causes IMP interference. The receiver is overloaded because the total signal power received by the receiver is too large. . In order to minimize these performance losses without modifying existing transmit and receive units, proper isolation between the GSM and WCDMA systems at the same site is required.

The main interferences corresponding to these three performance losses are spurious interference, intermodulation interference and blocking interference. Below we illustrate the mathematical models of these three kinds of interference. The principle of mutual interference between two co-located RF stations is shown in Figure 1:

Important RF devices related to mutual interference calculation between two co-located stations, including the transmit amplifier, transmit filter, transmit antenna, and receive filter, receiver, and receive antenna of the interfered station. Here, the difference between the RF levels from point A to point B is defined as the antenna isolation.

2.1 Spurious interference

The decrease in receiver sensitivity is due to an increase in the noise floor of the receiver. If the interfering base station has strong spurious emissions in the receiving band of the interfered base station, and the transmitting filter of the interfering base station does not provide sufficient out-of-band attenuation (the filter's cut-off characteristics are not good), the receiver noise threshold will increase. . The stray radiation output from the antenna connection of the interfering base station is attenuated by a certain isolation between the two base stations, so the spurious interference received at the antenna connection of the interfering base station is calculated according to the following formula:

IB=CTX-E Hysteresis-10log(WA/WB) (1)

Where IB is the interference level received at the antenna connection of the interfered base station; CTX is the spurious radiation level outputted by the interfering base station antenna connection; E is the antenna isolation; WA is the measurable bandwidth of the interference level; WB is the channel bandwidth of the interfered system.

2.2 Intermodulation interference

Intermodulation interference is caused by the nonlinearity of the system, and the intermodulation products generated by the synthesis of multiple carrier frequencies fall to the uplink frequency band of the adjacent WCDMA system, which reduces the signal-to-noise ratio of the receiver, mainly as the signal-to-noise ratio degradation and service quality of the WCDMA system. deterioration. The third-order intermodulation interference generated by two carriers of the same strength can be expressed as follows:

IMP3(dBm)=3PIN-2&TImes;TOI (2)

The PIN is the interfering carrier level at the input of the victim base station receiver; the TOI is the third-order cut-off point (dBm) defined at the receiver input, which is related to the characteristics of the receiver itself. Therefore, in order to minimize third-order intermodulation interference, the PIN should be lowered, and according to equation (3):

PIN=CA-EIMP3-LR_B (3)

Where CA is the maximum carrier transmit power (dBm) at the interfering base station antenna connection; LR_B is the attenuation (dB) of the receive filter of the interfered base station within the transmit bandwidth of the interfering base station; EIMP3 is the antenna isolation (dB).

Therefore, when the allowed third-order intermodulation interference is constant, the antenna isolation is determined by:

EIMP3=CA-LR_B-(IMP3+2&TImes;TOL)/3 (4)

2.3 Blocking interference

When a stronger power is applied to the receiver, it may cause the receiver to be overloaded, causing its gain to drop. The reason is that the amplifier has a linear dynamic range in which the output power of the amplifier increases linearly with the input power, and the ratio of the two powers is the power gain G. As the input power continues to increase, the amplifier enters the non-linear region and its output power no longer increases linearly with increasing input power, that is, its output power is lower than the expected value. The output power value, which is typically reduced to 1 dB below the linear gain, is defined as the 1 dB compression point of the output power, where the input power is defined as the 1 dB compression point of the input power. To prevent receiver overload, the total carrier power level received from the interfering base station needs to be below its 1 dB compression point. There are the following requirements for antenna isolation:

E blocking = CP_A-LR_B-CP_B (5)

CP_A: the total carrier power (dBm) at the interface of the interfering base station antenna;

LR_B: attenuation (dB) of the receive filter of the interfered base station within the transmit bandwidth of the interfering base station;

CP_B: the total carrier power (dBm) received at the antenna connection of the interfered base station;

E Barrier: Antenna isolation (dB).

In general, the most serious of the three types of interference is spurious interference, as long as spurious interference can be avoided, blocking interference and intermodulation interference can generally be avoided.

To ensure good system performance, the above three performance degradations must be avoided or minimized. Therefore, it is necessary to ensure good isolation between the antennas of the two co-located base stations. In general, the following three criteria should be observed in engineering for the above three types of interference:

(1) The intensity of the spurious emission signal received by the interfered base station from the interfering base station should be 10 dB lower than its receiving noise floor.

Assuming that the received noise floor of the interfered base station is NB (dBm), the noise generated by the spurious radiation of the interfering base station at the receiver of the interfered base station is NI (dBm), and the noise and spurs of the interfered base station itself The noise power accumulated noise power introduced by the interference is:

Ptotal=PB-PI=10NB/10+10NI/10 (6)

When NI = NB-10dB, the amount of noise degradation introduced by the interfered base station is:

Such noise degradation does not have a significant impact on the base station, so the spurious emission signal strength should be 10 dB lower than its receive noise floor.

(2) The third-order intermodulation interference (IMP3) level generated by the interfered base station should be 10 dB lower than the receiver noise limit for the same reason as the first criterion.

(3) The total carrier power received by the interfered station from the interfering station should be 5 dB lower than the receiver's 1dB compression point. This is mainly because the engineering point is often returned from the 1dB compression point in order to avoid the amplifier operating in the nonlinear region. Retreat 5dB.

If the isolation between systems can meet the above criteria, the sensitivity of the receiver of the interfered system will only drop by about 0.5 dB, which is acceptable for most communication systems.

In summary, the final cause of interference is highly dependent on the antenna isolation between the co-sites. In order to minimize performance loss without modifying existing transmit and receive units, proper isolation needs to be maintained between the co-sites.

The main frequency bands of WCDMA and the frequency bands of mobile existing GSM networks are shown in Table 1:

It can be seen from Table 1 that if the GSM and WCDMA stations are constructed, the GSM900 system is far away from the WCDMA band, and there is no intermodulation interference between the systems, as long as the base station meets the out-of-band spurious emission requirements for the co-station in the R99 protocol: <-96dBm/100kHz can be. At present, the performance of GSM900 base stations in most existing networks is satisfactory and superior to the common station requirements in the R99 protocol. The requirements for spatial isolation in engineering are very low, so this article will not discuss them in detail.

For the GSM1800 system, the transmission band is closer to the receiving band of the WCDMA band. The WCDMA of the two systems is the uplink frequency, the GSM1800 is the downlink frequency, and the downlink power is relatively large. The out-of-band spurious signals of the GSM1800 base station transmission channel are very It is easy to fall in the receiving channel of the WCDMA base station, which will raise the level of receiving noise of the WCDMA base station, make the uplink of the WCDMA system worse, reduce the sensitivity, and affect the network coverage. In addition, signal overload or intermodulation interference will also lead to system performance. Decline. So the problem is mainly concentrated on the interference between GSM1800 and WCDMA.

4.1 Impact of GSM1800 on WCDMA

(1) Stray interference

Noise floor of WCDMA receiver:

Nfloor(dBm)=NO(dBm/Hz)+W(dBHz)+NF(dB)

In the above formula: NO: The noise spectral density is due to the thermal motion of electrons, and the calculation formula is: NO=KT.

K is the Boltzmann constant (equal to 1.38 & TImes; 10-23J/K), and T is the absolute temperature (290K). Since J=W&TImes;s, 1W=1000mW=30dBm, convert KT to dBm to get:

NO=KT=10log(1.38×10-23×290)+30dBm×s=-174dBm×s

W: The bandwidth of the WCDMA system, which has a value of 3.84 MHz, that is, 10 log (3840 kHz) = 65.8 dBHz.

NF: The noise figure of a WCDMA receiver used to measure the extent to which the SNR is reduced after the signal passes through the receiver. The noise figure is an attribute of the receiver itself. The noise figure of the WCDMA base station receiver is about 4 dB.

Therefore, the noise floor of the WCDMA base station receiver:

Nfloor(dBm)=-174+65.8+4=-104dBm

There are two versions of the GSM technical specifications, and their requirements for working out-of-band spurs are shown in Table 2: