The way of PIM3 to -150dBc

Transcription

1 The way of PIM3 to -150dBc 1. What is PIM? PIM is a form of passive inter-modulation distortion thatis an undesired, non-linear, signal energy generated as a bi-product of two or more carriers sharing the same down-link path in wireless networks. Due to network hardware configurations, this multi-carrier interaction can cause significant interference in the up-link receive band, which can lead to reduced receiver sensitivity. To the mobile phone user, this often translates to a loss in audio fidelity in conversations, decreased data speeds, or, in extreme circumstances, dropped calls or an inability to make or receive calls or utilize data services. An on-site PIM test is a comprehensive measure of linearity and construction quality. PIM shows up as a set of unwanted signals created by the mixing of two or more strong RF signals in a non-linear device, such as in a loose or corroded connector, or in nearby rust. Other names for PIM include the diode effect and the rusty bolt effect. This pair of formulas can predict PIM frequencies for two carriers: nf1 mf2 nf2 mf1 F1 and F2 are carrier frequencies and the constants n and m are positive integers. When referring to PIM products, the sum of n + m is called the product order, so if m is 2 and n is 1, the result (2+1=3) is referred to as a third order product. Typically, the third order product is the strongest, causing the most harm, followed by the fifth and seventh order products, which also cause significant harm. Because PIM amplitude becomes lower as the order increases, higher order products typically are not strong enough to cause direct frequency problems, but they usually assist in raising the adjacent noise floor. Once this raised noise floor crosses into the Rx band, it then has an open door (and sometimes gain via an LNA) into the BTS. A standard PIM test will almost always test for the 3rd order because it is almost always the most predictable. 2. What causes PIM? Ferromagnetic materials, when in the current path, exhibit a non-linear voltage-to-current ratio. This non-linear effect is accentuated at higher power levels because of increased current density. Looking at Ohm s law from the perspective of power helps clarify the fact that the squaring effect of current results in a higher magnetic flux, which makes metals with high bulk resistivity, such as, iron, steel and nickel exhibit a magnet-like memory effect. This effect is better known as magnetic hystereses. Metals that exhibit this asymmetrical magnetic flux are often the main contributor of PIM energy. Poor metal-to-metal contact junctions can create additional nonlinearities resulting in

2 PIM. Such nonlinearities can come from under-torqued male-to-female DIN 7/16 mates, as well as irregular contact surfaces, such as poorly manufactured connectors and surface metal oxidation. Oxidation (corrosion) creates tiny air gaps, which promote voltage potential barriers, in turn resulting in a non-linear voltage-to- current ratio, sometimes referred to as the diode effect. 3. How to calculate? 1) Okumura-Hata model Okumura-Hata modelis used extensively for coverage prediction during the design and planning ofwireless networks. L Where BS Okumura-Hata A B lg f lg( h ( h ) [ lg( h L Okumura-Hata = Path loss h BS = Height of base station antenna in meters (m) h = Height of mobile station antenna in meters (m) a h =Mobile station height correction factor f= Frequency of Transmission in megahertz (MHz). C city = Antenna height correction factor in different areas d= Distance between the base and mobile stations in kilometers (km) ,150MHz f 1500MHz A 46.30,1500MHz f 2000MHz 26.16,150MHz f 1500MHz B 33.90,1500MHz f 2000MHz )] lg d C (1.1lg f 0.7) h 1.56lg f ( h ) 8.29 lg(1.54 h ) 1.1, f 200MHz middle or small city lg(11.75 h ) 4.97, f 400MHz big city When mobile station height is 1.5m (common value),antenna height correction factor in different areasare in small differences, if we suppose the frequency is 900MHz, we can get a h = If the base station antenna height is 30m, the formula can be simplified as:l = lgd + a(h ) city 2) PIM3 s Influence on base station coverage Before calculating coverage, assume the following items: (1) GSM transmitter transmit power is 2 43dBm (two carriers, 20W per carrier). Although the base station may actually operate in the multi-carrier case and each carrier power is less than 20W, but its total power is still close to 46dBm; in the case of multi-carrier will produce more intermodulation signal, and multiple intermodulation signals superimposed on each other, this leads multi-carrier intermodulation and two-carrier intermodulation their levels are similar when in the same total power. For simplicity, use 2 43dBm two-carrier to analog base

3 station transmit power is reasonable. (2) In the signal transmission and transmission process, intermodulation is generated only in antenna part, other places do not produce intermodulation. (3) Antenna intermodulation is defined based on 2 43dBm two carrier. (4) Antenna intermodulation linear changes with the input power (unit: dbm) and the slope is 3, that isoutput changes 3dB when input power change 1dB. Although the existing literature proved that the passive intermodulation with the input power linear change, the device varies the number is different, and in general is less than 3, but does not prevent the change slope is set to 3. (5) Consider a simple base station system, without tower TMA applications. According to the above assumptions, first analysis of an ideal base station (only with system thermal noise) coverage effected by the passive intermodulation of the situation, the specific calculation steps for base station coverage is shown in Table 1. GSM base station which without any interferenceits cover radius is 2.77 km. If the antenna intermodulation is 2 43dBm, the coverage radius is shortened to 2.03km, shortened by 26.8%. Using the method shown in Table 1, we first calculate the effect of different antenna intermodulation values on the coverage of base stations. Table 1 Ideal base station coverage calculation GSM Base Station Downlink power 43dBm a GSM feed loss 4dB b GSM antenna input power 39dBm c=a-b GSM antenna maximum gain 14dB d Antenna Passive Intermodulation Index -140dBc@2 43 e Antenna Passive Intermodulation Distortion IIP3 113dBm f=[3 43-( )]/2 Antenna Passive Intermodulation Level -109dBm g=3c-2f Base station receiver passive intermodulation power PIM3-113dBm h=g-b Thermal noise spectrum density -174dBm/Hz i Thermal noise bandwidth 53dB j=10lg( ) Receiver noise figure 5dB k Receiver thermal noise -116dBm L=i+j+k GSM antenna to receive the same frequency interference level -123dBm/200K m Base station receiver with frequency interference level -113dBm n=m+d-b Receiver noise+ PIM3-109dBm o=10lg(10 h/ l/ n/10 ) Receiver signal to noise ratio Eb/N t 9dB p Processing allowance 3dB q GSM base station receiving sensitivity -97dBm R=o+p+q GSM mobile phone antenna port maximum input power 33dBm s GSM mobile phone antenna maximum gain -5dB t

4 GSM mobile phone maximum equivalent isotropic radiated power EIPR 28dBm U=s+t Maximum link loss 135dB v=u-r+d-b Maximum cell radius 1.8km w=10 (v-126.4)/35.2 Figure 1. Effect of passive intermodulation on coverage (no interference) Secondly, it analyzes the influence of passive intermodulation on the coverage of base station that has some kind of interferences. Assuming a GSM base station with the same frequency interference, and the same frequency interference weaken the base station receiver sensitivity by 3dB, that is to say, sensitivity changes from -104dBm to-101dbm, coverage calculation process is shown in Table 2. Figure 2 shows that in the circumstance of same frequency interference, the effect of passive intermodulation on the base station coverage. It can be seen that other disturbances alleviates the influence of intermodulation interference on the reception sensitivity to a certain extent. The effect of passive intermodulation on the coverage is reduced, and in the case of 2 43dBm antenna, the coverage radius is reduced by 13.3 %, far below the ideal base station of 26.8%. Table 2 base station coverage calculation (with frequency interference) GSM Base Station Downlink power 43dBm a GSM feed loss 4dB b GSM antenna input power 39dBm c=a-b Antenna Passive Intermodulation Index -140dBc@2 43 d Antenna Passive Intermodulation Distortion IIP3 113dBm e=[3 43-( )]/2 Antenna Passive Intermodulation Level -109dBm f=3c-2e Base station receiver passive intermodulation power PIM3-113dBm g=f-b Thermal noise spectrum density -174dBm/Hz h Thermal noise bandwidth 53dB i=10lg( ) Receiver noise figure 5dB j Receiver thermal noise -116dBm k=h+i+j Receiver noise+ PIM dBm l=10lg(10 k/ g/10 ) Receiver signal to noise ratio Eb/N t 9dB m Processing allowance 3dB n GSM base station receiving sensitivity -99.2dBm o=l+m+n

5 GSM mobile phone antenna port maximum input power 33dBm p GSM mobile phone antenna maximum gain -5dB q GSM mobile phone maximum equivalent isotropic radiated power EIPR 28dBm r=p+q GSM base station antenna maximum gain 14dB s Maximum link loss 137.2dB t=r-o+s-b Maximum cell radius 2.0km u=10 (t-126.4)/35.2 Figure 2.Impact of passive intermodulation on coverage (with frequency interference) 4. Conclusion It can be seen from above, when the intermodulation is less than 2 43dBm, the impact of passive intermodulation on the coverage is less than 5%. If the intermodulation is more than 2 43dBm, the coverage radius of the antenna reduced sharp.that is why the high performance passive devices including the antenna intermodulation requirements are less than 2 43dBm.