ABSTRACT
This analysis focus on PIM interference under multi-band multi-signal input in mobile communi-cation system. Unlike single band system that only odd order PIM (especially 3rd order) should be concerned, in multi-band multi-signal ca, both odd and even order PIM could be interference s o u r c e b e c a u s e o f m o r e c o m p l i c a t e d intermodulation, more IMPs generated and more receive bands. Especially, the 2nd order PIM may interfere more rious to receiving channel for its strong magnitude. In duplex indoor distribute system, the PIM interference is a potential problem to GSM900, DCS1800, CDMA, and even 3G sys-tem wireless rvices, becau the PIM frequencies may fall in receive bands and interfere to the receiv-ing channels. In radio system design and wireless channel assignment, precautions must be taken to minimize the PIM level and to avoid interfere to receiving channels. For practical u, the lower order possible PIM interference to 2G and 3G systems is calculated.
Key words: PIM , IMP, interference, Multi-band multi-signal, 2G and 3G systems
PIM Interference Analysis under Multi-band Multi-signal Input in Duplex Indoor Distribution System
Pauling Huang, Wang Baohua , Senior Engineer, China Unicom Email:, wangbao
I. INTRODUCTION
Passive Intermodulation (PIM) is a co-existent phe-nomena in radio system. When two or more signals transmit through passive components (e.g. antennae,connectors, combiners, couplers, isolators, switches,diplexers and cable asmbly), the nonlinear respon,a decaying ries of PIM products (IMPs) of all orders, are generated. If some of IMPs are at the same frequencies and strong enough with receiving signals,they interfere to them. The physical mechanism of PIM and precautions to minimize PIM level are simply described in ction 2.
In modern communication system, high power (e.g. 43dBm for GSM) transmitting signal IMPs may interfere to the nsitive receiving signals (e.g. mini-mum -104dBm for GSM) if they are at the same frequency. This may deteriorate the communication quality, reduce capacity, make bad calls or even drop calls [1~5]. Multi-band multi-signal input will ad-dress the problem becau of more complicated intermodulation, more IMPs generated and more receive bands.
Most of discussions on PIM assume the input signals in a single frequency band, so it’s reasonable they just concern about the odd order PIM (e.g. 3rd order, ab. IP3), becau only odd order PIM may
fall
in the receive band. However, in current mobile networks, sites and indoor distribution systems are often shared by veral mobile networks which belong to different frequency bands. In this situation, both even and odd order of PIM may fall in receive bands [1, 7, 8]. So under multi-band multi-signal input, more IMPs may be interference source ex-cept for IP3.
PIM is a rapid decaying ries by its order, the lower the order is, the stronger of its magnitude. The 2nd order intermodulation product (IP2) is much stranger than IP3. If IP2 falls in a receiving channel, it can interfere to receiving signals more rious. Theoretical analysis of PIM is described and 4 lower order modulation components are deducted in ction 3.
With the communication technology development and the increasingly demand of wireless rvice, there are more and more communication networks coming into u. In China, there are at least 5 mobile networks now. It is hard to find suitable places to locate all ba transceiver stations BTSs for all systems parately, and even harder to get ap-proval by landlord and residents. As a result, co-locating (for different operators) or sharing (for the same operator) sites and antenna systems are commonly appreciative. For indoor coverage, more and more indoor distribution systems have to be shared by all networks.
In crowded co-locating BTS site and multiple system shared indoor duplex antenna distribution system, there exist a large number of passive compo-nents (antennae, connectors, cable, etc.) and steel pole, tower, rusted bolts that may cau PIM. In a famous casino in Macau, a rious PIM interference in indoor distribution system made it can not make any call. Finally by adjusting some input signal channels, the system worked normally. In China, many sites are co-locating and most of the indoor distribution systems are often shared by veral networks, so PIM is inevitable and worth of carefully study. For practical u, the important 4 lower orders IMPs that possibly interfere to 2G and 3G system are calculated under the prenting of 2G system signals input in ction 4.
II. CAUSES OF PIM AS PHYSICAL
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In radio systems, passive components are usually assumed electrical linearity. In other word, signals between output and input are linear relationship. But if the input signals are strong enough, the passive components behave weak nonlinearity, producing high order harmonics and intermodulation products. In particular, when the components contain the magnetic material or are rusted, polluted by chemi-cal material or looly connected, the PIM can be more obvious.银鱼仔
As described by many studies[5,8,10] it is ex-tremely complex about the mechanism of nonlinearity in the passive components and usually divide into three kinds:
1. Contact nonlinearity: Caud by different conductor’s contact surfaces, the possible reasons are as following: (1) The contact surface between two conductors exist microscale concave-convex, only some small bulges connect. This caus the surface electric currency nonuniformity, and the contact resistance changes; (2) The conductor sur-face covers a thin layer oxidation which caus “the diode effect”. When the voltage comes to a certain intensity, the tunnel effect achieves; (3) The nonuniformity rust eclips on surface that caus the surface current density nonuniformity; (4) Sol-dering contamination and oxide on the connection surface etc.
搜索引挚2. Material nonlinearity: Ferromagnetic material, non-linear dielectric medium, conductor corrosion and environment temperature change may cau the surface resistance change. The incident electromag-netic wave respon voltage, electric current change non-linearly or achieve the magnetic hysteresis, so the cond radiating waves have been distorted. The commonly ud non-linear material includes: Nickel, iron, cobalt and their alloy material, lanthanon material, aluminum magnesium copper alloy, ferro-
magnetic material, ferrous oxide compound material, non-linear dielectric medium material etc.
3. Surface effect: The conductor surface microscale roughness, pollution and the welding remains make the reflected electromagnetic wave non-linear. Contact nonlinearity induces IP3 level lower than -100dBc and easily influenced by movement or vibration; Magnetic hysteretic material nonlinearity induces IP3 level from 90 to 100dBc, but it has nothing to do with external force [12]. Obviously, the material nonlinearity caud the PIM is stranger. In GSM network with 20W (43dBm) transmitting power, the IP3 level is usually from -90 to -120dBm [8]. No data could be found yet about IP2 level. But it is reasonable that IP2 is much stronger than IP3 as IP2 is lower in order than IP3. so IP2 can more rious interfere to nsitive receiving signal if they at the same frequency.
To minimize PIM interference, some precautions should be taken into account in mobile network planning, construction and wireless channel assignment:
1. Avoid to u nickel, iron and the ferromagnetic material or their coat on passive component, the electricity silver-plating or copper-plating may re-duce PIM level.
2. U less connectors as possible, and u welding connection rather than screw connection. If screw connectors have to be ud, cleanup the connect interface and screw tightly to ensure enough con-tact area.
3. Avoid to us braid covered flexible jumper, great angle curve, inten extrusion distortion, make the cable lay straight as possible.
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4. Make enough space between transmitting and receiving antennas to avoid transmitting signal PIMs going through receiving path.
5. Change some channels to avoid lower order PIM frequencies falling into receiving channels.
III. PIM MATHEMATICAL ANALYSIS According to nonlinear system theory, the Volterra integral commonly ud to express nonlinear output [13, 14, 15], but PIM is non-time-depend weak nonlinear phenomenon, so the PIM products y(x)
may simplify as Taylor ries [1, 3, 10]:
1 The magnitude ak reduces rapidly with the in-crea of k, the higher of k is the much smaller ak, that is a
1
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2
>>a
3
>>....... y(x) is convergent. Therefore, usually only veral lower terms take into consideration, the higher terms are neglected. Here below we discuss 4 lower terms.
For N sine signals input x:
(2)
In complex form:
Where 3
(3) substitutes into (1) :
4
Where,, m
i
is integer or zero.
From (1), (4):
5
When k = 1, from (5):
6 (6) is linear output component, proportional to input signals x.
When k = 2, from (5):
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In (7), the first item is zero order (direct current) components, the cond item is the cond order harmonics, and the third item is IMPs.
When k = 3, by (5) :
8
In (8) , the first item is linear components, the cond item is the 3rd order harmonics, the third item is 3rd order IMPs with arbitrary two out of all input signals, the 4th item is 3rd order IMPs with arbitrary three out of all input signals.
When k = 4, from (5):
9
In (9), the 1st item is the linear components, the 2nd item is the 2nd order harmonics, the 3rd and 4th item are 2nd order IMPs, the 5th item is the 4th or-der harmonics, the 6th item is the 4th order IMPs with arbitrary 1 signal and an other signal’s 3rd or-der harmonics , the 7th item is 4th order IMPs with arbitrary 2 signal’s 2nd order harmonics, the 8th item is the 4th order IMPs with arbitrary 2 signals and an other signal’s 2nd harmonics. The last term is the 4th order IMPs with arbitrary 4 signals.
For k>4, y
k
is more complicated but small of its magnitude. From the above discussion we can come to the conclusions:
1. The lower the order is, the much stranger of IMP magnitude, IP2 level is the strangest.
2. The multi-signals modulation can have all pos-sible mixed products including every order IMP and harmonic.
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3. The higher order modulation can have the same and lower order harmonics and IMPs.
4. The even order modulation has the even order IMPs and harmonics, the odd order modulation has the odd order IMPs and harmonics.
5. All higher order modulations have contribution to the same parity lower order IMPs and harmonics.
IV. PIM IN INDOOR DISTRIBUTION
SYSTEM OF MOBILE
COMMUNICATION
According to the overas statistics, there is approxi-mately 80% communication traffic occurred in indoor, and the percentage is increasing with the growth of 3G technology and the demand of wire-less data communication. Therefore, indoor distri-bution system pays more and more important role to ensure high quality and heavy traffic load of in-door communication. So PIM interference in indoor distribution system must be concerned significantly. There are usually two fundamental modes in in-door distribution system design: Duplex and Sim-plex [16]. In duplex mode, there is only one t of a
冬日暖ntenna and cable asmbly shared by signal recep-tion and transmission (fig. 1 A). In simplex mode,
two ts of antenna and cable asmbly are con-
