Summary of Technical Information

Transcription

1 Summary of Technical Information

2 I. UMTS - the Future Mobile Communication System page 3 1. Network Planning page 3 2. WCDMA Technology and RET Benefits page 4 3. Conclusion page 6 II. Antenna Isolation with Site Sharing page 7 1. Factors Influencing the Isolation Value page 7 2. Save Distance between two Panel Antennas page Optimised Minimum Distance between two Antennas page 10 III. Advanced Dipole Technology page Introduction page Kathrein s dipole based Xpol-antenna design page Typical measurements page CPR against azimuth page 16 III. Indoor Environment page Typical room page Indoor antennas just convert the RF power page Unobtrusive design page Flexible signal distribution with Splitters and Tappers page Advanced Indoor System Dual Band page 21 IV. Downtilting of antennas page Downtilting the vertical pattern page Optimum downtilt angles page Consequences regarding the electrical parameters page 27 VI. Passive Intermodulation at Base Station Antennas page Introduction page What is Intermodulation? page Where do intermodulation products come from? page Why is intermodulation a problem? page What solutions are there? page 33

3 UMTS the Future Mobile Communication System The four letters UMTS, the abbreviation for Universal Mobile Telecommunication System, are already well known among the general public. They stand for high data transmission rates and multi media applications. The start of this new system has been postponed many times due to general delays in the technology as well as scepticism in carrying out such huge investments. But the thumbs are now up and many licensees have to fulfil regulations regarding a minimum coverage before the end of While the end users do not care so much about the used technology, UMTS means a big step forward compared to GSM. The main technology for implementing the 3. Generation of mobile systems will be WCDMA (Wideband Code Division Multiple Access). The applied frequency range is MHz, which contains two paired blocks of 60 MHz each. Network Planning The technologies used with GSM and UMTS have Traditionally with GSM, the downtilt angle has to a big influence on network planning and the be altered only when the network structure required network optimisation due to some changes e.g. by adding new sites, which happens essential differences. may be once or twice a year. In this case it is In both cases, the vertical pattern downtilt plays a acceptable to send out installation teams to sites major role concentrating the radiated power into to change the mechanical or electrical adjustable the cell to be covered and controlling the downtilt angles of the antennas. interference from adjacent cells. Comparison Access Frequency plan Hand over Cell size GSM TDMA (Time Division Multiple Access) separation of the subscribers by time slots certain frequencies per cell registration only in one cell (hard hand over) fixed UMTS WCDMA (Wideband Code Division Multiple Access) separation of the subscribers by codes the same full bandwidth in each cell registration in two or more cells (soft hand over) variable 3

4 With UMTS, there is a complex relationship between capacity, coverage and interference. It is expected that the electrical downtilt of the antennas has to be modified several times a day! It is clear that the previous technologies cannot provide the fast and permanent access to vary the downtilt angle of the antennas. This led to the concept of a remote electrical downtilt (RET) controlled from a central location within the network e.g. the operational and maintenance center (OMC). WCDMA Technology and RET Benefits In essence, CDMA uses the same frequency band in each cell with the unpleasant disadvantage for a specific subscriber that all the other subscribers are noise and cause interference. Consequently, power levels in CDMA networks are kept to a minimum in order to reduce this interference. The power levels might even be below the noise level, and a certain subscriber can only be identified by using codes. consequently reducing the coverage area and with it the number of subscribers. This process will continue until the power control is recovered. Power adjustment and cell breathing To keep the noise low within a cell, the transmit power of the downlink (base station) is also altered. For each subscriber, the base station has to provide exactly the right minimum power. This requires an extensive and fast power adjustment. The effect of a variable coverage area due to an increased load and noise is called cell breathing. The graph below describes the relationship between number of users, noise increase and cell range. Cell breathing and noise increase in UMTS voice If the load in the cell rises, either by an increased number of subscribers, or by higher transfer data rates, the power and with it the noise level will grow and finally hinder communication. The base station gets at its limit concerning power adjustment and responses by turning down the power, cell range [km] With RET it is possible to partly compensate this effect and to optimise the power distribution in critical areas. number of users noise increase source: HU Berlin 4

5 Antenna coverage BTS High traffic or soft handover area BTS BTS Decreasing the downtilt Increasing the coverage BTS BTS BTS Soft hand over To improve the low power uplink situation, subscribers may be registered in more than one cell. That means the weak signals are received in two or three cells and added up by the system. It is estimated that approximately 30 % 40 % of the subscribers will be in such a soft-handover condition. This technology provides some benefits regarding the uplink levels but on the other hand it eats up capacity. The amount of soft handover can be adjusted by RET changing the overlapping areas in the network. High traffic area Skilful planning, adding RET features to your network, may increase capacity in high traffic areas. For example, during rush hours the network can concentrate on train stations or airports. Furthermore, the network can be adopted to meet the temporary requirements of special events like music festivals, exhibitions or major sporting events. Network expansions If a network grows due to an increased number of subscribers, additional sites are generally added in between the already existing ones. To avoid interference between the sites, downtilt angles have to be reset by installation teams who have to visit each individual site. With RET this adaptation could be carried out centrally from the OMC without any work at the site. 5

6 Conclusion According to equipment suppliers and OEM s, remote electrical tilt will become a major feature of UMTS networks. For the operators, the decisive question with respect to the implementation of RET systems is the level of investment required. The corresponding tenor forecast is that RET will be payed off quite quickly: due to the achieved network optimisation, up to 20 % of WCDMA equipment can be saved the network will show lower bit failure rates and a smaller amount of drop calls The network operators, especially those in Europe, have more experience with GSM than with CDMA and now face the problem with various new sites of how to decide in advance, whether or not to use RET. The Kathrein concept to upgrade the RET function with already installed antennas considers this dilemma and allows the operators to postpone the decision until tests have been performed. 6

7 Antenna Isolation with Site Sharing Due to the environmental restrictions and growing shortage of available sites, site-sharing has become more and more regular. Apart from static aspects, isolation between the antennas on the same site is the biggest problem. To get different systems with two separate antennas working properly, an isolation of at least db between both networks is necessary. This isolation cannot be achieved by the antennas alone. It must be generated with the combination of filter isolation together with the isolation of the antennas. The required isolation offered, from the antennas should be at least 30 db. Factors Influencing the Isolation Value For the isolation values, different influencing factors have to be considered: Electrical specifications: With the same mechanical settings at a site, variations of the electrical specifications impact the isolation: Frequency: Antennas are not filters! They also receive frequencies out of the band they are specified for. However, for these frequencies they show worse VSWR values. The resulting mismatch creates an attenuation called mismatch loss, that increases the isolation between two antennas. Therefore, antennas operated in different frequencies have higher isolation values than antennas operated in the same frequency band. Polarisation: The lowest isolation figures apply, when two antennas have the same polarisation. If the polarisation is different, the isolation values increase. Taking one antenna with vertical and one with slanted polarisation, mainly the vertical component of the slanted polarisation is responsible for the isolation. Due to the fact that the amplitude of this vertical component is 3 db smaller compared to the complete signal, the isolation is approx. 3 db higher. Half-power beam width: With two antennas side by side and facing into the same direction, radiation against each other (orthogonal to the main beam) finally determines the isolation. The broader the half-power beam width, the higher the radiation level at +/ 90. Consequently the isolation decreases with a growing half-power beam width of the two antennas. (see picture 1, next page) Electrical tilt: The electrical tilt is achieved by feeding the dipoles with unequal phases of a signal. The different phases lower the coupling between two antennas, resulting in higher isolation values for antennas equipped with fixed or adjustable electrical tilt, rather than for antennas without electrical tilt. (see picture 2, next page) Mechanical settings: Keeping the electrical specifications of two antennas constant at a site, also variations of the mechanical settings influence the isolation: 7

8 Vertical or horizontal separation: Antennas have very dedicated radiation patterns with nulls above and below the antennas main beam. This results in a very small radiation level towards an antenna that is directly above or below. Therefore, two vertical separated antennas show higher isolation values than two horizontally separated antennas at the same distance. (see picture 3, next page) Angle: The signal level behind the antenna is much smaller than the one in front or even at +/ 90. If now two antennas do not point into the same direction, but are separated through an angle (e.g. 120 ) between them, the mutual level of radiation becomes less. For this reason, the isolation grows with the azimuth angle between the two antennas. Pole-/Wall-Mounting: Despite the relatively high front-to-back ratio of panel antennas, the influence of a large plane (e.g. building fascade) behind the antenna cannot be completely neglected. The reflections from the surface usually result in a slightly smaller radiation pattern than normal, decreasing the level of radiation towards the neighbouring antenna. Isolation db Distance a/m : XPol A-Panel dBi 9 T : XPol F-Panel dBi 0 8 T Isolation db Distance a/m : XPol A-Panel dBi 8 T : XPol F-Panel dBi 0 8 T Picture 1: Isolation values for different half-power beam width s Isolation db T 2 T 4 T 6 T 8 T a Distance a/m : XPol F-Panel dBi 0 8 T : XPol F-Panel dBi 0 8 T Picture 2: Isolation values for different downtilt angles 8

9 Therefore, two same antennas mounted on a wall show higher isolation values than if being mounted on a pole. (see picture 4, below) Design: One of the biggest influencing factors is the design of the antennas, since the current at the edges of the reflector significantly influences the isolation between two antennas. These currents depend on the construction and the kind of the radiating elements used (e.g. dipole, patch). Therefore, isolation values of one manufacturer may not be used for antennas from another. Kathrein antennas, with their proven dipole construction, are designed for high isolation values. Isolation db Distance a/m, Distance b/m : XPol F-Panel dBi 2 T Horizontal separation Vertical separation a Horizontal separation b Vertical separation Picture 3: Vertical / horizontal separation Isolation db Antennas on pipe masts Antennas close to reflective structure a a Distance a/m : XPol F-Panel dBi 2 T Antennas on pipe masts Antennas close to reflective structure Picture 4: Pole / wall mounting 9

10 Save Distance between two Panel Antennas There is a standard question of network planners about the required minimum save distance for two panel antennas in order to achieve isolation values of more than 30 db. Save distance a for an isolation value of 30 db: a Vertical separation: The isolation values for vertical separation are always quite good, therefore typically only the minimum possible distance is needed. Horizontal separation: The minimum save distance depends on the wavelength and on the horizontal half-power beamwidth: Minimum distance a Horizontal half-power beam width 2 λ λ 90 3 λ λ 120 (λ = wavelength) Angle separation: Taking a 120 angle, 30 db of isolation are already reached with the minimum mechanical distance. Optimised Minimum Distance between two Antennas However, the stated save distance (see above) is only a save distance and not the optimised minimal possible distance. This distance may only be found with measurements. Kathrein has done a number of isolation measurements for typical site configurations, that are available for our customers. In these measurements we have measured values up to 50 db. Values of more than 50 db also depend on the special site due to reflections from buildings or parts of the pole. Therefore, these values can no longer be seen as typical. 10

11 Advanced Dipole Technology 1. Introduction The dipole is the oldest and most approved radiating element in the field of mobile communication. It is the basis for nearly every professional antenna type such as the yagi antenna, the log. periodic antenna, and particular the panel antenna. The latest development of panel antennas leads to the sophisticated technology of slanted dual polarization (Xpol). Is the dipole technology suitable to fulfil the growing and stringent requirements? Can these odd metal structures still compete with newer solutions like the patch radiator on a printed board? The answer is definitely yes! This article will show that the dipole technology more than other concepts provide the flexibility to perfect certain characteristics without the effect of destroying others. This feature is specific important for the design of dual band cross-polarized antennas. 2. Kathreins s dipole based Xpol-antenna design 2.1 General description Electrical : Xpol antennas consist of two independently working slanted dipole systems, one for +45 polarization and the other for -45 polarization. The dipoles are symmetrically positioned in front of a reflector screen. Both the power distribution and the impedance transformation are carried out by a low loss cable harness. Additional elements for beam-shaping and isolation perfect the design. Mechanical: The radome consists of a completely closed selfsupporting fiber-glass profile, into which the metal parts are inserted. There are no drill-holes at all in the profile, which is closed by two end caps with short sealing rings. This concept offers ideal permanent protection against environmental influences and increases the mechanical stability. The improved separation of the electrical and the mechanical function facilitates the optimization of particular performances. 11

12 2.2. Outstanding characteristics Symmetrical construction Xpol antennas are available with horizontal half power beam widths of 65 and 90. Starting from a standard vertical polarized antenna, the required dipole-pair for 65 and the single dipole for 90 are rotated by +45 and -45, resulting in orthogonal polarizations (see fig. 1). While the dipoles of the 90 type form an X on which the expression Xpol antenna is based, the basic 65 dipole system is a rhomb. Both designs are fully symmetrical referred to the center line of the reflector screen, which is the basic condition for symmetrical horizontal radiation patterns. Fig. 1: General construction of Xpol-antennas 65 Half-power Beam Width 90 Half-power Beam Width Reflector Dipole system Feeding harness

13 2.2.2 Beam-shaping The dipole technology offers a high flexibility in modeling the radiation patterns. Beam width and shape are defined by the dipole position to the reflector and the reflector dimensions. Particular the vertical edges of the reflector screen have a decisive influence on vertically polarized components. For slanted polarizations, consisting of vertical and horizontal components, parasitic elements in the reflector screen as further beam-shaping elements are added, which mainly have an effect on horizontally polarized components. Thus already the patterns of the basic dipole system are optimized, which means a great benefit in combining them. The quality of the resulting pattern is improved regarding sidelobes and gain, and the required number of single elements is minimized (see item ). In addition, with the separate adjustability of the vertical and the horizontal components, the resulting polarizations are controllable. Orthogonal polarizations provide the best polarization diversity gain results, therefore the horizontal radiation patterns for the vertical and the horizontal component are standard measurements for Xpol antennas. If the patterns half power beam widths and thereby the gain values resp. the amplitudes are identical, the polarizations are orientated +/- 45 and consequently orthogonal (fig. 2). Fig. 2: Vertical (V) and horizontal (H) components and resulting polarizations: a) Equal amplitudes (V=1/H=1)? orthogonal polarizations + 45 V V H -H b) Different amplitudes (V=1/H=0.7)? non-orthogonal polarizations V V -35 H -H 13

14 A perfect polarization orthogonality results in a high cross-polar ratio (CPR), which is determined by measuring the horizontal radiation patterns with the operating polarizations +45 and -45. The CPR compares the level difference between the similar polarized signals (co-polar) and the dissimilar polarized signals (cross-polar) of the radiated wave. A high CPR stands for a high uncorrelation of the two signals and consequently for a good polarization diversity performance. The dipole design provides excellent values also apart from the main direction (coverage sector width +/- 60 ) and even at +/- 90 (see item 4)! High isolation between the two antenna systems The polarization diversity technology assigns both systems of an Xpol-antenna to work in the Rx- and Tx-mode simultaneously. Therefore a minimum isolation of 30 db between the antenna inputs is required. Kathrein s dipole design guarantees a min. isolation of 32 db. Measurements of each antenna during the production show a typical value of 35 db! Within the basic dipole system ( X and rhomb ), the symmetrical construction provides high isolation, while the isolation from one bay to the next is improved by patented decoupling measures Low-loss power distribution by cables Low-loss flexible semi-rigid coax cables distribute the power to each dipole and take care of the impedance transformation. The diameter of the cables (and the corresponding attenuation) varies with the application, diameters of 0.250?, 0.141? and 0.085? are in operation. That means, to reach the same gain values, antennas using a printed board power distribution have to compensate the higher losses by additional bays of radiating elements! This results in a roughly 20% higher vertical antenna length and a smaller vertical beam width. This system produces only a minimal attenuation, which will become apparent by comparing it with a printed circuit solution. As a standard the corresponding cross-section of the conductive lines is between the 0.085? and the 0.141? cable. In addition these lines are open and radiate a part of the power, which causes further losses. Another advantage of the cable harness is the flexibility regarding versions with electrical downtilt. The required variation of the phase relations between the radiating elements is carried out easily by changing the length of the cables. It is not necessary to redesign the entire antenna. 14

15 2.2.5 Low intermodulation products Since more than 15 years Kathrein is doing research on the reduction of intermodulation (IM) products. There was already a self-designed measuring device for IM products at 450 MHz with a dynamic range of 160 db in operation, when such a device was not available on the market. should look like. Kathrein antennas provide a typical 3 rd order IM-products attenuation of -150 dbc using two transmitters with an output power of 20 W (43 dbm) each. The extremely valuable experiences flowed into the antenna design and determine for example the applied material, the possible material combinations and how a contact between two parts Continuance of the electrical parameters against enviromental influences Antennas are confronted with all the environmental influences such as cold and hot temperatures, rain, ice, snow, lightning and high wind velocities. Kathrein antennas are well prepared, the mechanical design is based on the environmental conditions according to ETS goes together with the deterioration of electrical parameters like VSWR, isolation and CPR. The Kathrein dipole technology is highly resistant against rain, ice and snow. Dipoles are very slim structures with a small surface and therefore the occurring additional capacity is relatively low. Regarding the deviation of the electrical parameters, especially rain, ice and snow on the radome may cause problem because of their dielectric parameters. Due to the fact that the antenna depths became smaller and smaller, this dielectric load is very close to the radiating elements, working as an additional capacity. Consequently the operational frequency range is shifted, which Due to their larger surface, the capacity influence on patches is much higher. For example, a wet radome can change the isolation of a patch antenna significantly, while a dipole antenna reacts much more good natured. 15

16 3. Typical measurements The following antenna parameters have a decisive influence on the network and are important for the judgement of antennas : 1. Half power beam width for co-polar polarization 2. Half power beam width for vertical / horizontal polarization 3. Front-to-back ratio - co-polar 4. Front-to-back ratio - total power 5. Cross-polar ratio For a high cross-polar attenuation the half power beam widths of the three polarization components co-polar, vertical and horizontal are similar. This feature is perfectly performed by Kathrein s Xpolantennas and consequently there is no need for network planning reasons to differentiate between the above polarization components. These measurements also provide the front-to-back ratio, which is an important feature for the network planning. The front-to-back ratio can be determined as the worst case of either the vertical or the horizontal polarized components. It is only required to calculate the total power, if the two components have similar levels. In case of identical levels, the total power value is 3 db less compared to the individual components. Xpol dipole antennas provide typical front-to-back ratios of db total power. The following figures show the co-polar and cross-polar as well as the vertical and horizontal polarized patterns of 65 and 90 antennas. Beside the symmetry of the patterns, the scalar printout with a linear scale in db shows clearly the cross-polar ratio in each azimuth direction. The dipole design provides excellent values also apart from the main direction and even at +/- 90! Please note as well the high front-to-back ratio for the co-polar and the cross-polar signal. Fig. 3: Typical horizontal co-polar and cross-polar pattern for 65 beam width (measurement) relative gain [db] XPol A-Panel 800/ dBi horizontal radiation pattern 120 -sector co-pol cross-pol azimuth [deg] 16

17 Fig. 4: Typical 65 horizontal pattern of vertical and horizontal polarized component (measurement) relative gain [db] XPol A-Panel 800/ dBi horizontal radiation pattern 120 -sector hor. polarized vert. polarized azimuth [deg] Fig. 5: Typical horizontal co-polar and cross-polar pattern for 90 beam width (measurement) relative gain [db] XPol A-Panel 800/ dBi horizontal radiation pattern 120 -sector co-pol cross-pol azimuth [deg] 17

18 Fig. 6: Typical 90 horizontal pattern of vertical and horizontal polarized components (measurement) relative gain [db] XPol A-Panel 800/ dBi horizontal radiation pattern 120 -sector hor. polarized vert. polarized azimuth [deg] 4. CPR against azimuth As already mentioned, the dipole design provides excellent CPR values not only in main direction but even at +/- 90. It is important for the coverage of a standard sector, to rely on high CPR values and consequently on high diversity gains also at the sector edges, where the antenna gain is already considerably reduced. Fig. 7: CPR values against azimuth (according patterns fig. 3 and 4) XPol A-Panel 800/ dBi Cross Polar Ratio sector 25 CPR [db] azimuth [deg]

19 Indoor Environment Typical room 2,5 m Indoor antennas just convert the RF power 1. Lots of reflections from the walls, ceiling, floor, furniture and persons (see sketch above), destroy the free space radiation patterns and the corresponding antenna gain. 2. The dimensions of normal rooms do not fulfil the far field conditions (distance to the antennas more than 3 m for GSM 900, respectively 1.5 m for GSM 1800). 3. Therefore the measured far field patterns do not apply; specific radiation patterns and gains provide no benefit within closed rooms. Kathrein refers to this physical facts with its present indoor program with mainly two omni versions for ceiling mounting and one directional antenna for wall mounting. Apart from single band antennas, also multiband versions are available. Unobtrusive design Most clients prefer unobtrusive antenna appearance in indoor applications. Kathrein reacted on this demand by redesigning the most sold indoor antennas and The shape of the new models and adapts perfectly to the requirements of modern buildings. In addition these antennas are multiband types operating from MHz and suitable for UMTS as well. 19

20 Flexible signal distribution with Splitters and Tappers Especially for the signal distribution within bigger buildings with lots of indoor antennas, it is necessary to design an indoor network with more or less similar signal levels in all floors. Therefore Kathrein provides 2-, 3- and 4-way splitters and splitters with unequal power splitting ( Tappers ). The resulting distribution attenuation (valid for both directions), are given with the following survey: Splitters equal power ratio Tappers 2-way 2- way 1-3 db Power splitting 4/1 4-1 db 0 db 0 db 1-3 db 1-7 db 3- way 1-4,7 db Power splitting 10/1 10-0,4 db 0 db 1-4,7 db 0 db 1-4,7 db 1-10,4 db 4- way 1-6 db Power splitting 32/1 1-6 db 0 db 0 db 1-6 db 1-6 db ,1 db -15,1 db 20

21 Advanced Indoor System Dual Band Similar signal levels in all floors (without cable losses) 6. Floor 4-way Splitter K ,4 db 5. Floor -6,4 db 2-way Splitter K ,4 db 4-way Splitter K ,4 db 4. Floor 2-way Tapper 4/1 K ,4 db 4 1-9,4 db 2-way Splitter K ,4 db 3. Floor 2-way Tapper 4/1 K ,4 db 4 1-8,4 db 3-way Splitter K Floor 2-way Tapper 4/1 K ,4 db 4 1-7,4 db -13,1 db 4-way Splitter K Floor 2-way Tapper 10/1 K relative signal strength ,4 db 0 db -10,4 db -13,4 db omni antenna for ceiling mounting ,4 db 2-way Splitter K directional antenna for wall mounting Combiner GSM 900 Base Station GSM 1800 Base Station 21

22 20 15 Antennen. Electronic Downtilting of antennas 1. Downtilting the vertical pattern Network planners often have the problem that the base station antenna provides an overcoverage. If the overlapping area between two cells is too large, increased switching between the base station (handover) occurs, which strains the system. There may even be disturbances of a neighbouring cell with the same frequency. In general, the vertical pattern of an antenna radiates the main energy towards the horizon. Only that part of the energy which is radiated below the horizon can be used for the coverage of the sector. Downtilting the antenna limits the range by reducing the field strength in the horizon and increases the radiated power in the cell that is actually to be covered. 1.1 Mechanical downtilt The simplest method of downtilting the vertical diagram of a directional antenna is a mechanical tipping to achieve a certain angle while using an adjustable joint. (see Figure 1) But the required downtilt is only valid for the main direction of the horizontal radiation pattern. In the tilt axis direction (+/-90 from main beam) there is no downtilt at all. Between the angles of 0 and 90 the downtilt angle varies according to the azimuth direction. This results in a horizontal half-power beam width, which gets bigger with increasing downtilt angles. The resulting gain reduction depends on the azimuth direction. This effect can rarely be taken into consideration in the network planning (see Figure 2). Fig. 1: Mechanically downtilted A-Panel Fig. 2: Changes in the horizontal radiation pattern when various downtilt angels are used (compared to the horizon) db MECHANICAL 0 DOWNTILT 22

23 1.2 Electrical downtilt In general, the dipols of an antenna are fed with the same phase via the distribution system. By altering the phases, the main direction of the vertical radiation pattern can be adjusted. Figure 3, shows dipols that are fed from top to bottom with a rising phase of 70. The different phases are achieved by using feeder cables of different lengths for each dipole. The electrical downtilt has the advantage, that the adjusted downtilt angle is constant over the whole azimuth range. The horizontal half-power beam width remains unaltered (see Figure 4). However, the downtilt angle is fixed and cannot be changed. Figure 3: Phase variations for a fixed el. downtilt? = 0? = 70 Figure 4: Changes in the radiation pattern using various downtilt angles ? = ? = 210? = db ELECTRICAL 1.3 Adjustable electrical downtilt With this technique it is possible to combine the advantages of the mechanical downtilt (i. e. adjustment possibility) with those of electrical downtilt (horizontal half-power beam independent of downtilt angle). Instead of using different fixed cables to achieve the various phases for the dipoles, mechanical phase-shifters are used. Figure 5: Phase diagram of an adjustable phase-shifter P = 1 P = 2 + +? +? P = 3.5 P = 2 P = 1 -? - -? Phase-shifter 23

24 These phase-shifters can be used to set various downtilt angles which remain constant over the whole azimuth range. The adjustment mechanisms can be positioned either on the rearside (Eurocell panels) or on the bottom (F-Panels, A-Panels) of the antenna. Figure 6: Downtilt adjusting mechanism (with scale) for A-Panels 2. Optimum downtilt angles The optimum tilt angle for a particular antenna depends on the vertical radiation pattern, especially on the half-power beam width, and therefore also on the actual length of the antenna. 2.1 How to calculate the optimum downtilt angle In standard applications the purpose of using a downtilt is to limit the field strength in the horizon. Considerable limitation is achieved if the radiated power in the horizon is limited by 6 db. This means that one can easily predict the smallest efficient tilt angle by simply tilting the vertical radation pattern until the field strength in the horizon is reduced by 6 db. But there is also a second important point when calculating the optimum downtilt angle. Apart 24 from the main beam, vertical radiation patterns also have two or more side lobes depending on the number of dipoles within the antenna (see Figure 7). Maximum field strength reduction in the horizon is achieved if the minimum between the main beam and the first side-lobe is orientated towards the horizon.

25 Figure 7: Typical vertical radiation pattern First upper side-lobe Main beam If the tilt angle is set too high, the field strength is not reduced, but is increased again by the first side-lobe. 2.2 Small antennas vertical half-power beam width 70 As the Figure 8 shows, the minimum tilt angle that would be efficient lies at around 50 (power in the horizon reduced by 6 db). Using such an angle, the antenna would beam more or less directly into the ground. Therefore the use of a downtilt with very small antennas (i.e. length up to 500 mm) can not be recommended. Figure 8: Minimum efficient tilt angle for small antennas

26 2.3 Standard antennas vertical half-power beam width 13 The minimum efficient tilt angle for these antennas (length 1.3 m) lies at 8. At an angle of 19 the first side-lobe lies on the horizon. This provi- Figure 9: Minimum efficient tilt angle for standard antennas des a good range of angles for the efficient tilting of standard antennas. Figure 10: First side-lobe lies on the horizon Long antennas vertical half-power beam width 6.5 The minimum efficient tilt angle for these antennas (length 2.6 m) lies at around 3 4. At an angle of 8 9 the first side-lobe lies on the horizon. This provides a good range of angles for the efficient tilting of long antennas. Figure 11: Minimum efficient tilt angle for long antennas

27 2.5 High downtilt angles for special locations For some special locations (e.g. on the tops of high mountains, on the roof-tops of tall buildings or for coverage in the street below etc.) a very high downtilt angle might be necessary. To achieve such high downtilt angles, a combination of mechanically and electrically downtilted antennas is also possible. 3. Consequences regarding the electrical parameters Taking all the above into account, it is easy to imagine, how very sophisticated the development of electrically adjustable downtilt antennas is, since intensive measurements have to be carried out. All the electrical parameters must fulfil the specifications with every single downtilt angle. Electrical values such as those for side-lobe suppression, isola-tion, cross-polar ratio, intermodulation or beam tracking are especially critical. Kathrein s lengthy and outstanding experience with vertical polarized electrical adjustable antennas has enabled us to fully optimize the characteristics of the new X-polarized and dual-band X-polarized antenna models. 27

28 Passive Intermodulation at Base Station Antennas 1. Introduction If a base station antenna transmits two or more signals at a time, non-linearities can cause interferences, which may block one or more receiving channels of the base station antenna. This can result in a connection breakdown to a mobile. Figure 1: Base station communicating with two mobiles The risk for this problem to occur increases with the number of transmitting (Tx) frequencies connected to one base station antenna. With the standard XPol-antennas 2 Tx-antennas are combined (see Figure 2). Figure 2: XPol antenna with two duplexers T x1 R xa T x2 R xb The latest technology using dual-band, dual-polarised (XXPol) antennas, now again doubles the number of antennas and hence also the number of carriers in one radome, to combine both the 900 and 1800 MHz systems. But this also means 28 a further possible increase in interferences problems. These interference problems are called Intermodulation.

29 2. What is Intermodulation? Intermodulation (IM) is an undesirable modulation which leads to unwelcome alterations to the high frequency carrier output. An input signal put into a linear passive device at a certain frequency f 1 will produce an output signal with no modification to the frequency. Here only the amplitude and the phase can be modified. However, if the same signal is put into a passive device with non-linear transmission characteristics, then this will result in distortions to the time-scale, leading to changes in the frequency. This means that, in addition to the carrier frequency f 1, several harmonics are produced: 2 f 1, 3 f 1, 4 f 1,..., n f 1. Moreover, if the input signal contains two or more frequency components, f 1 and f 2, the output signal will generate a spectral composition. In addition to the harmonics, this new spectral composition also includes all possible frequency combinations. These frequency combinations can be expressed by the equation: IMP = nf 1 ± mf 2 IMP: Inter Modulation Products n,m = 1, 2, 3,... Only the IMP > 0 are physically relevant. The order of the IMP can be equated as: O = n + m There are IMP of even and odd orders. The products of even orders have a large spacing to the original Tx frequencies and therefore cause no problems with single band antennas. The most troublesome IMP are those of the odd orders: Intermodulation products of even orders Intermodulation products of odd orders 2 nd Order f 1 + f 2 / f 2 f 1 3 rd Order 2f 1 f 2 4 th Order 2 f f 2 / 2 f 2 2 f 1 5 th Order 3f 1 2f 2 7 th Order 4f 1 3f 2 Large spacing compared to the original frequencies Close to the original frequencies Since the IMP frequencies of the odd orders lie very close to the original frequencies, they can appear within the received signal band-width and thereby degrade the overall communication system. 29

30 Figure 3: Input signals Level f1 f2 f Frequency Figure 4: IM spectrum of odd orders Level Rx Tx f 1 f 2 2f 1 f 2 2f 2 f 1 3f 1 f 2 3f 2 f 1 4f 1 f 2 4f 2 f 1 f f f f f f f f f Frequency 3. Where do intermodulation products come from? If high-power signals of different frequencies exist, any device with non-linear voltage-current characteristics will generate intermodulation products. The level will depend on the degree of the non-linearity and on the power-ratings of the incident frequencies. 30

31 There are two main categories of non-linearities: Contact non-linearities at metal/metal joins Contact non-linearities arise where discontinuities exist in the current path of the contact. They may have various causes and are not normally visible to the naked eye. The following are potential causes: Surface condition of the join, e.g. dirt, surface textures,... Electron tunnelling effect in metal insulator metal joins Contact mating: Poor contact spring force or poor contact quality Material and surface-plating non-linearities Non-linear conductive materials or treated surfaces (e.g. the treatment of copper foils on printed circuit boards (PCB s) patch antennas on PCB) Magneto-resistance effect in non-magnetic materials Non-linearity due to non-linear dielectric Non-linearity due to variations of permeability into ferromagnetic materials Material non-linearity is an important source of intermodulation products if two or more signals pass through ferro-magnetic material. But the result of a poor contact join is of far more significance! 4. Why is intermodulation a problem? Current mobile telephone systems are designed to operate with a transmitting frequency range Tx and a slightly shifted receiving frequency range Rx. Problems arise when intermodulation products occur in the receiving Rx frequency range (see also Figure 4) which degrade the reception performance. The following example for GSM 900 shows that, under certain conditions, the intermodulation products of 3 rd, 5 th and even 7 th or higher orders may fall in the receiving band. GSM 900 Tx Band Rx Band MHz MHz Intermodulation Products f 1 f 2 f IM 3 rd Order 2f 1 - f MHz 958 MHz 914 MHz 5 th Order 3f 1-2f MHz 956 MHz 902 MHz 7 th Order 4f 1-3f MHz 952 MHz 908 MHz 31

32 The most disturbing intermodulation products in the GSM 900 and 1800 systems are those of the 3 rd order. These are the products with the highest power level and also the ones that lie closest to the original transmitting frequencies. These products may block the equivalent Rx channels. It is therefore absolutely essential to keep the IMP s to a minimum level below the sensitivity of the receiving equipment. These products are measured as Intermodulation Levels in either dbm or dbc. The total intermodulation level compared to a power-rating of 1 mw is expressed in dbm: IM = 10 log P IMP3 [dbm] On the other hand, dbc is defined as the ratio of the third order intermodulation product to the incident Tx carrier signal power: IM = 10 log(p IMP3 /P Tx [dbc] The levels of intermodulation products according to the GSM standard are shown in the following table: Level of IM products accord. GSM Standard (3 rd order) Referred to two carriers of 20 W each (43 dbm) IM attenuation of Kathrein antennas < 103 dbm < 146 dbc Typically < 150 dbc A comparison of the carrier level and the level of the IMP expressed in distances clearely illustrates this fact: Comparison Average distance earth sun Equivalent distance Carrier IM Product 0 dbm 150 dbm 150 Mill. kilometer 0,15 mm 32

33 5. What solutions are there? In view of all the facts mentioned, the following points must be taken into consideration when designing passive devices such as antennas, cables and connectors: All components such as feeder cables, jumpers, connectors etc. must fulfil the IM standards. All connectors must have good points of contact. Particular materials such as copper, brass or aluminium are recommended. Other materials like steel and nickel should to be avoided in the signal path. Material combinations with a high chemical electrical potential should not be used as any thin corrosion layer between the materials will act as a semi-conductor. All points of contact should be well-defined and fixed. All cable connections should be soldered. Engineers at KATHREIN have been researching ways of reducing intermodulation (IM) products for more than 15 years now. Long before other such devices became available on the market, Kathrein developed a company-designed IM product measuring device for the 450 MHz frequency with an operating sensitivity of 160 dbc. Kathrein s long-standing and extremely valuable experience is incorporated into all our antenna designs and helps to determine for example the best material to use, all possible material combinations and also what a point of contact between two antenna parts should look like. Kathrein antennas typically show a 3rd order intermodulation product attenuation of 150 dbc, where two transmitters each with an output power-rating of 20 W (43 dbm) are used. As explained earlier, there is an increased risk of intermodulation with XX-pol. antennas since four Tx antennas are used. IMP s of the 2nd order may also cause problems with XX-pol. antennas due to the combination of the 900 and the 1800 MHz frequencies. Kathrein has therefore introduced a 100% final test rate for intermodulation products in their serial production of all XX-pol. antennas. 33

34 internet: KATHREIN-Werke KG Phone Fax Anton-Kathrein-Straße 1-3 P.O. Box Rosenheim Germany