Antenna arrangement with interleaved antenna elements

Description of equivalent WO 2007011295 A1

ANTENNA ARRANGEMENT WITH INTERLEAVED ANTENNA ELEMENTS

0002Technical field

0003The present invention relates to an antenna arrangement with interleaved antenna elements for multiple frequency band operation, especially for mobile communication systems, as defined in the preamble of claim 1. The invention also relates to an antenna system being adapted to communicate through a communication link with a base station.

0004Background -to -the invention

0005Present antenna arrays used for transmitting and receiving RF (Radio Frequency) signals in mobile communication systems are normally dedicated to a single frequency band or sometimes two or more frequency bands. Single frequency band antennas have been used for a long time and normally include a number of antenna elements arranged in a vertical row. A second row of antenna elements needs to be added beside the first row if the operator in a network wants to add another frequency band using single frequency band antennas. However, this requires enough space to implement and the arrangement may also be sensitive to interference between the RF signals in the different frequency bands.

0006These drawbacks have been partially resolved by prior art arrangements 10 which are schematically shown in figures IA and IB.

0007In figure IA two types of antenna elements 11, 12 have been arranged alternatively in a column. A first antenna element 11 is a dual band antenna element which operates in two different frequency bands FBi and FB₂, a second antenna element 12 is an antenna element which operates in only one frequency band FBi. A drawback with this prior art embodiment is that the frequency bands FBi and FB₂ will couple to each other due to the closeness of the parts making up the antenna element 11.

0008Therefore, this kind of configuration is only suitable when the frequency bands have a big separation, for example if FB₂ is approximately twice the frequency as FBi. If the frequency bands are too close, filters with high Q values, for example cavity filters which consume space and are relatively expensive and heavy, must be used very close to the antenna elements .

0009The prior art arrangement shown in figure IB, as disclosed in US 6,211,841 (Nortel), is formed by an array including first antenna elements, 11a, which are positioned in two parallel columns 13a, 14a and operate in a first, lower frequency band, and second antenna elements 12a, which are alternately located in two adjacent columns 13a, 15a and operate in a second, higher frequency band. One of these adjacent columns (13a) is the same as one of the columns accommodating the first antenna elements 11a, whereas the other column 15a is located between the columns 13a, 14a. By locating the antenna elements 11a, 12a in parallel, spaced apart columns side by side, it has been made possible to achieve the desired low coupling even between frequency bands which are relatively close to each other, namely up to a quotient of about 2/3.

0010In US 6,844,863 B2 (Andrew Corporation), an arrangement with interleaved arrays of antenna elements is disclosed. Here, the various arrays deliberately couple to each other in a common frequency band.

0011Accordingly there is a need for a new antenna arrangement that will operate in two or more frequency bands with a reduced coupling between the frequency bands without using filters close to the elements or, if filters are needed, using filters with low Q values, such as micro strip or strip line filters, which are small in size and relatively cheap to implement.

0012Summary of the invention

0013An object with the present invention is to provide a multiple frequency band antenna arrangement, and an antenna system, that will reduce the coupling between different frequency bands while at the same time minimizing the space needed compared to prior art antennas.

0014The object is achieved for a multiple frequency band antenna arrangement which is connectable to a transceiver for transmitting and receiving RF signals in at least two separate frequency regions . The antenna arrangement has at least two sets of antenna elements arranged on a reflector. A first set of antenna elements is arranged in a column and operates in a first frequency region, whereas a second set of antenna elements is likewise arranged in a column and operates in a second frequency region. According to the present invention, the first and second sets of antenna elements are interleaved along and positioned on a straight line so as to form a single column, said first and second frequency regions including first and second frequency bands, respectively, which are separate and substantially non-overlapping but relatively close to each other, and the distance between adjacent antenna elements in said column, operating in different frequency bands, are substantially the same along said column and is smaller than the wavelength λ of the centre frequency of the highest one of said first and second frequency bands.

0015The object is also achieved by an antenna system being adapted to communicate through a communication link with a base station, wherein the antenna system comprises an antenna arrangement, and means for controlling the phase and amplitude of transmitting signals and receiving signals to/from antenna elements in said antenna arrangement.

0016An advantage with the present invention is that an isolation of more than 30 dB between the frequency bands can be obtained, without the use of cavity filters even if the frequency bands are close to each other.

0017Another advantage with the present invention is that it is easy to configure an antenna having a desired selection of frequency bands .

0018Still another advantage with the present invention is that the size of the antenna arrangement is maintained small compared to prior art arrangements.

0019Further objects and advantages are obvious by a skilled person from the detailed description below.

0020Brief description of the drawings

0021Fig. IA shows a schematic representation of a prior art dual band antenna arrangement.

0022Fig. IB shows, schematically, another prior art dual band arrangement .

0023Fig. 2A shows a schematic representation of a dual band antenna arrangement according to the present invention.

0024Fig. 2B shows a modified version of the arrangement of fig. 2A.

0025Fig. 2C illustrates the separation of the two frequency bands being used in the dual band antenna arrangement. Fig. 3 shows a perspective view of a first embodiment of a dual band antenna arrangement according to the present invention.

0026Fig. 4 shows a perspective view of a second embodiment of a dual band antenna arrangement.

0027Fig. 5 shows a perspective view of a third embodiment of a dual band antenna arrangement.

0028Fig. 6 shows a perspective view of a first embodiment of a multi band antenna arrangement.

0029Fig. 7 shows a schematic representation of the multi band antenna arrangement in figure 6.

0030Fig. 8 shows a block diagram illustrating the signal path in an antenna system, including an antenna arrangement according to the invention.

0031Fig. 9 shows schematic representation of a second embodiment of a multi band antenna array including additional filters.

0032Fig. 10 shows a schematic representation of a third embodiment of a multi band antenna array.

0033Fig. 11 shows an antenna system, including a multi band antenna according to the invention.

0034Detailed description of the preferred embodiments

0035The prior art antenna arrangements shown in figures IA and IB have been described above in the background to the invention.

0036Figure 2A shows a schematic representation of a dual band antenna arrangement 20, according to the present invention, operating in two frequency regions including first and second frequency bands FB₁ and FB₂ which are separate and substantially non-overlapping but relatively close to each other. The antenna elements 21 (marked with continuous lines) operating in the lower frequency band FBi is of a first type and the antenna elements 22 (marked with dashed lines) operating in the higher frequency band FB₂ is of a second type.

0037The modified version of the dual band antenna arrangement 25, shown in figure 2B, is basically the same as the one shown in figure 2A, the only difference being that cross polarised antenna elements 26 are interleaved with linear y polarised antenna elements 27.

0038In figure 2C there is illustrated how the two frequency bands are "substantially non-overlapping". The input reflection coefficient for the antenna elements 21 (figure 2A) in the lower frequency range is represented by the S-parameter Sn, whereas the input reflection coefficient for the antenna elements 22 in the higher frequency range is represented by the C-parameter S₂₂- In practice, the reflection coefficient should be less than -15 dB (R_max) • Moreover, the cross-coupling coefficient between the two frequency ranges should also be low, say less than -20 dB (C_max) . By the use of these criteria, we can define the operative frequency bands FBi and FB₂, as shown schematically in figure 2C. Thus, although the respective frequency does in fact overlap partially, the selected frequency bands FBi and FB₂ are separate and distinct from each other.

0039The first and second frequency bands should have centre frequencies being related as follows:

00402/3 < fl/f2 < 3/2, fl ≠ f2 and typical examples of possible centre frequencies are fl = 850 MHz, f2 = 900 MHz; fl = 1800 MHz, f2 = 2000 MHz; fl = 1900 MHz, f2 = 2100 MHz; fl = 2000 MHz, f2 = 2500 MHz.

0041The antenna elements could be patches, dipoles, cross polarized antenna elements, dielectric resonator antennas (DRA) or any other type of antenna elements available to the skilled person. The essential feature of the invention is that each antenna element operates in only one frequency band and that they are arranged on a reflector in an interleaved configuration along a straight line, in a single column, as illustrated in figure 2.

0042Figures 3, 4 and 5 show different embodiments of the schematic representation in figure 2.

0043Figure 3 shows a dual band antenna arrangement 30 having a first type of antenna elements 31 implemented as a- double patch antenna element transmitting and receiving within a lower frequency band FBi. A second type of antenna element 32 is implemented as a patch antenna element transmitting and receiving within a higher frequency band FB₂. An example of a lower frequency band could be 1710-2170 MHz and an example of a higher frequency band could be 2.5-2.7 GHz. Both types of antenna elements are known to those skilled in the art.

0044An intermediate distance "x", between the centres of two adjacent antenna elements, is substantially the same for all antenna elements in the array, which for the frequency bands exemplified above is in the range 0.3-0.7 λ (λ = the wavelength of the centre frequency of the highest one of the two frequency bands) or 28-54 mm. A first distance "y", between antenna elements 31 that operate within the same frequency band, namely the lower frequency band, is in the range of a distance that corresponds to 0.5-0.9 lambda (λ) of the centre frequency of that (lower) frequency band. Likewise, a second distance "z", between antenna elements 32 that operate within the higher frequency band, is in the range of a distance ^■ that corresponds to 0.5-0.9 lambda (λ) of the centre frequency of that (higher) frequency band. The distance y may be different from the distance z, but since this will give rise to un- desired effects, it is preferred that the distance y is equal to z. As an example y and z are selected to be approx. 100 mm each.

0045The embodiment described in connection with figure 3 contains types of antenna elements that are rather large and there may be a problem concerning the appearance of grating lobes that will occur when two antenna elements are placed too far from each other.

0046This effect has been considered in the embodiments illustrated in figures 4 and 5.

0047In figure 4, a perspective view of a second embodiment of a dual band antenna array 40 is shown. The dual band antenna array 40 contains two types of antenna elements, a first type 41 for the lower frequency band and a second type 42 for the higher frequency band. As an example, the first type of antenna elements 41 only receives RF signals within a range of 1920-1980 MHz and the second type of antenna elements 42 only transmits RF signals within a range of 2110-2170 MHz, which leaves a suppressed frequency band of 130 MHz therebetween. Thereby a traditional antenna for the UMTS band is replaced by a dual band antenna with separate antenna elements for the R_x band and T_x band, respectively, so that simplified T_x and R_x radio chains can be realized.

0048Both types 41 and 42 of antenna elements are made of a DRA (Dielectric Resonator Antenna) which are considerable smaller than conventional patch antennas. The drawback with the DRA is that they might have a narrow bandwidth compared to other types of antenna elements, but if used only for reception or transmission they will operate in a desired way. The size of the DRA compared to patches, as described in connection with figure 3, will minimize the appearance of grating lobes since the antenna elements can be placed closer together compared to the antenna elements described in connection with figure 2.

0049In figure 5, a perspective view of a third embodiment of a dual band antenna array 50 is shown. The dual band antenna array 50 contains two types of antenna elements, a first type 51 for the lower frequency band and a second type 52 for the higher frequency band. As an example, the first type of antenna elements 51 transmits and receives RF signals within a range of 1710-2170 MHz, which is similar to the antenna element 31 described in connection with figure 3. The second type of antenna elements 52 transmits and receives RF signals within a range of 2.5-2.7 GHz, which is the same frequency band as antenna element 32 (fig. 3) operated within.

0050A difference between the previously described antenna element 32 and the antenna element 52 is the type of antenna element being used. In the third embodiment described in connection with figure 5, a DRA is used as the second type of antenna element. Although the DRA might have a narrow bandwidth, the second antenna element will be sufficient to ensure proper operation. To reduce the coupling between adjacent antennas elements (and thereby lower the requirements/need of filters) , a shielding wall 53 is provided between each antenna element 51, 52, with the distances (x, y and z) maintained as described in connection with figure 3. Dielectric Resonator Antennas (DRA) are preferably used for the higher frequency band due to the narrow bandwidth.

0051Figures 6 and 7 show an embodiment of a multi band antenna array 60 of the present invention including three different frequency bands. This embodiment includes three types of antenna elements, a first type 61 for a lower frequency band FBi a second type 62 for a middle frequency band FB₂ and a third type 63 for a higher (or even lower) frequency band FB₃. As examples, the following combinations of centre frequencies fl, f2, f3 are possible:

0052fl = 850 MHz, f2 = 900 MHz, f3 = 1800 MHz; fl = 850 MHz, f2 = 900 MHz, f3 = 1900 MHz; fl = 850 MHz, f2 = 900 MHz, f3 = 2000 MHz; fl = 1800 MHz, f2 = 2000 MHz, f3 = 2500 MHz; fl = 1800 MHz, f2 = 2000 MHz, f3 = 2500 MHz; fl = 2000 MHz, f2 = 2500 MHz, f3 = 900 MHz.

0053There are five patch antenna elements 61 with three square- shaped DRA 62 interleaved with the three of the lowest patch antenna elements 61, and three circular-shaped DRA 63 interleaved with the three of the highest patch antenna elements 61. This results in a single column with eleven interleaved antenna elements operating at three separate frequency bands. The presence of DRA makes it possible to include shielding walls 64 between each antenna element in the column to minimize the grating lobes.

0054The distances between adjacent antenna elements are substantially the same as discussed in connection with figure 3. An intermediate distance "x", between the centres of two adjacent antenna elements, is substantially the same for all antenna elements in the colum. A first distance "y", between two antenna elements 61 that operate within the lower frequency band, is preferably a distance that corresponds to 0.5-0.9 lambda of the centre frequency of the lower frequency band, i.e. 1940 MHz in this example. A second distance "z", between two antenna elements 62 that operate within the middle frequency band, is preferably a distance that corresponds to 0.5-0.9 lambda of the centre frequency, i.e. 2.35 GHz in this example, of the middle frequency band. A third distance "w", between two antenna elements 63 that operate within the higher frequency band, is preferably a distance that corresponds to 0.5-0.9 lambda of the centre frequency, i.e. 2.6 GHz in this example, of the higher frequency band.

0055The distances y, z and w may be differ somewhat from each other, but since this will give rise to undesired effects, it is preferred that the distances y, z and w are equal to each other.

0056Figure 8 shows a block diagram illustrating the signal path in an antenna system 80 according to the present invention. The signal path can be divided into a transmission path T_x and a reception path R_x that are connected to a separate antenna element 81 and 82 for each path as illustrated in the drawing or a common antenna element (not shown) .

0057The reception path R_x comprises a band pass filter BPi to filter out the desired Radio frequency (RF) band connected in series with an optional low pass filter LP to remove spurious resonances before the filtered RF signal is fed into a Low Noise Amplifier LNA. The amplified RF signal is frequency shifted to an IF (Intermediate Frequency) signal using a Local Oscillator LO and a mixer 83. The IF signal is thereafter converted to a digital signal using an arrangement including an Analogue-to-Digital Converter (ADC) . There are three different arrangements shown in figure 8. The first option includes a Wideband A/D Converter W/ADC that converts the complete RF band into a digital stream of 16 s/c (samples/chip) . The second option includes several single carrier A/D Converter SC/ADC that together converts the complete RF band into a digital stream of 16 s/c.

0058The 16 s/c digital signal in the first and second option is thereafter fed into a digital filter DF and a Digital Down Converter DDC. The DDC converts the 16 s/c signal to a 7 s/c signal which is fed to a digital phase shifter DPS which receives control signals, preferably in digital form. The control signals are received from a connected base station (not shown) through a communication line, such as a fibre 85. DPS controls the phase cp and amplitude a. of the digitized IF signal. The signal from the DPS is fed into a summation module 84 together with signals from other optional antenna elements.

0059The third option for converting the IF signal to a digitized signal include an analogue phase shifter APS, to which control signals, preferably in analogue form, are fed that are received from a connected base station (not shown) through a communication line, such as a fibre 85. APS controls the phase φ and amplitude α of the IF signal which is digitized using a following Analogue-to-Digital Converter ADC which converts the signal into a digital stream of 16 s/c. The 16 s/c digital signal in the third option is thereafter fed into a digital filter DF and a Digital Down Converter DDC. The DDC converts the 16 s/c signal to a 7 s/c signal and is fed into the summation module 84 together with signals from other optional antenna elements. Digital I and Q signals of 2 s/c are thereafter sent to the base station through the fibre 85. Communication through the fibre may use CPRI-standard communication protocols.

0060The base station also supplies a digital I and Q signal of 1 s/c for transmission to a splitter 86. The signal can be controlled in a digital or an analogue way, both being described in connection with figure 8.

0061In a digital option the signal from the splitter 86 is fed to a Digital Phase Shifter DPS, which is supplied with digital control signals for controlling the phase φ and amplitude α of the transmission signal from the base station through the fibre 85. The signal is then fed to a device 87 for Digital Up Conversion DUC, a Digital Predistortion PDP and Crest Factor Reduction CFR is thereafter connected to the digital transmission signal. The DUC converts the signal to 16 s/c from 7 s/c. The DPD is used to obtain a linear signal after the signal is amplified and CFR is used to limit the peak in the signal to optimize the performance of the amplifier AMP. The digital signal is thereafter processed in a Digital/Analogue Converter DAC to an IF transmission signal.

0062In an analogue option the signal is fed to a device 87 for Digital Up Convertion DUC, a Digital Predistortion PDP and Crest Factor Reduction CFR is thereafter connected to the digital transmission signal. The digital signal is thereafter processed in a Digital/Analogue Converter DAC to an IF transmission signal, and is thereafter fed to an Analogue Phase Shifter APS, which is supplied with analogue control signals for controlling the phase φ and amplitude α of the transmission signal from the base station through the fibre 85. The signal is then frequency shifted to a RF transmission signal using a local oscillator LO and a mixer 88. The RF transmission signal is amplified in' an amplifier AMP with a following optional filter F. A band pass filter BF₂ completes the transmission path, where the desired radio frequency band is selected before transmission via the antenna element 82. The RF signal is sensed before the band pass filter BF₂ and frequency shifted to an IF feedback signal using a local oscillator LO and a mixer 89. The IF feedback signal is converted to a digital signal, using a Digital-to-Analogue Converter DAC, and fed into the DPD in the device 87. The same local oscillator LO is used for the transmission path.

0063In the example, different antenna elements 81, 82 are used for transmission and reception of the signals, but naturally a common antenna element may be used for both transmission and reception.

0064Figure 9 shows a schematic representation of a second embodiment of a multi band antenna array 110 including additional filters LP, BP, and HP to provide a better isolation between the operating frequency bands FBi, FB₂, and FB₃ for the antenna arrangement.

0065The antenna arrangement 110 comprises two types of antenna elements, where a first antenna element 111 is a dual band antenna element receiving RF signals in a first frequency band FBi, and transmitting RF signals in a second frequency band FB₂. The RF signals received in the first frequency band FBi is fed to a low pass filter LP, or a band pass filter for low frequencies, and thereafter to a first transceiver circuit Tl. Transmitting RF signals from the first transceiver circuit Tl are fed to a band pass filter BP and thereafter to the dual band antenna element 111. The second type of antenna element 112 is operating within a third, higher frequency band FB₃, i.e. both receiving and transmitting RF signals within FB₃. RF signals to/from the antenna element 112 is fed through a high pass filter HP, or a band pass filter for high frequencies, to/from a second transceiver circuit T2. Transceiver circuits Tl and T2 are connected to a base station BS (not shown) .

0066Suppression means in the form of metallic strips 113 are arranged between each antenna element 111, 112, to shield the antenna elements from each other. Each metallic strip is fastened to the reflector 114 in an isolating way, e.g. using a dielectric material disposed therebetween. The filters will provide an increased isolation of more than 30 dB, whereas the construction in itself may only give an isolation of 15-20 dB.

0067Only one filter is provided for all antenna elements operating within a frequency band in this embodiment, and in figure 14 another embodiment is illustrated wherein a separate filter is used for each antenna element.

0068Figure 10 shows a schematic representation of a third embodiment of a multi band antenna arrangement 115, comprising three types of DRA antenna elements 116, 117, and 118. These elements are interleaved in such a way that two antenna elements of different type are arranged between two antenna elements of the same type. The distances y, z, and w are preferably the same as described in connection with figure 6 and the distances x between adjacent antenna element 116, 117 and 118 is preferably equal to each other.

0069A suitable means to further increase the isolation between the frequency bands in a multi-band antenna is illustrated in figure 11. The figure shows a communication system 100 having a dual band antenna arrangement 101, such as any of those illustrated in connection with figures 2A, 2B, 3, 4, and 5, with a low pass filter, (or band pass filter) , LP between each antenna element 102 operating in the low frequency band and the transceiver circuitry Tl for the low frequency band, and a high pass filter, (or band pass filter) , HP between each antenna element 103 operating in the high frequency band and the transceiver circuitry T2 for the high frequency band. Each transceiver circuitry Tl, T2 is illustrated in connection with figure 8 and is connected to a base station BS, which is connected to the PSTN as is well-known to a person skilled in the art .

0070The antenna system 100 also includes a device for Remote Electrical Tilt RET, which is controlled by the base station BS. RET controls an actuator 104 that will change the electrical tilt of the lobes from the antenna 101, as is well- known to those skilled in the art.

0071If the antenna arrangement 101 includes an antenna arrangement with more than two frequency bands, such as the embodiment shown in figures 6, 7, and 13, then each antenna element operating at an intermediate frequency band is provided with a band pass filter to increase the isolation to the lower and higher frequency bands. The filters will provide an increased isolation of more than 30 dB, whereas the construction in it self may only give an isolation of 15-20 dB.

0072The feeding of the antenna elements may include probe feeding, aperture feeding for all types of contemplated antenna elements, such as Patch antennas, DRA, Dipole antennas, cross polarized antennas.