Abstract:
An antenna diversity system includes at least two antennas. Each antenna may be connected via a common connection point to a receiver by a respective switch which presents a low impedance connection between the antenna and receiver in the on state and a substantially reactive load to the antenna in the off state. Selection of appropriate impedances for the off state load enables the antennas to function as an array with a variety of beam patterns depending on the state of the switches. Cycling through a sequence of switch states steers the antenna beam providing improved resistance to fading and multipath effects. Alternatively, the antennas may be connected to a hybrid coupler, which enables two beam patterns to be available simultaneously for signal quality measurement and comparison.

Description:
FIELD OR THE INVENTION 
     The present invention relates to an antenna diversity system having a beam steering arrangement, and more particularly to an antenna diversity circuit, capable of generating a range of antenna beam patterns, and to a method of operating such a system. Applications of such a system include wireless data links in a business or domestic environment, and cordless and cellular telecommunications systems such as DECT (Digital Enhanced Cordless Telecommunications) and GSM (Global System for Mobile communication). 
     BACKGROUND OF THE INVENTION 
     For convenience of description the present invention will be described with reference to an antenna diversity system used with a receiver, however it should be understood that the invention is equally applicable to use with a transmitter owing to the reciprocity between transmitting and receiving antennas. 
     In a complex propagation environment such as within a building, signal fading is a common occurrence. The fading is caused by the mutual interference of multiple signals arriving along propagation paths of different lengths. If the interference is destructive the received signal strength may be reduced below the level required for adequate reception. 
     Within a building there is often a significant difference in the direction of propagation of the signals, with the result that a receiver having an antenna which can be directed towards a good signal while rejecting other signals will experience less severe fading and therefore enjoy better quality reception. 
     The propagation environment within a building is typically varying rapidly, for example because of the movement of people in the vicinity of the propagation paths. Hence, for the potential improvement offered by a directional antenna to be realised it must be able to be steered rapidly, which therefore requires electronic steering. 
     Adaptive antenna systems comprising a plurality of antennas are known. For example, published PCT Application WO 95/33312 discloses an arrangement for selecting two antennas from a plurality of directional antennas using two receivers. A first signal quality metric is measured for a communication signal received from a first antenna, and a second signal quality metric is measured for the communication signal received from a second antenna. When the difference between the first and second signal quality metrics exceeds a threshold a receiver is coupled to a third antenna and a third signal quality metric is measured. Based on the three metrics a primary and secondary antenna are selected from which to receive the communication signal. Steering is realised in this system by changing the selected directional antenna. However, such known systems have the disadvantage of requiring at least two complete receivers which makes them relatively expensive. 
     An example of an antenna diversity arrangement requiring only a single receiver is provided in our PCT application WO 97/08774 (PHB 34000) which provides a transceiver having two antennas and an arrangement for selecting the antenna providing the strongest received signal. While providing some benefits, such an arrangement makes no attempt to use the antenna which is not selected, and cannot selectively reject multipath signals. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to effect antenna diversity in a manner which selectively rejects multipath signals. 
     According to a first aspect of the present invention there is provided an antenna diversity system comprising at least two antennas, each of the at least two antennas being coupled to beam forming means, and control means for controlling the beam forming means to provide at least two alternative predetermined antenna beam patterns. 
     According to a second aspect of the present invention there is provided a method of operating an antenna diversity system comprising at least two antennas, each of the at least two antennas being coupled to beam forming means, and control means for controlling the beam forming means, the method comprising operating the control means to provide at least two alternative predetermined antenna beam patterns. 
     The present invention is based upon the recognition, not present in the prior art, that a variety of beam patterns can be formed by an antenna diversity arrangement comprising a plurality of antennas. Each beam pattern is produced by one or more of the antennas being connected to a transceiver while the remainder of the antennas are terminated by an appropriate impedance. Only a single transceiver is required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 illustrates a typical indoor multipath environment; 
     FIG. 2 is a schematic diagram of an antenna diversity system for use with two antennas; 
     FIG. 3 is a graph of the antenna patterns of an antenna diversity system; 
     FIG. 4 is a diagram of an implementation of an antenna diversity circuit with two antennas; 
     FIG. 5 is a schematic diagram of an antenna diversity system using a hybrid coupler; 
     FIG. 6 is a schematic diagram of an antenna diversity system using a hybrid coupler for signal quality measurement only; and 
     FIG. 7 is a diagram of an antenna diversity system with three antennas. 
     In the drawings the same reference numerals have been used to indicate corresponding features. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A simple indoor scenario is illustrated in FIG.  1 . It comprises two rooms  100 , each having walls  102  with doorways  104 . A transmitter  106  is located in one room and a receiver  108  in the other room. Three possible signal paths between the transmitter  106  and receiver  108  through doorway  104  are shown: a first signal path  110  which is direct; a second signal path  112  which undergoes one reflection from a wall  102 ; and a third signal path  114  which undergoes two reflections from walls  102 . In practice there will be a very large number of signal paths, having various numbers of reflections from walls  102  and transmissions through walls  102 . As a result of such reflections and transmissions the signal strength at the receiver  108  will vary for different paths. 
     To avoid interference between the signals  110  to  114  it is desirable to have a directional antenna arrangement which selectively receives signals coming from a particular direction. Further, it is desirable to able to steer the directional antenna arrangement. For example, if the antenna is aimed to receive the direct signal  110  and that signal is blocked (for example by someone moving into the signal path  110 ), the antenna could be steered to receive an alternative signal  112 ,  114 . 
     FIG. 2 is a schematic diagram of an antenna diversity system  200  comprising first and second antennas  202 ,  212 . The antennas are preferably substantially omni-directional, and may be of any convenient type, for example monopole, dipole or microstrip patch. Antennas  202  and  212  may be of identical or non-identical types. The first antenna  202  is connected to a first switch  204 . In the on state the switch  204  routes the signal from the antenna  202  to a receiver (Rx)  224  via a first contact  206  and a common connection point  222 . In the off state the switch  204  provides a termination  210  to the antenna via a second contact  206 . The termination  210  is chosen to be a reflective termination, absorbing no power, and should therefore have a substantially reactive impedance. The second antenna  212  is connected similarly to a second switch  214 , and from there either via a first contact  216  to the common connection point  222  or via a second contact  218  to a termination  220 . 
     Signal quality measurement means (SQM)  226  are provided to enable detection of a fade or other problem with the received signal. Any convenient method can be used in the implementation of said means, for example detection of received radio signal strength or bit error rate. A controller  228  (CON) is provided to set the state of the switches  204 ,  214 , in response to information from the receiver  224  and signal quality measurement means  226 . 
     If both switches  204 ,  214  are in the on state, and if there are equal electrical lengths from the antennas  202 ,  212  to the common point  222 , the radiation pattern will show a maximum response in the broadside direction, the antennas forming a two-element broadside array. 
     If the first switch  204  is on, connecting the first antenna  202  to the common connection point  222 , while the second switch  214  is off, presenting a reflective termination  220  to the second antenna  212 , the pair of antennas can be made to behave as an end-fire two element array. The first antenna  202 , which is switched on, is the feed element of the array while the second antenna  212  behaves as either a director or a reflector, depending on the phase of the reflective termination  220 . If the distance between the two antennas  202 ,  212  is chosen to be up to half of a wavelength at the frequency (or centre frequency) of interest then an end-fire configuration can be produced, with optimum results for a distance of a quarter of a wavelength. The end-fire configuration has a unidirectional pattern along the axis of the array. The direction of maximum gain depends on the phase of the reflective termination  220 . If the termination  220  has a positive reactance this makes the second antenna  212  behave as a reflector, which in general is the preferred arrangement. 
     Reversing the first and second switches  204 ,  214  reverses the roles of the first and second antennas  202 ,  212 , reversing the direction of maximum gain. 
     These three combinations of the first and second switches  204 ,  214  (on-on, on-off and off-on) allow the antennas  202 ,  212  to be operated in two end-fire and one broadside array configurations. 
     The general expression for the antenna pattern of an n-element linear antenna array is 
     
       
           e =sin  nu /sin  u   
       
     
     where e is the gain of the antenna array, normalised to a maximum gain of unity, n is the number of antenna elements, and u is given by 
     
       
           u =( d  cos φ+ a )/2 
       
     
     where d is the element separation (in wavelengths), a is the phase shift between elements at the frequency (or centre frequency) of interest, and φ is the angle measured clockwise relative to the axis of the array. For the broadside array pattern a=0, while for the end-fire array patterns a=±π/2. 
     The combination of two antennas in an array provides a 3 dB improvement in antenna gain compared to either a single antenna or to simple antenna space diversity (where the antenna providing the strongest signal is selected). However, the radio frequency (RF) circuitry is no more complicated than that required to implement antenna space diversity, as no additional phase shifting elements are required to cause the antenna beam to steer. 
     FIGS. 3A to  3 C show polar plots of antenna response patterns produced by the three combinations of the switches  204 ,  214  for an arrangement where the two antennas are separated by a distance of a third of, a wavelength. In all plots the axis of the array is along the x axis, i.e. from left to right on the page, with the first antenna  202  on the left and the second antenna  212  on the right. FIG. 3A shows the pattern when the switches  204 ,  214  are both on, which is a broadside pattern symmetrical about the x axis. FIG. 3B shows the pattern when the first switch  204  is on and the second switch  214  is off, which is an end-fire pattern with maximum gain in the negative x direction. Finally, FIG. 3C shows the pattern when the first switch  204  is off and the second switch  214  is on, which is an end-fire pattern with maximum gain in the positive x direction. 
     The gain differentiation between the antenna patterns is quite significant over a range of angles, which will be effective in providing an improvement in fading behaviour. An effective strategy to sweep the beam, moving the direction of maximum gain 90° at a time, is to drive the circuit so that it moves from end-fire left to broadside to end-fire right to broadside and so on. Such behaviour could be obtained by having a four-state circuit which cycles the state of the switches  204 ,  214  from on-off to on-on to off-on to on-on. On detection of a fade, the four-state circuit can be switched to its next state. 
     A variety of RF switches can be used. A switch should preferably have an on to off ratio of at least 15 dB, and the on-state loss should preferably be low. When off the switch should be reflective, i.e. it should be either a short or an open circuit or have any value of reactance with little or no resistive part. To make the second antenna behave as a reflector the appropriate termination phase can be achieved by a suitable network (for example a length of transmission line) between the antenna and the switch. 
     Suitable switches include PIN diodes and GaAs FET switches. Switching the gate bias to a low noise FET amplifier makes a good RF switch for this purpose. The inherent impedance mismatch caused by the FET when it is biased off forms a reflective termination. The idealised two-way switches  204 ,  214  shown in FIG. 2 are not necessary for a practical implementation. 
     Some applications may require an omnidirectional radiation pattern which cannot be achieved by any of the arrangements of switches described above, as shown by FIG.  3 . Such a radiation pattern is however possible with minor modifications to the arrangement described above. In this modified circuit the second switch  214  has a third switch position in which the second antenna  212  is parasitically tuned to have no effect. When the second switch  214  is in this third position and the first switch  204  is in its first position, the first antenna  202  is connected to the receiver. In this configuration the antenna pattern is simply that of the first antenna  202  alone, which is preferably chosen to be omnidirectional. 
     An antenna diversity circuit according to the present invention is shown in FIG. 4, based on an arrangement disclosed in our PCT application WO 97/08774. It comprises a substrate  410  of, for example, Duroid or FR 4 glass fibre. On one side of the substrate  410  is provided a first elongate end-fed metallic dipole  412 . In use it is intended that the dipole be arranged vertically such that the effective part of the dipole is the upper section having a length corresponding substantially to a quarter of the wavelength of the frequency (or centre frequency) of interest. The elongate dipole is formed in microstrip. 
     On the reverse side of the substrate  410 , also formed in microstrip, is a ground plane  414  and a second dipole comprising first and second elements  416 ,  417  in microstrip which are connected to the ground plane  414  at a distance corresponding substantially to a quarter of a wavelength from the free end of the first dipole element and extend away therefrom. Each of the first and second elements  416 ,  417  has a length corresponding to a quarter wavelength of the frequency (or centre frequency) of interest. The first and second elements  416 ,  417  are inclined relative to the longitudinal axis of the first dipole element. The elements  414 ,  416 ,  417 ,  426 ,  427  on the reverse side of the substrate are drawn with dashed lines. 
     The dipole elements  412 ,  416 ,  417  form the first printed antenna. The second printed antenna is of the same type, with dipole elements  422 ,  426  and  427 . The antennas are laid out on the substrate  410  such that the first dipole elements  412 ,  422  are separated by a distance corresponding to substantially one third of a wavelength of the frequency (or centre frequency) of interest. The feed lines of these two dipole elements are of the same width as the dipoles and comprise a 50 ohm line. A common feed point  222  is provided at substantially the mid-point of this line. The first and second elements  416 ,  417  and  426 ,  427  of the second dipole are on the opposite side of the substrate  410 . PIN diodes  434 ,  436  are connected to the feed lines of the first dipole elements  412 ,  422  at a position remote from their main radiation region. These PIN diodes are also connected to respective low-pass filters  438 ,  440  comprising capacitive stubs  442 ,  444  which are coupled to terminals  446 . 
     In use the common feed point  222  is connected to the input of a radio receiver  224 , and a controller  228  is provided to set the energisation of the PIN diodes  434 ,  436 . When a fade is detected by signal quality measurement means  226  the controller  228  changes the state of the PIN diodes as required to produce the next antenna pattern, for example acting as a four-state circuit as described above. 
     Measurements on a practical implementation of the antenna diversity circuit shown in FIG. 4, having an operating frequency of 5.8 GHz, gave results for antenna patterns in substantial agreement with the theoretical computations detailed above. 
     A schematic diagram of an alternative embodiment of an antenna diversity system is shown in FIG. 5, in which two radiation patterns are available simultaneously. In this embodiment the first and second antennas  202 ,  212  are not connected directly to switches  204 ,  214  but instead are connected respectively to first and second inputs  503 ,  505  of a 90° hybrid coupler (HC)  502 . An example of a suitable hybrid coupler for use in a DECT system is a Murata LDC30B030GC1900, which is sufficiently small to fit in any handset and has a loss of less than 1 dB. 
     A first output signal  504  from the hybrid coupler  502  provides a first beam signal that is the same as would be obtained from the first antenna  202  when the second antenna  212  has a reflective termination, because of the 90° phase shift applied by the hybrid coupler  502 . A second output signal  506  from the hybrid coupler  502  provides a second beam signal that is the same as would be obtained from the second antenna  212  when the first antenna  202  has a reflective termination. Hence if antennas  202 ,  212  are omnidirectional and separated by a distance of a third of a wavelength, the antenna response patterns for the outputs  504 ,  506  are those illustrated in FIGS. 3B and 3C. Alternative response patterns could be generated using different antenna spacings and couplers having phase shifts different from 90°. 
     Two directional couplers (DC)  508  enable a sample of each output signal  504 ,  506  to be tapped off for use by a signal quality comparator (SQC)  510 . The comparator  510  may for example comprise circuitry to measure the received radio signal strength of each signal, or some other suitable quality parameter such as bit error rate. 
     If required the directional couplers  508  could be replaced by switches. This alternative implementation has the disadvantage that the comparator  510  cannot compare signal qualities while the receiver  224  is receiving signals, but avoiding the use of directional couplers may be preferable in some applications. 
     A switching arrangement identical with that of FIG. 2, comprising switches  204 ,  214  as described above, may be used to select the output from one of the directional couplers  508  for connection to the common connection point  222  and receiver  224 . A controller (CON)  512  sets the state of the switches  204 ,  214  in response to information from the receiver  224  and signal quality comparator  510 . Differences from the arrangement of FIG. 2 are that only one of the switches  204 ,  214  can be on at a time, and the unused beam must be terminated in a matched load  514  (normally 50 ohms) for correct operation of the hybrid coupler  502 . 
     Both the switching embodiment (of FIG. 2) and the hybrid coupler embodiment (of FIG. 5) are particularly suitable for use in Time Division Multiple Access (TDMA) systems such as DECT, in which signals are transmitted in time frames, each comprising a preamble and a data section. In such a system, signal quality measurements or comparisons can be performed during reception of the preamble in a frame enabling the best beam pattern to be selected for reception of the data section. 
     Such implementations also allow use at high velocity, by reducing the time delay between signal measurement and implementation of diversity switching. This enables the switching to be performed on a per-frame basis, avoiding the need to attempt to determine the temporal fading characteristics of the received signal. 
     A further advantage is that the directionality of the two beams reduces the delay spread in the received signals, thereby reducing inter-symbol interference and making it possible to achieve higher data rates. 
     Both the switching and hybrid coupler embodiments are also suitable for transmission, in which case the receiver  224  is replaced by a transceiver. The beam last selected for reception could be used for transmission, taking advantage of the symmetry of the radiation channel in a time division duplex system. The transmitted signal will have the advantage of a 3 dB directive gain over that of a single omnidirectional antenna, thereby giving enhanced coverage. 
     In a frequency division duplex system the radiation channel may not be symmetric, depending on the frequency difference between uplink and downlink transmissions and other characteristics of the radio propagation environment. It may therefore be better simply to use an omnidirectional beam pattern for transmission in some circumstances. 
     A disadvantage of the hybrid coupler embodiment of FIG. 5 is that 3 dB of power is dissipated by the load  514  on the unused output of the hybrid coupler  502  during reception. However, the performance is no worse than that with a single omnidirectional antenna, because of the directive gain in each beam of approximately 3 dB. 
     FIG. 6 illustrates a second hybrid coupler embodiment which alleviates this problem. The embodiment is similar to that of FIG. 5, but instead of the antennas  202 ,  212  being connected directly to the respective inputs  503 ,  505  of the hybrid coupler  502  they are connected to switches  602 ,  604 . For sampling and comparison of the quality of received signals the switches  602 ,  604  connect the antennas  202 ,  212  to the inputs  503 ,  505  of the hybrid coupler  502 , whose outputs  504 ,  506  are connected directly to the signal quality comparator  510 . 
     For normal reception and transmission the switches connect the antennas  202 ,  212  to terminals  606 ,  608  which connect directly to switches  204 ,  214 , bypassing the hybrid coupler  502 . The remainder of the circuit operates in the same way as the switching embodiment of FIG. 2 described above, with a transceiver (Tx/Rx)  610  in place of the receiver  224 . The settings of the switches  204 ,  214  are determined from measurements made by the comparator  510  when connected to the outputs  504 ,  506  of the hybrid coupler  502 . 
     If it is desired to have continuous comparison of received signal quality from the different beams, the switches  602 ,  604  can be replaced by directional couplers  508 . 
     The method in accordance with the present invention could also be applied to antenna diversity systems having more than two antennas. For example, one problem with the two antenna arrangement detailed above is that the broadside pattern, shown in FIG. 3A, is symmetrical about the axis of the array. FIG. 7 is a plan view of one possible arrangement of three antennas that overcomes this limitation. The arrangement comprises a first antenna  202  and a second antenna  212 , separated by a maximum distance of a half of a wavelength at the frequency (or centre frequency) of interest, together with a third antenna  702 , halfway between the first and second antennas  202 ,  212  but separated laterally by a maximum distance of a half of a wavelength at the frequency (or centre frequency) of interest. 
     This arrangement retains the end-fire capability, in directions A and B, of the two antenna array, while adding the ability to generate antenna patterns in directions C and D. 
     It will be apparent to those skilled in the art, by using the teachings in accordance with the present invention, that the addition of further antennas would enable the generation of a greater number of directional antenna patterns. 
     Although the antenna diversity system was described above in relation to use with a receiver, it will be apparent that it could equally well be used with a transmitter. For example, in a two way cordless communication system, such as DECT or GSM, the antenna beam pattern which is optimum for reception of signals at a handset from a base station will also, by reciprocity, be optimum for transmission of signals from the handset to the base station. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in antenna diversity systems and methods of operating such systems, and which may be used instead of or in addition to features already described herein. 
     In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.