METHOD AND APPARATUS FOR TESTING WITH SIMULATED MOVING MOBILE STATIONS

Abstract: Method and apparatus for realistic testing in a radio communication system, wherein a plurality of antenna elements are distributed to form an antenna group. A local control unit controls a switching unit to selectively connect a first stationary radio station to the antenna elements, one at a time, in a cyclically repeated sequence such that, during a radio communication with a second stationary radio station, a relative movement is emulated between the antennas of the radio stations. The antenna group distribution and the switching sequence are selected such that the emulated antenna movement creates a Rayleigh fading effect in the communicated raio signals due to reflections in a pyhsical radio environment, giving rise to multipath interference.

Description: Method and Apparatus for Testing with Simulated Moving Mobile Stations FIELD OF THE INVENTION The present invention relates generally to testing in radio communication systems. More specifically, the invention relates to an antenna arrangement and a method for simulating relative movement between two communicating radio stations for testing in radio communication systems using the air interface. BACKGROUND OF THE INVENTION In a radio telecommunication system, a mobile switching center (MSC) is linked to a plurality of base stations by a series of transmission links. The base stations are geographically distributed to form an area of coverage to provide radio communication with mobile stations in that area. Each base station is designated to cover a specified area, known as a cell, in which a two-way radio communication over a specified radio channel can take place between the base station and a mobile station. Equipment manufacturers have long recognized the importance of testing various radio network equipment prior to installation in the field. Furthermore, designers and operators of radio communication systems sometimes find it valuable to test system performance, e.g., when developing new algorithms, operational parameter settings or frequency planning configurations, in order to verify the functionality of the system. By way of example, in order to evaluate new functions for power regulation and/or handoff procedures before installation in live systems, it is desirable to have them tested in a laboratory environment which simulates a live system as realistically as possible. In prior testing methods, the air interface has been simulated in testing procedures using a coaxial cable to connect the mobile station to the base station to form a coaxial network air interface, which is described in detail in U.S. Patent No. 5,465,393 granted to Frδstrom et al. entitled: "Simulated Air Interface System for Simulating Radio Communication," issued on November 7, 1995 to the present assignee, the disclosure of which is incorporated herein by reference. A disadvantage of using such a coaxial test network is that the signals transmitted between the mobile station and the base station are unrealistically isolated from other interfering signals, such as those from other mobile communications and reflections on physical objects of their own signals. It is thus desirable to include the air interface in such test systems in order to recreate real-life conditions such that interfering radio signals, log-normal and multipath fading influence the results. Indeed, those factors are highly interesting to handle in tests since they often have a negative effect on the system performance. However, there can be problems with such factors during testing due to the inherent randomness associated with the radio path, thus leading to a lack of repeatability in testing procedures. One approach is to use actual mobile stations which are operated by a number of people acting as testers and moving around in order to create, for example, multipath fading and handoff events. However, this technique is personnel demanding and cumbersome, since it is difficult to exactly repeat a moving pattern of a plurality of mobiles. These problems have been partially addressed in co-pending U.S. Patent Application Serial No. 09/195,671 entitled: "Air Interface Based Wireless Telecommunication Test System, " filed on November 19, 1998, the disclosure of which is incorporated herein by reference. Therein, an indoor antenna arrangement for testing wireless telecommunication systems is described. The arrangement includes a plurality of individual antennas which are deployed in a fixed pattern within a confined testing area such as a building or a laboratory. Further, a plurality of base stations are selectively connected to the antennas via an antenna matrix such that a cell is activated when the antenna matrix makes a connection between a base station and an individual antenna. The antenna matrix can be controlled to automatically activate cells according to a specified testing procedure, thereby making the cells shift from antenna to antenna in a step wise fashion. This enables the use of stationary (non-moving) mobiles which are thus simulated to move into and out off cells as the cells instead 'move' from one antenna to the next. An exemplary test system of this type is illustrated in Fig. la where a room is shown having twenty-one antennas A1-A21 distributed therein. In an exemplary snapshot in time, a predefined cell pattern is present according to Fig. la such that every fifth antenna Al, A6, All and A16 is active, their associated radio coverage areas or cells being schematically represented by circles 11 - 14 respectively. A mobile 10 is shown placed in a fixed location, currently within cell 11, communicating with a base station connected to antenna Al. Further mobiles may be placed in various fixed locations within the room of Fig. la. Movement of the stationary mobile 10 is simulated by shifting the cell pattern in a step-wise manner such that each base station changes antenna by means of an antenna matrix, not shown. A shift in the clockwise direction, for example, is indicated by arrows. As the cell pattern moves relative to the stationary mobile, the received signal strength of the current connection in cell 11 weakens which the mobile perceives as a movement in its position. When a candidate cell 14 following the current cell 11 moves toward the stationary mobile 10, the received signal strength of the candidate cell 14 becomes greater than that of the current cell 11, thereby triggering a handoff from cell 11 to cell 14. During tests, the communicating stations will in this way experience a fairly realistic, step- wise log-normal fading effect on a received radio signal, sometimes resulting in handoffs. Since a test sequence including a series of cell activations can be repeated exactly, it is possible to run the same test over and over again, using, e.g., different system parameter settings or different algorithms, and compare the results. However, using stationary antennas in the indoor antenna arrangement described above will not in a realistic way include the effects from multipath fading, so-called Rayleigh fading, which term will be used hereinafter. In this arrangement, the individual base station antennas are placed in a room at comparatively large intervals such that during periods between the antenna shifts, the distance between two communicating stations is constant and the received signal strength is stable. In real-life, when the mobile continuously moves through a physical environment, the received radio signal strength will fluctuate due to Rayleigh fading, since the received signal typically is the sum of both the direct signal and various reflected signals. The multipath interference of these signals will cause a signal strength fluctuation of the received signal which is illustrated in Fig. lb, where the received signal strength is shown as a function of the distance between a transmitter and a receiver. For example, at points where a reflected signal and a direct signal are out of phase, there will be a signal strength minimum or dip 101 since the two signals will then interfere destructively with each other, and vice versa when the two signals are in-phase, as is well known in the art. Such fading dips due to one reflection will thus occur at intervals of one wavelength. For example, if a carrier frequency of 850 MHZ is used, the dips will occur at intervals of approximately 35 cm. In Fig. lb, it is also illustrated with a dotted line 102 that the signal strength gradually decreases with an increasing distance, which is attributable to the log-normal fading. In view of the foregoing, it is desirable to provide a method and a radio communication testing system that more closely resemble real-life conditions with regards to the air interface. In particular, it would be a great advantage to create the effects of Rayleigh fading during tests which employ stationary antennas for both of two radio stations in communication with each other. SUMMARY OF THE INVENTION According to exemplary embodiments of the present invention, these and other objects are achieved by providing a method and an apparatus for testing in radio communication systems, wherein an antenna unit comprises a plurality of antenna elements which are distributed in a predetermined pattern, forming an antenna group. This antenna unit also includes a switching unit and a local control unit controlling the switching unit to selectively connect a first stationary radio station to the antenna elements, one at a time, in a sequence such that, during a radio communication with a second stationary radio station, a relative movement is emulated between the active antenna of the first radio station and the antenna of the second radio station. Preferably, the switching sequence is repeated cyclically. The predetermined distribution pattern of the antenna elements and the sequence are selected such that said emulated relative movement creates a multipath or Rayleigh fading effect on the radio signals communicated between the first and the second radio stations due to reflections of the transmitted radio signals in a physical environment, giving rise to multipath interference. According to a preferred embodiment, the first stationary radio station is a base station and the second stationary radio station is a mobile station. The plurality of antenna elements at the first stationary radio station may be distributed to form, e.g., a substantially straight line, a circle or an arc. The invention further provides a system for testing radio communication with a plurality of stationary radio stations located in a physical environment, including surfaces capable of reflecting radio signals, wherein the stationary radio stations each comprises an antenna arrangement for creating Rayleigh fading in communicated radio signals. BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying figures, of which: FIGURE la illustrates an indoor antenna arrangement for testing wireless telecommunication systems according to co-pending U.S. Patent Application Serial No. 09/195,671. FIGURE lb illustrates a Rayleigh fading pattern that a moving mobile station typically experiences during a call in a real-life radio communication system. FIGURE 2 illustrates a part of a test system including an antenna arrangement in accordance with exemplary embodiments of the present invention. FIGURES 3a and 3b illustrate alternative antenna group configurations according to the present invention. FIGURE 4a is a block diagram illustrating a part of an exemplary test system including control units for controlling a plurality of antenna groups according to one aspect of the present invention. FIGURE 4b is an alternative block diagram to that of Fig. 4a. FIGURE 5 illustrates more closely the local control unit and switching unit of Fig. 4a or 4b according to one preferred implementation of the present invention. FIGURE 6 illustrates a plurality of antenna groups distributed in a test room. DETAILED DESCRIPTION The invention will now be described in connection with Fig. 2 which illustrates the function of the invention in a simplified form. In the figure is shown a base station 200, a mobile station 204, wherein the base station 200 and the mobile station 204 are in radio communication with each other, and a reflecting object 205, e.g., a wall in a room. The base station 200 is connected to an antenna unit 201 including an antenna switch 202 and a plurality of antenna elements 203. The antenna switch 202 selectively connects a transmitting path of a transceiver (not shown) in the base station 200 to the plurality of antenna elements 203, in this example eight antenna elements 203.1 - 203.8 evenly distributed along a straight line. The switch 202 connects base station 200 to each antenna element 203 in a predetermined sequence, e.g., from 203.1 to 203.8 and then backwards from 203.8 to 203.1. This switching sequence can be cyclically repeated such that an oscillating movement back and forth of the base station antenna is emulated, giving rise to a Rayleigh fading effect to the radio signals received at the mobile station 204, as will be explained below. The distance d between the two most widely spaced antenna elements 203.1 and 203.8 is, in this example, selected to be approximately * 2 wavelength of the carrier wave used, such that when antenna element 203.1 is active, the direct signal 206 and the reflected signal 207 from the base station 200, are in-phase with each other when reaching the mobile 204. When the antenna switch connects the base station transceiver to antenna element 203.8, the direct signal 208 and the reflected signal 209 will be out of phase when reaching the mobile 204, thus interfering destructively with each other resulting in a fading dip. In this simplified example, it is assumed that the reflecting object 205 is far away from the plurality of antenna elements 203 compared to the distance d between antenna elements 203.1 and 203.8. The reflected signals 207 and 209 will then travel approximately the same distance, whereas the traveling distances of the direct signals 206 and 208 will differ with λ h wavelength. It is further preferable to use equal cable length from a switching point 210 to each antenna element 203.1- 203.8. Otherwise, compensations can be made for different delays attributable to signal propagation in the cables. In the example above, a radio transmission from the base station to the mobile station was discussed. Of course, the Rayleigh fading effect will also occur for a radio transmission from the mobile station to the base station if the receiving path of the base station transceiver is switched in the above-described manner. It is also possible to implement the antenna unit at the mobile station. Those skilled in the art will appreciate that the actual reflected signals will include a multitude of reflections at different angles from various physical object surfaces in the testing environment, giving rise to a more complex fading pattern. Although a straight line distribution of the plurality of antenna elements or antenna group 203 is shown in Fig. 2, other distribution patterns are possible, such as a circle as shown in Fig. 3a, or an arc as shown in Fig. 3b. Of course, further variations are possible within the scope of the present invention. In order to achieve a realistic Rayleigh fading effect, it is important to properly select an antenna distribution pattern, including the number of antenna elements in the antenna group and the distance d between the two most widely spaced antenna elements in the group, as well as a switching sequence for emulating the antenna movement. In an exemplary embodiment, at least eight antenna elements are included in the antenna group and the distance d is on the order of V% - 2 wavelengths which will provide a reasonable life-like Rayleigh fading effect, although more or fewer than eight antenna elements may be used depending on test requirements. Another factor which influences the Rayleigh fading effect in tests is the rate of the sequential switching between the antenna elements. A high switching rate simulates a fast-moving mobile and vice versa. The switching rate can be controlled with high accuracy and within a wide range in the present invention, thus enabling a wide spectrum of test cases, as will be described below. An exemplary configuration for controlling a plurality of antenna groups to create Rayleigh fading in a test system is illustrated in the block diagram of Fig. 4a. Therein, three base stations 41 are shown, each being selectively connected by an antenna switch 43 to an antenna element in a separate antenna group 42 for radio communication. Each antenna switch 43 is controlled by a local control unit 44 and the three local control units 44 are further controlled by a common central control unit 45. It is to be understood that any number of base stations with associated antenna groups may be used for different tests. Each local control unit 44 receives control signals including operational instructions from the central control unit 45 and the local control unit 44 controls the antenna switch 43 to switch the antenna element connection accordingly. The control signals received from the central control unit 45 may include a switching rate, a clock pulse and a switching sequence, i.e., in which order the antenna elements are to be connected. In Fig. 4a, the central control unit 45 is connected to each local control unit 44 by means of a separate connection line 46. Alternatively, the connections can be accomplished with a so-called Daisy chain, as illustrated in fig 4b, wherein the central control unit 45 is connected to a first local control unit 44.1 , which in turn is connected to a second local control unit 44.2 and so forth. The control signals transmitted from the central control unit 45 then include a local control unit address and control signals are received in a serial manner by the local control units 44 but action is only taken by the addressed local control unit. A control instruction or signal may include more than one local control unit address. Fig. 5 shows a block diagram illustrating in more detail a control arrangement according to an exemplary implementation of the present invention. As in previous examples, a base station 51 is via a connection line 58 selectively connected to a group of antenna elements 52 by an antenna switch 53 which is controlled by a local control unit 54. A central control unit 55 transmits control signals, such as operational instructions including parameters defining a switching rate and a switching sequence, to the local control unit 54. Furthermore, the central control unit 55 may transmit control signals including a clock pulse controlling the switching rate from a master clock (not shown) residing in the central control unit 55. The clock pulse may be sent continuously or at selected intervals. Alternatively, the clock pulse may be generated in a local clock residing in each local control unit. The control signals transmitted by the central control unit 55 are received by a program logic device 56 in the local control unit 54. The program logic device 56 controls a set of switching drivers 57 of which each driver connects and disconnects a corresponding switching element in the antenna switch 53 to an antenna element of the antenna group 52. The switching elements can be of a PIN-diode type and the switching drivers can be PIN-diode drivers, which components are well-known in the art. The local control unit 54 may be put into at least two different operation modes: a programming mode when the local control unit 54 receives instructions from the central control unit 55, and a run mode when the local control unit 54 performs the switching in accordance with the received instructions. During the run mode, the local control unit 54 may also receive control signals including a clock pulse from the central control unit 55. The present invention provides a realistic environment including the air interface for the testing of equipment or functions in a radio communication system using stationary radio stations. Since real antennas are used, a multitude of realistic factors are included in the tests, such as interference and fading. Furthermore, using stationary radio stations provides for consistent environmental conditions that are suitable for comparing results from repeated tests. In particular, the invention enables tests where a wide range of different mobile speeds can be simulated and in which a realistic Rayleigh fading effect is included without using any physically moving parts. Another benefit is that since no moving parts need to be involved, the need for testing personnel as well as for operation and maintenance of mechanical moving structures is eliminated. The invention can advantageously be used in combination with the test system of the above-mentioned U.S. Patent Application Serial No. 09/195,671, thus providing the effects of both fluctuating Rayleigh fading and log-normal fading resulting in, for example, handoff events and power regulation mechanisms for stationary mobiles. In that case, each individual antenna of the above- mentioned U.S. Patent Application Serial No. 09/195,671 may be substituted with an antenna unit according to the present invention as illustrated in Fig. 6. Al, A2 ... denote different antenna units which are deployed in a fixed pattern within a confined testing area 61. A plurality of base stations (not shown) are selectively connected to the antenna units Al, A2... via an antenna matrix (not shown) to activate cells in a first predetermined sequence. In Fig. 6, only one cell is shown in two positions 62, 63 for simplicity. The full circle 62 illustrates the cell coverage area when antenna unit A2 is active by being connected to a base station (not shown). When antenna unit A3 is activated instead by shifting the base station connection, the cell is shifted to the dotted circle 63, as indicated by an arrow, whereby a stationary mobile station 64 becomes inside the cell. During the active periods, each active antenna unit emulates a mobile station movement by selectively connecting the base station to the antenna elements within the antenna unit in a second predetermined sequence in accordance with the invention as described above. While the invention has been described with reference to specific exemplary embodiments, the description is only intended to illustrate the inventive concept and should no t be taken as limiting the scope of the invention. Various alternatives, modifications and equivalents may be used without departing from the spirit of the invention, which is defined by the appended claims.

Claims: CLAIMS 1. A method of testing in a radio communication system including a first stationary radio station which comprises a transceiver, an antenna unit comprising an antenna switch and a plurality of antenna elements forming an antenna group, wherein said antenna elements are distributed in a predetermined pattern, the method comprising the steps of: connecting, by said antenna switch, said transceiver selectively to each of the antenna elements in a predetermined switching sequence; and communicating radio signals between said first stationary radio station and a second stationary radio station, wherein said predetermined antenna distribution pattern and said predetermined switching sequence are selected such that said communicated radio signals are subject to Rayleigh fading. 2. The method of claim 1, wherein said predetermined switching sequence is cyclically repeated. 3. The method of claim 1 , wherein said first stationary radio station is a base station and said second stationary radio station is a mobile station. 4. The method of claim 1, wherein said predetermined antenna distribution pattern includes a distance between the antenna elements on the order of Vz - 2 wavelengths of a carrier frequency used for the communication of said radio signals. 5. The method of claim 4, wherein said distance between the antenna elements is the distance between the two most widely spaced antenna elements of said antenna group. 6. The method of claim 1, wherein said connecting step is performed with a predetermined switching rate associated with a simulated speed of the second radio station relative to the first radio station. 7. The method of claim 1 , wherein said predetermined distribution pattern is in the form of a substantially straight line. 8. The method of claim 1, wherein said predetermined distribution pattern is in the form of a circle. 9. The method of claim 1 , wherein said predetermined distribution pattern is in the form of an arc. 10. The method of claim 1, wherein said connecting step further comprises the step of: controlling, by a local control unit, said antenna switch to connect said transceiver selectively to each of the antenna elements according to said predetermined switching sequence. 11. The method of claim 10, further comprising the step of: receiving control signals in said local control unit from a central control unit, wherein the central control unit transmits control signals to further local control units, each controlling a separate antenna group. 12. The method of claim 11, wherein said control signals include operational instructions. 13. The method of claim 12, wherein said operational instructions include parameters defining a switching rate and said predetermined switching sequence. 14. The method of claim 11, wherein said control signals include a clock pulse. 15. A system for testing radio communication, comprising: a first stationary radio station including a transceiver, an antenna unit including an antenna switch and a plurality of antenna elements being distributed in a predetermined pattern forming an antenna group; a second stationary radio station in radio communication with said first radio station, wherein said antenna switch is controlled to connect said transceiver selectively to each of the antenna elements in a predetermined switching sequence such that said radio communication is subject to Rayleigh fading. 16. The system of claim 15, wherein said first stationary radio station is a base station and said second stationary radio station is a mobile station. 17. The system of claim 15, wherein said predetermined antenna pattern includes a distance between the antenna elements in the order of Vi - 2 wavelengths of a carrier frequency used for said radio communication. 18. The system of claim 17, wherein said distance between the antenna elements is the distance between the two most widely spaced antenna elements of said antenna group. 19. The system of claim 15, wherein said predetermined distribution pattern is in the form of a substantially straight line. 20. The system of claim 15, wherein said predetermined distribution pattern is in the form of a circle. 21. The system of claim 15, wherein said predetermined distribution pattern is in the form of an arc. 22. The system of claim 15, further comprising: a local control unit for controlling said antenna switch to connect said transceiver selectively to each of the antenna elements according to said predetermined switching sequence. 23. The system of claim 22, wherein the local control unit controls said antenna switch to cyclically repeat said predetermined switching sequence. 24. A system for testing radio communication, comprising: a plurality of radio stations, each comprising a transceiver and an antenna unit including an antenna switch, a local control unit and a plurality of antenna elements being distributed in a predetermined pattern forming an antenna group; at least one surface capable of reflecting radio signals; and a central control unit for transmitting control signals to each local control unit, wherein the local control unit controls said antenna switch to connect said transceiver selectively to each of the antenna elements in a predetermined switching sequence such that communicated radio signals are subject to Rayleigh fading. 25. The system of claim 24, wherein said control signals include operational instructions. 26. The system of claim 25, wherein said operational instructions include parameters defining a switching rate and said predetermined switching sequence. 27. The system of claim 24, wherein said control signals include a clock pulse. 28. The system of claim 24, wherein said plurality of radio stations are base stations. 29. The system of claim 28 further comprising at least one stationary mobile station, wherein said communicated radio signals are received by said at least one stationary mobile station. 30. The system of claim 28, wherein said communicated radio signals are received by at least one of said base stations. 31. The system of claim 24, wherein each local control unit controls the antenna switch to cyclically repeat said predetermined switching sequence. 32. An apparatus for creating Rayleigh fading in a radio communication between two radio stations, comprising: a plurality of antenna elements being distributed in a predetermined pattern forming an antenna group; an antenna switch; a local control unit for controlling said antenna switch to connect a transceiver in one of said radio stations selectively to each of said antenna elements according to a predetermined switching sequence such that said radio communication is subject to Rayleigh fading. 33. A system for testing radio communication, comprising: a plurality of antenna units, each comprising an antenna switch, a local control unit and a plurality of antenna elements being distributed in a predetermined pattern forming an antenna group; at least one base station; an antenna matrix, wherein the antenna matrix selectively connects said at least one base station to the antenna units for activating cells in a first predetermined sequence such that said radio communication is subject to log-normal fading and wherein the local control unit controls said antenna switch to selectively connect said at least one base station to each of said antenna elements according to a second predetermined switching sequence such that said radio commumcation is subject to Rayleigh fading. 34. The system of claim 33, wherein the local control unit controls the antenna switch to cyclically repeat said second predetermined switching sequence. 35. The system of claim 33 further comprising at least one stationary mobile station.

Inventors: FROESTROEM TORE, GREEN PER, FROESTROEM, TORE, GREEN, PER
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2001
Main classification: H01Q3
IPC codes: H01Q3, H04B7, H04W24