Over-the-air test

0001Over-the-air test

0002Field

0003The invention relates to an over-the-air testing of a device in an an- echoic chamber. Background

0004When a radio frequency signal is transmitted from a transmitter to a receiver, the signal propagates in a radio channel along one or more paths having different angles of arrivals, signal delays and powers, which causes fadings of different durations and strengths in the received signal. In addition, noise and interference caused by other transmitters interfere with the radio connection.

0005A transmitter and a receiver can be tested using a radio channel emulator emulating real circumstances. In a digital radio channel emulator, a channel is usually modeled with a FIR filter (Finite impulse Response filter), which generates convolution between the channel model and an applied signal by weighting the signal, delayed by different delays, with channel coefficients, i.e. tap coefficients, and by summing the weighted signal components. The channel coefficients are functions of time to correspond to the temporal behaviour of a real channel. A traditional radio channel emulator test is performed via a conducted connection such that a transmitter and a receiver are coupled together via a cable.

0006Communication between a subscriber terminal and a base station of a radio system can be tested using an OTA (Over The Air) test where a real subscriber terminal is surrounded by a plurality of antennas of an emulator in an anechoic chamber. The emulator which may be coupled to or act as a base station emulating paths between the subscriber terminal and the base station according to a channel model, in the test, the direction of a path depends on the direction of an antenna, and hence the directions of paths are limited and there is a need for a better OTA test solution. Brief description of the invention

0007An object of the invention is to provide an improved method. According to an aspect of the invention, there is provided a method of communicating with an electronic device under test through a simulated radio channel of an emulator. The method further comprises forming a beam of a signal of a path of a simulated radio channel with at least two antenna elements of a plurality of antenna elements coupled to an emulator in an anechoic chamber.

0008According to another aspect of the invention, there is provided a testing system, the testing system comprising an emulator having a simulated radio channel for communicating therethrough with the electronic device. The testing system comprises a plurality of antenna elements couplable to an emulator; and the emulator is configured to form a beam of a signal of a path of a simulated radio channel with at least two antenna elements of the plurality of antenna elements in an anechoic chamber.

0009According to another aspect of the invention, there is provided a computer program product encoding a computer program of instructions for executing a computer process for communicating with an electronic device under test through a simulated radio channel of an emulator. The process comprises: forming a beam of a signal of a path of a simulated radio channel with at least two antenna elements of a plurality of antenna elements coupled to an emulator in an anechoic chamber.

0010The invention provides several advantages. The direction of paths may be more freely controlled, and an effect of the antenna of the DUT is in- eluded in the test. Additionally, complex radio channel scenarios can be modeled.

0011List of drawings

0012In the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which Figure 1 illustrates a propagation of a radio signal; Figure 2 illustrates a power azimuth spectrum of reception beams, Figure 3 illustrates a power azimuth spectrum of transmission beams, Figure 4 shows a measurement configuration in an OTA test chamber,

0013Figure 5 shows a beam to be modeled by the antenna elements, Figure 6 shows a group of antenna elements and associated antenna group switching network, Figure 7 shows a DLJT surrounded by groups of antenna elements, Figure 8 presents controlling delays of antennas in a MIMO configuration,

0014Figure 9 presents controlling delays of antennas in an OTA chamber, and Figure 10 shows a flow chart of a method.

0015Description of embodiments

0016Figure 1 illustrates a propagation of a radio signal between a transmitter and a receiver. The transmitter 100 may comprise an antenna 102 of at least one antenna element 104 to 110. The antenna may be, for example, ULA (Uniform Linear Array) antenna where the spacing between the antenna elements is constant, for example half a wavelength of the radio signal. In this example, the transmitter 100 may be a base station of a radio system. Correspondingly, the receiver 112 may comprise an antenna 114 of at least one antenna element 116 to 122. In this example, the receiver 112 may be a sub- scriber terminal of a radio system. When the transmitter 100 transmits a radio signal, a transmission beam 124 may be directed to an angle øj and its angle spread may be δφ which may be xδf^d , where x is a real number larger than zero and δf^ύ is a standard deviation of the angle φi. The transmission beam

0017124 may hit at least one cluster 126, 128 which reflects and/or scatters the ra- diation. Each cluster 126, 128 may have a number of active regions 1260 to 1264, 1280 to 1286 which predominantly reflect and/scatter in the cluster 126, 128. A cluster 126, 128 may be fixed or moving, and a cluster 126, 128 may be a natural or man-made object such as a building, a train, a mountain etc. The active regions may be some finer structural features on an object. The reflected and/or scattered beam may be directed towards the antenna 114 of the receiver 112. The antenna 114 may have a reception angle ψi and its angle spread may be δ_ψ which may be y<5*^td , where y is a real number larger than zero and ό^ is a standard deviation of angle <p_t. The beam

0018130 reflected and/or scattered from a cluster 126 may then be received. Simi- larly, the antenna 114 may also have a beam from a reception angle φ₂ and its angle spread may be δ_ψ2 . The propagation from a transmitter 100 to a receiver 112 via at least one cluster 126, 128 causes an additional delay to a signal with respect to a signal traveling straight along a line-of-sight.

0019Clusters 126, 128 in a radio channel are responsible for multi path propagation. It can be approximated that a path and a cluster 126, 128 have a correspondence such that one received path comes from one cluster. Hence, a radio channel may be described by cluster powers, delays, nominal AoA (Angle of Arrival) and AoD (Angle of Departure), and angle spreads of clusters at both arrival and departure ends. Additionally, information on the receiver and transmitter antenna arrays is required. The information may include values of parameters of an antenna array geometry and an antenna field pattern (beam). Also the subscriber terminal velocity vector and/or the cluster Doppler frequency component may be needed.

0020Table 1 presents an example of a clustered delay line model of a radio channel in an urban environment. Clusters 1 and 3 have three active regions which have different delays and powers.

0021Tablei . Non-line-of-sight clustered delay line model, urban macro- cell.

0022<img file="WO2010040887A1_D0001.tif" /> An ASD (Angle Spread of Departure) may be assumed constant for all clusters, ASD = 2° in this example. Correspondingly, an ASA (Angle Spread of Arrival) may be assumed constant for all clusters, ASA = 15° in this example. Additionally, XPR (Cross Polarization Power ratio) may also be assumed constant for all clusters, XPR = 7 dB in this example. They may also be different for different clusters.

0023An impulse response estimate H_UiS,n(t, T) of a radio channel may be expressed in a mathematical form as follows:

0024<img file="WO2010040887A1_D0002.tif" />

0025where Ftχ_jS is a transmission antenna field pattern (i.e. transmission beam), Frx,u is a reception antenna field pattern (i.e. reception beam), d_s is a distance between the antenna elements in a ULA transmission antenna, d_u is a distance between the antenna elements in a ULA reception antenna, k is a wave number (k = 2π/λo, where λo is a wavelength of the radio signal), P_n means a cluster power, M means the number of active regions in a cluster, m is an index of an active region, n is an index of a cluster, Φ_n,_m is a constant phase term of a scatterer n,m, v_π,_m is a Doppler frequency of an active region having index n,m and ris a delay.

0026A Doppler frequency of an active region having index n,m can be expressed as:

0027„ l> π,m - H^cosfcv, -^βv) , _m [£■)

0028A₀ where v is a velocity vector and ||v|| is a relative speed between an active re- gion and the receiver.

0029The impulse response estimate in equation (1) may be simplified, when the receiver antenna is assumed omnidirectional, in the following form

0030<img file="WO2010040887A1_D0003.tif" />

0031Figure 2 illustrates a power azimuth spectrum of reception beams from five clusters. In Figure 2 the x-axis is angle in degrees and the y-axis is power in decibels. The five beams 200, 202, 204, 206 and 208 are received at different angles of arrival. The beams 200, 202, 204, 206 and 208 may be received at different moments of time i.e. at least one of them may have a different delay with respect to the other beams. Figure 3 shows a power azimuth spectrum of transmission beams to the same five clusters according to the example in Figure 2. In Figure 3, the x- axis is angle in degrees and the y-axis is power in decibels. The five beams 300, 302, 304, 306 and 308 are transmitted at only a slightly different angles of departure since the reflecting and/or scattering clusters are only slightly dis- persed in the angle.

0032Figure 4 presents an OTA test chamber. The test chamber may be an anechoic room. A DUT 400 such as a subscriber terminal may be surrounded by antenna elements 402, 404, 406, 408, 410, 412, 414 and 416 which are coupled to an emulator 418 which may be, for example, EB (Elek- trobit) Propsim® C8. The emulator 418 may comprise a processor, a memory and a suitable computer program. In this example, there are eight antenna elements in a circle separated by a constant angle of 45°. In general, there may be at least two antenna elements 402 to 416 and they may be separated from each other by a separation angle Aθ. When there are at least three an- tenna elements 402 to 416, the separation angle Δ0 may be the same or different for any two successive antenna elements 402 to 416. The antenna elements 402 to 416 may be at the same or different distances from the DUT 400 and the antenna elements 402 to 416 may be placed only in a sector instead of a full angle or a full solid angle. The DUT 400 may also have one or more elements in the antenna.

0033Communicating with the DUT 400 over the air enables testing an antenna design, polarization and placement effects in such a way that path directions may be freely included in the testing. That is not possible if a cable connection is used between the emulator 418 and the DUT 400. The emulator 418 has a channel model for the test. The channel model may be selected by a person accomplishing the test. Additionally, interference and noise may be input to the test in a desirable manner and to a desirable extent. The channel model used may be a play back model based on a recorded channel from a real radio system or it may be an artificially generated model or it may a combination of a play back model and an artificially generated model. Assume now that the emulator 418 is coupled to or acts as a base station of a radio system and the antenna elements 402 to 416 are transmitting to the DUT 400 which acts as a receiving subscriber terminal of the radio system. It may be assumed that DUT antenna characteristics are unknown and that information may be ignored in the following example. The OTA antenna elements 402 to 416 may be assumed to be at angles θk of directions from the DUT, where k is 1, .... K, where K is the number of antenna elements. The angular spacing of the antenna elements 402 to 416 may be constant &amp;+i - θn = Aθ.

0034A geometric channel model in the emulator 418 may be mapped on the OTA antenna elements 402 to 416. The emulator 418 simulates the situation where the transmitted radiation from the base station hits clusters. The emulator 418 also forms a reflected and/or scattered beam from each cluster and divides the departure power and delay of the cluster suitably to the at least one antenna element 402 to 416. Hence, the antenna elements 402 to 416 are controlled to reproduce reflected and/or scattered beams of clusters.

0035Often an angle of a beam representing a reflected and/or scattered beam from a cluster differs from an angle 6k of an antenna element 402 to 416 more than a threshold which may be, for example, 1°. Then such a beam may be transmitted using at least two antenna elements 402 to 416.

0036In an embodiment, the power of a simulated cluster may be divided between two antenna elements on the basis of antenna angles θ|_&lt; and a cluster angle Ip_n. An angle &amp; of an antenna element k closest to a cluster angle ^_n may be found according to the following mathematical equation

0037<img file="WO2010040887A1_D0004.tif" /> where min means minimum value of the expression among all values of øj, int means an integer value of the division (including 0). The value of k is mm _{θ +} -Aθ - φ_n j i ⁱ 2 int . The second antenna element k + 1 may then be the

0038Aθ ^y

0039one having an angle fo + AΘ = θk+i. Hence, the selected antenna elements may be those between which the beam reflected and/or scattered from a cluster at least mainly is with respect to the DUT 400.

0040A weight w_nk+ifor each antenna element 402 to 416 may be calculated in the following manner <img file="WO2010040887A1_D0005.tif" />

0041where i is either 0 or 1, k is index of an antenna element closest to an angle cρ_n of a cluster n. The power P_n of a cluster n to an antenna element k is multiplied by a weight w_n,k such that P_k + P_k+i = P_n-

0042Assume now 8 antenna elements in a circle around a DUT, i.e. K = 8 and Aθ = 45°, a single base station antenna, a single cluster, cluster power 2, AoA ψn = 37°. A power P_k for antenna element 402 (antenna k) becomes P_k = p_nw_π1 = = 2.0 0.1778 = 0.3556 <img file="WO2010040887A1_D0006.tif" /> And a power P_k+i for antenna element 404 (antenna k + 1 ) becomes Pk₊I = 2.0 0.8222 = 1.6444 <img file="WO2010040887A1_D0007.tif" />

0043Figure 5 illustrates the beam 500 formed by the antenna elements 402, 404 with the calculated power division. The signals fed to different antenna elements may also be phase shifted with respect to each other such that a directional power spectrum may be modified. The phase shifting may be performed by weighting the base band signals with suitable complex coefficients which set powers and relative delays of the signals. The phase shifting may also be performed by delaying the radio frequency signals with respect to each other. For example, desired delays may be selected suitably from a bank of digital delays (for example digital finite impulse response filter structure). Different beams of different paths of the simulated radio channel may be formed at different moments of time. A beam of a path of the simulated radio channel may be formed at different moments of time. A plurality of different beams of different paths of the simulated radio channel may be formed at a moment of time.

0044Figure 6 presents a group 600 of antenna elements. In an embodiment, the antenna may comprise at least one group 600 of antenna elements 6002, 6004, 6006, 6008, 6010. Hence, in place of the antenna element 402, for example, there may not only be one antenna element but several elements 6002, 6004, 6006, 6008, 6010. Each antenna element 402 to 416 may comprise, for example, five elements. In general, in place of an antenna element 402 to 416 there may be a group 600 of at least two antenna elements 6002, 6004, 6006, 6008, 6010.

0045A mapping to OTA antenna elements may be more simple and more accurate if a single OTA antenna element is replaced by a group 600 of antenna elements 6002, 6004, 6006, 6008, 6010. Assume that a group comprises G antenna elements 6002, 6004, 6006, 6008, 6010. The number of elements 6002, 6004, 6006, 6008, 6010 to be fed in each antenna group 600 may be selected on the basis of a channel model arrival (per cluster) azimuth spread. Each group may be fed by a single emulator output port, and antenna elements 6002, 6004, 6006, 6008, 6010 of each group may be connected to the emulator with a switching network 620 which may comprise at least one splitter, combiner, attenuator and/or phase shifter. In an embodiment, the switching (i.e. selection of antenna elements) may be similar for all groups and it may to be done only once per measurement.

0046On the basis of the signal from the emulator a beam controller 622 may control how many antenna elements of a group are needed for a beam. In general, any positive integer number of antenna elements up to the maximum may be used.

0047In an embodiment, an odd number of elements may be used. For example, with G = 5 choices may be one, three or five elements, depending on the scenario of the channel model. If there are narrow clusters in the channel model, three elements may be enough for the beam. If the clusters are wider, maximum number of elements may be used for the beam.

0048The selection of antenna elements in a group may be expressed in a mathematical form as follows:

0049Z' = &lt; Z (6) <img file="WO2010040887A1_D0008.tif" />

0050where Z = G - 2j and j is 0, ...,(G - 3)/2, round means rounding to a closest integer value of the division (the minimum value is 1).

0051A mapping of the channel model to an OTA antenna may be per- formed by applying the following rules. Set each of the clusters to appropriate emulator channels and OTA antenna elements depending on a nominal direction of a cluster. Selection of the OTA antenna elements for a cluster n may be made by taking closest OTA antenna group centre θk for a nominal AoA ψn of a cluster. Select the number of antenna elements, for example Z', within a group by a switch 622.

0052Figure 7 presents a DUT 400 surrounded by groups 600 to 614 of antenna elements. In this example, each group 600 to 614 has three antenna elements. A beam 700 may be formed using a group 602. With eight groups and five elements in each group a full circle may be covered with uniformly located antenna elements. If a cluster is extremely wide requiring a very wide beam, for example wider than Aθ, the cluster may be mapped to more than one antenna group.

0053Several groups may also be used to form a beam. The groups may be applied in the same manner as what is described relating to equations (4) and (5) for selecting two antenna elements. Then, instead of selecting two antenna elements, two groups of antenna elements may be selected for a beam. In Figure 7, a beam 700 may be formed using groups 600 and 602.

0054In an embodiment, fixed weights may be implemented for antenna elements such that, for example, Gaussian or Laplacian shaped cluster power azimuth spectrum can be replicated.

0055A reception using at least two antenna elements is performed in a corresponding manner. Hence, the method may be applied in both uplink and downlink. Assume now that the antenna elements 402 to 416 are receiving signals from the DUT 400. Signals received by the at least two antenna ele- ments 402 to 416 may be combined in the emulator 418 for forming a reception beam of a signal of a path of a simulated radio channel. The combining may comprise weighting the power from the two antenna elements or group of antenna elements using weights w_nk+i calculated in equations (4) and (5). Additionally, the shape and direction of the beam may be weighted using complex coefficients or another sort of phase shifting.

0056The embodiments may be applied in 3GPP (Third Generation Partnership Project) LTE (Long Term Evolution), WiMAX (Worldwide Interoperability for Microwave Access), Wi-Fi and/or WCDMA (Wide-band Code Division Multiple Access). In the MIMO (Multiple In Multiple Out) which is also a possi- ble application, signals are divided to antenna elements in a different manner with respect to the present embodiments. Figure 8 shows a MIMO configura- tion having two transmit antenna elements 800, 802 and two receive antenna elements 804, 806. There are two delay taps 808, 810 representing different paths in delay elements 814 to 820 of an emulator 812. Signals from each transmit antennas 800, 802 are fed to delay elements 814 to 820 delaying the signals with the same delays (taps 808, 810). The outputs of delay elements 814 and 820 which delay with both delays (taps 808, 810) are combined and fed to the antenna element 806. Correspondingly, the outputs of delay elements 816 and 818 which also delay with both delays (delay taps 808, 810) are combined and fed to the antenna element 804. Figure 9 shows an example of a present embodiment. Also in this example there are two transmit antenna elements 900, 902 and two receive antenna elements 904, 906 of a plurality of antenna elements in an anechoic chamber 922 of the OTA test. There are two delay taps 908, 910 representing different paths in delay elements 914 to 920 of an emulator 912. A signal from a transmit antenna 900 is fed to delay elements 914, 916. The delay element 914 delays the signal with a delay corresponding to the delay tap 908 and the delay element 916 delays the signal with a delay corresponding to the delay tap 910,

0057A signal from a transmit antenna 902 is fed to delay elements 918, 920. The delay element 918 delays the signal with a delay corresponding to the delay tap 910 and the delay element 920 delays the signal with a delay corresponding to the delay tap 908. The outputs of delay elements 914 and 920 which delay with the same delay (delay tap 908) are combined and fed to the antenna element 906. Correspondingly, the outputs of delay elements 916 and 918 which delay with the same delay (delay tap 910) are combined and fed to the antenna element 904. Hence, different delay taps are fed to different antenna elements 904, 906 if they represent a different AoA.

0058Figure 10 presents a flow chart of the method. In step 1000, a beam of a signal of a path of a simulated radio channel is formed with at least two antenna elements of a plurality of antenna elements coupled to an emulator in an over-the-air chamber.

0059The embodiments may be implemented, for instance, with ASIC or VLSI circuits (Application Specific Integrated Circuit, Very Large Scale Integration). Alternatively or additionally, the embodiments of method steps may be implemented as a computer program comprising instructions for executing a computer process for communicating with an electronic device under test through a simulated radio channel of an emulator. The emulator may control on the basis of the electronic circuits and/or the computer program the use of the antenna elements and the formation of beams in the anechoic chamber.

0060The computer program may be stored on a computer program dis- tribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, an electric, magnetic, optical, infrared or semiconductor system, device or transmission medium. The computer program medium may include at least one of the following media: a computer readable medium, a program storage medium, a record medium, a computer readable memory, a random access memory, an erasable programmable read-only memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunications signal, computer readable printed matter, and a computer readable compressed software package. Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in several ways within the scope of the appended claims.