WO2014053591A1

WO2014053591A1 - Multiple antenna

Info

Publication number: WO2014053591A1
Application number: PCT/EP2013/070613
Authority: WO
Inventors: Julien PERRUISSEAU-CARRIER; Mohsen YOUSEFBEIKI
Original assignee: École Polytechnique Fédérale de Lausanne
Priority date: 2012-10-05
Filing date: 2013-10-03
Publication date: 2014-04-10
Also published as: GB201217835D0

Abstract

A multi-input, multi-output antenna comprises an electrically conductive body, generally symmetrical about a plane of symmetry. An active port (Feed point) is located on the plane, a first parasitic port (1) is located on one side of the plane, and a second parasitic port (2) is located on the other side of the plane, generally symmetrically to the first parasitic port. An RF module is connected to the active port (Feed point), and each of the first and second parasitic ports (1, 2) are connected to a respective load (6, 7) of variable impedance. The design of the antenna is intended for beamspace MIMO that can be effectively incorporated in a compact, low-cost mobile terminal.

Description

MULTIPLE ANTENNA

Background to the Invention

[0001] This invention relates to a multiple antenna, also known as a multi-input multi-output (MIMO) antenna.

[0002] An always important challenge in the design of a wireless communication system is the increasing demand for better performance and higher data rates. As a consequence, traditional SISO (single input, single output) systems, employing a single antenna for transmission and a single antenna for reception, have been substituted with MIMO counterparts in current and future wireless communication systems, such as 802.1 In, WiMAX , or 3GPP-LTE. The use of multiple transmit and receive antennas in MIMO systems introduces an extra spatial dimension in addition to the time: the signals sampled in the spatial domain at both transmitting and receiving ends are combined in order to add diversity (for improving the quality and the reliability) and/ or achieve multiplexing (for increasing the data rate).

[0003] Thus, MIMO techniques allow a dramatic increase in performance, by significant improvements in the reliability and/ or transmission rate. In brief, such techniques achieve a major increase of the main figure of merit of wireless communication, which can be qualitatively expressed as "Datarate/ (Power x Bandwidth)". In the context of the ever increasing number of wireless devices deployed (i.e. higher interferences), the demand for higher data-rates, and the scarce spectrum nd energy resources, the use of MIMO techniques has become indispensable.

[0004] On the other hand, the performance enhancements obtained through MIMO techniques bring certain associated drawbacks in comparison with SISO systems. Classical MIMO transceivers require an RF branch (for demodulation and d o w n -c on v s ion at the receiver and modulation and up-con version at the transmitter) as well as independent multidimensional signal processing in each baseband path. These requirements generate at each MIMO terminal an extra cost, size, and power consumption. Therefore, classical MIMO transmission may often not be supported due to the practical limitations of mobile platforms in terms of cost, power consumption and size.

[0005] Multiple antennas: A common assumption in the study of traditional MIMO systems is to consider several uncorrelated antennas at each end of the link. If this is to be a reasonable assumption, the different antenna elements should in general be placed far enough away from each other to reduce mutual coupling among them and obtain almost decorrelated MIMO channels. However, it is usually a significant challenge to place multiple anten as within a small device while maintaining good isolation betwee them. Insufficient anten a spacing at small platforms degrades the performance of the MIMO communication systems, due to increased channel correlation. The available approaches aiming at reducing the mutual coupling among the ports of multiple antenna systems can be categorized into two main techniques: pattern orthogonality methods and decoupling (decorrelating) networks. However, these techniques come with other drawbacks (e.g. increased loss and complexity of decorrelating networks) and in any case the issue of multiple RF chains, explained below, remains.

[0006] Multiple RF chains: In order to implement any spatial multiplexing and diversity MIMO scheme, traditionally multiple radio frequency chains are required, resulting in greater complexity, higher power consumption and increased hardware cost. Therefore, utilization of a single RF front-end in a MIMO system, where a single RF path is used instead of multiple parallel RF paths, is of a great importance at both transmitter and receiver sides. This would result in an RF section that has lower hardware complexity and cost, more compact size, and reduced power consumption. There are different design alternatives for implementation of single RF front-end MIMO systems. The most common approaches are the antenna selection technique, analogue combining, time division multiplexing, and code division multiplexing:

Table 1: Summary of single-RF front end techniques

[0007] The above-men Honed techniques provide design alternatives for many practical MI MO systems, aiming at single-RF front-end architectures. However, as bandwidth expansion should be avoided at the transmitter side (due to resultant adjacent channel interference), time-division multiplexing and code- modulated path-sharing techniques cannot be applied for signal multiplexing in MIMO transmitters. The antenna selection technique is a mere diversity technique, in which the best channel is selected. The analog antenna combining approach is also restricted to the receiving side. In conclusion none of the techniques discussed so far allow the multiplexing of signals using a single-RF chain without in tolerable drawbacks related to multiple switching within a single symbol period. Next we describe a novel technique which addresses this problem, which is the one to which the invention relates.

[0008] Beamspace MIMO concept: Recently, a novel technique called beam s pace-MI MO has been proposed by A. Kalis, A. Kanatas, and C. Papadias, "A Novel Approach to MIMO Transmission Using a Single RF Front End," IEEE Journal on Selected Areas in Communications, vol. 26, pp. 972-980, 2008; O. Alrabadi, C. Papadias, A. Kalis, and R. Prasad, "A universal encoding scheme for MIMO transmission using a single active element for PSK modulation schemes," IEEE Transactions on Wireless Communications, vol. 8, pp. 5133-5142, 2009; and O. N. Alrabadi, J. Perru issea u -Carrier, and A. Kalis, "MIMO Transmission Using a Single RF Source: Theory nd Antenna Design," IEEE Trans. Microw. Theory Tech. nd IEEE Trans. Antennas Propag., Joint Special Issue on MIMO Technology, vol. 60, pp. 654-664, 2012.

[0009] Figures la and lb depicts schematic representations of classical and beamspace MIMO transmitters respectively. The classical MIMO transceiver has two separate branches, independently 'assigning' the signals s i and s2 to the antenna's radiation patterns 131 and B2, as symbolized in Figure la. Figure lb represents the beam-space MIMO concept. In this approach, instead of driving different symbol streams to different active antenna elements as in the traditional MIMO transmitting mode, symbols are mapped directly into the beamspace domain of a single active antenna, modulating orthogonal basis patterns. In other words, each datastream si, s2 is assigned to a virtual antenna of radiation pattern Bl, B2. This is achieved by designing the system such that the instantaneous pattern of a single antenna, GT, is at any instant in time decomposable in an orthogonal basis formed by Bl and B2, such that GT=sl*Bl+s2*B2, i and s2 taking any value required by the considered modulation (e.g. for BPSK s i and s2 can independently take any of the values + 1 / - 1). Using a traditional MIMO receiver, it is then possible to retrieve the transmitted information at a rate that is equal to the corresponding classical MIMO transmitter.

[0010] To implement the beam-space MIMO concept, the documents cited above proposed the use of a so-called 'parasitic antenna' shown in Figure 2, which comprises one active antenna connected to a single RF branch, and multiple parasitic (passive) antenna elements loaded by variable reactive impedances. By controlling the reactance loads via a DC control, basic antenna properties, such as the antenna beam -pattern, can be reconfigured. Switched parasitic antennas had already been utilized in the past for angular pattern diversity on the receiver (or transmitter) side, where different weakly-correlated beam-patterns can be switched during one single symbol period to provide diversity (similar to antenna selection technique, but in the beamspace domain). They have also been used for analog beamforming and null steering.

[0011] However their use for beamspace MIMO was first suggested in the aforementioned documents. The central active element is fed with a RF signal modulated by the first baseband datastream, while simultaneously the parasitic elements are reconfigured by a second baseband signal, as shown in Figure 3. The baseband control signal has information about the other datastream to be transmitted over the air. In this way, it has been shown that the input datastream can be mapped onto an orthogonal set of basis functions in the wavevector domain via a single radio and compact array dimensions.

[0012] In the seminal work of Kalis et al, the authors proposed the idea of mapping the signals onto a set of predefined weakly correlated beams (cardioids) in the far-field of a switched parasitic antenna. The design method was demonstrated for BPSK and QPSK signaling but is approximate and more important is only valid for ideal omnidirectional elementary radiators, which is highly unrealistic.

[0013] In Alrabadi et al (2009), orthogonal basis functions were obtained by decomposing the Euler functions comprising the far-field of the switched parasitic antenna, and the design method extended to any PSK order signaling. However, the design is still based on simple analytical description of the elementary and only in a 2D plane, making the applicability of the approach virtually irrelevant.

[0014] Alrabadi et al (2012) demonstrated a switched parasitic array system for transmitting two BPSK signals at 2.6 GHz. This was the first experimental demonstration of the Beamspace MIMO concept. The switched parasitic antenna consists of three printed dipoles (central active surrounded by two passive elements) and thus follows the initial concept in the above references. The variable loads (realized by PIN diodes and SMD components) were optimized for maximizing the average rate of communication at a target frequency of 2.6 GHz (by simultaneously maximizing the transmit efficiency and minimizing the power imbalance between the basis functions). Using this antenna the first successful MIMO transmission with a total rate of 820 kbps has been conducted over the air, using a single RF source in an indoor office environment.

[0015] Though demonstrating the concept of beamspace MIMO for the first time, the known implementations discussed above (multi-element arrays of monopoles or dipoles) are not suitable for an actual implementation in compact, low-cost mobile devices. Indeed, despite having much smaller inter-element spacing than conventional MIMO arrays (in order to achieve significant coupling between the different elementary elements), the elements themselves remain too large to be utilized in modern compact wireless devices. In general, the requirement of utilizing several disconnected radiators (active and passive) is inconvenient, not only in terms of size but also in terms of easy fabrication, influence of the user on the antenna performance, etc. On the other hand, the prevalent exacting requirements of modern communication devices demand that the antenna should be treated as a component of the whole platform rather than an isolated element within the design process.

Summary of the Invention

[0016] The present invention provides a multi-input, multi-output antenna according to claim 1 and to a method of manufacturing a MIMO antenna according to claim 9 or claim 10. Optional features of the invention are set out in the dependent claims.

[0017] The invention relates to the design of an antenna for beamspace MIMO that can be effectively incorporated in a compact, low-cost mobile terminal.

Rather than a multi-element structure in which each element is regarded as a distinct and physically separate radiator, a compact multi-port integrated antenna structure is used, as symbolized in Figure 4 in the case of a three-port radiator integrated into a device platform.

[0018] The known beamspace Ml MO designs discussed above considered a given a-priori antenna structure, based on which the variable load impedance parasitic values are optimized. With this approach the reactance values required for the optimum operation of the antenna system differ from those of available control elements, e.g. PIN diode switches. The known designs involve realization of desired variable loads by a circuit embedding such available control devices as well as other passive elements. The proper design and operation of this load control circuit is the most critical part for the actual implementation of a switched antenna structure, in terms of both design effort nd system performance.

[0019] A switched antenna structure for beamspace multiplexing of PSK signals that uses con trol elements (for instance PIN diodes) directly as the variable load avoids the design of the aforementioned variable load circuit. This is achieved by optimizing the antenna structure itself for the desired loads (for example the impedances corresponding to each of the two states of the PIN diode), rather than the other way around.

[0020] Although this condition put more burdens on the design of the antenna structure, reducing the complexity of the load circuit actually reduces the overall design complexity and manufacturing cost, while also increasing the certainty and consistency in the proper operation of the beamspace MI MO terminal.

[0021] Such an approach is easily implemented for BPSK modulation, and is fully validated as shown below. It could also be extended to higher order modulation by using a simplified load, consisting for instance in two diodes connected in parallel. [0022] The implementation of higher-order PSK modulations using the beam- space Ml MO has already been considered in prior art. Indeed, as explained earlier:

[0023] Kalis et al proposed the idea of mapping the signals onto a set of predefined, almost-orthogonal beams (cardioids) in the far-field of a switched parasitic antenna. BPSK and QPSK modulation were demonstrated as a proof of concept.

[0024] Alrabadi et al (2009) extended this. Orthogonal basis functions were obtained by decomposing the Euler functions comprising the far-field of the switched parasitic antenna, and the design method extended to any PSK order signaling.

[0025] However in both cases the design method is based on a 2D far-field description of idealistic omnidirectional radiators, making the approach inapplicable in practical cases.

[0026] Thus both these works on higher order PSK modulation are based on assumptions that are in general not realistic, and that simply not apply to the radiating structure of the present invention. The design method described below for higher order PSK modulation is valid for any radiating structure and ensures the best possible performance while using simple passive imaginary loads.

[0027] In addition, we also state here for the first time that while the complexity of the variable loads increases with the order of the modulation when the two datastreams have the same the same order (the number of required dynamic load values correspond to the order of modulation), it is also possible to increase the modulation order of one of the streams (the one directly fed to the active port) while keeping the load complexity unchanged.

[0028] The novel design procedure can be as follows:

[0029] In step (a), design a three-port antenna structure, fulfilling the physical requirements of the application. The antenna system must have an axis of symmetry such that the center port is excited by a single-RF module. Two other ports are terminated using purely imaginary loads whose values will be determined in terms of scattering parameters and active port radiated fields of the structure.

[0030] In step (b), extract the scattering parameters and active port radiated fields of the structure using a full-wave simulation. This data will be used to obtain the values of reactive loads as well as the performance of antenna system in beamspace multiplexing. If the structure is lossless, there is no need for the knowledge of radiated fields.

[0031] It can be demonstrated that in the case of a PSK modul tion with the order higher than two (QPSK, 8-PSK, ...), the values of all reactive loads are interconnected. In other words, the basis functions for beamspace multiplexing, the power imbalance ratio between them, the reflection coefficient of the system, and the values of reactive loads can be calculated in step (c) as a function of one of the loads.

[0032] As mentioned, for each given reactive load, the other reactance values can be calculated. In step (d) the optimum reactance set is found for the best performance of the antenna system in terms of return loss, power imbalance r tio, nd the ergodic capacity, while considering the possibility/ simplicity of practical realization of the reactive loads.

[0033] In step (e), if the optimum solution is not satisfactory, the designer can change some parameters of the design and investigate the effects on the performance.

Brief Description of the Drawings

[0034] The prior art has been described, and the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: [0035] Figures la and lb schematically show classical MIMO and beamspace MI MO respectively;

[0036] Figure 2 is a diagram of a three-element switched pa asitic antenna;

[0037] Figure 3 schematically shows a single branch transmitter based on beamspace MIMO;

[0038] Figure 4 shows a generalized 3-port impedance-loaded radiator; according to the invention;

[0039] Figure 5 is a flow chart exemplifying design steps according to the invention;

[0040] Figures 6a and 6b show an antenna that has been constructed according to the invention;

[0041] Figures 7a and 7b show radiation patterns of the antenna of Figures 6a and 6b;

[0042] Figure 8 shows reflection coefficients of the antenna of Figures 6a and 6b;

[0043] Figures 9 and 10 show simulated angular basis functions of the antenna of Figures 6a and 6b;

[0044] Figures 11a and lib show how reactive loads are permuted in the radiator of Figure 4;

[0045] Figure 12 shows a 3-element dipole array;

[0046] Figure 13 shows required values of reactive loads for QPSK beamspace multiplexing;

[0047] Figure 14 shows the power imbalance ratio of the antenna structure for QPSK beamspace multiplexing; and

[0048] Figure 15 shows the reflection coefficient of the antenna structure for QPSK beamspace multiplexing. Detailed Description of Particular Embodiments

[0049] An antenna according to the invention has been used to demonstrate the beamspace multiplexing of two BPSK signals using an integrated multi-port antenna and simplified variable loads, usable in actual compact mobile devices. Respective sides of the designed and fabricated built-in switched antenna are shown in Figures 6a and 6b. The compact integrated antenna structure, comprising ports 0, 1, 2, is realized in close proximity to a grounded printed circuit board (PCB) of a hypothetical mobile device, having a ground plane 4 on plastics substrate 5. Two mirrored beam patterns are produced while the control elements change their states. As mentioned earlier, it can be shown that an arbitrary antenna system that has a single RF input but has the capability of creating mirror image pattern pairs will be capable of transmitting two BPSK signals s i and s2, simultaneously.

[0050] Parts of antenna system are directly printed on both sides of the ungrounded portion of the PCB, while the other par is easily realized b the use of copper strips.

[0051] The central port 0 is directly connected to a single-RF chain through coaxial excitation, while the two parasitic ports 1, 2 are terminated by variable load circuits. At each parasitic port, only one control element, i.e. a PIN diode 6, 7, is employed as the load circuit for switching between the instantaneous beam patterns. Each PIN diode is connected through a via 8 and an RF choke 9 to the ground plane 4.

[0052] This particular antenna is designed to be used in a frequency range of 1920-1980 MHz for uplink and a frequency range of 21 10-2170 MHz for downlink. The antenna is fed through a perpendicular 50 Ω coaxial cable. The embodiment has been optimized according to the method described above. Each parasitic port is loaded b a single PIN diode. An optimization approach has been employed for the embodiment to obtain a desired performance in terms of the ergodic data rate.

[0053] The simulated and measured radiation patterns of the embodiment in the plane of the device for both states of the switching system, at 1.95 GHz, are depicted in Figures 7a (state I) and 7b (state II). Figure 8 shows the comparison between the reflection coefficients of the structure. There is a very good agreement between the measurement and simulation results. Moreover, the desired mirror image pattern pair is also obtained when the state of the PIN diodes changes.

[0054] We can then calculate the basis functions as well as the upper bound of the average throughput for simultaneously transmitting two BPSK signals via the antenna structure. The calculated power imbalance ratios are 0.96, 0.75 and 1.02 at 1.92 GHz, 1.95 GHz and 1.98 GHz, respectively. This values yield an upper- bound ergodic capacity of 4.4-4.6 b/s/Hz for a transmit signal to noise ratio {P a ) of 10 dB within the upload frequency range. Simulations show a radiation efficiency of 76-82% and 87-89% for upload and download frequency bands, respectively. The corresponding angular basis functions in the plane of the terminal at 1920 MHz are shown in Figure 9. Figure 10 depicts the distribution of the magnitude of normalized electric field over full space for two basis beam patterns at 1920 MHz.

Theoretical Background of the Design Method

[0055] Assume a symmetric three-port antenna structure, shown in Figure 4, excited through the central port while the other two ports are terminated by purely imaginary loads Xi and X₂. The structure can be fully expressed by a 3-by- 3 scattering matrix and three port patterns. The analytical expressions for the reflection coefficient and total radiated field can be obtained as follows:

Γ = So o + Soi(£i + h) (1) Ε(Ω) = £₀Ε₀(Ω) + ^,(Ω) + £₂Ε₂(Ω) (2) where Γ = (/¾ - Ζο) / (/¾ + ¾), the reflection coefficient of the load connected to port i of the structure, E, (Ω) is the active element pattern of the port, and

¹ Ι-^+Γ^-Γ,Γ^-ί')

[0056] By permuting the reactive loads of the passive ports as /¾ ;¾] <→ [jXi jXi] (that corresponds to permuting Γι <→ Γ₂) as depicted in Figures 12a and 12b, (3) implies that [h€2] <→ [h€1]. Defining the notations (¹²) and (²¹) to present these two different termination states, the total radiated fields are obtained as follows:

E ^{U) (Ω) = E₀ (Ω) + £_t E_x (Ω) + £ ₂ E₂ (Ω)

E⁽²¹⁾ (Ω) = Ε₀(Ω) + £₂ £·, (Ω) + ^ E₂ (Ω) (4) which implies that £^(2!)(Ω) is a mirrored version of Ε^{12)(Ω) with regard to the axis of symmetry. It can be shown th t the definition of angular functions B; and BIT as follows:

creates an orthogonal basis for decomposing the instantaneous radiation fields. Using (4), the basis functions can be rewritten directly in terms of three port patterns:

¾Ω) = [1 (£_{ί +}£₂)/2 (£_{ί +}£₂)/2][ (Ω) ¾Ω) Έ(Ω)]¹ β, (Ω) = [0 (ί₂ - ί ) Ι 2 ( ₂ - ^) / 2] [/?₀ (Ω) £, (Ω) _; (Ω)]^; (6)

[0057] To be capable of transmitting two signals si and si simultaneously using the beamspace MI MO concept, supposing the active port (port 0) is excited by the first signal, these two signals must be decomposed into the orthogonal basis functions. So it is necessary to choose the values of reactive loads of two passive ports, [jXxi jXxi] (corresponding to [ Γ _Λι Γ ,2]), in such a way that the following desired radiation field is obtained: s, E_x (Ω) = 5, B, (Ω) + s₂ B_JJ (Ω) (7)

[0058] Note that the excitation of the active port using the first data stream is a simplifying choice but is potentially not the only solution. Using (6), (2) and (3), the desired loads can be found by solving the following equations:

where for the sake of simplicity si/ s\ - . It can be easily shown that the reflection coefficient of the structure at the port 0 remains constant for different signal combinations (obtained using (1)). Since the real part of a complex load degrade the efficiency of the antenna structure, only purely imaginary solutions are interesting. Applying the condition of purely imaginary loads (that is equivalent to |Γ_χ1| = [Γ_Χ2 | = 1 in (8), Xi and ₂ (reactive loads for definition of basis functions) should satisfy the following two equations simultaneously:

[0059] In general, such a solution for Xi and X₂ does not exist. However, both equations in (9) converge to the same one when [a| = 1, namely for any PSK modulation. Therefore any BPSK modulation can be supported and the desired load determined accordingly.

[0060] To illustrate these results, consider a three-element antenna structure comprising dipole antennas, as shown in Figure 13. Applying the technique described in the previous sections, we can use the structure for QPSK beamspace multiplexing. The center element is driven by the first stream, while other two elements are terminated with switchable loads. The antenna is simulated using full-wave software (HFSS) when it has been seen as a three-port structure. The scattering parameters and the active element radiated fields are extracted.

[0061] Figure 14 shows the required value of X₂ for each given Xi. In addition, the obtained values for the other two loads for QPSK signaling are depicted. The power imbalance ratio between the basis functions is illustrated in Figure 15 and reflection coefficient at the center port in Figure 16.

Applications and Perspectives

[0062] Since the real part of a complex terminating load (i.e. the ohmic resistor) degrades the efficiency of the antenna system, the proposed technique will be confined to modulations whose chosen constellation points are positioned on the unit circle, such as PSK, MSK and GMSK. In the case of PSK modulation, since the phase representing the data signal is constant over the symbol period, the technique requires switching between the reactive elements only at the beginning of each symbol. For MSK and GMSK, continuous phase shift of the modulated signal is desired, and it is therefore necessary to continuously change the reactance of the terminating loads (for instance, b applying varying voltage to a varactor) even during the symbol period.

[0063] PSK is widely utilized in existing applications and standards due to the constant amplitude of the its modulated carrier signal. Particularly, the standards IEEE 802.1 la/b/g (WLAN/Wi-Fi), IEEE 802.15.1 (Bluetooth), IEEE 802.15.4 (ZigBee), IEEE 802.15.6 (WBAN) and ISO/IEC 1443 (identification cards) use a variety of PSK schemes depending on the application requirements such as noise/ interference resistance, spectral efficiency, hardware complexity, and power consumption. The IEEE 802.15.6 standard, targeting the WBAN medical and non-medical applications, also uses digital phase modulations such as GMSK and DPSK. Medical applications of WBAN appear in the healthcare area, such as collection and transmission of vital information on patients to a remote monitoring station. On the other hand, gaming and social networking applications of WBAN are increasingly considered.

[0064] The proposed multiplexing technique could thus help bringing MI MO benefits to such classes of applications at very low extra complexity. Since, as mentioned above, MIMO drastically improves the tradeoff "Data-rate/ (Power x Bandwidth)", depending on the specific need, the advantage can be either in terms of data rate, power, or bandwidth, all other parameters remaining equal. Moreover, the ability to transmit high data-rate bursts as enabled by the single-RF MIMO technique of the present invention might help to further reduce duty cycles in some very low power applications such as WBAN, thus providing an extra power saving.

[0065] The invention is suitable for open-loop MIMO principally. [0066] Finally, it is important to note that the hardware used for the technique can readily be used to achieve different functionalities, which is very useful in the context of the ever increasing demand for more advanced dynamic flexibility at all layers of the communication system, including dynamic selection of the operation mode. Indeed the developed antenna hardware can be used to sequentially operate for:

• Tx Ml MO multiplexing or space-time coding M1MO diversity (this is the baseline functionality described above);

• Tx: the antenna can also be used for SISO/SIMO transmission with the possibly of dynamically selecting the radiation patterns. This can be very useful as an alternative operation mode to the previous MI MO multiplexing/ diversity: since MIMO performs well for strongly scattered environment and SISO for Iine-of-sight, it can be useful to be able to switch between the two modes during operation in highly dynamic wireless environments, selecting the best available pattern;

• Receive pattern diversity (slow pattern reconfiguration to dynamically compensate for channel variation, or fast pattern change for oversampling in the receive mode). This can also be useful for analog diversity at the user terminal in Multi-user-MIMO (MU-MIMO).

[0067] Therefore the technique could enable significant improvement of the energy efficiency with just a limited increase in hardware complexity.

Claims

1. A multi-input, multi-output antenna, comprising an electrically conductive body, generally symmetrical about a plane of symmetry, the body having an active port located on the plane, a first parasitic port located on one side of the plane, and a second parasitic port located on the other side of the plane, generally symmetrically to the first parasitic port, an RF module being connected to the active port, and each of the first and second parasitic ports each being connected to a respective load of variable impedance.

2. An antenna according to claim 1, wherein the ports are provided at one or more edges of the electrically conductive body.

3. An antenna according to claim 1 or 2, wherein the electrically conductive body is generally planar.

4. An antenna according to claim 3, wherein the electrically conductive body comprises an electrically conductive layer that is supported on an insulative substrate.

5. An antenna according to any preceding claim, wherein additional ports are provided symmetrically about the plane of symmetry.

6. An antenna according to any preceding claim, wherein each load of variable impedance comprises a control element, the reactance of which is variable by switching between different states of the element.

7. An antenna according to claim 6, wherein each load of variable reactance comprises at least two control elements connected in parallel or in series.

8. An antenna according to claim 6 or 7, wherein the control elements comprise PIN diodes.

9. A method of manufacturing a MIMO antenna for use with phase shift keying of order higher than two, comprising the steps of: a. designing an antenna according to any preceding claim; b. employing a full-wave simulation to extract scattering parameters of the antenna; c. setting a purely imaginary load for the first parasitic port, and calculating basis functions for beamspace multiplexing, a power imbalance ratio between the basis functions, a reflection coefficient of the system, and value(s) of the other load(s), also considered to be purely imaginary; d. optimizing at least one parameter selected from return loss, power imbalance ratio and ergodic capacity by repeating step (c) whilst varying the reactive loads set and calculated; e. checking that the at least one parameter is satisfactory, and if not, amending the design of the antenna and returning to step (b); and f. making the antenna according to the original or amended design with the load values set in the last iteration of step (d).

10. A method of manufacturing a MIMO antenna for use with phase shift keying, comprising the steps of: a. designing an antenna shape according to any one of claims 1 to 8; b. employing a full-wave simulation to extract scattering parameters of the antenna; c. considering impedance values of off-the-shelf control elements, each of which is to be used as a load for one of the parasitic ports, and calculating basis functions for beamspace multiplexing, orthogonality of basis functions, a power imbalance ratio between the basis functions, a reflection coefficient of the system, and an ergodic capacity of the system; d. checking that at least one parameter selected from return loss, orthogonality of basis functions, power imbalance ratio and ergodic capacity is satisfactory, and if not, amending the design of the antenna and returning to step (b); and e. making the antenna according to the original or amended design with the control elements considered in step (c).

11. A method according to claim 9 or 10, wherein in step (b), active port radiated fields are calculated to cater for losses in the antenna structure.

12. A method according to claim 10 or claim 11, when dependent on claim 10, wherein binary phase shift keying is considered.

13. A method according to claim 10 or claim 11, when dependent on claim 10, wherein phase shift keying of order higher than two is considered.

14. A method according to claim 9, 10 or 11, wherein datastreams having phase shift keying modulation of mutually different order are considered.