Patent Application
20080129634
Embodiments of the invention relate generally to MIMO communications.
Wireless communication networks, such as those based on an IEEE (institute of Electrical and Electronics Engineers) 802.11 protocol (also known as Wi-Fi), can achieve greater data throughput using a technique known as multiple-input-multiple-output (MIMO). MIMO relies on multiple antennae to exploit multiple electromagnetic transmission paths available to radio signals traveling in a highly reflective indoor propagation environment. However, when deploying MIMO transmitters and receivers in an outdoor environment or any large open area where there is line-of-sight between the transmitter and the receiver, the communication reduces to essentially a point-to-point communication and the underlying multiple transmission paths required by a MIMO communications system are no longer present.
Methods and systems for exploiting orthogonal antenna polarizations which restore MIMO capability to an otherwise single path link are provided. In one embodiment, multi-polarization antennae and antennae arrays create multiple orthogonally polarized independent channels of communication which are transmitted and received by similar multi-polarization antennae, thereby taking advantage of the fact that orthogonally polarized electromagnetic waves travel independently and can be used as independent communication channels. Transmitters and receivers comprising the multi-polarization antennae behave as if independent communication channels are available in the same line-of-sight link, allowing an increase in bandwidth and providing a way to exploit MIMO in outdoor and other line-of-sight communication links.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will he apparent, however, to one skilled in the art that the invention can be practiced without these specific details.
Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.
Multiple-input-multiple-output (MIMO) communication is used in wireless communication networks, such as those based on an IEEE 802.11n protocol. MIMO wireless communications use multiple antennae to exploit the multiple paths available to radio signals traveling in a highly reflective indoor propagation environment. Outdoors, or in large open indoor spaces, there are far fewer reflections and thus less multipath propagation. Therefore MIMO systems rapidly lose their advantage over conventional wireless links in an outdoor environment. When deploying MIMO transmitters and receivers in an outdoor environment or any large open area where there is line-of-sight between the transmitter and the receiver, the communication reduces to essentially a point-to-point communication and the underlying multi-path assumptions of MIMO are no longer valid. In such a scenario, not only does MIMO fail to boost existing line-of-sight bandwidths, but the extra overhead of attempting to exploit MIMO outdoors actually costs throughput or power and MIMO becomes a burden.
The present embodiments disclose techniques which exploit orthogonal polarizations which, especially in outdoor point-to-point links, restore MIMO capability to an otherwise line-of-sight link. As disclosed herein, multi-polarization antennae and antennae arrays create at least two orthogonally polarized independent channels of communication which are transmitted and received by a similar multi-polarization antenna, taking advantage of the fact that orthogonally polarized electromagnetic waves travel independently and can be used as independent communication channels A MIMO transmitter or receiver comprising such multi-polarization antennae behaves as if it has at least two independent communication channels available to it in the same line-of-sight link. This allows an increase in bandwidth and provides a way to exploit MIMO in outdoor and other line-of-sight communication links. However, it is understood that the advantages of the present embodiments do not require a line-of-sight link.
While a particularly popular application of MIMO is for communications in accordance with the IEEE 802.11n and similar wireless protocols, it is to be understood that the multi-polarization MIMO techniques disclosed by the present embodiments can in general be applied to communications in accordance with other protocols. Thus, the techniques disclosed herein are not dependent upon any particular frequency range or any particular communication protocol or standard.
Processor 101 has access to a plurality of independent channels, one per antenna element 110 of the multi-polarization antenna 108. The number of independent channels may range between two to four depending on the number of antenna, elements 110 that comprise the multi-polarization antenna 108. In the embodiment shown in
In the embodiment of
The signals may carry data that is packetized (for example according to an IEEE 80211 or other packet-based communication protocol) or data that is not packetized. In the case of packetized data transmission, the processor 101 prepares the data for transmission as data packets. In the case of packetized data reception, the processor 101 receives the data as packets.
The disclosed techniques can be applied to any type of antenna that can accept at least two orthogonal inputs to produce at least two orthogonally polarized electromagnetic fields, such as microstrip patch antennae and aperture dish antennae which allow feeds with multiple polarities and can send or receive multiple signals according to the multiple polarities. Aperture dish antennae may achieve higher gains than patch antennae for point-to-point communication. By way of example and not limitation, the present techniques are hereinafter disclosed with reference to microstrip patch antennae. Generally, a microstrip patch antenna is etched on a two-layer printed circuit board with a ground plane layer and an antenna element layer. The antenna element is about ½ wavelength in length (representing the resonant length) and typically ¼ to 2 wavelengths wide. It is excited by a feed located at or near one edge. If made square, the antenna will resonate along both the vertical and horizontal axes.
The two feeds interact minimally and for practical purposes can be assumed to be independent channels. When a patch element is excited in one polarization, the fields and the currents on that element are independent and do not interact with the fields and currents flowing in the orthogonal direction. Furthermore, while a portion of the energy going into one polarization feed may leak out of the other polarization feed, it is generally small and as a practical matter can be ignored.
Note that a multi-polarization transmitter and receiver can maintain a high communication bandwidth between them as long as their polarizations substantially match, i.e. as long as their relative spatial orientations are such that the vertical and horizontal axes of their antennae are substantially aligned. One way of providing for this relative orientation is to have the receiver and transmitter antennae stationary and in a fixed and aligned orientation relative to each other.
Patch antenna elements can be arranged in many configurations to create antenna arrays for producing higher gain than a single element. Such an array can be used in place of the multi-polarization antenna 108 in the system shown in
In one embodiment, a combination of corporate and series feeding provides an efficient and elegant interconnection scheme providing the antenna elements 202 with feeds of two polarizations. The array is fed by two central energy feed lines 205 and 207, one per polarization. The rows are corporate fed by the central row feed 205 which branches out and is connected to one element 202 per row. Along the rows, the elements 202 are connected in series by row interconnects 206 and are series fed downstream from the corresponding branches of the central row feed 205. The columns are corporate fed analogously by the central column feed 207, with the elements 202 connected in series along the columns by column interconnects 208 and series fed downstream from the corresponding branches of the central column feed 207. As in the case of the single element 202, the two feeds 205 and 207 are independent and represent two independent channels. One advantage of the combination of corporate and series feeding is that complex routing from the feed lines 205 and 206 to individual antenna elements 202 is avoided.
A further advantage of the combining series and corporate feeds, as disclosed herein, is that it helps distribute the energy provided by the central feeds 205 and 207 more evenly across the antenna elements 202. Such even distribution is especially important in high gain antenna applications having an array comprising many antenna elements 202. In one embodiment comprising an n×n array, the central feeds 205 and 207 branch out such that they each split the power evenly across their n branches. The impedance of the antenna elements 202 is designed such that the first antenna element in a series fed sequence of w antenna elements 202 in a row (respectively column) removes only one n-th of the energy from its central row feed 205 branch (respectively column feed 207 branch) and radiates it, allowing the rest of the energy to travel past that antenna element to the remaining elements 202 in the row (respectively column).
The next antenna element in the row (respectively column) removes another one n-th of the supplied energy and radiates it, and so on, until the last element receives the last n-th and radiates it. This way, both feed lines 205 and 207 (for both polarizations) distribute their energy evenly across the elements 202 of the antenna array. The proper impedance of the patch elements 202 for a row (or column) can be determined by using a series of equations which take into account the impedance of the series connected antenna elements in the row (or column), as well as the impedance of their interconnects 206 (or 208), and by iterating until an acceptable approximation is reached, as should be obvious to one of ordinary skill in the art.
The multi-polarization feed technique is not limited to linear polarization, such as the above described vertical and horizontal polarization, but can also be applied using circular polarization.
In an alternative embodiment, the individual patch elements are altered to generate circular polarization, with a first polarization being a left hand polarization and a second orthogonal polarization being a right hand polarization. A patch antenna element can be made to generate circular polarization by adjusting its dimension very slightly, for example by feeding it from opposite corners. Since such circular polarizations are also independent, circularly polarized antennae and antenna arrays can be used in place of linearly polarized antennae to produce two independent channels and double the bandwidth.
It is to be understood that embodiments of the invention may be practiced using antennas other than patch antennas. For example, as is shown in
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the broad invention and that this invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art upon studying this disclosure. In an area of technology such as this, where growth is fast and further advancements are not easily foreseen, the disclosed embodiments may be readily modifiable in arrangement and detail as facilitated by enabling technological advancements without departing from the principals of the present disclosure or the scope of the accompanying claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 11/565,511 filed on 30 Nov. 2006.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11565511 | Nov 2006 | US |
| Child | 11685998 | US |
