WO2005041566A2

WO2005041566A2 - A diversity controller for a video receiver

Info

Publication number: WO2005041566A2
Application number: PCT/US2004/029467
Authority: WO
Inventors: James Coleman Veatch
Original assignee: The Windermere Group, Llc
Priority date: 2003-10-15
Filing date: 2004-09-10
Publication date: 2005-05-06
Also published as: WO2005041566A3

Abstract

A diversity controller (5) for a video receiver (10) includes a beamforming network (3) that combines received video signals at two or more antennae to provide a plurality of antennae combination signals. A high speed RF switch (4) is responsive to a received signal quality measure for asynchronously coupling one of the plurality of antennae combination signals to the video receiver.

Description

A DIVERSITY CONTROLLER FOR A VIDEO RECEIVER FIELD OF INVENTION The present invention relates to the field of communication systems and more particularly to the reception of RF signals in the presence of multipath interference. BACKGROUND OF THE INVENTION Communication devices are often exposed to a phenomenon known as multipath interference. Multipath interference happens when a transmitted signal and its reflections from surrounding objects, e.g., buildings, trees, walls, etc., arrive at a receiver with varying phases that distort proper reception of the transmitted signal at the receiver. Among other things, multipath interference is dependent on the spatial attributes of the antenna used during transmission and reception of the communication signals. In communication systems that utilize fixed antenna installations, proper positioning of antennae, e.g., a receiving antenna, a transmitting antenna, or both, can eliminate multipath interference. As such, moving the receiver antenna or the transmitter antenna within a range of quarter a wavelength can significantly impact signal communication quality for better or worse. Radio frequency (RF) communications systems use various types of modulation techniques to communicate information. For example, TV broadcasting systems use amplitude modulated (AM) and frequency modulated (FM) video signals. AM broadcast television video signals are used primarily for point-to-multipoint applications, where receive and transmit antenna positions are fixed. In such systems the relative position of transmit and receive antennae may be adjusted manually for optimum signal reception in the presence of multipath interference. Ultra-high frequency and microwave video systems use FM signals to transmit video signals, primarily because FM signals offer inherently better signal quality, higher sensitivity and longer range. Such FM systems often use a point-to-point arrangement where one or both transmitter and receiver antennas move relative to each other, for example, using antenna rotors that allow for adjustment of the antenna position, as done with antenna dishes in satellite communication, for example. One known way of mitigating the effects of multipath interference is to use directional antennae. Another method for dealing with multipath interference is by receiver diversity. One type of receiver diversity requires selecting a received signal having optimal received signal quality. In this type of video receiver diversity, two or more antennas are connected separately to a video receiver and a selector device selects the best signal at one of the receiving antennae to be connected to the receiver. Usually, prior art approaches avoid video interruption by synchronizing the video signal such that the antenna switching takes place between video frames logo, during vertical and horizontal blanking periods. For example, US Patent 5,410,748 discloses a space diversity receiver having a plurality of antennae. A signal sampling operation samples antenna signals for a very short period and supplies them to a signal processing circuit for selecting the best antenna signal. US Patent 5,335,010 discloses another type of antenna diversity for reception of television signals. A diversity processor is responsive to line synchronizing pulses during a line blanking interval for coupling the best video signal received at one or a combination of antennae to the receiver, h the case of an imminent picture disturbance, a control circuit produces an address signal so that a new antenna signal or a linear combination derived from the antenna signals is applied quickly to the television receiver by means of an antenna combiner. US Patent 5,818,543 teaches a diversity receiver that switches between at least two antennae when the noise level of a processed receiver signal exceeds a predetermined level. A diversity circuit extracts the noise from the receiver signal with a bandpass filter and compares the level of the noise to the predetermined level. When the noise exceeds the predetermined level, the diversity circuit sends a command signal to switching circuitry to switch from the antenna currently responsible for the receiver signal to another antenna. If the noise in the receiver signal antenna still exceeds the predetermined level, then the switching circuitry continues to switch between antennae until an acceptable noise level is achieved. A synchronization circuit is also provided if antenna switching is desired during the vertical or horizontal blanking intervals. Another type of receiver combines a plurality of received signals in order to optimally detect the desired information in the received signal. Known as beamforming, this method is used for improving the reception of a signal arriving from a desired location in the presence of noise and interfering signals from other locations. Beamforming networks combine multiple received signals at multiple receiving sensors, such as antennae. US Patent 6,577,353 discloses a plurality of combiners and a plurality of television tuners. One of the combiners applies the strongest combination of input video signals to one of the tuners for viewing, while the other tuner is used for testing various other combinations of input video signals are searched for a combination significantly stronger than the combination being viewed. This reference describes switching between outputs of a combination of multiple antennae using multiple tuners (receivers) to select the superior signal. However, the use of multiple tuners can make the system complex and costly. US Patent 6,115,419, discloses an adaptive digital beamforming receiver with pi./2 phase shift to improve signal reception. The disclosed apparatus and method automatically adapts a television signal within a television receiver to minimize (or null) interfering signals of the television receiver that are caused by the presence of multiple signal echoes created by obstacles in the room in which the antenna of the television receiver is located. It is also known to combine diversity and beamforming to solve intersymbol interference (ISI) problems encountered in high-speed underwater acoustic (UWA) communications due to multipath. US Patent 5,844,951 recognizes that while beamforming and diversity combining techniques can be used to mitigate ISI and fading caused by multipath propagation, these two techniques are generally considered to be fundamentally different approaches to solving the same problem. It would be appreciated that most of the above-described approaches to diversity control and reception of video signals are complex and costly. Therefore, there exists a need for a simple and low cost system that can improve video signal reception effectively in the presence of multipath interference without causing noticeable image distortion. SUMMARY OF INVENTION Briefly, according to the present invention, a video receiver combines receiver selection diversity and beamforming to improve video signal quality. A diversity controller according to the present invention is responsive to a received signal quality measure to couple the received video signals at a plurality of antennae to the video receiver. The diversity controller includes a beamforming network that operates in conjunction with a high speed RF switch to asynchronously select one of a plurality of antennae combination signals. The antennae combination signals are generated by combining received video signals at a number of antennae, preferably, in accordance with some of the more detailed features of the invention, as describe herein that utilize received signal directivity attributes. When the received signal quality measure falls below a threshold, a comparator circuit triggers an antenna-seeking algorithm that switches the receiver and couples its input to an antennae combination signal at such high speed, e.g., within 10 nanoseconds, that the images represented by video signals are not affected during switching. As such, the diversity controller of the invention eliminates the need for synchronizing the antenna switching and selection process with the received video signal, for example, with blanking intervals (horizontal and vertical). Furthermore, the high speed switching of the combination video signals obviates the need for separate receivers. Consequently, the received video signals can be coupled to a single receiver in an asynchronous manner. As herein defined, asynchronous means not being synchronized relative to the received video signal. According to some the more detailed features of the present invention, the plurality of antennae combination signals corresponds to a received signal directivity attribute, for example, from at least two different directions, e.g., bidirectional and tri-directional, or n directional. In one embodiment, the beamforming network comprises transmission lines that differ in length by ^X wavelength, e.g., one or more pairs of transmission lines that differ in length by % wavelength. According to other more detailed features of the invention, the beamforming network comprises a transmission line switching network that combines video signals received at least one pair of antennae, e., g., a pair of antennae that are separated from each other by % wavelength, and selects an antennae combination signal for coupling to the video receiver, for example, via transmission lines that differ in length by % wavelength. In an exemplary embodiment, such transmission lines are provided by coupling a ^XΛ wavelength transmission line to one of two equal length transmission lines. The antenna combination signal can be selected based on a switching algorithm, for example, one that selects the antenna combination signal sequentially. In one embodiment, the beamforming network comprises a ground plane for mounting two % wavelength separated antennae and coupled to the transmission lines via any one of coaxial cables, striplines, microstrips. A radio and method for receiving video signals according to the present invention combines video signals received at the plurality of antennae to provide a plurality of antennae combination signals. According to this aspect a switch signal is generated based on a received signal quality measure made at a video receiver for selectively coupling one of the plurality of antennae combination signals to the video receiver asynchronously at a speed that does not cause a noticeable image interruption related to the received video signal. Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a video receiver that incorporates the diversity controller of the present invention. FIG. 2 is a diagram depicting one embodiment of a beamforming network utilized in the diversity controller of the present invention. FIG. 3 is a polar chart depicting the directivity pattern created by the beamforming network of FIG. 2. FIG. 4 is a diagram depicting one embodiment of a beamforming network utilized in the diversity controller of the present invention. FIG. 5 is a block diagram of a conventional video receiver that can be used with the diversity system of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. One skilled in the art would recognize, however, that the invention might be practiced without some or all of these specific details using equivalent components. FIG. 1 depicts the block diagram of a diversity system according to an exemplary embodiment of the present invention. As stated above, this exemplary embodiment combines antenna selection diversity and beamforming to improve image and video signal quality, hi one embodiment, the diversity system of the present invention is used with a wideband video receiver 10, which can be any suitable video receiver currently available in the market. The diversity system of the present invention also includes a plurality of antennae 1A, IB, and 1C that couple to a beamforming network 3 via input ports 2A, 2B, and 2C. Although the exemplary embodiment of FIG. 1 shows three antennae, any number of antennae may be used in the diversity system of the present invention. As shown, the beamforming network 3 has a plurality of output ports 15 and 16 that couple a plurality of antennae combination signals to a multi-port, high-speed RF switch 4. A diversity controller 6 is responsive to a received signal quality measure, such as signal strength, bit error, signal-to-noise ratio , preferably, made at the video receiver 10, to couple the received video signals at the antennae 1A, IB and 1C to the receiver 10 in response to a switch signal;. When a received video signal fades or falls below a threshold, a comparator circuit 8 triggers an antenna-seeking algorithm in the diversity controller 5 that changes the switching state of the highspeed RF switch 4 at high speed, e.g., within 10 nanoseconds. Once triggered by the switch, the RF switch 4 causes a selected antennae combination signal, as presented in a first output port 15, to be disconnected from the video receiver 10. , Instead, another antennae combination signal is selected at a second output port 16 of the beamforming network 3. Consequently, due to the high speed switching action, a viewer watching images that are represented by the received video signals does see a noticeable image interruption. As stated above, this feature of the invention simplifies the diversity controller by eliminating the need for synchronizing the received video signal with horizontal and vertical blanking intervals and the need for additional receivers. Optionally, a user interface 7 may be employed to control the operation of the diversity control system for example by changing variables of the antenna-seeking algorithm 5 to optimize diversity operation under various conditions. In an exemplary embodiment, the diversity system of the present invention is a directional diversity system. As such, the beamforming network 3 provides antennae combination signals that correspond to a received signal directivity attribute. As herein used, received signal directivity attribute relates to a radiation directivity pattern. For example, in an n-directional diversity system, the antennae combination signals corresponds to n different radiation directivity patterns. As described further below, the beamforming network 3 comprises a transmission line switching system that provides a desired directivity pattern in accordance with the present invention. Depending on the application and physical requirement of the diversity system, transmission lines contained in the transmission line system can be any suitable transmission lines that meet the requirements of any communication application. For example, the transmission lines can be selected from a wide variety of coaxial cables, striplines, microstrips, etc. Such transmission lines can be coupled to the antennae 1 A, IB and 1C via suitable feed connectors, such as coaxial connectors, etc. In a bi-directional diversity system, the beamforming network 3 receives antenna signals from two quarter- wave vertical antennae 23 and 25. As shown in

FIG. 2, the beamforming network 3 comprises a ground plane 21 for mounting the two antennae 23 and 25, which are physically separated from each other by ^lA wavelength. The beamforming network 3 also comprises transmission lines that differ in length by ^λA wavelength, feeding the two VA wavelength separated antennae. The wavelength transmission lines 27 provide the % wavelength difference in length. As explained below, this arrangement creates a directional pattern that is predictable and depends on the physical orientation of the antennae 23 and 25 and the phasing of the transmission lines. FIG. 3 shows a typical radiation pattern for the transmission line system described in the beamforming network 3 of FIG. 2. It can be seen that while the directive gain provides only a modest 3 dB signal increase, there is significant signal rejection in the opposite direction. This property is useful in a multi-path environment, where multiple signal reflections arrive at the antennae 23 and 25 of FIG. 2 simultaneously causing destructive interference. The antenna array created by the beamforming network 3 increases signals in the primary direction and rejects other signal decreasing the potential for multi-path interference. The radiation pattern can be reversed simply by altering the transmission line lengths, without physically moving the antennae 23 and 25. In one exemplary embodiment, the transmission line system of the invention includes a network of switches and an array of transmission lines that differ in length by A wavelength. Using a proper diversity algorithm, the transmission line system of the invention can be used to select a desired receiver directivity attribute to improve video signal reception and image quality. The bi-directional diversity system described above can be extended to a quad-directional diversity system by adding a third ^XA wave vertical element that form an array of three orthogonal antennae that are within an orthogonal pattern. FIG. 4 shows an exemplary diagram with three-antennae physical configuration. Under this arrangement, received signals from individual omnidirectional antennae 31, 32 and 33 can be combined in the beamforming network 3 to form directional patterns in four perpendicular directions. The antennae and transmission lines can be arranged to provide received signals directly as well as via a ^lA wave transmission line to create an array of transmission line pairs that differ in length by Vi wavelength. The beamforming network comprises a transmission line switching network 35 that combines video signals received at least one pair of antennae and selects an antennae combination signal for coupling to the video receiver. The antenna combination signal is selected based on a switching algorithm, for example, sequentially. In one exemplary embodiment, the transmission line and switching network 35 sequentially switches the transmission line pairs in a defined sequence until optimum video signal reception is achieved. In one embodiment, a switch circuitry generates switching signals that select a sequence of transmission line pairs that differ in length by V* wavelength. This feature of the invention could be used to obtain information that indicate the direction (i.e., the quadrant) of the received video signal. For better results, and higher resolution direction fining capabilities, an antenna array with higher directivity like a yagi-uda or adding antennae elements to the phased array could be used. The beamforming network 3 can be operated extremely quickly using high-speed logic and switches. The beamforming network can also apply a single antenna element to the receiver in the event that a signal element provides a better signal. FIG. 5 depicts the block diagram of the receiver block of FIG. 1. The exemplary video receiver can operate in frequency ranges of 350-450 MHz, 1710-

1850 MHz, 2450-2484 MHz, 4400-4700 MHz and 5100-5900 MHz. As shown, the received video signals from the high speed antenna switch (4) are applied to the receiver via an antenna jack 26. An RF filter 11 removes unwanted signals and passes desired signals for amplification in the low-noise amplifier (LNA)13. A second RF filter 14 provides additional attenuation of unwanted signals. The RF signal is converted to the intermediate frequency (IF) by mixing the RF signal with the signal from the phase locked loop (PLL) synthesizer 15 in the mixer 16. Since mixing of this type produced sum and difference products, additional filtering is required in the IF filter 17. The IF signal is amplified and held at a constant level by the amplifier/limiter 16. Signal strength of the incoming signal is directly proportional to the current in the limiter and is used to drive the signal strength meter 20 and provide and output voltage proportional to the signal strength at a point 27. The discriminator 19 demodulates of the frequency-modulated signal and produces a base-band signal that is filtered to remove the audio sub-carrier and amplified to produce video output at the video jack 25. The audio sub-carrier demodulator 23 recovers the audio signal and creates an audio signal on the audio output jack 24. None of the above cited prior art teach or suggest a television receiver that includes multiple antennae coupled to a beam-former network that has a plurality of output ports that provide corresponding combinations of received video signals from multiple antennae, and a switching circuit that selects one of the output ports based on a received signal quality measure at high speed. The present invention can use a single receiver (tuner) coupled to one of multiple output ports of the beamformer network through a high-speed switch that operates asynchronously relative to the video signal for coupling an optimum received video signal at one of the ports of the beamformer network to the receiver in a way that does not interrupt the quality of the image the video signal represents. While certain exemplary embodiments have been described in this specification and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broader invention, and that this invention is not to be limited only to the specific constructions and arrangements shown and described, because various other changes, combinations, omissions, modifications, and substitutions, in addition to those set forth above and below, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims

WE CLAIM: 1. A diversity controller for a video receiver, comprising. a beamforming network that combines received video signals at two or more antennae to provide a plurality of antennae combination signals; and a high speed RF switch that is responsive to a received signal quality measure for asynchronously coupling one of the plurality of antennae combination signals to the video receiver.

2. The diversity controller of claim 1, wherein said received signal quality measure is measured at the video receiver.

3. The diversity controller of claim 1 , wherein the beamforming network comprises transmission lines that differ in length by VA wavelength.

4. The diversity controller of claim 1 , wherein the beamforming network comprises at least one pair of transmission lines that differ in length by VA wavelength.

5. The diversity controller of claim 1 , wherein said beamforming network comprises a transmission line switching network that combines video signals received at least one pair of antennae and selects an antennae combination signal for coupling to the video receiver.

6. The diversity controller of claim 1, wherein the at least one pair of antennae are separated from each other by V wavelength.

7. The diversity controller of claim 6, wherein the transmission line switching network comprise at least one pair of transmission lines that differ in length by VA wavelength.

8. The diversity controller of claim 1 , wherein the transmission line switching network couples at least one VA wavelength transmission line to one of two equal length transmission lines that produce the pair of transmission lines that differ in length by VA wavelength.

9. The diversity controller of claim 1, wherein the antenna combination signal is selected based on a switching algorithm.

10. The diversity controller of claim 1, wherein the switching algorithm selects the antenna combination signal sequentially.

11. The diversity controller of claim 1, wherein each of said plurality of antennae combination signals corresponds to a received signal directivity attribute.

12. The diversity controller of claim 1, wherein the received signal directivity attribute relates to at least two different directions.

13. The diversity controller of claim 1, wherein the transmission lines comprise at least one of coaxial cables, striplines, microstrips.

14. A video receiver, comprising: a plurality of antennae; an RF beamforming network that combines video signals received at the plurality of antennae to provide a plurality of antennae combination signals; a diversity controller that is responsive to a received signal quality measure made at the video receiver to provide a switch signal; and a high speed RF switch that is responsive to the switch signal for selectively coupling one of the plurality of antennae combination signals to the video receiver.

15. A method for receiving a video signal; comprising combining video signals received at the plurality of antennae to provide a plurality of antennae combination signals; generating a switch signal based on a received signal quality measure made at a video receiver; selectively coupling one of the plurality of antennae combination signals to the video receiver asynchronously at a speed that does not cause a noticeable image interruption.