WO2012052772A1

WO2012052772A1 - Wireless communications devices

Info

Publication number: WO2012052772A1
Application number: PCT/GB2011/052040
Authority: WO
Inventors: Mark Barrett
Original assignee: Bluwireless Technology Ltd
Current assignee: Bluwireless Technology Ltd
Priority date: 2010-10-21
Filing date: 2011-10-20
Publication date: 2012-04-26
Anticipated expiration: 2013-04-21
Also published as: GB201017736D0; GB2484703A

Abstract

A method and apparatus are disclosed for rapidly determining the direction of arrival of a radio frequency signal over an air interface (3) from a transmitter device (1) to a receiver device (2). The techniques described make use of quadrature baseband signals to provide inputs to a direction of arrival processor.

Description

WIRELESS COMMUNICATIONS DEVICES

The present invention relates to wireless communications devices, and, in particular, to beam acquisition in multiple antenna wireless communications devices.

BACKGROUND OF THE INVENTION

In order to obtain the necessary radio link budget for reliable wireless communications at high (gigabit) data rates at millimetre wave frequencies (30-100 GHz) it is necessary to employ phased array antennas for the transmitter and receiver between two devices to provide the required antenna gain. Typical configurations may range from 4 to 16 antenna elements (x2 for transmitter and receiver). The outputs (or inputs) from these antennas are combined to form a high gain beam pointing in the direction of the device to which communications are intended. Such high gain beams are needed in both transmit and receive directions to establish the necessary Signal to Noise Ratio (SNR) required for low error rate data transmission. A problem arises as to how to determine which direction to point the beam when the communications link is being established. One solution, as described in the IEEE 802.15.3c 60 GHz Wireless PAN and IEEE 802.1 1 ad Wireless LAN standards, is to use an iterative approach based on the use of a Beacon signal and a Contention Access Period during which the source and sink devices iteratively scan beams (like searchlights) and listen for the optimum (highest SNR) response. This is then used to lock the beam direction for the duration of the communication link together with a tracking loop which can update the beam direction (typically by selecting the beam forming weighting coefficients from a pre-defined set of beams defined in a beam codebook) as devices move relative to one another. The problem with this approach is the number of iterations and therefore the excessive time needed to acquire the optimum beams before any data transmission can occur between the two devices. Since the intended communication period between devices may be very short (measured in seconds) it follows that the acquisition time for link setup needs to be much lower than one second to maximise the efficiency of the data communication and minimise power consumption. It is, therefore, desirable to provide a technique and system that can speed up beam acquisition in comparison with previously considered techniques. SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method for rapidly acquiring a beam direction in a wireless communications system, the method comprising extracting in-phase and quadrature baseband signals for each of a plurality of receiver antenna structures, before such signals are subjected to beam forming processing, filtering such extracted in-phase and quadrature signals using a narrow band low pass filter, to produce filtered in-phase and quadrature signals for each antenna structure, subjecting the filters in-phase and quadrature signals to analogue to digital conversion to produce digital in- phase and quadrature signals for each of the antenna structures, determining direction of arrival information for a beam relative to the antenna structures from the digital in-phase and quadrature signals, and updating a beam forming process using the direction of arrival information.

According to another aspect of the present invention, there is provided a wireless communications device comprising a plurality of antenna structures, a baseband converter connected to receive radio frequency signals from the plurality of antenna structures, and operable to convert received radio frequency signals to produce respective pairs of in-phase and quadrature baseband signals therefrom, a beamformer connected to receive respective pairs of in-phase and quadrature baseband signals from the baseband converter, and operable to combine received pairs of in-phase and quadrature signals in dependence upon received weighting signals, to produce a single pair of beamformed in-phase and quadrature signals, a baseband processor connected to receive beamformed in-phase and quadrature signals, and operable to convert received analogue in-phase and quadrature signals into an output datastream, and a controller connected to receive respective pairs of in-phase and quadrature baseband signals from the baseband converter, and operable to calculate weighting signals for supply to the beamformer from in-phase and quadrature baseband signals received from the baseband converter.

In one example, such a wireless communications device further comprises a low-pass filter unit connected to receive baseband in-phase and quadrature signals, and operable to filter received baseband in-phase and quadrature signals, and a multiplexer for selecting a single in-phase and quadrature signal pair for processing.

Such a wireless communications device may further comprise an analogue to digital converter unit connected to receive a single in-phase and quadrature signal pair from the multiplexer, and operable to convert received in-phase and quadrature signals to digital in- phase and quadrature signals, and to supply such digital in-phase and quadrature signals to the controller for processing.

In such a wireless communications device, the analogue to digital converter unit may be provided by the baseband processor.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates direction of arrival estimation for a wireless communications system;

Figure 2 illustrates a phased array device embodying one aspect of the present invention for use in a wireless communications system;

Figures 3 to 5 illustrate respective components of the device of Figure 2;

Figure 6 illustrates a controller for use in a phased array device of Figure 2;

Figure 7 illustrates a second phased array device embodying an aspect of the present invention; and

Figure 8 is a flow chart illustrating steps in a method embodying another aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 illustrates a much simplified wireless communication system in which a transmitter device 1 communicates with a receiver device 2 over an air interface 3. The transmitter device 1 , and the receiver device 2 each include phased arrays of antenna structures, so that each can direct its transmit or receive beam in an appropriate direction. In the example in Figure 1 , the transmitter device 1 directs the transmit beam 4 at a transmit angle 5 to a predefined axis of the transmitter devicel . The receiver device 2 directs the receive beam 6 at a receive angle 7 to a predefined axis of the receiver device 2. It is to be noted that the predefined axes have no particular relationship to one another, which means that beam direction calculation must be performed at each of the transmitter and receiver devices 1 and 2. Any beam forming technique must be accurate, since the environment over which the devices communicate will usually be cluttered. In a typical array antenna implementation, the Half Power Beamwidth (HPBW) is likely to be between 15 and 30 degrees and so a pointing accuracy in the region of 2-5 degrees is required for obtaining a high quality communications link. Note also that the simplified system shown in Figure 1 effectively illustrates a nett transmission path from the transmitter device 1 to the receiver device 2, and does not explicitly illustrate reflected signal paths. Figure 2 illustrates a phased array device embodying one aspect of the present invention for use in a wireless communications system. The phased array device comprises four antenna structures 10, 12, H and 16, although it will be readily appreciated that any number of antenna structures can be used in devices embodying the present invention.

The antenna structures 10, 12, 14, 16 are connected with respective quadrature

down converters 20, 22, 24, 26, which convert received radio frequency signals to baseband frequency using local oscillator (LO) signals 21 , 23, 25, 27 provided by a local oscillator 45. Each downconverter 20, 22, 24, 26 produces an in-phase (I) and a quadrature (Q) signal for each antenna.

This receiver architecture preserves the spatial amplitude and phase of the incoming waveform for subsequent beam forming and signal demodulation functions. The analogue IQ outputs from each antenna element are then combined using an analogue beamformer. During reception of a beacon signal, the direction of the beam is controlled by respective analogue vector modulators 30, 32, 34, 36, whereby the IQ outputs of the receiver are adjusted in amplitude and phase for each antenna element. The outputs are adjusted using control signals provided by a controller 70, which operates to generate amplitude and phase coefficients for the beamformer.

The outputs are then combined in a combiner unit 40 to form a single analogue in-phase (I) output signal, and a single analogue quadrature (Q) output signal. These are then filtered before IQ analogue to digital (ADC) conversion by an ADC unit 50, the subsequent signal processing functions for data demodulation.

The various components of the phased array device of Figure 2 will be described in more detail with reference to Figures 3 to 6.

Figure 3 illustrates one of the downconverters 20. It will be appreciated that the other down converters 22, 24, and 26 are identical to that shown in Figure 3. The downconverter 20 includes an input amplifier 200 which is connected to the antenna structure 10, to receive incoming radio frequency signals therefrom. The input amplifier 200 amplifies the incoming radio frequency signal and supplies this amplified signal to first and second mixers 201 and 203. The first mixer combines the amplified signal with a local oscillator signal 21 , to produce an in-phase signal. The second mixer 203 combines the amplified signal with 90° phase shifted version of the local oscillator signal provided by a phase shifter 202. The second mixer 203 produces a quadrature signal. The in-phase and quadrature signals are supplied to respective low-pass filters 204 and 206, and the outputs of the low pass filters are provided to respective output amplifiers 206 and 207, which generate output in-phase (I) and quadrature (Q) signals respectively.

Each downconverter produces such in-phase (I) and quadrature (Q) signals l_n, Q_n, and these signal pairs are then provided to the respective beamformer 30, 32, 34, and 36. The beamformers produce respective pairs of beamformed signals l₃₀, Q30 ; I32 Q32 ; I34, Q34 ; I36, Q₃₆, which are then supplied to the combiner unit 40.

As shown in Figure 4, the combiner unit 40 receives the beamformed signals from the beamformers, and combines the in-phase signals together using combiners 401 , 403 and 405, to produce a combined output signal l₄₀. The quadrature signals are combined using combiners 402, 404 and 406, to produce a combined quadrature output signal Q₄₀. The combined in-phase and quadrature signals are provided to the ADC unit 50, which illustrated in Figure 5, and comprises first and second low pass filters 502 and 503 for receiving the in-phase and quadrature signals respectively. Filtered in-phase and quadrature signals are provided to respective analogue to digital converters 504 and 505 for conversion into digital in-phase and quadrature signals. These digital signals are then subjected to demodulation, decoding and other appropriate processing steps in order to produce a final datastream 60.

Figure 6 illustrates the controller 70 of Figure 2 that is responsible for producing the beamformer phase and amplitude control signals 31 , 33, 35 and 37. The controller comprises a series of low pass filters which receive respective in-phase and quadrature signals from the downconverters 20, 22, 24 and 26. Filtered in-phase and quadrature signals are then supplied to a multiplexer 704 which selects an in-phase/quadrature pair for supply to a processor 708, via an analogue to digital converter unit 706. The ADC unit 706 converts each of the in-phase and quadrature signals using respective converters 706I and 706Q. The processor 708 is operable to calculate the required phase and amplitude control signals for the beamformers 30, 32, 34, and 36. Several options exist for calculating the required amplitude and phase coefficients - for example either using a reference signal and a closed loop adaptive filter to adjust the beamformer weighting co-efficient to maximise the output level SNR or by using Direction of Arrival (DoA) algorithms to estimate the directions of arrivals of all wavefronts arriving at the array antenna. The required set of amplitude and phase coefficients for the beamformer can then be calculated directly from the estimated DoA.

Figure 7 illustrates a second example phased array device embodying the present invention. The Figure 7 example has much in common with the embodiment of Figure 6, and so Figure 7 has been simplified somewhat to aid understanding. An antenna array 102 is connected to a downconverter unit 104 which produces baseband signals from the received radio frequency signals. These baseband signals are supplied to a beamformer and combiner 106 which operates to form an appropriate beam shape, and to combine the in-phase and quadrature signals into a single I and Q pair of beamformed signals. The baseband signals are also supplied to a low-pass filter and multiplexer unit 108 which filters the baseband signals and selects a single I and Q pair for onward supply.

The beamformed I and Q signals, and the multiplexed I and Q signals, are supplied to switching and filtering units 1 10 and 1 12. A first such unit 1 10 deals with the in-phase (I) signals, and a second unit 1 12 deals with the quadrature (Q) signals. The switching and filtering units 1 10 and 1 12 select and filter either the beamformed signals or the multiplexed IQ signals from each of the individual antenna element receivers 102, which are then supplied to analogue to digital converters 1 14 and 1 16.

When the beamformed signals are selected, the resulting digital in-phase and quadrature signals are supplied to a digital processor 1 18, which operates to process the signals to produce a final output datastream 1 19, as required during the normal processing of the data communications link.

When the mutliplexed antenna element signals are selected, the resulting digital in-phase and quadrature signals are supplied to a controller 120. The controller 120 is operable to calculate the required phase and amplitude control signals 122 for the beamformer 106 using one of several DoA array signal processing methods as previously mentioned.

The example of Figure 7 has the advantage that a single pair of analogue to digital converters is required for both the beamformed signals, and for the antenna element control signals. Reducing the number of A D converters greatly reduces the space, cost and power requirements for the phased array device. The "main" or beamformed signal path typically has a bandwidth of approximately +/-1 GHz, whereas the antenna element control path preferably operates with a reduced bandwidth, determined by the low pass filters in the control signal path of approximately +/-10MHz. This x100 reduction in bandwidth leads to a +20 dB increase in signal to noise ratio (SNR) for a low bandwidth beacon signal. Such a beacon signal may comprise a sine wave or simple modulated digital sequence beam signal at an offset frequency in the region of 5 MHz. Note also that the need for such filtering for increased SNR is desirable for performance improvement and may not be required for simple low performance systems. Typical known implementations of array signal processing systems use a pair of dedicated IQ Analog to Digital Converters (ADC) connected to each of the antennas, downconverters and receivers. Such implementations are therefore significantly more complex, costing and consume more power than embodiments of the present invention. For example, as shown in Figure 7, the analogue IQ outputs from each antenna element are multiplexed to the input of the single IQ ADC already used for the beamformed signal path. Since the sample rate is very high (>2.6 GHz) any spatial error introduced by capturing the array wavefront of data across the antenna elements will be trivial. For example, for a 200nSec period, a 5 MHz signal sampled across 4 elements at 2.6 GHz the temporal phase error across the antenna elements will be .4 degs. This can be further reduced by sampling in reverse or random order when, for example, building the expectation estimate of the co-variance matrix across, for example, 10-12 temporal samples of the digitized antenna array output from the antenna elements. In summary, the re-use of the IQ ADC already needed for the processing of communication data stream for the purpose of rapidly estimating the DOA of the beam leads to signbificant reductions in system cost and power consumption. Having captured the 'array signal vector' of samples from the receive antenna array using the architecture described above, the direction of arrival azimuth and/or elevation angle of the source can be determined through one of several well known methods from the field of super-resolution direction finding. These may include eigenvalue subspace based methods such as the MUSIC algorithm or iterative maximum likelihood methods such as the IMP algorithm. Here super-resolution refers to the ability to resolve the direction of arrival of a source wavefront to sub-Rayleigh (-3dB) beamwidth levels - typically to <1 -2 degrees for a 4-8 element linear array or -3dB beamwidth of 15-30 degrees. Such algorithms would typically be rapidly executed on the high performance (10 GOPs+) baseband processor following the ADC in a typical implementation of a millimetre wave (e.g. 60 GHz) data transceiver device. Such DoA algorithms are well known to experts in the array signal processing field.

The accuracy of the direction finding algorithm is also greatly improved by the additional 20 dB of SNR achieved by the reduction in side channel bandwidth. Figure 8 shows steps in a method embodying another aspect of the present invention. A beacon signal is received (step A), and a baseband signal pair for one of the antenna structures is selected (using the multiplexer 108; 704) (step B). The selected signal pair is then subjected to analogue to digital conversion (step C), and the digital signals supplied to the processor 120, 708 (step D). If further antenna signals are available for processing, then the method returns to step B, and the next antenna signals are selected, converted and supplied to the processor (step E).

Once a required number of antenna signal pairs have been processed, the processor undertakes a direction of arrival calculation (step F), and then calculates (step G) the required amplitude and phase coefficients for the beamformer. These coefficients are then used for reception and transmission from the device.

The process described above is performed by the transmitter device and by the receiver device, using respective beacon signals received from the other device, to determine the complementary direction required to set the beamformer weighting coefficients for those devices.

The technique described above enables a reduction in the time taken for the process of capturing the beam directions between two devices, by reducing the number of processing steps required. Such a reduction can greatly accelerate communication link setup time compared to the iterative beam steering methods described in the IEEE 802.15.3c and 802.1 1 ad standards. Once the direction has been established, the array coefficients for the optimum beam for the transmitter device and for the receiver device can be calculated and applied to setup the required communication link. Therefore, a key aspect of successful imnplemetation of such a fast DOA acquisition technique requires efficient integration within the communications protocol and specifically the appropriate part of the protocol dedicated to 'Beam Forming (BF)'. For example, the IEEE 802.1 1 ad specification in which

beamforming forms part of the MAC protocol. In the specification, BF training is performed by the two communicating devices performing Sector Level Sweeps (SLS) followed by a Beam Refinement Protocol (BRP). The SLS operation requires the transmitting and receiving stations to sweep beams iteratively across a region of coverage until a pre- determined quality threshold is reached. Sector identification information is transmitted and used by the receiver as a measure of quality. A counter is used to moinitor the number of iterations during the SLS process until the link meets required quality criteria. If the number of counts exceeds a maximum specified then the process is terminated. Embodiments of the present invention are able to accelerate the SLS process described above by providing an accurate DOA estimate from the first Sector identification information transmissions, so that further iterations are not required to meet the required

communications link quality threshold. Depending on how far about the stations are separated in angle this can accelerate the BF process by several seconds. Such anaccelerated BF function can be implemented in the MAC function of the communications processor co-operating with the DF processor (120).

Claims

CLAIMS:

1 . A method for rapidly acquiring a beam direction in a wireless communications system, the method comprising: extracting in-phase and quadrature baseband signals for each of a plurality of receiver antenna structures, before such signals are subjected to beam forming processing; filtering such extracted in-phase and quadrature signals using a narrow band low pass filter, to produce filtered in-phase and quadrature signals for each antenna structure; subjecting the filters in-phase and quadrature signals to analogue to digital conversion to produce digital in-phase and quadrature signals for each of the antenna structures; determining direction of arrival information for a beam relative to the antenna structures from the digital in-phase and quadrature signals; and updating a beam forming process using the direction of arrival information.

2. A method as claimed in claim 1 , wherein the beam forming process is an interative process, and where determining the direction of arrival information serves to reduce the number of iterations in the beam forming process.

3. A wireless communications device comprising: a plurality of antenna structures; a baseband converter connected to receive radio frequency signals from the plurality of antenna structures, and operable to convert received radio frequency signals to produce respective pairs of in-phase and quadrature baseband signals therefrom; a beamformer connected to receive respective pairs of in-phase and quadrature baseband signals from the baseband converter, and operable to combine received pairs of in-phase and quadrature signals in dependence upon received weighting signals, to produce a single pair of beamformed in-phase and quadrature signals; a baseband processor connected to receive beamformed in-phase and quadrature signals, and operable to convert received analogue in-phase and quadrature signals into a digital output datastream; and a controller connected to receive respective pairs of in-phase and quadrature baseband signals from the baseband converter, and operable to calculate weighting signals for supply to the beamformer from in-phase and quadrature baseband signals received from the baseband converter.

4. A wireless communications device as claimed in claim 3, wherein such weighting signals serve to reduce a number of iterations performed by the beamformer.

5. A wireless communications device as claimed in claim 3 or 4, further comprising: a low-pass filter unit connected to receive baseband in-phase and quadrature signals, and operable to filter received baseband in-phase and quadrature signals; and a multiplexer for selecting a single in-phase and quadrature signal pair for processing.

6. A wireless communications device as claimed in claim 5, further comprising a single analogue to digital converter unit connected to receive a single in-phase and quadrature signal pair from the multiplexer, and operable to convert received in-phase and quadrature signals to digital in-phase and quadrature signals, and to supply such digital in-phase and quadrature signals to the controller for processing.

7. A wireless communications device as claimed in claim 6, wherein the analogue to digital converter unit is provided by the baseband processor.