System and method for transmit and receive antenna patterns calibration for time division duplex (TDD) systems

A method and system for calibrating transmit and receive antenna patterns for time division duplex (TDD) is described. A station in a communications network comprises at least two antennas configured to operate together for both transmitting and receiving modes. A phase setting difference between the transmit and receive antenna patterns may be determined and used to determine a relative amplitude setting for the transmit antenna pattern and/or a relative amplitude setting for the receive antenna pattern.

FIELD OF THE INVENTION

The present invention relates generally to wireless communication systems, and more specifically, to such systems configured to calibrate transmit and receive antenna patterns thereof.

BACKGROUND OF THE INVENTION

Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.

The term “time division duplex” (TDD) as used herein, is defined for the wireless communication systems is referred to in general for systems using the same frequency spectrum for methods of communications in a time division manner (for example, WiFi, and TDD-long term evolution (LTE) systems).

The term multiple input multiple outputs (MIMO) communication system as defined herein is a communication system that may be used to improve the spectral efficiency, for example, by applying multiple inputs multiple outputs (MIMO) schemes, beam-forming, or nulling (interference mitigation/management). These operations usually require transmitter to have the knowledge of channel state information (CSI) so that a set of pre-coding weights may be set to the multiple data streams to exploit the channels for the multiple spatial channel transmission or to the same data stream to perform the beam-forming or nulling.

Typically, the receiver can feed the CSI or even the preferred pre-coding matrix (index) back to the transmitter. These feedbacks can consume some available bandwidth of the transmission in the opposite direction and reduce the data throughput. If transmission in both directions operate in the same spectrum, like TDD systems, the channels through the air are reversible and the channel information can be estimated by the receive device and then applied to the device's transmission. However, a complete transmission channel should be from the transmitter baseband to the receiver baseband, which includes various components inside the transmitter (e.g., digital to analog converter (DAC), up converter, power amplifier, filter) and receiver such as duplexer, linear amplifier (LNA), down converter, filter, analog to digital converter (ADC). The transmit path and receive path may thus experience very different gain/loss and delays behavior, due to the different components used in both paths. Applying channel reciprocity without considering the different delay and gain/loss factors between the transmit/receive paths are therefore not valid and may not be accurate enough for the use by devices in TDD systems. These parameters may be factory calibrated. However, this calibration may be tedious and costly. In addition, one important element that jeopardizes reciprocity is antennas, which project slightly different radiation patterns at up and down links, due to differences in the Voltage Standing Wave Ratio (VSWR) in both directions. Antenna VSWR may not be practically calibrated in the factory, due to the cost and time of such procedures.

Methods other than factory-calibration known in the art are capable of self-calibrating on transmit and receive paths of the device used in TDD systems. However, these methods may need an extra receive path with attenuator to calibrate transmit/receive (Tx/Rx) paths precisely on various power levels. This may limit the use of the known methods for the devices that are not equipped with the extra receive path.

SUMMARY OF THE INVENTION

According to embodiments of the invention, a calibration method is provided to improve TDD system performance for effectively overcoming the aforementioned difficulties inherent in the art.

Embodiments of the present invention provide calibration of transmit and receive antenna patterns (e.g., beam peaks and nulls) for the devices in TDD systems, in which the channel reciprocity may be utilized so that the feedback of channel state information can be reduced or eliminated for beam-forming, nulling (e.g., interference mitigation) operations.

Some of the TDD operations, for example, beam-forming and nulling (e.g., interference mitigation/management) of a communication device may not need the complete calibration information of the device's Tx and Rx paths for using channel reciprocity. Instead, matching Tx and Rx antenna pattern (peak and null), e.g., finding the weighting offset between the two antenna patterns, may be sufficient for the TDD device to take advantage of the channel reciprocity on certain applications. Embodiments of the invention provide a method for calibrating a device's transmit and receive antenna patterns (e.g., obtaining the weighting offset for the two antenna patterns), disclosed herein for a TDD device (for example, WiFi) to enable the use of the channel reciprocity. Embodiments of this method, which may be termed calibration, may be used in a radiation test or in the field using live signals. When field processing is used, embodiments of the method may be done in a relatively static, stable and reliable environment e.g., Line-Of-Sight environment, high signal to noise plus interference (SINR), low mobility, minimal interference level, repetitive measurements that allow for averaging, and filtering out of elements that may distort the accuracy of the said calibration.

According to an embodiment of the invention, communication devices are provided in a TDD system. The communication devices may include mobile devices such as a user equipment (UE). A UE may include any end user device with wireless connectivity such as a smart telephone, a laptop or a tablet personal computer (PC). The communication devices are not limited to UEs and may comprise any station in a communication network. The communication devices may include a plurality of M antennas for beam-forming or nulling operations, for example having tunable phases and optionally also adjustable amplitude. Each antenna may be used for both transmission and receiving. A communication device in a system according to embodiments of the invention may also comprise a plurality of radio circuits configured to transmit and receive via said antennas using TDD, for example by transmitting and receiving on a common channel. The device may also comprise a computer processor such as a baseband processor configured to calculate a weight setting difference between the transmit and the receive antenna pattern. The antenna pattern may be a peak-null pattern of the plurality of antennas operating together. In some embodiments one or more components such as the baseband processor and radio circuits may be configured to carry out all or part of the present invention.

According to one embodiment of this invention, a communication device may perform a calibration processes to find the weight (for example, the relative phase setting or the relative amplitude setting or both) difference for matching transmit and receive antenna patterns (peaks and nulls) so that the channel reciprocity may be applied in the TDD system.

Embodiments of the invention also provide a method and system for configuring a station in a communications network comprising at least two antennas configured to operate together for both transmitting and receiving modes. A phase setting difference between the transmit and receive antenna patterns of said at least two of said antennas may be calculated or determined. This phase setting difference may be used to calculate or determine a relative amplitude setting for the transmit antenna pattern or a relative amplitude setting for the receive antenna pattern or both.

Embodiments of the invention also provide a method and system for configuring a station in a communications network as described above, in which a predetermined phase difference between receive and transmit antenna patterns is used to determine a relative amplitude setting for the transmit antenna pattern or a relative amplitude setting for the receive antenna pattern or both. One or both of these relative amplitude settings may be determined in order to achieve a null in a signal received from or transmitted to another station by said at least two antennas.

According to embodiments of the invention, a time division (TD) user equipment (UE) or a Wi-Fi station may select base transmitter station or access point (e.g. selecting SSID) signal, in a reliable environment as indicated above, and perform peak and null tuning on both transmissions by iteratively manipulating phases and amplitudes using a quality indicator, so that the maximum/the minimum is achieved.

A quality indicator may be data rate; so peak may be defined as amplitudes and phases setting to the two or more antennas which maximizes data rate, and null—as ones which minimize it; this may be applied to both to up and down links.

To calibrate the transmit beam, the station may upload a large file (for example with a constant packet length) at the same location for the receive beam calibration. For the receive path, peak/null may be established also via received signal level measurement, and therefore an iterative process may not be required.

Embodiments of the invention may start with an arbitrary phase (amongst two or more transmitting antenna) and equal amplitudes, and check the upload data rate (or the number of re-transmission time) until steady, then repeat the data rate measurements after changing to a new relative phase. The same may be done with relative amplitude. The transmit beam peak and null may then be found.

During the calibration, at WiFi protocol, if a collision happens (e.g., interferer shows up), a particular calibration data point may be given up.

The null may be difficult to obtain, for example if the received power has fallen below the noise level or the acknowledgement for a null transmission cannot be obtained. In that case according to embodiments of the invention, the peak relative phase may be reversed (e.g., minus 180 degrees) for the phase of the null of the two-antenna beam.

In addition, this calibration method may be applied to other TDD networks (for example a non WiFi network). Instead of SSID, the received system broadcast (for system message) channel power or pilot power may be used for UE receive beam calibration.

In a non-WiFi network, interferers may become an issue for the calibration. According to embodiments of the invention, a threshold of SINR may be set (e.g., 25 dB) for the receive beam peak. On the transmit beam peak, a reasonable data rate may be set as the threshold. Then according to embodiments of the invention the calibration may be calculated or determined to be valid only when there are data over these set thresholds.

According to one embodiment of the invention, the said calibration process may include measuring transmit and receive antenna patterns of the device; or finding out the weights (for example, the relative phase setting between two antennas or the relative amplitude setting or both) of the antenna pattern nulls or peaks. The calculation of weight setting difference may be based on nulls only.

According to one embodiment of the invention, the said antenna pattern measurements may be carried out with the direct radio frequency (RF) signal measurement or through system performance evaluation (for example, MCS—modulation coding set).

According to one embodiment of this invention, the said antenna pattern measurements (or calibration procedure) may be performed at the location that has line-of-sight view to another communication device in a communication system such as the WiFi access point (AP) (or base station). The location may also be close to the other communication device to minimize the interference and to mitigate the fading situation.

According to one embodiment of the invention, the line-of-sight view may be confirmed if the difference of received signal strength indicator (RSSI) from the receive antennas are within a pre-set range, such as threshold (for example, 3 dB), and if the fluctuation (due to fading) of RSSI from each of the receive antennas are within a pre-set threshold (for example, 3 dB).

According to one embodiment of the invention, the closeness of the calibrated device to an AP (or base station) may be confirmed if the received signal powers from each of the receive antennas are within a pre-set range (for example, −25 to −50 dBm in WiFi device calibration).

According to one embodiment of the invention, the receive antenna pattern may be obtained through for example a coherent combining process (for example, the maximum ratio combining) on the received signals from receive antennas, while varying the combining weight of the signals over a pre-set range (for example, varying the relative phase over a range of 0 to 360 degrees).

According to another embodiment, the weight setting for the null or peak of receive antenna pattern may be obtained by performing the channel estimation on the signal from each receive antenna.

A direct RF measurement for the transmit antenna pattern (transmit power vs. the relative phase between the transmit antennas) may not be feasible. According to one embodiment of the invention, the transmit antenna pattern may be represented by system performance parameter, for example, the measured data rate or feedback data rate or feedback channel information, instead of transmit RF power, versus the relative phase between the two antenna signals.

According to one embodiment of the invention, the communication device may transmit the same signal (same data from a large file) with equally amplitude (power) through the transmit antennas while changing the relative phase between the two signals (or antennas), and collects the system performance information (for example, MCS—modulation coding set) to represent the transmit antenna pattern.

According to one embodiment of the invention, for the said transmit antenna pattern measurement, the data rate (for example, MCS—Modulation and Coding Set fed back from base station, or the transmit data rate count when the fixed data rate transmission and re-transmission scheme is used) may be collected to represent the transmit antenna pattern.

The system parameters (for example, data rate) for a transmit null may not be reliably obtainable, due to the low signal-to-noise ratio at null. According to one embodiment of the invention, a computer processor may be configured to extrapolate from two reliable measured transmit data rates and their corresponding phase settings. For example the weight setting for the null may be extrapolated from the parameter slopes (for example, data rate changes) on the relatively reliable data on both sides of the null. The computer processor may be configured to calculate the offset of phase settings for transmit and receive antenna patterns by comparing the phase settings of the said transmit and receive nulls.

According to another embodiment, the weight setting for the null of an antenna pattern may be confirmed with the weight setting for the peak, which should occur at 180 degrees away from the null setting.

According to another embodiment of the invention, the antenna pattern calibration may be applied to the antenna pattern formed by any number of antennas.

Embodiments of the invention also provide a method comprising selecting a calibration site location for a station having plurality of antennas and radio circuit. The site location may have a line of sight view with another station in a communications network, such as an access point (AP). The method may then comprise monitoring a broadcast message from said another station (AP) and channel estimating on a receive signal from each radios/antennas representing a channel of the station. A phase difference Φ between the different channels may then be recorded and weights difference of the antennas calculated for the receive peak (RP) and receive null (RN) as ΦRP=−Φ and ΦRN=ΦRP+180 degrees, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention and as used herein, the following terms are defined with the following meanings, unless explicitly stated otherwise.

As defined above, in TDD a single channel (e.g., the same or overlapping frequency spectrum) is used for both downlink and uplink transmissions. For example, both stations and their served access point in WiFi systems use the same frequency channel alternatively in time to communicate each other. Alternatively TDD cellular communication, e.g., between a mobile device and a base station, may periodically alternate between uplink transmissions (e.g. from the mobile device to a base station for a predetermined uplink interval or period, such as, 5 milliseconds (ms)) and downlink transmissions (e.g. from a base station to a mobile device or UE for a predetermined downlink period, such as, 5 ms). The base station typically coordinates the alternating timing between uplink (UL) and downlink (DL) transmissions.

Embodiments of the present invention propose the calibration procedures to derive the correction factors (weight difference) on matching transmit and receive antenna patterns of a TDD device. This correction factor information may enable the device perform the beam-forming and/or nulling application for improving the spectral efficiency in TDD (e.g., WiFi) systems.

FIG. 1is a block diagram showing a system100according to some embodiments of the present invention. System100shows a WiFi (i.e., TDD) system with the unmatched transmit (uplink) and receive (downlink) antenna patterns. Station110may have a baseband processor124, M antennas120-1. . . ,120-M, and M radios, or radio circuits,122-1. . . ,122-M, each antenna may be used for both transmit and receive and any of the antennas120-1. . . ,120-M may be configured to operate together for both transmitting and receiving modes; Access Point (AP)140may also have multiple (M) antennas and radios. In transmit antenna pattern130-1, T indicates the relative transmit gain with the relative phase of transmit antennas set at Φ0. In receive antenna pattern130-2, R indicates the relative receive gain with the same relative phase (Φ0) set for the receive antennas. The transmit antenna pattern and the receive antenna pattern are not matched; with the same relative phase setting (Φ0), the transmit gain (T) is on the peak of the antenna patterns while the receive gain (R) is not.

FIG. 2is a block diagram200illustrating the same system in100but with different relative phase settings for transmit antennas and receive antennas (Φ0and Φ1, respectively) according some embodiments of the present invention. The receive antenna pattern230-2, is then matched to the transmit antenna pattern230-1, with these relative phase settings; both transmit and receive gains (T and R) are on the peaks of transmit and receive antenna patterns. The calibration is to find the difference between the relative phase settings (Φ0and Φ1) so that the TDD device may perform beam-forming and nulling without the feedback from AP (or base station) and thus improve the spectral efficiency of the TDD system. Embodiments of the invention include the calibration procedures.

Embodiments of the calibration procedures are configured to obtain the difference of weight (relative phase) settings between the transmit null (peak) and the receive null (peak).FIG. 3is a flowchart300illustrating example procedures for measuring receive antenna patterns according some embodiments of the present invention. Step310shows that the calibration (e.g., receive and transmit antenna pattern measurements) may be performed at the location where the device is, which preferably has a Line-Of-Sight (LOS) view to the AP (or base station) and close to the AP to minimize the impact of the multiple paths, interference, and fading. The line-of-sight view may be defined quantitatively. For example, LOS may be confirmed if the difference of received signal strength indicator (RSSI) from the receive antennas are within a pre-set threshold (for example, 3 dB) and if the fluctuation (due to fading) of received signal strength indicator (RSSI) from each of the receive antennas are within a pre-set threshold (for example, 3 dB). The closeness of the calibrated device to AP (or base station) may be defined quantitatively as well. For the example of the WiFi device calibration, the closeness may be confirmed if the received signal powers from each of the receive antennas are within a pre-set range (for example, −25 to −50 dBm).

For receive antenna pattern measurement, Step320shows that the device may monitor the constant signal broadcasted from AP (or base station), for example, the Service Set identifier (SSID) from WiFi AP or system broadcast message (including pilot) from base station. An initial relative phase ΦRimay be set in Step320as well. Step330shows that the device under calibration may measure the received signals from each receive antenna and coherently combine them with the weight (relative phase) setting. For example, the combined signal power C(ΦR)=a2+b2+2*a*b*cos(ΦR), where a and b are the received signal strength (amplitude) from each of the two antenna and ΦRis the relative phase setting (may be ΦRifor the initial measurement). Step340shows that the combined signal powers may then be averaged over many repeated measurements until the average value becomes stable (for example, the average values fluctuates within a pre-set value say, 1 dB or 20%). The collected signal strength may be discarded and not be input into the averaging if the discrepancy of signal strengths (a and b) exceed a pre-set threshold (for example 1 dB). Step350indicates that the received signal measurement and the signal combining using the same relative phase will be repeated if the averaged value is not relative stable. Once the stable averaged combined power becomes stable,360shows that the averaged combined power and the corresponding relative phase setting may be stored and a new relative phase may be set by adding the phase resolution (say, 10 degrees) to the current relative phase setting. Step370shows that the receive antenna patterns measurement is complete and may be stopped once the measurements have through the full range (360 degrees) of the relative phase settings. Otherwise, continue the measurement with the new phase setting.

The calibration is to find the weight (relative phase) between transmit and receive antenna patterns. On certain antenna spacing (for example, distance of the antennas is less than half of the wavelength), the antenna patterns may have a single null (or peak). The antenna pattern offset may be obtained by comparing the relative phase settings of transmit null (or peak) and receive null (or peak). The calibration may then only measure the phase setting for the null (or peak), instead of the measurement for the whole antenna pattern.FIG. 4is a flowchart400illustrating that the calibrated device may perform channel estimation to obtain the relative phase setting for the receive null and peak according some embodiments of the present invention. Thus the baseband processor may deduce the antenna pattern by applying channel estimation on the received signal from each antenna. Step410selects the calibration site that has a LOS view and close to AP (same location and orientation for the Tx antenna pattern measurement). The calibration device monitors the constant broadcast message that has a pilot (or reference signal), shown in Step420. Step430shows that the calibrated device may perform a channel estimation on the pilot or reference signal. Step440indicates that the relative phase Φ between the two (receive antennas) channels may be found and stored. The phase settings for the receive peak, ΦRP, and null, ΦRN, may then be found as:
ΦRP=−Φ  (1)
ΦRN=ΦRP+180(degrees)  (2)
In other words the receive null is deduced by adding 180 degrees to the phase setting of the receive peak.

Step460shows that the phase settings may be separately averaged over several channel estimations. Step470indicates that the receive null (peak) measurement is complete and may be stopped once the averaged values become stable. Stability can be achieved when, for example, the fluctuation of the averaged values is less than a pre-set value (say, 20%).

A direct RF signal measurement may not be feasible for the transmit antenna pattern. Instead, the system performance, for example, data rate evaluation, may be used for representing the transmit antenna pattern. A transmit antenna pattern may be presented by collected data rates versus the relative phase settings of the transmit antennas. For the transmit antenna measurement, the calibrated device may transmit the equal and constant amplitude signal through the transmit antennas and collect the data rate information for various relative phase settings (between the transmit antennas).

FIG. 5is a flowchart500depicting an example procedure to measure the transmit antenna pattern according some embodiments of the present invention. Step510indicates that transmit antenna measurement should be done at the same site and the same orientation for receive antenna pattern measurement. Step520shows that the calibrated device may perform User Datagram Protocol (UDP) to upload random data until transmit antenna measurement is completed. The data rate count may be based on the feedback data rate information or counted by the calibrated device itself, depending on the system data transmission scheme. Step530shows that if the transmit data rate is dynamically scheduled (based on the channel quality) by the receiver (e.g., base station), for example, in TDD-LTE system, the feedback data rate (MCS—Modulation/Coding Set) may then be used for representing the antenna pattern, shown on step540.

An embodiment of a procedure for the data rate collection based on feedback is shown inFIG. 6in flowchart600according some embodiments of the present invention. As opposed to LTE, for example, in a WiFi system, the transmit data rate is not scheduled by the receiver station, the calibrated device may then count the transmit data rate itself550.

FIG. 7is a flowchart700showing an embodiment of a procedure for self-counting on the transmit data rate. The data rate count may be limited by the system setup and/or the receiver capability. For example, the received signal (and SNR) may be too low to support any transmit data rate, due to that the transmission is on or around the transmit null. On the other hand, the received signal (and SNR) may far exceed the required SNR to support highest transmit data rate of the system setup when the transmission is on or around the transmission peak.560calculate the phase settings for transmit null and peak. Example detailed calculation and procedure are described inFIGS. 8 and 9.

For a system with dynamically scheduled transmit data rate by receiver, the feedback data rate information may be collected for representing the transmit antenna pattern. Flowchart600ofFIG. 6depicts an example procedure to collect the transmit data rate. Here the same signal with equal amplitude is transmitted through the transmit antennas, for example under the control of a baseband processor, while varying the relative phase between the two signals, and collects the data rate information for the said transmit antenna pattern measurement. Feedback information may be used to determine data rates for performing transmit antenna pattern measurement. Step610shows that the calibrated device set a relative phase ΦTiand equal amplitude for the two transmit signals/antennas for the UDP transmission. Step620indicates that the calibrated device monitor (and use) the feedback MCS until it becomes stable, For example, the feedback MCS is not changed for consecutive pre-set number (say, 10) of feedbacks. The relative phase setting and the feedback data rate (MCS) may then be stored, shown on630. Step640shows that a new phase setting by adding the pre-set phase resolution (say 10 degree) may be set for continuous UDP transmission. Step650shows that if data rate collection has through the whole range of the phase settings (360 degrees), the calibrated device may stop the data rate collection and proceed to calculate the phase setting for transmit null and peak shown on660. Otherwise, it continues the data rate collection with the new phase setting.

For a system with non-scheduled transmit data rate by receiver, the calibrated device may count its transmit data rate for representing the transmit antenna pattern. Flowchart700ofFIG. 7depicts an example procedure to self-count the transmit data rate. Step710shows that the calibrated device set a relative phase ΦTiand equal amplitude for the two transmit signals/antennas for the UDP transmission. In the example of WiFi system, the transmission may be with irregular data size and on the irregular time period, and may be even unsuccessful. Step720indicates that the calibrated device may sum the transmitted data for each successful transmission and calculate the data rate with the sum of the transmission periods until the aggregate data rate becomes stable. The relative phase setting and the stable aggregate data rate may then be stored, shown on730. Step740shows that a new phase setting by adding the pre-set phase resolution (say 10 degree) may be set. Step750shows that if data rate collection has through the whole range of the phase settings (360 degrees), the calibrated device may stop the data rate collection and proceed to calculate the phase setting for transmit null and peak shown on760. Otherwise, it continues the data rate collection with the new phase setting.

FIG. 8is a graph diagram800showing the example of the collected data rate and relative phase settings for obtaining the phase setting of the transmit null and peak according some embodiments of the present invention. In the example, R1and R2are the measured data rates in the midrange of the all measured data rates selected for calculate the phase setting for the transmit null. The reasons for selecting the data rates in the midrange are that the data rate measured around the transmit null may be too noisy (and unreliable) and the data rate measured around the transmit peak may be under-stated due to the system limitation and hence not accurate. Each measured data rate may have two phase settings; phase settings of Φ1and Φ4result in the measured data rate R1and Φ2and Φ3for R2, shown in800. These phase settings (Φ1, Φ2, Φ3, and Φ4) and the selected data rate (R1 and R2) may form two straight lines S1(810) and S2(820). S1and S2may then be presented as
S1:R=[(R1−R2)/(Φ1−Φ2)]*Φ+(R2*Φ1−R1*Φ2)/(Φ1−Φ2)  (3)
S2:R=[(R1−R2)/(Φ4−Φ3)]*Φ+(R2*Φ4−R1*Φ3)/(Φ4−Φ3)  (4)
The phase setting of transmit null, ΦTN, is the phase setting corresponding to the intersect point (830) of the two lines. Using Equation 3 and 4, the phase setting of transmit null may be obtained,
ΦTN=(Φ1*Φ3−Φ2*Φ4)/(Φ1−Φ2+Φ3,−Φ4)  (5)
840shows the phase setting for the transmit peak, which is 180 degree away from the transmit null.

FIG. 9is a flowchart900depicting an example procedure calculate the phase settings of the transmit null and peak according to some embodiments of the present invention. Step910shows that two data rates R1and R2in the midrange of all the measured data rates may be selected to calculate the phase setting of transmit null. Step920is the procedure to find the phase settings for the two selected data rate (Φ1, Φ4, Φ2, and Φ3). Step930shows that the phase setting of transmit null, ΦTN, may be calculated according Equation 5. In step940the phase setting for the transmit peak ΦTPmay be calculated,
ΦTP=(ΦTN+180)degrees  (6)

The phase offset of transmit and receive antenna pattern may be obtained by comparing the phase settings for transmit and receive nulls, ΦTNand ΦRN.

FIG. 10shows exemplary collected data A00for transmit and receive antenna patterns formed by two antennas according to some embodiments of the present invention. In this exemplary figure, A10and A20indicate receive and the transmit antenna patterns. From the phase settings of receive null (A30) and transmit null (A40), the calibration procedure according to embodiments of the present invention finds 30 degree phase shift between these two antenna patterns (A10and A20).

The foregoing describes the adjustment of relative phase between multiple transmitters, or between multiple receivers, to compensate for differences between transmit and receive paths. The matching of transmit and receive antenna patterns to achieve equivalent peaks and nulls can be further improved, especially the matching of nulls, by additionally adjusting the relative amplitudes of signals. The following describes the adjustment of amplitude with reference toFIGS. 11 to 14. Embodiments of the invention may be used to calculate or determine a relative amplitude setting for the transmit antenna pattern and a relative amplitude setting for the receive antenna pattern, or both. The difference between these two relative amplitude settings may be determined as a weight setting difference between transmit and receive antenna patterns. As with the phase setting difference described above, the amplitude setting difference may be determined based on nulls in the antenna patterns. The determination of amplitude setting is particularly useful for signal cancellation.

FIG. 11illustrates an example TDD MIMO system. Embodiments of the invention may be implemented in any station in a communications network.FIG. 11illustrates embodiments of the invention using a UE and a base station or access point by way of example. The roles of the UE and the base station may be reversed and each may be substituted for another station. The system shown inFIG. 11includes UE1110comprising, in this example, two antennas1111and1112and respective radios, or radio circuits,1113and1114. The principles of embodiments of the invention may be extended to any number M of antennas and radios. Any of the antennas may be configured to operate together for both transmitting and receiving modes. The system also comprises base station1150which may also have M antennas and radios, only one of which is shown inFIG. 11, antenna1151and radio1152. Each antenna on these two devices, UE1110and base station1150may be used for both transmit and receive, but not simultaneously, in the TDD MIMO operation. The channels available over the air (between the antennas) are reversible. Each radio1113,1114and1152comprises respective amplifiers for the transmit and receive paths. UE1110comprises a baseband processor1120and base station1150comprises a baseband processor1160.

The received signals at antennas1111and1112in receive mode respectively can be represented as:
r1=CR1*h1a*S r2=CR2*h2a*S(7)
where:S is the transmitted signalC represents the gain or loss caused by components inside the transmitter/receiverh represents the effect of the over-the-air channel.

A single antenna base station1150will receive two signals from antennas1and2in transmit mode, indicated as Y1and Y2inFIG. 11, which can be represented as:
Y1=ha1*CT1t1 andY2=ha2*CT2*t2  (8)
It can be assumed that
h1a=ha1andh2a=ha2(9)
since these are reciprocal over the air channels.

In order to achieve a transmit null from antennas1and2to antenna A, the signals Y1and Y2should cancel each other out. In other words the goal is to achieve, as near as possible:
Y1/Y2=−1 orY1=Y2*ej 180(10)
From equations 7 and 8 we can derive:
r1*t1=S*Y1*Cr1/Ct1andr2*t2=S*Y2*C r2/C t2  (11)
and
(r2*t2)/(r1*t1)=(Y2/Y1)*(Cr2*Ct1)/(cr1*Ct2)  (12)
We now define:
R12=r1/r2,T12=t1/t2,C12=(Cr2*Ct1)/(Cr1*Ct2)  (13)
and
C12=(CT1*CR2)/(CT2*CR1)=A12ejΦ12(14)
where A12and Φ12 are the unknown amplitude and phase corrections for channel reciprocity.

After the signals from two antennas have been adjusted to be out of phase, cancellation of the respective signals can be further improved by amplitude matching. Now suppose that for receive nulling, r1and r2have been set 180° apart via a received signal phase adjustment of θ12, and amplitude correction B12,
thenC12=1/(R12*T12)
becomes:C12Rx null=(r2*t2)(B12*r1*ejθ12*t1)  (15)
It is then useful to calculate or determine what adjustment of phase and amplitude is needed between transmit and receive modes in order to achieve phase nulling in transmit mode. The goal is to achieve Y1=Y2*ej 180, which is to adjust the transmit phase between t1and t2by Φ12 starting from receive nulling correction θ12, and the amplitude ratio by A12 from receive amplitude correction B12, so starting from equation (15)
C12Rx null+Tx null_=(r2*t2)/(B12*A12*r1*ej(θ12+Φ12)*t1)  (16)

A possible method of calculating or determining the amplitude ratio according to embodiments of the invention will now be described with reference toFIG. 12. Some or all of the operations ofFIG. 12may be performed in the baseband processor, e.g. baseband processor1120, for example by suitable programming of the baseband processor. At operation1210the relative phase settings for the receive and transmit nulls are determined. This may be done using any of the methods described above with reference toFIGS. 1 to 10. From this procedure it is possible to determine the relative phase between transmit and receive as:
Φ12=ΦTN−ΦRN(17)

ΦTNand ΦRNare the relative phase settings for transmit null and receive null determined by any of the procedures described above with reference toFIGS. 1 to 10and Φ12 is the phase correction for channel reciprocity, also termed “calibrated phase” or phase setting difference. The phase setting difference is an example of a weight setting difference that may be determined as described with reference toFIGS. 1 to 10. It will be appreciated that this phase adjustment or calibrated phase is intended to be applied on switching from reception to transmission (and a reverse adjustment is applied on switching back) to achieve, in the illustrated example ofFIG. 11, a null at the base station, or at least to minimize the amplitude of the combination of signals received by or transmitted to the two antennas1111and1112.

It is then possible to adjust the amplitude for both receive and transmit imbalances in order to achieve deep up and down (transmission and reception) nulls. According to embodiments of the invention this adjustment of amplitude is applied in addition to the phase adjustment to further improve the nulling effect of combining two signals. As with phase, it is possible to calculate or determine a correction of relative amplitude or calibrated relative amplitude, to be applied on switching from reception to transmission, or in reverse from transmission to reception.

In the subsequent operations ofFIG. 12, according to embodiments of the invention, the phase between transmission and reception is adjusted in order to match the transmit and receive antenna patterns, for example using the predetermined phase difference Φ12. According to embodiments of the invention, the process of calculating or determining the phase and amplitude corrections may be repeated at intervals.

At operation1220, a broadcast message from the base station or AP is monitored, for example this may be the SSID. At operation1230, channel estimation is performed on the received signal from each of the antennas, r1 and r2.

Next at operation1240, new weight settings B12 and θ12 (amplitude and phase, see equation 15) are determined for the receive null based on the channel estimation. These weight settings are for relative phase and amplitude between the two antennas1111and1112in receive mode. It should be noted here that both ΦRNand θ12 represent the relative phase between antennas to achieve a receive null. φRNis derived from a process of averaging described with reference toFIG. 4whereas θ12 may be based on one channel estimation.

Next, at operation1250, with the relative phase of signals t1 and t2 set to achieve a null by applying the phase calibration Φ12 determined at operation1210, an amplitude ratio between signals t1 and t2 is set and data is uploaded.

In order to determine a suitable amplitude ratio t1/t2 to be used, the channel estimation in operation1230may be performed before each collection of “transmit” data to obtain values of r1 and r2. The transmit signal ratio may be determined as:
(t1/t2)=(r2)/(r1*B12*Z*ej(θ12+Φ12))  (18)

Z is a real number representing the difference in amplitude ratio between transmit and receive antenna patterns and may be varied within a predetermined range in iterations of operation1250, for example −Y to +Y in dB. Operation1250may be performed with different values for t1/t2 at intervals, for example of one second.

The transmission date rate for each value of t1/t2 may be determined, for example based on feedback information received from the base station1150. The relationship between data rate and amplitude ratio t1/t2 may be registered and recorded or stored at operation1260. Operations1230to1260may be repeated until it is determined at operation1270that data rates for all values of Z have been recorded.

It will be seen from equation (18) that the amplitude ratio t1/t2 according to embodiments of the invention is based on the receive relative amplitude setting B12, and the receive relative phase setting θ12 as well as the receive/transmit phase setting difference Φ12. Also according to embodiments of the invention, the receive relative amplitude setting B12, and the receive relative phase setting θ12 are newly determined, for example based on channel estimation, for each iteration of operations1250and1260, transmitting and recording data rate.

Following this “blind search”, the lowest registered data rate may be identified and the corresponding amplitude ratio defined as the amplitude ratio setting for the transmit null. This may also be referred to as amplitude “imbalance” and may be measured in dB.

FIG. 13Ashows an example of results that may be achieved from variation of amplitude ratio. Note that if Z=Y (or −Y), i.e. the minimal data rate occurs on the edge of, or outside, the expected range, the range of Z may be extended until a minimum appears. The data shown in this figure has a clear minimum from which the amplitude ratio for the transmit null may be determined.

FIG. 13Bshows an example set of results with more than one minimum data rate, such as might occur due to system limitations, or data transmission being unsuccessful, due to low SNR at the null, where the “slope method” may be used to extrapolate the virtual minimum data rate and corresponding amplitude ratio.

It will be appreciated from the foregoing that having determined amplitude ratios or relative amplitudes r1/r2 and t1/t2 for transmit and receive, it is useful also to determine the difference between these two ratios, r1/r2 minus t1/t2, to be used as a weight setting difference between transmit and receive antenna patterns, in order to match the patterns and achieve a null at the base station1150.

FIG. 14is a graph illustrating the effect of the variation of amplitude ratio on the received signal strength. In the figure, the x axis represents relative phase in degrees between two signals and the y axis represents signal strength relative to the peak, measured in dB. The different curves represent the relationship between signal strength of the combined signal and relative phase for different relative amplitudes measured in dB. Thus for example the relationship at 0 dB, which is not possible to achieve in practice, results in a null at infinity, and the other curves show that a difference of only 0.1 dB in relative amplitude can make a difference of a higher order in strength of the combined signal.

The determination of relative amplitude in the manner described above, using feedback from the other station where the null is to be achieved, may be performed by suitable configuration of the baseband processor without the need for any additional hardware.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or an apparatus. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” For example, a baseband processor or other processor may be configured to carry out methods of the present invention by for example executing code or software.

Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. It will further be recognized that the aspects of the invention described hereinabove may be combined or otherwise coexist in embodiments of the invention.

The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.