Enhanced calibration for multiple signal processing paths in a frequency division duplex system

Calibrating signal processing paths for a plurality of transmission devices by obtaining calibration data for at least one of the signal processing paths for each of the transmission devices and determining a plurality of calibration weights from the calibration data for each of the transmission devices. A calibration variance is calculated between the plurality of calibration weights and it is determined if the calibration variance is below a calibration variance threshold. Additionally, a phase variation and a magnitude variation are calculated from the calibration data for each of the transmission devices and it is determined for each of the transmission devices if the phase variation is below a phase variation threshold and if the magnitude variation is below a magnitude variation threshold. Further, if the calibration variance is below the calibration variance threshold, and the phase variation is below the phase variation threshold and the magnitude variation is below the magnitude variation threshold for each of the transmission devices, then the plurality of calibration weights are applied to the at least one of the signal processing paths of each of the transmission devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of signal processing, and more specifically to calibration of multiple signal processing paths within a wireless network.

2. Description of the Related Art

A signal processing system, in for example a Frequency Division Duplex (“FDD”) system, includes a plurality of signal processing paths and requires a suitable mechanism to match characteristics of the individual signal processing paths to each other within a given pre-specified tolerance. Each of the signal processing paths also includes a transmitter (Tx) and/or a receiver (Rx) or an electrical/electronic/optical measurement system that allows an information/measurement signal with or without modulating a carrier to be processed through it. It is necessary for the plurality of processing paths to have electrical parameters of, for example, magnitude, phase and bulk delay through the individual processing paths to match each other within an acceptable tolerance, which may be different for the different processing paths.

Beamforming is a general signal processing technique used to control the directionality of the reception or transmission of a signal on a transducer array. Using beamforming, the majority of signal energy can be transmitted from a group of transducers (such as radio antennas) in a chosen angular direction. The present invention discloses a beamforming calibration system for use in a FDD system for matching characteristics of the individual signal processing paths to each other within a given pre-specified tolerance.

SUMMARY OF THE INVENTION

An embodiment of the invention relates to a method of calibrating signal processing paths for a plurality of transmission devices. The method includes obtaining calibration data for at least one of the signal processing paths and determining a plurality of calibration weights from the calibration data for each of the transmission devices. A calibration variance is calculated between the plurality of calibration weights and it is determined if the calibration variance is below a calibration variance threshold. Additionally, a phase variation and a magnitude variation are calculated from the calibration data for each of the transmission devices and it is determined for each of the transmission devices if the phase variation is below a phase variation threshold and if the magnitude variation is below a magnitude variation threshold. Further, if the calibration variance is below the calibration variance threshold, and the phase variation is below the phase variation threshold and the magnitude variation is below the magnitude variation threshold for each of the transmission devices, then the plurality of calibration weights are applied to at least one of the signal processing paths of each of the transmission devices.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates an exemplary system for carrying out an embodiment of the present invention. Broadly, the system includes an interface (IF) module102, a beamforming (BF) module104, two or more transmission devices108, a loop module106, and two or more antennas120.

The IF module102is used to interconnect the system with one or more modems112. Each type of modem will require a unique IF module102that is specifically designed to handle the unique interface and signaling requirements. The modems112are able to control the signal processing paths of the transmission devices108. The signal processing paths include both the Tx power output and Rx gain of the transmission devices108. Because the transmission devices108are used to carry signals from all the modems112simultaneously, control of the Tx power output and Rx gain cannot be accomplished by adjusting each transmission device output power and gain control. Instead, each of the transmission devices108is set to maximum Tx power and maximum Rx gain, and beamforming weights are applied to the system to obtain precise Tx power output control and Rx gain control for each transmission device108. Ideal beamforming weights are transmitted from the modems112to the IF module102. The IF module102is used to up-convert and down-convert signals from the modems112into a 30 MHz bandwidth that is used by the system and then the signals, including the ideal beamforming weights, are transmitted to the BF module104.

The BF module104is used to perform the main beamforming function including the calibration of the transmission devices108. The BF module includes a beamforming unit114, a calibration unit116, and a central processing unit (CPU)118.

The BF module104performs the multiply-accumulate functions necessary to control the Tx power output and Rx gain control of each of the transmission devices108. In the downlink direction, the BF module communicates with the modems112via the IF module102by means of 16-bit digital IF signal running at 60 MSamples/s. This digital signal is digitally down-converted to produce a baseband 32-bit i and q signal. This baseband 32-bit i and q signal is fed into the beamforming unit to produce a 32 bit i and q signal for each of the transmission devices108. These 32 bit i and q signals are sent out of the BF module104to the transmission devices108at a rate of 60 MSamples/s.

In the uplink direction, the BF module104receives 32-bit i and q signals from each of the transmission devices108. These signals are fed into the beamforming unit114and will produce a 32 bit i and q signal for each of the modems112. The signals are then digitally up-converted producing a 16-bit IF signal at 60 MSamples/s that is sent to modems112via the IF module102.

A calibration signal is created in the calibration unit116. The calibration signal is used to create an in-band signal used for calibration of delays through the transmission devices108. Calibration data is obtained by transmitting the calibration signal through each of the transmission devices108and looping it back to the calibration transmission device110, or transmitting the calibration signal through the calibration device110and looping it back through each of the transmission devices108. The received calibration data is processed by the CPU118to ensure the received calibration data is of good quality and to create calibration weights. After calibration weights are obtained, the beamforming unit114creates beamforming weights by combining the ideal beamforming weights with the calibration weights and the beamforming unit114applies the beamforming weights to the system.

Two or more transmission devices108are present in the system.FIG. 1depicts the system as having four transmission devices108though it should be readily understood that any number of two or more transmission devices108may be used. The transmission devices108provide the Frequency Division Duplex (FDD) channels used for beamforming and a calibration device110provides an additional channel used for calibration by sending and receiving a calibration signal that can be used to measure the differences between the transmission devices108.

In FDD systems, the transmitter and receiver operate at different frequencies. Additionally, there is no gap in the transmission and reception of data and, therefore, the calibration signal needs to be sent out at the same time as the data is being transmitted and received. Further, during the calibration process, the system must meet regulatory requirements. Specifically, the regulatory spectral emission mask specifications must be met in the transmit direction and spurious emission specifications must be met in the receive direction.

The loop module106is used to loop RF signals from the one of the transmission devices108to the calibration transmission unit110. The BF module104uses a calsel signal310to control the loop module106to determine which one of the transmission devices'108RF signals are looped to the calibration device110or vice versa. The loop module106must be carefully designed so as not to significantly impact differential phases of the multiple phase paths.

Referring now toFIG. 2,FIG. 2is a block diagram illustrating the transmission devices108and the calibration transmission device110. Along the Tx path202, the transmission devices108have a digital-to-analog (DAC) converter212to convert digital baseband i and q signals to radio frequency (RF) signals at a specified RF frequency. The RF signals are fed into filters216and an adjustable attenuator220before being transmitted to the loop module106. Conversely along the Rx path204of the transmission devices108, RF signals received from the loop module106are passed through an adjustable attenuator220and filters218before being inputted into an analog-to-digital (ADC) converter214to convert the RF signals to digital baseband i and q signals before being transmitted to the BF module104. The Tx path202of the calibration transmission device is similar to the Tx path202of the transmission devices except there is no adjustable attenuator220and the Tx path202of the calibration transmission device operates in the same frequency band as the Rx path204of the transmission devices. Additionally, the Rx path204of the calibration transmission device110is similar to the Rx path204of the transmission devices except the Rx path204of the calibration transmission device operates in the same frequency band as the Tx path202of the transmission devices.

Referring now toFIG. 3,FIG. 3is a block diagram illustrating the interconnections between the BF module104, the transmission devices108, the calibration device110, the loop module106, and the antennas120. For each of the transmission devices108along the Tx path202, RF signals pass through a power amplifier302before entering a diplexer306connected to the antennas120. Conversely, along the Rx path204, RF signals leave the diplexer306connected to the antennas120before entering a low noise amplifier304. A calsel signal310is used by the loop module to control a switch308to select whether the calibration transmission device110receives RF signals from one of the transmission devices108or transmits RF signals to one of the transmission devices108by means of directional couplers307.

FIG. 4is a representation of a channel bandwidth410of a spectrum allotted to a carrier404and calibration tones408inserted in the channel bandwidth410. The channel bandwidth410is the amount of spectrum that is allocated for the carrier404. The channel bandwidth410is typically regulated by a regulating body, such as the Federal Communication Commission (“FCC”), based on the type of signal being transmitted and, as such, a carrier404must conform to a spectral emission mask406. For example, a Universal Mobile Telecommunications System (“UMTS”) signal would be assigned a frequency band to operate in by the FCC and, additionally, would have to adhere to a prescribed channel bandwidth within the assigned frequency band. A typical channel bandwidth for a UMTS signal is 5 MHz. A spectral mask406is illustrated in the figure to define the boundaries of the channel bandwidth410. As shown, the carrier bandwidth402of the carrier404is typically less than that of the channel bandwidth410.

A pair of calibration tones408comprising the calibration signal are inserted in the spectrum between the edges of the carrier bandwidth402and the channel bandwidth410. The bandwidth of the calibration tones408must be narrow enough to fit within the gap in the spectrum between the channel bandwidth410and the carrier bandwidth402. The carrier404and the calibration tones408are produced as digital signals within the system at a fixed sampling rate. The bandwidth of the calibration tones408is determined based on the finite transmit time of the calibration tones408. The bandwidth of the calibration tones408can be approximately be determined by Equation (1), in which BWcaltoneis the bandwidth of the calibration tones408, fsis the sample rate, and N is the length of the calibration tones.

Additionally, because the bandwidth of the calibration tones408must be narrow enough to fit within the gap in the spectrum between the channel bandwidth410and the carrier bandwidth402, the bandwidth of the carrier402can be represented by Equation (2), in which BWchannelis the channel bandwidth410and BWcarrieris the bandwidth of the carrier402.

Combining Equation 1 and Equation 2, the length of the bandwidth of the calibration tones408can be readily determined and is represented by Equation (3).

As noted above, the calibration tones408consist at two frequencies within the gaps in the spectrum between the channel bandwidth410and the carrier bandwidth402. In determining the calibration tones408, a Hamming window is used to minimize the spectral leakage of the calibration tones408. The calibration tones408are determined by Equation 4, in which n=[0, 1, . . . N−1}, A is the amplitude of the calibration tones408, and kLand kRare the left and right indexes of the calibration tones408, and are determined by Equation (5) and Equation (6), respectively, where fcis the center frequency of the carrier404.

The amplitude of the calibration tones408, A, is chosen such that the calibration tones408are smaller than the carrier404and, as a result, will not significantly impact the power that is transmitted or the Peak-to-Average Power Ratio (PAR) of the combination of the carrier404and the calibration tones408within the channel bandwidth410. The amplitude, A, for the calibration tones408may be different if the carrier404is an uplink carrier or if the carrier904is a downlink carrier.

Referring now toFIG. 5,FIG. 5illustrates a flowchart describing a method of obtaining calibration data from each of the transmission devices108for each carrier. Beamforming requires the accurate control of the phase and amplitude of the signals to and from the antennas120. In order to achieve this accuracy, the transmission devices108are calibrated so that the differences in phase and amplitude between them can be compensated for. The calibration transmission device110is used to send and receive a calibration signal that can be used to measure the differences between the active transmission devices.

In Step502, a carrier is set as a specified carrier.

In Step504, it checked to see if the specified carrier is an uplink carrier or a downlink carrier. If the specified carrier is determined to be a downlink carrier, calibration data for the downlink carrier is obtained in Step506. If the specified carrier is determined to be an uplink carrier, calibration data for the uplink carrier is obtained in Step508. Steps506and508will be discussed in more detail in reference toFIG. 6andFIG. 7, respectively.

In Step510, the CPU118checks the quality of the obtained calibration data and determines calibration weights for each of the transmission devices for the specified carrier, and stores the calculated calibration weights in the calibration unit116. Step510will be discussed in more detail in reference toFIGS. 8 and 9.

In Step512, the method checks to see if calibration weights have been determined for all carriers. If not, in Step514, a next carrier is set as the specified carrier and the method returns to Step504. If calibration weights have been determined for all carriers, in Step516, the beamforming unit114determines beamforming weights for the system by multiplying ideal beamforming weights received from the modems112by the calculated calibration weights stored in the calibration unit116. The beamforming weights are applied to the Tx/Rx paths by the BF module104.

It should be noted that the calibration process should be performed in a manner such that it is robust to single failures of any one Tx and/or Rx chain. The process should be able to identify which chain, if any, has failed. Additionally, a failed transmission device should not be used as a reference transmission device and the SNR should not be checked for a failed transmission device.

FIG. 6is a flowchart illustrating a method of obtaining calibration data from a transmission device when a carrier is a downlink carrier corresponding to step506ofFIG. 5.

In Step602, a calibration signal comprising a pair of calibration tones is calculated and inserted in the spectrum between the edges of the specified carrier bandwidth and the channel bandwidth as discussed above in reference toFIG. 4.

In Step604, one of the transmission devices108is set as a specified transmission device.

In Step606, the specified carrier and the calibration signal are summed, transmitted through the Tx path of the specified transmission device, and looped back to the Rx path of the calibration transmission device110. A signal is captured at the Rx path of the calibration transmission device110containing the specified carrier, the calibration signal, and an amount of noise. A Hamming window is applied to the captured signal to reduce the amount of spectral leakage caused by the specified carrier, and a Fast Fourier Transform (“FFT”) is taken of the captured signal and stored as calibration data for the specified transmission device; Equation (7) illustrates the application of a Hamming window and a FFT to the captured signal.

The FFT transformed captured signal comprises a left index (kL) and a right index (kR) corresponding the pair of calibration tones of the calibration signal. Normalized calibration tones are determined for the specified transmission device by dividing captured calibration tones from a reference transmission device by the captured calibration tones for the specified transmission device; any of the transmission devices108may serve as the reference transmission device. A calibration weight for the specified transmission device is determined by taking the average of the normalized calibration tones.

At Step608, the method checks to see if the summed specified carrier and calibration signal have been sent through all the transmission devices108. If not, in Step610, a next transmission device is set as the specified transmission device and the method returns to Step604. Otherwise, the method proceeds to Step510inFIG. 5.

FIG. 7is a flowchart illustrating a method of obtaining calibration data from a transmission device when a carrier is an uplink carrier corresponding to step508ofFIG. 5.

In Step702, a calibration signal comprising a pair of calibration tones is calculated and inserted in the spectrum between the edges of the specified carrier bandwidth and the channel bandwidth as discussed above in reference toFIG. 4.

In Step704, one of the transmission devices108is set as a specified transmission device.

In Step706, the calibration signal is transmitted through the Tx path of the calibration transmission device110and looped back to the Rx path of the specified transmission device. A signal is captured at the Rx path of the specified transmission device containing the specified carrier, the calibration signal, and an amount of noise. A Hamming window is applied to the captured signal to reduce the amount of spectral leakage caused by the specified carrier, and a Fast Fourier Transform (“FFT”) is taken of the captured signal; Equation (7) illustrates the application of a Hamming window and a FFT to the captured signal.

The FFT transformed captured signal comprises a left index (kL) and a right index (kR) corresponding the pair of calibration tones of the calibration signal. Normalized calibration tones are determined for the specified transmission device by dividing captured calibration tones from a reference transmission device by the captured calibration tones for the specified transmission device; any of the transmission devices108may serve as the reference transmission device. A calibration weight for the specified transmission device is determined by taking the average of the normalized calibration tones.

At Step708, the method checks to see if the summed specified carrier and calibration signal have been sent through all the transmission devices108. If not, in Step710, a next transmission device is set as the specified transmission device and the method returns to Step704. Otherwise, the method proceeds to Step510inFIG. 5.

Referring now toFIG. 8,FIG. 8is a flowchart illustrating a method of calibrating signal processing paths according to one embodiment of the present invention.

In Step802, a new set of calibration data is obtained for each of the transmission devices108for a given carrier as described in above in reference toFIGS. 5-7. The set of calibration data for each of the transmission devices includes a left index (kL) and a right index (kR) corresponding the pair of calibration tones of the calibration signal. Normalized calibration tones are determined for the specified transmission device by dividing captured calibration tones from a reference transmission device by the captured calibration tones for the specified transmission device; any of the transmission devices108may serve as the reference transmission device.

In Step804, calibration weights are calculated for each of the transmission devices108and a calibration variance, Δcal, is determined for the calibration weights across all the transmission devices108. A calibration weight for a given transmission device is determined by taking the average of the normalized calibration tones for that transmission device. The calibration variance is the variation of the calibration weights across all the transmission devices108calculated in dB.

In Step806, a phase variation, Δp, and a magnitude variation, Δm, of the calibration data are calculated for each of the transmission devices108. The phase variation, Δp, is the variation in phase of the left index calibration tone, calikL, and the right index calibration tone, calikRand is illustrated in Equation (8).

The magnitude variation, Δm, is the variation of the absolute values of the left index calibration tone, calikL, and the right index calibration tone, calikRand is illustrated in Equation (9).

In Step808, the calibration variance is compared to a calibration variance threshold, and, for each of the transmission devices108, the phase variation is compared to a phase variation threshold and the magnitude variation is compared to a magnitude variation threshold. If the calibration variance is below the calibration variance threshold, and, for each of the transmission devices108, the phase variation is below the phase variation threshold and the magnitude variation is below the magnitude variation threshold, then the method proceeds to Step810; if not, the method proceeds to Step814. Examples of threshold values include a phase variation threshold of 5 degrees, a magnitude variation threshold of 0.5 dB, and a calibration variance threshold of 1 dB. Note that, however, the present invention is not limited to these values and other threshold values may be used based on the specific needs of a system.

In Step810, any active alarms are cleared, a log is updated, and the calibration weights are stored in the calibration unit116. In Step814, it is checked if the process flow has been looped more than 3 times. If the process has not been looped more than 3 times, the method proceeds to Step816and error details are logged before proceeding back to Step802. If the process has been looped more than 3 times, the method proceeds to Step812, at which an alarm is activated, the error details are logged, and previously determined calibration weights stored in the calibration unit116are used by the system.

Referring now toFIG. 9,FIG. 9is a flowchart illustrating a method of calibrating signal processing paths according to a second embodiment of the present invention. More specifically,FIG. 9illustrates an embodiment in which a signal-to-noise ratio (“SNR”) is calculated and used to determine if the received calibration data is of good quality. For example, in order to achieve a +/− 1 deg accuracy on the calibration results, the SNR should be at least 35 dB.

It should be noted that the method of calibration ofFIG. 9is similar to the method of calibration ofFIG. 8except the method of calibration ofFIG. 9includes additional steps of calculation of the SNR of the received calibration data, and comparison of the SNR to a predetermined threshold to determine if the received calibration data is of good quality. Identical steps described above in reference toFIG. 9will not be described below.

In Step902, the SNR is calculated for each of the transmission devices108. Equation (10) is used to calculate the SNR of the received calibration data, where kNL, and kNRare determined by Equation (11) and Equation (12), respectively.

In Step904, for each transmission of the devices108, the SNR is compared to a SNR threshold. As noted above, ideally the SNR should be greater than 35 dB to judge that the received calibration data is of good quality, therefore the SNR threshold is set to 35 dB. Please note, however, that while 35 dB is used as the SNR threshold, the present invention is not limited to these values and other threshold values may be used to determine if the received calibration data is of good quality based on the specific needs of a system.

If, for each of the transmission devices108, the SNR is greater than the SNR threshold, the received calibration data is judged to be of good quality and the method proceeds to Step804. However, if the received calibration data is not judged to be of good quality, the method proceeds to Step906.

In Step906, it is checked if the process flow has been looped more than 3 times. If the process has not been looped more than 3 times, the method proceeds to Step816and error details are logged before proceeding back to Step802. If the process has been looped more than 3 times, the method proceeds to Step812at which an alarm is activated, the error details are logged, and previously determined calibration weights are applied to the signal processing paths.

FIG. 10is a representative BF module104for calibrating multiple signal processing paths as shown in the system ofFIG. 1. InFIG. 10, the BF module104includes a memory1010, a processor118, user interface1002, application programs1004, communication interface1006and bus1008.

The memory1010can be computer-readable media used to store executable instructions, computer programs, algorithms or the like thereon. The memory1010may include a read-only memory (ROM), random access memory (RAM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), a smart card, a subscriber identity module (SIM), or any other medium from which a computing device can read executable instructions or a computer program. The term “computer programs” is intended to encompass an executable program that exists permanently or temporarily on any computer-readable medium. The instructions, computer programs and algorithms stored in the memory1010cause the BF module104to perform calibrating multiple signal processing paths as described in the system ofFIG. 1. The instructions, computer programs and algorithms stored in the memory1010are executable by one or more processors118, which may be facilitated by one or more of the application programs1004.

The application programs1004may also include, but are not limited to, an operating system or any special computer program that manages the relationship between application software and any suitable variety of hardware that helps to make-up a computer system or computing environment of the BF module104. General communication between the components in the BF module104is provided via the bus1008.

The user interface1002allows for interaction between a user and the BF module104. The user interface1002may include a keypad, a keyboard, microphone, and/or speakers. The communication interface1006provides for two-way data communications from the BF module104. By way of example, the communication interface1006may be a digital subscriber line (DSL) card or modem, an integrated services digital network (ISDN) card, a cable modem, or a telephone modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface1006may be a local area network (LAN) card (e.g., for Ethernet™ or an Asynchronous Transfer Model (ATM) network) to provide a data communication connection to a compatible LAN.

Further, the communication interface1006may also include peripheral interface devices, such as a Universal Serial Bus (USB) interface, a Personal Computer Memory Card International Association (PCMCIA) interface, and the like. The communication interface1006also allows the exchange of information across one or more wireless communication networks. Such networks may include cellular or short-range, such as IEEE 802.11 wireless local area networks (WLANS). And, the exchange of information may involve the transmission of radio frequency (FR) signals through an antenna (not shown).

Further, the above disclosure defines the signal processing paths as being the Tx or Rx path of a transmission device. It is noted that the present invention is not limited to such disclosure and the above disclosure may be easily modified to work in a system containing signal processing paths consisting of an electrical/electronic/optical measurements system that allows an information/measurement signal with or without modulating a carrier to be processed through it.

While an embodiment of the invention has been disclosed, numerous modifications and changes will occur to those skilled in the art to which this invention pertains. The claims annexed to and forming a part of this specification are intended to cover all such embodiments and changes as fall within the true spirit and scope of the present invention.