Patent Application
20080130607
Not Applicable.
Certain embodiments of the invention relate to wireless communication. More specifically, certain embodiments of the invention relate to a method and system for multimode DC offset compensation.
In some conventional systems, a radio frequency (RF) signal may be converted to an intermediate frequency (IF), and then from IF to a baseband signal, where the IF may be in the megahertz range. Generally, the RF signal may be mixed with a local oscillator signal that results in two sideband signals that are the sum of the frequencies of the two signals and the difference of the frequencies of the two signals. One of the two sideband signals may be chosen as an IF signal, and this IF signal may be the same for all received RF signals. Therefore, a radio that may receive a plurality of channels, such as an AM or FM radio, may tune to a particular station by changing the local oscillator signal frequency such that the IF remains constant. With a constant IF, most of the receive path may be common in the receiver. Other radios may convert a received signal directly from RF to a baseband signal to reduce IF circuitry.
The process of retrieving information from an RF signal may be referred to as demodulation. The presence of direct current (DC) offsets is undesirable if data is to be accurately recovered during demodulation. The DC offsets in the demodulated output of, for example, a frequency modulated (FM) system may occur when the local oscillator of a transmitting radio generates a signal with a slightly different frequency than the frequency of the signal produced by the receiving radio local oscillator. The DC offset due to frequency mismatch may adversely affect the data extracted from the IF or baseband signals.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.
A method and system for multimode DC offset compensation, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.
These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.
Certain embodiments of the invention relate to a method and system for multimode DC offset compensation. Aspects of the method may comprise accumulating a selected number of signal samples, determining an average of the accumulated signal samples, and keeping a running total of the average for use feedback signal generation. The signal samples may be, for example, samples of a WCDMA signal. By keeping track of the signal offset error, the signal offset error may be determined. For example the signal offset error may comprise a DC component and/or a sinusoidal component. The number of signal samples to accumulate for the average may be based on the signal offset error, which may be a residual DC value.
A threshold frequency may be determined, which may be based on, for example, analysis of receiver characterization. A number of signal samples may be selected to be used when the signal offset error is less than or equal to the threshold. A smaller number of signal samples may be selected and used when the signal offset error is greater than the threshold. The accumulated number of signal samples may be divided by the number of signal samples to determine an average signal offset error of the accumulated signal samples. An embodiment of the invention may utilize the number of signal samples to be, for example, 2N. Accordingly, dividing by 2N may comprise right shifting the value to be divided by N bits.
In operation, the transmission block 110 may be adapted to transmit RF signals over a wired or wireless medium. The receiver block 120 may be adapted to receive the RF signals and process it to a baseband signal that may be suitable for further processing, for example, as data or voice. For example, the low noise amplifier 122 may amplify the received RF signal, and the mixer 124 may mix the amplified RF signal with a signal from the local oscillator 126. The output of the mixer 124 may be a signal that is at a lower frequency than the received RF frequency. For example, the output of the mixer may be a baseband signal. In some instances, when the signal from the local oscillator 126 does not match the frequency of the RF carrier of the received RF frequency, the mixer 124 may introduce a bias signal. The bias signal may be a DC offset, or the bias signal may be a sinusoidal signal. It may be desirable to attenuate the bias signal.
The baseband processor 154 may comprise suitable logic, circuitry, and/or code that may be adapted to process analog baseband signals generated from the receiver front end (RFE) 152. The processor 156 may comprise suitable logic, circuitry, and/or code that may be adapted to control the operations of the receiver front end 152 and/or the baseband processor 154. For example, the processor 156 may be utilized to update and/or modify programmable parameters and/or values in a plurality of components, devices, and/or processing elements in the receiver front end 152 and/or the baseband processor 154. Control and/or data information may be transferred from a processor external to the RF receiver system 150 to the processor 156. Similarly, the processor 156 may transfer control and/or data information to at least one controller and/or processor external to the RF receiver system 150.
The processor 156 may utilize the received control and/or data information to determine the mode of operation of the RFE 152. For example, the processor 156 may select a specific frequency for a local oscillator, or a specific gain for a variable gain amplifier. Moreover, the specific frequency selected and/or parameters needed to calculate the specific frequency, and/or the specific gain value and/or the parameters needed to calculate the specific gain, may be stored in the system memory 158 via the processor 156. This information stored in system memory 158 may be transferred to the RFE 152 from the system memory 158 via the processor 156. The system memory 158 may comprise suitable logic, circuitry, and/or code that may be adapted to store a plurality of control and/or data information, including parameters needed to calculate frequencies and/or gain, and/or the frequency value and/or gain value.
The ADC 212 may comprise suitable logic, circuitry, and/or code that may enable conversion of analog signals to equivalent digital signals. The CMF block 214 may comprise suitable logic, circuitry and/or code that may be adapted to filter a digital baseband signal. The CMF block 214 may be used, for example, for receiving for WCDMA signal. The CMF block 214 may comprise a plurality of digital filters, which may be utilized to filter an in-phase (I) component and a quadrature (Q) component of the digital baseband signal. The digital filters may have a combined impulse response that may be, for example, square root raised cosine (SRRC), which may be required by, for example, the WCDMA specifications. A diagram illustrating exemplary sampling is described with respect to
In operation, the received signal may be processed by the receiver front end 152 to demodulate and downconvert the received RF signals. The output of the combiner 210 may logically represent the analog baseband signal that may result after the processing by the receiver front end 152. The offset signal Bias may be the DC offset introduced by various components of the receiver front end 152 during processing of the received RF signal. For example, a local oscillator (not shown) used to downconvert the received RF signal to a baseband signal may introduce a DC offset if the local oscillator frequency is not the same as a carrier frequency of the received RF signal.
The processed signal, which may be an analog baseband signal, may be communicated to the ADC 212. The ADC 212 may convert the analog baseband signal to a digital baseband signal. The number of bits for each sample may depend on design considerations. An exemplary number of bits may be eight signed bits for each analog sample. Accordingly, each sample may have a value in the range of, for example, −127 to +127. Other embodiments of the invention may use other ranges, for example, from −128 to +127, or from −127 to +128. The digital baseband signal may be communicated to the CMF block 214, which may appropriate filter the digital baseband signal. The CMF block 214, which may be, for example, in the RF receiver system 150, may perform digital filtering that corresponds to digital filtering by a corresponding CMF block (not shown) in, for example, the transmitter block 110. The CMF block 214 may, for example, receive data eight bits at a time, and output data eight bits at a time.
The digitally filtered signal may be communicated to the baseband processor 216 for appropriate processing, such as, for example, offsetting of the offset signal Bias introduced by the receiver front end 152. The output of the baseband processor 216 may be, for example, a digital baseband signal Sig_Out. The receive path may have been described for a single signal for exemplary purposes. However, an embodiment of the invention may receive, for example, I and Q signals from the receiver front end 152 represented by the combiner 210. Accordingly, the ADC 212, the CMF 214, and the baseband processor 216 may process the I signal independently of the Q signal. One architecture may comprise two separate blocks or circuitry to handle the I and Q signals.
The graph 260 may illustrate samples of the sinusoidal wave 252 having a zero DC offset at DC_0. The sinusoidal wave 252 may be sampled by, for example, the ADC 212. Accordingly, the samples at the sample points S1, S11, and S21, which may correspond to the zero crossings at time instants T1, T4, and T7, respectively, may have digital values of zero. The samples at the sample points S2 . . . S10 may comprise positive digital values, and the samples at the sample points S12 . . . S20 may comprise negative digital values. For example, if the samples at the sample points S1 . . . S21 are represented by an 8-bit number, the amplitude may range from −127 to +127. Other embodiments of the invention may allow, for example, a range from −128 to +127, or from −127 to +128.
The interpolator block 310 may comprise suitable logic, circuitry, and/or code that may enable interpolating digital values between digital inputs. For example, the interpolator block 310 may interpolate values to generate 16 digital output values from 4 digital inputs values. This may occur, for example, by oversampling from 4 samples per CDMA chip to 16 samples per CDMA chip. Each input may be 8 bits, and each output may be 17 bits, for example. The 8-bit inputs and the interpolated 8-bit values may be shifted left 9 bits to generate the 17-bit outputs. The combiner block 322 may comprise suitable logic, circuitry, and/or code that may enable combining of a plurality of inputs. For example, the combiner block 322 may combine the digital inputs from the interpolator block 310 and the negative values from the scaling block 324, and a digital value Rounding_Factor. The digital value Rounding_Factor may have a value that may be dependent upon a design of the baseband processor 216. For example, an embodiment of the invention may assign a value of 256 to the digital value Rounding_Factor. The Rounding_Factor may be communicated to the combiner block 322 by, for example, the processor 156. Other embodiments of the invention may, for example, make the value of the Rounding_Factor constant. The combiner block 322 may receive 17-bit inputs from the interpolator block 310 and the scaling block 324, and output a 17-bit value.
The scaling block 324 may comprise suitable logic, circuitry, and/or code that may enable scaling a digital input by a value indicated by a scaling input. For example, the digital input communicated by the accumulator 336 may be scaled appropriately by the Scaling_Factor input. Since the Scaling_Factor input may indicate a value that may be a power of 2, an embodiment of the invention may implement the scaling block 324 to, for example, shift left the digital input signal by an appropriate number of bits. Accordingly, the Scaling_Factor may be a number that indicates a number of bits to shift left. The Scaling_Factor may be communicated to the scaling block 324 by, for example, the processor 156. The scaling block 324 may scale an 8-bit input by the Scaling_Factor, where the Scaling_Factor may indicate an exponent as large as 9. Accordingly, the scaling block 324 may output a 17-bit value.
The de-scaling block 326 may comprise suitable logic, circuitry, and/or code that may enable dividing an input by an appropriate value. Various embodiments of the invention may implement the de-scaling block 326, for example, to shift right by an appropriate number of bits. For example, the number of bits to shift right may be 9 bits. Accordingly, the 17-bit input from the combiner block 322 may be reduced to an 8-bit output of the de-scaling block 326. The output of the de-scaling block may be the digital signal Sig_Out. The digital signal Sig_Out may be, for example, processed further by the baseband processor and/or communicated to other circuitry in the RF receiver system 150 for further processing to audio, video, and/or text for application systems, such as, for example, email.
The accumulator block 332 may comprise suitable logic, circuitry, and/or code that may enable receiving and accumulating 8-bit values of the digital signal Sig_Out. The accumulator block 332 may support signed 8-bit values. Accordingly, the total accumulated by the accumulation block 332 may increase or decrease with each 8-bit value received from the de-scaling block 326. The accumulator block 332 may accumulate N 8-bit inputs and dump the final accumulated value after the Nth input. Accordingly, the accumulator block 332 may have a value of zero after a dump of the accumulated value. The output may be communicated to the de-scaling block 334 and the error detector 340. The output of the accumulator block may be, for example, 18 bits. In an embodiment of the invention, N may be equal to, for example, 1024.
The de-scaling block 334 may comprise suitable logic, circuitry, and/or code that may enable dividing an input by an appropriate value, such as, for example, 1024. Accordingly, if the division factor is a power of two, an embodiment of the invention may implement the de-scaling block 334, for example, to shift right by an appropriate number of bits. If the dividing factor is 1024, the de-scaling block 334 may shift the input data to the right by 10 bits. Accordingly, the output of the de-scaling block 334 may be 8 bits. The 8-bit output of the de-scaling block 334 may be an approximate average of the 1024 8-bit inputs accumulated by the accumulator block 332. The output of the de-scaling block 334 may be communicated to the accumulator block 336.
The accumulator block 336 may comprise suitable logic, circuitry, and/or code that may enable receiving of 8-bit inputs and accumulation of the 8-bit inputs without dumping. Accordingly, the accumulator block 336 may generate an output that may be a DC offset of the received signal, and the DC offset may be constantly updated. The output of the accumulator block 336 may be, for example, 8 to 11 bits wide. The error detector 340 may comprise suitable logic, circuitry, and/or code that may enable processing the output of the accumulator block 332 to generate an error signal Offset_Error. The algorithm used for generating the error signal Offset_Error may be implementation dependent. The error signal Offset_Error may be communicated to a processor, for example, the processor 156. The processor 156 may determine whether the received error signal Offset_Error may be, for example, above an error threshold level. The error threshold level may be pre-determined, or dynamically determined depending on various transmission characteristics, such as, for example, signal to noise ratio, strength of received signals, and/or bit error rate. The processor 156 may, for example, adjust reception parameters of the RF receiver system 150 based on the error signal Offset_Error. The adjustment of the reception parameters is discussed in more detail with respect to
In operation, the digital data Sig_In may be received by the interpolator block 310 from, for example, the CMF 214 8 bits at a time, where each 8-bit data may represent a sample of an analog signal. The interpolator block 310 may process the digital data Sig_In to interpolate 16 bytes of data for every 4 bytes of the digital data Sig_In. For example, this may occur due to 4× oversampling from 4 samples per CDMA chip to 16 samples per CDMA chip. The interpolator block 310 may also filter each 8-bit data to generate a 17-bit data using the 512 FIR filter gain. Accordingly, the output of the interpolator block 310 may be 17 bits wide, for example. The 17-bit output from the interpolator block 310 may be communicated to the combiner block 322. The combiner block 322 may combine, or add, the 17-bit data from the interpolator block 310 with the negative value of the 17-bit data from the scaling block 324 and with the Rounding_Factor of 256. The result may be a 17-bit value that may be communicated to the de-scaling block 326. The de-scaling block 326 may shift right the 17-bit input by 9 bits. The result may be an 8-bit data that may be rounded to the nearest decimal integer due to the addition of the Rounding_Factor value of 256 by the combiner block 322.
The 8-bit data from the de-scaling block 326 may be communicated for further processing for presentation to a user of the RF receiver system 150. The 8-bit data from the de-scaling block 326 may also be communicated to the accumulator block 332. The accumulator block 332 may accumulate, or add, for example, 1024 8-bit data and then communicate the final accumulated data to the de-scaling block 324 and the error detector block 340. Since the 8-bit data being accumulated may be signed, the accumulated data may keep track of the offset signal Bias because the non-Bias values may tend to cancel each other.
After communicating the accumulated data, the accumulator block 332 may clear the accumulated data to zero. Accordingly, each accumulated data may be dependent on the next 1024 8-bit data received after communicating the previous accumulated data. The output of the accumulator block 332 may be 18 bits wide. The error detector block 340 may generate an error signal that may be used by, for example, the baseband processor 216 and/or the processor 156, for controlling operation of the RF receive system 150.
The de-scaling block 334 may, for example, shift right the 18-bit data from the accumulator block 332 by 10 bits. The resulting 8-bit data may represent a truncated average of the 1024 8-bit data accumulated by the accumulator block 332. Accordingly, the 8-bit data from the de-scaling block 334 may be an average residual DC offset value for the received 1024 samples. The 8-bit data from the de-scaling block 334 may be communicated to the accumulator block 336. The accumulator block 336 may accumulate the 8-bit data. The accumulator block 336 may accumulate the 8-bit data from the de-scaling block 334, thereby effectively integrating the average DC offsets. Accordingly, the output of the accumulator block 336 may be used as a feedback signal to reduce the DC offset in the digital input signal Sig_In received by the interpolator block 310.
The output of the accumulator block 336 may comprise 8+(9−P_DC_GAIN) number of bits. The output of the accumulator block 336 may be communicated to the scaling block 324. The scaling block 324 may scale the output from the accumulator block 336 by a factor of 2̂P_DC_GAIN. This may be accomplished, for example, by shifting left the output from the accumulator block by P_DC_GAIN number of bits. Accordingly, the result may be a 17-bit wide data. The 17-bit wide data may be communicated to the combiner block 322.
The value of P_DC_GAIN may be a fixed value in some embodiments of the invention. In other embodiments of the invention, a processor, such as, for example, the processor 156, may communicate the P_DC_GAIN value to the accumulator 336 and to the scaling block 324. Additionally, an assumption was made that the offset signal was a DC signal for ease of explanation. However, the invention need not be limited to reduction of a DC offset signal. Non-DC offset signals may also reduced using various embodiments of the invention.
The selector block 402 may comprise suitable logic, circuitry, and/or code that may enable selection of operation of either the accumulator block 404 or 406. The selector block 402 may also enable the switch to communicate an output from either the accumulator 404 or the accumulator 406 to either the de-scaling block 410 or the de-scaling block 412. The selector block 402 may also, for example, indicate to the de-scaling blocks 410 and 412 to enable operation of each block, and also whether each block may generate an output. Commands may be communicated to the selector block 402 for a specific selection and operation of the accumulators 404 and 406, and for the de-scaling blocks 410 and 412, for example, by the processor 156 or other circuitry in the baseband processor 216. The accumulator block 404 may accumulate, for example, 1024 samples while the accumulator block 406 may accumulate, for example, 512 samples. The de-scaling block 410 may de-scale by, for example, a factor of 1024 while the de-scaling block 412 may de-scale by, for example, a factor of 512.
In operation, the error signal Offset_Error from the error detector 340 may be monitored by, for example, the processor 156. The processor 156 may determine that the error signal Offset_Error may indicate that a different number of samples may be used for error correction. Accordingly the processor 156 may communicate appropriate commands to the selector block 402. The selector block 402 may then select an appropriate accumulator to accumulate data and an appropriate de-scaling block to generate an averaged error data.
For example, if the error signal Offset_Error varies relatively fast, for example, at a rate of 100 Hz, the processor 156 may determine that a smaller number of samples may be accumulated in order to provide a faster feedback. Accordingly, the processor accumulator block 406 and the de-scaling block 412 may be selected. However, if the error signal Offset_Error is relatively small, the processor 156 may determine that a larger number of samples may be accumulated. Accordingly, the processor accumulator block 404 and the de-scaling block 410 may be selected.
While an embodiment of the invention may have been described with respect to
In step 506, the accumulator block 404 and/or 406 may accumulate the data from the de-scaling block 326. The accumulator block 404 and/or 406 may accumulate an appropriate number of data. For example, the accumulator block 404 may accumulate data for 256 WCDMA chips, at four 8-bit samples per chip, and the accumulator block 406 may accumulate data for 128 WCDMA chips, at four 8-bit samples per chip. Since the output from the de-scaling block 326 may comprise sixteen 8-bit data for each WCDMA chip, the accumulator blocks 404 and 406 may accumulate one-fourth of the data communicated by the de-scaling block 326. Since the data may tend to cancel each other, the accumulated data may be considered to be accumulation of common bias, or offset.
The next step may be step 508 and step 514. In step 508, the accumulator blocks 404 and 406 may communicate the total accumulated data after the appropriate number of 8-bit data have been accumulated to the switch block 408. The switch block 408 may communicate the accumulated data from the accumulator blocks 404 and 406 to the appropriate de-scaling block 410 and 412, respectively. The de-scaling blocks 410 and 412 may right shift the received accumulated data to generate a truncated average. The truncated average may represent, for example, an average offset value for the respective number of samples accumulated by the accumulator blocks 404 or 406.
For example, the de-scaling block 410 may right shift the 18-bit data generated by the accumulator block 404 by 10 bits to generate an 8-bit data. The de-scaling block 412 may right shift the 17-bit data generated by the accumulator block 406 by 9 bits to generate an 8-bit data. The 8-bit data generated by the de-scaling blocks may be considered as, for example, an average of the offset error data received for accumulation by the accumulator blocks 404 and 406. After outputting the accumulated data, the accumulator blocks 404 and 406 may set the accumulated value to zero to start accumulation over again.
In step 510, the average offset value from the de-scaling block 410 or 412 may be communicated to the accumulator block 414. The accumulator block 414 may not dump and clear the accumulated value as the accumulator blocks 404 and 406. Rather the accumulator block 414 may constantly accumulate the average offset values. Accordingly, the accumulator block 414 may integrate the error over time to provide a feedback signal for correction of the offset in the input signal Sig_In. In step 512, the accumulator block 414 may output data to the scaling block 324. The data from the accumulator block 414 may be further scaled by the scaling block 324 to generate, for example, a 17-bit feedback data to the combiner block 322. In step 514, the error detector 340 may receive data from either the accumulator block 404 or 406, via the switch block 408. The error detector block 340 may generate an error signal that may be communicated to, for example, the processor 156. The processor 156 may determine whether to use, for example, the accumulator block 404 or 406.
In step 606, under control of the processor 156, the selector block 402 may be enabled to select specific blocks for the number of samples to be accumulated for feedback. Some embodiments of the invention may select between two different numbers of samples to accumulate, while other embodiments of the invention may allow dynamic configuration of the various blocks so as to provide an optimal number of samples to be accumulated. For example, the accumulator block 404 and/or 406 may be configured by, for example, the selector block 402 and/or the processor 156 to accumulate a specific number of samples. Similarly, the de-scaling blocks 410 and/or 412 may be configured by, for example, the selector block 402 and/or the processor 156 to de-scale by an appropriate value.
Aspects of a method and system for multimode DC offset compensation may comprise, for example, the processor 156 that selects a number of signal samples to be used for feedback signal generation. The accumulator block 404 and/or 406 may accumulate the selected number of signal samples, and the de-scaling block 410 and/or 412 may determine an average value of the accumulated signal samples. The accumulator block 414 may accumulate the average value from the accumulator block 404 and/or 406 to generate a running feedback error value. The signal samples may be, for example, samples of WCDMA signals.
The number of signal samples accumulated by the accumulator block 404 and/or 406 may be based on a signal offset error, or residual DC. For example, an embodiment of the invention may comprise two different numbers of signal samples to accumulate. A larger number of signal samples may be accumulated when the signal offset error is less than or equal to a threshold. A smaller number of signal samples may be accumulated when the signal offset error is greater than the threshold. In an embodiment of the invention, the accumulator 404 and/or 406 may accumulate a number of signal samples that may be equal to 2N. Accordingly, the de-scaling block 410 and/or 412 may right shift right the total value from the accumulator block 404 and/or 406 by N bits to determine the average of the number of signal samples.
Another embodiment of the invention may provide a machine-readable storage, having stored thereon, a computer program having at least one code section executable by a machine, thereby causing the machine to perform the steps as described above for multimode DC offset compensation.
Accordingly, the present invention may be realized in hardware, software, or a combination of hardware and software. The present invention may be realized in a centralized fashion in at least one computer system, or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein is suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein.
The present invention may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form.
While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.
