Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0069—Cell search, i.e. determining cell identity [cell-ID]
  • H04J11/0073—Acquisition of primary synchronisation channel, e.g. detection of cell-ID within cell-ID group
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0069—Cell search, i.e. determining cell identity [cell-ID]
  • H04J11/0076—Acquisition of secondary synchronisation channel, e.g. detection of cell-ID group
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/06—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0212—Channel estimation of impulse response

Definitions

• the present invention relates to a method for estimating and removing DC offset in a direct conversion receiver. Furthermore, the invention also relates to a receiver device, a computer program, and a computer program product thereof.
• Direct Current (DC) offset is a major problem in direct conversion receivers.
• a part of the Local Oscillator (LO) signal is down converted to a baseband signal which leads to unwanted DC offsets (see e.g. Rainer Gebberger, R. Krueger, B. Adler, J. Kissing, L. Maurer, G. Hueber and A. Springer, " LTE-Downlink Performance in the Presence of RF-Impairments", Proceedings of the 10th European Conference on Wireless Technology, pp. 189-192, October 2007).
• the DC offset can affect both cell search and data demodulation performance.
• LTE Long Term Evolution
• the cell search in Long Term Evolution (LTE) systems requires time domain correlation and unwanted DC offset can therefore destroy the correlation property of cell search synchronization signal.
• the DC sub-carrier is not used for data transmission in LTE systems, the Channel State Information (CSI) estimation around the DC component will be affected by the DC offset that directly impacts the performance of LTE system. It is thus very important to estimate and compensate the DC offset before the data demodulation and decoding at the receiver side.
• CSI Channel State Information
• the DC offset compensation methods mainly involves two prior art approaches, i.e.:
• Notch filter or high pass filter approach the notch filter or high pass filter is a digital filter circuitry that provides a notch at the frequency corresponding to the DC offset frequency and thus can remove the DC offset component from the received signal;
• Time domain received samples averaging approach - this method involves averaging received samples to estimate the DC offset.
• the notch filter or the high-pass filter approach requires a long transient time and it will distort the noise power spectrum density which adds complexity to whiten the distorted noise.
• the DC offset unit may need to support different bandwidth configurations which make the digital filter design more complex.
• An object of the present invention is to provide a solution which mitigates or solves the drawbacks and problems of prior art solutions.
• Another object of the invention is to provide an improved solution to the problem of estimating DC offset and mitigating the same.
• the above mentioned objects are achieved by a method in a direct conversion receiver for estimating and removing DC offset, said direct conversion receiver being arranged to receive radio communication signals in a wireless communication system, the method comprising the steps of:
• time domain signals comprise at least one first synchronisation signal
• the present method can also be executed in processing means of e.g. a computer.
• the method may be comprised in a computer program product.
• the above mentioned objects are also achieved with a direct conversion receiver device arranged to receive radio communication signals in a wireless communication system; the direct conversion receiver device further being arranged to:
• time domain signals comprise at least one first synchronisation signal
• the direct conversion receiver device may be modified so as to correspond to all different embodiments of the present method.
• the present invention provides an improved DC offset estimation method for direct conversion receivers which result in improved performance at the receiver. This is achieved by using the first synchronisation signal for estimation of a residual DC offset which in turn is used for computing a refined DC offset estimation having higher accuracy than prior art solutions. Further, according to preferred embodiments the present method jointly estimates the time domain CSI of the first synchronisation signal and the residual DC offset at the same time. Thereby, the accuracy of CSI estimation of the first synchronisation signal can be improved. The cell search procedure accuracy and Down Link (DL) data performance is also improved by the invention. Thus, complicated notch filter or high-pass filter approaches which need to support different bandwidth configurations or different modes in a multimode modem is also avoided with the present method.
• DL Down Link
• Fig. 1 illustrates a direct conversion receiver structure
• Fig. 2 shows the DL SCH structure in LTE FDD mode
• Fig. 3 illustrates the cell search procedure in LTE systems at the receiver side
• Fig. 4 illustrates an embodiment of a receiver structure according to the present invention
• Fig. 5 illustrates a residual DC offset estimation structure
• Fig. 6 illustrates the flow of DC offset estimation and compensation method according to an embodiment of the present invention.
• the present invention relates to a method in a direct conversion receiver for estimating and removing DC offset.
• the system performance can be substantially improved by providing accurate DC offset estimations.
• the estimation and removal of DC offset can also be combined with cell search procedures, such as determining frame head, Cell ID, Cyclic Prefix (CP) type, etc.
• cell search procedures such as determining frame head, Cell ID, Cyclic Prefix (CP) type, etc.
• each radio frame duration is 10 ms long and consists of 10 subframes (numbered from 0 to 9), and each subframe can be divided into two slots.
• each radio frame includes 20 slots numbered from 0 tol 9.
• each slot includes several Orthogonal Frequency-Division Multiplexing (OFDM) symbols depending on the CP type and subcarrier configuration (e.g. one slot consists of 7 OFDM symbols per slot in normal CP type with subcarrier spacing of 15 kHz).
• OFDM Orthogonal Frequency-Division Multiplexing
• the DL Synchronization Channel (SCH) in LTE is transmitted every 5 subframes in the centre/middle 6 Resource Blocks (RBs) regardless of the bandwidth configuration.
• the SCH includes two parts, namely: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS).
• PSS Primary Synchronization Signal
• SSS Secondary Synchronization Signal
• the SCH is transmitted at different position in the radio resources depending on the frame structure, i.e. Frequency Division Duplex (FDD) or Time Division Duplex (TDD) mode.
• FDD Frequency Division Duplex
• TDD Time Division Duplex
• the PSS In FDD mode the PSS is mapped on the last OFDM symbol in slot 0 and 10, and the SSS is mapped on the previous OFDM symbol adjacent to the OFDM symbol in which the PSS is mapped.
• a typical FDD DL SCH RB mapping is shown in figure 2.
• the PSS is mapped to the third OFDM symbol in subframes 1 and 6 and the SSS is mapped to the last OFDM symbol in slot 1 and 1 1.
• the PSS sequence p ⁇ n is generated from a frequency-domain Zadoff- Chu sequence according to:
• the root u is decided by the cell sector ID N jQ which can have three possible values: 0, 1 or 2.
• the mapping relationship between the root u and the cell sector ID NJ Q is defined in as follows: iV (2) u
• the cell search procedure relies on the SCH.
• PSS detection is applied on down sampled time domain samples (with sampling rate of 1.92 Mbps) to get the slot start position through a slicing correlation with possible local PSS sequences.
• a successive SSS detection is applied in the frequency domain to detect the frame head, Cell ID and CP type.
• the SSS detection is based on the correlation between received SSS frequency domain samples and all possible local SSS sequences at the receiver.
• the CSI estimation of the PSS can be used to conjugate multiply with the extracted SSS sequence before implementing a FFT to transform the SSS sequence into the frequency domain.
• Figure 3 illustrates the above described cell search structure.
• the present method in a direct conversion receiver involves receiving and down sampling time domain signals which are received from a transmitter in the wireless communication system.
• the time domain signals should comprise at least one first synchronisation signal for synchronisation of received data.
• a residual DC offset Ad based on the at least one first synchronisation signal is estimated and a refined DC offset estimation d is computed based on the estimated residual DC offset Ad .
• the refined DC offset estimation d is applied on the time domain signals so as to remove DC offset from the time domain signals. Thereby, improved performance is provided by the present method.
• the method also comprises the steps of: estimating a raw DC offset d based on the down sampled time domain signals; subtracting the estimated raw DC offset d from the down sampled time domain signals; and detecting the first synchronisation signal based on the subtracted down sampled time domain signals.
• the first synchronisation signal is detected based on the down sampled time domain signals which have been subtracted with the raw DC offset d .
• the first synchronisation signal is the PSS in LTE systems, which also means that a second synchronisation signal is the SSS for mentioned types of systems.
• the PSS and SSS are transmitted in the DL in the SCH.
• the present method further comprises the steps of: jointly estimating the residual DC offset Ad and CSI in the subtracted down sampled time domain signals based on the detected first synchronisation signal.
• time domain samples can be modelled as,
• xfnj is the transmitted time domain signal
• L ( L ⁇ I) is the channel time delay
• d is the DC offset to be estimated
• w[n] is the thermal noise.
• M is the total samples used for estimation of the raw DC offset d for the specific averaging time.
• the raw estimation is thus an initial DC offset estimation. While when the received signal suffers from frequency selective fading which is common in wireless communication environment or when the SNR is low, the raw estimation may have a fairly large estimation error that results in a residual DC offset remaining in the received signals after removing the raw DC offset estimation from the received signals.
• the PSS (the first synchronisation signal) based residual DC offset estimation is given as follows: after subtracting the raw estimation d from the received samples in Eq. (4), the time domain PSS signal can be modelled as,
• n k +L - ⁇ ,k + L,k + L + l,...,k + L + m - 2
• PSS[ri] is the time domain PSS signal
• h[l, n] is the l-t channel tap at sampling time n and L is the channel time delay
• d is the DC offset
• wfnjis the thermal noise
• k is the detected PSS start position
• H[L, Ad] (h[0], h[ ⁇ ],..., h[L - l],Ad) r - (H[L], Adf
• H[L,Ad] is a column vector containing the CSI vector H[L] and the residual DC offset Ad.
• W[m] is Additive White Gaussian Noise (AWGN)
• AWGN Additive White Gaussian Noise
• different matrix C[m] can be pre-calculated and stored.
• the corresponding matrix C[m] is chosen
• the joint PSS CSI and residual DC offset estimation method for H[L, Ad] described in Eq. (7) is just a simple matrix-multiplication between the chosen matrix C[m] and the received time domain PSS samples Y[m] from which the raw estimation d has been removed according to Eq. (5).
• residual DC offset Ad is taken into consideration for the CSI estimation H[L] the joint estimation H[L, Ad] renders a better estimation for the PSS CSI.
• PSS132 PSS[3 ⁇ ] . . PSS[24]
• the coefficients C[m] are in the form of 10x 64 matrix which can be calculated and stored for all cell sector IDs .
• Ad the estimated residual DC offset
• the estimation error of raw DC offset d obtained from averaging the received time domain samples results in a fairly large residual DC offset and thus degrade the performance. Therefore the refined DC offset d is a better estimation than the raw DC offset d .
• the PSS can be used to estimate the DC offset.
• the present method can be implemented in the following steps:
• step 5 Add the raw estimation of the DC offset d in step 1 and the residual DC offset estimation Ad in step 3 together to obtain a refined DC offset estimation for data demodulation of the received signals;
• the residual DC offset Ad and PSS CSI can be jointly detected.
• the refined DC offset estimation will be passed through an alpha filter before being used to compensate for the DC offset before demodulation and decoding. The alpha filter will further improve the DC offset estimation and therefore also the performance.
• the residual DC offset and CSI joint detection unit is described in the following text with reference to figure 5.
• the three possible filter matrices CfmJ are pre-computed and stored in a memory according to a pre-defined filter length m. After the PSS position and the cell sector
• the estimated PSS CSI can be used as the initial CSI of SSS to remove the channel affect before SSS detection.
• any method according to the present invention may also be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method.
• the computer program is included in a computer readable medium of a computer program product.
• the computer readable medium may comprises of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.
• the present invention also relates to a direct conversion receiver device corresponding to all embodiments of the present method.
• the receiver device comprises all necessary means and is arranged to perform the method according to the present invention which means that the device may e.g. include: processing means, signal input means, signal output means, sampling means, memory means, communication means, etc.
• the present invention is according to an embodiment used in a 3 GPP wireless communication system such as LTE or LTE Advanced. Hence, in this case the receiver device is a part of, or comprised in a user equipment (UE).
• UE user equipment

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• 2013
  • 2013-01-03 WO PCT/EP2013/050069 patent/WO2014106540A1/en active Application Filing
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