Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
  • H04W88/02—Terminal devices
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7073—Synchronisation aspects
  • H04B1/7085—Synchronisation aspects using a code tracking loop, e.g. a delay-locked loop
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/709—Correlator structure
  • H04B1/7093—Matched filter type
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/70703—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation using multiple or variable rates
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0024—Carrier regulation at the receiver end
  • H04L2027/0026—Correction of carrier offset
  • H04L2027/003—Correction of carrier offset at baseband only
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0044—Control loops for carrier regulation
  • H04L2027/0053—Closed loops
  • H04L2027/0057—Closed loops quadrature phase
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0044—Control loops for carrier regulation
  • H04L2027/0063—Elements of loops
  • H04L2027/0067—Phase error detectors
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W84/00—Network topologies
  • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
  • H04W84/10—Small scale networks; Flat hierarchical networks
  • H04W84/12—WLAN [Wireless Local Area Networks]

Definitions

• the invention generally relates to receivers in spread spectrum communications systems such as WLAN (Wireless Local Area Network) systems, and in particular to the phase error correction of received signals.
• WLAN Wireless Local Area Network
• a Wireless Local Area Network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN.
• WLAN devices Using radio frequency or infrared technology, WLAN devices transmit and receive data over the air, minimizing the need of wired connections.
• WLAN systems combine interconnectivity with user mobility.
• WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communications systems.
• the spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security.
• Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
• the standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard.
• the standard was extended to the 802.1 lb standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
• CCK Computer Code Keying
• a frequency error may result from a frequency offset of the radio frequency oscillators at the transmitter and the receiver.
• the received signal can be described as
• ⁇ e is the oscillator frequency difference between receiver and transmitter
• ⁇ p e is the difference in oscillator phase between the receiver and the transmitter
• FIG. 1 an error correction arrangement is schematically shown consisting of a frequency error correction unit 100 and a phase error correction unit 110.
• the frequency error correction unit 100 is used to compensate for the frequency difference, and the phase error correction unit 110 will then compensate for the residual phase error. This will now be described in more detail.
• This signal can be considered a signal with time dependent phase
• the phase error correction unit 110 has now the task to remove the remaining phase error such that the received signal is as close as possible to the transmitted signal, to minimize the probability of demodulation errors.
• An example of how the phase error correction unit 110 may operate is depicted in FIG. 2.
• the phase error correction unit shown in FIG. 2 includes an error correction module 200 that performs the following operation:
• ⁇ e (t) is the current estimate of the phase error.
• the error correction module 200 is controlled by means of an error signal received from the measurement module 210.
• the measurement module 210 measures the phase error of the output signal of the correction module 200 and tries to generate the error signal so as to minimize the phase difference ⁇ e (t) - ⁇ e (t) .
• FIG. 3 the state diagram (or constellation diagram) for a BPSK (Binary Phase Shift Keying) system is shown.
• the diagram has been rotated for explanatory reasons.
• the hollow symbols represent the "ideal" signal points whereas the cross mark represents the signal point of the received signal which is phase offset.
• the current phase difference between the ideal and the received constellation point is given by A ⁇ e (t). It is the task of the error measurement module 210 to determine this phase difference to generate the error signal as precise as possible.
• phase error can then be calculated according to
• FIG. 3 a range around the cross mark indicating the region where the measured constellation points will be randomly distributed with a certain probability, due to the additive noise.
• the region is shown to have a certain radius, and this radius will depend on the current channel conditions in the communication system.
• measuring the phase difference as shown above has the disadvantage that due to the additive noise, there will be a random measurement error.
• the measurement error may be up to 100% if the distance between the ideal signal point and the received signal point, i.e. the cross mark, in the constellation diagram does not exceed the radius of the noise region.
• the measurement module 210 cannot accurately generate an error signal if the phase difference cannot be measured precisely.
• the phase error correction in conventional receivers often operate insufficiently, leading to reduced reliability of the overall system, and reducing the settling time of the receiver.
• An improved receiver and operation method having increased control quality when performing a phase error correction, in particular in case of noisy data samples.
• a receiver in a spread spectrum communications system comprises a phase error correction unit connected to receive an input signal having a phase error and adapted to generate an output signal having a corrected phase error.
• the receiver further comprises a despreader that is adapted to despread a data signal.
• the despreader is connected to the phase error correction unit to provide the despread data signal to the phase error correction unit.
• the phase error correction unit is arranged for correcting the phase error dependent on the despreader data signal.
• said despreader is a CCK (Complementary Code Keying) matched filter.
• said CCK matched filter is implemented using a Walsh tree.
• said CCK matched filter comprises a CCK dccorrelator to provide a phase output of said CCK decorrelator to said phase error correction unit.
• said despreader comprises a Barker matched filter; a CCK (Complementary Code Keying) matched filter; and a selection unit connected to said Barker matched filter and said CCK matched filter, wherein said selection unit is adapted to select one of said Barker matched filter and said CCK matched filter to provide an output of the selected filter to said phase error correction unit.
• said selection unit is connected to receive a selection signal indicating which one of said Barker matched filter and said CCK matched filter is to be selected.
• the receiver further comprises a controller adapted to generate said selection signal dependent on a data transfer mode relating to a signal currently received by said receiver.
• said controller is adapted to keep track of the currently used data transfer mode, and is capable of switching the selection signal in case the data transfer mode changes.
• the receiver further comprises a controller adapted to generate said selection signal dependent on the data rate of said data signal to be despread.
• said controller is adapted to generate said selection signal to instruct said selection unit to select said Barker matched filter in case of a 1 Mbps or 2 Mbps data rate.
• said controller is adapted to generate said selection signal to instruct said selection unit to select said CCK matched filter in case of a 5.5 Mbps or 11 Mbps data rate.
• said controller is adapted to keep track of the currently used data rate, and is capable of switching the selection signal in case the data rate changes.
• the receiver is a WLAN receiver.
• an integrated circuit chip for processing spread spectrum data signals.
• the integrated circuit chip comprises a receiver according to any of the above embodiments.
• a method of operating a WLAN receiver comprises correcting a phase error in an input signal, and despreading a data signal.
• the phase error correction is performed dependent on the despread data signal.
• a method of operating a receiver in a spread spectrum communications system comprises correcting a phase error in an input signal, and despreading a data signal.
• the phase error correction is performed dependent on the despread data signal.
• correcting the phase error comprises generating an error signal based on said despread data signal, the error signal being indicative of the current phase error; and generating a corrected output signal based on said error signal.
• generating the error signal comprises integrating said despread data signal over time.
• said data signal is a signal generated by correcting the phase error of said input signal.
• despreading the data signal comprises operating a Barker matched filter.
• despreading the data signal comprises operating a CCK (Complementary Code Keying) matched filter.
• said CCK matched filter is implemented using a Walsh tree.
• said CCK matched filter comprises a CCK decorrelator to provide a phase output of said CCK decorrelator, the phase error correction being performed dependent on said phase output.
• despreading the data signal comprises operating a Barker matched filter; operating a CCK (Complementary Code Keying) matched filter; and selecting one of said Barker matched filter and said CCK matched filter, wherein the phase error correction is performed dependent on an output of the selected filter.
• selecting one of the filters comprising receiving a selection signal from a controller, the selection signal indicating which one of said Barker matched filter and said CCK matched filter is to be selected.
• the method further comprises operating said controller to generate said selection signal dependent on a data transfer mode relating to a signal currently received by said WLAN receiver.
• operating said controller comprises keeping track of the currently used data transfer mode; and switching the selection signal in case the data transfer mode changes.
• the method further comprises operating said controller to generate said selection signal dependent on the data rate of said data signal to be despread.
• operating said controller comprises generating said selection signal to select said Barker matched filter in case of a 1 Mbps or 2 Mbps data rate.
• operating said controller comprises generating said selection signal to select said CCK matched filter in case of a 5.5 Mbps or 11 Mbps data rate.
• operating said controller comprises keeping track of the currently used data rate; and switching the selection signal in case the data rate changes.
• the method further comprises correcting a frequency error in a received signal; wherein said input signal is a signal having a corrected frequency error.
• the method is adapted for operating an IEEE 802.1 lb compliant WLAN receiver.
• FIG. 1 is a schematic block diagram illustrating components in a conventional receiver, used for correcting a frequency and phase error
• FIG. 2 illustrates the main components of a conventional phase error correction unit
• FIG. 3 is a phase constellation diagram showing ideal and received signal points
• FIG. 4 is a block diagram of a phase error correction arrangement according to a first embodiment
• FIG. 5 is a block diagram of a phase error correction arrangement according to a second embodiment
• FIG. 6 is a block diagram of a phase error correction arrangement according to a third embodiment
• FIG. 7 is a flowchart illustrating the process of operating the arrangement according to one of the embodiments.
• FIG. 8 is a flowchart illustrating the filter selection process performed in the arrangement according to the third embodiment.
• a phase error correction unit 200, 400 including an error correction module 200 and a measurement module 400.
• the measurement module 400 issues an error signal to the correction module 200 where the error signal is used to correct the phase error.
• the construction of the phase error correction unit according to the first embodiment is similar to that of FIG. 2 but it is to be noted that the measurement module 400 may be different. This is because the measurement module 400 is connected to receive an output of the Barker matched filter 410, so that the measurement module 400 needs to be adapted to generate the error signal on the basis of the despread data signal that is output from the Barker matched filter 410.
• the measurement module 400 will then generate the error signal according to:
• the output of the Barker matched filter 410 will depend at any time not only on the real and imaginary parts of the currently received data sample but also on the respective values of up to ten previous data samples.
• the influence of the additive white noise will be averaged so that the measurement module 400 may generate the error signal with greater precision. This allows for better controlling the quality of the phase error correction compared with conventional, sample-based schemes.
• substantially no additional circuitry is needed for this purpose since a Barker matched filter 410 is usually already present in the receiver.
• CCK is a modulation technique implemented in the IEEE 802.11b standard for allowing data rates of 5.5 and 11 Mbps.
• the CCK modulation can generally be described as a modification of MOK (M-ary Orthogonal Keying) modulation using codes of a complex symbol structure.
• MOK M-ary Orthogonal Keying
• the CCK technology allows for multi-channel operation and employs the same chip rate and spectrum shape as the 802.11 Barker code spread functions.
• CCK can be considered as a form of M-ary codeword modulation where one of M unique signal codewords is chosen for transmission.
• the first embodiment uses a Barker matched filter 410 and is therefore best suited for correcting the phase errors in 802.11 compliant WLAN systems, or in 1 or 2 Mbps modes of 802.11b compliant WLAN receivers
• the technique of the second embodiment shown in FIG. 5 may be best used in 5.5 and 11 Mbps modes of 802.1 lb compliant receivers.
• the CCK matched filter 510 may be implemented using a Walsh tree.
• the Walsh tree may consist of a CCK correlator and a CCK comparator, and may use a soft decision maximum likelihood decoding. The comparator is used to find the local maximum.
• the despread signal which is output by the CCK matched filter 510 and provided to the measurement module 500, is the phase output of a CCK decorrelator.
• the arrangement of the second embodiment may achieve the same advantages as the first embodiment. That is, the Gaussian noise will be suppressed because by using a despread signal, the signal is raised from the noise floor. By avoiding a sample-based phase error correction, the measurement of the phase difference as well as the correction of the phase error may be performed more precisely. This reduces demodulation errors and leads to a better overall performance of the WLAN receiver.
• a multiplexer 610 is provided acting as a selection unit for selecting one of the Barker matched filter 410 and the CCK matched filter 510 dependent on the current transfer mode, i.e. on the data rate.
• the multiplexer 610 receives a selection signal from a controller 620 which keeps track of the modes so that the arrangement can switch from one filter to the other in case the data rate changes, e.g. from 2 to 5.5 Mbps.
• the measurement modules 400, 500, 600 may be adapted to integrate the phase difference to achieve an estimate of the current phase error:
• ⁇ e (t) a - ⁇ e (t - l) + b - A ⁇ e (t)
• Integration may be done using a low-pass filter, and the term integration as used herein may include any smoothing algorithm that averages over a given period of time, or calculates a weighted sum of a previously smoothed value and a current value. These techniques may even more suppress an influencing additive white noise.
• step 700 the received data signal is despread using either the Barker matched filter 410 or the CCK matched filter 510. It is to be noted that any other kind of despreading (or demodulating) technique may likewise be used.
• the signal may best be despread using the despreader that is already present in the receiver.
• the despread signal is then integrated in step 710, and an error signal is generated in step 720. It is to be noted that instead of integrating the despread signal and generating the error signal based thereon, an error signal may be generated without a preceding integration step. Moreover, in another embodiment, the error signal could be integrated.
• step 730 the phase error correction is performed in step 730, and the process returns to step 700 where the output of the correction module 200 is fed to the despreader 410, 510.
• step 800 the controller 620 determines the data rate. It is then checked in step 810 whether data transmission is performed in the 1 or 2 Mbps mode, and if so, the multiplexer 610 is instructed to switch to the Barker matched filter 410 (step 820). If it is determined in step 830 that data transmission is performed in the 5.5 or 11 Mbps mode, the multiplexer 610 will switched to the CCK matched filter 510 in step 840.
• phase error correction technique is provided that is applicable in WLAN systems and other spread spectrum communications systems and that may improve demodulation reliability and settling time by suppressing the influence of additive noise in the received data samples.
• the technique may be used in particular in 802.1 lb compliant systems. No restriction with respect to encoding techniques apply, so that the embodiments may achieve the mentioned advantages in BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) and other schemes including DQPSK (Differential QPSK).
• the present invention may advantageously be applied in computer components and may therefore be implemented in industrial processes and products.

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• 2003
  • 2003-02-28 JP JP2004510204A patent/JP2005528855A/ja active Pending
  • 2003-02-28 AU AU2003213622A patent/AU2003213622A1/en not_active Abandoned
  • 2003-02-28 CN CNB038122790A patent/CN100539559C/zh not_active Expired - Fee Related
  • 2003-02-28 EP EP03711305A patent/EP1510055A1/de not_active Withdrawn
  • 2003-02-28 WO PCT/US2003/006150 patent/WO2003103248A1/en not_active Ceased

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