Classifications

• • 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
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03012—Arrangements for removing intersymbol interference operating in the time domain
• • 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
• • 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
  • H04B1/7097—Interference-related aspects
  • H04B1/711—Interference-related aspects the interference being multi-path interference
  • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
• • 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
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L2025/03433—Arrangements for removing intersymbol interference characterised by equaliser structure
  • H04L2025/03535—Variable structures
• • 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
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L2025/03592—Adaptation methods
  • H04L2025/03726—Switching between algorithms

Definitions

• the invention relates in general to interference suppression in digital radio receivers.
• the invention relates to co-channel and adjacent channel interference suppression in digital radio receivers.
• a communication system can be seen as a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system.
• the communication may comprise, for example, communication of voice, data, multimedia and so on.
• the communication system may be circuit switched or packet switched.
• the communication system may be configured to provide wireless communication.
• cellular communication system refers to a system, where the coverage is provided by a plurality of cells.
• Communication devices communicate via a cellular communication system using one or more cells at a time.
• a communications device moving within the area of a cellular communication system typically changes cells depending on the quality of signals relating to the cells.
• Frequency re-use refers to the use of a certain frequency band in a nearby cell. Typically the aim is not to use the same frequency band in neighboring cells. In general, at least two cells using other frequency bands are between two cells using a same frequency band. Due to, for example, limited number of available frequencies it may be necessary to use a given frequency band in cell which are near each other, even neighbors to each other. Use of the same frequency band in a nearby cell (or, in general, by any radio transmitter) typically causes co-channel interference to users using this frequency band in a given cell. Use of adjacent frequency bands in nearby cells causes adjacent channel interference.
• the performance limiting factor is interference, rather than noise. Therefore, the capacity of the wireless communciation systems can be increased by the introduction of receivers with improved performance in interference limited scenarios.
• One way of obtaining co-channel and adjacent channel interference rejection in wireless communication systems is to use an antenna array taking advantage of spatial diversity. This technique is, however, generally not feasible in portable communications devices due to cost, complexity and size constraints.
• SAIC Single Antenna Interference Cancellation
• the schematic structure of a conventional receiver is shown in Figure 1.
• the received signal Rx(t) 11 is first filtered with a band-pass filter (Rx-Filter 21), its output r(t) 12 is synchronized and de-rotated in a synchronization block (Sync 22).
• the synchronized and de-rotated signal x(t) 13 is then used for channel and interference estimation in a channel estimator (Ch-Est 23).
• the channel estimator provides at least a channel estimate h 14.
• At least the synchronized and de-rotated signal x(t) 13 and the channel estimate h 14 are provided to the equalizer (Equalizer 24).
• the equalized signal z(t) 15 is input to a decoder ( Decoder 25) for decoding, and the decoder 25 gives the transmitted bit s'(t) 16.
• the equalizer may be replaced with another type of detector.
• WOO 193439 uses in-phase and quadrature components sampling, so that for each symbol there is one in-phase component sample and one quadrature component sample.
• the whitening operation in WOO 193439 is performed on the in-phase and quadrature components of the de- rotated signal x(t).
• a further problem with the whitening discussed in WOOl 93439 is that if the signal noise is white, i.e., we are operating in a sensitivity limited scenario, the whitening using a FIR filter may colour the already white noise, leading to degraded performance compared with a conventional receiver.
• the degraded performance is typically due to inaccuracies in the estimation of the FIR filter coefficients.
• the present invention aims to address at least some of the problems discussed above.
• a method for suppressing interference comprising providing a succession of samples relating to a succession of symbols, there being provided at least two samples per symbol, determining filter coefficients for a first whitening filter relating to suppression of intersymbol interference, whitening the succession of samples using the first whitening filter and the filter coefficients, and whitening the succession of samples using a second whitening filter, said second whitening relating to suppression of intrasymbol interference based on correlation properties of samples within a symbol.
• a computer readable medium containing executable computer program instructions which, when executed by a data processing system, cause said data processing system to perform a method as defined by the first aspect of the invention.
• a device for suppressing interference comprising means for receiving samples relating to symbols, there being provided at least two samples per symbol, and whitening means for whitening the received samples, said whitening means having a first whitening filter relating to suppression of intersymbol interference, means for determining filter coefficients for the first whitening filter, and a second whitening filter relating to suppression of intrasymbol interference based on correlation properties of samples within a symbol.
• a communications device comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the third aspect of the invention.
• a network element for a communication system comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the third aspect of the invention.
• a method for suppressing interference comprising providing a succession of samples relating to a succession of symbols, there being provided at least one sample per symbol, determining filter coefficients for a whitening filter based on a model jointly estimating filter coefficients and channel estimates, the whitening filter relating to suppression of intersymbol interference, whitening the succession of samples using the whitening filter and the determined filter coefficients, thereby providing a succession of whitened samples, and determining channel estimates based on the succession of whitened samples.
• a computer readable medium containing executable computer program instructions which, when executed by a data processing system, cause said data processing system to perform a method in accordance with the fourth aspect of the invention.
• a device for suppressing interference comprising means for receiving samples relating to symbols, there being provided at least one sample per symbol, a whitening filter relating to suppression of intersymbol interference, a joint estimator for determining filter coefficients and channel estimates, the joint estimator configured to input the filter coefficients to the whitening filter, and a channel estimator for determining channel estimates corresponding to the whitened samples output from the whitening filter.
• a communications device comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the sixth aspect of the invention.
• a network element for a communication system comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the sixth aspect of the invention.
• a method for suppressing interference comprising providing a succession of samples relating to a succession of symbols, there being provided at least two samples per symbol, determining filter coefficients for a whitening filter relating to suppression of intersymbol interference, and whitening the succession of samples using the whitening filter and the filter coefficients, wherein determining said filter coefficients for the whitening filter and whitening the succession of samples are carried out on the succession of samples in a fractionally spaced manner.
• a method for suppressing interference comprising providing a succession of samples relating to a succession of symbols, there being provided at least two samples per symbol, dividing the succession of samples into a set of symbol-spaced successions of samples, determining filter coefficients for a whitening filter relating to suppression of intersymbol interference, and whitening the succession of samples using the whitening filter and the determined filter coefficients, wherein determining said filter coefficients for the whitening filter and whitening the succession of samples are carried out in parallel on the set of symbol-spaced successions of samples.
• a computer readable medium containing executable computer program instructions which, when executed by a data processing system, cause said data processing system to perform a method in accordance with the seventh aspect or the eighth aspect of the invention.
• a device for suppressing interference comprising means for receiving samples relating to symbols, there being provided at least two samples per symbol, a whitening filter relating to suppression of intersymbol interference, and means for determining filter coefficients for the whitening filter, wherein said means for determining filter coefficients and said whitening filter are configured to process the succession of samples in a fractionally spaced manner.
• a device for suppressing interference comprising means for receiving samples relating to symbols, there being provided at least two samples per symbol, a whitening filter relating to suppression of intersymbol interference, and means for determining filter coefficients for the whitening filter, wherein the device is configured to divide the succession of samples into a set of symbol-spaced successions of samples, and the means for determining filter coefficients and the whitening filter are configured to operate using the set of symbol- spaced successions of samples.
• a communications device comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the tenth or the eleventh aspect of the invention.
• a network element for a communication system comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the tenth or the eleventh aspect of the invention.
• a method for suppressing interference comprising providing a succession of samples relating to a succession of symbols, there being provided at least one sample per symbol, determining filter coefficients for a whitening filter relating to suppression of intersymbol interference, and determining whether to whiten the succession of samples using the whitening filter and the determined filter coefficients based on the determined filter coefficients.
• a computer readable medium containing executable computer program instructions which, when executed by a data processing system, cause said data processing system to perform a method in accordance with the twelfth aspect of the invention.
• a device for suppressing interference comprising means for receiving samples relating to symbols, there being provided at least one sample per symbol, a whitening filter relating to suppression of intersymbol interference, means for determining filter coefficients for the whitening filter, and switching means for switching the whitening filter into use based on said filter coefficients.
• a communications device comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the fourteenth aspect of the invention.
• a network element for a communication system comprising means for receiving symbols over a radio interface, means for providing samples relating to the received symbols, and a device in accordance with the fourteenth aspect of the invention.
• Figure 1 shows, as an example, a schematic block diagram of a conventional receiver
• Figure 2a shows, as an example, a schematic block diagram of a receiver in accordance with an embodiment of the invention
• Figure 2b shows, as an example, schematically a possible implementation of a channel estimation and signal whitening block in accordance with an embodiment of the invention in some detail;
• Figure 2c shows, as an example, schematically a second possible implementation of a channel estimation and signal whitening block in accordance with an embodiment of the invention
• Figure 3a shows, as an example, schematically a channel estimation and signal whitening block having a joint estimator for filter coefficients and channel response;
• Figure 3b shows, as an example, schematically a channel estimation and signal whitening block employing a conventional channel estimator
• Figure 3c shows, as an example, schematically a channel estimation and signal whitening block employing a joint estimator for filter coefficients and a separate channel estimator for channel response;
• Figure 4a shows, as an example, schematically a channel estimation and signal whitening block employing parallel symbol-spaced sample processing
• Figure 4b shows, as an example, schematically combination of parallel symbol-spaced sample sequences using a combining filter
• FIGS. 5a and 5b show schematically handling of parallel symbol-spaced sample sequences
• Figure 6 shows, as an example, switching functionality relating to FIR whitening
• Figure 7 shows, as an example, a filter module for removing intrasymbol correlations
• Figure 8 shows, as an example, switching functionality relating to IQ whitening.
• Embodiments of the invention are applicable in a digital communications system employing a modulation scheme which can be represented by a real modulation alphabet.
• Some examples of applicable modulation schemes are Pulse Amplitude Modulation (PAM), Minimum Shift Keying (MSK) modulation, Gaussian Minimum Shift Keying (GMSK) modulation, and Binary Phase Shift Keying (BPSK) modulation, and offset Quadrature Amplitude Modulation (offset-QAM) like binary offset QAM and quaternary-offset QAM, which can be viewed as binary or quaternary PAM signal by applying a proper rotation to every symbol.
• PAM Pulse Amplitude Modulation
• MSK Minimum Shift Keying
• GMSK Gaussian Minimum Shift Keying
• BPSK Binary Phase Shift Keying
• offset-QAM offset Quadrature Amplitude Modulation
• those embodiments of the invention which do not employ IQ-splitted signal, are applicable to any modulation, not only binary modulation
• Embodiments of the invention may be used, for example, in Global System for Mobile communications (GSM) or in Code Division Multiple Access (CDMA) systems.
• GSM Global System for Mobile communications
• CDMA Code Division Multiple Access
• pilot symbols or other predetermined symbols are generally needed for determining proper filter coefficients for the FIR whitening filter.
• pilot symbols or other predetermined symbols are generally needed for determining proper filter coefficients for the FIR whitening filter.
• pilot symbols or other predetermined symbols (such as a training sequence) are generally needed for determining proper filter coefficients for the FIR whitening filter.
• pilot symbols or other predetermined symbols such as a training sequence
• the description below assumes a single antenna receiver, and this is the situation in which the embodiments of the invention are most useful.
• the embodiments can, however, be easily extended to more than one receiver antenna, and the samples received from different antennas can be treated equivalently as fractional samples.
• the algorithm would be the same if two samples per symbol are available from a single antenna or if one sample per symbol is available
• Interference suppression is obtained by a digital processing of the signal, which can be classified as a filter or a succession of filtering operation on the digital signal with the aim to whiten the interference.
• the term whitening filter refers to a filter or succession of filters.
• a whitening filter in accordance with the described embodiments may combine the performance gain obtained by using a fractionally spaced processing and the performance gain obtained by splitting the received signal into its real and imaginary parts and processing this signal with a multidimensional filter.
• the real part of the signal is often referred to as the in-phase (I) component
• the imaginary part of the signal is often referred to as the quadrature (Q) component.
• the schematic structure of a receiver 200, where whitening of the received signal is performed, is shown in Figure 2a.
• the receiver 200 includes a RX filter 210, a synchronising and de-rotating unit Sync 220, a channel estimation and signal whitening block 230, an equaliser 240 and a decoder 250.
• the RX filter 210, the Synch unit 220, and the decoder may be similar to those units in a conventional receiver shown in Figure 1.
• the equaliser 240 may be similar as the equaliser 24 in the conventional receiver shown in Figure 1.
• the equalizer may be replaced with another type of a detector.
• the whitening operation in the channel estimation and signal whitening block 230 takes in most of the embodiments into account intersymbol correlation of noise and interference by modelling the noise and interference together as an autoregressive (AR) process and therefore assuming Infinite Impulse Reponse (HR) and intrasymbol correlation relating to noise and interference.
• the channel estimation and whitening block 230 thus typically contains a first whitening filter 231, which is a Finite Impulse Response (FIR) filter for removing intersymbol correlation of noise and interference, and a second whitening filter 232 for taking into account intrasymbol correlation.
• FIR Finite Impulse Response
• the second whitening filter employs correlation information about multiple samples related to the same symbol, for instance obtained by oversampling the signal or by considering as independent samples the in phase and quadrature signal component related to the same symbol, in other words, in-phase and quadrature signal components within a symbol.
• signals originating from multiple receiver antennas may be used for suppressing intrasymbol correlation.
• an equalizer 240 needs a signal to be equalized and a channel estimate corresponding to the signal to be equalized. If the signal input to the equalizer is filtered, also the channel estimate needs to be filtered.
• the channel estimate corresponding to the filtered signal may be obtained, for example, by determining a channel estimate based on a signal before the filtering and then filtering this channel estimate using the same filter with which the signal is filtered. As an alternative example, the channel estimate corresponding to the filtered signal may be determined based on the filtered signal.
• Figures 2b and 2c shows two examples, but it is appreciated that these figures show the functionality typically present in the channel estimation and signal whitening block 230 and they are not intended to show the only possible arrangements of this functionality.
• the second whitening filter 232 in Figures 2b and 2c could contain functionality for estimating the channel h corresponding to the whitened signal y .
• the second whitening filter may filter a channel estimate h ' input to the second whitening filter.
• Figure 2b shows, as a first example, one possible implementation of the channel estimation and signal whitening block 230 in some more detail.
• the channel estimation and whitening block 230a in Figure 2b contains the first whitening filter 231 and the second whitening filter 232.
• the whitened signal y(t) output from the first whitening filter 231 is input to the second whitening filter 232.
• the first whitening filter 231 is a FIR filter
• the channel estimation and signal whitening block 230 contains functionality for determining filter coefficients for the FIR filter. In Figure 2b this is shown as a Filter coefficients block 233.
• the channel estimation and signal whitening block 230 contains functionality for estimating channel impulse responses.
• the block 230a contains a first channel estimator 234a, which takes the whitened signal y(t) as input and provides a channel estimate h ' corresponding to the whitened signal as output.
• the channel estimate h ' is input to the second whitening filter 232, which outputs a further whitened signal y .
• Figure 2b there is a second channel estimator 234b for estimating a further channel estimate h corresponding to the further whitened signal y .
• Figure 2c shows, as a second example, a second possible implementation of the channel estimation and signal whitening block 230 in some more detail.
• the block 230b contains a first channel estimator 234a before the first whitening filter 231.
• the channel estimate h output from the first channel estimator 234a is filtered with the first whitening filter, for obtaining the channel estimate h ' to be input to the second whitening filter 232.
• the block 230b contains a second channel estimator 234b for estimating a further channel estimate h corresponding to the further whitened signal y output from the second whitening filter 232.
• Figures 2b and 2c show the whitening using the first whitening filter 231 to occur before the whitening using the second whitening filter 232, the order of these two filters may be reversed. Furthermore, some embodiments of the invention may discard the second filtering block 232 and corresponding channel estimation. In this case, the filtered signal y(t) and the corresponding channel estimate h ' are typically input to the equalizer or other symbol detector.
• Figures 2b and 2c show the functionality typically present in the channel estimation and signal whitening block 230. It is not intended to show the only possible order of this functionality. For example, as is explained below, the channel estimation and filter coefficients may be determined jointly.
• the filter coefficients A for the FIR filter in accordance with the above described model need to be determined for whitening the received signal.
• Figures 3a, 3b and 3c show some examples of determining the filter coefficients A.
• the filter coefficients A are input to the FIR whitening filter 231.
• the whitened signal y and the whitened channel response h' may be input to the second whitening filter 232 or, if the second whitening filter is omitted, to the equalizer 240.
• the channel response h is first estimated in a first channel estimator 234a, which may be a conventional least square channel estimator or any other suitable channel estimator.
• the received transmitted symbol is a known symbol, for example, a received transmitted pilot symbol.
• the channel estimator 234a provides a channel estimate h.
• the received signal is reconstructed in the signal reconstructor 302 using the channel estimate h and the known symbol a.
• the noise estimator 303 it is possible to provide, in the noise estimator 303, a noise estimate ⁇ as the difference between the received
• L-I transmitted signal and the reconstructed signal e.g. hit
• x(t) - ⁇ a(t - /)/?(/) x(t) - ⁇ a(t - /)/?(/) .
• filter coefficients A may then be obtained as a function of the noise estimate ⁇ in the filter coefficient estimator 304.
• the filter coefficients A are input to the FIR whitening filter 231. It is possible to determine the channel estimate corresponding to the filtered signal y using the channel estimate h and the filter coefficients A, for example, by filtering the channel estimate h with the FIR whitening filter 231, if the block 231 also receive the channel estimates h as input as Figure 2c shows. If a second filter 232 is present in the Channel estimator and signal whitening block 330b, the filtered channel estimate h ' may be input also to the second filter 232. Alternatively a second channel estimation for obtaining the channel estimate h ' is performed from the signal y(t). As a further alternative, the second whitening filter 232 may contain functionality for determining h ' based on h and A or based on the signal y(t).
• a third alternative Channel estimator and signal whitening block 330c shown is shown in Figure 3c.
• the joint estimator 301' is used for determining only the filter coefficients A.
• the filter coefficients are then provided to the FIR whitening filter block 231 and the signal is whitened.
• a first channel estimator 234a provides a channel estimate h' ' .
• the filtered signal y and the corresponding channel estimate h ' may be input to the equaliser.
• the block 330c may include a second whitening filter 232 and a second channel estimator 234b.
• this second whitening filter may be left out if it is, for example, expected that sufficient interference suppression can be obtained using only the FIR whitening filter. This may be the case, for example, when oversampling is used.
• a further reason for leaving the second whitening filter out is that the structure of the receiver can be kept simple. This is applicable especially for the structure shown in Figure 3 c, but may apply also for other structures of the channel estimation and signal whitening block 230.
• the joint estimator 301 ' in Figure 3c may be the same joint estimator 301 as in Figure 3a. In this case, the whitened channel response K may be simply ignored. In the following it is discussed, how to determine the coefficients A and the whitened channel K jointly. Then it is also discussed how to efficiently determine only the filter coefficients A in the joint estimator 301' without determining the whitened channel response.
• the signal may be split in real and imaginary parts a(f - I) Re ⁇ (/) ⁇ (2)
• the noiseless received signal can be expressed as:
• H' ⁇ 2 A:+i ⁇ ⁇ W ⁇ H ⁇ >K+L ⁇ is the channel filtered with the prediction error filter (i.e. the whitened channel).
• the length of the channel increases linearly with the filter order K.
• the filter length is two times the length of the same filter where the splitting in real and imaginary part is not employed while the length of the filtered channel is the same.
• the same model is applicable also without IQ splitting, for instance in the presence of multiple samples per symbols deriving from an oversampled signal or an interpolated signal or multiple reception of the signal by multiple antennas.
• IQ splitting a more effective interference suppression in the case of binary modulation can be achieved, but if the signal is not IQ splitted the method can also be used for other than binary modulations.
• Equation 13 where the two first columns of matrix W contain ones and zeros. These two first matrix columns of matrix W relate to real and imaginary parts of the sample relating to the current symbol (or current time instant).
• the FIR filter coefficients A can be determined without determining the whitened channel impulse response at the same time in the following way.
• a(t) are the known pilot symbols (or training sequence bits).
• the matrix E does not contain terms varying from burst to burst and can therefore be pre-calculated.
• Equation 25 can be further simplified by applying the QR decomposition to the matrix E.
• Equation 26 Equation 26
• R is an upper triangular matrix with same rank as the matrix M.
• oversampling may be used.
• the signal received in the RX filter 210 is a digital signal, and front end filtering, down conversion and analog-to-digital conversion with oversampling is performed before the RX filter. Processing of oversampled data in the joint estimator 301, 301' and in the FIR whitening filter 231 is discussed next.
• multiple samples per symbol may be obtained by interpolation, or they may be available because of multiple replicas of the signal received by multiple antennas. In the following we will describe in more detail various options of dealing with multiple samples per signal where the number of available samples per symbol is denoted as NSPS.
• Extending the previous formulation to the fractionally spaced domain may be done in various ways.
• a first option is to consider equation 1 where the discrete variable t span a fraction of the duration of the symbol period, and then following the already described steps.
• the prediction error is given by
• NSPS Numberer of Samples per Symbol
• FIG. 4a This parallel symbol-spaced solution is shown schematically in Figure 4a, where a channel estimation and signal whitening block 430a is shown to contain a demultiplexer 401 for dividing the sequence of oversampled samples into NSPS sequences of symbol-spaced samples (in Figure 4a, as an example, into four sequences of symbol-spaced samples).
• Each symbol-spaced sample sequence is input to a respective joint channel and filter coefficient estimator 301.
• the filter coefficients A from these estimators are input to the respective first whitening filters 231.
• the NSPS parallel sequences of the whitened samples y are input to the second whitening filter 232.
• the output from the (possible NSPS) second filter(s) 232 may be an oversampled whitened signal or, equivalently, NSPS streams of symbol-based sequences of whitened samples. This output may be sent to an equalizer that can handle an oversampled signal or NSPS parallel streams.
• the output from the second filter(s) 232 may be combined using a combining filter, which takes as input multiple parallel stream (or equivalently an oversampled signal) and gives a single stream symbol spaced as output.
• This combining filter may be done in several ways, for instance as a matched filter or the feedforward filter of an MMSE_DFE equalizer.
• the second filter 232 contains functionality for combining oversampled signal into a sequence of symbol-based whitened samples.
• Figures 5a and 5b shows schematically the demultiplexing and multiplexing discussed above.
• NSPS parallel symbol-spaced sequences of samples
• Figure 4b shows an example with a matched filter 403. It is possible that there is a second filter 232 between the parallel first whitening filters 301 and the matched filter 403.
• a further solution is to use a formulation, which resembles in-phase and quadrature component processing.
• the signal is "fractional spaced splitted" instead of being IQ splitted, that is the same processing that was followed on the IQ splitted signal can be performed on a signal where fractionally spaced samples are arranged similarly.
• the prediction error is given by
• multiple samples per symbol may be processed in series (that is, in one fractionally spaced sequence) for example in the order defined by one of the equations 30, 35 or 40 above.
• the multiple samples per symbol may, alternatively, be processed in connection with at least the FIR whitening by dividing the samples in symbol spaced streams and by processing the symbol spaced streams in parallel, typically independently of each other.
• Equations 30 to 44 in fractional spaced processing methods the dimensions of the matrix Z(t) grow, whereas in the parallel symbol spaced method the matrix Z(t) is processed NSPS times.
• the size of the matrix Z (t)Z(t) which has to be inverted grows.
• the more data columns are added the more the matrix Z (t)Z(t) becomes close to singular and the inversion may become instable. This problem can of course be mitigated by adding to the matrix a small regularization term.
• Poor performance of a whitening receiver in a sensitivity limited scenario can be alleviated by assessing whether the noise is white or not and discarding signal whitening in the case of white noise.
• the FIR filter coefficients A behave in a certain manner (possess values exhibiting certain characteristics). This manner can be determined, for example, by simulations.
• a decision to use FIR whitening may be based on the properties of the FIR filter coefficients A.
• a sensitivity detector refers to a block assessing the properties of the FIR filter coefficients A .
• Figure 6 shows, as an example, a channel estimation and signal whitening block 630 using a FIR filter.
• the block 630 contains a joint channel and filter coefficient estimator 301, but any other alternative for determining the FIR filter coefficients may be used (see, for example, Figure 2b, 2c, 3b or 3c).
• the block 630 further contains a FIR whitening filter 231 and a sensitivity detector 601.
• the sensitivity detector 601 makes use of simple mathematical expressions in order to assess the FIR filter coefficients.
• the sensitivity detector 601 determines, based on the FIR filter coefficients, that the noise is more or less white, it switches in use the channel estimator 23. This way, when the noise is white and there is no need to remove co-channel or adjacent channel interference, whitening is not applied, but a (traditional) channel estimation is performed.
• the decision about using the FIR filter may change from burst to burst. In a sensitivity scenario, whitening will rarely take place, that is, will take place in a small fraction of the bursts. In an interference scenario, whitening will take place very frequently, that is, in a large fraction of the bursts.
• determining whether to use FIR whitening based on the FIR filter coefficients can be used with any of the above described filters using a FIR whitening filter.
• the second whitening filter relating to intrasymbol whitening may have associated functionality for switching on and off the use of the second whitening filter.
• the switches relating to the first and second whitening filters operate independently of each other. If only one whitening filter has associated switching functionality, the other filter may be in use continuously. In some situations it may be feasible to have the switches operating in concert with each other. For example, the following co-operational cases may be feasible.
• Case 1 Whitening using the first and the second whitening filters is applied only when sensitivity detectors associated with both filters detect coloured noise (interference). If only one sensitivity detector (or none of them) detects coloured noise, no whitening is applied.
• Case 2 Whitening using the first and the second whitening filters is applied if either one (or both) of the sensitivity detectors detects coloured noise.
• Case 3 Only the sensitivity detector relating to the first whitening filter is present but it controls the use of the first and second whitening filters.
• Case 4 Only the sensitivity detector relating to the second whitening filter is present but it controls the use of the first and second whitening filters.
• the sensitivity detector 601 applies at least one metric for assessing the FIR filter coefficients.
• a metric is a factor, whose value is dependent on FIR filter coefficients. As discussed below, a metric may be dependent on the complex numbers from pairs of FIR filter coefficients. It is possible to determine a range of metric values, which corresponds to white noise, for example by simulations. This range of metric values depends on the specific metric. If the sensitivity detector 601 judges that a metric value is within the range corresponding to the white noise, the FIR whitening is discarded. If a metric value is outside the range corresponding to the white noise, the FIR whitening is employed. It is clear to a skilled person that the metric value range corresponding to white noise may be determined as a threshold which the metric should exceed or below which a metric value should remain, depending on the definition of the metric.
• Ci ⁇ i ⁇ +j ⁇ >3
• C 2 Ai t2 +jA ⁇ A
• C 3 A 2 , ⁇ +j ⁇ 2 ,3, ...
• C 8 A 4 , 2 +jA 4 ⁇ 4 .
• complex numbers are formed from pairs of FIR filter coefficients.
• 2K-I 2K-1 m ⁇ Mn /,(
• an interference scenario (coloured noise) is detected if the following assertion is true: m ⁇ m thr OK mi > U 1 OR ... OR m L > a L where 1 ⁇ L ⁇ 2K-l.
• a sensitivity limited scenario (white noise) is detected.
• the thresholds, m thr and ⁇ y ... ai depend on the signal levels in the system and must be tuned for a particular implementation. Likewise, the constants L, U n and V n and the functions/, must be selected appropriately.
• a GSM GMSK receiver with an oversampling rate of 2.
• Dx 1 IC 1 7 I
• D 2 1 IC 2 7 I
• D, 1 IC 3 7 I
• An interference scenario (coloured noise) is detected if the following assertion is true: m ⁇ mt h r OR mi > a t hr OR nt2 > a t /, r - If the assertion does not hold, a sensitivity limited scenario (white noise) is detected.
• a second specific example of a metric m used in a sensitivity detector 601 is the following.
• the metric m is formed as the sum of the squared Euclidean distances between the actual FIR filter coefficients and reference coefficients C k -
• N is an integer constant between 1 and the AR process order multiplied with the oversampling factor, i.e. l ⁇ iV ⁇ 2 ⁇ for 2 times oversampling.
• the reference coefficients C k , l ⁇ k ⁇ N, are constant complex numbers.
• an interference scenario (coloured noise) is detected if the following assertion is true: m ⁇ nithr
• m thr depends on the signal levels in the system and must be tuned for a particular implementation. Likewise, N and the c ⁇ s must be tailored to the implementation.
• Threshold value: m thr -1.0.
• Sensitivity switching using a sensitivity detector can significantly improve sensitivity performance of a receiver employing FIR whitening without hampering interference performance very much. Gains of up to 2.0 dB have been demonstrated for sensitivity performance in a realistic receiver for GSM phones.
• the degradations in interference scenarios were limited to 0.3 dB for single interferer cases (co-channel interference or adjacent channel interference). Negligible degradations ( ⁇ 0.01 dB) were seen in a complex interference scenario with a mixture of several GMSK modulated co- and adjacent channel interference contributions. Such complex scenarios are the most realistic interference scenarios, and the losses in the single interferer cases are not important.
• the second whitening filter 232 operates with two samples per symbol and IQ splitting.
• the IQ splitting is also optional.
• the number of samples per symbol can be increased also by means of interpolation if additional samples are not available directly from the received signal Rx(t).
• the intrasymbol correlation can be handled by using correlation properties of samples relating to the same symbol.
• the following notation uses a signal y as input signal to the intrasymbol whitening filter. This is not intended to limit the signal processing to cases, where there is a FIR whitening filter before the intrasymbol whitening filter. As mentioned above, the order of these filters may be reversed.
• the second whitening filter 232 is a filter for filtering the sampled received signal to remove co-channel interference and noise so as to produce a filtered sampled signal ⁇ k (i.e. a succession of filtered signal samples).
• R H is the 4x4 noise plus interference correlation matrix given by: in which E[...] is the mathematical operation of taking the ensemble average. Since the interference arises from a cyclo stationary random process, the expectation operation can be replaced by a time average, i.e.
• the Hermitian matrix R may be the noise plus interference correlation matrix
• SVD Singular Value Decomposition
• W V ⁇ n .
• a whitened impulse response H 0 is also provided by the IQ filter, this time acting on the impulse response H' o :
• each 4 x 1 vector h m may be computed as:
• Such a module would not decimate the signal in case of a fractionally spaced equalizer, but in case of a symbol-spaced equalizer, the module would decimate the signal, and would do so by filtering the signal with a filter equivalent to the feed-forward filter of a DFE equalizer or by a polyphase matched filter.
• the equalizer following the intrasymbol whitening filter 732 may be a trellis detector, and the input of the trellis detector is symbol spaced.
• This symbol spaced input can be obtained by post processing the output of the whitening filter W by a pre-filter (designed for white noise) equivalent to the feed forward filter of a Decision Feedback Equalizer (DFE).
• DFE Decision Feedback Equalizer
• R noise plus interference correlation matrix
• a Forney metric can be used in the trellis Viterbi equalizer. See e.g. "Unification of MLSE Receivers and Extensions to Time Varying Channels"; by G. Bottomley, S. Chennakesku; in IEEE Trans. Inform. Theory; vol. 46, pp 464-472, April 1998.
• the equalizer could employ the Ungerboeck metric, also described in the above paper by Bottomley et al.
• Ungerboeck metric also described in the above paper by Bottomley et al.
• the output of the whitening filter W can be decimated to one sample per symbol by employing a polyphase matched filter, matched to the whitening desired impulse response.
• the equalizer can operate with more than one sample per symbol, and in such a case there is yet another alternative implementation. If the equalizer can operate with more than one sample per symbol, instead of using the intrasymbol whitening filter , the same result can be obtained by modifying the metric inside the equalizer to take into account the noise correlation. More specifically a modified branch metric given by:
• Some embodiments of the invention also provide a mechanism for enabling and disabling IQ whitening (either performed as described above or using other techniques). Such a switching mechanism is useful because, it turns out, IQ whitening to suppress co-channel interference is remarkably effective for binary modulated co- channel interference-limited channels, but causes some performance loss in sensitivity (white noise) limited channels.
• the invention thus provides a switching algorithm that can be used to dynamically switch between IQ whitening and non-whitening in a receiver.
• the switching is based on examining relative values of different components of the noise plus interference correlation matrix R r; 5 as explained below.
• the switching is based on examining expectations E[...] of different products i ⁇ and ⁇ i 4+1 , as also explained below.
• the idea behind the switching mechanism provided by the invention is to determine whether the noise present is white noise (in such case the whitening is turned off), or non white noise, i.e. whether it is temporally correlated (in such case the whitening is turned on).
• the IQ whitening filter (with truncated autocorrelation) requires the computation of the 4x4 (2-IQ split, 2-over-sampling) correlation matrix R /( from the noise samples. So in the preferred embodiment, the metric for switching on and off whitening is derived as a function of the elements of R n .
• the temporal correlation (between the fractional samples, i.e. the samples within one symbol) can be expressed as a combination of the elements of the correlation matrix R 11 , as follows:
• the temporal correlation can be obtained from the elements of the correlation matrix, and the correlation between the real and imaginary components of a sample does not become zero.
• the metric depends on the relative values of different components of the noise plus interference correlation matrix R,, . Then, if M tc > ⁇ tc for some predetermined threshold ⁇ tc , the channel is categorized as temporally correlated and whitening is performed; otherwise whitening is disabled.
• the switching is based on examining expectations E[...] of different products i ⁇ i fc and i ⁇ i A+1 .
• E[...] of different products i ⁇ i fc and i ⁇ i A+1 we evaluate the (numerical) value, and also the (numerical) value, Then, using as a metric for determining whether to whiten the quantity M n defined by:
• M n ⁇ , (60) we switch to whitening when M n is greater than some predetermined threshold ⁇ n
• Both of the embodiments for switching described above are based on observation of the second order statistics of the interference signal. Other embodiments may depend a more complicated measure.
• the sensitivity detector 801 switches to the module 802 for filtering without whitening, and otherwise switches to the module 732 for whitening.
• the module 802 may be discarded, especially if, for example, intersymbol whitening functionality is residing in the filter block 800 before the switch 801 or after the block 732.
• a filter where there is a first whitening filter for suppressing intersymbol interference and a second whitening filter for suppressing intrasymbol interference, there may be a sensitivity detector and a switch (that is, switching functionality) relating to each filter. This has been discussed above in connection with switching on/off the intersymbol whitening filter.
• the channel estimation and signal whitening block in accordance with embodiments of the invention may be implemented using software, hardware or a suitable combination of these.
• embodiments of the invention may be implemented as an Application-Specific Integrated Circuit (ASIC), designed for a particular application.
• ASIC Application-Specific Integrated Circuit
• embodiments of the invention may be implemented as program code for a programmable Digital Signal Processing (DSP) chip.
• DSP Digital Signal Processing
• interference suppression in accordance with embodiments of the inventions may be used in communications devices and in network elements of various communication systems.
• the terms communications device and network element refer here to any devices that are provided with functionality to receiving signals over a radio interface and processing the received signals.
• Some specific examples of the communications systems, where embodiments may be used are cellular communications systems, such as GSM or UMTS (Universal Mobile Telecommunications System).

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