WO2010066287A1

WO2010066287A1 - Apparatus for rejecting co-channel interference and corresponding method, computer program and computer-readable medium

Info

Publication number: WO2010066287A1
Application number: PCT/EP2008/067131
Authority: WO
Inventors: Mauri NISSILÄ; Markku Vainikka
Original assignee: Nokia Siemens Networks Oy
Current assignee: Nokia Solutions and Networks Oy
Priority date: 2008-12-09
Filing date: 2008-12-09
Publication date: 2010-06-17
Anticipated expiration: 2011-06-09

Abstract

An aspect of the invention relates to an apparatus comprising: a definer (202, 204, 206) configured to define symbol decisions on the basis of pre-determined signal samples obtained from a received signal including data and reference symbols, a storage circuitry (210) configured to store the symbol decisions, a determiner (212) configured to determine channel estimates on the basis of the signal samples, a regenerator (214) configured to regenerate a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, a delayer (220) configured to delay the signal samples, and a subtractor (216) configured to subtract the regenerated signal from the delayed signal samples for reducing effects of interference.

Description

APPARATUS FOR REJECTING CO-CHANNEL INTERFERENCE

AND CORRESPONDING METHOD, COMPUTER PROGRAM AND COMPUTER-READABLE MEDIUM

Field

The exemplary and non-limiting embodiments of this invention relate generally to an apparatus, method, computer program and computer-readable medium.

Background art

The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with dis- closures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.

Multi-antenna systems, also known as adaptive array antennas, smart antennas, etc., are usually implemented with antenna arrays. Multiple- input and multiple-output (MIMO) refers to systems using multiple antennas both at a transmitter and receiver. Other multiple antenna systems also exist, such as a single-input and multiple-output (SIMO).

The use of muiti-antenna systems is of a great interest and several techniques have been introduced to combine diversity antennas or different antenna branches of antenna arrays in reception. One example of such a combining technique is interference rejection combining (IRC).

Co-channel interference, which is typically generated by neighboring ceils, is often unknown to the receiver. Nevertheless, the impact of the co- channel interference to a signal of interest may be reduced even significantly by the interference rejection combining technique. One example of !RC techniques may be as follows: the co-channel interference and the receiver noise are Jointly modeled as a colored Gaussian noise, the statistical properties of the colored noise are estimated based on parametric description of the noise pro- cess, and the estimated parameters are used to specify the whitening filter for the combined signals of co-channel interference and receiver noise. Brief Description

The following presents a simplified brief description in order to pro- vide a basic understanding of some aspects of the invention. This brief description is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. An aspect of the invention relates to an apparatus comprising: a de- finer configured to define symbol decisions on the basis of pre-determined signal samples obtained from a received signal including data and reference symbols, a storage circuitry configured to store the symbol decisions, a determiner configured to determine channel estimates on the basis of the signal samples, a regenerator configured to regenerate a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, a delayer configured to delay the signal samples, and a subtracter configured to subtract the regenerated signal from the delayed signal samples for reducing effects of interference. A further aspect of the invention relates to a method comprising: defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols, storing the symbol decisions, determining channel estimates on the basis of the signal samples, regenerating a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, delaying the signal samples, and subtracting the regenerated signal from the delayed signal samples for reducing effects of interference.

A still further aspect of the invention relates to a computer-readable medium having stored thereon a data structure, comprising: defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols, storing the symbol deci- sions, determining channel estimates on the basis of the signal samples, regenerating a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, delaying the signal samples, and subtracting the regenerated signal from the delayed signal sam- pies for reducing effects of interference.

A stil! further aspect of the invention relates to an apparatus comprising: means for defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols, means for storing the symbol decisions, means for determining chan- nel estimates on the basis of the signal samples, means for regenerating a transmitted signal corresponding to the received signal by using the stored symboi decisions and the channel estimates, means for delaying the signal samples, and means for subtracting the regenerated signal from the delayed signal samples for reducing effects of interference. According to an embodiment of the invention, the apparatus further comprises; a reception unit configured to receive a signal including data and reference symbols transmitted, and a sampler configured to sample the received signal for obtaining signal samples.

According to a further embodiment of the invention, the apparatus uses a most recent channel estimate.

According to a further embodiment of the invention, the apparatus estimates the channel after the whitening filter.

According to a further embodiment of the invention, the apparatus filters the channel estimates by the whitening filter. According to a further embodiment of the invention, the apparatus comprises a module.

According to a further embodiment of the invention, the apparatus comprises a network element.

A further aspect of the invention is an apparatus (user device, net- work element, etc.) which is configured to perform functionality of an apparatus according to any embodiment of the invention. An aspect of the invention is a program containing an executable code configured to perform a method according to any embodiment of the invention when executed in a computing device.

Although the various aspects, embodiments and features of the in- vention are recited independently, it should be appreciated that all combinations of the various aspects, embodiments and features of the invention are possible and within the scope of the present invention as claimed.

Brief description of the drawings

In the following the invention will be described in greater detail by means of exemplary embodiments with reference to the attached drawings, in which

Figure 1 shows a simplified block diagram illustrating exemplary system architecture;

Figures 2A and 2B show an exemplary apparatus according to an embodiment of the invention; and

Figure 3 shows an exemplary flow chart according to an embodiment of the invention.

Detailed description of some embodiments

Exemplary embodiments of the present invention will now be de- scribed more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements, Although the specification may refer to "an^B, "one", or "some" embodiments) in several locations, this does not necessarily mean that each such reference is to the same embodiments), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Embodiments are applicable to any user terminal, server, node, host, corresponding component, and/or to any communication system or any combination of different communication systems that support required functionality.

The protocols used, the specifications of communication systems, servers and user terminals, especially in wireless communication, develop rap- idly. Such development may require extra changes to an embodiment. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, embodiments.

In the following, different embodiments will be described using, as an example of a system architecture whereto the embodiments may be ap- plied, an architecture based on Evolved UMTS terrestriaf radio access (E- UTRA, UMTS - Universal Mobile Telecommunications System) without restricting the embodiment to such an architecture, however.

Many different radio protocols to be used in communications systems exist. Some examples of different communication systems are the Uni- versal Mobile Telecommunications System (UMTS) radio access network (UTRAN or E-UTRAN), Long Term Evolution (LTE₁ the same as E-UTRA)₁ Wireless Local Area Network (WLAN)₁ Worldwide Interoperability for Microwave Access (WiMAX), Bluetooth®, Personal Communications Services (PCS) and systems using ultra-wideband (UWB) technology. Figure 1 is a simplified system architecture only showing some elements and functional entities, all being logical units whose implementation may differ from what is shown. The connections shown in Figure 1 are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the systems also comprise other functions and structures. It should be appreciated that the functions, structures, elements and the protocols used in or for group communication, are irrelevant to the actual invention. Therefore, they need not to be discussed in more detail here. Figure 1 shows a part of a radio access network of E-UTRA. The communications system is a cellular radio system which comprises a base sta- tion (or node B) 100, which has bi-directional radio links 102 and 104 to user devices 106 and 108. The user devices may be fixed, vehicle-mounted or portable. The user devices 106 and 108 may refer to portable computing devices. Such computing devices include wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: mobile phone, multimedia device, personal digital assistant (PDA), handset, The base station includes transceivers, for instance. From the transceivers of the base station, a connection is provided to an antenna unit that establishes bi-directional radio links to the user devices. The base station is further connected to a controller 110, a radio network controller (RNC)₁ which transmits the connections of the devices to the other parts of the net- work. The radio network controller controls in a centralized manner several base stations connected to it. The radio network controller is further connected to a core network 112 (CN). Depending on the system, the counterpart on the CN side can be a mobile services switching centre (MSC), a media gateway (MGW) or a serving GPRS (general packet radio service) support node (SGSN), etc.

It should be noted that in future radio networks, the functionality of an RNC may be distributed among (possibly a subset of) base stations.

The embodiments are not, however, restricted to the system given as an example but a person skilled in the art may apply the solution to other communication systems provided with the necessary properties. Different radio protocols may be used in the communication systems in which embodiments of the invention are applicable. The radio protocols used are not refevant regarding the embodiments of the invention.

The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet.

In modern communication systems, packet-switched traffic is becoming more and more important. Delivery of digital data over mobile networks as well as Internet Protocol-based (IP-based) person-to-person communication combining different media and services into the same session increases the use of packet-switched services.

High Speed Packet Access, HSPA, which is also usable in relation to embodiments, is able to provide high data rate transmission to support mul- timedia services. HSPA brings high-speed data delivery to 3rd generation (3G) terminals. HSPA includes High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA). HSUPA uses a packet scheduler and it operates on a request-grant principle that is a user terminal requests a permission to send data and the scheduler decides on resource allocation. Co-channel interference or CCI is crosstalk from different radio transmitters using a same or neighbouring frequency. Co-channel interference, which is typically generated by neighboring cells, is often unknown to the receiver. Nevertheless, the impact of the co-channel interference to a signal of interest may be reduced even significantly by the interference rejection combining technique. One example of IRC techniques may be characterized as follows: the co-channel interference and the receiver noise are jointly modeled as a colored Gaussian noise, the statistical properties of the colored noise are estimated based on parametric description of the noise process, and the estima- ted parameters are used to specify the whitening filter for the combined signals of co-channel interference and receiver noise.

In order to estimate the statistical properties of the co-channel interference and receiver noise signal (CCIN signal), the estimate of a desired signal may be subtracted from the received signal (the received signal in this con- text refers to the sampled received signal). An estimate of the CCIN signal is herein referred to as a residual signal. The residual signal may be used for specifying and updating coefficients of a whitening filter or an MMSE equalizer.

Figures 2A and 2B are block diagrams of exemplary apparatuses according to some embodiments of the invention. Although the apparatuses have been depicted as one entity, different modules and one or more memory buffers may be implemented in one or more physical or logical entities. Each of the apparatuses may be an entity or a part of one, such as a base station or another network element. An example of other network element is an ad-hoc network element (either a mobile router or a mobile network node). The apparatus may also be a user terminal or operably coupled to it.

The user terminal is a piece of equipment or a device that associates, or is arranged to associate, the user terminal and its user with a subscription and af- lows a user to interact with a communications system. The user terminal presents information to the user and allows the user to input information. In other words, the user terminal may be any terminal capable of receiving information from and/or transmitting information to the network, connectable to the network wirelessfy or via a fixed connection. Examples of the user terminal include a personal computer, a game console, a laptop (a notebook), a personal digital assistant, a mobile station (mobile phone), and a line telephone.

The apparatuses may be implemented by using one or more processors including required hardware. Other options for implementation also ex- ist. Generally the processor fs a central processing unit, but the processor may be an additional operation processor. The processor may comprise a computer processor, application-specific integrated circuit (ASiC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out one or more functions of an embodiment. It should be appreciated that the apparatuses may comprise other units than those depicted in Figures 2A and 2B. However, they are irrelevant to the actual invention and, therefore, they need not to be discussed in more detail herein.

The apparatuses are welf-suited to generate a residual signal in cases where a training signat (piiot data) is not time-multiplexed with data symbols (examples of such systems are WCDMA or HSUPA system) or where the time-multiplexed pilot sequence is too short for the proper operation of an IRC-receiver. If the piiot and data symbols are transmitted simultaneously, they are made distinguishable at a receiver by orthogonal spreading codes. The apparatuses are arranged to implement a computationally efficient method for generating a residual signal which may then be used for calculating noise and interference covariance estimates and/or coefficients of a whitening filter (typically a linear whitening filter, LWF) or of an equalizer. Basically, in the apparatuses, the residual signal is generated by subtracting an es- timated desired signaf from received signal samples, thus yielding an estimate of potential co-channel interference (CCI) signal that may have disturbed the desired signal. In this application, the concept of "desired signal" denotes a copy of a transmitted signal targeted to a selected user multiplied with a channel convolution matrix.

Instead of estimating a desired signal from the time-multiplexed pilot section of the received signal, the apparatuses may regenerate the desired signal by exploiting the decision feedback where either hard or soft symbol decisions are fed back prior to or after decoding. Pilot symbols are an example of reference symbols.

The apparatuses may buffer the symbol decisions and, with the aid of the buffered symbol decisions and typically the most recent channel esti- mate, regenerate and subtract the desired signal from an appropriately delayed block of received signal samples.

Thus, coefficients for a whitening filter or an MMSE equalizer may be computed based on previous symbol decisions. Hence, the requirement of an iterative processing on the same set of received signal samples can be re- laxed and the typical receiver operations, such as whitening filtering, equalization and soft demodulation, need to be carried out only once per each received signal block. Further, since the proposed IRC scheme operates on the basis of decision feedback, the apparatus may be adjusted to all wireless systems (or systems combining wired and wireless systems) impaired by the presence of a CCI signal. Particularly suitable the scheme is for systems where pilot symbols are not time-multiplexed with data symbols, but at transmitted at least partly simultaneously. Examples of such systems are WCDMA and HSUPA.

The example of Figure 2A presents a block diagram of an IRC receiver concept with two reception (diversity) antennas, in different impiementa- tions, the number of reception antennas may vary being any number from 1 to N. The apparatus may be applied to both spatial and spatio-temporal IRC implementations.

Signals Rx1 and Rx2 represent received signal samples obtained from the reception antennas via radio frequency (RF, front-end) processing and analogue-to-digital converters. If the apparatus is a unit, module, component etc, it may be operably coupled to the front-end parts and the converters 200 of the apparatus it is located in or otherwise coupled to. Such apparatuses may be base stations or other network nodes, for instance.

Signals Rx1 and Rx2 pass through whitening filter 202. A whitening filter typically performs filtering both in spatial and time domains. One example of a whitening filter is a Wiener filter. The whitening filter is designed to whiten (i.e. decorrelate) coloured noise in the received signal.

The whitened received samples (RxW1, RxW2) are fed to frequency- domain or time-domain equalizer (FDE) 204 which typically operates on a block-basis. The radio channels in radio systems are usually multipath fading channels, which cause iπtersymbol interference (ISI) in the received signal. To remove ISf from the signal, equalizers may be used. Equalization is usually estimation of an equalizer filter, which is the inverse of a channel impulse response. The estimated equalizer is then convolved with the received signal to obtain an estimation of the transmitted signal. The FDE block may also include other functions, such as de- scrarnbling and de-spreading.

In this example, the output of the FDE is conveyed to the soft de- moduiator 206 which produces soft symbol decisions (hard decisions may also be used as an option) to be further exploited by the channel decoder (for in- stance a Turbo decoder 208), Soft bits indude a bit decision and information on the reliabiiity of the decision. In principle, the purpose of the demodulator is to select the bit sequence actually transmitted. The selection may be carried out by searching for the point on the constellation diagram which is closest (in a Euclidean distance sense) to that of the received symbol. Estimates for transmitted symbols are then obtainable by utilizing information about bit sequences represented by the constellation points.

The soft decisions (afso hard decisions are suitable) are stored in a memory (or buffered) 210 for the purpose of regenerating the desired signal as seen by the reception antennas. For the signal regeneration, also channel estimates pertaining to the desired user are required. The channel estimates are also used in determining equalizer filter coefficients (channel impulse response). The channel is esti- mated from the received signal sample either before or after the whitening filter. Usually the channel estimation is based on the known sequence of symbols, such as a pilot sequence, which is unique for a certain transmitter. Thus, the channel estimator is able to determine channel estimates by exploiting the known transmitted symbols and the corresponding received samples. In this example, least-squares (LS) channel estimation techniques may be used in a channel estimator 212.

The channel estimates <ChE1 , ChE2) are filtered by the whitening filter yielding signals ChEWI and ChEW2, which are fed to the FDE for the purpose of calculating the equalizer coefficients,

The desired signal is regenerated in block 214 based on channel estimates ChE1 and ChE2, and subtracted in block 216 from the delayed and buffered received signal for obtaining residual signal samples. Delaying and buffering are carried out in block 220. The delay is adjusted in a manner that the delay due to the desired signal regeneration via the decision feedback is compensated.

Coefficients for the finear whitening filter are calculated in block 218. The calculation is based on the residual signal, The coefficients are typically selected to diminish the effects of interference and noise in a received signal, A residual signal sample at time corresponding to the reception antenna may be as follows:

Resn(k) = Exn(k-D)-y_B(k) , (1)

Wherein n denotes an ordinal number of a signal k denotes a block of residual signal samples (A =I, .,.,K)

Rx denotes a received signal sample, D denotes a delay of the received signal , and y_n denotes a regenerated desired signal.

The regenerated desired signal y_n may be obtained as follows: y_n = s*ChEn, (2) Wherein y_π denotes a regenerated desired signal, έ denotes a soft or hard symbol decision,

* denotes a convolution, ChEn denotes a channel estimate, and n denotes an ordinal number of a signal or a channel estimate.

Other options aiso exist for obtaining a regenerated signal than that depicted by equations (1) and (2).

It is understood that the signal regeneration may also include other operations, such as spreading, scrambling, and code-multiplexing of control and data information.

The example of Figure 2B presents a block diagram of an IRC receiver concept with two reception (diversity) antennas. In different implementations, the number of reception antennas may vary being any number from 1 to N. The apparatus may be applied to both spatial and spatio-temporal IRC implementations. In this second example, received signal samples are assumed to include coloured noise and no whitening is explicitly carried out.

Signals RxI and Rx2 represent received signal samples obtained from the reception antennas via radio frequency (RF, front-end) processing and analogue-to-digital converters. If the apparatus is a unit, module, component etc, it may be operably coupled to the front-end parts and the converters 200 of the apparatus it is located in or otherwise coupled to. Such apparatuses may be base stations or other network nodes, for instance.

In this second example, frequency- domain or time-domain equali- zation is carried out jointly with noise whitening by selecting the MMSE equalizer coefficients appropriately. In other words, in the first example, whitening and equalization are carried out by separate functional units whereas in the second example, they are performed jointly by one functional block.

Thus the received signal samples Rx1 and Rx2 are fed to a Mini- mum Mean Square Error (MMSE) equalizer 222. An equalization filter can be used to diminish the effects of channel-induced distortion. Typically, an MMSE criterion seeks filter tap weights (coefficients) that minimize the mean-square error between the desired output from the equalizer and its actual output. As stated above, received signal samples are assumed to include coloured noise while fed to the MMSE equalizer 222.

After equalization, the received samples are fed to a soft demodula- tor 206 which produces soft symbol decisions to be further exploited by the channel decoder (for instance a Turbo decoder 208).

The soft decisions (also hard decisions are suitable) are stored in a memory (or buffered) 210 for the purpose of regenerating the desired signaf as seen by the reception antennas. For the signal regeneration, also channel estimates pertaining to the desired user are required. Usually the channel estimation is based on the known sequence of symbols, such as a pilot sequence, which is unique for a certain transmitter Thus, the channel estimator is able to determine channel estimates by exploiting the known transmitted symbols and the corresponding received samples. In this example, feast-squares (LS) channel estimation techniques may be used in a channel estimator 212.

The desired signal is regenerated in block 214 based on channel estimates Ch E 1 and ChE2, and subtracted in block 216 from the delayed and buffered received signal for obtaining residual signal samples. Delaying and buffering are carried out in block 220. The delay is adjusted in a manner that the delay due to the desired signal regeneration via the decision feedback is compensated. tn block 224, noise and interference matrix is calculated based on the residual signal. The MMSE filter is updated on the basis of the noise and interference calculation.

Let us consider the MMSE equalization in the case where the noise is assumed to be coloured.

The system model may be as follows: r = Hs+n , (3) wherein r denotes a sequence of received samples,

H denotes a convolution matrix of a channel, s denotes a symbof decision, and n denotes a zero-mean Gaussian noise vector, ft is further assumed that noise and interference covariance matrix (calculated in block 224) can be expressed as follows: Q = £{nn^ff}, (4) wherein

£ denotes probability, n denotes a zero-mean Gaussian noise vector, and

^B denotes a Hermitian matrix (complex-conjugate). The formulation implies that the receiver noise is coloured.

The output of the MMSE equalizer may be expressed as: y =H^H (HH* +Q)-V, (5)

Wherein

H denotes a convolution matrix of a channel, Q noise and interference covariance matrix,

^H denotes a Hermitian matrix (complex-conjugate), and r denotes a sequence of received samples. The apparatus may include: a definer (206 in the examples of Figures 2A and 2B) configured to define symbol decisions on the basis of prede- termined signal samples obtained from a received signal including data and reference symbols, a storage circuitry (buffer 210 in the examples of Figures 2A and 2B) configured to store the symbol decisions, a determiner (212 in the examples of Figures 2A and 2B) configured to determine channel estimates on the basis of the signal samples, a regenerator (214 in the examples of Figures 2A and 2B) configured to regenerate a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, a delayer (220 in the examples of Figures 2A and 2B) configured to delay the signal samples (the signal samples are typically stored after sampling for delaying), and a subtracter (216 in the examples of Figures 2A and 2B) configured to subtract the regenerated signal from the delayed signal samples for reducing effects of interference. The apparatus may further include a recep- tion unit (200 in the examples of Figures 2A and 2B) configured to receive a signal including data and reference symbols transmitted, a sampler (200 in the examples of Figures 2A and 2B) configured to sample the signal for obtaining signal samples, Another embodiment of the apparatus may include means (206 in the examples of Figures 2A and 2B) for defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols, means (210 in the examples of Figures 2A and 2B) for storing the symbol decisions, means (212 in the examples of Figures 2A and 2B) for determining channel estimates on the basis of the signal samples, means (214 in the examples of Figures 2A and 2B) for regenerating a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, means (220 in the examples of Figures 2A and 2B) for delaying the signal samples, and means (216 in the examples of Figures 2A and 2B) for subtracting the regenerated signal from the delayed signal samples for reducing effects of interference. The apparatus may further include means (200 in the examples of Figures 2A and 2B) for receiving a signal including data and reference symbols, and means (200 in the examples of Figures 2A and 2B) for sampling the received signal for obtaining signal samples.

The effects of interference (typically co-channel interference) may be reduced by determining the coefficients of the whitening filter or MMSE equalizer appropriately on the basis of the regenerated signal (block 218 in the examples of Figures 2A and 2B). Typically, a block of K residual signal sam- pies ( k = 1, ...,K) is used to calculate the new values for the coefficients. Initially the coefficients for the whitening filter may be defined such as the whitening filter will be an all-pass filter.

The units may be software and/or software-hardware and/or firmware components (recorded indelibly on a medium, such as read-only-memory or embodied in hard-wired computer circuitry, for example).

The storage circuitry (210 in the examples of Figures 2A and 2B) may include volatile and/or non- volatile memory and typically stores content, data, or the like. The memory may be, for example, a random access memory (RAM), hard drive, or other fixed data memory or storage device. Further, the memory, or part of it, may be a removable memory detachably connected to the apparatus. The techniques described herein may be implemented by various means so that an apparatus implementing one or more functions of a corresponding mobile entity described with an embodiment comprises not only prior art means, but also means for implementing the one or more functions of a corresponding apparatus described with an embodiment and it may comprise separate means for each separate function, or means may be configured to perform two or more functions. For example, these techniques may be implemented in hardware (one or more apparatuses), firmware (one or more apparatuses), software (one or more modules), or combinations thereof. For a firmware or software, implementation can be through modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in any suitable, processor/computer-readable data storage medium(s) or memory unit(s) or articie(s) of manufacture and executed by one or more processors/computers. The data storage medium or the memory unit may be implemented within the processor/computer or external to the processor/computer, in which case it can be communicatively coupled to the processor/computer via various means as is known in the art.

In the foflowing, en embodiment of a method will be described by means of Figure 3.

The steps/points, signaling messages and related functions dβscri- bed above in Figure 3 are in no absolute chronological order, and some of the steps/points may be performed simultaneously or in an order differing from the given one. Other functions can also be executed between the steps/points or within the steps/points and other signaling messages sent between the illustrated messages. Some of the steps/points or part of the steps/points can also be left out or replaced by a corresponding step/point or part of the step/point. The embodiment starts in block 300. In block 306, symbol decisions are defined on the basis of the signal samples. The symbol decisions are hard or soft decisions. Soft decisions include a bit decision and information on the reliability of the decision. In principle, the purpose of the demodulator is to select the bit sequence actually transmitted. The selection may be carried out by searching for the point on the constellation diagram which is closest (in a Euclidean distance sense) to that of the received symbol. Estimates for transmitted bits are then obtainable by utilizing information about bit sequences represented by the constellation points. In block 308, the symbol decisions are stored for example in a buffer.

In block 310, channel estimates are determined on the basis of the signal samples. The channel estimates are also used in determining equalizer filter coefficients (channel impulse response). The channel may be estimated from the received signal sample either before or after a whitening filter. Usually the channel estimation is based on the known sequence of symbols, such as a pilot sequence, which is unique for a certain transmitter. Thus, the channel estimator is able to determine channel estimates by exploiting the known transmitted symbols and the corresponding received samples. In this example, least-squares (LS) channel estimation techniques may be used.

In block 312, a transmitted signal corresponding to the received signal is regenerated by using the stored symbol decisions and the channel estimates, instead of estimating a desired signal from the time-multipiexed pilot section of the received signal, the apparatus may regenerate the desired signal by exploiting the decision feedback where either hard or soft symbol decisions are fed back prior to or after decoding. The regenerated desired signal y_n may be obtained as depicted by equations (1) and (2). Other options also exist.

In block 314, the signal samples are delayed. The delay is adjusted in a manner that the delay due to the desired signal regeneration via the deci- sion feedback is compensated. The signal samples are typically stored after sampling for delaying. In block 316, the regenerated signal is subtracted from the delayed signal samples for reducing effects of interference. The effects of interference (typically co-channel interference) may be reduced by determining the coefficients of a whitening filter or MMSE equalizer appropriately on the basis of the regenerated signal. Typically, a block of K residual signal samples

(k ~ 1,...,Z) is used to calculate the new values for the coefficients. Initially the coefficients for the whitening filter may be defined such as the whitening filter will be an all-pass filter.

Another embodiment further includes receiving a signal including data and reference symbols (block 302) and sampling the received signal for obtaining signal samples (block 304). The reference symbols may be a training sequence or pilot symbols. The pilot symbols may be time-multiplexed or transmitted simultaneously with data symbols. The idea of the training sequence or pilot symbols is to transmit a known sequence with which channel cha- racteristics, such as a channel impulse response can be determined. in the sampling, a continuous signal is changed to a discrete signal comprising a sequence of samples.

The embodiment ends in block 318. The embodiment is repeatabie in many ways. One example is shown by arrow 320. An embodiment provides one or more computer programs embodied on one or more distribution media, comprising program instructions which, when ioaded into an electronic apparatus, constitute the apparatus as explained above. The computer program may execute the instructions for receiving a signal including data and reference symbols, sampling the received signal for obtaining signal samples, defining symbol decisions on the basis of the signal samples, storing the symbol decisions, determining channel estimates on the basis of the signal samples, regenerating a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates, delaying the signal samples, and subtracting the regenerated signal from the delayed signal samples for reducing effects of interference.

The computer program may be in a source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier (also called a computer-readable medium), which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. Depending on the pro- cessing power needed, the computer program may be executed in a single electronic digital computer or it may be distributed amongst a number of computers.

The units, typically implemented by one or more processors (see Figures 2A and 2B) may be configured to store programming such as execu- table code or instructions (e.g., software or firmware), electronic data, databases, or other digital information, and may include processor-usable media.

Exemplary processor-usable media may include any one of physical media such as electronic, magnetic, optical, electromagnetic, infrared or semiconductor media. Some more specific examples of processor-usable media in- elude, but are not limited to, a portable magnetic computer diskette, such as a floppy diskette, zip disk, hard drive, random-access memory, read only memory, flash memory, cache memory, or other configurations capable of storing programming, data, or other digital information.

Processor-usable media may be embodied in any computer pro- gram product or article of manufacture. The processor-usable media can contain, store, or maintain programming, data or digital information for use by or in connection with an instruction execution system. For example, exemplary processor-usable media may include any one of physical media such as electronic, magnetic, optical, electromagnetic, infrared or semiconductor media. Some more specific examples of processor-usable media include, but are not limited to, a portable magnetic computer diskette, such as a floppy diskette, zip disk, hard drive, random-access memory, read only memory, flash memory, cache memory, or other configurations capable of storing programming, data, or other digital information. It wili be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. An apparatus, comprising: a definer configured to define symbol decisions on the basis of predetermined signal samples obtained from a received signal incfuding data and reference symbols; a storage circuitry configured to store the symbol decisions; a determiner configured to determine channei estimates on the basis of the signal samples; a regenerator configured to regenerate a transmitted signal corres- ponding to the received signal by using the stored symbol decisions and the channel estimates; a delayer configured to delay the signal samples; and a subtractor configured to subtract the regenerated signal from the delayed signal samples for reducing effects of interference.

2. The apparatus of claim 1 , the apparatus further comprising; a reception unit configured to receive a signal including data and reference symbols transmitted; and a sampler configured to sample the received signal for obtaining signal samples.

3. The apparatus of any preceding claim, wherein a most recent channel estimate is used.

4. The apparatus of any preceding claim, wherein the channel is estimated after the whitening filter.

5. The apparatus of any preceding claim, wherein the channel esti- mates are filtered by the whitening filter.

6. The apparatus of any preceding claim, the apparatus comprising a module,

7. The apparatus of any preceding claim, the apparatus comprising a network element.

8. A method comprising: defining symbol decisions on the basis of predetermined signal samples obtained from a received signal incfuding data and reference symbols; storing the symbol decisions; determining channel estimates on the basis of the signal sampies; regenerating a transmitted signal corresponding to the received sig- na! by using the stored symbol decisions and the channel estimates; delaying the signal samples; and subtracting the regenerated signal from the delayed signal samples for reducing effects of interference.

9. The method of claim 8, the method further comprising receiving a signal including data and reference symbols; and sampling the received signal for obtaining signal samples.

10. The method of claims 8 or 9, wherein a most recent channel estimate is used.

11. The method of any preceding claim 8 to 10, wherein the determining of channel estimates is carried out after the whitening filter.

12. The method of any preceding claim 8 to 11, further comprising filtering the channel estimates by the whitening filter.

13. A computer program comprising program code means adapted to perform any of steps of claims 8 to 12 when the program is run on a computer or on a processor.

14. A computer-readable medium having stored thereon a data structure, comprising: defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols; storing the symbol decisions; determining channel estimates on the basis of the signal samples; regenerating a transmitted signal corresponding to the received sig- πal by using the stored symbol decisions and the channel estimates; delaying the signaf samples; and subtracting the regenerated signal from the delayed signal samples for reducing effects of interference.

15. The computer-readable medium of claim 14, the method further comprising receiving a signal including data and reference symbols; and sampling the received signal for obtaining signal samples.

16. The method of claims 14 to 15, wherein a most recent channel estimate is used.

17. The method of any preceding claim 14 to 16, wherein the determining of channel estimates is carried out after the whitening filter.

18. The method of any preceding claim 14 to 17, further comprising filtering the channel estimates by the whitening filter.

19. An apparatus comprising: means for defining symbol decisions on the basis of predetermined signal samples obtained from a received signal including data and reference symbols; means for storing the symbol decisions; means for determining channel estimates on the basis of the signal samples; means for regenerating a transmitted signal corresponding to the received signal by using the stored symbol decisions and the channel estimates; means for delaying the signal samples; and means for subtracting the regenerated signal from the delayed sig- nal samples for reducing effects of interference.