WO2007032715A1

WO2007032715A1 - Method using reliability measures corresponding to channel parameters in adaptive scheduling

Info

Publication number: WO2007032715A1
Application number: PCT/SE2005/001352
Authority: WO
Inventors: Stefan Felter; Karl J. Molnar; Leonid Krasny
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2005-09-15
Filing date: 2005-09-15
Publication date: 2007-03-22

Abstract

The present invention relates to a method and arrangement for improving the channel state condition information that is provided between transmitter and receiver units in a wireless communication system. The present invention introduces a statistical measure, e.g. an uncertainty value of a quantity or its prediction value, for parameters that describe a communication channel and define said measure under consideration of possible impacts on said channel. A reliability measure is provided for the communication channel from a transmitter unit to a receiver unit representing an estimation of appropriate channel parameters. The reliability measure is either retrieved at the receiver unit and based on direct channel observations or indirectly retrieved at the transmitter unit based on feedback information or conclusions from the behaviour of the reverse channel. The present invention can be implemented either in a radio base station or in a user equipment of a wireless communication system.

Description

METHOD USING RELIABILITY MEASURES CORRESPONDING TO CHANNEL PARAMETERS IN ADAPTIVE SCHEDULING

FIELD OF THE INVENTION

The present invention relates to a method and arrangements for achieving an improved and adaptive scheduling in telecommunication networks.

BACKGROUND OF THE INVENTION

Future wireless communication systems will offer several and different methods for transmission and reception of data, which will be provided for within the same communication standard. As an example, communication from a system unit to a user equipment in said system can be performed by help of two distinct transmission schemes: MIMO and beamforming.

MIMO (multiple input, multiple output) denotes a transmission approach with a multiple antenna scenario, where both transmitter and receiver use several antennas with separate modulation and demodulation for each antenna and interfering channels with radio links between all pairs of transmitter and receiver antennas. The antennas at each end of the communications circuit are combined to minimize errors and optimize data speed.

In a beamforming transmission approach the transmission power for two or more antennas is concentrated on a more or less narrow section within which a receiver unit is assumed.

In ^"the case of two or more different transmission approaches that could be selected, it can turn out to be efficient to select different transmission schemes depending on various conditions within the system, e.g. with regard to the environment, traffic situation, etc., and adapt the transmission scheme in accordance with changes of said conditions .

Figure 1 shows an illustration of a part of a communication system including, e.g., a radio base station as a transmitter unit 11 and, e.g., a mobile user equipment as a receiver unit 12 communicating with each other via a multipath channel 14. For many radio channels the signal transmitted by the transmitter unit 11 reaches the receiver unit 12 via more than one path. For example, in a mobile communication environment, the transmitted signal is reflected by a variety of scatterers 13 such as buildings, trees, or moving vehicles. Thus, in addition to the direct path 141 from the transmitter to the receiver, there are several other indirect paths 142, arising from the presence of scatterers 13 , that contribute to the composition of the received signal . The contributions from these indirect paths exhibit different signal attenuations and time delays relative to that from the direct path, i.e. they may interfere with the contribution from the direct path either constructively or destructively at the receiver input. The signal components caused by the direct path 141 and the indirect paths 142 are denoted multipath components and illustrated by a complex channel filter function either in the time domain as a channel impulse response or, correspondingly, as a channel frequency response in the frequency domain. The channel impulse response illustrates the contributions of the various multipaths in terms of complex parameters, which can be reproduced, e.g., as an amplitude value and a phase value or any other appropriate representation of complex values. The channel impulse response also denotes the delay times τ, for said various multipaths. Correspondingly, the channel frequency response illustrates the frequency selectivity of the multipath channel . In the time domain, for instance, the channel filter function can be defined as a complex channel impulse response h(τ) , which can be represented by a set of sub- functions h_n(τ) . When applying, e.g., a summing function for modelling of the complete channel impulse response from the various sub-functions this can be expressed in a sufficient approximation as

N

∑h_n(τ) =h(τ) n=0

The set of sub-functions h_n{τ) preferably comprises a kind of ranking with respect to their significance for the- representation of the channel. h_o(τ) , for instance, may include the most essential channel components, e.g. the largest channel component or the first component in time, i.e. the path with the shortest time delay. The accuracy of the function depends on the number N of sub-functions as each sub-function provides additional components of the channel impulse response.

In order to make an appropriate choice of the transmission method it is most necessary to obtain a good knowledge of the channel conditions between the transmitters and receivers. Typically in FDD-systems, the user equipments measure the channel state condition information and provide this information as feedback information back to the transmitting unit. Preferably, this feedback information is delivered sufficiently often and at regular instances of time. It may consist, e.g., of a set of values describing or updating said parameters of the channel impulse response or, in case of insufficient channel state condition information, at least some other kind of feedback information relating to the quality of the communication channel, e.g. by help of a signal-to-interference and/or noise ratio. For all measurements, however, one must keep in mind that such channel measurements are made during one time period but will be used during another, later time period. One approach for mitigating the effects of this delay in using channel information is to apply prediction values of the channel state at the time when it is to be used. However, this still does not solve the problem of uncertainty for said channel state condition information.

SUMbIARY OF THE INVENTION

As future communication systems will allow different methods for transmission and reception of data within the same communication standard, it is highly desirable to be able to always select the best-suited transmission approach for communication.

This, however, involves not only the problem to determine channel conditions sufficiently well but also to retrieve a measure for the reliability of said channel conditions.

Therefore, it is the object of the present invention to achieve a method and arrangement for improving the channel state condition information that is provided between transmitter and receiver units in a communication system.

It is the principal idea of the present invention to introduce a statistical measure, e.g. an uncertainty value of a quantity or its prediction value, for parameters that describe a communication channel and define said measure under consideration of possible impacts on said channel.

This idea is realised by the method and arrangement according to the present invention by providing a reliability measure for the communication channel from a transmitter unit to a receiver unit representing an estimation of appropriate channel parameters . The reliability measure is either retrieved at the receiver unit and based on direct channel observations or indirectly retrieved at the transmitter unit based on feedback information or conclusions from the behaviour of the reverse channel .

When derived by the receiver unit, the reliability measure can be forwarded by means of an information element, which is introduced in an appropriate communication at least between transmitter unit and receiver unit; said information element comprising information on the reliability of the channel or channel prediction information.

The present invention allows for an adaptive scheduling of resources in a communication system in a more controlled and robust way. For instance, signalling of the prediction uncertainty provides a more accurate choice of whether to use an open-loop or close-loop transmission system resulting in a significantly improved RAN-performance .

In open-loop systems the information on channel prediction uncertainty will improve the reliability of choosing the set of antennas to transmit.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

For a better understanding, reference is made to the following drawings and preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a transmitter unit and a receiver unit communicating with each other via a multipath channel.

Figure 2 shows a channel impulse response with uncertainty distribution for one of the sub-functions .

Figure 3 shows a first embodiment of the present invention.

Figure 4 shows a second embodiment of the present invention.

DETAILED DESCRIPTION

The method and arrangement according to the present invention intends to improve the reliability of information regarding the communication channels between transmitting and receiving units . Within the scope of the present invention transmitting units can be either included in the radio base station or in a mobile user equipment and, accordingly, the receiving units can be included either in mobile user equipments or the radio base stations, respectively. For a radio base station including the transmitting unit the present invention thus addresses the downlink as the communication channel for which a reliability metric of the channel information shall be improved. Accordingly, for a mobile user equipment including the transmitting unit this communication channel refers to the uplink.

As stated above, the reliability of information regarding a communication channel addresses the underlying problem that various kinds of channel state condition information is measured at one certain instance of time but used at another, later instance of time. Assuming that an appropriate channel model is used and assuming a not too high level of thermal noise, this approach would provide reliable information only for time-invariant systems . However, it is understandable that air-borne communication channels are impacted by various influences making them a highly time-variant system. Such influences relate, e.g., to environmental conditions such as the terrain shape or to weather conditions impacting the propagation between transmitter and receiver. This impact is even strengthened by the behaviour of the user equipments using said communication channels, partly due to their position within said environment but also due to higher derivates of their movement pattern, e.g. their speed. It is neither possible to solve this principal problem by help of prediction values for the channel state parameters as the quality of the predication values is also impacted by the time-variant system and, thus, suffers the same lack of reliability.

The present invention therefore suggests introducing a reliability measure for one or more parameters that describe the communication channel between a transmitter and a receiver unit, i.e. parameters relating to the channel state condition information or other quality measures that can be derived for said communication channel. In principal, there are two basic embodiments to retrieve the reliability measure: According to a first embodiment of the present invention it is the receiving unit which determines channel parameters for the communication channel from which it receives data from the transmitter unit, calculates the reliability measure representing the uncertainty of the channel parameter estimation, and reports said measure back to the transmitter unit. According to a second embodiment of the present invention, the transmitter unit concludes the reliability measure for the communication channel to a receiver unit, e.g. by analysing received feedback information or appropriate signal patterns that are received at known instances of time from the receiver unit. Finally, yet another, preferred, embodiment of the present invention is a combination of the first and second embodiment, which thus uses both information on the reliability measure as reported by the receiving unit and as concluded at the transmitter unit.

The transmitter unit can then apply the retrieved reliability measure as one input information, e.g., to decide on the appropriate transmission scheme for data transmission to a specific receiver unit. Other relevant information that the transmitter unit takes into account include, e.g., the expectation value for said communication channel. The selection of a transmission scheme is influenced partly by the degree of reliability in general but also in view of specific parameters, e.g. how specifically the location and/or behaviour of the user equipment can be determined.

The reliability measure ' as introduced above can thus be regarded as a prediction of the uncertainty for certain channel parameters. The following denotes several alternatives to derive such a reliability measure:

According to one conceivable alternative the reliability measure for a selection of at least some distinct channel parameters can be expressed by help of a distribution function for said parameters or an appropriate representation of such a distribution function. The distribution function thus illustrates the range of the various prediction values for said selected parameters . Figure 2 for instance shows on two axes, Re{/z(r)} and Im{h(τ)} , the complex channel impulse response h{τ) , whereby the complex representation allows distinguishing magnitude and phase value of the complex channel filter taps h_x(τ) and h_n{τ) , and on the other axis the delay time r for said parameters \(τ) and h_n{τ) . In said figure the reliability- measure is illustrated, e.g., for the parameter H₁(T) of the channel impulse response by a distribution with regard to the delay time τ and/or the distribution of the representations of the complex parameter /J₁(V) . The distribution of the prediction values Ji₁(T) thus forms an uncertainty space around the true value and time instance of \(τ) . This distribution is illustrated in figure 2 by denoting the intervals for the distribution of \(τ) along the Re{&(r)}-axis 21, along the Im{/z(r)}~axis 22, and along the T -axis 23. The distribution functions within these intervals along the various axes can be further described by certain key characteris.ing parameters depending on the complexity- of . said function. When assuming, e.g., a Gaussian distribution. for the channel filter parameters h,(τ) and assuming a ^' correct estimation of the expectation value, the . distribution could be represented by a value for the standard deviation σ,-². Accordingly, an uncertainty space could also be imagined around the other parameters h_n(τ) .

Another alternative for deriving a reliability measure provides a measurement of the channel prediction error. As explained above, the reliability of the channel state condition information is dependent on measurement errors and delays in using a time-varying, fading, and dispersive channel. One approach for mitigating the effects of delay in using the channel state condition information is to predict the channel state at the time when it is used. A decision, e.g., about which transmission technique to use in the communication between two network units is then advantageously not only based on the estimate of the channel state condition information but also on its reliability in view of the physical radio channel properties, e.g., as mentioned above. Thus, a measure of the magnitude of the channel prediction error can be used as additional information for improving the channel state condition information. The reliability measure is in that case a distribution function of the deviations of the prediction values .

It is yet another alternative to apply an appropriate channel measurement as reliability measure, either instead of measurements on the channel impulse response or, preferably, to establish an initial reliability measure, which can be refined later, e.g. by help of the reliability measures as mentioned in the embodiments above. Such radio channel measurements can be, e.g., a signal-to-noise and/or interference measure (SNIR) . Another measure can relate to a Doppler frequency measurement for the parameters, which can indicate a relation to the speed of a moving user equipment and thus indicate the appropriate transmission method that should be selected. In a simple approach it would also ,be possible to assume, as a starting point, that the communication channels in both directions, i.e. downlink or uplink, have the same radio characteristics and apply this for determining a reliability measure.

According to the first embodiment of the present invention it is the receiver unit that derives the reliability measure as described above. Then, in a next step it is suggested that the receiver unit introduces an information element in an appropriate communication between receiver and transmitter unit for transmitting the derived reliability information. Said information element must be defined with regard to its content, i.e. the kind of information to be sent, and the circumstances when and how to send said information. Preferably, the information element is included in an already existing message protocol between receiver and transmitter unit. Said element may be sent either periodically or, alternatively, on demand of the initiating transmitter unit or another hierarchically higher network unit. Yet another alternative is to demand such information in case of a certain parameter passing a threshold value, i.e. indicating a considerable change in one of the radio conditions .

Regarding the kind of information to be sent, the state of the art discloses channel prediction uncertainty that is described by the prediction error variance. However, with regard to dispersive channels not only the flat fading channel should be considered as there might be multiple channel taps that can potentially be used for multi-antenna beamforming. For example, in a TDCF concept,, there may be N /measured channel taps at the mobile unit but only;, M<N; • taps

•would actually be used for beamforming. Consequently,,_.; there are a number of options to characterise the prediction

^■ error. These are the error variance, or standard .variance, for each measured channel tap, the sum of the error variance of the measured channel taps, or the maximum error variance of the channel taps. Of course, instead of reporting the above information for all channel taps the information could be restricted to the best M channel taps .

In an advantageous alternative, the information element combines the reliability measure that is to be transmitted with one or more other appropriate channel related parameter for estimating the channel and the uncertainty of said estimation.

When considering how often said information is reported, it is one issue where the channel prediction is to be performed. If the mobile terminal performs the prediction it can compute the prediction error information and transfer only the necessary information back to the base station. Alternatively, if the base station performs the prediction, the relevant information, i.e. the channel state, must be reported back to the base station. This means that the relative frequency of reporting this information is dependent on the specific architecture. Additionally, the frequency might be governed by the underlying propagation conditions. For instance, when assuming a slow fading the reporting can be made less often. The following provides several conceivable alternatives for reporting:

The transport mechanism for sending said information element can be done, e.g., along the following mechanisms: The information can be represented either as absolute values or differential updates of the prediction error. The information can be sent in a coded or uncoded manner.

According to a second embodiment of the present invention it is the transmitter unit that derives the reliability measure for the communication channel to a receiver unit. This can be done, e.g. by analysing feedback information from the receiver unit or appropriate signal patterns, e.g. pilot signals, that are received at known instances of time from the receiver unit. As the transmitter unit is interested in the reliability of the communication channel to a receiver unit but is only able to measure on the communication channel in the reverse direction, i.e. from said receiver unit, it is necessary to apply suitable models comprising assumptions for the communication channels in both directions such that the behaviour of the communication channel from the receiver unit with regard to such received signal patterns can be transformed into a comparable behaviour as it would be experienced on the reverse communication channel. Then, the transmitter unit can conclude a prediction value for one or more channel parameters in order to indirectly determine a reliability measure for the communication channel to the receiver unit .

According to a preferred embodiment of the present invention a transmitter unit is able to make use of both the reliability measure as reported from receiver units and as concluded from measurements of the reverse communication channel. This can be done, e.g., by means of combining the reliability measures as retrieved above.

Claims

1. A method in a first network unit (311) of a communication system (10) , said first unit (311) comprising means for receiving data on a first communication channel (321) from at least a second network unit (312) of the communication system (10) and means for transmitting data on a second communication channel (322) to said second unit (312),

c h a r a c t e r i s e d b y

determining a reliability measure for one or more channel parameter predictions describing the first communication channel (321) ,

introducing an information element in a communication protocol message on the second communication channel (322), said information element comprising a representation of said reliability measure.

2. The method according to claim 1, whereby the reliability measure consists of a distribution function for one or more predicted channel parameters .

3. The method according to claim 2, whereby said one or more channel parameters are selected from the channel impulse response .

4. The method according to claim 2 or 3 , whereby said distribution function can be represented by a value of its standard deviation.

5. The method according to claim 1, whereby the reliability measure includes the channel prediction error for the first communication channel (321) .

6. The method according to one of claims 1-5, whereby the reliability measure includes a measure of the signal-to- noise and/or interference ratio for the first communication channel (321) and a measure of the Doppler frequency for said parameters .

7. The method according to claim 6 , whereby said measures of the signal-to-noise and/or interference and the Doppler frequency are applied to achieve initial values for the reliability measure.

8. The method according to claim 6, whereby the reliability measure includes a prediction of a movement pattern for the first network unit (311) .

9. The method according to claim 1, whereby said information element is transmitted periodically.

10. The method according to claim 1, whereby said information element is transmitted on demand.

11. The method according to claim 1, whereby said information element is transmitted on occurrence of passing a threshold for a certain parameter.

12. The method according to claim 1, whereby said second network unit (312) determines on reception of said information element the transmission scheme to be used for data transmission to said first network unit (311) .

13. The method according to one of claims 1-12, whereby the reliability measure is combined with one or more other appropriate channel related parameter for estimating the channel and the uncertainty of said estimation.

14. The method according to one of claims 1-13, whereby the first network unit (311) is a mobile user equipment and the second network unit (312) is a radio base station.

15. The method according to one of claims 1-13, whereby the first network unit (311) is a radio base station and the second network unit (312) is a mobile user equipment.

16. A method in a first network unit (411) of a communication system (10) , said first unit (411) comprising means for receiving data on a first communication channel

(421) from at least a second network unit (412) of said communication system (10) and means for transmitting data on a second communication channel (422) to at least said second network unit (412),

c h a r a c t e r i s e d b y

deriving expected values for one or more channel parameter predictions describing the first communication channel (421),

retrieving, based ^■ on said expected values, a reliability measure for the second communication channel (422) ,

determining, depending on said reliability measure and expected value, a transmission scheme for reliably transmitting data on said second communication channel (422) .

17. The method according to claim 16, whereby the first network unit (411) retrieves a reliability measure from an information element, which is included in a communication protocol message received on said first communication channel (421) .

18. The method according to claim 16, whereby the first network unit (411) retrieves a reliability measure by

measuring signal patterns of a known time relation that are received on the first communication channel (421) , transforming the measured signal patterns by a channel model representing the second communication channel (422) ,

predicting from the transformed signal patterns one or more channel parameters for determining a reliability measure for the second communication channel (422) .

19. The method according to claim 18, whereby the first network unit (411) retrieves a measure of the signal-to- noise and/or interference ratio for the first communication channel (421) and transforms this value into a corresponding value for the second communication channel (422) .

20. The method according to claim 16 and 17, whereby the first network unit (411) derives a combined reliability measure from the reliability measures according to claim 16 and 17.

21. The method according to claim 20, whereby the first network unit (411) retrieves a combined reliability measure by means of a weighted combination of said reliability ■ measures .

22. The method according to claim 16, whereby the first network unit (411) is a radio base station and the second network unit (412) is a mobile user equipment.

23. The method according to claim 16, whereby the first network unit (411) is a mobile user equipment and the second network unit (412) is a radio base station.

24. A first network unit (311) in a communication network (10) , said first unit (311) comprising means (3111) for receiving data on a first communication channel (321) from at least a second network unit (312) in said network (10),

c h a r a c t e r i s e d i n means (3112) for determining a reliability measure for one or more channel parameter predictions describing the first communication channel (321) ,

means (3113) for providing an information element comprising a representation of said reliability measure,

means (3114) for including said information element in a communication protocol message on the second communication channel (322) .

25. The first network unit according to claim 24, whereby said network unit (311) consists of a mobile user equipment.

26. The first network unit according to claim 24, whereby said network unit (311) consists of a radio base station.

27. A first network unit (411) of a communication system (10), said first unit (411) comprising means (4111) for receiving data on a first communication channel (421) from at least a second network unit (412) of said communication system (10) and means (4112) for transmitting data on a second communication channel (422) to at least said second network unit (412) ,

c h a r a c t e r i s e d i n

means (4113) for deriving an expected value for one or more channel parameter predictions describing the first communication channel (421) and means (4114) for retrieving a reliability measure for the second communication channel (422),

means (4115) for determining, depending on said reliability measure and expected value, a transmission scheme for reliably transmitting data on said second communication channel (422) .

28. The first network unit according to claim 27, whereby said first network unit (411) consists of a mobile user equipment .

29. The first network unit according to claim 27, whereby the first network unit (411) consists of a radio base station.