CHANNEL CAPACITY

TECHNICAL FIELD The invention relates to reference signals used in wireless communication systems. More particularly, the invention relates to a method, computer program and computer program product for assessing reference signal length as well as to reference signal length assessment devices. BACKGROUND

In mobile or cellular communication systems downlink (DL) pilots, or reference signals (RS) of predefined and known characteristics are regularly transmitted by the infrastructure access points (AP) or base stations (BS). The RS:s are used (e.g. measured) by both idle and active mobile stations or user equipment (UE), for the purpose of mobility measurements, cell association, and as reference for channel state information (CSI) estimation and data demodulation, or supporting channel state dependent scheduling algorithms . For example, in systems such as Long Term Evolution (LTE) systems, some of the reference signals are called cell specific reference signals (CRSs). CRSs have a predefined pattern that covers the entire frequency band and are transmitted four times per millisecond (assuming two antenna ports).

It is also possible that predefined pilots or reference signals are

transmitted by mobile stations or user equipment (UE) to the network node (uplink, UL). In this case, the wireless AP or BS estimates the uplink channel and assumes channel reciprocity to estimate the DL channel as well. This method is usable in time division duplex (TDD) systems, where the switching between UL and DL transmissions happens within the coherence interval of the channel. UL or DL reference signals are thus useful both in frequency division duplex (FDD) and TDD systems for channel state information acquisition both at the wireless transmitter and receiver nodes. In both cases (DL or UL reference symbols or pilot based channel estimation), the number of reference symbols, the transmit power used for reference signal transmission and consequently the total transmit energy spent on reference or pilot symbols represent a fundamental trade off. Although the problem of determining the power ratio between the reference symbols and data symbols is well known, existing technologies fail to provide a mechanism that can maximize the channel capacity for transmission over uncertain channels with channel state information (CSI) at the transmitter and the receiver (CSIT/CSIR). While the problem of single or multiple input multiple output (SIMO or MIMO) communication over a channel known at the transmitter and receiver is well understood, the problem of efficient use of reference signal symbols and reference signal power is not addressed by existing CSI acquisition techniques. For example, in current Long Term Evolution (LTE) systems, the UE transmits demodulation reference symbols (DMRS) when transmitting data in the uplink. The number and power of the DMRS symbols are standardized and not necessarily optimal in all scenarios. This means that if for instance the reference signal length is extended far beyond what is necessary for channel estimation, it is clear that the data transmission capacity is reduced because too much emphasis is focused on reference signal transmission. If on the other hands the reference signal length is too short, a substandard channel estimation maybe the result, which may in turn lead to undesirable retransmissions being required. There has been some work on SIMO and MIMO communication in the context of uncertain channel state information as for instance is evident in "Capacity of multi-antenna Gaussian channels", by I. E. Telatar, European Transactions on Telecommunications, Vol. 10, pp. 585-595, Nov 1999. This paper studied the problem of communication over uncertain Gaussian channels under the assumption that the channel realization is available at the receiver but not the transmitter. However, in practice, the wireless transmitter and receiver do not have full knowledge of the channel. A channel estimation extension of this paper was presented by B. Hassibi and B. M. Hochwald in "How much training is needed in multiple-antenna wireless links?", IEEE Transactions on Information Theory, Vol. 49 , pp. 951 - 963, April 2003, where the problem of power ratio between the training and data symbols was studied under average and per symbol power constraints over the channel coherence time (the time where the channel is roughly constant). The crucial assumption of average power constraint allows for spending only one symbol on training, since the power is only limited by the total power resource available during the coherence time. For the case of per symbol power constraints, the power ratio is harder to determine.

The problem with the above-described solutions is that the number of reference signal symbols and their transmit power level and thereby the total energy spent on transmitting pilot symbols and their ratio to the energy spent on data transmission are not considered in any cohesive way, which may lead to a substandard system capacity usage

There is thus a need for improving the system capacity usage and then especially the system capacity usage with regard to reference signal length. SUMMARY

The invention is therefore directed towards improving system capacity usage with regard to reference signal length.

This object is according to a first aspect achieved by a reference signal length assessment device for a wireless communication network. The device comprises a processor acting on computer instructions through which the reference signal length assessment device is operative to:

obtain a channel covariance of a communication channel between a transmitter and a receiver,

obtain a resource block length T used in the communication channel, obtain a power constraint P, and

determine a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity C(T _T ).

At least one of the receiver and transmitter is a part of the wireless communication network. Furthermore, the power constraint is a constraint concerning the power used per symbol transmitted in the communication channel and the function is a function of the channel covariance, the power constraint P, the resource block length T and a reference signal length Τ _τ .

The object is according to a second aspect achieved through a method of assessing the length of a reference signal to be used in a communication channel between a transmitter and a receiver. The method is performed in a reference signal length assessment device and comprises:

obtaining a channel covariance of the communication channel, obtaining a resource block length T used in the communication channel, obtaining a power constraint P, and

determining a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity C(T _T ), In the method at least one of the receiver and transmitter is a part of the wireless communication network, the power constraint is a constraint concerning the power used per symbol transmitted in the communication channel and the function is a function of the channel covariance, the power constraint P, the resource block length T and a reference signal length Τ _τ .

The object is according to a third aspect achieved through a reference signal length assessment device for a wireless communication network comprising:

means for obtaining a channel covariance of a communication channel between a transmitter and a receiver ,

means for obtaining a resource block length T used in the communication channel,

means for obtaining a power constraint P, and

means for determining a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity C(T _T ).

At least one of the receiver and transmitter is a part of the wireless communication network. Furthermore, the power constraint is a constraint concerning the power used per symbol transmitted in the communication channel and the function is a function of the channel covariance, the power constraint P, the resource block length T and a reference signal length Τ _τ .

The object is according to a fourth aspect achieved through a computer program for assessing the length of a reference signal to be used in a communication channel between a transmitter and a receiver, where at least one of the transmitter and receiver is a part of a wireless

communication network. The computer program comprises computer program code which when run in a reference signal length assessment device, causes the reference signal length assessment device to:

obtain a channel covariance of the communication channel,

obtain a resource block length T used in the communication channel, obtain a power constraint P, and

determine a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity C(T _T ).

Furthermore, in the computer program the power constraint is a constraint concerning the power used per symbol transmitted in the communication channel and the function is a function of the channel covariance, the power constraint P, the resource block length T and a reference signal length Τ _τ . The object is according to a fifth aspect achieved through a computer program product for assessing the length of a reference signal used in a communication channel between a transmitter and a receiver, where at least one of the transmitter and receiver is a part of a wireless

communication network. The computer program product comprises a data carrier with computer program code according to the fourth aspect.

In a first variation of the first aspect, the reference signal length

assessment device when determining a number of reference signal symbols is further configured to optimize the function of the channel capacity

C(Tx).

In a corresponding variation of the second aspect the determining of the number of reference signal symbols to be used comprises optimizing the function C(T _T ) of the channel capacity.

In a second variation of the first and second aspects, the covariance is expressed in the form of a covariance matrix C and the function C(T _T ) is a function of the difference between the resource block length T and the reference signal length Τ _τ and an expression comprising the covariance matrix C as well as an inverse matrix comprising elements of the covariance matrix. In a third variation of the first and second aspect, the capacity C(T _T ) is determined as

C{T _T ) - (T - Γ _τ )(1ο¾ \PC + I _m \ - Iog ₂ |PC + I _m \) with

where I _m is an identity matrix.

In a fourth variation of the first and second aspect, the covariance is valid during a coherence time interval and the resource block length T is shorter than or equal to the coherence time interval. In a fifth variation of the first aspect, the reference signal length

assessment device is further configured to provide determined number of reference signal symbols to the transmitter for setting the reference signal length used in the transmission to the receiver. In a corresponding variation of the second aspect, the method further comprises providing the determined number of reference signal symbols to the transmitter for setting the reference signal length used in the transmission to the receiver. In a sixth variation of the first aspect, the reference signal length assessment device when obtaining the power constraint is configured to determine the power constraint P based on the maximum transmit power PT_MAX of and the number of symbols simultaneously transmitted by the transmitter.

In a corresponding variation of the second aspect, the obtaining of the power constraint comprises determining the power constraint P based on the maximum transmit power PT_MAX of and the number of symbols simultaneously transmitted by the transmitter. The reference signal length assessment device may comprise the transmitter. Alternatively it may comprise the receiver. It may also comprise the channel estimator.

The reference signal length assessment device may also be physically placed in different locations. It may be provided in a mobile station.

Alternatively it may be located in the wireless communication network, like in an access node such as a base station or a core network node.

However, it may just as well be provided in a completely different environment, such as in a cloud computing environment with which a wireless communication network node or the mobile station

communicates. The invention has a number of advantages. It improves the wireless channel capacity and thereby helps to improve the overall system capacity and spectral efficiency. In this way it is possible to obtain a resource use that is considerably improved with regard to reference signal length.

Thereby the number of reference signals used may be a number that provides a better compromise between channel estimation quality and channel data usage compared to earlier. The invention provides a robust way of determining an improved power allocation of pilots or reference signals in order to improve the data rate with a given channel covariance, per symbol power constraint, and resource block-length.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in relation to the enclosed drawings, in which: fig. l schematically shows a wireless communication network comprising a base stations that communicates with a mobile station,

fig.2 schematically shows a reference signal interval of a radio resource used for communication between the base station and the mobile station, fig. 3 shows a block schematic of a first realization of a reference signal length assessment device,

fig. 4 shows a block schematic of a second realization of a reference signal length assessment device,

fig. 5 shows a flow chart of a number of method steps being performed in the reference signal length assessment device in a first embodiment of a method for assessing the length of a reference signal,

fig. 6 shows a flow chart of a number of method steps being performed in the reference signal length assessment device in a second embodiment of a method for assessing the length of a reference signal, and

fig. 7 shows a computer program product comprising a data carrier with computer program code for implementing functionality of the reference signal length assessment device.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the invention with unnecessary detail.

The present invention concerns the determination of a number of reference signal symbols to be used in the estimation of a communication channel in a wireless communication network, where the wireless communication network may for instance be a mobile communication network like a Long-Term Evolution (LTE), Universal Mobile

Telecommunications System (UMTS) and Global System for Mobile Communications (GSM). These are just a few examples of networks where the invention may be implemented. Other types of networks that may be used are Wireless Local Area Networks (WLAN).

Fig. l schematically shows a wireless communication network, which may be a network according to any of the above described types. Furthermore, the exemplifying communication network is in this case a mobile communication network MN 10 comprising a base station BS 14 providing coverage of a cell Ci 12.

In fig. 1 there is also shown a mobile station MS 16 which is indicated as being located within the cell Ci. Because of this the mobile station 12, which in many systems is also termed user equipment (UE) is able to communicate with the base station 14. It may more particularly

communicate in an uplink communication channel ULC and a downlink communication channel DLC, where either may be part of a resource assigned to communication between the base station 14 and the mobile station 16. A resource may in this respect be a time slot. It may also be a time slot and a frequency.

The base station 14, which is often termed eNodeB or just NodeB, is furthermore provided in a part of the mobile communication network 10 termed access network or radio access network. In the mobile

communication network 10 there may also be a core network. This is not central to the various aspects to be described and has therefore been omitted.

Fig. 2 schematically shows a resource RES assigned for communication between the base station 14 and the mobile station 16. The resource RES is in this example a resource assigned as the downlink communication channel DLC, which may thus be a time slot or a time slot and a frequency. In this resource RES, the base station 14 transmits a reference signal, which in the downlink is termed a pilot signal PS. It can be seen that in fig.2 the pilot signal PS is only transmitted in a limited part of the resource RES, which is a reference signal interval RSI. The rest of the resource is used for data, which data is often some sort of content that a user of the mobile station is interested in receiving or delivering. It should here be realized that there may be a similar interval RSI used in the uplink.

However, in this case the signal sent may be termed reference signal. It should also be realized that a pilot signal is a type of reference signal.

Aspects of the invention are directed towards assessment of the length of the reference or pilot signal used in such a resource. Because of this, there is provided a reference signal length assessment device, which may be provided as a part of the mobile station, the base station or even as a part of some other entity which supports the mobile station 16 or the base station 14 and which the base station or mobile station therefore

communicates with in order to perform reference signal length

assessment. In the case of performing reference signal length assessment for a base station, the reference signal length assessment device may be provided in another wireless communication network node such as in another access network node such as in another base station or a base station controller. It may also be provided in a core network node.

Alternatively, either if a mobile station or a base station is supported, the reference signal length assessment device may be provided as a part of a cloud computing environment, such as a part of processing blade of a computer centre.

Fig. 3 shows a block schematic of a first way of realizing the reference signal length assessment device RSLAD 18. It maybe provided in the form of a processor PR 20 connected to a program memory M 22. The program memory 22 may comprise a number of computer instructions

implementing the functionality of the reference signal length assessment device 18 and the processor 20 implements this functionality when acting on these instructions. It can thus be seen that the combination of processor 20 and memory 22 provides the reference signal length assessment device 18. As mentioned above this device maybe a part of a mobile station, a base station or a circuit blade in a cloud computing environment.

Fig. 4 shows a block schematic of a second way of realizing the reference signal length assessment device RSLAD 18, which may specifically be realized in a mobile station or a wireless access point such as the base station. The reference signal length assessment device 18 may comprise a reference signal length assessment unit RSLAU 22, an optional power constraint determining unit 24, an optional channel estimator 26 and a wireless interface WI 28. The reference signal length assessment unit 22 is connected to the wireless interface 28, while the power constraint determining unit 24 and the channel estimator 26 are each connected between the wireless interface 28 and the reference signal length assessment unit 22. The wireless interface 28 does typically comprise a radio circuit and therefore comprises transceiver circuitry. This is in fig. 3 indicated through the inclusion of a receiver RE 30 and a transmitter TR 32, where the receiver 30 is a radio receiver configured to communicate with another entity via one or more communication channels and the transmitter 32 is configured to communicate with another entity, with advantage the same entity over one or more other communication channels. It should here be realized that the reference signal length assessment device 18 may be connected to one or more antennas.

The units and blocks in fig. 4 may be provided as software blocks for instance as software block in a program memory, but also as a part of dedicated special purpose circuits, such as Application Specific Integrated Circuits (ASICs) and Field-Programmable Gate Arrays (FPGAs). It is also possible to combine more than one unit or block in such a circuit. In the example in fig. 1, the mobile station 16 is communicating with the base station 14, either in the uplink over the uplink communication channel ULC or in the downlink over the downlink communication channel DLC or both. In the case of the downlink communication a transmitter TR in the base station 14 transmits in the downlink channel DLC, which transmissions are received by a receiver in the mobile station 16. In the case of uplink transmission a transmitter TR in the mobile station 16 transmits to the base station in the uplink channel ULC, which transmissions are received by a receiver RE in the base station 14. In both cases, in uplink or downlink, communication is carried out via an assigned resource RES on which the communication channel is implemented. Such a resource is furthermore typically assigned by the base station 14. In some systems another node may take care of the resource assignment. It should also be realized that it is possible that several resources are used

simultaneously in the downlink as well as uplink between the mobile station 16 and the base station 14.

As is well-known, in order to communicate with an acceptable quality, there is performed a channel estimation by a channel estimator both with regard to uplink and downlink communication. In channel estimation the influences of the environment such as fading, interference from other communication channels etc. are considered, Furthermore, the reference signals are used as input to these channel estimations. This is as such well- known. The reference signals are made up of a sequence of known symbols. The properties of the channel can then be estimated based on how the known symbols of a reference signal sequence are changed, where the quality of the estimation increases with the number of symbols used. In short, the more reference signal symbols that are used, the better the channel estimation is.

However, the more reference signal symbols that are transmitted in a resource, the more the available amount of useful data is reduced. This can readily be seen in fig.2. The length of the pilot signal PS and thus the size of the reference signal interval RSI increases if the number of reference signal symbols increase. It can also be seen that if the reference signal interval RSI increases then the space left in the resource RES for useful data decreases.

Furthermore, the transmission requires power. This also means that there is a substantial amount of power used when transmitting a reference signal. Power is, especially in a mobile station, often a limited resource. The consequences of this can be that an unjustifiable amount of power and bandwidth is used for transmitting of reference signals.

On the other hand, the use of too few reference signal symbols is also undesirable. This will lead to a substandard channel estimation, which in turn may lead to loss of data and unnecessary retransmissions being made. This situation may thus also lead to a waste of power and resources.

There is therefore in view of what is mentioned above, a need for better usage of reference signal symbols. Aspects of the invention address this problem. A first embodiment will now be described with reference being made also to fig. 5, which shows a flow chart of method steps being performed in a method of assessing the length of a reference signal to be used in a communication channel, where the method steps are being performed by the reference signal length assessment device 18, which device may thus be implemented in the base station 14 or the mobile station 16. For ease of understanding the invention, it is furthermore assumed that the reference signal length assessment device 18 performing the method is the device in fig. 3, which is placed at one end of the communication channel, either in the base station 14 or in the mobile station 16.

Aspects of the invention are applicable to communication over uncertain channels with channel state feedback at the transmitter, where the receiver at the receiving end of the communication channel makes an estimate of the channel by using a known sequence of training symbols. This channel estimate is then transmitted back to the transmitter. For example, in the mobile communication network, the mobile station 16 can feedback the channel estimate based on the measurement of reference symbols transmitted by the base station 14. The channel estimate is furthermore based on a covariance, which covariance with advantage may be expressed as a covariance vector, possibly together with a noise vector. The communication channel may furthermore be a slowly varying channel. In this case the values of a covariance matrix may be valid in a channel coherence time interval, after which a new estimation needs to be done and consequently a new covariance determined.

The method starts with the reference signal length assessment unit 22 obtaining a channel covariance of the communication channel for which the reference signal symbols are being assessed, step 34. A channel covariance may be provided by a channel estimator. As an example the communication channel may be the downlink communication channel DLC being provided via the resource RES. In this example the transmitter of the channel would be provided in the base station 14 and the receiver in the mobile station 16. Furthermore, in this case the channel covariance could be provided by a channel estimator in the mobile station 16. In case the reference signal length assessment unit 22 is provided in the mobile station 16, the channel estimator 26 would be this channel estimator used for estimating the communication channel. Consequently this unit 26 could then directly send the channel covariance to the reference signal length assessment unit 22. In case the reference signal length assessment device 18 would be provided at the base station 14 instead, then the channel covariance may need to be sent from the channel estimator in the mobile station 16 to the receiver 30 of the wireless interface 26, and forwarded therefrom to the referenced signal length assessment unit 22. Furthermore, a channel estimation is normally performed for both the uplink channel and the downlink channel, knowledge about the

corresponding channel covariance may already exist at both ends of the communication channel. Furthermore, the channel covariance maybe determined by a channel estimator using known techniques. The channel covariance may additionally be changing slowly. The reference signal length assessment unit 22 also obtains a resource block length T, step 36. This length is typically determined by the base station 14 and may be provided by conventional radio circuits of the wireless interface of the base station 14. In case the reference signal length assessment device 18 is located in the base station 14, then the wireless interface 28 may determine this length and provide the information directly to the reference signal length assessment unit 22. In case the reference signal length assessment device 18 is a part of the mobile station 16, then the information may be received from the base station 14 and the receiver 30 of the wireless interface 28. It is also possible that the resource block length T is known on both sides of the communication channel, i.e. in both the base station 14 and the mobile station 16, for instance in relation to the setting up of the communication channel. It is furthermore possible that the length is system specific and defined beforehand. In both of the latter mentioned cases, it is thus possible that knowledge of the resource block length T is known by the reference signal length assessment device 18 itself. Typically, the block length T is also much shorter than the estimated coherence time of the channel. However, it may at least be less than or equal to the (estimated) channel coherence time.

The reference signal length assessment unit 22 also obtains a power constraint P, step 38. The power constraint P maybe a constraint concerning the power used per symbol transmitted in the communication channel. The power constraint P may thus be a measure of the amount of power required for transmitting a symbol. The power constraint may be determined by the power constraint determining unit 24. Alternatively it may be determined by the device at the other end of the communication channel and received therefrom via the same or another communication channel and the receiver 30 of the wireless interface. One possible way to determine the power constraint is through calculating it based on the maximum transmit power of the base station 14 and the number of simultaneously transmitted symbols, for example the number of subcarriers used in an orthogonal frequency division system. In the example of the downlink channel, the power constraint P may with advantage be determined by the base station 14 and if the reference signal length assessment device 18 is provided in the mobile station 16, then the constraint may be transmitted from the base station 14 to the mobile station 16. Alternatively, a power constraint determining unit 24 in the mobile station 16 may determine the power constraint, in which case the base station 14 may provide the information needed for this

determination. After this has been done the reference signal length assessment unit 22 determines a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity C(T _T ), step 40, which function is a function of the channel covariance, the power constraint P, the resource block length T and the reference signal length Τ _τ . The function may optionally also comprise a noise term. As the only variable that has not been set is the reference signal length, the

determining may be performed through investigating this function with regard to different values of the reference signal length. The function may with advantage be a function of the difference between the resource block length T and the reference signal length Τ _τ and an expression comprising the covariance, for instance in the form of a covariance matrix C. The function may with advantage be a function where the above-mentioned difference is multiplied with an expression involving the covariance. When the covariance is provided as a covariance matrix, the expression may furthermore involve the covariance matrix as well as an inverse matrix comprising elements of the covariance matrix.

It is here furthermore possible that the determining of the symbols comprises optimizing the function, for instance maximising the function with regard to the reference signal length. The optimising may as an example be performed through obtaining a derivative of the function with regard to the reference signal length, i.e. dC(T _T )/dT _T and selecting a length corresponding to a zero value thus represents a maximum. The reference signal symbol number corresponding to this optimization may then be determined to be the number of reference signal symbols to be used in channel estimation.

In case the transmitter 32 is the transmitter of the channel, the number is then forwarded to it for being used in transmitting reference signals on the communication channel. However, in case the transmitter of the channel is at the other end of the channel, which is the case for the downlink channel DLC and the reference signal length assessment device 18 placed in the mobile station 16, then the number is transmitted to the device at the opposite end via the transmitter 32, which in the example thus means that the number is transmitted from the mobile station 16 to the base station 14.

The channel capacity that the transceiver maximizes may be the worst case capacity, in the sense that given a noise covariance, the device 18 maximizes the minimal capacity over all distributions of the measurement noise under a fixed noise covariance matrix known at both the transmitter and receiver. The main advantage of the proposed solution is that it improves the wireless channel capacity and thereby helps to improve the overall system capacity and spectral efficiency.

In this way it is possible to obtain a resource use that is considerably improved with regard to reference signal length. Thereby the number of reference signals used may be a number that provides a better compromise between channel estimation quality and channel data usage compared to earlier. The invention provides a robust method that determines an improved power allocation of pilots or reference signals in order to improve the data rate with a given channel covariance, per symbol power constraint, and block-length. The method provides a robust way of determining the data/pilot power ratio in order to guarantee a data rate for transmission over uncertain channels where feedback of channel state information is provided from the receiver to the transmitter.

Now a second embodiment will be described with reference being made to fig. 6, which shows a flow chart of a method for assessing the length of a reference signal to be used in a communication channel and being performed in the reference signal length assessment device 18. In this case the mobile communication system may be an LTE system employing

OFDM. Just as in the first embodiment, the reference signal length assessment device 18 operates in relation to one end of the communication channel that is provided between the base station 14 and the mobile station 16. It may thus assess the reference signal length in respect of either a base station or a mobile station, as well as in respect of an uplink channel or in respect of a downlink channel.

The method yet again starts with the reference signal length assessment unit 22 obtaining the channel covariance of the communication channel, step 42. In this example the covariance is expressed as a covariance vector C. Also in this case the covariance vector C maybe received from the local channel estimator 26 or from radio circuits at the other end of the communication channel. The reference signal length assessment unit 22 also obtains the resource block length T, step 44. The block length T may be set by the base station 14 and this setting may also be known by the mobile station 16, in which case it may also be known by the wireless interface 28 of the reference signal length assessment device 18. As an alternative the block length T may be received by the reference signal length assessment device 18 via the wireless interface 28.

In case the reference signal length assessment device 18 is provided in the base station 14, then the wireless interface 28 may determine this length and provide the information directly to the reference signal length assessment unit 22. In case the reference signal assessment unit is a part of the mobile station 16, the information maybe received from the base station 14 via the receiver 30 of the wireless interface 28. In any event the resource block length T is shorter than or equal to the coherence time interval and with advantage substantially shorter. The reference signal length assessment unit 22 also obtains the power constraint P. In this embodiment the power constraint determining unit 24 determines the power constraint P. It should however be realized that this step is optional. The power constraint could be determined by another entity, for instance a radio circuit at the opposite end of the

communication channel, which entity then transmits the power constraint to the reference signal length assessment device 18.

In order to determine the power constraint P, the power constraint determining unit 24 obtains the maximum output power PT_MAX of the transmitter transmitting on the channel, step 46. This value may be received from the transmitter that transmits on the communication channel, which maybe the transmitter 32 in the wireless interface 28 or the transmitter at the opposite end of the communication channel. The value may also be known at both ends of the communication channel. The power constraint determining unit 24 also obtains the number of active subcarriers used in the OFDM scheme, step 48, which is a measure of the number of symbols simultaneously transmitted by the base station 14. This number is typically provided by the base station 14. In case the reference signal length assessment device 18 is provided in the base station 14 it may thus already have this information, for instance in the wireless interface 28. In this case the wireless interface 28 may directly send the information to the power constraint determining unit 24. Otherwise the power constraint determining unit 24 may receive the information from the base station via the receiver 30 of the wireless interface 28.

Thereafter the power constraint determining unit 24 determines the power constraint P, step 50, which determination is made based on the maximum output power PT_MAX and the number of simultaneously used subcarriers. The power constraint P may more particularly be determined as the maximum power PT_MAX divided by the number of used subcarriers in order to determine a measure of the maximum available power per symbol transmitted in the communication channel.

After the power constraint P has been determined, the power constraint determining unit 24 forwards it to the reference signal length assessment unit 22.

The reference signal length assessment unit 22 then inserts all of the obtained values in a function of the channel capacity, step 52. The covariance matrix C, the resource block length T and the power constraint P are thus all inserted into the function. They are more particularly inserted into the function:

C(T _T ) - (Γ - T )(Iog ₂ \PC + J _w j - log ₂ \PC + I _m \) with ί = ( ^{Ρί +} έ ^ί' ") ^" ' ^£: where Τ _τ is the reference signal length and I _m is the identity matrix of equal size as the covariance matrix C, |PC +I _m | is the determinant of the vector addition PC + I _m , C is a matrix based on an inverse of a matrix formed by the combination of the covariance matrix C and the identity matrix I _m and I PC + Im I is the determinant of the vector addition PC+ I _m .

The identity matrix I _m may in this case represent a noise term or a noise covariance matrix. Since all the variables except for the reference signal length Τ _τ are known, it can be seen that the capacity is in fact a function of the reference signal length Τ _τ . In order to assess the number of symbols to be used in the reference or pilot signal, the reference signal length assessment unit 22 then optimises this function. The optimization is in this case performed through maximising the capacity with regard to reference signal length, step 54.

The optimising may be performed through deriving the function with regard to the reference signal length, i.e. dC(T _T )/dT _T and selecting a length corresponding to a zero value for which the function has a maximum. The reference signal symbol number corresponding to this optimization is then determined or set to be the number of reference signal symbols to be used in the communication channel. Thereby the number of sequential reference signal symbols corresponding to the reference signal length are directly obtained as the optimal pilot signal number, step 56. This number is then provided to the transmitter that transmits on the communication channel for being used as a reference signal, step 58.

As can be seen from what is described above, the reference signal length assessment device 18 provides a function where the transmitter or receiver (or both) of the communication channel gets information about the block-length T over a coherence time interval, the covariance matrix C of the channel and the per symbol power constraint P. The channel capacity function C(T _T ) is then optimized with respect to the reference signal length Τ _τ and thereby also the optimal number of pilot training symbols (pilots) to be used is obtained.

In the second embodiment the function C(T _T ) is an exact expression of the channel capacity as a function of the channel covariance matrix C, a noise covariance matrix and the reference signal length Τ _τ used during a coherence time interval. This exact expression is thus used to determine the number of pilot symbols that are needed by finding an optimal reference signal length. The optimal reference signal length then directly corresponds to an integer number of pilot symbols that maximizes the channel capacity. This exact expression can be used by the reference signal length determining device to determine the optimal number of reference symbols which results in a higher channel capacity than in existing systems. Thereby the number of reference signals used are the ones that provide the best compromise between channel estimation quality and reference signal length or channel data usage. The second embodiment thus provides a robust method that determines an optimal power allocation of pilots or reference signals in order to maximize the data rate with a given channel covariance, per symbol power constraint, and block- length.

The computer program code of a reference signal length assessment device may be in the form of computer program product for instance in the form of a data carrier, such as a CD ROM disc or a memory stick. In this case the data carrier carries a computer program with the computer program code, which will implement the functionality of the above-described

synchronisation assisting device. One such data carrier 6o with computer program code 62 is schematically shown in fig. 7.

The reference signal length assessment device may also be considered to comprise:

means for obtaining a channel covariance of a communication channel between a transmitter and a receiver, where at least one of the receiver and transmitter is a part of the wireless communication network,

means for obtaining a resource block length used in the communication channel,

means for obtaining a power constraint, which is a constraint concerning the power used per symbol transmitted in the communication channel, and

means for determining a number of reference signal symbols to be used for channel estimation employing a function of the channel capacity, which function is a function of the channel covariance, the power constraint, the resource block length T and a reference signal length.

The means for determining a number of reference signal symbols may be further considered to comprise means for optimizing the function of the channel capacity.

The reference signal length assessment device may be further considered to comprise means for providing the determined number of reference signal symbols to the transmitter for setting the reference signal length used in the transmission to the receiver.

The means for obtaining a power constraint maybe further considered to comprise means for determining the power constraint based on the maximum transmit power of and the number of symbols simultaneously transmitted by the transmitter.

While the invention has been described in connection with what is presently considered to be most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various

modifications and equivalent arrangements. Therefore the invention is only to be limited by the following claims.