WO2010031892A1

WO2010031892A1 - Link adaptation in communication network applying demodulate-and-forward relay nodes

Info

Publication number: WO2010031892A1
Application number: PCT/FI2008/050516
Authority: WO
Inventors: Haifeng Wang; Jing Xu; Haitao Xiong; Lin TANG
Original assignee: Nokia Corporation
Priority date: 2008-09-18
Filing date: 2008-09-18
Publication date: 2010-03-25

Abstract

There is provided a method, an apparatus and a computer program for improving transmission efficiency in a relay enhanced communication network. The method, comprises: determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate- and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the target node via the demodulate-and-forward relay node, on the basis of the determined transmission efficiency.

Description

Link Adaptation in Communication Network Applying Demodu- late-and-Forward Relay Nodes

Field

In general, the invention relates to performing link adaptation in a re- (ay enhanced wireless communication network.

Background

It is expected that relays will be widely employed in future communication networks. In relay enhanced networks, the transmission may occur from a transmitter to a receiver via a relay node or a relay station. The relay node is generally placed at the edge of the cell covered by a central node, such as a base station, in order to extend the coverage area of the central node and to increase the capacity/throughput at the cell-edge. Further, a relay node may increase the capacity at shadowed areas in the cell as well as in the locations where the traffic demand is high such as in airports or other hot spots, for example. In addition, the relay node may be applied to reduce the average radio transmission power of user equipment attached to the relay node.

Currently, there are various relay transmission schemes that can be employed. In an amplify-and-forward protocol an amplify-and-forward relay node first receives a signal from the source node, then scales the power of the signal up or down and finally forwards the signal towards a target node. Another exemplary relaying protocol applies selective decode-and-forward method, in which the received data at the relay node is decoded and retransmitted to the target node only if the data is correctly received through cy- die redundancy check or a similar error detecting code.

A demodulate-and-forward relay scheme performs a hard decision of the received, demodulated, symbol at the relay node at a first phase and then modulates and forwards the data to the target node. Since the relay node always forwards the data to the target node in a static manner, a network scheduler may pre-allocate radio resources accordingly. If the relay node makes an erroneous hard decision, the erroneous symbol may propagate to the target node as the relay node forwards the symbol. Consequently, the pre- allocated radio resources may be applied for erroneous transmission and the transmission efficiency may drop. Thus, novel solutions for improving the transmission efficiency and reducing the error propagation in demodulate-and- forward relay enhanced communication networks are needed.

Brief description

An object of the invention is to improve the transmission efficiency of the demodulate-and-forward relay enhanced communication network.

According to an aspect of the invention, there is provided a method, comprising determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodu- late-and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the ba- sis of the determined transmission efficiency.

According to an aspect of the invention, there is provided an apparatus, comprising a controller configured to determine a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate-and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate- and-forward relay node and a target node; and to select the modulation format pair for data transmission between the source node and the demodulate-and- forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency,

According to an aspect of the invention, there is provided an apparatus, comprising controlling means for determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate-and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate- and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the demodulate-and- forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency. According to an aspect of the invention, there is provided a computer program product, embodied on a computer-readable storage medium and comprising a program code which, when run on a processor, executes the method comprising determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate-and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency.

Embodiments of the invention are defined in the dependent claims.

List of drawings En the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which

Figure 1 illustrates a cellular communication network applying demodulate-and-forward relay nodes;

Figure 2 shows very general architecture of the demodulate-and- forward relay enhanced communication network, according to an embodiment of the invention;

Figure 3 shows a virtual channel;

Figure 4 illustrates an apparatus capable of performing a joint link adaptation in the demodulate-and-forward relay enhanced communication network, according to an embodiment of the invention;

Figure 5 illustrates a method for performing the joint link adaptation according to an embodiment of the invention; and

Figure 6 presents a detailed description on how to select a modulation format pair for data transmission, according to an embodiment.

Description of embodiments

The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single em- bodiment. Single features of different embodiments may also be combined to provide other embodiments.

Figure 1 illustrates a cellular communication system applying de- modulate-and-forward relay nodes. A demodulate-and-forward relay node 106 providing coverage to a cell 104 is placed at the edge of a cell 100 covered by a central node 102. The purpose of the demodulate-and-forward relay node 106 may be, for example, to extend the coverage area of the central node 102 and to increase the capacity/throughput at the cell-edge. The demodulate-and- forward relay node 106 may forward a signal from a source node to a target node. The source node may be, in a downlink transmission, a central node 102 such as a base station, an evolved node B as in an evolved UMTS terrestrial radio access network (E-UTRAN), a radio network controller (RNC) or any other apparatus capable of controlling a radio communication within the cell 100. In the downlink case, the target node may be, for example, any user equipment 110 such as a mobile phone, a palm computer, or any other apparatus capable of interacting with a radio communication network. In an uplink transmission, the user equipment 110 may be the source node and the central node 102 may be the target node.

Even though the description from now on is given for a downlink transmission, embodiments of the invention may be straight forwardly applied in an uplink transmission as well. In the downlink transmission, the central node 102 (source node) may transmit information to the demodulate-and- forward relay node 106 as shown in Figure 1. Let us denote the connection between the central node 102 and the demodulate-and-forward relay node 106 as a relay link 112. The demodulate-and-forward relay node 106 may demodulate and detect the received symbol from the central node 102. Further, the demodulate-and-forward relay node 106 may modulate the symbol and forward it to the user equipment 110 via an access link 114.

In the demodulate-and-forward relay enhanced communication net- work, the central node 102 may, according to prior art, pre-allocate radio access resources and determine the transmission format separately and independently for the relay link 112 and the access link 114. The transmission format may comprise the modulation format, such as symbol constellation, used in modulators of the central node 102 and the demodulate-and-forward relay node 106. The modulator in the central node 102 may apply, for example, a binary phase shift keying (BPSK) for the relay link 112, and the modulator at the relay node 106 may apply, for example, quadrature phase shift keying (QPSK) or 16-quadrature amplitude modulation (16-QAM) for the access link 114. The embodiments of the invention aim in performing a joint adaptation of the relay and access links 112 and 114, respectively. The joint adaptation may denote, for example, selecting the modulation format for the links jointly.

A very general architecture of a relay enhanced communication network according to an embodiment of the invention is shown in Figure 2. Figure 2 shows only the elements and functional entities required for understanding the communication network. Other components have been omitted for reasons of simplicity. The implementation of the elements and functional entities may vary from that shown in Figure 2. The connections shown in Figure 2 are logical connections, and the actual physical connections may be different. It is apparent to a person skilled in the art that the relay enhanced communication network also comprises other functions and structures. As explained earlier, due to the static forwarding scheme applied in the relay node 106, in which the demodulate-and-forward relay node always forwards the data to the target node, the error resulting from the hard decision may propagate to the user equipment 110. According to an embodiment of the invention, a joint link adaptation may be performed, which is robust to the error propagation resulting from an erroneous hard decision at the demodulate-and- forward relay node 106. The link adaptation may jointly determine the modulation formats for the relay link 112 and the access link 114. For example, one conservative strategy to avoid the erroneous decision is to apply the most robust modulation format for the first link to the demodulate-and-forward relay node 106. However, the most robust modulation format will result in low spectrum efficiency or low channel quality utilization. If an optimistic strategy is adopted, a high modulation will be applied for the first link and the probability of error decision will be high. This may also result in low spectrum efficiency due to high amount of erroneous decisions. Furthermore, we can see that the spec- trum efficiency is a function of the modulation formats for the relay link 112 and the access link 114. In other words, a separate modulation format selection may not take full use of the two links.

The description related to Figure 2 is given for the downlink transmission, wherein a source node may be the central node 102 and a target node may be the user equipment 110. However, embodiments of the invention may be applied in the uplink transmission as well. A controller 200 of the central node 102 may determine a transmission efficiency for each candidate modulation format pair M1 , M2, the modulation format pair M1 , M2 comprising a first modulation format M1 for a first radio propagation channel between the source node and the demodulate-and- forward relay node 106 and a second modulation format M2 for a second radio propagation channel between the demodulate-and-forward relay node 106 and the target node. Thus, in the downlink transmission, the first radio propagation channel may be the relay link 112 and the second radio propagation channel may be the access link 114. The transmission efficiency may be, for example, a metric for spectral efficiency.

The candidate modulation format pair M1, M2 may be any combination of two modulation formats. For example (BPSK; QPSK)₁ (BPSK;16-QAM), (16-QAM; 8-PSK) are possible combinations. The controller 200 may choose one candidate modulation format pair M1, M2 and determine the transmission efficiency for that modulation format pair M1 , M2. Then, the controller may select another candidate modulation format pair and determine the transmission efficiency for that modulation format pair, At the end, the controller 200 may have determined transmission efficiency for each candidate modulation format pair MI, M2. The controller 200 may further select the modulation format pair M1 ,

M2 for data transmission between the source node and the demodulate-and- forward relay node 106 and between the demodulate-and-forward relay node 106 and the target node, on the basis of the determined transmission efficiency. Thus, a first modulator 202 and a second modulator 212 may apply the selected modulation format pair prior to data transmission. In other words, the first modulator 202 may apply the first modulation format M1 for the first radio propagation channel between the source node and the demodulate-and- forward relay node 106 and the second modulator 212 may apply the second modulation format M2 for the second radio propagation channel between the demodulate-and-forward relay node 106 and the target node. By jointly selecting the modulation formats M1, M2 for the relay link 112 and the access link 114, respectively, the error due to possible erroneous hard decision at the demodulate-and-forward relay node 106 may be reduced.

The controller 200 may further perform other signal-processing op- erations, such as interleaving, encoding, etc. The central node 102 may further comprise an antenna and a transceiver for transmitting data via the relay link 112. The central node 102 may transmit a data stream via the relay link 112 to the demodulate-and-forward relay node 106 after having modulated the data stream with the first modulation format M1 at the first modulator 202.

The demodulate-and-forward relay node 106 may, after having re- ceived the transmitted symbol, demodulate and detect the symbol at a first demodulator 210. The first demodulator 210 may demodulate the symbol by using the same modulation format as the first modulator at the central node 102. The detection may be based on the hard decision. That is, a symbol distance or a similar metric may be applied in detecting the received and demodu- lated data symbol.

The demodulate-and-forward relay node 106 may then perform modulation transform at the second modulator 212 by applying the second modulation format M2 of the selected modulation format pair M1, M2 to a data stream. After having modulated the data stream, the demodulate-and-forward relay node 106 may forward the data to the user equipment 110 via the access link 114. The demodulate-and-forward relay node 106 may thus comprise an antenna and a transceiver for accessing the access link 114. Since the demodulate-and-forward relay node 106 may not perform channel (de)coding and (de)interleaving, the latency of the demodulate-and-forward relay node 106 is low.

A demodulator 220 of the user equipment 110, denoted as a second demodulator 220, may perform demodulation of the received data symbol by using the same modulation format as the second modulator 212. The controller 222 may also perform other signal processing operations, such as de- interleaving, decoding, etc.

The controllers 200 and 222 may be implemented with separate digital signal processors provided with suitable software embedded on a computer readable medium, or with separate logic circuits, such as an application specific integrated circuit (ASIC). The controllers 200 and 222 may comprise input/output (I/O) interfaces such as computer ports for providing communication capabilities. The input/output interfaces may perform signal-processing operations for enabling a physical channel connection, if needed.

The controller 200 may generate a virtual channel when determining the transmission efficiency. Figure 3 shows a block diagram of the virtual channel 300. The virtual channel may depict the functionalities of the first modulator 202, the relay link 112, the first demodulator 210 and the second modulator 212, according to an embodiment of the invention. The following description regarding the virtual channel 300 refers to Figures 2 and 3. Even though the first demodulator 210 and the second modulator 212 are physically located in the demodulate-and-forward relay node 106, the virtual channel 300 generated by the controller 200 of the central node 102 may depict their functionalities and, thus, the controller 200 may obtain knowledge of the detected data symbol X"MI after the hard decision at the point 304 and the modulated data symbol X⁶W at point 306. The superscript ^Θ denotes an estimate. The subscript MI denotes the first modulation format M1 and the subscript _M2 denotes the second modulation format M2. That is, the controller 200 may virtually modulate the data symbol x at point 320 with the first modulation format M1 in block 202, virtually transmit the modulated data symbol x_M1 at point 302 through the relay link 112, demodulate and detect the symbol x_m at the first demodulator 210 in order to obtain a hard decision estimate X⁶ _MiOf the x_Mi, and to modulate the estimate X⁸Mi at the second modulator 212 in order to obtain

Further, the central node 102 may virtually generate a data symbol x_M2 at point 312 representing the correct output of the second modulator 212. That is, the controller 200 may virtually modulate the data symbol x in the third modulator 310 using the second modulation format M2 of the selected modulation format pair M1, M2. Thus, the third modulator 310 may be identical to the second modulator 212.

Thus, the controller may have knowledge regarding the correct data symbol x, the data symbol x_Mi modulated with the first modulation format M1 , the estimate of the data symbol X⁶Mi modulated with the first modulation format M1, the estimate of the data symbol VW modulated with the second modulation format M2 and the error-free data symbol of the second modulator x_M2 modulated with the second modulation format M2.

The controller 200 may take a reliability metric pi for symbol detec- tion at the demodulate-and-forward relay node 106 into account when determining the transmission efficiency for each candidate modulation format pair M1, M2. That is, the controller 200 may choose one candidate modulation format pair M1 , M2 and determine the pi for that modulation format pair M1 , M2. Then, the controller may select another candidate modulation format pair and determine the pi for that modulation format pair. At the end, the controller 200 may have determined pi for each candidate modulation format pair M1, M2. The controller 200 may determine the reliability metric pi for the symbol detection by calculating a normalized correlation coefficient between at least one transmitted data symbol x_Mi from the source node and the at least one detected data symbol X⁶ _Mi corresponding to the transmitted data symbol at the demodulate-and-forward relay node. That is,

where E denotes the expectation value and * is the complex conjugate. Since the values of pi depend on the channel quality between the source node and the demodulate-and-forward relay node, the values for the pi corresponding to a certain first modulation format M1 may be stored in a look-up table as a function of a channel quality metric, such a signal-to-noise ratio (SNR). Thus, the controller 200 may store values for the reliability metric pi for the symbol detection into the look-up table in order to use the look-up table when preparing data transmission. Similarly, the controller 200 may take a reliability metric ρ₂ for a modulation transform at the demodulate-and-forward relay node into account when determining the transmission efficiency for each candidate modulation format pair M1, M2. The controller may generate the virtual channel for depicting the functionalities of the demodulate-and-forward relay node, and deter- mine the reliability metric P₂ for the modulation transform by calculating a normalized correlation coefficient between at least one transmitted, modulated, data symbol from the demodulate-and-forward relay node and at least one virtually modulated data symbol. That is,

Further, since the X⁶W may depend on the reliability of the hard decision, represented by the pi, the values for the p₂ corresponding to a certain modulation format pair M1 , M2, may be stored in a look-up table as a function of the pi. Thus, the controller 200 may store values for the reliability metric p₂ for the modulation transform into the look-up table in order to use the look-up table when preparing data transmission.

Without loss of generality, the received data symbol y at the user equipment 110 may be determined as

Y = h_ue ⁾f_m + /7_UΘ = Λ_uep₂XM2 +

- P2XM2) + flue, (3) where h_ue is the channel coefficient for the second radio propagation channel between the demodulate-and-forward relay node 106 and the user equipment

110, that is the access link 114, and n_ue is a noise term. Based on this equa- tion, the controller 200 may determine a channel quality metric, such as a SNR of a radio propagation channel between the source node and the target node, that is, between the central node 102 and the user equipment 110 in the downlink transmission case. Let us denote this SNR as end-to-end (e2e) SNR. The e2e SNR may be computed as

SNR_e2e(M1 , M2) = (Ip₂ /?ua|²) / [(1-P2²)+σ_ue ²], (4) where σ_ue ² is the variance of the noise at the user equipment 110. It can be seen that the SNR_e2e is only dependent on the modulation formats M1 , M2 corresponding to the modulation formats used for the relay link 112 and the ac- cess link 114, respectively. Thus, the controller 200 may take the reliability metric P₁ for the symbol detection and the reliability metric p₂ for the modulation transform into account in Equation (4). In order to calculate Equations (1) to (3), the controller 200 may acquire the channel state information (CSI) for the first radio propagation channel and the second radio propagation channel, i.e. for the access link 114 and for the relay link 112. The CSI may be obtained through pilot signals received from the user equipment 110 and the demodu- late-and-forward relay node 106, or similar well-known methods.

Thus, the controller 200 may have determined the channel quality metric, such as the e2e SNR, for each candidate modulation format pair M1 , M2. Consequently, the controller 200 may determine the transmission efficiency for each candidate modulation format pair M1, M2 on the basis of a channel quality metric obtained from the acquired channel state information. Further, the determination of the transmission efficiency for each candidate modulation format pair M1 , M2 may be performed on the basis of the channel quality metric for the corresponding candidate modulation format M1 , M2. As explained, the channel quality metric may be, for example, the signal-to-noise ratio of the radio propagation channel between the source node and the target node. That is, the channel quality metric may be a combined SNR of the relay link 112 and the access link 114. Consequently, the modulation format pair M1, M2 may be determined as

M1, M2, C = arg max (1-BLER(SNR_e2e,C)0-τB/ OVT₂)], (5) where BLER denotes the block error rate, L_TB is the size of the transmitted data block, C is a selected code rate and Ti and T₂ are the time durations in symbol level for the relay and access links 112 and 114, respectively. Thus, the term [L_TB / (Ti+T₂)] denotes the maximum data rate. As shown, the BLER may be computed for a certain code rate C, which is the same for the relay link 112 and the access link 114. Arg max is the value of the argument for which the value of the given expression attains its maximum value. That is, the argument of the arg max -function in equation (5) may produce a metric for the transmission efficiency. As a result, the modulation format pair M1, M2 most appropriate for the prevailing channel conditions may be determined. Thus, the reliability metrics pi and p2 may have an impact to the selection of the modulation format pair M1 , M2 through Equations (1 ) to (5).

Embodiments of the invention may be applied in the uplink transmission as well. Referring to Figure 2 and the uplink transmission, the central node 102, after having selected the modulation format pair M1, M2, may transmit information regarding the selected modulation format pair M1, M2 to the user equipment 110 and the controller 222, and the second modulator 220 of the user equipment 110 may apply the information when preparing data transmission in an uplink direction. Similarly, the central node 102 may trans- mit information regarding the selected modulation format pair M1, M2 to the demodulate-and-forward relay node 106.

Figure 4 shows an apparatus 400 capable of performing the joint link adaptation in the demodulate-and-forward relay enhanced communication network according to an embodiment of the invention. Figure 4 shows only the elements and functional entities required for understanding the apparatus. Other components have been omitted for reasons of simplicity. The implementation of the elements and functional entities may vary from that shown in Figure 4. The connections shown in Figure 4 are logical connections, and the actual physical connections may be different. The apparatus 400 may be, for ex- ample, a base station, an evolved node B as in E-UTRAN, a radio network controller, etc. In general, the invention may be applied in any apparatus capable of controlling radio access in a communication network. The joint link adaptation may measure the reliability of the hard decision and the modulation transform performed at the demodulate-and-forward relay node. Thus, the er- ror propagation may be efficiently reduced.

The apparatus 400 may comprise a controller 402, which may determine the transmission efficiency for each candidate modulation format pair M1, M2 and select the modulation format pair M1 , M2 on the basis of the determined transmission efficiency. The controller 402 may select, for example, the modulation format pair M1, M2, which has the highest transmission efficiency, The controller 402 may be implemented with separate digital signal processors provided with suitable software embedded on a computer readable medium, or with separate logic circuits, such as the ASIC. The controller 402 may comprise an interface such as a computer port for providing communica- tion capabilities. The interface may perform signal-processing operations for enabling a physical channel connection, if needed.

The apparatus 400 may further comprise a memory 404, which may store the look-up tables for at least one of the following: the look-up table for the pi as a function of the SNR of the relay link 112 and the modulation format applied in the first modulator 202 of Figure 2, and the look-up table for the p₂ as a function of the pi and the modulation format pair M1 , M2 to be used in the first modulator 202 and the second modulator 212 of Figure 2. Further, the memory 404 may store a look-up table for the modulation format pair M1 , M2 as a function of the e2e SNR. Through the look-up tables, the implementation complexity of the embodiments of the invention may be low.

The apparatus 400 may further comprise a modulator 406 for modulating the data to be transmitted via an antenna 410 to the demodulate-and- forward relay node. Thus, the apparatus may comprise a transceiver 408 for accessing the radio frequency air interface via the antenna 410. The modulator 406 may apply the first modulation format M1 of the modulation format pair M1 , M2 in the modulation. The data to be transmitted to the demodulate-and- forward relay node may comprise information regarding the second modulation format M2 to be used in the modulator of the demodulate-and-forward relay node. The apparatus 400 may need to obtain knowledge of the channel state information (CSI) in order to obtain the pi. However, this may be performed through pilot signals received from the user equipment and the demodulate-and-forward relay node, or similar well-known methods.

Figure 5 shows a method for performing the joint link adaptation ac- cording to an embodiment of the invention. The method starts in step 500.

In step 502, the method may comprise determining the transmission efficiency for each candidate modulation format pair M1 , M2, the modulation format pair M1 , M2 comprising the first modulation format M1 for the first radio propagation channel between the source node and the demodulate-and- forward relay node and the second modulation format M2 for the second radio propagation channel between the demodulate-and-forward relay node and a target node.

Step 504 comprises selecting the modulation format pair M1 , M2 for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency. The method ends in step 506.

Figure 6 shows a detailed description on how to select the modulation format pair M1 , M2 for data transmission between the source node and the target node via the demodulate-and-forward relay node, according to an embodiment. The method begins in step 600.

In step 602, the method comprises selecting a candidate modulation format pair M1, M2. The candidate modulation format pair M1 , M2 may be any combination of two modulation formats. For example (BPSK; QPSK), (BPSK; 16-QAM), (16-QAM; 8-PSK) are possible combinations. The method may further acquire channel state information for the first radio propagation channel and the second radio propagation channel.

Step 604 comprises determining the first reliability metric pi for the symbol detection at the demodulate-and-forward relay node for the candidate modulation format M1. The reliability metric pi may represent the reliability of a correct detection at the demodulate-and-forward relay node. The pi may be determined with equation (1) and the virtual channel explained earlier. The values for pi may be stored in a memory to lower the computational complexity. Thus, the value pi for the corresponding candidate modulation format pair M1, M2 and the acquired channel state information may be determined from the look-up table.

Step 606 comprises determining the second reliability metric ρ₂ for a modulation transform at the demodulate-and-forward relay node for the candidate modulation format pair M1, M2. The reliability metric ρ₂ may represent the reliability of a correct modulation transform at the demodulate-and-forward relay node. The modulation transform denotes the change from the modulation format M1 applied in the first modulator and the first demodulator to the second modulation format M2 applied in the second modulator and the second demodulator. The p2 may be determined with equation (2) and the virtual channel. The values for p₂ may be stored in a memory to lower the computational complexity. Thus, the value p₂ for the corresponding to the candidate modulation format pair M1, M2 and the previously determined pi may be determined from the look-up table.

Step 608 comprises calculating the channel quality metric, such as the e2e SNR, representing the SNR of the channel between the source node and the target node. The channel quality metric may be determined with equation (4). Thus, the step 608 may comprise determining the channel quality metric for the candidate modulation format pair M1 , M2 on the basis of the reliability metric for the modulation transform and the acquired channel quality information for the second radio propagation channel (the access link 114 of Figure 2 in the downlink transmission).

In step 610, the method may determine the transmission efficiency, such as the spectral efficiency, for the candidate modulation format pair M1, M2 on the basis of the channel quality metric obtained from the acquired channel state information. A metric for the transmission efficiency may be, for ex- ample, the block error rate (BLER) presented with equation (5).

In step 612, the method may check whether the transmission efficiency has been determined for all the candidate modulation formats MI₁ M2. If the transmission efficiency has not been determined for all of the candidate modulation format pairs, the method continues in step 602, in which another candidate modulation pair will be selected and the transmission efficiency will be determined for it according to steps 604-610. That is, the method may comprise repeating the determination of the reliability metrics, the channel quality metric and the transmission efficiency for each candidate modulation pair.

If the method determines in step 612 that the transmission efficiency has been obtained for each candidate modulation format pair M1, M2, the method continues in step 614, in which the method comprises selecting the modulation format pair M1, M2. The selected modulation format pair M1 , M2 may the one that produces highest transmission efficiency. The first modulation format M1 may then be used in the modulator of the source node and the second modulation format M2 may be used in the modulator of the demodu- late-and-forward relay node. The method ends in step 616.

Parts of the method presented in Figure 6 may be computed beforehand and, for example, a look-up table for modulation format pairs as a function of the e2e SNR may be generated and accessed for selecting appro- priate modulation format pair M1 , M2 prior to data transmission. Embodiments of the invention may be implemented as computer programs. The computer programs comprise instructions for executing a computer process for performing the joint adaptation for the demodulate-and- forward relay enhanced communication networks. The computer program may carry out, but is not limited to, the tasks related to Figures 2 to 6.

The computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, an electric, magnetic, optical, infrared or semiconductor system, device or transmission medium. The computer program medium may include at least one of the following media: a computer readable medium, a program storage medium, a record medium, a computer readable memory, a random access memory, an erasable programmable read-only memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunications signal, computer readable printed matter, and a computer readable compressed software package.

Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.

Claims

1. A method, comprising: determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first moduiation format for a first radio propagation channel between a source node and a demodulate- and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency.

2. The method of claim 1 , further comprising: acquiring channel state information for the first radio propagation channel and the second radio propagation channel; and determining the transmission efficiency for each candidate modulation format pair on the basis of a channel quality metric obtained from the acquired channel state information.

3. The method of claim 2, wherein the channel quality metric is a signal-to-noise ratio of a radio propagation channel between the source node and the target node.

4. The method of any of claims 1 to 3, wherein the transmission efficiency is a metric for spectral efficiency.

5. The method of any of claims 1 to 4, further comprising: taking a reliability metric for symbol detection at the demodulate- and-forward relay node into account when determining the transmission efficiency for each candidate modulation format pair.

6. The method of claim 5, further comprising: determining the reliability metric for the symbol detection by calculating a normalized correlation coefficient between at least one transmitted symbol from the source node and the at least one detected symbol corre- sponding to the transmitted symbol at the demodulate-and-forward relay node.

7. The method of any of claims 6 to 7, further comprising: storing values for the reliability metric for symbol detection into a look-up table in order to use the look-up table when preparing data transmls- sion.

8. The method of any of claims 1 to 7, further comprising: taking a reliability metric for a modulation transform at the demodu- late-and-forward relay node into account when determining the transmission efficiency for each candidate modulation format pair.

9. The method of claim 8, further comprising: generating a virtual channel for depicting the functionalities of the demodulate and forward relay node; and determining the reliability metric for the modulation transform by calculating a normalized correlation coefficient between at least one transmitted symbol from the demodulate-and-forward relay node and at least one virtually modulated symbol.

10. The method of any of claims 8 to 9, further comprising: storing values for the reliability metric for the modulation transform into a look-up table in order to use the look-up table when preparing data transmission.

11. The method of any of claims 1 to 10, further comprising: acquiring channel state information for the first and second radio propagation channels; determining a reliability metric for symbol detection for a candidate modulation format pair from a first look-up table on the basis of the acquired channel quality information for the first radio propagation channel; determining a reliability metric for a modulation transform for the candidate modulation format pair from a second look-up table on the basis of the determined reliability metric for the symbol detection; determining the channel quality metric for the candidate modulation format pair on the basis of the reliability metric for the modulation transform and the acquired channel quality information for the second radio propagation channel; determining the transmission efficiency for the candidate modulation pair; repeating the determination of the reliability metrics, the channel quality metric and the transmission efficiency for each candidate modulation pair; and selecting the modulation format pair with highest transmission efficiency for data transmission.

12. An apparatus, comprising: a controller configured to: determine a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate- and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and select the modulation format pair for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency.

13. The apparatus of claim 12, wherein the controller is further configured to: acquire channel state information for the first radio propagation channel and the second radio propagation channel; and determine the transmission efficiency for each candidate modulation format pair on the basis of a channel quality metric obtained from the acquired channel state information.

14. The apparatus of claim 13, wherein the channel quality metric is a signal-to-noise ratio of a radio propagation channel between the source node and the target node.

15. The apparatus of any of claims 12 to 14, wherein the transmission efficiency is a metric for spectral efficiency.

16. The apparatus of any of claims 12 to 15, wherein the controller is further configured to take a reliability metric for symbol detection at the de- modulate-and-forward relay node into account when determining the transmission efficiency for each candidate modulation format pair.

17. The apparatus of claim 16, wherein the controller is further configured to determine the reliability metric for the symbol detection by calculating a normalized correlation coefficient between at least one transmitted symbol from the source node and the at least one detected symbol corresponding to the transmitted symbol at the demodulate-and-forward relay node.

18. The apparatus of any of claims 16 to 17, wherein the controller is further configured to store values for the reliability metric for symbol detection into a look-up table in order to use the look-up table when preparing data transmission.

19. The apparatus of any of claims 12 to 18, wherein the controller is further configured to take a reliability metric for a modulation transform at the demodulate-and-forward relay node into account when determining the transmission efficiency for each candidate modulation format pair.

20. The apparatus of claim 19, wherein the controller is further configured to: generate a virtual channel for depicting the functionalities of the demodulate and forward relay node; and determine the reliability metric for the modulation transform by calculating a normalized correlation coefficient between at least one transmitted symbol from the demodulate-and-forward relay node and at least one virtually modulated symbol.

21. The apparatus of any of claims 19 to 20, wherein the controller is further configured to store values for the reliability metric for the modulation transform into a look-up table in order to use the look-up table when preparing data transmission.

22. The apparatus of any of claims 12 to 21, wherein the controller is further configured to: acquire channel state information for the first and second radio propagation channels; determine a reliability metric for symbol detection for a candidate modulation format pair from a first look-up table on the basis of the acquired channel quality information for the first radio propagation channel; determine a reliability metric for a modulation transform for the candidate modulation format pair from a second look-up table on the basis of the determined reliability metric for the symbol detection; determine the channel quality metric for the candidate modulation format pair on the basis of the reliability metric for the modulation transform and the acquired channel quality information for the second radio propagation channel; determine the transmission efficiency for the candidate modulation pair; repeat the determination of the reliability metrics, the channel quality metric and the transmission efficiency for each candidate modulation pair; and select the modulation format pair with highest transmission efficiency for data transmission.

23. An apparatus, comprising: controlling means for: determining a transmission efficiency for each candidate modulation format pair, the modulation format pair comprising a first modulation format for a first radio propagation channel between a source node and a demodulate- and-forward relay node and a second modulation format for a second radio propagation channel between the demodulate-and-forward relay node and a target node; and selecting the modulation format pair for data transmission between the source node and the demodulate-and-forward relay node and between the demodulate-and-forward relay node and the target node, on the basis of the determined transmission efficiency.

24. The apparatus of claim 23, further comprising controlling means for: acquiring channel state information for the first radio propagation channel and the second radio propagation channel; and determining the transmission efficiency for each candidate modulation format pair on the basis of a channel quality metric obtained from the acquired channel state information.

25. The apparatus of claim 24, wherein the channel quality metric is a signal-to-noise ratio of a radio propagation channel between the source node and the target node.

26. The apparatus of any of claims 23 to 25, wherein the transmission efficiency is a metric for spectral efficiency.

27. The apparatus of any of claims 23 to 26, further comprising controlling means for: acquiring channel state information for the first and second radio propagation channels; determining a reliability metric for symbol detection for a candidate modulation format pair from a first look-up table on the basis of the acquired channel quality information for the first radio propagation channel; determining a reliability metric for a modulation transform for the candidate modulation format pair from a second look-up table on the basis of the determined reliability metric for the symbol detection; determining the channel quality metric for the candidate modulation format pair on the basis of the reliability metric for the modulation transform and the acquired channel quality information for the second radio propagation channel; determining the transmission efficiency for the candidate modulation pair; repeating the determination of the reliability metrics, the channel quality metric and the transmission efficiency for each candidate modulation pair; and selecting the modulation format pair with highest transmission efficiency for data transmission.

28. A computer program product, embodied on a computer-readable storage medium and comprising a program code which, when run on a processor, executes the method according to any of claims 1 to 11.