WO2010000309A1

WO2010000309A1 - Method and arrangement in a telecommunication system

Info

Publication number: WO2010000309A1
Application number: PCT/EP2008/058434
Authority: WO
Inventors: Robert Baldemair; Stefan Parkvall; Anders FURUSKÄR
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-07-01
Filing date: 2008-07-01
Publication date: 2010-01-07

Abstract

The present invention relates to a method for uplink transmission in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink. The method comprising the steps of: receiving a resource allocation message from the network; determining whether said resource allocation fulfills at least one predefined condition that implicitly indicates which access scheme to be used in uplink transmission; if a condition for implicit indication is fulfilled, selecting access scheme according to a pre-defined rule, applying the selected access scheme in the uplink transmission. Hereby, in case of fulfillment of any of the predefined conditions, no explicit signaling of information regarding the usage of DFT-spread OFDM or regular OFDM is required. The invention furthermore relates to a terminal capable of performing said method, a method in a network node, and a network node.

Description

Method and Arrangement in a Telecommunication System

FIELD OF THE INVENTION

The present invention relates to the alternate use of DTF-spread OFDM and regular OFDM in uplink transmission in an OFDM based telecommunication system.

BACKGROUND

In release 8 of the LTE standard, uplink transmission is based on DFT (Discrete Fourier Transform) spread OFDM, also known as single carrier FDMA. DFT- spread OFDM may be realized through applying a DFT pre-coder before the input to the IFFT (Inverse Fast Fourier Transform) in a regular OFDM modulator. This is illustrated in Fig 1.

DFT-spread OFDM reduces the peak-to-average-power-ratio (PAPR) of the transmitted signal as compared to a regular OFDM signal. This enables transmitting at higher average power, which in turn improves coverage.

A receiver for DFT-spread OFDM is slightly more complex than one for regular OFDM. Typically an equalizer is required to mitigate frequency selective fading. In case of very wideband allocations this equalizer may lead to noise enhancements.

For MIMO receivers, the relative complexity difference is larger since typically the symbols are not independent of each other. For this reason, it is likely that a regular OFDM mode will be introduced in LTE standards beyond release 8.

There are currently no existing systems supporting both DFT- spread OFDM and regular OFDM. SUMMARY

It is the object of the present invention to provide a solution that solves the problem of controlling the usage of DFT-spread OFDM and regular OFDM in a system supporting both DFT-spread OFDM and regular OFDM, and how to efficiently provide this information to the terminals.

The present invention is based on the understanding that the DFT precoder may be by-passed, i.e. regular OFDM should be used, under certain conditions. For example, depending on the adopted MIMO scheme the advantage of reduced PAPR in DFT- spread OFDM may also become less pronounced.

More specifically, a first aspect of the invention relates to a method in a terminal, for uplink transmission in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising the steps of

- receiving a resource allocation message from the network;

- determining whether said resource allocation fulfills at least one predefined condition that implicitly indicates which access scheme to be used in uplink transmission;

- if a condition for implicit indication is fulfilled, selecting access scheme according to a pre-defined rule;

- applying the selected access scheme in the uplink transmission.

Hereby, in case of fulfillment of any of the predefined conditions, no explicit signaling of information regarding the usage of DFT-spread OFDM or regular OFDM is required. Instead, said information is implicitly coupled to said predefined conditions. Thus, an efficient usage of the control channel is achieved and the overhead signaling is reduced. Furthermore, the implicit information provided to the terminal makes the decoding process performed by the terminal less complex than if an explicit bit always were to be included for this purpose.

According to a specific embodiment of the invention, at least one condition defines a first subset of said predefined conditions, whereby fulfillment of said first subset implies that regular OFDM should be used. One condition defining said first subset may be that the signaled resource allocation is non- contiguous . Another such condition defining said first subset may be that single-user MIMO (Multiple Input Multiple Output) is used.

According to a specific embodiment of the invention, at least one condition defines a second subset of said pre-defined conditions, whereby fulfillment of said second subset implies that DFT-spread OFDM should be used. One condition defining said second subset may be that the signaled resource allocation is non-contiguous . Another such condition defining said second subset is that MIMO is not used.

A second aspect of the invention relates to a method in a network node for allocating resources to a terminal in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising the steps of

- determining whether a resource allocation message fulfills at least one predefined condition that implicitly indicates to the terminal which access scheme to use in uplink transmission;

- if no such predefined condition is fulfilled, selecting which access scheme to be used by the terminal in uplink transmission;

- signaling to the UE an indication of the selected access scheme to be used in the uplink transmission.

Thus, only when the usage of DFT-spread OFDM or regular OFDM as access schemes in the uplink cannot be implicitly indicated to the terminal, i.e. when no condition for implicit indication is fulfilled, is the network node required to actively select which scheme to be used and provide this information to the terminal.

The invention furthermore relates to a terminal capable of performing uplink transmission in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising

- a receiver capable of receiving a resource allocation message from the network;

- a processor capable of determining whether at least one predefined condition is fulfilled for said resource allocation, said condition implicitly indicating which access scheme to be used in uplink transmission, and selecting access scheme dependent on fulfillment of said condition according to a pre-defined rule,- and;

- a transmitter capable of applying the selected access scheme in the uplink transmission. The invention furthermore relates to a network node capable of allocating resources to a terminal in a cellular telecommunication network supporting both DFT- spread OFDM and regular OFDM as access schemes in the uplink, comprising

- a processor capable of determining if at least one predefined condition is fulfilled for a resource allocation to be sent to a terminal, said conditions implicitly indicating to the terminal which access scheme to be used in uplink transmission, and of selecting which access scheme to be used in uplink transmission in case no such condition is fulfilled; and

- a transmitter capable of signaling to the terminal an indication of the selected access scheme.

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

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of preferred embodiments as illustrated in the drawings.

Figure 1 (prior art) illustrates application of a DFT precoder;

Figure 2 illustrates an exemplified embodiment of a method according to the invention performed by a terminal;

Figure 3 illustrates an alternative embodiment of a method according to the invention performed by a terminal; Figure 4 illustrates an exemplified embodiment of a method according to the invention performed by a network node ;

Figure 5 illustrates schematically an exemplified terminal according to the invention;

Figure 6 illustrates schematically an exemplified network node according to the invention.

DETAILED DESCRIPTION

The present invention can be exemplified in the following non- limiting description of an embodiment of the invention.

In the transmitter comprised in a terminal, the use of regular OFDM or DFT-spread OFDM can be realized by simply by-passing the DFT precoder for use of regular OFDM. The usage of DFT-spread OFDM and regular OFDM would preferably be controlled by the network, e.g. the base stations. There are, however, exceptions to this principle, which will be described later in this description.

A straightforward solution for providing information on precoder usage to the terminal would be to always explicitly indicate the DFT-precoder usage to the terminals.

However, such explicit signaling of precoder usage has drawbacks such that it may require an extra bit in the PDCCH, and that new formats for the PDCCH messages need to be defined. This in turn would increase the complexity in the decoding process carried out by the terminal, since the new formats would be added to the already existing message formats, thereby doubling the workload to be performed in the decoding process.

According to the invention, it is observed that the preferred use of DFT-spread OFDM and regular OFDM is dependent on one or several side conditions, expressed in terms of the resource allocation message. In this context, the term "resource allocation" should be understood as information to the terminal regarding for example terminal identification such as RNTI (Radio Network Temporary Identifier) , allocation expressed in time and/or frequency and/or code, modulation and coding scheme (MCS) , MIMO configuration and power control .

The basic concept of the invention is to implicitly couple the DFT-precoder usage to such conditions. By defining these conditions to the terminal, explicit signalling of the usage of DFT-spread OFDM or regular OFDM as access schemes in the uplink can in many cases be avoided.

According to a specific embodiment of the invention, a set of side conditions (C) may divided into three subsets:

Cl) In this subset regular OFDM should always be used

C2) In this subset DFT spread OFDM should always be used

C3) In this subset either DFT spread OFDM or regular OFDM should be used.

When the terminal is allocated resources that fulfil the one or more conditions defining the subsets Cl or C2 , the terminal is implicitly made aware what access scheme it should use in accordance with a predefined rule, and thus selects the appropriate access scheme based on the information implicitly provided in the resource allocation message. If the resource allocation message neither fulfils the conditions defining Cl nor the conditions defining C2, subset C3 is applied, whereby explicit signalling to the terminal of precoder usage is used.

In the following, examples of conditions defining the subset Cl are provided. Thus if conditions defining Cl is fulfilled, the DFT precoder may be by-passed, i.e. regular OFDM is used.

1) The signalled resource allocation is non-contiguous, since this is not possible with a DFT- spread OFDM signal;

2) Single-User MIMO is used;

3) The terminal transmission power is below a certain level, since when the terminal transmission power is low, PAPR is no issue and there is no need to reduce the PAPR by using DFT-spread OFDM. This is an example of when the usage of DFT-spread OFDM and regular OFDM is not controlled by the network. In this case the terminal may signal an indication on precoder usage to the network, e.g. the base station, or alternatively the base station may use blind detection to identify OFDM usage applied by the terminal .

4) The modulation order and/or code rate exceeds/exceed a predefined threshold (e.g. corresponding to 16QAM, code rate 3/4) . Terminals using higher modulation scheme are most likely not power limited and may gain more from OFDM than loose from lost single carrier property.

5) The bandwidth (even contiguous) of the allocated resource exceeds a certain bandwidth. In situations over very frequency selective channels (and if bandwidth is large likelihood of variations is large) DFT-spread OFDM suffers from noise enhancements. In the following, examples of conditions defining the subset C2 are provided. Thus if conditions defining C2 is fulfilled, DFT-spread OFDM will be applied in uplink.

1) Contiguous allocations 2) Non MIMO

Thus, in case of allocations belonging to subset Cl or C2 , the terminal uses the implicitly signalled precoder usage. In a specific embodiment, this behaviour would be standardised and 'hard coded' in the terminal.

In cases where conditions for providing implicit information on precoder usage, i.e. the conditions defining subset Cl or C2, are not fulfilled, a third subset C3 will be applied, which means that information on usage of DFT precoder will be explicitly provided to the terminal. This can be made in different ways, the most straight forward way is, as previously mentioned, signalling the information by using a bit in PDCCH message. Other ways to do this include:

1) Using 'new' formats of the resource allocation message, e.g. :

a. If the resource allocation (number of allocated resource blocks) is not a product of the factors 2, 3, and 5 (which is the case for LTE release 8) .

b. Blindly from new DCI (Downlink Control Information) format (i.e. format of the PDCCH message) . For example, if current format 0 implies DFT-spread OFDM, new format OA (which may support non-cont allocation) implies the use of regular OFDM. This implication, i.e. the mapping of DCI formats and use of DFT-spread and regular OFDM, would be defined in the standard. 2) Using special Radio Network Temporary Identifiers

(RNTIs,) such that terminals with capability for both DFT-spread OFDM and regular OFDM receive a first and a second RNTI, Radio Network Temporary ID. The usage of the first RNTI indicates to the terminal that regular OFDM is to be used, while usage of the second RNTI indicates to the terminal that DFT-spread OFDM is to be used.

3) Usage is configured by RRC via RRC signalling. This type of signalling may for example be used for configuring the terminal to use a specific access scheme in the uplink over a longer time frame. For example, the terminal may be configured via RRC signalling to always use DFT-spread OFDM in case conditions for implicit determination are not fulfilled, as long as no contradicting configuration signalling is received via RRC. RRC signalling may also be used together with the conditions for providing the information implicitly, to determine if implicit determination of OFDM vs. DFT-spread OFDM should be made or if for example DFT-spread OFDM should always be used, until the terminal is configured otherwise.

According to a specific embodiment, the subsets, here exemplified as Cl, C2, and C3 , would be defined in the standard, so that resource allocations belonging to Cl and C2 do not contain explicit DFT precoder usage information, whereas resource allocations not fulfilling the conditions for implicitly providing the information, thus in this example belonging to subset C3 , do contain such information.

Exceptions to this principle may occur. For example may an explicit indicator be received although one of the conditions Cl or C2 are fulfilled. To resolve such 'error occurrences', it may be defined in the standard, that an explicit signalling of precoder usage will override an implicit indication also in cases where conditions for implicit indication are fulfilled.

In case of resource allocations belonging to subset C3 , a network node, e.g. the base station, will decide on what precoder usage it prefers, and indicate this explicitly to the terminal as previously described. The base station may take this decision dependent on for which purpose, e.g. high average bit rate, the resource allocation is optimized. The decision of precoder usage can thus be compared to the decisions on coding, power level etc taken by the base station each time a terminal is scheduled.

It should be noted that any of the subsets Cl and C2 given as examples herein may be "empty" in the meaning that there is no condition defining such subset. For example, in a case where subset C2 is defined by at least one condition for implicit indication of use of DFT precoder, whereas subset Cl is empty, this would mean that either DFT spread OFDM, as implicitly indicated by fulfilment of C2 , or explicit signalling would be used, and that regular OFDM should never be used.

Specific embodiments of the invention will now be discussed with reference to Figures 2-6.

Fig. 2 illustrates a method performed by a terminal for selection of uplink access scheme. In step 200, the terminal receives a resource allocation message from the network, typically from a base station such as an eNodeB in case the invention is carried out in an LTE system. In step 201, the terminal determines whether the resource allocation message includes an explicit indication on which uplink access scheme to use, that is if the DFT precoder should be used or bypassed. If such explicit indication is included, the terminal selects the indicated precoder usage according to step 202. There are different ways on how such explicit indications may^¬ be carried out as described in the previous section regarding subset C3. If, on the other hand, no such explicit indication is included in the message, the terminal will in step 203 determine fulfilment by the resource allocation message of one or more predefined conditions defining usage or non-usage of the DFT precoder, thus implicitly indicating to the terminal the access scheme to be applied in the uplink. In step 204 the terminal will select uplink access scheme in accordance with said predefined conditions. In step 205, the terminal applies the selected access scheme in uplink transmission .

Fig. 3 illustrates an alternative embodiment, where the terminal may be configured via RRC signalling which uplink access scheme to be used in case the conditions for implicit indication of precoder usage are not fulfilled. Step 300, where the terminal receives a resource allocation message is identical to step 200 in Fig. 2. Thereafter, in step 301, the terminal determines the fulfilment of conditions for implicit indication on precoder usage . If such conditions are fulfilled, see step 302, the terminal will select uplink access scheme in accordance with step 303, which is identical to step 204 in Fig. 2. If, on the other hand, step 302 results in that no such conditions are fulfilled, the terminal will according to step 304 look for an explicit indication on precoder usage. Such indication may be included in the resource allocation message, or the terminal may already be configured via RRC signalling as to which access scheme to be used in case of non-fulfilment of the conditions for implicit indication. In step 305, the terminal selects access scheme according to the explicit indication on precoder usage. In step 306, the terminal applies the selected access scheme in uplink transmission.

Fig. 4 illustrates a method in a network node for providing indication on precoder usage to a terminal. In step 400, a resource allocation message is created. Such message comprises information to the terminal regarding for example terminal identification such as RNTI (Radio Network Temporary- Identifier) , allocation expressed in time and/or frequency and/or code, modulation and coding scheme (MCS) , MIMO configuration and power control. In step 401, the network node determines if the resource allocation message fulfils the predefined conditions for implicit indication of precoder usage. If such conditions are fulfilled, the network node is not required to take any action since the resource allocation message will implicitly indicate precoder usage to the terminal, see step 402. However, if no such conditions are fulfilled, and if precoder usage in case of non-fulfilment of the conditions for implicit indication is not already configured via RRC signaling, see step 403, the network node will in step 404 select which access scheme to be used for uplink transmission, and signal an explicit indication on this selection to the terminal in step 405. If, however, the terminal is already configured via RRC signaling as to which access scheme to be used in case of non-fulfillment of the conditions for implicit indication, no action is required from the network node at this stage, see step 406.

Fig. 5 illustrates schematically a terminal 500 such as a telephone, computer, handset etc supporting both DFT-spread OFDM and regular OFDM in the uplink. For the sake of clarity, features comprised in a terminal that are not directly related to this invention will not be discussed in the following and have been omitted in the schematic figure. The terminal 500 comprises a receiver 501 capable of receiving a resource allocation network from the network. The terminal furthermore comprises a processor unit 502 that is capable of determining fulfillment of one or more predefined conditions, implicitly indicating which access scheme to be used in uplink transmission, i.e. the precoder usage. Said processor unit is furthermore capable of selecting access scheme dependent on the fulfillment of said predefined conditions. A transmitter 503 will then apply the selected access scheme in uplink transmission.

Fig. 6 illustrates schematically a network node 600, such as a base station, comprising a processor 601 that is capable of determining if at least one of a set of predefined conditions for implicitly indicating precoder usage to the terminal is fulfilled for a resource allocation message to be sent to a terminal. Said processor unit is furthermore capable of applying an algorithm for selecting which access scheme to be used in uplink transmission in case no conditions for implicitly indicating the precoder usage to the terminal are fulfilled. A transmitter 602 will then in the case referred to above signal an explicit indication of the selected access scheme to the terminal 500.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive.

Claims

1. Method in a terminal for uplink transmission in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising the steps of

- receiving a resource allocation message from the network;

- applying the selected access scheme in the uplink transmission.

2. The method according to claim 1, wherein at least one condition defines a first subset out of pre-defined conditions, and wherein fulfillment of said first subset implies that regular OFDM should be used.

3. The method according to claim 2, wherein one condition defining said first subset is that the signaled resource allocation is non-contiguous .

4. The method according to claim 2 or 3 , wherein one condition defining said first subset is that single-user MIMO (Multiple Input Multiple Output) is used.

5. The method according to any of claims 2 - 4, wherein one condition defining said first subset is that the terminal transmission power is below a certain level.

6. The method according to any of claims 2 - 5, wherein one condition defining said first subset is that the modulation order and/or code rate exceeds/exceed a predefined threshold.

7. The method according to any of claims 2 - 6, wherein one condition defining said first subset is that the bandwidth of the allocated resource exceeds a certain bandwidth.

8. The method according to claim 1, wherein at least one condition defines a second subset out of pre-defined conditions, and wherein fulfillment of said second subset implies that DFT-spread OFDM should be used.

9. The method according claim 8 , wherein one condition of said second subset is that the signaled resource allocation is non-contiguous .

10. The method according claim 8 or 9, wherein one condition of said second subset is that MIMO is not used.

11. The method according to any of claims 1-10, wherein if said resource allocation does not fulfill any predefined condition for implicit indication, the terminal receives signaling from the network indicating which access scheme to be used in the uplink transmission.

12. The method according to claim 11, wherein said signaling comprises an explicit bit in a Physical Downlink Control Channel, PDCCH, message indicating which access scheme to be used.

13. The method according to claim 11, wherein said resource allocation message has a certain format indicating which access scheme to be used.

14. The method according to claim 11, wherein terminals with capability for both DFT-spread OFDM and regular OFDM receives a first and a second RNTI, Radio Network Temporary ID, where usage of the first RNTI indicates that regular OFDM is to be used, while usage of the second RNTI indicates that DFT- spread OFDM is to be used.

15. The method according to claim 11, wherein said terminal receives indication on which access scheme to use via RRC configuration.

16. Method in a network node for allocating resources to a terminal in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising the steps of

17. The method according to claim 16, wherein said indication comprises an explicit bit in a Physical Downlink Control Channel, PDCCH, message indicating which access scheme to be used.

18. The method according to claim 16, wherein a resource allocation message that is sent to the terminal has a certain format indicating which access scheme to be used.

19. The method according to claim 16, wherein UEs with capability for both DFT-spread OFDM and regular OFDM are given a first and a second RNTI, Radio Network Temporary ID, where usage of the first RNTI indicates that regular OFDM is to be used, while usage of the second RNTI indicates that DFT-spread OFDM is to be used.

20. The method according to claim 16, wherein said terminal is configured via RRC signaling on which access scheme to use.

21. A terminal capable of performing uplink transmission in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising

- a transmitter capable of applying the selected access scheme in the uplink transmission.

22. A network node capable of allocating resources to a terminal in a cellular telecommunication network supporting both DFT-spread OFDM and regular OFDM as access schemes in the uplink, comprising

a transmitter capable of signaling to the terminal an indication of the selected access scheme.