WO2008095334A1

WO2008095334A1 - Peak to average power ratio reduction in multi- carrier systems

Info

Publication number: WO2008095334A1
Application number: PCT/CN2007/000358
Authority: WO
Inventors: Jaap Van De Beek; Oskar Mauritz; Branislav Popovic; Mattias Wennstr-M
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2007-02-02
Filing date: 2007-02-02
Publication date: 2008-08-14
Also published as: CN101361305B; CN101361305A

Abstract

An improved Peak to Average Power Ratio (PAPR) reduction in a multi-carrier communication is disclosed. According to the present invention, bit-inversion is performed on a transmitted signal if necessary based on a total PAPR value. If PAPR reduction is determined necessary, a transmitter first determines, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted. Then, each bit determined to be inverted is inverted, thereby creating a bit inverted encoded information signal. An indicator is further set to a value indicating that at least one bit has been inverted, and the bit inverted encoded information signal and the indicator are transmitted to a receiver. If PAPR reduction is determined not necessary, the transmitter instead first sets an indicator to a value indicating that no bit has been inverted, and then transmits said encoded information and the indicator to a receiver.

Description

PEAK TO AVERAGE POWER RATIO REDUCTION IN MULTI- CARRIER SYSTEMS

Field of the invention

The present invention relates to a method for possible Peak to Average Power Ratio (PAPR) reduction in a multi-carrier communication system including a transmitter and a receiver, said method determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value.

The present invention also relates to a transmitter and a receiver, respectively, arranged for possible Peak to Average Power Ratio (PAPR) reduction in such a multi-carrier communication system.

The present invention also relates to an associated telecommunication system.

Background of the invention

The invention may be implemented in any communication link based on multi-carrier modulation, such as OFDM modulation, and employing channel coding. The 3GPP LTE standard, currently being developed, is an example of such a communication system. There is, however, a number of other systems in which the present invention may be implemented. The invention is in this document for simplicity reasons described in terms of an OFDM 3GPP

LTE system, but may, as is clear to a skilled person, also be implemented in any other of the systems mentioned above.

Multi-carrier communication methods, as Orthogonal Frequency Division Multiplexing (OFDM), have been adopted in many wireless standards. Signals transmitted in these systems tend to have large signal dynamics, especially when the signals are transmitted without having been adjusted in order to mitigate the large signal dynamics.

An OFDM signal is a multiplex of modulated carriers. In particular, each sample of the signal is a weighted sum of similarly distributed random variables, causing a Rayleigh-like amplitude distribution when the number of OFDM subcarriers becomes large. In order to transmit large signal peaks represented by the tails of this distribution, a high power amplifier at a transmitter must support a very large dynamic range which, in general, is expensive and power inefficient.

A high Peak to Average Power Ratio (PAPR) may thus be a problem in OFDM systems. The above mentioned efficiency problems related to high PAPR values further lead to a non- optimized energy consumption. Energy consumption is always an important factor in communication systems, especially in the uplink, since mobile terminals have limited energy resources, but also in the downlink. High PAPR values also result in heat generation in transmitter circuits and thus lead to energy losses.

Methods that reduce the Peak to Average Power Ratio (PAPR) of a transmitted signal, hereafter denoted total PAPR, will make transmission equipment cheaper and will also provide power savings.

The PAPR related problems and reduction techniques have been addressed in numerous prior art publications and patents. Some prior art solutions are based on coding at the expense of data rate (bandwidth). Other solutions are based on scrambling or phase manipulations, at the expense of heavily coded side information to be transmitted. A third class of solutions, so called tone reservation methods, reduce peak power by inserting signals in unused subcarriers. In wireless systems, this comes at the expense of additional power consumption and bandwidth as these subcarriers could otherwise have been used for data transmission. Finally, there exists a fourth class of solutions, the so called non-bijective constellations methods.

Of these prior art solutions, the third class of solutions, the tone reservation methods, is the most relevant one in the context of the present invention, along with for example prior art documents [1] and [2] from the fourth class, the non-bijective constellation methods. There is a list of prior art documents at the end of this specification.

However, the above mentioned prior art methods for reducing PAPR are non-optimal regarding efficient use of radio resources, such as subcarrier usage and transmission power usage. For example, in tone reservation methods, some carriers are reserved for transmission of signals mitigating PAPR for the total OFDM signal, i.e. mitigating a total PAPR. This might reduce PAPR, but at the same time the method has some disadvantages. One disadvantage is that some carriers are only used for PAPR mitigation reasons and are not used for carrying information, which lowers the throughput of the system. Another disadvantage is that a part of the total transmission power is used for these reserved carriers, instead of using all of the total transmission power on carriers that carry information.

One further drawback of the above mentioned prior art solutions is that these methods do not take into consideration the fact that PAPR not always has a high value and that PAPR therefore not always is a problem in the system. In a multiuser downlink of a cellular system, the transmission power of a base station transmitter is usually limited by a certain maximum power level. However, power generally fluctuates over time and the base station does not transmit at a power level near its maximum all the time. The PAPR related problem is thus not always present even for a fixed power amplifier having a certain dynamic range. In the above-mentioned prior art methods, such as in the tone reservation methods, carriers are reserved for PAPR reduction the whole time, whether or not there is a need for PAPR reduction. In situations of low levels of PAPR, this is a waste of radio resources.

Some of the above mentioned problems have been solved previously, as indicated in the prior art document [3]. This document [3] discloses PAPR reduction in a transmitter by the use of bit inversion of a predetermined bit of the channel encoded data when PAPR of the

OFDM signal is greater than a reference value. In a receiver, the transmitted signal is then detected by the use of, among other components, a channel decoder circuit. The channel decoder circuit is meant to compensate, by decoding the received signal, for the distortion caused by inversion of the bit in the transmitter.

The solution shown in prior art document [3] thus alleviates some of the problems stated above, but is still a non-optimal solution regarding PAPR reduction and signal reception quality in the receiver. The solution shown in prior art document [3] thus introduces a problem relating to the exactness of the PAPR reduction and a problem relating to detection of the information in the signal received at the receiver.

Summary of the invention

It is an object of the present invention to provide a PAPR reduction that solves the above stated problem. Accordingly, the present invention aims to provide a more efficient PAPR reduction than the PAPR reduction known in the prior art, while, at the same time, providing a high quality detection of the information signal in the receiver.

The object is achieved by a method where the transmitter performs the following steps if PAPR reduction is determined necessary: determining, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted, inverting each bit determined to be inverted, thereby creating a bit inverted encoded information signal, setting an indicator to a value indicating that at least one bit has been inverted, and transmitting said bit inverted encoded information signal and said indicator to said receiver; whereas said transmitter performs the following steps if PAPR reduction is determined not necessary: setting an indicator to a value indicating that no bit has been inverted, and transmitting said encoded information signal and said indicator to said receiver.

The object is also achieved by a method where the receiver performs the following steps: receiving an input signal and an indicator transmitted from said transmitter, said input signal possibly including at least one inverted bit, detecting a value of said indicator, and detecting said input signal based on said value of said indicator.

The object is also achieved for a transmitter being arranged to perform the following steps if PAPR reduction is determined necessary: determining, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted, inverting each bit determined to be inverted, thereby creating a bit inverted encoded information signal, setting an indicator to a value indicating that at least one bit has been inverted, and transmitting said bit inverted encoded information signal and said indicator to said receiver; whereas said transmitter is arranged to perform the following steps if PAPR reduction is determined not necessary: setting an indicator to a value indicating that no bit has been inverted, and transmitting said encoded information signal and said indicator to said receiver.

The object is also achieved for a receiver being arranged to perform the following steps: receiving an input signal and an indicator transmitted from said transmitter, said input signal possibly including at least one inverted bit, detecting a value of said indicator, and detecting said input signal based on said value of said indicator.

The object is also achieved for a telecommunication system including at least one transmitter and/or receiver as defined above.

The PAPR reduction according to the present invention is characterized in that it is only performed if it is actually needed. There is no need for, as was the case in some prior art solutions, having a lot of radio resources constantly reserved for PAPR purposes. Therefore, the available radio resources will be utilized more efficiently.

Further, in the PAPR reduction according to the present invention, a subset of bits is predefined or selected, and it is evaluated, for each bit in the subset, whether that bit should be inverted or not. This evaluation takes into consideration which effect a possible inversion of a bit has on the total PAPR value for the output signal. These evaluations can, according to the invention, be performed in a more or less exhaustive way and the invention thus results in a very exact PAPR reduction.

Also, according to the present invention, an indicator is set and transmitted to each receiver of a transmitted signal. The set indicator value indicates whether bit inversion has been performed or not. The receiver may then use this information to enhance the performance on the detection procedure in the receiver. The present invention thus improves quality of the detection of the encoded information signal in the receiver.

In an embodiment of the invention, user specific indicators are used, making it possible for a transmitter to select to which receivers bit inversion is to be used. This is advantageous since different users are in different communication situations and also since different receivers have different capabilities of detecting bit inverted signals. By using a set of flags, user specific indicators, PAPR reduction can be adapted to a multiuser situation.

Further, the use of user specific indicators makes it possible for each user to know if bit- inversion has been performed on signals transmitted to that specific user. By this, no receiver is forced to try to detect the received signal as if it had been bit-inverted even though the received signal had not been bit-inverted, as could be the case if only one single indicator is used for all receivers. Thus, the use of a set of user specific indicators improves throughput, since receiver detection is improved.

In an embodiment of the present invention, Hybrid Automatic Repeat Request (H-ARQ) is used in the system. When H-ARQ is used, the present invention uses different subsets or different mappings for each or the retransmissions. This has the advantage that diversity is added to the transmissions/retransmissions, which is very favorable since it increases the effective code rate.

In an embodiment of the present invention, subsets are chosen such that sign bits are included in the subsets. To include sign bits in the subset has the advantage that when this sign bit is inverted it has a big influence on the resulting total PAPR value, i.e. the PAPR value of the transmitted signal.

According to other embodiments of the present invention, subsets are predefined or adaptively selected by the transmitter. The subset may also be selected such that its bits are evenly spread out over the carriers used. Another possibility is to select the subset such that bit inversion is performed on bits transmitted to receivers having good reception conditions, such as good SNR (Signal to Noise Ratio). These different ways of selecting the used subset have different advantages. The multiple possibilities for selecting the subsets have the advantage that one selection may be chosen that results in a good PAPR reduction.

Detailed exemplary embodiments and advantages of the PAPR reduction according to the invention will now be described with reference to the appended drawings illustrating some preferred embodiments. Brief description of the drawings

Fig. 1 shows a transmitter according to the present invention.

Fig. 2 shows Frequency Division Multiplexing (FDM) of users in the system.

Fig. 3 shows a receiver according to the present invention.

Fig. 4 shows a constellation diagram for 16-QAM.

Fig. 5 shows the result of an evaluation simulation of the invention.

Fig. 6 shows a flow diagram of the method of the present invention.

Detailed description of preferred embodiments

The present invention aims to perform a more exact PAPR reduction than the prior art solutions described above.

At a transmitter, operating according to the present invention, a predefined subset of the transmitted bits is defined to be a subset of bits being candidates to bit inversion. Bit inversion means that a "0" is replaced by a "1" or a "1" is replaced by a "0". According to the invention, when the PAPR increases above a threshold, the transmitter invokes PAPR-reduction and determines for each of the bits in the predefined subset if bit inversion should take place or not for that bit, the choice of inversion or non-inversion being subject to evaluation of a total PAPR value of the resulting OFDM signal (symbol).

When performing the PAPR reduction by using bit inversion, much less radio resources are wasted than in, for instance, prior art tone reservation methods, since all carriers may be used for conveying information. Further, the choice of inversion or non-inversion for each bit of the subset, while considering a resulting PAPR value, makes the PAPR reduction according to the present invention very efficient, since bit inversion may be chosen for the specific bit or bits having a great influence on the resulting PAPR value. An exact determination whether each bit in the subset should be inverted or not may thus be achieved according to the present invention, which is advantageous. The subset of transmitted bits is here not "reserved" in the sense that no data is transmitted on their positions. Rather, the transmitted bits in these predefined positions are subject to incidental bit inversion. Only when the PAPR calls for counter-measures this bit inversion is applied. This is advantageous from a radio resource point of view.

This way of determining from time to time whether inversion is to be performed has similarities with incidental puncturing of the encoded data message in a system using puncturing. The method of inverting bits differs from puncturing in that bits are here being inverted whereas being erased in puncturing. Further, when using puncturing, the receiver adds "O"-bits in the positions where bits were erased in the transmitter. In the decoder, these punctured bits are then restored through the strength of the channel code.

The transmitter according to the present invention may further transmit a flag, including at least one bit, as side information indicating whether any bit inversions are present in the current OFDM symbol or not. In the case of multi-user scheduling in the frequency domain, Frequency Division Multiplexing (FDM), the flag is broadcasted in order to inform all receivers that receive data from the current OFDM symbol if bit inversion has been performed or not. Further, in the multi-user case, a number of bits may be used for this flag when user specific flags are used, these user specific flags being transmitted to each specific user to inform each user whether or not bit inversion has been performed on information transmitted to that user.

A receiver receiving a bit inverted signal and knowing the predefined subset of transmitted bits subject to incidental bit inversion may perform detection in at least two ways:

1. Perform detection as normal and neglect that any of these bits may be inverted.

2. Treat the subset of bits in a special way, using the a priori knowledge that these bits may have been inverted in the detection.

The present invention performs detection according to the second alternative above, i.e. by using the information relating to the bit inversion transmitted by the transmitter in the detection procedure in the receiver.

For the receiver, it is of course a big advantage to know whether inversion has been performed on any of the bits it is receiving. Especially, if the receiver knows which bits that are part of the subset, for which the bits possibly may have been inverted, then the receiver may rely less on these bits than on the rest of the received bits when the receiver performs channel decoding. That is, if the receiver knows that a number of bits, having known positions in the received signal, may have been inverted, then the receiver may utilize this knowledge to perform a more efficient and exact channel decoding.

Fig. 1 shows a transmitter according to the present invention. At the transmitter, a channel encoder transforms the sequence of information bits into a sequence of encoded bits. These encoded bits are then mapped (by interleaving, mapping to tones and modulation constellation symbols) on the transmitted bits. Typically, the channel encoder adds redundancy to the information bits used for error correction. Convolutional codes, block codes, turbo codes and LDPC (low Density Parity Check) codes are examples of such codes. As is clear for a person skilled in the art, essentially any error correcting code may be used for this channel coding.

A predefined subset of the resulting sequence of transmitted bits is then candidates for bit inversion, i.e. a "O"-bit may be turned into a "1" and vice versa. The predefined subset is either set by an a priori definition, i.e. the subset is always fixed and known to both transmitter and receiver, or is set using signalling, i.e. the transmitter selects a subset and signals to the receiver information indicating the particular subset to be used. The signalling of subset information may be done by the use of a side-channel, broadcast channel, control channel, or the like.

The transmitter determines then for each of these transmitted bits whether bit inversion should be performed or not, subject to evaluation of the PAPR of the resulting signal (i.e. OFDM symbol). The decision of which one of more bits, if any, of the subset bits that should be inverted can be made in a number of ways according to the invention. It can be done either through exhaustive search, i.e. by checking all possible combinations of inversion and non- inversion of the bit/bits in the subset used and then verifying the PAPR of the OFDM symbol they generate, or through the employment of some suboptimal, possibly iterative, algorithm. Such a suboptimal algorithm may decide the value of each bit by evaluating its effect on the PAPR. A suboptimal algorithm may preferably be performed in a sequential order, such that inversion or non-inversion is first decided for a first bit in the subset thereby setting the first bit to this decided value, whereafter inversion or non-inversion is decided for a second bit in the subset, and so on, each of these decision being made while considering their effect on a total PAPR value of the output signal.

Such an intelligent choice, according to the invention, of which bit/bits of the bits in the subset of bits to invert has a positive effect on the PAPR reduction. This leads to a more efficient PAPR reduction than the one shown in the prior art document [3].

The transmitter according to the invention may transmit, as side-information, a flag indicating whether bit inversion of the transmitted bits is present or not in the current OFDM symbol (or multiple of OFDM symbols). This flag message may be a 1-bit flag broadcasted to all users or may also be a user specific flag sent individually to each receiver. As is described below, in the latter case, a flag containing at least one bit may be sent individually to each user, resulting in a total number of bits used for these flags corresponding to the number of receivers that are transmitted an individual flag.

When the indicator flag is sent individually to each receiver, the transmitter transmits, as side-information, a set of receiver specific flags, typically 1 bit each, to each receiver individually, indicating whether any bit inversions are present or not in the subcarriers containing data for this particular receiver in the current OFDM symbol. This may be very useful in a multi-user situation, as will be shown in the following.

Fig. 2 shows Frequency Division Multiplexing (FDM) of users in the system. In a multiuser situation, a number of users, that is a number of receivers, are present in the system. These different users may typically have different reception conditions at a given moment. It may not always be an optimal solution to perfoπn bit-inversion on signals transmitted to all of these users, especially since the bit-inversion results in a slight decoding degradation in the receivers. If some receivers have poor reception condition, it may therefore be disadvantageous to add to these reception problems by also adding the slight degradation related to bit-inversion for these specific users.

In a multiuser situation, a transmitter, such as a Base Transceiver Station (BTS), Node B or the like, in accordance with the present invention, may be able to choose to which ones of these receivers it should perform bit inversion on the encoded information signal. Bit inversion may be performed on encoded information signals transmitted to all of the receivers the transmitter communicates with and it may also be performed on encoded information signals transmitted to a limited number of these receivers.

That is, the subset of bits to be inverted may be chosen such that bit inversion is performed on bits transmitted to a number of receivers being less than the total number of receivers the transmitter is transmitting to. By this, the transmitter can control which users in the system that have to perform bit inversion. Typically, if there are receivers that are not able to perform a proper detection of a signal having inverted bit, bit inversion is not performed on signals transmitted to these receivers. More specifically, in an embodiment of the invention, signals transmitted to receivers having poor SNR (Signal to Noise Ratio) conditions should preferably not be bit-inverted. Bit inversion should instead preferably be performed on signals transmitted to receivers having good reception conditions, such as good SNR. This selection of users, to which bit-inversion should be performed, enhances the total throughput in the system.

In order to fully take advantage of this user selective bit-inversion, there is also a further optimization problem that has to be solved. If user selective bit-inversion is used, such that bit-inversion is performed on signals to some, but not all, of the receivers, then if only one indicator is used to all the receivers, this indicator must be chosen such that it indicates that bit inversion is used. This indication indicated by the indicator is then correct for the receivers receiving bit-inverted signals, but is not correct for the receivers receiving signals that have not been bit-inverted.

If a receiver believes that bit-inversion has been performed on a received signal even though it has not been performed, the receiver will perform weighting and decoding of the signal as if it had been bit-inverted. This would lead to degradation in reception performance and overall system throughput.

In a multiuser situation, a user specific indicator is therefore, according to an embodiment of the present invention, transmitted from the transmitter to each of the receivers, indicating where bit inversion has been performed on signals transmitted to that specific user. Of course, this user specific indicator may have the same value for a number of users if their signals have been dealt with in the same way by the transmitter. These user specific indicators, such as flags, makes it possible for each user to weight and/or decode the received signal, based on the information whether bit-inversion has been performed for each user, in such a way that the problems relating to use of a single flag in a multi-user system are alleviated. The use of a set of user specific indicators thus enhances reception performance and throughput in the system.

In the case of FDM of the users and user multiplexing within an OFDM symbol, some of the receivers can thus optionally be chosen to be affected by bit inversion PAPR reduction technique, by the use of individual indicator flags to receivers. This has a big advantage in its ability to adapt bit inversion to the needs of different users in the system as well as to the capabilities of different users to detect bit inverted signals such that the resulting signal has a high quality.

If a retransmission method, such as Hybrid Automatic Repeat Request (H-ARQ), is used in the system, the mapping of the encoded bits on the transmitted bits may be different for each retransmission. By this changing mapping, the encoded bits subject to possible bit inversion change in each retransmission, thereby introducing diversity which increases the effective code rate of the encoded message. The mapping may be predefined and dependent on the H-ARQ transmission/retransmission number. By this the receiver knows which encoded bit/bits that may possibly be inverted just from knowing the transmission/retransmission number.

According to another embodiment of the present invention, different predefined subsets are used for each ARQ transmission/retransmission. These subsets are predefined for each transmission/retransmission number, such that the receiver knows which encoded bit/bits that may possibly be inverted just from knowing the transmission/retransmission number.

The embodiments including retransmissions thus have diversity advantages and increase the effective code rate.

Fig. 3 shows a receiver according to the present invention. The receiver according to the invention knows the predefined subset of transmitted bits, either by a priori knowledge or through signalling from the transmitter. The receiver thus knows the subset of bits subject to possible bit inversion and can therefore perform detection of the received signal either by the use of normal detection or by treating the particular subset of bits in a special way. In the normal detection procedure, the fact that bits may have been inverted is simply ignored by the receiver. Inverted bits may then be corrected by the channel decoder by the robustness of the error correcting code used. Inverted bits are hereby treated in the same way as bits corrupted by a noisy transmission channel.

According to the present invention, the procedure of special treatment of subset bits is used. The receiver hereby uses the a priori knowledge that the bits in the subset may have been inverted. This knowledge may be used to assign lower weights to the soft detected values of the received signal before employing the signal to the channel decoder. This way of assigning lower weights to possibly inverted bits enhances the efficiency of the channel decoding of the receiver.

This weight assigning method is here performed similarly as bit weighting is performed in a receiver in a system using puncturing. In a receiver using puncturing, a number of bits are added to the signal in the positions where bits were punctured in the transmitter. These added bits are then assigned lower weights than the other bits in the signal, typically they are assigned a zero weight corresponding to total lack of knowledge of the value of the added bits. Thereafter channel decoding is performed on the signal. The weighting before decoding in a puncturing system has a number of decoding efficiency advantages. The weighting of the bits according to the present invention has the same clear advantages from the weighting step as the weighting in the puncturing system, although puncturing, i.e. deletion of bits in the transmitter followed by addition of bits in the receiver, is not performed in the PAPR reduction according to the invention.

Fig. 4 shows a typical constellation diagram for 16-QAM. From studying this constellation diagram it can be understood, as will hereafter be described, that inversion of different bits within a symbol may have different influences on the output signal and thus on the PAPR of the output signal. Each symbol in this constellation diagram has four bits. It can be seen that each of the symbols corresponding to constellation points on the left half of the diagram, on the left side of the Q-axis, has a "0" as its first symbol bit, whereas each symbol corresponding to constellation points on the right half of the diagram, on the right side of the Q-axis, has a "1" as its first symbol bit. Analogously, each of the symbols corresponding to constellation points on the upper half of the diagram has a "0" in its third position, whereas each of the symbols corresponding to constellation points on the lower half of the diagram has a "1" in its third position. These bits of a symbol having the same value for all symbols on one half of the diagram, but a different value for all symbols on the other half of the diagram are denoted sign bits.

According to an embodiment of the present invention, the subset of bits to be inverted may be chosen in such a way that it includes sign bits. This has the advantage that an inversion of such a sign bit results in a movement from one constellation point on one side of the constellation diagram to another constellation on the opposite side of the diagram. This movement thus results in a long Euclidean distance movement to a constellation point in the other half of the diagram, which has a greater impact on the output signal, and thus the PAPR, than a short Euclidean distance movement within one half of the diagram.

As an example relating to the 16-QAM constellation diagram of fig. 4, if we invert the first bit in the symbol "0000", we would get a bit inverted symbol "1000". If we invert the second bit in the symbol "0000", we would instead get a bit inverted symbol "0100". From the diagram, it may easily be seen that the inversion of the first bit of the symbol results in a movement of constellation points having a longer Euclidean distance than if the second bit had been inverted. Constellation points for "0000" and "0100" are next to each other in the diagram, whereas constellation points for "0000" and "1000" are of opposite sides of the diagram. The movement to the opposite side of the diagram is likely to have a greater influence on the resulting signal and thus on the amplification of the signal than the movement to a constellation next to the original one.

By taking this knowledge into consideration, subsets may thus, according to the present invention, be selected such that a wanted PAPR reduction is achieved, by inversion of the bits in the subset.

Here, the modulation scheme 16-QAM with Gray mapping has been used for illustrating the influence different bits of a symbol have on PAPR. Of course, this also applies to other modulation schemes, where corresponding relationships between different bits, constellation points and PAPR are present. Thus, this applies to essentially any modulation scheme having its constellation points and mapping defined such that inversion of at least one bit in a modulation symbol results in a change of constellation point to a constellation point in another quadrant. In other words, this applies to all modulation schemes having sign bits. The present invention may be implemented using essentially any modulation scheme and is not restricted to 16-QAM. The present invention may also be implemented using a modulation scheme having another mapping than Gray mapping, if this other mapping is defined such that sign bits are present.

According to an embodiment of the present invention, the transmitter may adaptively select the subset of bits that is considered to be inverted, and signal information identifying this subset to the receiver. When the transmitter adaptively selects a subset to be used it may take into consideration momentary PAPR conditions as well as the influence different bits have of the PAPR reduction (as has been described above). By doing so the transmitter may, according to the invention, in a very exact way adapt the PAPR reduction process to the momentary PAPR conditions in the system.

Further, in a step of the method of the present invention, it is decided which bit or bits in a subset that should be inverted. From the discussion above, it can be understood that also a choice between different bits to invert within a predetermined subset of bit may result in different changes in PAPR. Since inversion of different bits results in different movements in the constellation diagram, also the choice between bits to invert within a subset will have an influence on the PAPR reduction. Thus, by choosing which bit or bits in a subset to invert while considering the total PAPR for the output signal, a very efficient PAPR reduction may be performed according to the present invention.

Further, the subset may also be selected such that its bits are evenly spread out over the carriers used. This has a positive effect on the total PAPR value, i.e. the PAPR of the output signal.

A transmitter and a receiver for implementing the transmitter method steps and receiver method steps according to the invention will be described below.

The transmitter according to the invention ^"is arranged for determining if a PAPR reduction is necessary. The transmitter is further arranged with means for performing, if necessary, bit inversion, while considering a total PAPR value, for selected bits in a subset and for transmitting a bit inverted signal along with an indicator indicating that bit inversion has been performed to a receiver. The receiver according to the invention is arranged for receiving an input signal possibly including bits inverted by the transmitter. The receiver is further arranged with means for detecting an indicator value and for detecting the input signal based on the detected indicator.

The transmitter and receiver of the invention may be adapted to perform any of the steps of the method of the invention. A trivial requirement is of course that such a step involves a transmitter or a receiver, respectively. The invention also includes a communication system including at least one transmitter and one receiver according to the present invention.

Fig. 5 shows the result of an evaluation simulation of the invention. It is assumed in these simulations that the PAPR reduction method in the invention is applied in all OFDM symbols, hence no on/off switching using a side-information flag is implemented.

The simulations are performed for OFDM symbols with 256 tones and 16-QAM modulation on each tone. 8 sign bits or 4 tones are used for PAPR reduction.

Fig. 5 illustrates the improvement in PAPR. Note that for the method according to the invention, 99.9% of the OFDM symbols have a PAPR of less than 9.5 dB while for the prior art method "tone reservation" this figure is about 9.9 dB and for the reference system where no PAPR reduction is performed, this figure is 10.5 dB. Thus a transmitter implementing the invention can employ a 1 dB lower power backoff than a reference transmitter without PAPR reduction. In practice, given a maximum power level where linear amplification can be ensured, this means that the average power can be boosted with 1 dB. This in turn means that users in the network experience a 1 dB higher SNR than in the reference system.

Note that the cost for the transmission of the side information bit is not incorporated in the performance curves. However, the transmission of a single flag (one bit for all users or 1 bit per user) indicating whether bit-reversion is employed is usually insignificant compared to the number of transmitted data bits in an OFDM symbol.

The receiver is not able to completely compensate for the detection of inverted bits that is employed at the receiver. The loss, however, in terms of block-error rate is only about 0.2 dB at 0.01 block-error rate (BLER) for the invention. Note also that the PAPR reduction method was used in all OFDM symbols so this BLER value shows the worst case BLER degradation. Thus, to maintain BLER performance the receiver must raise the average power with 0.2 dB. Since the PAPR reduction is about 1 dB, the net effect is then a 0.8 dB PAPR gain.

Fig. 6 shows a flow diagram of the transmitter method steps of the present invention. In the first step of the flow diagram, the transmitter determines if reduction of PAPR is necessary, based on a total PAPR value, i.e. based on PAPR of the output signal.

If PAPR is determined necessary, the transmitter first determines, in a following step, while considering the total PAPR value, for each bit in a subset of bits of the encoded information signal if that bit is to be inverted. The transmitter then inverts each bit determined to be inverted, in a following step. The transmitter thereby creates a bit inverted encoded information signal. The transmitter further sets at least one indicator to a value indicating that at least one bit has been inverted, and then finally transmits the bit inverted encoded information signal and the at least one indicator to the at least one receiver in a last step.

If PAPR reduction is not determined necessary , the transmitter instead jumps directly to the step of setting an indicator to a value indicating that no bit has been inverted, and thereafter transmits the encoded information signal, without having performed bit inversion on it, and the indicator to the receiver. This is also the reason for the use of the term "possible Peak to Average Power" in the claims, "possible" here means that PAPR reduction is performed if necessary.

The PAPR reduction according to the invention may be modified by those skilled in the art, as compared to the exemplary embodiments described above.

Prior art documents

[1] M. Sharif, C. Florens, M. Fazel, B. Hassibi, "Amplitude and Sign Adjustment for Peak- to-Average-Power Reduction", IEEE Transactions on Communications, Vol. 53, no. 8, August 2005.

[2] B.S. Krongold and D.L. Jones, "PAR Reduction in OFDM via Active Constellation Extension", IEEE Transactions on Broadcasting, Vol. 49, no. 3, September 2003

[3] US patent application US 2006/0120269 Al, Nam-Il Kim et al.

Claims

1. Method for possible Peak to Average Power Ratio (PAPR) reduction in a multi- carrier communication system including a transmitter and a receiver, said method determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value, characterized in that said transmitter performs the following steps if PAPR reduction is determined necessary: determining, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted, inverting each bit determined to be inverted, thereby creating a bit inverted encoded information signal, setting an indicator to a value indicating that at least one bit has been inverted, and transmitting said bit inverted encoded information signal and said indicator to said receiver; whereas said transmitter performs the following steps if PAPR reduction is determined not necessary: setting an indicator to a value indicating that no bit has been inverted, and transmitting said encoded information signal and said indicator to said receiver.

2. Method as claimed in claim 1, wherein said encoded information signal or said bit inverted encoded information signal is transmitted from said transmitter to at least two receivers in said system.

3. Method as claimed in claim 2, wherein said step of setting said indicator includes setting a single value intended for all receivers that receive the signal from said transmitter, indicating that bit inversion is used for all said receivers.

4. Method as claimed in claim 3, wherein said single value is broadcasted to all said receivers.

5. Method as claimed in claim 2, wherein said step of setting said indicator includes setting a receiver specific value for each receiver indicating that bit inversion is used or is not used for that specific receiver.

6. Method as claimed in claim 5, wherein said receiver specific value is transmitted to each of said receivers.

7. Method as claimed in claim 2, wherein said subset of bits is chosen such that possible bit inversion is performed on bits transmitted to a limited number of receivers, said limited number of receivers being less than a total number of receivers that receive the signal from said transmitter.

8. Method as claimed in claim 7, wherein said subset of bits is chosen such that possible bit inversion is performed on bits transmitted to receivers having good reception conditions.

9. Method as claimed in claim 7, wherein said subset of bits is chosen such that possible bit inversion is performed on bits transmitted to receivers having good SNR (Signal to Noise Ratio).

10. Method as claimed in claim 1, wherein said subset of bits is fixed and known to both said transmitter and said receiver.

11. Method as claimed in claim 1 , wherein said subset of bits includes at least two bits of at least one carrier of said multi-carrier communication system.

12. Method as claimed in claim 11, wherein said bits of said subset are essentially evenly distributed on the carriers of said multi-carrier communication system.

13. Method as claimed in claim 1, wherein said subset of bits is variable.

14. Method as claimed in claim 13, wherein said subset of bits to be used is chosen at said transmitter.

15. Method as claimed in claim 14, wherein information relating to said chosen subset is signaled from said transmitter to said receiver.

16. Method as claimed in claim 1, wherein said system uses retransmission error correction, wherein a different subset of bits is used for each retransmission.

17. Method as claimed in claim 1, wherein said system uses retransmission error correction, wherein a different mapping of encoded bits on transmission bits is used for each retransmission.

18. Method as claimed in any one of claims 16 or 17, wherein said retransmission error correction is Hybrid Automatic Repeat reQuest (HARQ).

19. Method as claimed in claim 1 , wherein said step of determining for each bit in said subset if it is to be inverted includes selection of one combination out of all relevant combinations of inversions and non-inversions for each bit in said subset, said selection being based on said total PAPR value.

20. Method as claimed in claim 1, wherein said step of determining for each bit in said subset if it is to be inverted includes determining sequentially for each bit if is to be inverted or not, based on said total PAPR value.

21. Method as claimed in claim 1, wherein said subset of bits of said encoded information signal is selected based on a potential impact inversion of the bits of said subset has on said PAPR value.

22. Method as claimed in claim 21, wherein said subset of bits of said encoded information signal is selected such that inversion of at least one bit in said subset results in a change of constellation point in a modulation scheme, from a constellation point in one quadrant to another constellation point in another quadrant.

23. Method as claimed in claim 22, wherein said subset of bits includes at least one sign bit.

24. Method as claimed in claim 1, wherein the inversion of bits is performed after a channel encoder of said transmitter and before a mapper of said transmitter.

25. Method as claimed in claim 1, wherein the inversion of bits is performed after a mapper of said transmitter.

26. Method as claimed in claim 1, wherein PAPR reduction is determined necessary if said total PAPR value exceeds a PAPR threshold.

27. Method for possible Peak to Average Power Ratio (PAPR) reduction in a multi- carrier communication system including a transmitter and a receiver, said method determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value, characterized in that said receiver performs the following steps: receiving an input signal and an indicator transmitted from said transmitter, said input signal possibly including at least one inverted bit, detecting a value of said indicator, and detecting said input signal based on said value of said indicator.

28. Method as claimed in claim 27, wherein said step of detecting said value of said indicator includes detecting a single value broadcasted to all receivers that receive the signal from said transmitter.

29. Method as claimed in claim 27, wherein said step of detecting said value of said indicator includes detecting a receiver specific value transmitted to at least one specific receiver.

30. Method as claimed in claim 27, wherein, if the detected value of said indicator indicates that bit inversion has been performed, said step of detecting said input signal includes weighting bits of the input signal before channel decoding.

31. Method as claimed in claim 30, wherein said weighting is performed based on said value of said indicator.

32. Method as claimed in claim 31, wherein said weighting is performed such that said at least one bit in said subset, including bits possibly being inverted, is assigned a low weight.

33. Method as claimed in claim 27, wherein information relating to a subset including bits possibly being inverted is received and detected.

34. A transmitter arranged for possible Peak to Average Power Ratio (PAPR) reduction in a multi-carrier communication system including a transmitter and a receiver, said transmitter being arranged for determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value, characterized in that said transmitter is arranged to perform the following steps if PAPR reduction is determined necessary: determining, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted, inverting each bit determined to be inverted, thereby creating a bit inverted encoded information signal, setting an indicator to a value indicating that at least one bit has been inverted, and transmitting said bit inverted encoded information signal and said indicator to said receiver; whereas said transmitter is arranged to perform the following steps if PAPR reduction is determined not necessary: setting an indicator to a value indicating that no bit has been inverted, and transmitting said encoded information signal and said indicator to said receiver.

35. The transmitter as claimed in claim 34, wherein said transmitter further is arranged for: transmitting said encoded information signal or said bit inverted encoded information signal to at least two receivers in the system, and setting said indicator such that a receiver specific value for each of said receivers is set, indicating that bit inversion is used or is not used for that specific user.

36. A receiver arranged for possible Peak to Average Power Ratio (PAPR) reduction in a multi-carrier communication system including a transmitter and a receiver, said transmitter being arranged for determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value, characterized in that said receiver is arranged to perform the following steps: receiving an input signal and an indicator transmitted from said transmitter, said input signal possibly including at least one inverted bit, detecting a value of said indicator, and detecting said input signal based on said value of said indicator.

37. A telecommunication system arranged for possible Peak to Average Power Ratio

(PAPR) reduction in a multi-carrier communication system including a transmitter and a receiver, said transmitter being arranged for determining if reduction of PAPR, by inversion of at least one bit of an encoded information signal sent from said transmitter to said receiver, is necessary, based on a total PAPR value, characterized by that said telecommunication system includes at least one of the following components: a transmitter arranged to perform the following steps if PAPR reduction is determined necessary: determining, while considering said total PAPR value, for each bit in a subset of bits of said encoded information signal if that bit is to be inverted, inverting each bit determined to be inverted, thereby creating a bit inverted encoded information signal, setting an indicator to a value indicating that at least one bit has been inverted, and transmitting said bit inverted encoded information signal and said indicator to said receiver; whereas said transmitter is arranged to perform the following steps if PAPR reduction is determined not necessary: setting an indicator to a value indicating that no bit has been inverted, and transmitting said encoded information signal and said indicator to said receiver; and a receiver arranged to perform the following steps: receiving an input signal and an indicator transmitted from said transmitter, said input signal possibly including at least one inverted bit, detecting a value of said indicator, and detecting said input signal based on said value of said indicator.