WO2018127504A1

WO2018127504A1 - Polar coding with dynamic adaptation of the frozen bit pattern

Info

Publication number: WO2018127504A1
Application number: PCT/EP2018/050098
Authority: WO
Inventors: Baris GÖKTEPE; Cornelius Hellge; Thomas Wirth; Thomas Schierl; Thomas Fehrenbach
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2017-01-04
Filing date: 2018-01-03
Publication date: 2018-07-12

Abstract

A polar decoder includes an input to receive a polar encoded codeword over a channel. The polar decoder includes a processor to decode the received polar encoded codeword to obtain a decoded codeword. Selected bits of the decoded codeword are frozen in accordance with a frozen bit pattern. The processor determines a quality of the transmission over the channel, and, responsive to the quality, initiates a modification of the frozen bit pattern.

Description

POLAR CODING WITH DYNAMIC ADAPTATION OF THE FROZEN BIT

PATTERN

Description

The present invention concerns decoders and encoders for polar encoded codewords as well as the transmission of polar encoded codewords in a communication system, for example a wireless communication system. Fig. 1 is a schematic representation of an example of a network infrastructure, such as a wireless communication network or wireless communication system, including a plurality of base stations eNB₁ to eNB₅, each serving a specific area surrounding the base station schematically represented by the respective cells lOC to 100₅. The base stations are provided to serve users within a cell. A user may be a stationary device or a mobile device. Further, the wireless communication system may be accessed by loT devices which connect to a base station or to a user. loT devices may include physical devices, vehicles, buildings and other items having embedded therein electronics, software, sensors, actuators, or the like as well as network connectivity that enable these devices to collect and exchange data across an existing network infrastructure. Fig. 1 shows an exemplary view of only five cells, however, the wireless communication system may include more such cells. Fig. 1 shows two users UE1 and UE2, also referred to as user equipment (UE), that are in cell 100₂ and that are served by base station eNB₂. Another user UE₃ is shown in cell 100₄ which is served by base station eNB₄. The arrows 102^ 102₂ and 102₃ schematically represent uplink/downlink connections for transmitting data from a user UE UE₂ and UE₃ to the base stations eNB₂, eNB₄ or for transmitting data from the base stations eNB₂, eNB₄ to the users ΌE^, UE₂, UE₃. Further, Fig. 1 shows two loT devices 104₁ and 104₂ in cell 100₄, which may be stationary or mobile devices. The loT device 104^ accesses the wireless communication system via the base station eNB₄ to receive and transmit data as schematically represented by arrow IO6₁. The loT device 104₂ accesses the wire- less communication system via the user UE₃ as is schematically represented by arrow 106₂.

The wireless communication system may be any single-tone or multicarrier system based on frequency-division multiplexing, like the orthogonal frequency-division multiplexing (OFDM) system, the orthogonal frequency-division multiple access (OFDMA) system defined by the LTE standard, or any other iFFT-based signal with or without CP, e.g. DFT-s- OFDM. Other waveforms, like non-orthogonal waveforms for multiple access, e.g. filter- bank multicarrier (FBMC), generalized frequency division multiplexing (GFDM) or universal filtered multi carrier (UFMC), may be used. In a communication network as shown in Fig. 1 there may be communication channels which have substantially constant channel properties or which have channel properties that change only slowly over time. For example, a wireless backhaul channel connecting one or more of the base stations may have substantially constant channel properties. Likewise, a channel between an loT device, like a sensor device located at a fixed posi- tion, may communicate to a base station over a communication channel having substantially constant channel properties. In such scenarios, data to be transmitted may be encoded using polar codes.

Polar codes are error-correcting codes that allow for encoding and decoding operations to be performed with low complexity. For polar codes, as the code length grows large, the respective channels seen by individual ones of the bits in an information vector approach either a pure-noise channel or a pure-noiseless channel. The polar codes are constructed by identifying the indices of the bits in the information vector that see noiseless channels, also referred to as information bits. The remaining indices are set to predetermined values known by both the encoder and decoder, also referred to as frozen bits. The overall channel includes information bit channels on which the information bits are transmitted, and frozen bit channels for which the values are predetermined and are known at the encoder and at the decoder. Fig. 2 is a schematic, high-level diagram for a transmission scheme using polar encoding and successive cancelation (SC) decoding. An encoder 200 receives a vector u including information bits u₀ to u_N to be transmitted. The encoder 200 encodes the bits to be transmitted using a polar recursive encoding yielding the codeword vector x including the values x₀ to XIM which are transmitted over a channel 300. At the decoder 400 the transmitted codeword y including the values y₀ to y_N is received, on the basis of which the SC decoder 400 produces the original information vector u including the information bits u₀ to u_N. Some of the bits have a fixed value that is shared between the encoder 200 and the decoder 400 so that for such frozen bits the bit value is known at the decoder 400, independent of the actual information received over the channel for this bit, referred as a fro- zen bit channel. The polar codes are constructed on the basis of a channel model, e.g., an AWGN (additive white Gaussian noise) channel, for a certain SNR (signal to noise ratio). On the basis of the model, the frozen bits are calculated once for the overall channel, i.e., a frozen bit pattern to be used is determined. However, since realistic channels may differ from ideal channel models, the frozen bit selection may be suboptimal for the real channel.

It is an object of the present invention to provide an approach which allows for an improved communication using polar coded codewords.

This object is achieved by the subject matter as defined in the independent claims. Embodiments are defined in the dependent claims.

Embodiments of the present invention are now described in further detail with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic representation of an example of a wireless communication system including a plurality of base stations;

Fig. 2 is a schematic, high-level diagram for a transmission scheme using polar encoding and successive cancelation (SC) decoding; Fig. 3 is a schematic representation of a polar decoder in accordance with an embodiment of the present invention;

Fig. 4 is a schematic representation of a transmission scheme in accordance with embodiments of the inventive approach;

Fig. 5 is a flow chart of a method for a capacity estimation in accordance with an embodiment of the present invention; and

Fig. 6 illustrates an example of a computer system on which units or modules as well as the steps of the methods described in accordance with the inventive approach may execute. In the following, preferred embodiments of the present invention are described in further detail with reference to the enclosed drawings in which elements having the same or simi- lar function are referenced by the same reference signs.

In accordance with embodiments of the present invention, the problem with a non-optimal or suboptimal frozen bit selections is addressed. In accordance with the inventive approach, initially, the frozen bits or the frozen bit pattern is used that has been calculated, for example, on the basis of a channel model of a real channel over which the encoder and the decoder or devices including the encoder and/or decoder are assumed to communicate with each. During operation, the performance of the respective bit channels is monitored so as recognize an insufficient performance of info bit channels and/or a good performance of frozen bit channels. For example, the original frozen bit pattern are deter- mined on the basis of a certain realization of a channel model. However, it may turn out that for the real channel over which the data is actually transmitted, one or more of the information bit, which were considered pure-noiseless or substantially noiseless channels, turn out to be not as noiseless as assumed. Such channels may be recognized as being not suited for the transmission of information bits. Likewise, it may be determined that frozen bit channels correctly transmit the bit value which is known at the decoder, i.e., the originally assumed pure or substantially noisy channel turns out to have good transmission characteristics. In accordance with the inventive approach, the decoder recognizes the insufficient performance of the information bit channels and/or the good performance of the frozen bit channels and causes a modification of the frozen bit pattern, e.g., the decoder may signal to the encoder to adapt the frozen bit indices used when encoding the data to be transmitted, i.e. tells the encoder to adapt its frozen bit pattern.

Embodiments of the present invention provide a polar decoder having input to receive a polar encoded code word over a channel, wherein selected bits of the polar encoded code word are frozen in accordance with the frozen bit pattern. Further, a processor is provided to decode the received polar encoded code word and to evaluate the quality of the transmission. Responsive to the determined quality, the processor may initiate the change of the frozen bit pattern. Further embodiments provide a polar encoder having an input to receive data to be encoded, and a processor to generate a polar encoded codeword by encoding the received data using a polar code. Selected bits are frozen in accordance with a frozen bit pattern. Responsive to a control signal received from a polar decoder, the polar encoder operates in accordance with a modified frozen bit pattern.

Embodiments of the present invention provide a polar decoder as schematically depicted in Fig. 3. The polar decoder 400 includes an input 402 to receive a polar encoded codeword y over a channel, and a processor 404 to decode the received polar encoded codeword y to obtain a decoded codeword u that may be provided to an output 406 pf the polar decoder 400. For decoding the codeword y, the processor 404 may include or implement a decoding unit 408. Selected bits of the decoded codeword u are frozen in accordance with a frozen bit pattern. The processor 404 determines a quality of the transmission over the channel, and, responsive to the quality, initiates a modification of the frozen bit pattern. For determining the quality, the processor 404 may include or implement a quality determining unit 410. The polar decoder may further include a memory or a storage 412 holding an initial frozen bit pattern or, later, the modified bit pattern which is provided to the decoding unit 410, which may include an SC decoder for decoding the codeword y.

In accordance with embodiments, the quality determining unit 410 may evaluate a plurality of transmitted code words to judge the transmission quality. When the transmission quality is determined to be insufficient, the quality determining unit 410 may generate a control signal c to be output at a feedback output 414 coupled to the encoder via a separate control line or via the channel. The control signal may indicate to the encoder a modified frozen bit pattern to be used for the following transmission(s). For example, in case it is determined that the transmission quality is not sufficient, the quality determining unit 410 may change or adjust the initial or currently used frozen bit pattern stored in the memory 412. In accordance embodiments, the quality determining unit 410 may evaluate the respective information bit channels and frozen bit channels so as to determine as to whether one of the frozen bit channels becomes an information bit channel and/or as to whether one of the information bit channels becomes a frozen bit channel. On the basis of this evaluation, the frozen bit pattern may be adjusted and signaled to the encoder.

In accordance with embodiments, the quality determining unit 410 may keep track of unsuccessfully decoded codewords. For example, the decoding unit 408 may send a non- acknowledgment (NACK) message to the encoder in case the codeword y is not de- codable. The NACK message triggers a retransmission of the previously sent codeword y. On the basis of a number of requested retransmissions, e.g., over a predefined time period, an insufficient transmission quality may be determined, and a modification of the fro- zen bit pattern may be initiated. In accordance with other embodiments, the transmitted and retransmitted codewords may be compared to determine erroneous bit channels. For example upon detecting a number of transmission errors in a specific bit channel and/or upon detecting that a specific frozen bit channel has only a low number of transmission errors, an adjustment of the currently used frozen bit pattern may be initiated.

In accordance with further embodiments, the decoder 400 may only signal to the encoder that a modification of the frozen bit pattern is desired. The actual modification of the currently used frozen bit pattern may be performed at the encoder. The modified frozen bit pattern may be signaled back to the decoder 400 to be used as currently used frozen bit pattern for the following transmissions. In accordance with embodiments, the control signal may indicate the bit channels to be changed. In accordance with yet other embodiments, for adjusting or modifying the frozen bit pattern the quality determining unit 410 may randomly modify the frozen bit pattern or randomly selecting a new frozen bit pattern from a set of predefined frozen bit patterns.

Fig. 4 is a schematic representation of a transmission scheme in accordance with further embodiments of the inventive approach described herein. The decoder 400 may be a SC decoder including the decoding unit 408 receiving from the channel 300 the codeword y. Further, the decoding unit 408 receives from the memory 412 the frozen bit pattern F. Initially, the memory 412 stores the initial polar code/frozen bits. The same initial polar code/frozen bits is used at the encoder 200. The decoding unit 408 decodes the codeword y and generates the information vector u that may be provided to the output 406. In the embodiment of Fig. 4, the quality determining unit 410 estimates a channel capacity. The quality determining unit 410 receives the codeword y and the information vector u.. In accordance with embodiments of the present invention, the quality determining unit 410 estimates for each bit u, of the information vector u the channel capacity. This estimation may be done iteratively over a predefined period of time. The decoder 400 further includes an adjustment unit 416, and the quality determining unit 410 provides to the adjustment unit 416 an estimated channel capacitor vector J. The adjustment unit 416cause an adjustment of the frozen bits or the frozen bit pattern using the estimated channel capacitor vector J. The adjustment unit 416 may generate an updated or adjusted frozen bit pattern F that is provided to a feedback unit 418 that may include the memory 412. The adjusted frozen bit pattern F may replace the currently used frozen bit pattern stored in the memory 412. The feedback unit 418 signals the adjusted frozen bit pattern to the encoder 200. In accordance with embodiments, instead of signaling the entire frozen bit pattern, the de- coder 400 may only signal the changes between the currently used frozen bit pattern and the adjusted frozen bit pattern, as is indicated by AF in the feedback channel 450.

The encoder 200 includes an encoding unit 202 receiving the information u to be transmit- ted over the channel 300. A polar encoding unit 202 is provided receiving the information to be transmitted as well as the frozen bit pattern F yielding, at the output, the codeword x to be transmitted over the channel 300. The encoder 200 includes a feedback unit 204 holding the frozen bit pattern F. Upon receiving, via the feedback line 450, from the decoder 400 an adjusted or updated frozen bit pattern, the currently used frozen bit pattern, that is stored in feedback unit 204 of the encoder 200 is replaced by the updated or adjusted frozen bit pattern. Subsequent transmissions use the updated frozen bit pattern for generating the codeword x.

In accordance with embodiments of the present invention, the quality determining unit 410 may iteratively estimate the channel capacity for each bit u, over a predefined time, i.e., for the bit channel carrying bit u, the channel capacity is estimated over time. In accordance with embodiments, when the estimated channel capacity after a specific observation period is below a predefined threshold, the bit is chosen as a frozen bit by the adjustment unit 416. In a similar way, when determining that the channel capacity is above the threshold or reaches the threshold, a bit currently considered a frozen bit may be adjusted to be an information bit by the adjustment unit 416.

In accordance with another embodiment, the adjustment unit 416 processes the values of the vector J, which indicate the respective channel capacity values estimated over a pre- defined period of time for each of the bit channels carrying the respective bits u,. The adjustment unit 416 may sort the channel estimate values and select the bit channels associated with the best ones of the values, up to the code rate, as data or information bit channels, and the remaining bits are frozen bits. In the embodiment depicted in Fig. 4, the interface between the decoder 400 and the encoder 200 transmitting the adjusted frozen bit pattern F may be an interface separate from the channel 300, for example an over the air interface. In accordance with other embodiments, instead of providing a separate feedback path 450, the feedback may also be signaled to the encoder 200 using the channel 300. Fig. 5 is a flow chart of a method for a capacity estimation in accordance with an embodiment of the present invention. As SC-based decoders, like the decoder described with reference to Fig. 4, use a successive decoding process, i.e., they decode bit by bit by using the previously decoded bits for decoding the next bit, this process is used to estimate the bit channel capacities. Regardless of the decoding result, the receiver saves the initially received vector 500 for the capacity estimation. The second vector used is the corrected vector 502 that may be obtained either by an initial successful decoding or by additional retransmissions and further decoding attempts. For estimating the channel capacities several methods may be used. In accordance with one embodiment a first method com- pares at 504 the faulty decoded vector from the initial transmission to the corrected vector 502. Since the decoding is performed successively, the first not matching bit position may be counted as an bit error to update at 506 the statistics for capacity estimation. In accordance with another embodiment a second method may employ a more complex estimation unit. For the estimation, the SC decoder computes at 504 likelihood ratios for all bit positions, also for the frozen bit positions as if they were information bits, by putting the corrected bits for the bit positions which were required for that bit position. The receiver may perform a channel estimation for each bit channel based on the computed likelihood ratio since it knows the correct result. The inventive approach, as described in the above embodiment, is advantageous over conventional approaches in that the use of a static frozen bit pattern is avoided. Thereby, non-optimal transmission of information are avoided, which are due to the fact that the frozen bit pattern is determined on the basis of channel models, which may be different from realistic channels used for actually transmitting information. For example, although the channels are considered substantially static, nevertheless, slow changes may result in changes in the bit channels which differ from the model assumed. Also, differences in the environment of the communication system which have not been taken in consideration when modeling the channel, or that may change over time, may cause channels initially found to be suitable as information bit channels to be no longer sufficient and, vice versa, frozen bit channels may turn out to be suitable for information transmission. In accordance with the inventive approach, the static use of bit patterns and the suboptimal transmission resulting therefrom is avoided by monitoring the performance of the respective bit channels, for example on the basis of the above-mentioned channel capacity, so that the frozen bit pattern may be dynamically adjusted or changed responsive to the detection of changes in the channel, thereby allowing for an improved transmission of data. Although some aspects of the described concept have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or a device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus.

Various elements and features of the present invention may be implemented in hardware using analog and/or digital circuits, in software, through the execution of instructions by one or more general purpose or special-purpose processors, or as a combination of hardware and software. For example, embodiments of the present invention may be implemented in the environment of a computer system or another processing system. Fig. 6 illustrates an example of a computer system 600. The units or modules described above in Fig. 3 and in Fig. 4 as well as the steps of the methods performed by these units may execute on one or more computer systems 600. The computer system 600 includes one or more processors 602, like a special purpose or a general purpose digital signal processor. The processor 602 is connected to a communication infrastructure 604, like a bus or a network. The computer system 600 includes a main memory 606, e.g., a random access memory (RAM), and a secondary memory 608, e.g., a hard disk drive and/or a removable storage drive. The secondary memory 608 may allow computer programs or other instructions to be loaded into the computer system 600. The computer system 600 may further include a communications interface 610 to allow software and data to be transferred between computer system 600 and external devices. The communication may be in the form electronic, electromagnetic, optical, or other signals capable of being handled by a com- munications interface. The communication may use a wire or a cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels 612.

The terms "computer program medium" and "computer readable medium" are used to generally refer to tangible storage media such as removable storage units or a hard disk installed in a hard disk drive. These computer program products are means for providing software to the computer system 600. The computer programs, also referred to as computer control logic, are stored in main memory 606 and/or secondary memory 608 Computer programs may also be received via the communications interface 610. The computer program, when executed, enable the computer system 600 to implement the present invention. In particular, the computer program, when executed, enable processor 602 to implement the processes of the present invention, such as any of the methods described herein. Accordingly, such a computer program may represent a controller of the computer system 600. Where the disclosure is implemented using software, the software may be stored in a computer program product and loaded into computer system 600 using a removable storage drive, an interface, like communications interface 610.

The implementation in hardware or in software may be performed using a digital storage medium, for example cloud storage, a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.

Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the present invention may be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The pro- gram code may for example be stored on a machine readable carrier.

Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for per- forming one of the methods described herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet. A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A further em- bodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.

In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus. The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.

Claims

1. A polar decoder, comprising: an input configured to receive a polar encoded codeword over a channel; and a processor configured to decode the received polar encoded codeword to obtain a decoded codeword, wherein selected bits of the decoded codeword are frozen in accordance with a frozen bit pattern, wherein the processor is configured to determine a quality of the transmission over the channel, and, responsive to the quality, to initiate a modification of the frozen bit pattern.

2. The polar decoder of claim 1 , wherein the processor is configured to determine the quality of the transmission on the basis of a number of retransmissions requested by the polar encoder in response to receipt of a non-decodable polar encoded codeword.

3. The polar decoder of claim 2, wherein the processor is configured to initiate the modification of the frozen bit pattern responsive to the number of retransmissions reaching a threshold.

4. The polar decoder of one of claims 1 to 3, wherein the polar encoded codeword includes a plurality of bits transmitted over a plurality of bit channels of the channel, and wherein, to determine the quality of the transmission, the processor is configured to determine a performance of the bit channels.

5. The polar decoder of claim 4, wherein, to determine the performance of the bit channels, the processor is configured to determine a correct and/or an incorrect transmission of a bit over a bit channel.

6. The polar decoder of claim 5, wherein the processor is configured to initiate the modification of the frozen bit pattern responsive a number of correct bits or incorrect bits reaching a threshold.

7. The polar decoder of claim 5 or 6, wherein the plurality of bit channels includes information bit channels and frozen bit channels, and wherein the processor is configured to initiate the modification of the frozen bit pattern responsive to a number of correct bits received over one or more frozen bit channels reaching a first threshold or a number of incorrect bits received over one or more information bit channels reaching a second threshold.

8. The polar decoder of claim 4 or 5, wherein the plurality of bit channels includes information bit channels and frozen bit channels, and wherein the processor is configured to determine an insufficient performance of an information bit channel and/or a good performance of a frozen bit channel.

9. The polar decoder of one of claims 4, 5 and 8, wherein, for determining the performance of a bit channel, the processor is configured to estimate a channel capacity of the bit channel, and to determine the performance of the bit channel using the estimated channel capacity.

10. The polar decoder of claim 9, wherein the polar encoder is a SC decoder, and, to estimate a channel capacity of the bit channel, the processor is configured to compare a faulty decoded vector from an initial transmission to a corrected vector, the corrected vector obtained by an initial successful decoding or by additional retransmissions and further decoding attempts, and to update statistics for capacity estimation using a first not matching bit position.

11. The polar decoder of claim 9, wherein the polar encoder is a SC decoder, and, to estimate a channel capacity of the bit channel, the processor is configured to compute likelihood ratios for all bit positions using an initially received vector and a corrected vector, the corrected vector obtained by an initial successful decoding or by additional retransmissions and further decoding attempts, and to update statistics for capacity estimation using a first not matching bit position.

12. The polar decoder of one of claims 9 to 11 , wherein the processor is configured to iteratively estimate the channel capacity over a predefined period of time.

13. The polar decoder of one of claims 1 to 12, wherein, to initiate the modification of the frozen bit pattern, the processor is configured to cause randomly modifying the frozen bit pattern,

randomly selecting a new frozen bit pattern from a set of predefined frozen bit patterns,

selectively adjusting one or more bit channels from an information bit channel to a frozen bit channel or from a frozen bit channel to an information bit channel.

The polar decoder of claim 13, wherein, to selectively adjust the frozen bit pattern, the processor is configured to adjust one or more of the plurality of bit channels or all of the plurality of bit channels.

The polar decoder of claim 14, wherein, to selectively adjust the frozen bit pattern, the processor is configured to determine a channel capacity value for each of a plurality of the bit channels, to order the channel capacity values, and to use the bit channels having the best channel capacities, up to the rate of the polar code, as information bit channels.

The polar decoder of claim 15, wherein the processor is configured to use the remaining bit channels as frozen bit channels.

The polar decoder of claim 15 or 16, wherein the processor is configured to determine the channel capacity value for all of the bit channels.

18. The polar decoder of one of claims 1 to 17, wherein the polar encoded codeword is provided by a polar encoder, the polar decoder comprising: a signaling unit configured to signal the modification of the frozen bit pattern to the polar encoder.

19. The polar decoder of claim 18, wherein the signaling unit is configured to signal the modified frozen bit pattern to the polar encoder.

20. The polar decoder of claim 19, wherein the signaling unit is configured to signal only chances between a currently used frozen bit pattern and the modified frozen bit pattern to the polar encoder.

21. The polar decoder of one of claims 18 to 20, wherein the signaling unit is configured to send a control signal to cause the modification of frozen bit pattern at the polar encoder.

22. The polar decoder of one of claims 1 to 21 , comprising an output configured to output the decoded codeword.

23. A polar encoder, comprising; an input configured to receive data to be encoded; and a processor configured to generate a polar encoded codeword by encoding the received data using a polar code, wherein selected bits of the polar encoded codeword are frozen in accordance with a frozen bit pattern, wherein, responsive to a control signal received from a polar decoder, the polar encoder is configured to operate in accordance with a modified frozen bit pattern.

24. The polar encoder of claim 23, wherein the control signal received from a polar decoder includes the modified frozen bit pattern or the changes when compared to the currently used frozen bit pattern, and wherein the processor is configured to encode data using the received modified frozen bit pattern.

25. The polar encoder of claim 23, wherein, responsive to the control signal received from a polar decoder, the processor is configured to modify the frozen bit pattern and to signal the modified frozen bit pattern to the polar decoder.

26. The polar encoder of claim 25, wherein, to modify the frozen bit pattern, the processor is configured to cause

randomly modifying the frozen bit pattern,

27. A communication system, comprising: a polar encoder of one of claims 23 to 26; a polar decoder of one of claims 1 to 22; and a communication channel configured to transmit the polar encoded codeword form the polar encoder to the polar decoder.

28. The communication system of claim 27, comprising a feedback channel configured to transmit the control signal form the polar decoder to the polar encoder.

29. The communication system of claim 28, wherein the polar decoder is configured to transmit the control signal to the polar encoder over the communication channel.

30. A method, comprising: receiving a polar encoded codeword over a channel; decoding the received polar encoded codeword to obtain a decoded codeword, wherein selected bits of the decoded codeword are frozen in accordance with a frozen bit pattern; determining a quality of the transmission over the channel; and responsive to the quality, initiating a modification of the frozen bit pattern.

31. A method, comprising: receiving data to be encoded; generating a polar encoded codeword by encoding the received data using a polar code, wherein selected bits of the polar encoded codeword are frozen in accordance with a frozen bit pattern; and responsive to a control signal received from a polar decoder, operating in accordance with a modified frozen bit pattern.

32. A method, comprising: generating, at a polar encoder, a polar encoded codeword by encoding data to be transmitted using a polar code, the polar code operating in accordance with a frozen bit pattern; transmitting the polar encoded codeword over a channel; receiving, at a polar decoder, the polar encoded codeword and decoding the received polar encoded codeword, wherein selected bits of the decoded codeword are frozen in accordance with the frozen bit pattern; determining a quality of the transmission over the channel; responsive to the quality, initiating a modification of the frozen bit pattern; and operating the polar encoder with the modified frozen bit pattern.

33. A non-transitory computer program product comprising a computer readable medium storing instructions which, when executed on a computer, carry out the method of one of claims 30 to 32.