WO2010076699A1 - Demodulation of ofdm qam signals with channel estimation errors - Google Patents

Demodulation of ofdm qam signals with channel estimation errors Download PDF

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Publication number
WO2010076699A1
WO2010076699A1 PCT/IB2009/055565 IB2009055565W WO2010076699A1 WO 2010076699 A1 WO2010076699 A1 WO 2010076699A1 IB 2009055565 W IB2009055565 W IB 2009055565W WO 2010076699 A1 WO2010076699 A1 WO 2010076699A1
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Prior art keywords
constellation
receiver
error
channel
transmitter
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PCT/IB2009/055565
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French (fr)
Inventor
Gunes Kurt
Coskun Sahin
Original Assignee
Turkcell Iletisim Hizmetleri Anonim Sirketi
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Priority to SE1150722A priority Critical patent/SE1150722A1/en
Publication of WO2010076699A1 publication Critical patent/WO2010076699A1/en
Priority to FI20115772A priority patent/FI20115772L/en
Priority to FI20116245A priority patent/FI20116245A/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/06—Receivers
    • H04B1/10—Means associated with receiver for limiting or suppressing noise or interference
    • H04B1/1027—Means associated with receiver for limiting or suppressing noise or interference assessing signal quality or detecting noise/interference for the received signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2626—Arrangements specific to the transmitter only
    • H04L27/2646—Arrangements specific to the transmitter only using feedback from receiver for adjusting OFDM transmission parameters, e.g. transmission timing or guard interval length
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/38—Demodulator circuits; Receiver circuits
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
    • H04L2025/03375—Passband transmission
    • H04L2025/03414—Multicarrier
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
    • H04L2025/03375—Passband transmission
    • H04L2025/0342—QAM
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain

Definitions

  • the present invention relates to a method, wherein channel estimation errors observed in unequal amplitude constellations during the additive noise and frequency equalization process in OFDM (Orthogonal frequency-division multiplexing) based multi-carrier systems are compensated, and which comprises a transmitter and a receiver.
  • OFDM Orthogonal frequency-division multiplexing
  • Performance degradation due to channel estimation error in OFDM based multi- carrier systems is a known problem in the art.
  • the severity of the performance degradation depends on the method used during channel estimation and the physical channels used during data transmission.
  • the entire frequency band (B Hz) is divided into N subchannels.
  • the incoming serial data sequence (X n ) is first error correction coded in the error control coder (103) and then converted into a parallel structure at the serial to parallel converter (104). Subsequently, pilot tone(s) is/are added to the signal based on the used communication standard. Then an N point IFFT (Inverse Fast Fourier Transform) block (105) converts the signal into time domain. After the signal is passed through the parallel to serial converter (106), a prefix and/or suffix is added to the signal by the cyclic prefix/suffix adder (107) in order to protect the data against channel dispersion. Finally, the signal is converted into analog form by passing it through a digital to analog converter (D/A converter) (108) ( Figure 1).
  • D/A converter digital to analog converter
  • Each subcarrier has a bandwidth of B/N Hz.
  • Each subcarrier can be modulated by a digital modulation technique such as binary shift keying, M-ary phase shift keying (MPSK) or M-ary quadrature amplitude modulation (M-QAM).
  • MPSK M-ary phase shift keying
  • M-QAM M-ary quadrature amplitude modulation
  • the received signal is first passed through an analog to digital (A/D) converter. Then the cyclic prefix and/or suffix are removed from the signal to get rid of the dispersion that might be added by the channel.
  • A/D analog to digital
  • Equalization in OFDM systems can simply be performed on each subcarrier by using a single tap frequency domain equalizer (single tap FDE).
  • Frequency domain equalizers are frequently used in OFDM based systems since they are simple to implement.
  • Channel taps are estimated based on transmission of a known data sequence. This process is called data aided or pilot based channel estimation.
  • the received OFDM symbol at the output of the FFT block can be denoted as;
  • Equation 2 is the received symbol sample.
  • the received OFDM symbol at the output of the FFT block can also be expressed as;
  • H ⁇ represents n th channel tap
  • I ⁇ represents interchannel interference on the n th channel
  • W ⁇ represents additive white Gaussian noise (AWGN) component.
  • AWGN additive white Gaussian noise
  • Channel estimation is performed by using the known pilot symbols.
  • the channel estimation can basically be obtained by the following equation
  • P 1 denotes the i th pilot symbol.
  • the channel taps used in FDE can be obtained by averaging the per symbol channel estimates (5) as follows:
  • the estimation error associated with the channel tap estimate, H n causes performance degradation in OFDM systems.
  • Equation 8 there is a noise term scaled by the constellation amplitude of the transmitted symbol.
  • a 64-QAM constellation is used.
  • a regular 64-QAM constellation is shown in Figure 4.
  • the average symbol energy in the constellation is normalized to 1.
  • Received samples of this constellation through ideal channels with 3 pilot symbols are shown in Figure 5.
  • constellation points with higher amplitude values are subject to more noise during channel estimation.
  • the solution methods provided in the state of the art for the problem of improving accuracy of channel estimation are generally based on transmission of a known data sequence named as pilot symbols.
  • the objective of the present invention is to realize a method wherein OFDM based receiver and transmitter are used in which the performance degradation resulting due to channel estimation error is reduced.
  • Another objective of the invention is to realize a method which employs an OFDM based receiver where symbol error is reduced by making use of the unequal amplitude constellation points, and a transmitter operating in accordance with the said receiver.
  • a further objective of the invention is to realize a method which employs an OFDM based receiver where the error rates of the high amplitude signals are reduced and a transmitter.
  • Figure 1 shows the block diagram of a transmitter of the state of the art.
  • Figure 2 shows the block diagram of an embodiment of the receiver of the present invention.
  • FIG. 3 shows the block diagram of the transmitter operating in accordance with the receiver of the present invention.
  • Figure 4 is the graphic showing the line up of the points of a regular 64-QAM constellation in the state of the art.
  • Figure 5 is the graphic showing the line up of the points of a regular 64-QAM constellation of the state of the art when noise is added thereto.
  • Figure 6 is the flowchart of the calculation of noise ratio in the invention.
  • Figure 7 is the flowchart of the operation of the constellation generator.
  • Figure 8 is the graphic showing the line up of the points of 64-QAM constellation points distorted with the invention.
  • Figure 9 is the graphic showing the error rates of the 64-QAM constellation points distorted with the invention and the 64-QAM constellation points of the prior art.
  • the inventive receiver (1) comprises an analog to digital converter (2), a cyclic prefix/suffix remover (3), a serial to parallel converter (4), a N-FFT block (5), a channel estimator (6), a frequency domain equalizer (7), a demodulator (8), a serial to parallel converter (9), an error control decoder (10) and a channel error processing unit (11).
  • the transmitter (21) operating in accordance with the inventive receiver (1) comprises an error control coder (22), a modulator (23), a serial to parallel converter (24), a N-IFFT block (25), a parallel to serial converter (26), a cyclic prefix/suffix adder (27) and a digital to analog converter (28).
  • the noise ratio in the receiver (1) and the transmitter (21) is measured by the following steps;
  • the pilot symbol being received by the receiver (1) (102), The demodulator (8) calculating the observed noise ratio (103), The channel error processing unit (11) taking the proportion of the symbol amplitude to observed noise ratio (104),
  • the channel error processing unit (11) calculating the expected noise ratio for the untransmitted symbols (105).
  • a demodulator (8) is used in the invention for reducing symbol error.
  • the decision boundaries are enlarged. How much noise is expected in which symbol is determined by calculating the noise ratio.
  • the decision boundaries are distorted in accordance with the expected noise in the demodulator (8) in the receiver (1).
  • the receiver (1) in one embodiment of the invention comprises a constellation generator (12).
  • the receiver (1) decides for the most suitable constellation by means of the constellation generator (12).
  • the decision boundaries and the constellation to be used are determined by the following steps performed by the constellation generator (12):
  • the performance degradation observed in high amplitude symbols due to the fact that more noise occurs therein is determined by way of trying specific constellations by using a feedback channel between the receiver (1) and the transmitter (21).
  • the constellation observed to have minimum error rate is selected to be used in the demodulator (8) in the receiver (1).
  • the constellation that is to provide the maximum performance is distorted by using a linear model. Performance test are conducted by trying various values of the used parameters and the constellation with the maximum performance is selected to be used in the system. In this solution, the distorted constellation is located in the receiver (1) while the distorted decision boundaries are located in the modulator (23) in the transmitter (21).
  • An example embodiment of the invention is realized for a regular 64 QAM (Quadrature Amplitude Modulation) constellation.
  • the constellation decision boundaries are distorted such that they will be unequal.
  • the decision boundaries are distorted in the demodulator (8).
  • the decision boundaries are calculated by using the equal Euclidean distance principle.
  • the constellation points with higher amplitudes are separated from the rest of the constellation points.
  • the distorted constellation provides a gain of 0.5 dB in a non- fading channel relative to the regular constellation ( Figure 9).
  • the distortion in this figure is realized in line with the observed additive error amount. Amplitudes of the symbols with the highest amplitudes are further increased. This is because more additive noise is observed in these symbols depending on the channel estimation error. In order for this noise not to cause symbol error, high amplitude symbols should be separated as much as possible from the symbols that are close to them. This can be achieved by increasing the distance between the high amplitude symbols.
  • Distortion of the constellation is decided according to the error amount to be added to the transmitted data ( Figure 6).
  • the error distribution factor to be added to the data depends on the medium through which the data is transmitted.

Abstract

The present invention relates to a method, wherein channel estimation errors observed in unequal amplitude constellations in the additive noise and frequency equalization process in OFDM (Orthogonal frequency-division multiplexing) based multi-carrier systems are measured by means of a receiver and a transmitter, new constellations are generated according to the said measurements, and the errors are compensated.

Description

DEMODULATION OF OFDM QAM SIGNALS WITH CHANNEL ESTIMATION
ERRORS
A METHOD USED IN OFDM COMMUNICATION
Field of the Invention
The present invention relates to a method, wherein channel estimation errors observed in unequal amplitude constellations during the additive noise and frequency equalization process in OFDM (Orthogonal frequency-division multiplexing) based multi-carrier systems are compensated, and which comprises a transmitter and a receiver.
Prior Art
Performance degradation due to channel estimation error in OFDM based multi- carrier systems is a known problem in the art. The severity of the performance degradation depends on the method used during channel estimation and the physical channels used during data transmission.
In OFDM based systems, the entire frequency band (B Hz) is divided into N subchannels.
In the transmitter, the incoming serial data sequence (X n) is first error correction coded in the error control coder (103) and then converted into a parallel structure at the serial to parallel converter (104). Subsequently, pilot tone(s) is/are added to the signal based on the used communication standard. Then an N point IFFT (Inverse Fast Fourier Transform) block (105) converts the signal into time domain. After the signal is passed through the parallel to serial converter (106), a prefix and/or suffix is added to the signal by the cyclic prefix/suffix adder (107) in order to protect the data against channel dispersion. Finally, the signal is converted into analog form by passing it through a digital to analog converter (D/A converter) (108) (Figure 1). Each subcarrier has a bandwidth of B/N Hz. Each subcarrier can be modulated by a digital modulation technique such as binary shift keying, M-ary phase shift keying (MPSK) or M-ary quadrature amplitude modulation (M-QAM).
On the receiver side, the received signal is first passed through an analog to digital (A/D) converter. Then the cyclic prefix and/or suffix are removed from the signal to get rid of the dispersion that might be added by the channel.
Equalization in OFDM systems can simply be performed on each subcarrier by using a single tap frequency domain equalizer (single tap FDE). Frequency domain equalizers are frequently used in OFDM based systems since they are simple to implement. Channel taps are estimated based on transmission of a known data sequence. This process is called data aided or pilot based channel estimation.
The baseband model of transmitted OFDM symbol can be denoted as; ik = ^∑S ^^^ , fe = 0,l , ^ r Λ? - l (1) where, N represents the FFT size, n indicates frequency domain index, k indicates time domain index and {λ'N} indicates the transmitted symbol/bit sequence.
The received OFDM symbol at the output of the FFT block can be denoted as;
Figure imgf000004_0001
rk in Equation 2 is the received symbol sample.
The received OFDM symbol at the output of the FFT block can also be expressed as; In equation 3 , Hκ represents nth channel tap, I^ represents interchannel interference on the nth channel, and W^ represents additive white Gaussian noise (AWGN) component.
Channel estimation is performed by using the known pilot symbols. The channel estimation can basically be obtained by the following equation
H., = ≤-
"i (4)
In equation 4, P1 denotes the ith pilot symbol.
Where the number of pilot symbols used in the system is L, the channel taps used in FDE can be obtained by averaging the per symbol channel estimates (5) as follows:
Figure imgf000005_0001
The received samples at the output of FDE are
-» «, ViJJ ■ K ' K (6)
As can be seen from the above given explanations, the estimation error associated with the channel tap estimate, Hn, causes performance degradation in OFDM systems.
As a result of the experimental and analytical studies conducted, it is observed in the state of the art applications that in the OFDM based communication methods, the noise added to the transmitted data and the interchannel interference (ICI) occur more in constellation points with high amplitudes than the points that are close to the origin where the real (inphase) and virtual (quadrature) amplitudes in the quadrature axis are zero (Figure 4 and 5).
**» = ff» ÷ΔW» (V) In equation 7, ΔHn represents the channel estimation error. Then the received symbol can be expressed as
Figure imgf000006_0001
In equation 8, there is a noise term scaled by the constellation amplitude of the transmitted symbol. In order to demonstrate this effect, a 64-QAM constellation is used.
A regular 64-QAM constellation is shown in Figure 4. The average symbol energy in the constellation is normalized to 1. Received samples of this constellation through ideal channels with 3 pilot symbols are shown in Figure 5. As can be observed from this figure, constellation points with higher amplitude values are subject to more noise during channel estimation.
The solution methods provided in the state of the art for the problem of improving accuracy of channel estimation are generally based on transmission of a known data sequence named as pilot symbols.
One of the applications in the state of the art is disclosed in United States patent document US6327314. The said document relates to improving the channel estimation process for a more precise channel frequency response.
Another application in the state of the art is disclosed in United States patent document US2004240376. In the said document, a virtual training symbol is created and processed for correcting the channel estimation error. Summary of the Invention
The objective of the present invention is to realize a method wherein OFDM based receiver and transmitter are used in which the performance degradation resulting due to channel estimation error is reduced.
Another objective of the invention is to realize a method which employs an OFDM based receiver where symbol error is reduced by making use of the unequal amplitude constellation points, and a transmitter operating in accordance with the said receiver.
A further objective of the invention is to realize a method which employs an OFDM based receiver where the error rates of the high amplitude signals are reduced and a transmitter.
Detailed Description of the Invention
The method realized to fulfill the objective of the present invention is illustrated in the accompanying figures wherein,
Figure 1 shows the block diagram of a transmitter of the state of the art.
Figure 2 shows the block diagram of an embodiment of the receiver of the present invention.
Figure 3 shows the block diagram of the transmitter operating in accordance with the receiver of the present invention.
Figure 4 is the graphic showing the line up of the points of a regular 64-QAM constellation in the state of the art.
Figure 5 is the graphic showing the line up of the points of a regular 64-QAM constellation of the state of the art when noise is added thereto.
Figure 6 is the flowchart of the calculation of noise ratio in the invention.
Figure 7 is the flowchart of the operation of the constellation generator. Figure 8 is the graphic showing the line up of the points of 64-QAM constellation points distorted with the invention.
Figure 9 is the graphic showing the error rates of the 64-QAM constellation points distorted with the invention and the 64-QAM constellation points of the prior art.
The parts in the figures are each given a reference numeral where the numerals refer to the following:
1. Receiver
2. Analog to digital converter
3. Cyclic prefix/suffix remover
4. Serial to parallel converter
5. N-FFT block
6. Channel estimator
7. FEQ- Frequency Domain Equalizer
8. Demodulator
9. Serial to parallel converter
10. Error control decoder
11. Channel error processing unit
12. Constellation generator
21. Transmitter
22. Error control coder
23. Modulator
24. Serial to parallel converter
25. N-IFFT block
26. Parallel to serial converter
27. Cyclic prefix/suffix adder
28. Digital to analog converter
The inventive receiver (1) comprises an analog to digital converter (2), a cyclic prefix/suffix remover (3), a serial to parallel converter (4), a N-FFT block (5), a channel estimator (6), a frequency domain equalizer (7), a demodulator (8), a serial to parallel converter (9), an error control decoder (10) and a channel error processing unit (11).
The transmitter (21) operating in accordance with the inventive receiver (1) comprises an error control coder (22), a modulator (23), a serial to parallel converter (24), a N-IFFT block (25), a parallel to serial converter (26), a cyclic prefix/suffix adder (27) and a digital to analog converter (28).
Signals with higher amplitude values are subject to more additive noise and this invention is realized for this reason for unequal amplitude constellations.
The performance degradation observed in high amplitude symbols due to the fact that more noise occurs therein is corrected in the present invention by using a feedback channel between the receiver (1) and the transmitter (21).
The noise ratio in the receiver (1) and the transmitter (21) is measured by the following steps;
The pilot symbol being transmitted by the help of the transmitter
(2I) (IOl),
The pilot symbol being received by the receiver (1) (102), The demodulator (8) calculating the observed noise ratio (103), The channel error processing unit (11) taking the proportion of the symbol amplitude to observed noise ratio (104),
The channel error processing unit (11) calculating the expected noise ratio for the untransmitted symbols (105).
A demodulator (8) is used in the invention for reducing symbol error. In the demodulator (8) the decision boundaries are enlarged. How much noise is expected in which symbol is determined by calculating the noise ratio. The decision boundaries are distorted in accordance with the expected noise in the demodulator (8) in the receiver (1).
The receiver (1) in one embodiment of the invention comprises a constellation generator (12). In this embodiment, the receiver (1) decides for the most suitable constellation by means of the constellation generator (12). In this embodiment, the decision boundaries and the constellation to be used are determined by the following steps performed by the constellation generator (12):
adapting the calculated noise ratio to the used constellation (201), calculating the distances between constellation points that should be used in accordance with the noise ratios (202), generating a new constellation (203), controlling whether the same error ratio is provided with the generated constellation (204), if the same error ratio is provided with the generated constellation, ensuring that the constellation is used in the transmitter (21) and the corresponding decision boundaries are used in the receiver (1) (205), if the same error ratio is not provided with the generated constellation, returning to step 203 and continuing to generate new constellations.
In another embodiment of the invention, the performance degradation observed in high amplitude symbols due to the fact that more noise occurs therein is determined by way of trying specific constellations by using a feedback channel between the receiver (1) and the transmitter (21). The constellation observed to have minimum error rate is selected to be used in the demodulator (8) in the receiver (1). In an alternative embodiment, the constellation that is to provide the maximum performance is distorted by using a linear model. Performance test are conducted by trying various values of the used parameters and the constellation with the maximum performance is selected to be used in the system. In this solution, the distorted constellation is located in the receiver (1) while the distorted decision boundaries are located in the modulator (23) in the transmitter (21).
An example embodiment of the invention is realized for a regular 64 QAM (Quadrature Amplitude Modulation) constellation. In this embodiment, the constellation decision boundaries are distorted such that they will be unequal.
The decision boundaries are distorted in the demodulator (8). The decision boundaries are calculated by using the equal Euclidean distance principle. In the demodulator (8), the constellation points with higher amplitudes are separated from the rest of the constellation points.
As a result of this alternative of the invention, the distorted constellation provides a gain of 0.5 dB in a non- fading channel relative to the regular constellation (Figure 9). The distortion in this figure is realized in line with the observed additive error amount. Amplitudes of the symbols with the highest amplitudes are further increased. This is because more additive noise is observed in these symbols depending on the channel estimation error. In order for this noise not to cause symbol error, high amplitude symbols should be separated as much as possible from the symbols that are close to them. This can be achieved by increasing the distance between the high amplitude symbols.
Distortion of the constellation is decided according to the error amount to be added to the transmitted data (Figure 6). The error distribution factor to be added to the data depends on the medium through which the data is transmitted.
It is possible to develop a wide variety of embodiments of the inventive method. The invention cannot be limited to the examples described herein and it is essentially according to the claims.

Claims

1) A method used in OFDM communication, which employs a receiver (1) comprising an analog to digital converter (2), a cyclic prefix/suffix remover (3), a serial to parallel converter (4), a N-FFT block (5), a channel estimator (6), a frequency domain equalizer (7), a demodulator (8), a serial to parallel converter (9), an error control decoder (10) and a channel error processing unit (11); and a transmitter (2) comprising an error control coder (22), a modulator (23), a serial to parallel converter (24), a N-IFFT block (25), a parallel to serial converter (26), a cyclic prefix/suffix adder (27) and a digital to analog converter (28); characterized in that measurement of the noise ratio is carried out by the following steps;
- The pilot symbol being transmitted by the help of the transmitter (2I) (IOl),
- The pilot symbol being received by the receiver (1) (102),
- The demodulator (8) calculating the observed noise ratio (103),
- The channel error processing unit (11) taking the proportion of the symbol amplitude to observed noise ratio (104),
- The channel error processing unit (11) calculating the expected noise ratio for the untransmitted symbols (105).
2) A method according to Claim 1, characterized in that, in order to reduce the symbol error, the decision boundaries are distorted in the demodulator (8) in line with the expected noise.
3) A method according to Claim 1 or Claim 2, wherein a constellation generator (12) is used in the receiver (1); and characterized in that the constellation generator (12) performs the following steps;
- adapting the calculated noise ratio to the used constellation (201), - calculating the distances between constellation points that should be used in accordance with the noise ratios (202),
- generating a new constellation (203),
- controlling whether the same error ratio is provided with the generated constellation (204),
- if the same error ratio is provided with the generated constellation, ensuring that the constellation is used in the transmitter (21) and the concerned decision boundaries are used in the receiver (1) (205),
- if the same error ratio is not provided with the generated constellation, returning to step 203 and continuing to generate new constellations.
4) A method according to Claim 1 or Claim 2, wherein, through trying specific constellations by using a feedback channel between the receiver (1) and the transmitter (21), the constellation observed to have the minimum error rate is selected to be used in the demodulator (8) in the receiver (1).
PCT/IB2009/055565 2008-12-30 2009-12-08 Demodulation of ofdm qam signals with channel estimation errors WO2010076699A1 (en)

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SE1150722A SE1150722A1 (en) 2008-12-30 2009-12-08 Demodulation of OFDM QAM signals with channel estimation error
FI20115772A FI20115772L (en) 2008-12-30 2011-07-29 Demodulation of QFDM QAM signals with channel estimation errors
FI20116245A FI20116245A (en) 2009-05-11 2011-12-08 Constellation Editing for OFDM

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TR2008/10013A TR200810013A1 (en) 2008-12-30 2008-12-30 OFDM is a method used in communication.
TR2008/10013 2008-12-30

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6327314B1 (en) 1998-04-01 2001-12-04 At&T Corp. Method and apparatus for channel estimation for multicarrier systems
US20040240376A1 (en) 2003-05-30 2004-12-02 Agency For Science, Technology And Research Method for reducing channel estimation error in an OFDM system

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
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