Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
  • H04L1/0056—Systems characterized by the type of code used
  • H04L1/0071—Use of interleaving
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector
• • 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
  • H04L25/06—DC level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection
  • H04L25/067—DC level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection providing soft decisions, i.e. decisions together with an estimate of reliability
• • 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
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
  • H04L1/0045—Arrangements at the receiver end
  • H04L1/0054—Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms

Definitions

• the present invention relates to a demodulator circuit in a communication system using a multi-carrier modulation scheme.
• the invention relates in particular to the effective demodulation of a received signal in a COFDM demodulator for a digital television receiver.
• COFDM Coded Orthogonal Frequency Division Multiplexing
• Each carrier can be encoded to carry a symbol containing more than one bit, for example by using a rectangular constellation modulation system such as 16-QAM, as is known to a skilled person.
• a rectangular constellation modulation system such as 16-QAM
• FIG. 1 An exemplary 16-QAM constellation diagram is shown in Figure 1. As is known to a skilled person, each point on the 16-QAM constellation diagram corresponds to a 4-bit symbol. The symbols are normally assigned to the constellation points using Gray coding, in which symbols with similar most significant bits are grouped together.
• the carriers are modulated in accordance with successive symbols to be transmitted and each received signal is demodulated in the receiver to the corresponding symbol using the constellation diagram.
• the signal received will not correspond exactly with a constellation point because of interference or noise in the channel between the transmitter and the receiver. In this situation the receiver must demodulate the received signal to the symbol corresponding to the constellation point which is most likely to have been transmitted.
• the level of confidence which can be placed on the demodulated or de-mapped information is proportional to the distance or separation of the received signal from the expected constellation point .
• the closer the received signal is to a constellation point the more confidence can be placed in the de-mapped symbol.
• Gray coding When Gray coding is used, the level of confidence that can be placed in a particular de-mapped bit varies from bit to bit within the symbol.
• the information from the de-mapper is passed to a Viterbi decoder which decodes the bits.
• This soft decision information can be input into a soft decision Viterbi decoder.
• a soft decision Viterbi decoder maintains a history of many possible transmitted sequences and builds up a view of their relative likelihoods.
• the Viterbi decoder selects a "0' based on the maximum likelihood. In this way the Viterbi decoder can exploit information relating to the expected reliability of each bit based on the proximity of each bit to the expected constellation point.
• One problem with television broadcasting is the existence of multi-paths arising either as a result of the reception at the receiver of multiple copies of the signal emitted from a single transmitter, or as a result of the reception of signals from a number of transmitters all broadcasting the same signal.
• the existence of multi-paths is equivalent to a frequency selective channel response.
• the conventional analog television signals act as narrow interfering signals within the signal bandwidth of the digital television signal.
• This frequency selective channel response characteristic results in the large number of different carriers used in COFDM modulation having different signal to noise ratios (SNR) .
• SNR signal to noise ratios
• CSI channel state information
• the Viterbi decoder may effectively record that no information is available regarding that bit by disregarding, or "puncturing" the corresponding bit or bits.
• the transmitted data is coded using a convolutional code, which introduces redundancy in the signal in order to allow error correction of the signal to be achieved.
• the effect of the puncturing of data bits in the Viterbi decoder as indicated above, is merely to reduce the effective code rate of the signal. If a sufficiently robust code is used, the effective reduction in code rate resulting from the puncturing of bits can be tolerated, thus avoiding an impact on the decoded signal quality.
• Previously known demodulator circuits are complex since the calculated channel state information must be fed forward to the Viterbi decoder through the deinterleaver .
• the invention seeks to at least alleviate some of the problems associated with the previously known arrangements.
• the invention provides means for demapping data effectively using channel state information.
• a demodulator circuit for demodulating a multi- carrier modulated signal, having de-mapping means for de- mapping the received signal to generate decision data, the decision data being generated in a manner dependent on channel state information for the respective signal carrier; and a decoder for decoding the received bits from the decision data received from the de-mapping means.
• a receiver circuit for receiving a multi- carrier modulated signal comprising means for establishing channel state information for the carriers of the multi- carrier modulated signal from the displacements of received signals on that carrier from the corresponding nearest expected signal, over a number of received signals.
• a method for demodulating a multi-carrier modulated received signal comprising the steps of: de- mapping the received signal to generate decision data, the decision data being generated in a manner dependent on channel state information for the respective signal carrier; and decoding received bits from the decision data received from the de-mapping means.
• Figure 1 is an exemplary diagrammatic representation of a
• Figure 2 illustrates a typical variation in carrier CSI for a COFDM signal with co-channel analogue television interference
• FIG. 3 is a diagrammatic representation of the main components of a demodulator circuit in accordance with the invention.
• Figure 4 is an exemplary diagrammatic representation of a 16-QAM constellation diagram, showing the soft decision regions between the points on the constellation grid; and
• Figure 5 is a flow chart showing the de-mapping method in accordance with one embodiment of the invention.
• Figure 3 is a diagrammatic representation of the main components of a demodulator circuit in accordance with an embodiment of the invention.
• the demodulator circuit 100 consists of a de-mapping block 1, a bit and symbol de-interleaving block 2 and a Viterbi decoder 3.
• the received de-modulated signal 101 is applied to the input of the de-mapping block 1 and the de-mapping block 1 de-maps the received signal and outputs decision data 102 comprising an estimate of the transmitted data together with an indication of the level of confidence to be placed in the estimate.
• the decision data 102 output from the de-mapping block 1 is input to the bit and symbol de-interleaving block 2.
• the de-interleaved decision data 103 is input to the Viterbi decoder 3 which generates a demodulated output 104 based on the input decision data.
• the de-mapping block 1 comprises a channel state information (CSI) calculation block 11 in addition to a de- mapper 12.
• the CSI calculation block 11 determines the channel state information for each carrier used in the multi-carrier modulated signal.
• the channel state information calculated by the CSI calculation block 11 is used by the de-mapper 12 to generate decision data for a received signal using that carrier in a manner dependent on the channel state information for the respective carrier.
• the de-mapper 12 may generate decision data in which the soft decision data is selected in dependence on the distance to nearest constellation point.
• the de-mapper 12 may generate decision data in which the soft decision data is selected in dependence on the channel state information.
• the channel state information is incorporated by the de-mapper 12 in the decision data, and all the CSI functions are contained within the de-mapping block 1.
• the channel state information is not necessary to pass channel state information through the de- interleaving process to the Viterbi decoder as separate information, resulting in reduced data path widths and reduced power requirements.
• the channel state information may be calculated in a number of ways in accordance with the invention. However an advantageous method for calculating the channel state information in the CSI calculation block 11 will now be described with reference to Figure 4.
• FIG. 4 shows a constellation grid for a 16 QAM system, which is described herein as an exemplary transmission system.
• each constellation point A on the constellation diagram represents the phase/amplitude characteristic of an expected signal (i.e. a possible transmitted signal) .
• the actual received signal is normally different from the expected constellation part, as indicated by the received signal point B.
• the separation of the received signal point B from the nearest possible transmitted constellation point A in the I/Q signal space is evaluated and is accumulated over a number of received signals for that carrier.
• the quantized distances d ⁇ and d Q in the I and Q axis directions between the received signal point B and the nearest constellation point A are calculated and for each carrier are summed over a number of symbols received on that carrier.
• the accumulated value can be used as a measure of the SNR, i.e. as the channel state information (CSI) for that carrier.
• the actual distance from the received point B to the nearest constellation point A is calculated for each received symbol and the channel state information for a particular carrier is determined by accumulating this distance over a number of symbols received for that carrier.
• the channel state information can be also be established by using averaging or other combination techniques on the distances measured for the individual received symbols.
• the value established in accordance with the method described above can be taken as a measure of the CSI of the channel, since when the channel is relatively noisy the signal point B corresponding to the received symbol is likely to be at a greater distance in the I/Q plane from the expected constellation points A than when the channel has less noise.
• the CSI value established as outlined above is advantageously used in the establishment of the soft decision data to be used during de-mapping of the data in the de-mapping block 1 shown in Figure 3.
• the CSI value for the channel carrying a signal to be demodulated is compared with a first threshold Tl (step SI) . If the CSI value is less than the first threshold Tl, the carrier can be considered to have a good signal to noise ratio, and therefore it can be expected that the received constellation points should be close to the transmitted constellation points. In this situation, therefore, the input signal may be de-mapped in accordance with known techniques according to the position of the received signal in the constellation grid and soft decision data representing the quantized distance between the received constellation point and the expected constellation point may be assigned to the de-mapped bits (step S2) .
• the CSI value is greater than the first threshold Tl in step 1, the CSI value is compared to a second, higher, threshold T2 in step S3. If Tl ⁇ CSI value ⁇ T2, the carrier can be considered to have a slightly worse signal to noise ratio. In this situation, the signal is de-mapped to the nearest constellation point, and soft decision data for each de-mapped bit is assigned in dependence on the position within the Gray coding, for example (step S4). Since symbols having similar most significant bits are grouped together in a Gray coding constellation diagram, more confidence can be placed in the most significant bits of the received symbol than in the least significant bits.
• the CSI value is compared to a third, yet higher, threshold T3 in step S5. If T2 ⁇ CSI value ⁇ T3, the carrier can be considered to have a relatively bad signal to noise ratio, and therefore although data is de-mapped using the nearest constellation part, each data bit is given soft decision data indicating the lowest possible confidence level (step S6) .
• the carrier can be considered to have a sufficiently bad signal to noise ratio that no reliance can be placed on the decoded bits. In this situation all data bits are punctured (step S7) .
• soft decision data for the de-mapped data may be generated in the de-mapper by initially generating first data based on the distance between the received signal point and the nearest expected constellation point and then by altering said first data, depending on the channel state information for that carrier, to obtain the soft decision data.
• This method is particularly advantageous when implemented in an arrangement in which the separation of the received signal point and the nearest expected constellation point is determined to establish the channel state information for that carrier, since the calculation of the separation of the received signal point and the nearest expected constellation point can be used both in the generation of the soft decision data and also to update the channel state information for that carrier.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Power Engineering (AREA)
• Quality & Reliability (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
• Time-Division Multiplex Systems (AREA)

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• 1999
  • 1999-10-07 GB GB9923763A patent/GB2355164B/en not_active Expired - Fee Related
• 2000
  • 2000-09-26 EP EP00977260A patent/EP1224784A2/de not_active Withdrawn
  • 2000-09-26 WO PCT/US2000/040993 patent/WO2001026318A2/en not_active Ceased

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