Classifications

• • 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
• • 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
  • H04L2025/03592—Adaptation methods
  • H04L2025/03598—Algorithms
  • H04L2025/03611—Iterative algorithms
  • H04L2025/03617—Time recursive algorithms

Definitions

• the present invention relates to decision feedback adaptive channel equalizers and is particularly concerned with equalizers for orthogonal frequency division multiplexer (OFDM) channels
• An object of the present invention is to provide an improved OFDM adaptive channel equalizer.
• a method of OFDM channel equalization comprising the steps of: deriving a coefficient value from a received reference value; receiving subsequent symbols in dependence upon the coefficient value; and updating the coefficient value in dependence upon a weighted average of an optimal coefficient value for the current symbol and the previous coefficient value.
• a means for deriving a coefficient value from a received reference value means for receiving subsequent symbols in dependence upon the coefficient value; and means for updating the coefficient value in dependence upon a weighted average of an optimal coefficient value for the current symbol and the previous coefficient value.
• a first branch for deriving a channel coefficient value from a received reference value
• a second branch for deriving an equalized output value in dependence upon the coefficient value and for adapting a previous channel coefficient value in dependence upon a weighted average of the coefficient value and an optional coefficient value for a current equalized output value and received signal; and an output for the equalized output value.
• Fig. 1 illustrates in a functional block diagram a known OFDM transceiver
• Fig. 2 illustrates in a functional block diagram a known OFDM non-adaptive channel equalizer
• Fig. 3 illustrates in a functional block diagram a known OFDM adaptive channel equalizer
• Fig. 4 illustrates in a functional block diagram an OFDM decision-feedback adaptive channel equalizer in accordance with an embodiment of the present invention
• Figs. 5a and 5b graphically illustrate weighting factor nulling criteria for
• Fig. 1 there is illustrated in a functional block diagram a known
• the OFDM transceiver 10 includes an OFDM transmitter 12, an OFDM receiver 14, a data input 16, a data output 18 and an input/output port 20.
• the OFDM transmitter 12 includes a scrambler 22, a channel coder 24, an interleaver 26, a symbol mapper 28, and inverse fast Fourier transform (IFFT) 30 and a guard interval insertion 32.
• the OFDM receiver 14 includes a synchronization and guard interval stripper block 34, a fast Fourier transform (FFT) 36, an equalizer & sheer 38, a deinterleaver 40, a channel decoder 42, and a descrambler 44.
• FFT fast Fourier transform
• the data input at 16 is first scrambled by the scrambler 22 to ensure that the modulated signal will exhibit a flat spectrum similar to white noise.
• the scrambled data is passed through the channel coder 24 that performs forward error correction (FEC) encoding.
• FEC forward error correction
• the encoded data in interleaved by the interleaver block 26 to ensure frequency and eventually time diversity and mapped into an OFDM symbol (onto different carriers) by symbol mapper 28.
• the OFDM symbol which is list of complex (IQ modulated) values on different carriers, is then converted from the frequency domain to the time domain by the inverse fast Fourier transform
• the IFFT 30 output is pre-pended with a guard interval (GI) or a cyclic prefix by GI Insertion 30 and sent to RF conversion and amplification and to antenna via the input/output port 20.
• GI guard interval
• the OFDM receiver 14 implements the reverse operations in the reverse order of the OFDM transmitter 12.
• the GI stripper 34 removes the GI
• the FFT 36 converts from time to frequency domain
• the equalizer & slicer 38 performs symbol de- mapping.
• the receiver ensures time and frequency synchronization with the transmitter in the synchro block 34 and compensates for the channel impairments by use of the equalizer 38.
• a known OFDM non-adaptive channel equalizer An OFDM channel equalizer is required to invert the channel frequency response. The OFDM spreads data on several carriers thus equalization is needed on each carrier separately. OFDM with cyclic prefix (also called guard interval) avoids inter-symbol interference by setting the duration of the cyclic prefix longer than the channel impulse response. In such a case, the channel equalizer is reduced to one-tap filter on each carrier.
• the OFDM non-adaptive channel equalizer 50 includes an input 52 for received complex values X m>n and an output 54 for received sliced data
• a received reference value Xo, n is applied to a complex invertor 56, while remaining received complex values are applied to a complex multiplier 58.
• the output from the inverter 56 is applied to a second complex multiplier 64 having the other input set to the known transmitted reference value Yo, n and the output to the coefficient C 0 , n is stored in memory 60.
• the complex multiplier 58 takes the remaining received complex values C ⁇ , n , C 2 , n , . and multiplies them with the coefficient C 0 , n from memory 60.
• the output of mixer 58 is applied to slicer 62, the output of which is applied to the equalizer output 54.
• the output from the inverter 56 is applied to a second mixer 64 having input for a known transmitted reference value
• the equalizer There are two techniques to send reference information for the equalizer.
• One is scattered pilots. Each reference signal (called pilots) is sent on a subset of carriers in each symbol. Positions of the pilots change from symbol to symbol such that, on each carrier, a reference is sent periodically.
• the second technique is preamble or midamble technique in which reference signals are sent on all carriers in a special symbol. That is a set of reference signals that precludes the data-carrying OFDM symbols (preamble) and/or that are inserted periodically between data-carrying OFDM symbols (midamble).
• preamble precludes the data-carrying OFDM symbols
• midamble midamble
• the reference signals pilots or pre/mid-ambles
• the equalizer coefficient initialized during reference symbols on the upper branch is used unchanged to correct the following data symbols on the lower branch until another reference symbol is received.
• the known non-adaptive channel equalizer has several drawbacks. Noise from the reference symbol adds to the noise of data symbols through the equalizer coefficient. A noise burst during one reference symbol affects the overall signal to noise ratio (SNR) for all data symbols until the next reference symbol. Also, in variable channels, reference symbols must be sent more often, creating a significant overhead, hence reducing channel efficiency.
• SNR signal to noise ratio
• Fig. 3 there is a known adaptive channel equalizer using a least means squared algorithm.
• the OFDM channel equalizer of Fig. 3 adds further components to the second branch in order to update the coefficients for every received data symbol based on the difference between the output and input of the sheer 62.
• the lower branch contains the subtracter 76 that calculates the difference between inputs 72 and 74.
• the output of subtracter 76 is applied to a complex multiplier 78 with connection 80 from the input to provide an output at 82 applied to a ⁇ weighting function 84, the output of which is summed in adder 86 with the previously stored coefficients via 88 to provide updated coefficients, stored via 90.
• An adaptive equalizer can track variable channels. It also has advantages in fixed channels since it takes the initial estimate of the channel response obtained from the reference symbol and refines it during the data symbols.
• the effective signal noise (SNR) continuously improves during the data symbols.
• the adaptation improves system immunity to noise in general and especially to noise bursts that occur during a reference symbol.
• LMS-adaptation The cost of introducing the LMS-adaptation is significant since it requires an additional multiplier, two adders, a constant multiplication and an additional access to the coefficient memory.
• the multiplier brings most of the added cost in a hardware implementation.
• C m- ⁇ ;n and X m)n are used in two different places in the equalizer structure. In a pipeline hardware architecture this double usage requires either additional registers or double accesses to the input and coefficient memories to compensate for the pipeline delay.
• LMS-like algorithms can become unstable for large values of ⁇ , thus ⁇ has to be much smaller than 1 (e.g. typically less than 0.01).
• equalizer tracking speed is proportional to ⁇ , stability concerns limit the tracking speed of the equalizer and consequently the performance improvements could be provided by the equalizer.
• the OFDM channel equalizer 100 includes an input 52 and an output 54, an upper branch including an inverter 102 and a multiplier 104 and a coefficient output 106 and a lower branch including a multiplier 108 and an input from coefficient memory 110.
• Output from the multiplier 112 is applied to a sheer 114 whose output is coupled to the equalizer output 54.
• Input to the sheer 114 is subtracted in the subtractorl 18 from the output from the slicer 114.
• the output of the subtracter 118 is applied to a ⁇ weighting function 120 that also receives output via 122 from the slicer 114.
• the weighted output is applied to a second adder 126 to which is applied the output from the multiplier 108 via 124.
• the output from the second adder 126 is applied to a multiplexer 128 whose output 132 is applied to the multiplier 104.
• This rule calculates the updated coefficient value as a weighted average between the previous coefficient value and the optimal coefficient for the current symbol.
• ⁇ can take any value between 0 and 1 without compromising stability.
• ⁇ should be increased close to 0.5.
• the adaptive equalizer of Fig. 4 does not update a coefficient when the corresponding received data symbol is too noisy. Received symbols are considered too noisy when they do not fall within the given neighbourhood of one of the ideal constellation points. An example of such neighbourhoods for QPSK and 16QAM are depicted in Fig. 5 a and 5b, respectively.
• the proposed equalizer of Fig. 4 can use a ⁇ value close to 0.5 thereby achieving a much higher tracking speed than the LMS-like algorithms of Fig. 3.
• the adaptive equalizer in accordance with the embodiments of the present invention uses one multiplier less than the LMS equalizer of Fig. 3 and has a simpler architecture because C m - ⁇ >n and X m , n are used only once in the structure. This further reduces the number of registers. Also, there is only one port that writes into the coefficient memory so an additional multiplexer is saved.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
• Noise Elimination (AREA)

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• 2002
  • 2002-08-13 EP EP02754028A patent/EP1419628A2/de not_active Withdrawn
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