Classifications

• • 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
  • H04L27/3809—Amplitude regulation arrangements
• • 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
  • H04L25/0224—Channel estimation using sounding signals
• • 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
  • H04L25/03178—Arrangements involving sequence estimation techniques
  • H04L25/03248—Arrangements for operating in conjunction with other apparatus
  • H04L25/03292—Arrangements for operating in conjunction with other apparatus with channel estimation circuitry

Definitions

• the present invention relates to methods and systems for demodulating a quadrature amplitude modulated (QAM) signal. It will be convenient to describe an illustrative embodiment of the invention in relation to the demodulation of an orthogonal frequency division multiplexed (OFDM) signal, however the present invention should not be considered as being limited to this exemplary implementation.
• QAM quadrature amplitude modulated
• the received signal In a receiver of a communications system the received signal must be demodulated to extract the transmitted data bits from the received radio signal.
• the receiver will typically process the received signal in a block-wise fashion using a common QAM detection threshold for a block of equalised data symbols.
• QAM quadrature amplitude modulation
• the present inventors have ascertained that in certain circumstances this scheme yields sub-optimal performance. For example, if the equalised data symbols carry the signal strength information which can be used to improve channel decoding performance, use of a common QAM threshold will not yield good performance due to the amplitude fluctuation of equalised data symbols.
• the present invention provides a method of demodulating a signal on which is modulated, using a quadrature amplitude modulation scheme, a plurality of information symbols.
• the method includes determining at least one QAM detection threshold corresponding to one or more of said information symbols; and demodulating each information symbol on the basis of at least one respective QAM detection threshold.
• the information symbols modulated on the signal can have a plurality data symbols arranged between a pair of successive pilot symbols, in which case it is preferable that at least one common QAM detection threshold is determined for a less than the number of symbols between said successive pilot symbols.
• At least one QAM detection threshold is determined in respect of an individual information symbol.
• step (b) determining an amplitude value in respect of one or more information symbols between at least two of said pilot symbols on the basis of the amplitude values determined in step (a).
• the amplitude value in respect of the information symbols between the pilot symbols can be determined by extrapolating or interpolating between the values determined in step (a).
• the signal is an orthogonal frequency division multiplexing symbol.
• the present invention provides an apparatus for demodulating a signal including a plurality of information symbols which are modulated using a quadrature amplitude modulation scheme, the apparatus including ⁇
• the apparatus can further include channel estimation means configured to estimate a channel response of the signal; and means for equalising the signal on the basis of the channel response determined by the channel estimation means.
• the means for determining at least one QAM detection threshold can be configured to implement a method according to an embodiment of first aspect of the present invention.
• the present invention provides a receiver adapted to receive a signal which is modulated using a quadrature amplitude modulation scheme.
• the receiver include an apparatus according to an embodiment of the second aspect of the present invention.
• FIG. 1 is a block diagram of a QAM Demodulator operating in accordance with an embodiment of the present invention!
• FIG. 2 is a block diagram of a portion of a simplified OFDM receiver subsystem including a QAM Demodulator operating in accordance with an embodiment of the present invention.
• Figure 3 illustrates an exemplary data structure of an OFDM system.
• the QAM Demodulator of the preferred embodiment uses a QAM demodulation threshold that depends on the data symbol strength to produce soft bits and signal strength information in respect of each symbol.
• each data symbol to be demodulated has its own QAM detection threshold derived either from the equalised pilot symbols or from the channel estimates.
• a channel decoder such as Turbo decoder, can take advantage of the signal strength information and improve the system performance.
• Figure 1 shows a block diagram of a QAM Demodulator forming part of a receiver, which is made in accordance with a first embodiment, which uses a floating QAM detection threshold.
• the QAM Demodulator 100 includes five main processing blocks, namely an MRC Equalisation Block 102, a Channel Estimator Block 104, a QAM Threshold
• the QAM Demodulator 100 receives, as its input raw pilot and data symbols and data representing a QAM demodulation type and pilot to data power ratio.
• the data output from the Demodulator 100 are demodulated soft- bits for further processing.
• the Channel Estimation Block 104 is used to estimate the channel response at each data symbol position using the relevant pilot symbols.
• the channel estimates are used to correct phase distortion in data symbols caused by channel impairments. Because the data carried by pilot symbols are known to the receiver, they can be used to estimate the channel responses. Any known channel estimation scheme can be used for this function.
• the MRC Equalisation Block 102 receives raw data symbols and equalises them by multiplying them by the complex conjugate of the channel estimates generated by the Channel Estimation Block 104. This operation corrects phase distortion in the data symbols. The amplitude distortion in data symbols are not corrected so that the signal strength information of the received data symbols is retained and can be used in later channel decoding.
• the Threshold Estimator 106 estimates the signal strengths and produces a reference QAM detection threshold for each equalised data symbol or subset of the data symbols using the equalised pilot symbols or channel estimates. Detailed algorithms used for the estimation depend on application. An exemplary algorithm applicable to an OFDM system will be described below. Because the amplitude distortion is not corrected in the MRC Equalisation Block 102, the amplitudes of equalised data symbols will fluctuate significantly under fading conditions. Consequently, a common QAM detection threshold for a block of equalised data symbols will not perform well.
• QAM Threshold Calculation Block 108 One or more QAM demodulation thresholds may be required to demodulate a data symbol, depending on the QAM modulation type used, eg. 16-QAM or 64-QAM.
• Block 108 derives these thresholds for each data symbol using the reference QAM threshold determined by the QAM Threshold Estimator 106.
• the thresholds are determined on the basis of the QAM modulation type and the pilot'to-data power ratio which are provided as inputs to the demodulator 100.
• QAM Demodulation Block 110 In the QAM Demodulation Block 110 each equalised data symbol is demodulated using the QAM detection thresholds for the data symbol by the QAM Threshold Calculation Block 108. As noted above, the number of thresholds required to demodulate each data symbol depends on the QAM modulation type such as 16-QAM or 64-QAM.
• the QAM Demodulation Block 110 outputs demodulated soft-bits for further processing in the receiver.
• FIG. 2 shows a simplified OFDM receiver subsystem, including a QAM Demodulator according to an embodiment of the present invention.
• the Receiver 200 is configured to receive an OFDM base band symbol as an input 202.
• the Cyclic Prefix Deletion Block 204 removes the Cyclic Prefix added between OFDM symbols by the transmitter.
• the signal is then converted to a frequency domain signal by Fast Fourier Transform (FFT) Block 206 and the demodulated signal is output for further processing by a Transport Processing Block 210 and subsequent stages (not shown).
• FFT Fast Fourier Transform
• the QAM Demodulator 208 includes a single processing block, termed the Demodulation Threshold Calculation Block 212 that performs the combined functionality of the QAM Threshold Estimator 106 and QAM Threshold Calculation Block 108 of Figure 1.
• Figure 3 shows an example of the data structure 300 for an OFDM system, where each pilot eg. 302 or data symbol eg. 304 is positioned in a T-F (Time- Frequency) grid 306. It should be noted that Figure 3 is simply an example to be used for convenience and embodiments of the present invention can be applied to any OFDM system using any pilot and data structures.
• T-F Time- Frequency
• the data structure 300 shown in Figure 3 is applicable to an OFDM signal both before and after Equalisation.
• pilot and data symbols are termed “raw” pilot and data symbols.
• pilot and data symbols are called “equalised” pilot and data symbols.
• the columns eg. 308 represent OFDM symbols transmitted across a plurality of frequency sub-carriers, and rows of data symbols eg. 310, represent data symbols transmitted in successive time intervals on the same sub-carrier.
• Block 214 are used to calculate the reference QAM threshold in Block 212 in a similar manner to that as shown in Figure 1. Because pilot symbols generally occupy only a small portion of Time-Frequency grid in an OFDM data structure, it may be practical to use a sophisticated algorithm to estimate the signal strength at pilot symbol positions first, and then interpolate and/or extrapolate to determine the signal strength at data symbol positions on the basis of the signal strength of pilot symbol positions. The signal strength at data symbol positions will be used as reference QAM threshold.
• the QAM Threshold Estimator (106 of Figure 1 or 212 of Figure 2) operates as follows-
• a Finite Impulse Response (FIR) filter is used to smooth the equalised pilot symbols with appropriate boundary handling;
• the smoothed pilot symbols from Step 1 are converted from complex number values into real amplitudes! 3.
• each equalised data symbol in the T-F grid will have a corresponding QAM reference threshold which can be used to calculate one or more corresponding QAM demodulation thresholds for the data symbol.
• the partition of the functional blocks illustrated above is for illustration purposes only, and the functional split between blocks may vary in other implementations.
• the pilot-to-data power ratio adjustment can be performed either in the "QAM Threshold Estimator” or in the "QAM Threshold Calculation".
• the present inventors believe that it is computationally more efficient to make this adjustment in the "QAM Threshold Calculation" block.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

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• 2007
  • 2007-12-12 WO PCT/JP2007/074347 patent/WO2008078613A1/en not_active Ceased
  • 2007-12-12 JP JP2009524633A patent/JP5418835B2/ja not_active Expired - Fee Related
  • 2007-12-12 CN CN200780047575.7A patent/CN101569152B/zh not_active Expired - Fee Related
  • 2007-12-12 US US12/520,392 patent/US8284869B2/en not_active Expired - Fee Related
  • 2007-12-12 EP EP07850833.0A patent/EP2119167A4/en not_active Withdrawn

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