Classifications

• • 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
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7073—Synchronisation aspects
  • H04B1/7087—Carrier synchronisation aspects
• • 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
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/711—Interference-related aspects the interference being multi-path interference
  • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
  • H04B1/7117—Selection, re-selection, allocation or re-allocation of paths to fingers, e.g. timing offset control of allocated fingers
• • 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
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/711—Interference-related aspects the interference being multi-path interference
  • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
  • H04B1/712—Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop

Definitions

• this motion introduces a time varying Doppler shift on the signals received over the channel which may be observed as a change in the frequency of signals received over the channel.
• the change in frequency, or Doppler shift is related to the velocity of the mobile terminal and the angle between the direction of motion of the mobile terminal and the direction of arrival of the signal at the receiver.
• the change in frequency hereinafter frequency offset
• frequency offset may be as high as 1.6 kHz for a mobile terminal moving at 900 km/h and communicating in the 1.9 GHz band. It is an established fact that this frequency shift, if not compensated for, will greatly degrade the overall system performance.
• Embodiments of the invention provide methods and apparatus which estimate frequency offset correction in a manner which is suitable in environments which may involve high speeds .
• Decimation/summation can be performed at various points throughout the frequency offset determination, before or after application of the frequency offset correction. In most circumstances, the frequency offset correction will be estimated from a plurality of despread finger output sequences.
• the entities which track and process resolvable ultipath components are commonly known as "Rake fingers" or simply “fingers".
• a plurality of correlation functions each adapted to perform a respective correlation on respective input signals each of which is a function of a respective one of the plurality of despread finger output sequences to produce a corresponding plurality of correlation outputs, the correlation being performed between the respective input signal and a delayed version of the respective input signal.
• a combiner combines the plurality of autocorrelation outputs to produce a combined correlation output, and an instantaneous frequency offset determining function determines an instantaneous frequency offset as a function of the combined correlation output.
• the apparatus may further include a low-pass filter adapted to perform low-pass filtering on the instantaneous frequency offset to produce a filtered frequency offset.
• the instantaneous frequency offset determining function is a function substantially mathematically equivalent to a scalar multiplied by an arctangent of the imaginary part of the correlation output over the real part of the correlation output.
• the frequency offset correction is converted to a phase correction output with a numerically controlled oscillator.
• a closed loop arrangement is provided wherein the frequency offset correction is fed back and applied prior to the correlation function.
• the input signal is based upon a component which has known content, for example a pilot channel signal. In other embodiments, the input signal is based upon an input with unknown content which must therefore be estimated prior to correlation.
• pilot channel phase rotation correction circuit adapted to apply the phase rotation correction to the pilot channel and/or a DLL (delay locked loop) phase rotation correction circuit adapted to apply the phase rotation correction to a DLL, and/or a finger ' sample phase rotation correction circuit adapted to apply a phase rotation correction circuit to finger data samples, and/or a searcher phase rotation correction circuit adapted to apply a phase rotation correction to a searcher.
• DLL delay locked loop
• Another embodiment of the invention provides a' CDMA receiver adapted to include any of the above summarized functionality.
• Figure 1 is a block diagram of a frequency offset correction circuit according to a first embodiment of the invention
• Figure 2 is a block diagram of a frequency offset correction circuit according to a second embodiment of the invention
• Figure 3 is a schematic of circuitry using the phase rotation correction output produced by the circuitry of Figure 1 or 2 to apply correction to a pilot channel;
• Figure 4 is a schematic of circuitry using the phase rotation correction output produced by the circuitry of Figures 1 or 2 in adjusting timing in a delay lock loop;
• Figure 5 is a schematic of circuitry which applies the phase rotation correction output produced by the circuitry of Figure 1 or Figure 2 to correct data symbols;
• Figure 8 is a schematic diagram of modifications to the circuitry of Figure 1 to accommodate CDMA signals which do not have a pilot channel .
• the invention provides systems and methods adapted to compensate for the frequency offset between a mobile terminal and a base station, as well as to compensate for the Doppler frequency shift introduced by high-speed moving airplanes .
• the two parts may be implemented separately or together, and in any suitable fashion with appropriate combinations of hardware, software, DSP, ASICs, FPGA, processors etc .
• the invention is' applied in the context of receivers for
• an assumption which is made is that all the finger paths of the same user undergo approximately the same frequency shift and that this frequency shift changes relatively slowly with the time.
• the estimation period might be selected for example to equal the duration of a CDMA frame which is 20 ms in some systems.
• the frequency offset algorithm works on pilot signals output by fingers after PN despreading in CDMA receivers.
• the pilot signal is a component of the received signal which for the most part has known contents at . the receiver. More generally any component with known content could be used.
• the signal may also contain unknown content such as is the case with the pilot channel which also includes power control information which should be removed.
• the pilot signal x(n) from one finger can be expressed as follows:
• An intermediate signal s (n) is produced by a multiplier 16 which multiplies x(n) by c (n) * (the complex conjugate of the nth known PN code chip) as follows :
• the output rt 0 t(k) of combiner 30 is fed to an arctan functional block 32 which computes an estimate of the frequency offset according to:
• arctan functional block 32 can be implemented in any suitable way, including but not limited to table look-up, DSP etc.
• u(n) is a noise component which can be approximated by:
• F(m + 1) (1- ⁇ ) F(m) + ⁇ f(m) ⁇
• / (m) is the estimated frequency offset during the mth estimation period
• ⁇ is a parameter of the first order IIR 34.
• F(m+1) is the filter output which will be used to correct the frequency offset in the next frame.

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

Applications Claiming Priority (3)

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• 2001
  • 2001-09-07 AU AU2001287452A patent/AU2001287452A1/en not_active Abandoned
  • 2001-09-07 WO PCT/CA2001/001247 patent/WO2002027956A2/en not_active Ceased
  • 2001-09-07 EP EP01966908A patent/EP1325564A2/de not_active Withdrawn

Non-Patent Citations (1)

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