Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Patent
1995-11-28
1997-11-25
Chin, Stephen
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
375325, 375349, 375368, 370514, H03D 100, H04L 2706
Patent
active
056920156
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a coherent detector and a coherent detection method for demodulating a signal in a receiver employed in digital mobile communications or the like.
BACKGROUND ART
Various methods are employed for detecting digital signals in connection with modulation methods. Among these, a coherent detection method, which achieves detection using local oscillation in synchronism with a carrier frequency at a transmitting side, exhibits the best characteristics under Gaussian noise environment. In other words, the coherent detection method demands a minimum received-signal-power-to-noise ratio that fulfills a particular error rate. It is necessary, however, to fast estimate the transfer function of a propagation path at a receiving side to obtain the absolute phase of the transmission carrier required for the coherent detection. This is because the transfer function of the propagation path fluctuates sharply with time.
An interpolation coherent detection method is known as a method for carrying out the coherent detection by estimating the transfer function of the propagation path. For example, it is disclosed in Seiichi Sampei, "Fading Compensation for 16QAM in Land Communications", The Transactions of the Institute of Electronics, Information and Communication Engineers of Japan B-II, Vol. J72-B-II pp. 7-15, January 1989, or in its revised version, S. Sampei, et al. "Rayleigh Fading Compensation for QAM in Land Mobile Radio Communications", IEEE Transactions on Vehicular Technology, VOL. 42. No. 2, MAY 1993.
FIG. 1 illustrates a format of a signal used in such interpolation coherent detection. A transmitting side transmits a transmitted signal including a pilot signal P periodically inserted thereinto. The pilot signal has a pattern which is known both to transmitting and receiving sides, and contains one or more known symbols. One pilot signal P and an information symbol set (information signal) D sandwiched by successive two pilot signals P constitute one frame.
FIG. 2 shows a conventional receiver. Radio waves received by an antenna 1 are band-limited by a BPF (BandPass Filter) 2 to such an extent that an intended received signal suffer no distortion. The band-suppressed received signal is corrected to a normal level signal by an AGC (Automatic Gain Control) circuit 3, and the offset frequency between the carrier and a local oscillator is coarsely reduced by an AFC (Automatic Frequency Controller) 4. The BPF 2 is provided for ensuring the normal operation of the AGC 3 and the AFC 4.
Subsequently, the received signal undergoes quasi-coherent quadrature detection by a quasi-coherent quadrature detector 5 using a local signal from a local oscillator 6, which has the same frequency as the carrier of the received signal. The output of the quasi-coherent quadrature detector 5 is supplied to an interpolation compensator 9 through an LPF (LowPass Filter) 7 and an A/D converter 8. The LPF 7 is provided for suppressing noise from external bands and interference from adjacent channels. The interpolation compensator 9 estimates for each information symbol a transfer function by an interpolation method using the pilot signals, and compensates individual information symbols using the estimated transfer functions. The compensated signal undergoes decision by a decision block 10. Thus compensating each information symbol with the estimated transfer function enables the absolute phase detection. As a typical interpolation method, a first-order interpolation using two pilot signals, or a second-order interpolation using three pilot signals is generally used.
When the received signal includes noise, more accurate estimation of the transfer functions could be achieved by increasing the number of symbols per pilot signal, thereby reducing the estimation error of the transfer functions. Estimation of the transfer function associated with each information symbol can be carried out by applying the first-order or second-order Gaussian interpolation to the transfer functions estimated from th
REFERENCES:
patent: 5257312 (1993-10-01), Therssen et al.
patent: 5329547 (1994-07-01), Ling
patent: 5442646 (1995-08-01), Chadwick et al.
patent: 5544156 (1996-08-01), Teder et al.
Higashi et al, "Performance of Coherent RAKE Detection Using Interpolation on DS/CDMA", IEICE Technical Research Report, vol. 94, No. 312, pp. 57-62, Oct. 28, 1994.
Sampei, Seiichi, "Rayleight Fading Compensation Method For Multi-level QAM For Land Mobile Communications", Seasonal Report of Communication Synthesis Institute, vol. 37, No. 1, pp. 87-98, Feb. 1991.
Sampei, Seiichi, "Rayleight Fading Compensation Method For 16-QAM Modem in Digital Land Mobile radio Systems", IEICE Transactions, vol. J72-B-II, No. 1, pp. 7-15, Jan. 1989.
Adachi Fumiyuki
Higashi Akihiro
Ohno Koji
Sawahashi Mamoru
Chin Stephen
NTT Mobile Communications Network Inc.
Vo Don
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