JPH11136299A - Demodulation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily eliminate the effect of Doppler shift by calculating an uncertain variable of a Doppler shift component based on in-phase and orthogonal components obtained by sampling a received signal for a prescribed period in order to demodulate the received signal with the effect of the Doppler sift and to generate a base band signal. SOLUTION: A MODEM circuit 2 includes a detection calculation section 2-4 for uncertain variable Δf of a Doppler shift component and an elimination calculation section 2-5 for the Doppler shift component of a received signal. A MODEM control section 2-1 allows the detection calculation section 2-4 to sample in-phase and orthogonal components I, Q of the received signal in a timing of a prescribed sampling period Δt, obtains a mean value of Δf in a preamble of the received signal which is defined as Δf. The elimination calculation section 2-5 eliminates the Doppler shift component from the received signal based on the Δf to generate a base band signal. Thus, the hardware can be of a simple configuration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for eliminating the effects of Doppler shift in a communication system such as a portable telephone system using a low orbiting satellite which performs wireless communication between wireless stations moving at a relatively high speed. Things.

[0002]

2. Description of the Related Art In recent years, a mobile satellite communication telephone system using a low orbit satellite several hundred kilometers above the ground has been proposed. When this telephone system is used, a portable radio terminal on the ground performs direct radio communication with a low-orbit satellite and can communicate with a telephone terminal located in another area on the ground via the satellite. A paging service is also available.

[0003]

However, the low orbiting satellite is not a geostationary satellite but moves at a high speed in orbit as viewed from the ground. Therefore, the effect of the Doppler shift on the communication between the ground station and the low orbiting satellite is not significant. I can't ignore it. Therefore, it is desired to provide a communication technique which can be easily implemented in a mobile satellite communication telephone system and eliminates the influence of Doppler shift.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems of the prior art, the present invention provides a method for demodulating a Doppler-shifted received signal received by a radio unit to generate a baseband signal. The predetermined period (Δt) and the period (Δ
calculating means for calculating an uncertain variable (Δf) of the Doppler shift component based on the in-phase component I and the quadrature component Q obtained by sampling the received signal at t) and removing the Doppler shift component from the received signal in advance; It is characterized by.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram of the configuration of a radio receiver section showing an embodiment of the present invention. A receiving circuit (RF section) 1 for receiving a QPSK-modulated microwave using TDMA / FDMA, and It includes a modem (modulator / demodulator) circuit 2 for demodulating a signal in which the influence of the Doppler shift on the desired wave is not negligible.

The received signal after the quadrature demodulator 1-1 for the signal including the carrier of the signal not including the Doppler shift component in the RF unit 1 is represented by the following equation (1).

[0007]

(Equation 1)

[0008] With respect to equation (1), a complex conjugate COS (ωt) -j · sin (ωt) is used to remove the fundamental component of the carrier.
When the product of the above is obtained, the result becomes Equation 2, and phase information can be extracted.

[0009]

(Equation 2)

[0010] Here, the received signal including the Doppler shift component is expressed by Equation 3.

[0011]

(Equation 3)

Similarly, the complex conjugate cos (ω
t) −j · sin (ωt) to obtain the product.

[0013]

(Equation 4)

Equation 4 is a signal including a Doppler shift component and a baseband signal, and Δω, that is, Δf takes an indefinite value. Unless Δf is removed, the baseband signal cannot be demodulated.

Therefore, when sampling a received signal including a Doppler shift component at a sampling frequency fs (sampling period Δt = 1 / fs) and demodulating a baseband signal, first consider the product of the above complex conjugate. When the reference carrier component is removed using Equation (5), Equation 5 is obtained at the first sample time t1.

[0016]

(Equation 5)

Similarly, for t 2,..., T n, Equation 6 is obtained.

[0018]

(Equation 6)

Here, since the sampling period .DELTA.t is constant, t2 = t1 + .DELTA.t. If .DELTA.t is sufficiently small, it can be approximated to .theta.2 = .theta.1 + .DELTA..theta.

[0020]

(Equation 7)

The complex conjugate cos (ωt) − of the signal containing the Doppler shift component obtained at time t 1 in the equation (7)
If the product of j · sin (ωt) is obtained, Expression 8 is obtained.

[0022]

(Equation 8)

In this operation, the values of cos (ΔωΔt + Δθ) and sin (ΔωΔt + Δθ) are obtained, and the baseband modulation frequency fb and the sampling period Δt = 1
Since / fs is a known value, it can be obtained by Δθ = 2πfb / fs, so the only uncertain variable of the Doppler shift component in the above equation is Δf.

Then, Equation 9 is derived.

[0025]

(Equation 9)

As described above, Δf, which is an uncertain variable of the Doppler shift component, can be obtained from the amount of change from the immediately preceding detection signal. By demodulating the complex conjugate equation for the received signal as Equation 10 using Δf obtained in this way, the baseband signal cos θt + j · sin θt is obtained.

[0027]

(Equation 10)

The method of obtaining Δf and removing the Doppler shift component shown here is valid only in a training sequence such as a preamble in which a baseband signal does not include a phase component. In an actual demodulator, the received signal is subjected to a complex transformation (Hilbert transformation or the like) using a digital filter to obtain values (I and Q) of an in-phase component and a quadrature component, or to pass through a band-pass filter. In addition, it is necessary to consider variations in the calculated values due to the transient characteristics of the filter. Also, it must be assumed that the length of the training sequence is not constant, or that it cannot always be received from the training sequence.

Based on the above-mentioned method, the modem circuit 2 detects the uncertain variable Δf of the Doppler shift component by the arithmetic unit 2
-4 and an operation unit 2-5 for removing Doppler shift components from the received signal. According to the flowchart of the embodiment of the present invention shown in FIG. 2, the Doppler shift component is removed from the received signal and demodulated to a baseband signal. FIG. 3 shows an embodiment of the operation units (2-4 and 2-5). In this example, the modem control unit 2-1 causes the arithmetic unit 2-4 to sample (S1) the in-phase component I and the quadrature component Q of the received signal at the timing of the predetermined sampling period Δt. The modem control unit 2-1 obtains an average of Δf in the preamble of the received signal (training sequence period of communication) (S2), and determines this as Δf (S3). Based on the obtained Δf, the arithmetic unit 2-5 removes the Doppler shift component from the received signal (S4), generates a baseband signal, and performs QPSK demodulation (S4).
5) Then, the data is transferred to the upper layer via the FIFO circuit 3 (S6).

[0030]

As described above, according to the present invention, the Doppler shift amount can be measured from the received signal containing the Doppler shift component in the desired wave, and the hardware can be simply configured.
Only the desired information can be extracted. Moreover, the present invention can be realized only by arithmetic processing using a DSP or the like without using a PLL circuit having a large circuit scale. Furthermore, if only the reception band of the external RF unit is changed, it can be used in various frequency bands.

[Brief description of the drawings]

FIG. 1 is a diagram of an embodiment of a configuration of a radio receiver according to the present invention.

FIG. 2 is a flowchart of a reception process according to an embodiment of the present invention.

FIG. 3 is a block diagram of a calculation unit showing one embodiment according to the present invention.

[Explanation of symbols]

1: Receiving circuit (RF unit) 1-1: Amplifier 1-2: Multiplier 1-3: Filter
1-4: Limiter 1-5: Quadrature demodulator 1-6: Filter 1-7: Synthesizer 2: Modem circuit 2-1: Modem control unit 2-2: A / D converter 2-
3: Filter 2-4: Δf detection operation unit 2-5: Doppler shift component removal operation unit 2-6: Equalizer 2-7: Filter 2-8: QPSK
Demodulator 3: FIFO circuit

Claims (1)

[Claims] 1. A predetermined period (Δt) and an in-phase component obtained by sampling the reception signal at the predetermined period (Δt) in order to generate a baseband signal from a reception signal affected by Doppler shift received by a radio unit. A demodulation method comprising calculating means for calculating an uncertain variable (Δf) of a Doppler shift component based on I and a quadrature component Q and removing a Doppler shift component from the received signal in advance.