JPH0716166B2 - Wireless system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diversity type wireless system in a digital wireless communication system.

[0002]

2. Description of the Related Art In digital radio communication, especially mobile communication, fading is always present, so that it is not easy to secure the quality of received signals. Therefore, the diversity method is often used. For example, a spatial diversity system that uses two or more antennas separated so that the spatial correlation of the reception level is low and switches to a higher level to receive, or a carrier that differs from two or more transmission base stations by a specific frequency. A transmission diversity system or the like is used in which the frequency is modulated with the same baseband signal and simultaneously transmitted, and the receiving section receives the signal with one antenna. In the former case, two receiving antennas are required, and further a circuit for comparing these two receiving levels and a circuit for selecting a signal based on this are required. Therefore, the circuit configuration becomes complicated. Further, in the latter case, there is a drawback that the system configuration becomes complicated because it is necessary to secure a constant frequency difference between a plurality of base stations that are distant from each other in distance and to keep the time difference between the base stations of the modulated baseband signal small. .

[0003]

In order to solve the above-mentioned problems of the complicated system configuration, the present invention transmits an FM signal from one transmitting section, receives it by one antenna at the receiving section, and directly performs FM demodulation. It is an object of the present invention to provide a wireless system that obtains a diversity effect by doing so.

[0004]

In order to solve the above problems, the present invention provides a radio system for transmitting and receiving digital FM signals, in which the frequencies of two signals having a frequency difference such that the correlation is low in a spatial propagation path. A digital FM signal having a transmission IF signal at a frequency that is half the difference is used as a transmission local oscillator (hereinafter referred to as “local oscillation”) signal to generate two Rs related to an image.
A transmission unit that frequency-converts and outputs the F signal, and two RF signals that are transmitted from this transmission unit are received and differ from the transmission station originating frequency by a frequency that is an integral multiple of half the bit rate of the transmission baseband signal. It is characterized by comprising a receiving unit for converting two RF signals received as a signal originating from a receiving station into signals of the same frequency into the same IF band signal and performing FM demodulation.

[0005]

According to the present invention, a frequency signal, which is half the frequency difference between two signals separated in frequency so as to reduce the correlation in the spatial propagation path, is FM-modulated by a digital baseband signal as information and transmitted. Frequency conversion is performed by a local oscillation signal that is converted into a required RF frequency band, and two signals of image-related frequencies generated as a result are transmitted from an antenna as transmission signals, and in the reception unit, both signals are received by one antenna and transmitted. By performing frequency conversion of a signal having a frequency offset from a signal transmitted from a transmitting station as a receiving station by an integral multiple of half the frequency of the bit rate of a baseband signal, and subjecting the converted IF signal to ordinary FM demodulation, a simple The structure achieves the diversity effect.

[0006]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a structural explanatory view showing an embodiment of the present invention, in which 1 is a transmission baseband signal, 2 is an FM modulator, 3 is a transmission station oscillator, 4 is a mixer for converting transmission frequency, and 5 is transmission. Antenna, 6 are transmission signals of two frequencies, 7 is a receiving antenna, 8 is a receiving station, 9 is a mixer for receiving frequency conversion, 1
Reference numeral 0 is an FM demodulator, 11 is a reproduced baseband signal, IN is an input terminal, and OUT is an output terminal. The diversity method basically consists of two technologies. (1) Technology for configuring multiple transmission lines with low correlation (2) Technology for synthesizing signals from multiple transmission lines

[0008] As a transmission path configuration method, a space diversity method, a frequency diversity method, a time diversity method, an antenna directivity diversity method, a polarization diversity method, a multiple wave path diversity method, etc. have been devised. The signal combining method includes a selective combining method, an equal gain combining method, and a maximum ratio combining method, and phase control combining is required when performing equal gain combining and maximum ratio combining in the RF or IF band. Although any of these combining methods can be applied to both analog signals and digital signals, a square law combining method has been devised as a method to be applied only to digital signals.

In the present embodiment, the two image-related signals are set to have a small correlation.
Each has undergone independent fading. For this reason,
If both signals input from one antenna are returned to the same IF signal as they are, the phase of each IF signal randomly changes, so that the diversity effect does not occur. Figure 1
In, the output IF signal of the FM modulator 2 modulated by the transmission baseband signal 1 supplied to the input terminal IN is expressed as follows. s _i (t) = cos {2πf _i t + φ (t)} (1) Here, φ (t) is generally expressed as follows as is well known as a signal representing a digital FM modulation component.

[0010]

[Equation 1] Here, Δf _d is the modulation frequency shift amount, and T _b is the reciprocal of the bit rate f _b of the baseband signal and is the period of 1 bit.

[0011]

[Equation 2] That is, it is a signal subjected to FSK of ± Δf _d with respect to the IF frequency. Therefore, it can be expressed as follows. s _i (t) = cos {2πf _i t ± Δf _d } (2) Here, the signal from the transmitting station 3 is expressed as follows. l _t (t) = cos (2πf _t1 t) (3) The output transmission from the mixer 4 is expressed by equations (1) and (3) as follows: s (t) = cos {2π (f _t1 ± f _i ) t ± φ ( t)} Compound number same order (4)

[0012] That is, the image-related two-frequency component is output. The phases of the modulation information of these two signals rotate in opposite directions. When Expression (4) is expressed in the same manner as Expression (2), the high sideband signal s _u (t) is s _u (t) = cos {2π (f _t1 + f _i ) t ± Δf _d } Low sideband signal s _L (t) is

[0013]

[Equation 3] And these are transmitted from the transmission antenna 5 as the transmission signal 6. Since the two components of the signal input from the receiving antenna 7 undergo fading independently, they can be expressed as follows based on the equation (4). The high sideband signal is r _u (t) = R ₁ cos {2π (f _t1 + f _i ) t + φ (t) + θ ₁ } (5) The low sideband signal is r _L (t) = R ₂ cos {2π (F _t1 −f _i ) t−φ (t) + θ ₂ } (6) Here, R ₁ and R ₂ : independently Rayleigh fluctuations θ ₁ and θ ₂ : independently random fluctuations and uniform distribution. Now, assuming that the difference between the receiving station originating frequency and the transmitting station originating frequency, that is, the frequency offset amount is Δf, the signal originating from the receiving station 8 is expressed as follows. l _r (t) = cos {2πf _r1 t} = cos {2π (f _t1 + Δf) t}

The output of the reception mixer 9 and this signal and equation (5)
Alternatively, since it is a low-frequency component of the multiplication output with the signal represented by (6), it becomes as follows (here, the coefficient of the calculation result is "1" excluding fading fluctuation). Converting the IF signal from the high frequency signal is r _iu (t) = L _{[r u (t) × l r} (t) ] _{= R 1 cos {2π (f} i -Δf) t + φ (t) + θ 1} = R _{1 cos {2π (f i -Δf} ) t ± Δf d + θ 1} (7) converts the IF signal from the low band signal r _iL (t) = L _{[r L (t) × l r} (t) ] = _{R 2 cos {2π (f i} + Δf) t + φ (t) -θ 2} = R 2 cos {2π (f i + Δf) t ± Δf d -θ 2} (8) the converted signal in the mixer r (t) Is a composite signal of these, the following is obtained from equations (7) and (8). _{r (t) = r iu (} t) + r iL (t) = R 1 cos {2π (f i -Δf) t + φ (t) + θ 1} + R 2 cos {2π (f i + Δf) t + φ (t) -θ ₂ } = Rcos {2πf _i t + φ (t) + Θ} (9) R and Θ in the equation (9) are obtained by the following equations.

[0015]

[Equation 4]

Next, the 1-bit average CNR of the IF signal of equation (9) is obtained. It is assumed here that the quasi-stationary assumption holds. That is, it is assumed that the fading change speed is sufficiently slower than the signal speed and there is almost no fading change within one signal period. Actually, the bit rate under consideration is several hundreds b / s or more, and the maximum fading Doppler frequency of fading is 4 to 50H even in the 900 MHz band mobile phone.
z, this assumption is fully satisfied because it is within 1 Hz for indoor propagation. The average CNRγ within 1 bit is calculated by the following equation. Here, if the thermal noise power density is N, from equation (10)

[0017]

[Equation 5] Under the quasi-stationary assumption, R ₁ , R ₂ , and
Since θ ₁ and θ ₂ are constant, γ is as follows (where θ = θ ₁ + θ ₂ ).

[0018]

[Equation 6]

The third term is affected by the random phase due to fading in the equation (11). Here, the condition described in the claims, that is, the signal having a frequency different from the frequency transmitted from the transmitting station by an integral multiple of half the frequency of the transmission baseband signal is set as the receiving station.
Will satisfy the following equation. Δf = k · f _b / 2 (k = 1, 2, 3, ...: Integer) ∴2πΔfT _b = kπ

At this time, si of the third term in the equation (11)
n (2πΔfT _b ) becomes 0, and as a result, only the first and second terms are left, and only the terms not affected by fading remain as follows. γ = R ₁ ² / 2N + R ₂ ² / 2N Let γ _u and γ _L be the reception CNRs in the case of normal one-wave reception, respectively. Γ _u = R ₁ ² / 2N, γ _L = R ₂ ² Since it is / 2N, γ = γ _u + γ _L , and a CNR equal to the maximum ratio combining method is obtained.

[0021]

As described above, the present invention does not perform phase control and weighted synthesis in the carrier frequency or IF frequency band, but simply performs FM detection of the two waves received from one antenna as they are, and then performs phase control synthesis diversity. There is an advantage that the effect of

[Brief description of drawings]

FIG. 1 is a structural explanatory view showing an embodiment of the present invention.

[Explanation of symbols]

1 ... Transmission baseband signal, 2 ... FM modulator, 3 ... Transmitting station originating, 4 ... Mixer, 5 ... Transmitting antenna, 6 ... Two frequency transmitting signals, 7 ... Receiving antenna, 8 ... Receiving station originating, 9 ...
Mixer, 10 ... FM demodulator, 11 ... Reproduction baseband signal.

Claims (1)

[Claims] 1. A wireless FM system for transmitting and receiving a digital FM signal, comprising a digital FM signal having a transmission IF signal at a frequency which is half the frequency difference between two signals having a frequency difference such that the correlation is low in a spatial propagation path. A transmitter that frequency-converts two RF signals related to an image by a transmitter local oscillator signal and sends the RF signals, and
Two RF signals are received, and a signal having a frequency different from the transmission local oscillator frequency by an integral multiple of half the bit rate of the transmission baseband signal is used as the reception local oscillator signal. A wireless system comprising a receiving unit for converting into a signal and performing FM demodulation.