OFDM receiver and doppler frequency estimating circuit

1. An OFDM receiver comprising:

a tuner which receives a radiofrequency signal subjected to orthogonal frequency division multiplex modulation therein and converts the same to an intermediate frequency signal;

a variable gain amplifier which amplifies the intermediate frequency signal in accordance with a gain control signal;

an analog-digital converter which converts the signal amplified by the variable gain amplifier to a digital signal;

a fast Fourier transformer which transforms the digital signal into signals in a frequency domain corresponding to a plurality of carriers;

an equalizer which processes the signals outputted from the fast Fourier transformer in accordance with a Doppler detection signal thereby to make synchronization between the carriers so as to produce received data;

a level control circuit which generates a control signal corresponding to the gain control signal, for bringing an average level of the digital signal to a predetermined level, based on the digital signal;

a digital-analog converter which analog-converts the control signal to produce the gain control signal; and

a Doppler frequency detector which extracts frequency components of the control signal thereby to produce the Doppler detection signal.

2. The OFDM receiver according to claim 1 , wherein the equalizer comprises a plurality of equalizers different in characteristic and a selector which selects one of the equalizers in accordance with a select signal, and

wherein the Doppler frequency detector outputs a select signal for the selector, based on the produced Doppler detection signal.

3. The OFDM receiver according to claim 1 , wherein the equalizer is capable of controlling a characteristic by setting a parameter externally, and

wherein the Doppler frequency detector outputs a parameter for the equalizer, based on the produced Doppler detection signal.

4. The OFDM receiver according to claim 1 , 2 or 3 , wherein the Doppler frequency detector includes:

a differential unit which calculates a rate of change in the control signal,

a 0-cross detection unit which outputs a pulse signal when the rate of change calculated by the differential unit changes from positive to negative or from negative to positive,

a pulse counter which counts the pulse signal outputted from the 0-cross detection unit every predetermined period and outputs a count value thereof, and

a smoothing unit which removes high frequency components of the counted number outputted from the pulse counter and outputs the Doppler detection signal.

5. An OFDM receiver comprising:

a tuner which receives a radiofrequency signal subjected to orthogonal frequency division multiplex modulation therein and converts the same to an intermediate frequency signal;

a first variable gain amplifier which amplifies the intermediate frequency signal in accordance with a first gain control signal;

an analog-digital converter which converts the signal amplified by the first variable gain amplifier to a digital signal;

a first gain control unit which generates a first control signal corresponding to the first gain control signal, for bringing an average level of the digital signal to a predetermined level, based on the digital signal;

a digital-analog converter which analog-converts the first control signal to produce the first gain control signal;

a second gain control unit which generates a second control signal, based on the digital signal;

a memory which holds the digital signal therein by a predetermined cycle;

a second variable gain amplifier which reads the digital signal held in the memory and amplifies the same in accordance with the second control signal;

a fast Fourier transformer which transforms an output signal of the second variable gain amplifier into signals in a frequency domain corresponding to a plurality of carriers;

a plurality of equalizers which process the signals outputted from the fast Fourier transformer thereby to make synchronization between the carriers so as to produce received data;

a first reception variation detection unit which generates a first Doppler detection signal in accordance with the first control signal;

a second reception variation detection unit which generates a second Doppler detection signal in accordance with the second control signal;

a fading frequency estimating unit which weight adds the first and second Doppler detection signals thereby to calculate a fading frequency; and

a selection unit which selects one from the equalizers according to the fading frequency and selects received data of the selected equalizer as the received signal.

6. The OFDM receiver according to claim 5 , wherein a filter which removes unnecessary components contained in the digital signal outputted from the digital-analog converter is provided between the digital-analog converter, and the second gain control unit and the memory.

7. The OFDM receiver according to claim 5 , wherein the first and second gain control units and reception variation detection units respectively hold the values of the digital signals in order from the larger ones thereof every predetermined time, combines the values of the digital signals from the maximum of the held values of digital signals to arbitrary order thereof to calculate a received power estimated value, and generate the first and second Doppler detection signals, based on a variation in the received power estimated value.

8. The OFDM receiver according to claim 6 , wherein the first and second gain control units and reception variation detection units respectively hold the values of the digital signals in order from the larger ones thereof every predetermined time, combines the values of the digital signals from the maximum of the held values of digital signals to arbitrary order thereof to calculate a received power estimated value, and generate the first and second Doppler detection signals, based on a variation in the received power estimated value.