Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-12-18
2004-06-08
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Receivers
Automatic frequency control
C355S117000, C455S257000, C455S192200
Reexamination Certificate
active
06748036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radio receiving method and apparatus for receiving a radio modulation signal of a PSK (Phase Shift Keying) system.
2. Description of the Related Art
Conventionally, a PSK system is known as one of modulation techniques used to communicate a digital signal by radio. In the PSK system, two values of a digital signal are represented with phase data of a modulation signal, and, for example, if the number of modulation phases is 2
M
, then a digital signal of M bits can be communicated by radio at a time.
As a radio communication apparatus in which such a PSK system as described above is used for communication by radio, for example, a digital cellular apparatus is available. A digital cellular apparatus corresponds to an apparatus wherein a radio transmission apparatus and a radio receiving apparatus of a PSK system are integrated with each other, and receives a radio modulation signal of a PSK system transmitted from a base station, demodulates the modulation signal into an audio signal and outputs the audio signal. Further, it modulates an audio signal inputted thereto into a modulation signal of a PSK system and transmits it by radio to the base station.
In a digital cellular apparatus at present, the &pgr;/4 shift QPSK is adopted as the modulation system for a modulation signal. According to the &pgr;/4 shift QPSK, a phase variation is shifted by &pgr;/4 from that of the QPSK wherein phase modulation is performed at intervals of 90 degrees and digital data of 2 bits are represented by four points on a complex plane. Since a modulation signal does not pass the origin of the complex plane as seen in
FIGS. 17 and 18
, fluctuation of envelope values is reduced.
However, with a digital cellular apparatus which is carried by and moved with a user, when it receives by radio and demodulates a modulation signal of the PSK system transmitted thereto, by a multi-path effect arising from reflections of radio waves from buildings therearound or a Doppler effect arising from movement of the apparatus itself, the frequency of the modulation signal which originally is fixed is fluctuated.
Therefore, in a digital cellular apparatus or the like, it is necessary to adjust the reception frequency of a modulation signal. When the reception frequency is to be adjusted in this manner, in a popular digital cellular apparatus, a radio modulation signal received once by a communication antenna is converted into an I/F signal of a predetermined frequency by an IF amplifier, and a rising edge of the I/F signal is detected by an edge detection circuit.
Such rising edges are counted by a counter circuit in response to a reference clock of a reference oscillator, and phase data is detected from the modulation signal in a cycle of a symbol clock by a register circuit based on a result of the counting. An error of the phase data is sampled out in the cycle of the symbol clock and cumulatively added for an interval of a period of a time slot, and the reference clock of the reference oscillator is varied by a PLL (Phase Locked Loop) circuit so that the phase error cumulatively added in this manner may converge within a predetermined allowance range to adjust the reception frequency.
After the adjustment of the reference frequency of the modulation signal is completed in such a manner as described above, frame synchronization of the modulation signal should subsequently be established. In this instance, since, in the modulation signal, predetermined bit data is contained at a predetermined position of a frame, the predetermined bit data is detected from the received modulation signal to establish frame synchronization.
More particularly, first bit data and second bit data are sampled out for each frame in a cycle of the symbol clock from the modulation signal first, and then a frame correlation value is calculated from the first bit data while CRC calculation is executed with the second bit data. Then, frame synchronization is established in response to results of the calculation.
After the adjustment of the reception frequency is completed and frame synchronization of the modulation signal is established in such a manner as described above, since a condition wherein phase data can be detected well from the received radio modulation signal is reached, the phase of the received radio modulation signal can be demodulated into a digital signal to reproduce a speech signal and so forth.
It is to be noted that some of digital cellular apparatus at present apply a diversity system for such radio reception of a modulation signal as described above. In a digital cellular apparatus which adopts this diversity system, a pair of communication antennae are arranged in a predetermined spaced relationship from each other and a radio modulation signal is received by the communication antennae. Electric field strengths of ratio reception of the communication antennae are detected individually, and one of the communication antennae which exhibits a higher one of the electric field strengths is used for radio reception of the apparatus.
While such a digital cellular apparatus or the like as described above can receive and demodulate a radio modulation signal of a PSK system transmitted thereto by radio, radio receiving apparatus at present have various disadvantages.
For example, while the digital cellular apparatus or the like samples out and cumulatively adds the phase error of a modulation signal in a cycle of a symbol clock upon starting of radio communication in order to adjust the reception frequency of the modulation signal as described above, this complicates processing for the calculation of the phase error. Further, if the reception condition deteriorates in the proximity of a sampling timing, then a result of the sampling becomes noise, and consequently, the speed until the adjustment of the reception frequency is completed drops.
Further, while the digital cellular apparatus or the like samples out, in order to establish frame synchronization, bit data for each frame in the cycle of the symbol clock from the modulation signal and executes calculation of a frame correlation value and CRC calculation, also this drops the speed in which frame synchronization is established if the reception condition deteriorates in the proximity of a sampling timing.
Particularly, while the adjustment of the reception frequency is repeated in a predetermined cycle, at a point of time when frame synchronization is established, a phase error is sampled out from the modulation signal at a particular timing corresponding to a synchronizing frame. Accordingly, if the sampling timing is in the proximity of a phase variation of the modulation signal, then it is difficult to detect the phase error well and the reception performance is deteriorated.
With a digital cellular apparatus at present, it is prescribed to adjust the reception frequency until the frequency deviation becomes smaller than 0.3 ppm (part per million). However, in order to realize this, a reference oscillator is required to have a highly stable output frequency, and this drops the productivity of digital cellular apparatus.
In order to overcome the disadvantage just described, it is a common practice to perform AFC (Automatic Frequency Control) processing for detection data of a phase error to produce digital data of a plurality of bits, convert the digital data into analog data and input resulting data to a TCXO (Temperature Compensated Crystal Oscillator) of the VC (Voltage Controlled) type so that the frequency of a reference clock to be outputted from the TCXO of the VC type is adjusted with a high degree of accuracy.
However, with a conventional digital cellular apparatus, adjustment of the reception frequency with the reference clock adjusted in this manner is executed fully even at a point of time before frame synchronization is established immediately after operation of the apparatus is started. Consequently, high grade data processing is executed even from a point of time at whic
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