1. Field of the Invention
The present invention relates to a communicating apparatus for and a communicating method of performing an asynchronous communication with a base station for example.
2. Description of the Related Art
As a communicating apparatus of this kind, there is a communicating apparatus using the CDMA (Code Division Multiple Access) method. For example, according to a movable body communicating system using the CDMA method on the business level, such a system structure is employed that a plurality of base stations are arranged for respective service areas, and that the asynchronous communication is performed between each base station and a communicating apparatus (i.e., a portable information terminal) carried by each user by means of wireless communication.
From the base station to the portable information terminal, a signal based on a format shown in FIG. 7 is transmitted for example. Namely, on the side of base station, a signal in which one wireless frame is constituted by 16 slots and a division signal called as a “long code mark symbol” to divide intervals of respective slots is inserted, is generated and transmitted. The portable information terminal receives a signal from the base station, detects a position of the long code mark symbol in the received signal, generates a base band signal by performing an inverse-spreading process by a predetermined spread code series signal synchronous with the long code mark symbol, and reproduces the data in each slot by decoding the base band signal.
In order that the portable information terminal performs the asynchronous communication by selecting one appropriate base station from among a plurality of base stations and decodes the data in each slot in the received signal at a best condition, it is important for the portable information terminal itself to accurately detect the position of the long code mark symbol within the received signal so as to match the generating timing of the spread code series signal with the base station side.
In order to detect the position of the long code mark symbol, a receiving circuit shown in FIG. 8 is equipped in the above mentioned portable information terminal, and a process to establish the synchronization (synchronization capture) between the base station side and the terminal information terminal side is performed in advance of the actual start of the communicating operation such as a voice communication, a data communication and so on.
In FIG. 8, the receiving circuit is provided with an antenna 1 for receiving an electric wave from a base station, and a slot search circuit 3 to which the signal received by the antenna is inputted through an RF (Radio Frequency) circuit 2. The slot search circuit 3 is provided with a matched filter 4, an adder 5, a memory unit 6 and a peak judging unit 7.
The matched filter 4 is a correlating device, performs a correlating calculation between a received signal Sin from the RF circuit 2 and the predetermined code series data (i.e., the data in the same series as the long code mark symbol), and detects the position of the long code mark symbol on the basis of a phase shift amount when the correlation value becomes the maximum.
More concretely, as shown in FIG. 7, by dividing one slot term by a time width τ, which is one tenth of a chip duration Tc (=Tc/10), into 2560 points, the matched filter 4 performs the above mentioned correlating calculation, by using this time width τ as the phase shift amount, to thereby obtain the correlation value for every point i=1 to 2560 in the one slot term.
However, since the base station spreads the transmission signal to a wide band by a spectrum-spreading process, the S/N (Signal to Noise ratio) of the received signal is deteriorated at the portable information terminal, and the S/N of the correlation value is also deteriorated. Thus, it is generally difficult to accurately detect the position of the long code mark symbol on the basis of the correlation value obtained by the correlating calculation for just one slot term.
Therefore, the matched filter 4 repeats the correlating calculation for a plurality of slot terms (e.g., 32 slot terms), and the adder 5 adds and accumulates each calculated correlation value for each point i (i.e., for each phase shift amount) to thereby obtain the accumulated additional values each having a high S/N.
In order to obtain the accumulated additional value having the high S/N, 2560 memory areas AP(1) to AP(2560) are prepared to store the 2560 correlation values corresponding to the 2560 points in advance. When the matched filter 4 calculates the 2560 correlation values for each slot term, the adder 5 accumulates these newly calculated 2560 correlation values and the 2560 correlation values already stored in the memory unit 6 correspondingly for each point i (i.e., for each phase shift amount). Then, the matched filter 4 assigns and stores the 2560 accumulated correlation values again to the memory areas AP(1) to AP(2560). Then, by repeating those processes of the correlating calculation and the accumulation for a plurality of slot terms, a distribution of the accumulated additional value having the high S/N as shown in FIG. 9 is generated.
The peak judging unit 7 detects the greatest 20 accumulated additional values and the positions of the points i corresponding to those 20 accumulated additional values, from among the 2560 accumulated additional values, which have been accumulated over a plurality of slot terms and are finally stored in the memory areas AP (1) to AP(2560), to thereby generate a histogram as shown in FIG. 10. Further, on the basis of 20 points k in this histogram, the peak judging circuit 7 judges the position of the long code mark symbol in the received signal Sin, and establishes the synchronization (synchronization capture) to perform the asynchronous communication by selecting an appropriate base station. Then, by supplying this judgment result output to the spread series signal generator (not illustrated) equipped in the signal wave-detecting circuit, and by synchronizing the generation timing of the spread series signal to inverse-spread the received signal Sin with the position of the long code mark symbol, the base band signal decodable at the best condition is generated.
However, in the above mentioned portable information terminal, since a large amount of accumulated additional values i.e., the 2560 accumulated additional values are calculated, the memory unit 6 having a large memory capacity is equipped. Thus, there is a problem of an increase of the electric consumption, an increase of the cost and an increase of the apparatus size.
Especially, in order to accurately detect the position of the long code mark symbol by the above mentioned correlating calculation, it is desirable to improve the phase resolution of the correlation value by decreasing the time width τ (or the phase shift amount) as short as possible with respect to the chip duration Tc. However, as the time width τ is made the shorter, the total number of the points i in one slot term increases. Thus, there is such a problem that the memory unit 6 having an enormous memory capacity is necessary in accompaniment with the increase of the total number of the points i.
In case that the chip duration Tc with respect to a bit duration T i.e., a spreading ratio T/Tc is large, since the total number of the points i in one slot term increases, there is such a problem that the memory unit 6 having an enormous memory capacity is necessary in accompaniment with the increase of the total number of the points i.