Pulse or digital communications – Spread spectrum
Reexamination Certificate
2000-04-21
2004-05-18
Bayard, Emmanuel (Department: 2734)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06738412
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mobile communications device, a communications system, and a communications method for use in mobile communication, and more particularly, to a mobile communications device, a communications system, and a communications method which utilize the spread-spectrum communication technique.
In a mobile communications system utilizing a commonly-employed spread-spectrum communications system in which communication is established between a cell station and a mobile station, a system of the cell station fixes a transmission chip rate fc of the communication, and a transmission symbol rate fv is determined and fixed each time a call involving an individual mobile station is established. More specifically, a spreading rate N (=despreading rate Nr) of spread-spectrum communication is fixed each time a call involving a mobile station is established. Here, the spreading rate N (=despreading rate Nr) corresponds to a ratio of a chipping rate to a symbol rate.
In the common spread-spectrum communications system, communication between a cell station and a mobile station is synchronized and maintained, by means of the mobile station fixing the chipping rate fc for spreading/despreading.
In order to address a problem of a loss of synchronism of chips between a cell station and a mobile station stemming from high-speed movement of the mobile station, as well as to effect RAKE receiving for improving reception sensitivity, data to be used for defining the relationship between a phase and a correlation output over a spreading code period are prepared in the form of a delay profile. The delay profile corresponds to a relationship between the phase &psgr; and a correlation output P(&psgr;) determined by means of measurement of a correlation output P over all phases within the spreading code period.
FIG. 10
is a plot showing the relationship between a phase &psgr; and a correlation output P(&psgr;). As shown in
FIG. 10
, from the delay profile a plurality of phases &psgr;i for which a correlation output P(&psgr;i) is sufficiently greater than noise are determined as a receiving paths. At the time of normal demodulation, the initial phase of a despreading code is synchronized with the phase &psgr;i, thus effective despreading a received signal. Here, the phase &THgr; of the largest correlation output P (&PSgr;0=&THgr;) is particularly called a “principal wave.”
As mentioned above, in the common spread-spectrum communications system, a mobile station periodically prepares a delay profile, thus detecting and updating the phase &PSgr;i of a despreading operation to be performed at the time of a normal demodulation operation.
A correlation output P is determined by means of setting into a parallel correlator a despreading code (=a spreading code) C having the characteristic of a pseudorandom number, inputting a received BB signal into a matched filter, where the signal is despread, and detecting a resultant correlation output.
As mentioned above, in the common spread-spectrum communications system, there is prepared a delay profile for defining the relationship between a phase and a correlation output over the entire a spreading code period, to thereby maintain synchronization between a mobile station and a cell station; that is, to prevent loss of synchronism between chips, which would otherwise be caused by a high-speed movement of the mobile station. Preparation of the delay profile (see
FIG. 10
) for preventing loss of synchronism between chips, which would otherwise be caused by high-speed travel of the mobile station, is performed at a period Td shorter than a period T during which loss of synchronism between chips arises.
FIG. 11
is a plot showing the relationship between the correlation output P and a phase T. As shown in
FIG. 11
, in order to prevent a failure to detect a path, a phase-shift unit Ts to be used for determining a correlation output P for preparing a delay profile must be narrower than a phase width Tp in which a correlation is output.
In the common spread-spectrum communications system, a transmitter uses a code C having the characteristic of a pseudorandom number as a spreading code, whereas a receiver despreads a received code while the spreading code C transmitted from the transmitter is used as a despreading code.
FIGS. 12A and 12B
are plots showing the relationship between the correlation output P and the phase &THgr;.
FIG. 12A
is a discrete model case and shows the relationship between a correlation output P and a phase &THgr;, and
FIG. 12B
is a continuous model case and shows the relationship between a correlation output P and a phase &THgr;.
For the sake of simplicity, there will now be described a discrete model case, in which chips are in phase with each other and the width of one step phase ts of a sample to be correlated corresponds to one chip bit (or a chipping period Tc). Here, assume that the phase width Tp of the discrete model corresponds to one chip bit (or the chipping period Tc) and that a chip rate is taken as Fc.
FIG. 13
is an illustration showing the relationship between a despreading code C and a spreading code of a received signal in a case where chips are in phase with each other. As shown in
FIG. 13
, in the conventional spread-spectrum communications system, in a case where the despreading code C is synchronized with a received signal, the chance of a match arising between bit data assumes a value of 100%, and the chance of a disparity arising between bit data assumes a value of 0%. An expected value of a “match/disparity” for each bit assumes a value of 1 (i.e., 100%).
FIG. 14
shows the relationship between the despreading code C and the spreading code of the received signal in a case here chips are out of phase with each other by one bit. As shown in
FIG. 14
, in a case where the despreading code C is out of phase with the spreading code of a received signal; for example, where the despreading code C is out of phase with the spreading code of a received signal by only one bit, the chance of a match arising between transmitted data and received data is ½ (50%) and the chance of a discrepancy arising between transmitted data and received data is ½ (50%), because the despreading code C has the characteristic of a pseudorandom number. Accordingly, an expected value of a “match/disparity” for each bit assumes a value of 0 (i.e., 0%).
However, as shown in
FIG. 11
, in the common spread-spectrum communications system, the number of times (N) detection and calculation of a correlation output required for preparing a single delay profile is performed assumes a theoretical minimal value of N/2 (times) and actually assumes a value of 4N (times), with respect to the number of bits (N) of the spreading code C. Consequently, a CPU (central processing unit) or a DSP (digital signal processor) of the mobile station requires a much longer processing time and a larger processing load is imposed thereon. Further, in order to store the resultant detected and calculated correlation output P(&psgr;),memory requires larger storage capacity, thereby rendering the spread-spectrum communications system costly.
Longer processing time required for and a larger processing load imposed on a CPU or a DSP and an increase in the storage capacity of memory result in an increase in the power to be dissipated by a mobile station, thus hindering lengthening of a communication time or a call-await time.
In the common spread-spectrum communications system, in principle, only the mobile station can effect a countermeasure against lack of synchronism between chips, thus imposing difficulty in effecting an effective countermeasure against lack of synchronism between chips.
SUMMARY OF THE INVENTION
The present invention has been conceived in light of the drawbacks of the background art and is aimed at providing a mobile communications system, a communications system, and a communications method which effectively and readily enable selection of processing capability and resour
Bayard Emmanuel
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
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