Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-09-05
2004-02-24
Vo, Nguyen T. (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S574000, C370S329000, C370S335000, C370S342000, C375S137000, C375S140000, C375S145000
Reexamination Certificate
active
06697622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communication system for carrying out multiple access using spread spectra, and particularly to a neighboring cell search method applied to handover control during communication or to zone re-selection control during the idle mode in the system and a mobile station constituting the system.
2. Description of the Related Art
A mobile communication system like a widespread mobile phone system offers its services by dividing the entire service area into rather small radio zones called cells. As shown in
FIG. 1
, such a system comprises a plurality of base stations
111
for covering divided radio zones (cells), and mobile stations
112
for communicating with the base station by estabishing radio channels. Direct Sequence CDMA (DS-CDMA) is a scheme for a plurality of users to carry out communications using the same radio frequency band by transmitting information through second modulation that spreads a conventional information data modulation signal with a high rate spreading code. The radio signal of each user is identified by a spreading code assigned to the user.
In the mobile communication system, the spreading code used for the spreading usually consists of a combination of two types of spreading codes: a “first spreading code group” with the same period as an information symbol period and commonly assigned to all the base stations; and a “second spreading code” with a considerably longer period than the information symbol period and uniquely assigned to each of the base stations.
FIG. 2
is a schematic diagram illustrating a method of using the spreading codes in the mobile communication system to which the present invention is applied. In
FIG. 2
, the upper layer represents a scrambling code layer
202
with a long period uniquely assigned to individual base stations, and the lower layer represents a channelization code layer
204
with a short period commonly assigned to all the base stations. The signals transmitted from the base stations are identified using long period scrambling codes uniquely assigned to the individual base stations. A plurality of codes are defined as the scrambling codes for the entire system, and system designers select the codes to be assigned to the base stations among them.
For the mobile stations to demodulate information transmitted from the base stations, they must receive the information in synchronization with the timing of the spreading code repeated periodically at the transmitting side. In particular, as for the scrambling codes, detection of the timing requires a long time because of the long period. Accordingly, it is important for the mobile station to detect the repetition timing of the scrambling codes to demodulate perch channels of the base stations. In the present specification, the repetition timing of the scrambling codes is referred to as “phase”. It is not necessary to detect the absolute phase in practice, but to find the relative difference in the timing between the scrambling codes of the base stations, that is, the phase difference. Thus, the term “phase” refers to the relative phase between the scrambling codes in the present specification.
FIG. 3
is a schematic diagram illustrating timing relationships between the scrambling codes associated with signals sent from the base stations to a mobile station.
FIG. 3
illustrates a case of an inter-cell asynchronous mobile communication system, in which synchronization between the base stations are not necessarily required, and the timing of the scrambling codes received by the mobile station differs for each base station. On the contrary, in an inter-cell synchronous system establishing synchronization between the base stations, the timing of the scrambling codes is exactly adjusted to the timing assigned in advance to the base stations. Accordingly, the relative timing of the scrambling codes between the base stations is fixed and unchangeable. Comparing the inter-cell asynchronous system with the inter-cell synchronous system, the former has an advantage over the latter that it does not require any timing source such as the GPS (Global Positioning System) which is necessary for the synchronous system, and hence is more flexible in extending the system or the like.
The radio signal transmitted from a base station at certain transmission power travels through space with a certain attenuation, and arrives at a receiving site. Since the attenuation the radio signal undergoes increases with the distance from the transmitting site to the receiving site, it is common that a perch channel transmitted from a distant base station is received at a lower received level, and a perch channel transmitted from a near base station is received at a higher received level. In practice, however, the propagation loss is not determined only by the distant, but varies because of such conditions as the geography and buildings. As a result, the received power of the perch channels from the base stations fluctuate sharply with the move of the mobile station. In the condition in which the received levels of the perch channels from the base stations fluctuate sharply, perch channels received above a certain required received level alter incessantly. This is because the received level of the current perch drops suddenly, or the received level of a perch unreceivable increases abruptly above the receivable level. Thus, to receive the signals from the base stations with better quality, it is important for the mobile station to continuously monitor the perches from the base stations, and to select the best base station.
In the asynchronous mobile communication system, a mobile station must search for a perch quickly whose spreading code and phase are unknown. As a method of searching for a phase, there is one called “3-step cell search” disclosed in a document by K. Higuchi, M. Sawahashi go, and F. Adachi, “Fast Cell Search Algorithm In Inter-Cell Asynchronous DS-CDMA Mobile Radio”, IEICE Trans. Commun., Vol. E81-B, No.7, July 1998. The method provides a “masked symbol” to part of the perch channel which undergoes double spreading by a channelization code and a scrambling code. Here, the “masked symbol” is spread only by the channelization code without using the scrambling code.
FIG. 4
is a schematic diagram illustrating a structure of a perch channel.
First, the mobile station despreads the received signal using a channelization code
404
commonly used by all the base stations. This enables the mobile station to detect a peak at the timing of a masked symbol
408
of the received signal independently of the types of the scrambling codes (first step).
Subsequently, in response to the timing extracted at the first step, the mobile station detects a scrambling code group code
406
superimposed at the same position as the masked symbol
408
, and identifies the group to which the scrambling code belongs which is used by the base station in connection with the reception (second step).
Finally, using the scrambling codes belonging to the group determined at the second step, the mobile station identifies the scrambling code
402
used by the base station (third step).
In the system to which this method is applied, a lot of scrambling codes are divided into groups in advance. In contrast, in the inter-cell synchronous system, since the phase differences of the scrambling codes between the base stations are known in advance, and hence the searching timing can be limited to a fixed timing width (search window), the power consumption or time taken for the cell search can be saved.
The conventional search method in the inter-cell asynchronous system, however, requires more power consumption and time for the cell search than the inter-cell synchronous system, presenting a problem of exhausting the battery power of the mobile terminal quickly. On the other hand, employing the inter-cell synchronous system to simplify the cell search of the mobile station presets problems of hindering making full use of the above menti
Higashi Akihiro
Ishikawa Yoshihiro
Onoe Seizo
Yunoki Kazufumi
Nguyen Huy D
NIT DoCoMo, Inc.
Vo Nguyen T.
Workman Nydegger
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