Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2001-04-27
2004-12-07
Gelin, Jean (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S013400, C370S318000
Reexamination Certificate
active
06829489
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a communication system adaptable to a radio communication, such as a mobile communication and a satellite communication. More particularly, this invention relates to a communication system, a transmitter, a receiver, and a communication method capable of reducing characteristics deterioration at compressed mode transmission in a CDMA (Code Division Multiple Access) communication system.
BACKGROUND ART
A conventional communication system will be explained here. For example, in the CDMA cellular system, the same carrier frequency is repetitively used within a cell, and handover between the frequencies is not necessary within the same cell. However, when the CDMA cellular system coresides with the existing system, handover between different carrier frequencies becomes necessary. Three concrete examples of handover between different carrier frequencies are given below.
The first example is the handover of the frequency between adjacent cells. When different carrier frequencies are assigned to adjacent cells, because the traffic is heavy due to of an increase of the number of subscribers, handover becomes necessary between such cells. The second example is the handover of the frequency between cells of the umbrella structure. For example, when the umbrella structure is formed, different carrier frequencies are assigned to large and small cells, and the handover is necessary between these cells. The third example is the handover of the frequency between the third generation system represented by the W(Wide)-CDMA system and the second generation system represented by the current cellular phone system.
When the handover takes place under the foregoing conditions, it is necessary to detect the power of different frequency carriers. In order to perform such detection, the receiver must be able to detect two frequencies. However, if the receiver is to detect two frequencies, the necessary structure makes the arrangement of the receiver either larger in size or complex in structure.
The handover method includes two types: handover led by a mobile station (Mobile Assisted Handover: MAHO) and handover led by a network (Network Assisted Handover: NAHO). In NAHO, the load on the mobile station is less as compared to MAHO, however, synchronization with each mobile station becomes necessary in the base station. Furthermore, in NAHO, in order to trace each mobile station separately, the arrangement of the base station
etwork becomes complex and huge.
Thus, MAHO is preferable from the point of view from the mobile station. However, in order to judge whether the handover should take place or not, intensities of two different frequency carriers have to be observed. Different from the TDMA (Time Division Multiple Access) system used in the second generation, the CDMA cellular system generally uses continuous transmission for both transmission and reception. Hence, in order to observe the intensities of two different frequency carriers, the transmission or reception timing has to be suspended to observe the other frequency unless a receiving device capable of handling two frequencies is prepared.
Accordingly, in the conventional communication system, a technique related to a compressed mode has been proposed, in which transmission information in the normal mode is time compressed, so that the compressed information is transmitted in a shorter time and the other frequency carrier is observed in the remaining time. One example is described in PCT Unexamined patent Publication No. 8-500475 entitled as “Discrete Transmission for Seamless Handover in DS-CDMA System”. This publication discloses means to attain a compressed mode for cutting a data transmission time shorter by reducing a spreading factor of used spreading codes.
The compressed mode disclosed in the above publication will be explained in brief.
FIG. 20
is a view showing a transmission example in the normal mode and compressed mode in the conventional CDMA cellular system. Vertical axis represents the power rate/transmission power and horizontal axis represents time. Compressed mode transmission is interposed between normal transmission frames. For example, in case of transmission in the compressed mode, a non-transmission time is set within a descending frame (compressed frame). The time length can be set arbitrarily. The non-transmission time is used as an idle time during which the intensity of the other frequency carrier is measured. As has been discussed, in the conventional CDMA cellular system, interposing the idle time between the compressed mode frame transmissions allows slot transmission.
Also, at the foregoing compressed mode transmission, the transmission power increases with a time ratio between the idle time and frame (compressed mode frame) transmission time. Thus, as is shown in
FIG. 20
, the compressed mode frame is transmitted at higher transmission power than the normal transmission frame. Consequently, it is possible to maintain the transmission quality at the frame transmission in the compressed mode.
Besides the foregoing publication, the references as to the compressed mode include Gustafsson, M. et. al., “Compressed Mode Techniques for Inter-Frequency Measurements in a Wide-band DS-CDMA System”, Proc. of 8th IEEE PIMRC, '97. The latter publication discloses means to attain the compressed mode in case of increasing a coding rate, using multi-code transmission, or using multi-bit transmission modulation system, such as 16 QAM, besides the case of reducing the spreading factor.
On the other hand, in the conventional CDMA cellular system, in order to solve the “perspective (near-and-far) problem” that an undesired signal from a nearby station interferes with a desired signal from a remote station, transmission power control to the mobile station is effected so that the reception power in each base station will be equal. Hence, in the conventional CDMA cellular system, the channel state that is changed with time by adverse affect, such as fading, is corrected, so that not only can the required communication quality be secured at the receiver station, but also the line capacity can be utilized efficiently. The following description will describe the transmission power control in the conventional communication system with reference to the accompanying drawings.
FIG. 21
is a view showing the transmission power control at the normal mode transmission in the conventional communication system. To begin with, the receiver station determines reception power on the target such that meets the required communication quality, that is, target power. Here, the required communication quality on the target is not limited to the reception power, and may be a power ratio (SIR: Signal-to-Interference Ratio) between a desired signal and an interference signal instead. Then, the receiver station compares the power of the received desired signal with the target power, and if the former is greater than the latter, the receiver station sends a transmission power control command (TPC) to the transmitter station to lower the transmission power, and if the former is smaller than the latter, the receiver station sends a TPC command to the transmitter station to increase the transmission power. Upon receipt of the TPC command, the transmitter station changes the transmission power by using prescribed power amplitude: &Dgr; in accordance with the content in the TPC command. Here, the transmission power control is performed per time unit called as a slot to follow a change in the channel state (channel state) shown in the drawing. Either a fixed value or a value that varies in accordance with a certain rule is given as the value of &Dgr;.
FIG. 22
is a view showing the transmission power control at the compressed mode transmission in the conventional communication system. The target power is not changed between the normal mode transmission and compressed mode transmission for ease of explanation. However, in general, there is a case that the set value of the target power is changed so as to ensu
Murai Hideshi
Yamamoto Kazushi
Yano Yasuhiro
Gelin Jean
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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