Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
2000-04-14
2003-09-30
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S522000, C455S067150, C370S318000, C370S320000
Reexamination Certificate
active
06628924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly to a method for controlling a reference value for used in the closed loop control performed between a base station and a mobile station.
2. Description of Related Art
In a code division multiple access (hereinafter referred to as CDMA) mobile communications system, a service area is composed of a plurality of areas. The CDMA mobile communications system performs a two-way radio communication by means of CDMA transmission between a base transceiver station (hereinafter referred to as BTS) disposed at each area and a mobile station (hereinafter referred to as MS) located in an area.
In the CDMA transmission, a transmitting station performs modulation of a transmission signal before spreading the signal band to a wide band using a spreading code that is one of spreading codes that are perpendicular to each other and are previously assigned to every MS, and then transmission is performed. On the other hand, at a receiving station, the received wide-band signal is despread to an original narrow-band signal using the same spreading code before the narrow-band signal is demodulated. Thus, since the spreading codes allow identification of individual communication channels, the same frequency bands are used for all the channels in the CDMA mobile communications system. Further, in the CDMA mobile communications system, a plurality of BTSs can be connected to the MS at the same time. This causes smooth area switching in area boundaries by hand over processing, which is one of the advantages obtained in the CDNA mobile communications system.
On the other hand, in such a CDMA mobile communications system using the same frequency band and different spreading codes, a signal generated by a spreading code becomes a cause of interference noise for a signal generated by another spreading code. Thus, in order to realize the high sensitivity between the BTS and the MS while keeping the interference with other signals at the minimum level, the mutual transmission power control is employed using the signal to interference ratio (hereinafter referred to as SIR), which is defined as a ratio of the receiving power of a desired signal to the receiving power of another signal.
FIG. 1A
shows an example of schematic configuration of a CDMA mobile communications system. The service area of this CDMA mobile communications system includes a plurality of areas, in which N base transceiver stations BTSs
10
1
to
10
N
are located, respectively, and are connected to a host station, which is a radio network controller (hereinafter referred to as RNC)
11
through network channels. Further, a mobile station (MS)
12
performs radio communication with a plurality of BTSs that are located in the network area by means of CDMA transmission. The MS
12
may be connected to a plurality of BTSs depending on its location and performs radio communication with the BTS having the highest sensitivity at that time. Such a state is defined as a diversity handover (hereinafter referred to as DHO) state.
The MS
12
performs the high-speed closed loop control with the BTS in charge for the purpose of transmission power control. In the description below, it is assumed that the MS
12
is connected to BTSs
10
1
and
10
N
. The high-speed closed loop control is a transmission power control performed by an instruction of increase or decrease of transmission power to the opposed station between the MS
12
and each of the BTSs
10
1
and
10
N
when transmitting an uplink frame signal from the MS
12
to the BTSs
10
1
and
10
N
and when transmitting downlink frame signals from the BTSs
10
1
and
10
N
to the MS
12
.
FIG. 1B
shows a frame format of a signal transmitted between the MS and the BTSs. A frame signal
11
which is applicable to both uplink and downlink frame signals is composed of a plurality of slots
13
1
to
13
N
, A transmitter power control (hereinafter referred to as TPC) signal
15
is positioned at the specific location of a predetermined slot
13
, among the M slots. The TPC signal
15
is composed of, for example, 2 bits. When the TPC signal is “
11
”, it indicates an instruction of increase in the transmission power and, when the signal is “
00
”, it indicates an instruction of decrease in the transmission power.
Such a TPC signal is generated in a mobile station side or abase transceiver station side. Here in this case, it is assumed that the TPC signal is generated in BTS side. Such a BTS receives an uplink channel frame signal from the MS and compares the receiving SIR of this uplink signal with a predetermined referents SIR (hereinafter referred to as Sref) to perform the transmission power control far the MS. The reference SIR Brat is a predetermined reference value fox attaining a desired frame error rate allocated to every BTS. Hereafter, a frame error rate is referred to as FER.
More specifically, the BTS monitors an uplink frame signal received from a MB and, when receiving an uplink frame signal, measures SIR of the received uplink frame signal. If the measured SIR is not smaller than the reference SIR Sref, then it is determined that the received signal level is high enough to satisfy the desired FER. However, in the case where the transmission power of the MS is too great, it may cause interference to communications of other MSs located in the same area, resulting in deteriorated communication quality as a whole. Therefore, in the case of sufficiently high received signal level, the BTS generates the TPC signal instructing the MS to decrease in transmission power and adds it to the downlink frame signal. On the other hand, if the measured SIR is smaller than the reference SIR Sref, then the BTS generates the TPC signal instructing the MS to increase in transmission power and adds it to the downlink frame signal. When receiving the TPC signal from the MTS, the MS decreases or increases its transmission power depending on the received TPC bits.
Thus, in the CDMA mobile communications system, the transmission power of the MS is controlled from the opposite station (here, BTS) by adding the TPC signal generated based on the uplink frame signal to the downlink frame signal between the BTSs and MS. Such a control is called as a high-speed closed loop control.
Regarding the high-speed closed loop control, several controllers and control methods have been proposed. For example, “Mobile station transmission power controller” In Japanese Patent Unexamined Publication No. 6-132782 discloses a technique of maintaining communication quality depending on the distribution of mobile stations. More Specifically, BTS calculates an evaluated SIR value which is the difference between a local station SIR value and an average SIR value of overall service areas, and then compares the local station SIR value with the upper limit and lower limit threshold values, whereby a transmission power required to the opposite party is set to a value proportional to the SIR evaluated value based on the comparison result.
Further, “Transmission Power Control Method” in Japanese Patent Unexamined Publication No. 6-276130 discloses a technique in which d transmission power from the opposite party is set to a range which does not exceed predetermined upper limit value and lower limit value reflecting the updated amount &Dgr;P which is the difference between the target SIR value and a local SIR value at the present time and the transmission power is controlled so that SIR values of the adjacent BTSs become the same level.
Further, “Transmission Power Control Method and Transmission Power Control Apparatus” in Japanese Patent Unexamined Publication No. 8-32514 discloses a technique in which a transmission power control is carried out by the above-mentioned high-speed closed loop control which can be usually performed with high accuracy and, when a change of surrounding propagation situation occurs such that a received power is rapidly increased, the optimized transmission power can be obt
NEC Corporation
Rampuria Sharad
Sughrue & Mion, PLLC
Trost William
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