Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-05-25
2002-06-11
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000, C455S063300, C370S252000
Reexamination Certificate
active
06405052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for power control in code division multiplex access (CDMA) mobile communication systems, and more particularly to a method for power control during call acquisition in CDMA mobile communication systems.
2. Description of the Related Art
In CDMA mobile communication systems all users called mobile stations share the same frequency band when communicating with one or more base stations.
As mobile stations and base stations communicate simultaneously two separate frequency bands are provided, one band for the forward link (from the base station to the mobile station) and one band for the reverse link (from the mobile station to the base station). Each channel on the forward link or reverse link is separated by a particular spreading code by which a narrow band payload information (e.g. data or voice) is spread over the considerably larger channel bandwidth and transmitted. The receiver is able to recover the payload information of the transmitted spreaded signal by correlating it with the particular spreading code, while other not correlated signals (e.g. interference and noise) included in the received spreaded signal are weakened. Thus, signal interference ratio (SIR) can be enhanced by the process gain which is proportional to the quotient of channel bandwidth and narrow band payload information bandwidth. Such a CDMA system is well known in the art.
Since the frequency band on the forward link or on the reverse link respectively is used by all mobile stations, they cause interference to each other. The capacity of CDMA cellular communication systems, which means the maximum number of mobile stations being able to communicate with a base station simultaneously, is limited by the maximum permitted bit error rate (BER) of the decoded despreaded signal in the receiver which is inversely proportional to the SIR. Hence, the system capacity is limited by the interference.
In a highly dynamic environment of a mobile communication system where establishing and releasing of calls, movement of the mobile station, fading, shadowing, handoff, and other effects cause a permanent fluctuation of the received SIR an efficient power control method is necessary to maintain high quality communication and maximum system capacity.
It is well known to those skilled in the art that a reverse linkpower control method typically controls the transmitted power of the mobile station so that the required BER for each mobile station at the base station can be achieved with a minimum SIR.
A so-called open loop power control compensates for the path loss between the mobile station and the base station. The mobile station measures the received input power on the forward link and adjusts its transmission power level accordingly. A mobile station which is more distant from the base station or one which experiences a stronger fading loss will transmit with more power than a mobile station which is for example close to the base station. Obviously the near-far effect can be mitigated.
As previously mentioned, the forward and the reverse link use different frequencies implying an independent fading characteristic, and thus only open. loop power control is insufficient. Using a so-called closed loop power control method takes that problem into account. The base station measures the SIR of the received reverse link signal from a particular mobile station and compares it to a threshold and sends a power control adjustment signal on the forward link to the particular mobile station requesting it to adjust its transmission power. Typically, the power control adjustment is sent by one or more power control bits. If the SIR is less than the threshold, the particular mobile station will be requested to increase its transmission power level by a small amount (e.g. 1 dB). On the other hand, in case the SIR is larger than the threshold, the mobile station will be requested to decrease its transmission power level by a small amount (e.g. 1 dB). Typically, power control bits are sent about every millisecond. That period thus is generally referred as power control group.
Further, the level of the threshold is controlled in such a manner that the required BER is achieved with the lowest possible SIR. In other words, the threshold is increased when the actually measured BER is larger than the required BER, and in the case when the actually measured BER is smaller than the required BER, the threshold is decreased.
Similarly, in forward link power control the mobile station measures its received SIR on the forward link against a threshold, and accordingly, sends a kind of power adjustment request to the base station on the reverse link.
FIG. 5
shows a simplified protocol between a mobile station and a base station during call acquisition when establishing a forward and reverse link communication channel as it is well known in the art.
Upon request, the base station initializes a forward and a reverse link communication channel S
10
and starts transmitting on the forward link S
11
. A channel assignment order will be sent to the mobile station causing the mobile station to initialize its forward and reverse link communication channel process S
20
and start the acquisition of the forward link S
21
. After having received and decoded the forward link communication channel S
22
, the mobile station starts sending a preamble signal on the reverse link communication channel and starts the closed loop power control process S
23
, and thereafter waits for the base station to send an acknowledgement indicating that the preamble has been received. After sending the channel assignment order, the base station starts the acquisition of the reverse link communication channel S
12
and starts the closed loop power control S
13
. Finally, after succeeding in receiving and decoding the preamble S
14
, the base station sends an acknowledgement to the mobile station.
FIG. 1
shows a receiver and transmitter configuration used in a conventional closed loop power control method in a base station of CDMA mobile communication system.
A multipath signal transmitted by a mobile station is received by an antenna
101
, and relayed to a down converter
103
via a radio frequency distributor
1
.
02
. The signal is down-converted into a digital IF signal which is input into the demodulator
104
. Each correlator
14
1
,
14
2
, . . . ,
14
n
applies the spreading code determined by the mobile station to the input signal of the demodulator
104
to extract the multipaths which are combined in the summer
140
. The output signal of the summer
140
is coupled to the decoder
105
, while the SIR of the output signal is measured and sent to the detector
106
. The decoder
105
finally processes the original payload information signal, and determines the communication quality by measuring the BER of the decoded payload. The BER is output to the detector
106
.
The detector
106
relays the BER and SIR information to the closed loop power controller
107
. The closed loop power controller
107
runs the closed loop power control algorithm.
FIG. 2
depicts the flow chart of a conventional closed loop power control algorithm S
300
that is initiated after the base station has sent the channel assignment order S
13
. After initialization of the BER threshold and SIR threshold with nominal values S
301
, the SIR value received from the detector
106
is compared to the SIR threshold. If the SIR is larger than the SIR threshold, decrement power control bits are inserted into the forward link signal
109
which cause a decrease in the mobile station's transmission power S
303
. If the SIR is smaller than the SIR threshold the power control bits will force the mobile station to increase its transmission power level S
304
.
Further, the BER is compared to its BER threshold as well S
305
. If the BER is lower than the BER threshold the SIR threshold value is decreased which in the long term, causes a higher transmission power of the mobile station. On the other hand, a BER la
Gantt Alan T.
Hunter Daniel
NEC Corporation
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