Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-02-17
2001-08-07
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S422100, C455S069000
Reexamination Certificate
active
06272354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for adjusting transmit power during call set-up in a cellular radio system comprising in each cell at least one base station and a group of subscriber equipments communicating with one or several base stations, and in which method the signal-to-noise ratio between a subscriber equipment and a base station is estimated before an actual connection is set up.
BACKGROUND OF THE INVENTION
The present invention is applicable for use in all cellular radio systems and in so-called WLL (Wireless Local Loop) systems that are interference-limited, and especially in a radio system utilizing code division multiple access. Code division multiple access (CDMA) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example spectral efficiency and the simplicity of frequency planning.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the different users can be distinguished from one another in the receivers on the basis of the spreading code of each user.
Matched filters provided in the receivers are synchronized with a desired signal, which they recognize on the basis of a spreading code. The data signal is restored in the receiver to the original band by multiplying it again by the same spreading code that was used during the transmission. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other. In practice the spreading codes are not orthogonal, and the signals of the other users hinder the detection of the desired signal by distorting the received signal. This mutual interference caused by the users is called multiple access interference.
The more simultaneous users there are in the system, the greater the multiple access interference. Therefore the capacity of the CDMA cellular radio system is restricted by the above-described mutual interference caused by the users. The interference can be reduced by keeping the power levels of the signals from the terminal equipments as equal as possible in the base station receiver by means of accurate power control. The base station then monitors the powers of the received signals and transmits power control commands to the terminal equipments.
A known problem in interference-limited systems is the selection of the transmit power used at the beginning of the communication over the connection. A known solution is to start transmitting with a minimum power that is then increased by fixed steps. The disadvantage of this method is that it does not take into account the loading situation of the cell which may vary greatly, but it operates in a similar way in every situation.
In another known method, disclosed in Finnish patent application 952,396 that is incorporated herein by reference, the signal-to-noise ratio is estimated between a subscriber equipment and a base station in the uplink direction before the actual connection is established, and the transmit power is selected so that the desired signal-to-noise ratio is obtained. The disadvantage of this method is that since the terminal equipment starts transmitting directly with a power that is different from the minimum power, the transmission interferes with the other terminal equipments, which in turn must increase their transmit power in order to obtain a sufficient signal-to-noise ratio. In such a case, the total interference in the network increases needlessly.
SUMMARY OF THE INVENTION
The purpose of the present invention is to implement a method with which the correct transmit power is selected for obtaining the required signal-to-noise ratio and wherein the transmit power is increased to the desired level in such a way that no interference is caused to the other connections, taking into account the loading situation of the cell.
This is achieved with a method of the type described in the preamble, characterized in that a value with which the desired signal-to-noise ratio is obtained is calculated for the transmit power of the transmitter, and that a certain initial value is calculated for the transmit power by means of the power-control parameters of the system and a given time constant, and that the transmission is started with the calculated initial value, and that the transmit power is increased from the initial power to said value during the given time constant.
The invention also relates to a cellular radio system comprising in each cell at least one base station and a group of subscriber equipments communicating with one or several base stations, and in which cellular radio system at least some transceivers comprise means for estimating the signal-to-noise ratio between a subscriber equipment and a base station before an actual connection is set up.
The cellular radio system according to the invention is characterized in that at least some transceivers comprise means for calculating for the transmit power of the transmitter a value with which the desired signal-to-noise ratio is obtained, and means for calculating a certain initial value for the transmit power of each transmitter by means of the power-control parameters of the system and a given time constant, and that at least one transmitter comprises means for starting transmission with the calculated initial value, and means for increasing the transmit power from the initial power to said value during the given time constant.
The arrangement according to the invention can be applied in both transmission directions, i.e. for adjusting the initial power of the transmitter of both the base station and the subscriber equipment.
In an arrangement according to the preferred embodiment of the invention, the base stations measure the total interference of the signals received from the terminal equipments and transmit a pilot signal with a predetermined transmit power. For the purpose of call establishment, the terminal equipments measure the power levels of the pilot signals arriving from the base stations that are in a list of nearby base stations maintained by the terminal equipment. The attenuations over the connections between the terminal equipment and the base stations are estimated by means of the pilot signals, and the signal-to-noise ratio between the subscriber equipment and the base station in the uplink transmission direction is estimated on the basis of the total interference measured at the base station, the aforementioned attenuation over the connection, and the assumed transmit power of the terminal equipment.
In the method according to the invention, the total interference in the network is smaller than previously. Minimization of the total interference is essential in interference-limited systems, especially in CDMA systems. In systems where the traffic load varies, the optimal power level for call establishment can be obtained in each situation by utilizing the method according to the invention. The power level is not increased by fixed steps, but the rate of increase depends on the situation. The traffic load varies especially in systems with several different types of connections, such as speech and data connections, that have considerably different capacity requirements.
REFERENCES:
patent: 5216692 (1993-06-01), Ling
p
Chow C.
Hunter Daniel
Nokia Mobile Phones Ltd.
Perman & Green LLP
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