Pulse or digital communications – Receivers – Interference or noise reduction
Patent
1997-11-14
2000-07-18
Chin, Stephen
Pulse or digital communications
Receivers
Interference or noise reduction
375316, 342373, 455561, 455562, H04B 710
Patent
active
060917881
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a base station equipment for receiving and transmitting a signal of a desired user, in which the signal to be received may arrive at the equipment along a plurality of paths with a plurality of delays.
2. Description of Related Art
The present invention is applicable for use in a data transmission system applying any multiple access method, but especially in a cellular system using 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. An example of a known CDMA system is the broadband cellular radio standard, EIA/TIA IS-95.
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, recognized based on 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. These signals appear as noise with respect to the desired signal. The spreading codes of the system are preferably selected such that they are mutually orthogonal, i.e., they do not correlate with each other.
In a typical mobile phone environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal on the surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays. In the CDMA, the multipath propagation can be exploited during the reception of the signal in the same way as diversity. The receiver generally used in a CDMA system is a multibranch receiver structure where each branch is synchronized with a signal component which has propagated along an individual path. Each branch is an independent receiver element, the function of which is to compose and demodulate one received signal component. In a conventional CDMA receiver, the signals of the different receiver elements are combined advantageously, either coherently or incoherently, whereby a signal of good quality is achieved.
CDMA systems can also apply a soft handover wherein a mobile station may simultaneously communicate with several base stations by utilizing macrodiversity. The connection quality of the mobile station thus remains high during the handover and the user does not notice a break in the connection.
Interference caused by other connections in the desired connection appears in the receiver as noise that is evenly distributed. This is also true when a signal is examined in an angular domain according to the incoming directions of the signals detected in the receivers. The interference caused by the other connections in the desired connection also appears in the receiver as distributed in the angular domain, i.e., the interference is rather evenly distributed into the different incoming directions.
The capacity of the CDMA, which can
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Keskitalo Ilkka
Laiho-Steffens Jaana
Muszynski Peter
Chin Stephen
Ghayour Mohammad
Nokia Telecommunications Oy
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