Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-11-25
2001-10-02
Ngo, Ricky (Department: 2731)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C375S144000, C375S148000, C375S349000
Reexamination Certificate
active
06298050
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to systems and methods for cancelling interference and more particularly to a system and method for cancelling the extra interference created during position location in a CDMA cellular system.
BACKGROUND OF THE INVENTION
The link from a mobile station to a basestation in 3G (third generation) CDMA (code division multiple access) systems is a multiple access radio channel that uses CDMA as the access scheme. In this scheme many users share the same radio channel and each user is uniquely identified by a spreading code.
In 3G CDMA, the mobile transmitter in the mobile station is used by the end user to access the network. It may provide voice only, data only, simultaneous voice and data, and location services for many different environments including indoor, low mobility, full mobility, and fixed wireless. To support the various end user services, the mobile transmitter divides its channel into multiple subchannels. One of these subchannels is used to manage the air interface resource and aid in signal detection. This is the pilot subchannel. The remaining three subchannels are used to carry user data and to implement the air interface protocol.
The pilot subchannel carries pilot channel bits which have the characteristic that they are known at the receiver, and also carries PC (power control) bits which are easily detectable at the receiver, while the remaining channels carry information which is unknown at the receiver.
It is a well known fact that due to the nature of CDMA, the signals transmitted by one user will occupy the same bandwidth as signals transmitted by other users and in fact all the signals of other users may be modelled as a form of interference.
It is sometimes a requirement to determine the location of a user in a cellular network. Furthermore, the FCC has mandated that emergency calls (911 calls) made from cellular telephones shall be physically locatable to within 125 metres by the year 2001. This mandate was created so that emergency crews (fire, police, etc.) would be able to locate the caller in an emergency situation.
Locating a user in a cellular network is basically a triangulation process. The user's signal is received by several fixed base stations and the difference in the time of arrival of the signal at the base stations is used to estimate the user's position. For this system to work, the base stations must be in physically different locations; the physical location of the base stations must be known; and at least three base stations must be able to detect the user's signal. The first two criteria are usually satisfied by most cellular networks. The final criteria poses a problem in power controlled CDMA networks.
In a power controlled CDMA network (i.e., the 3G Cellular Network), a user's transmitted power is controlled by the base station with which it is communicating. The control algorithm is designed to minimize the amount of power transmitted by the mobile such that it still meets the minimum voice and data quality criteria. This is done in a CDMA network to minimize the amount of interference that each user adds to the other users also communicating with the base station. In the position location context, this has the unfortunate side effect of reducing the user's transmitted power to the point where only one base station can detect the signal, hence making position location impossible.
One way to solve the low power problem is for the mobile to increase its power when it needs to be located. This is a realistic solution since only emergency calls are located and there should only be a small percentage of emergency calls on a system at any one time. Unfortunately, increasing a mobile's power during an emergency call increases the amount of the interference that the other users see and thus degrades the quality of their signals.
Any substantial reduction in interference in a CDMA system is very important because this results in an effective increase in the system signal to noise ratio, and thereby allows a system to be deployed with fewer base stations. One of the most significant costs in deploying a CDMA network is simply that of obtaining rights to install base stations the desired locations. Any decrease in the number of base stations required, even at the expense of a slightly increased cost per basestation, would be desirable.
Many interference cancellation schemes have been proposed for eliminating the entire contribution of all other users. Such schemes are very complex because they require some sort of estimate of the received signals of the other users, and in many cases this estimate is very difficult to produce due to the user's weak signal strength.
SUMMARY OF THE INVENTION
This invention provides a technique for removing the extra interference created by the user being located during an emergency call when that user raises his transmitter power. By removing the interference the quality of the other users' signals on the network is not degraded.
Removing the extra interference created by the user being located during an emergency call (the interfering user) is a multiple step process. First, the interfering user is detected and decoded. This is a simple process since the interfering user's signal is stronger than all the other users signals. In detecting the interfering user, an estimate of the channel characteristics is made to determine the strength, number, and position of the multipath components of the user's signal. This channel information is used to reconstruct the interfering user's signal. Then, the reconstructed signal is subtracted from the composite received signal. This removes the interfering user's signal from the composite received signal, thus improving the signal-to-noise ratio of the other users on the system.
Preferably, the interference cancellation in this invention is only implemented when the interfering user's signal is much stronger than all the other user's individual signals.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
According to a first broad aspect, the invention provides an interference cancellation circuit for cancelling interference due to a particular user signal forming part of a digitized baseband composite signal when the particular user signal satisfies a predetermined constraint, the circuit comprising: a baseband detection unit for detecting one or more detected data channels from said particular user signal and making an air interface channel characterization which is an estimate of an air interface channel over which the particular user signal was transmitted; user signal regeneration circuitry for generating an interfering user signal reconstruction which is an estimate of said particular user signal by modulating said one or more data channel streams taking into account said air interface channel characterization; delay circuitry for delaying the digitized baseband composite signal an amount substantially equal to the time it takes the baseband detection unit and user signal regeneration circuitry to create interfering user signal reconstruction; and subtraction circuitry for subtracting the interfering user signal reconstruction from the delayed digitized baseband composite signal to create a corrected digitized baseband composite signal.
According to a second broad aspect, the invention provides a CDMA receiver comprising: an antenna for receiving an RF composite spread spectrum signal over an air interface; RF down conversion and A/D conversion circuitry for converting the composite spread spectrum signal into a digitized composite baseband signal; an interference cancellation circuit for cancelling interference due to a particular user signal forming part of a digitized baseband composite signal when the particular user signal satisfies a predetermined constraint, the circuit comprising: a baseband detection unit for detecting one or more detected data channels from said particular user signal and ma
Newson Paul
van Heeswyk Frank Martin
Ngo Ricky
Nortel Networks Limited
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