Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-11-25
2001-12-25
Pham, Chi (Department: 2731)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S346000
Reexamination Certificate
active
06333947
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an interference cancellation system and method and to a CDMA receiver including an interference cancellation circuit, and more particularly to a system and method for cancelling interference in CDMA receivers due to pilot channels or other known components of a received wideband signal.
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 transmission content into four channels. One of these channels, namely the pilot channel, is used to manage the air interface resource and aid in signal detection. The remaining three channels are used to carry user data and to implement the air interface protocol.
The pilot channel carries pilot bits which have the characteristic that they are known at the receiver, and also carries PC (power control) bits which though unknown at the receiver are easily detectable at the receiver, while the remaining subchannels 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. About 30% of the interference caused by other users is attributable to the pilot channels and 70% is attributable to the remaining information channels. Many interference cancellation schemes have been proposed for eliminating the entire contribution of other users. Such schemes are very complex because some sort of estimate of the received signal for other users including unknown components must be made.
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 basestations. One of the most significant costs in deploying a CDMA network is simply that of obtaining rights to install basestations at the desired locations. Any decrease in the number of basestations required, even at the expense of a slightly increased cost per basestation, would be desirable.
SUMMARY OF THE INVENTION
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 known information contained in a component of a digitized baseband composite signal, the circuit comprising: known information detection and air interface characterization circuitry for detecting said known information stream, demodulating said component and making an air interface characterization which is an estimate of an air interface channel over which the component was transmitted; digitized baseband component regeneration circuitry for generating an estimated component which is an estimate of said component of the digitized baseband composite signal by modulating said known information stream 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 known information detection and air interface characterization circuitry and component regeneration circuitry to create the estimated component; and subtraction circuitry for subtracting the estimated component from the delayed digitized baseband composite signal to create a corrected digitized baseband composite signal.
According to a second broad aspect, the invention provides an interference cancellation circuit for cancelling interference due to particular components of a composite baseband digitized signal which contains both known and unknown information, the circuit comprising: known information detection and air interface characterization circuitry for detecting said known information, demodulating each said particular component and making an air interface characterization which is an estimate of an air interface channel over which the particular component was transmitted; estimation circuitry for making an estimate of the unknown information in each said particular component containing known information; combining circuitry for combining for each said particular component the known information with the estimated unknown information to make a respective combined information; digitized baseband component regeneration circuitry for generating an estimated component for each said combined information which is an estimate of the respective particular component of the digitized baseband composite signal by modulating said combined information taking into account said air interface characterization; delay circuitry for delaying the digitized baseband composite signal an amount substantially equal to the time it takes the known information detection and air interface characterization circuitry and component regeneration circuitry to create the estimated components; and subtraction circuitry for subtracting the estimated components from the delayed digitized baseband composite signal to create a corrected digitized baseband composite signal.
According to a third 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 pilot channel components of the baseband digitized composite signal which contain both known pilot bits and unknown power control bits, the interference cancellation circuit comprising: a) pilot channel detection and air interface and characterization circuitry for detecting the pilot channel components present in the composite baseband digitized signal and making a respective channel estimate of an air interface over which each said pilot channel component was received; b) power control bit detection circuitry for estimating the power control bits in said pilot channel components; c) combining circuitry for combining for each pilot channel component in a respective combined information the pilot bits with the associated estimated power control bits; d) pilot channel reconstruction circuitry for creating a regenerated baseband digitized pilot channel component for each said combined information which is an estimate of the pilot channel component as it would have been received taking into account the respective channel estimate; e) delay circuitry for delaying the digitized baseband composite signal an amount substantially equal to the time it takes to make the estimates of the pilot channel components; and f) subtraction circuitry for subtracting the estimates of the pilot channel components from the delayed digitized baseband composite signal to produce a corrected composite digitized baseband signal; a baseband detection unit for each user connected to receive the corrected composite digitized baseband signal and the channel characterization information for that user, for producing one or more received data streams for that user.
According to a fifth broad aspect, the invention provides a method of reducing interference due to particular components of a composite digitized baseband signal, each particular containing a respective known information and a respective unknown information, the method
Newson Paul
van Heeswyk Frank Martin
Nortel Networks Limited
Pham Chi
Phu Phuong
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