System and method for cancelling the extra interference created during position location in a CDMA cellular system

• US 6,298,050 B1
• Filed: 11/25/1998
• Issued: 10/02/2001
• Est. Priority Date: 11/25/1998
• Status: Expired due to Term

• Current Assignee: Blackberry Limited
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Newson, Paul, van Heeswyk, Frank Martin
• Primary Examiner(s): NGO, RICKY QUOC
• Application Number:
  US09/199,302
• Time in Patent Office: 1,042 Days
• Field of Search: 370/208, 370/209, 370/204, 370/252, 370/311, 370/313, 370/328, 370/335, 370/342, 370/441, 370/479, 370/349, 375/130, 375/140, 375/141, 375/144, 375/147, 375/148, 375/260, 375/267, 375/346, 455/456
• US Class Current: 370/335
• CPC Class Codes: H04B 1/7107 Subtractive interference cancellation
• CPC Class Codes: H04B 1/711 the interference being multi-path interference
• CPC Class Codes: H04B 2201/70715 with application-specific features
• CPC Class Codes: H04B 7/005 Control of transmission; Equalising
• CPC Class Codes: H04W 52/283 Power depending on the position of the mobile
• CPC Class Codes: H04W 52/288 taking into account the usage mode, e.g. hands-free, data transmission, telephone

Abstract

Systems and methods for cancelling interference in received CDMA signals due to signals of users involved in position location in the received signals which have been digitized to form a composite digitized baseband signal which would otherwise interfere with the detection of other user'"'"'s data streams. The particular user'"'"'s signals are detected, and an estimate of the air interface for that user is made. The user'"'"'s signals are then passed through a model of the air interface, and the resulting output is subtracted from the composite digitized baseband signal, thereby removing interference due to that user'"'"'s signals.

Claims

1. 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.

2. A circuit according to claim 1 wherein said air interface characterization comprises an estimated phase shift, an estimated attenuation and an estimated timing for each of several multipaths in the particular user signal.

3. A circuit according to claim 1 wherein said digitized baseband composite signal contains a plurality of user signals, the circuit further comprising:

control circuitry for identifying whether one of said plurality of user signals satisfies said constraint and for establishing the one of said plurality of user signals as said particular user.

4. A circuit according to claim 3 wherein said control circuit comprises:

receive signal strength comparison circuitry for identifying whether one of the plurality of user signals has a receive signal greater than at least one other of the user signals by at least a predetermined amount, and selecting said particular user signal as the one of the plurality of user signals.

5. The circuit of claim 1 wherein said first baseband detection unit comprises:

a searcher for identifying multipaths forming said user signal; and

a plurality of tracker fingers each for tracking a respective multipath identified by the searcher and producing a respective multipath estimate consisting of an amplitude, phase and timing of the multipath.

6. The circuit of claim 1 wherein said particular user signal contains a plurality of data channels, and said first baseband detection unit comprises channel specific detection circuits for each of said data channels.

7. The circuit of claim 5 wherein said user signal regeneration circuitry comprises channel specific circuitry for processing each of said data channels and a combiner for combing the processed data channels into a multiplexed channel;

for each multipath estimate a multipath reconstructor comprising;

a) a modulator including code spreading and FIR filtering for multiplying the multiplexed channel by I and Q spreading signals in accordance with the timing estimate for that multipath estimate;

b) a phase rotation function for applying a phase shift equal to the phase for that multipath estimate;

c) an attenuation function for applying an attenuation equal to the amplitude for that multipath estimate;

summing circuitry for adding together an output of each multipath reconstructor.

8. A circuit according to claim 5 wherein said data channels includes a known information channel which is used to assist in detecting the user signal and in identifying multipaths of the user signal.

9. The circuit of claim 8 wherein the composite digitized baseband signal contains signals which are in accordance with the CDMA IS-95 3G standard, and the known information channel comprises the pilot channel of the particular user signal.

10. The circuit of claim 8 wherein said baseband detection unit comprises a complete CDMA IS-95 3G baseband detection unit with circuitry for detecting a CDMA IS-95 3G pilot channel component, and for demodulating power control bits in the pilot channel component, a fundamental channel, a supplementary and a control component;

and

wherein said user signal regeneration circuitry comprises a complete CDMA IS-95 transmit channel specific block, and for each multipath a multipath reconstructor comprising;

a) a modulator including code spreading and FIR filtering for multiplying the multiplexed channel by I and Q spreading signals in accordance with the timing estimate for that multipath estimate;

b) a phase rotation function for applying a phase shift equal to the phase for that multipath estimate;

c) an attenuation function for applying an attenuation equal to the amplitude for that multipath estimate;

summing circuitry for adding together an output of each multipath reconstructor.

11. A circuit according to claim 1 further comprising circuitry for cancelling one or more additional user signals, the circuitry comprising for each additional user signal:

an additional baseband detection unit;

an additional user signal regeneration circuitry for generating a respective interfering user signal reconstruction;

subtraction circuitry for subtracting all interfering user signal reconstructions from the delayed digitized baseband composite signal.

16. A receiver according to claim 11 wherein said interference cancellation circuit has an input identifying any user(s) involved in position location, and wherein said particular user must be such a user involved in position location.

12. 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 making an air interface channel characterization which is an estimate of an air interface channel over which the 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;

subtraction circuitry for subtracting the interfering user signal reconstruction from the delayed digitized baseband composite signal to create a corrected digitized baseband composite signal; and

a baseband detection unit for each user signal other than said particular user signal connected to receive the corrected composite digitized baseband signal and the channel characterization information for that user signal, for producing one or more received data streams for that user signal.

13. A receiver according to claim 12 wherein said first baseband detection unit comprises:

a searcher for identifying multipaths forming said user signal; and

a plurality of tracker fingers each for tracking a respective multipath identified by the searcher and producing a respective multipath estimate consisting of an amplitude, phase and timing of the multipath.

14. A receiver according to claim 13 wherein said data channels includes a known information channel which is used to assist in detecting the user signal and in identifying multipaths of the user signal.

15. A receiver according to claim 14 wherein said baseband detection unit comprises a complete CDMA IS-95 3G baseband detection unit with circuitry for detecting a CDMA IS-95 3G pilot channel component, and for demodulating power control bits in the pilot channel component, a fundamental channel, a supplementary and a control component;

and

wherein said user signal regeneration circuitry comprises a complete CDMA IS-95 transmit channel specific block, and for each multipath a multipath reconstructor comprising;

a) a modulator including code spreading and FIR filtering for multiplying the multiplexed channel by I and Q spreading signals in accordance with the timing estimate for that multipath estimate;

b) a phase rotation function for applying a phase shift equal to the phase for that multipath estimate;

c) an attenuation function for applying an attenuation equal to the amplitude for that multipath estimate;

summing circuitry for adding together an output of each multipath reconstructor.

17. A method of reducing interference due to a particular user signal forming part of a composite digitized baseband signal comprising:

selecting one of a plurality of user signals within said composite digitized baseband signal to the said particular user signal on the basis of one or more constraints;

detecting one or more data channels in the particular user signal and making an estimate of an air interface over which the particular user signal was received;

passing the detected data channels through a model of the air interface which is based upon the estimate of the air interface to produce an interfering user signal reconstruction;

delaying the composite digitized baseband signal to produce a delayed composite digitized baseband signal an amount of time substantially equal to how long it takes for the steps of detecting and passing;

subtracting the interfering user signal reconstruction from the delayed composite digitized baseband signal to produce a corrected composite digitized baseband signal.

18. A method according to claim 17 further comprising the steps of:

monitoring a signal power of each of said plurality of user signals;

comparing the signal powers to each other, and selecting the user signal having the highest power as said particular user only if it is higher in power than all other signals by a predetermined amount; and

after selecting said particular user, deselecting the particular user when its power is no longer higher in power than all other signals by said predetermined amount.

19. A method according to claim 17 further comprising the steps of:

maintaining information identifying which if any user signals pertain to users involved in position location;

selecting the user signal of a user involved in position location as said particular user signal.

20. A method according to claim 18 further comprising the steps of monitoring a signal power of each of said plurality of user signals wherein said particular user signal to be selected must have a power greater than at least one other user signal by a predetermined amount.