Apparatus and methods for controlling short code timing offsets in a CDMA system

• US 6,813,257 B1
• Filed: 06/26/2000
• Issued: 11/02/2004
• Est. Priority Date: 06/26/2000
• Status: Expired due to Fees

• Current Assignee: CDC Propriete Intellectuelle
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Ameduri, Scott A., Haverty, Michael B., Emmons, Thomas Peter Jr., Gross, Jonathan H.
• Primary Examiner(s): Nguyen, Chau
• Assistant Examiner(s): NG, CHRISTINE Y
• Application Number:
  US09/605,663
• Time in Patent Office: 1,590 Days
• Field of Search: 370/342, 370/335, 370/350, 370/503, 375/140
• US Class Current: 370/335
• CPC Class Codes: H04B 1/708 Parallel implementation
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception
• CPC Class Codes: H04B 7/18506 Communications with or from aircraft, i.e. aeronautical mobile service

Abstract

A CDMA cellular communications network (400, FIG. 4) includes one or more aircraft (410), which relay pilot channel and control channel signals between base transceiver stations (BTS'"'"'s 406, 413) and subscriber units (401). Over the control channel, the BTS transmits a handoff candidate list to the subscriber units. The handoff candidate list identifies candidate BTS'"'"'s to which the subscriber unit, theoretically, could hand off. In addition, the list indicates at which offsets the subscriber units should search for short codes transmitted by the candidate BTS'"'"'s over the pilot channel. Based on the path length between the BTS, aircraft, and subscriber unit, the candidate BTS'"'"'s actually generate their short codes at an offset that is equal to or earlier than the offset reported to the subscriber unit by some delta. In addition, the BTS'"'"'s can impose a variable delay on the generated short code bits to compensate for variations in the path delay as the aircraft flies in its flight pattern.

Claims

1. A method for controlling transmission of a pseudo noise (PN) code in a code division multiple access (CDMA) communication system, which includes at least one base transceiver station (BTS) and at least one subscriber unit, the method comprising:

transmitting, by a first BTS, data representing a first offset that indicates a time when a subscriber unit should search for a PN code in a signal transmitted by a second BTS;

generating the PN code, by the second BTS;

transmitting, by the second BTS, a modulated signal that includes the PN code at a time specified by a second offset, wherein the second offset specifies a different time than the first offset;

delaying modulation and transmission of the PN code by a delay time, wherein the delay time is based on a time-varying path length of a communication path between the second BTS and the subscriber unit; and

storing the PN code in a buffer prior to the modulation and transmission, and wherein delaying comprises altering a number of PN code bits in the buffer.

2. The method as claimed in claim 1, wherein transmitting the modulated signal comprises periodically transmitting a modulated short code at times specified by the second offset.

3. The method as claimed in claim 1, wherein the time specified by the second offset is earlier than a time specified by the first offset by a value of delta, and the delta is a function of a path length of a communication path between the second BTS and the subscriber unit.

4. The method as claimed in claim 3, wherein the communication path includes an aircraft that relays the modulated signal between the second BTS and the subscriber unit, and the path length equals about a sum of a length of a first link between the second BTS and the aircraft and a length of a second link between the aircraft and the ground.

5. The method as claimed in claim 4, wherein the aircraft attempts to follow a flight pattern, and the delta changes periodically with time based on a position of the aircraft along the flight pattern.

6. The method as claimed in claim 1, wherein transmitting the data comprises transmitting a handoff candidate list to the subscriber unit, wherein the handoff candidate list indicates first offsets for one or more candidate BTS'"'"'s, including the second BTS, to which the subscriber unit may be able to hand off, and the one or more BTS'"'"'s transmit modulated PN codes at times specified by second offsets that are different from times specified by the first offsets.

7. The method as claimed in claim 1, wherein delaying modulation and transmission comprises:

changing a frequency of a clock that regulates a time at which bits of the PN code are provided to a modulator.

8. A method for controlling transmission of a pseudo noise (PN) code in a code division multiple access (CDMA) communication system, which includes at least one base transceiver station (BTS) and at least one subscriber unit, the method comprising:

identifying areas where cells provided by multiple BTS'"'"'s overlap;

determining a first offset, which indicates a time when a subscriber unit should search for a PN code transmitted by a first BTS, so that a PN code transmitted by the first BTS at a time specified by the first offset will not interfere with a PN code transmitted by another BTS;

determining a second offset, which indicates a time at which the first BTS should transmit the PN code, wherein the second offset is different from the first offset; and

sending information identifying the second offset to the first BTS.

9. The method as claimed in claim 8, further comprising:

determining additional offsets for additional BTS'"'"'s so that PN codes transmitted by the additional BTS'"'"'s will not interfere; and

generating a handoff candidate list that correlates the first offset with the first BTS and the additional offsets with the additional BTS'"'"'s, wherein sending the information comprises sending the handoff candidate list to the first BTS.

10. The method as claimed in claim 8, wherein determining the second offset comprises:

calculating a path length of a communication path between the first BTS and the ground;

calculating a path delay for a signal transmitted along the communication path, based on the path length; and

determining the second offset as approximately the first offset minus the path delay.

11. The method as claimed in claim 10, wherein the communication path includes an aircraft that relays signals between a second BTS and the subscriber unit, and determining the path length comprises calculating a sum of a length of a first link between the second BTS and the aircraft and a length of a second link between the aircraft and the ground.

12. The method as claimed in claim 10, wherein the path delay changes based on a location of the aircraft along a flight pattern, the method further comprising:

calculating a delay that the first BTS should impose on the PN code before transmitting the PN code to the subscriber unit; and

sending information identifying the delay to the first BTS.

13. A base transceiver station (BTS) of a code division multiple access (CDMA) communication system, the BTS comprising:

a pseudo noise (PN) code generator that generates a PN code at a time specified by a first offset, wherein the PN code includes multiple bits;

a modulator that modulates the multiple bits into a modulated signal;

a radio frequency (RF) transmitter, coupled to the modulator, that transmits the modulated signal to a subscriber unit, wherein the subscriber unit has been informed to search for the PN code at a time specified by a second offset, and wherein the second offset is different from the first offset;

the RF transmitter transmits the modulated signal to the subscriber unit via an aircraft that relays the modulated signal to the subscriber unit, and wherein a delta between the first offset and the second offset is based on a path delay corresponding to a path length of a path between the BTS and the aircraft and a path length of a path between the aircraft and the subscriber unit.

14. The BTS as claimed in claim 13, further comprising:

a buffer for storing the multiple bits prior to modulating the multiple bits, effectively delaying modulation and transmission of the PN code by a delay time, wherein the delay time is based on a time-varying path length of a communication path between the second BTS and the subscriber unit.

15. The BTS as claimed in claim 14, further comprising:

a clock that regulates times at which bits of the PN code are provided to the modulator.

16. The BTS as claimed in claim 13, further comprising:

a system interface that receives information identifying the first offset from a control center.