CA2619679A1

CA2619679A1 - Quadriphase spreading codes in code division multiple access communications

Info

Publication number: CA2619679A1
Application number: CA002619679A
Authority: CA
Inventors: Branislav M. Popovic
Original assignee: Individual
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1998-06-16
Filing date: 1999-06-11
Publication date: 1999-12-23
Anticipated expiration: 2019-06-11
Also published as: CA2619679C; CA2620101A1; CA2620101C

Abstract

Optimal code sequences are generated for use in spreading and de-spreading functions in a code division multiple access (CDMA) communications system. In particular, a family of quadriphase spreading codes is employed that provides a maximal number of spreading codes to achieve a high capacity in the CDMA
communications system while at the same time having a minimal peak cross-correlation between any two spreading codes within that family to ensure cross-correlation interference is kept at or below acceptable levels. That optimal quadriphase spreading code family is the S(2) family of four phase code sequences of length L = 2m-1, where m is an integer greater than or equal to 5. The size of the S(2) family of quaternary spreading codes is (L
+ 2)(L + 1)2, and the maximum cross-correlation is 1 + 4.sqroot.(L +1). The spreading codes are preferably allocated to base stations using specific code subsets of the S(2) family having the same cross-correlation properties of the S(O) and/or S(1) family of codes.
Spreading codes are advantageously extended by one or more code symbols as necessary or otherwise desirable. For example, to support variable transmission rate services, it is desirable to employ spreading codes whose length may be expressed as an integer multiple of each spreading factor in the mobile communications system. Since individual spreading codes have a length of 2m-1, one code symbol is added to the generated spreading code.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

I. In a mobile communications system including plural base stations for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base stations, a method comprising:
allocating a first subset of the particular spreading code family to a first base station;
and allocating a second subset of the particular spreading code family to a second base station;
wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.
2. The method of claim 1, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(O) code family and an S(1) code family.
3. The method of claim 2, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2..., L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base stations.
4. The method of any one of claims 1 to 3, further comprising:
for a particular call, assigning a mobile station associated with the first base station an assigned code from the first subset of spreading codes;
associating a flag with the assigned code;
setting the flag to a first value when the mobile station is involved in the call;
setting the flag to a second value after a prescribed time expires; and prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.
5. The method in claim 4, further comprising:
setting the flag to the first value at the beginning of the call; and measuring the prescribed time from the beginning of the call.
6. The method in claim 4, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station.
7. The method in claim 6, further comprising:
setting the flag to the first value when the mobile is involved in the call;
measuring the prescribed time from a time associated with the handover; and if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.
8. In a mobile communications system including base station having plural sectors for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base station, a method comprising:
allocating a first subset of the particular spreading code family to a first base station sector; and allocating a second subset of the particular spreading code family to a second base station sector;
wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.
9. The method of claim 8, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(O) code family and an S(1) code family.
10. The method of claim 9, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2..., L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base station sectors.
11. The method of any one of claims 8 to 10, further comprising:
for a particular call, assigning a mobile station associated with the first base station sector an assigned code from the first subset of spreading codes;
associating a flag with the assigned code;
setting the flag to a first value when the mobile station is involved in the call;
setting the flag to a second value after a prescribed time expires; and prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.
12. The method of claim 11, further comprising:
setting the flag to the first value at the beginning of the call; and measuring the prescribed time from the beginning of the call.
13. The method of claim 11 or 12, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station sector.
14. The method of claim 13, further comprising:
setting the flag to the first value when the mobile station is involved in the call;
measuring the prescribed time from a time associated with the handover; and if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.