Method of quadrature spreading
The invention provides a method of quadrature spreading suited for use in a software implementation of a code division multiple access communication (CDMA) system. The method includes providing for a bit of short code, adding a bit of each of N data channels together to obtain a combined channel value (X in ), and performing a spreading operation to obtain a spread channel value.
1. A method of quadrature spreading by an M bit processor in a code division multiple access communication (CDMA) system, the method comprising:
adding a bit of each of N data channels together to obtain a combined channel value represented by K bits where K is the smallest number of bits which is required to represent N in binary and which is a factor of M;
packing S combined channel values into an M bit combined channel data element, where S×K=M;
extending M/N bits of short code to M bits by repeating each bit of short code N times to produce an extended short code; and
XORing the combined channel data element with the extended short code to obtain an N channel M bit spread channel value, wherein the XORing comprises a spreading operation:
X
out
=
{
X
in
if
the
bit
of
short
code
=
0
N
-
X
in
if
the
bit
of
short
code
=
1
and wherein said spreading operation is performed by:
providing constant data having M bits in which a binary sequence representing the number of channels N is repeated S times;
performing a first manipulation of said combined channel data by logically AND-ing said combined channel data with said short code data to thereby obtain first manipulated combined channel data;
performing a second manipulation of said combined channel data by (a) subtracting said combined channel data from said constant data to obtain first intermediate data and (b) logically AND-ing said intermediate data with the inverse of said short code data to thereby obtain second manipulated combined channel data;
logically OR-ing said first and second manipulated combined channel data; and subtracting said logically OR-ed first and second manipulated combined channel data from said constant data to obtain said spread channel data.
2. A method as claimed in claim 1 , wherein said spreading operation is performed by:
providing constant data having M bits in which a binary sequence representing the number of channels N is repeated S times;
performing a first manipulation of said combined channel data by logically AND-ing said combined channel data with the inverse of said short code data to thereby obtain first manipulated combined channel data;
performing a second manipulation of said combined channel data by (a) subtracting said combined channel data from said constant data to obtain intermediate data, and (b) logically AND-ing said intermediate data with said short code data to thereby obtain second manipulated combined channel data, and
logically OR-ing said first and second manipulated combined channel data to obtain said spread channel data.
3. A method as claimed in claim 1 wherein N is 4.
4. A method as claimed in claim 1 wherein M is the native number of bits of a processor used to carry out a method.
5. A method as claimed in claim 4 where M is 32.
6. A processor programmed to carry out the method of claim 1 .
7. A program which when executed by a processor will cause said processor to carry out the method of claim 1 .
8. A program which when executed by a 32-bit digital signal processor will cause said 32-bit digital signal processor to carry out the method of claim 1 wherein M=32.