hannel coding have proved to be an effective format for exploiting the properties of the Hadamard matrix to deliver beneficial performance gains over simpler radio systems. 3.3.5 OVSF Codes in W-CDMA The orthogonal variable spreading factor codes used in W-CDMA were originally conceived as a reordering of the Walsh codes used in IS95 CDMA with the added twist that user data rates could be changed every 10 milliseconds with users being moved between different lengths of spreading code (hence the 'variable' description used). This is shown in Table 3.7. The section in bold indicates that this is a tree-structured code. The codes to the right are longer copies of the codes to the left. The bold segment denotes a branch of the tree stretching from left to right. At SF4 (which means spreading factor 4) four users can be supported on each of 4 codes at, say a theoretical data rate of 960 kbits. The code tree then extends rightwards to SF256 (not shown for reasons of space and sanity) which would theoretically support 256 users each with a data rate of 15 kbits/s. As the data rate changes, potentially every frame (every 10 milliseconds), users can be moved to the left or right of the code tree. However, if a user is at SF4, no users can be on Making Telecoms Work Table 3.7 OVSF Codes in W-CDMA +1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1 +1+1+1+1+1+1+1+1 +1+1+1+1+1+1+1+1-1-1-1-1-1-1-1-1 +1+1+1+1 +1+1+1+1-1-1-1-1+1+1+1+1-1-1-1-1 +1+1+1+1-1-1-1-1 +1+1+1+1-1-1-1-1-1-1-1-1+1+1+1+1 +1+1-1-1+1+1-1-1+1+1-1-1+1+1-1-1 +1+1-1-1+1+1-1-1 +1+1-1-1+1+1-1-1-1-1+1+1-1-1+1+1 +1-1-1-1 +1+1-1-1-1-1+1+1+1+1-1-1-1-1+1+1 +1+1-1-1-1-1+1+1 +1+1-1-1-1-1+1+1-1-1+1+1+1+1-1-1 +1-1+1-1+1-1+1-1+1-1+1-1+1-1+1-1 +1-1+1-1+1-1+1-1 +1-1+1-1+1-1+1-1-1+1-1+1-1+1-1+1 +1-1+1-1 +1-1+1-1-1+1-1+1+1-1+1-1-1+1-1+1 +1--1+1--1--1+1-1+1 +1-1+1-1-1+1-1+1-1+1-1+1+1-1+1-1 +1-1-1+1+1-1-1+1+1-1-1+1+1-1-1+1 +1-1-1+1 +1-1-1+1+1-1-1+1 +1-1-1+1+1-1-1+1-1+1+1-1-1+1+1- +1-1-1+1-1+1+1-1+1-1-1+1-1+1+1- +1-1-1+1-1+1+1-1 +1-1-1+1-1+1+1-1-1+1+1-1+1-1-1+1 codes to the right on the same branch. 