Channelisation (1/3)
• Channelisation codes are orthogonal codes, based on Orthogonal
Variable Spreading Factor (OVSF) technique
• The codes are fully orthogonal, i.e., they do not interfere with each
other, only if the codes are time synchronized
• Thus, channelisation codes can separate the transmissions from a
single source
• In the downlink, it can separate different users within one cell/sector
• Limited orthogonal codes must be reused in every cell
• Problem: Interference if two cells use the same code
• Solution: Scrambling codes to reduce inter-base-station interference

Channelisation (2/3)
• In the uplink, it can only separate the physical channels/services of one
user because the mobiles are not synchronised in time.
• It is possible that two mobiles are using the same codes.
• In order to separate different users in the uplink, scrambling codes are
used.
• The channelisation codes are picked from the code tree as shown in
next slide.
• One code tree is used with one scrambling code on top of the tree.
• If c4,4 is used, no codes from its subtree can be used (c8,7 , c8,8 , …).