Introduction to WCDMA


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Properties of the Spread Spectrum

• Transmission bandwidth is much larger than information bandwidth.
• Bandwidth does not depend on the informational signal.
• Processing gain = Transmitted bandwidth/ Information bandwidth.
• Classification:
– Direct sequence: Data is scrambled by user specific pseudo noise code at the
transmitter side.
– Frequency Hopping: The signal is spread by changing the frequency over the
transmitted time of the signal:
• Fast frequency hopping.
• Slow frequency hopping.
– Time Hopping: The data is divided into frames, that itself are divided into time
intervals. The data is burst is hopped over the frames by utilising code sequences.

Background of SS

• First publications late 40s.
– Patent proposal in 1941.
• 1949 C. Shannon and R. Pierce develop basic ideas of CDMA.
• First applications 50s.
– Military with very low C/I, Anti-jam.
• RAKE receiver patent 1956.
• Cellular applications proposed late 70s.
• Investigations for cellular use 80s.
• IS-95 standard 1993.
– Commercial introduction in 1995.
• 1997/1998 3G technology choice in ETSI/ARIBA/TTA … .

Codes (1)
• Requirements for the spreading codes:
– Good auto-correlation properties. For separating different paths.
– Good cross-correlation properties. For separating different channels.
Channelisation codes used for channel separation from the same source.
• Same codes from all the cells.
• Short codes: used for channel separation in Uplink and Downlink.
– Othogonality property, reduce interference.
– Different spreading factors, different symbol rates.
– Limited resource, must be managed.
– Do not have good correlation properties, need for additional long code.
Scrambling codes.
• Long Codes:
– Good correlation properties.
– Uplink: different users.
– Downlink: different BS.