08-17-2017, 12:39 AM
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.