Wireless Technologies Workshop
A. D. Gore
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Code Division Multiple Access
Ashutosh Deepak Gore
adgore@ee.iitb.ac.in
EESA Lecture Series
11th August 2005
Wireless Technologies Workshop
A. D. Gore
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Challenges in wireless mobile communications
• Higher loss rate: R < C = B log2(1 + SN ) [Shannon]
• Restrictive allocation of frequency bands - costly spectrum
• Lower transmission rates
• Higher delay and delay jitter
• Lower security and simpler active attacking
• Locating a user and routing the call
Wireless Technologies Workshop
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Characteristics of the wireless channel
• Time-varying channel
• Interference
• Path loss
Pr =
Pt
f2rα
where 2 < α ≤ 4
• Fading due to multipath
• Shadowing
Wireless Technologies Workshop
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Cellular concept
• Spatial channel reuse
• Handoff
• Pilot channel
• Logical coverage area is a circle
• Hexagons completely cover the area, which leads to minimum number of base sta-
tions
• Sectoring reduces inter-cell interference
• Downlink / Forward link: BS to MS
• Uplink / Reverse link / Multiple access: MS to BS
Wireless Technologies Workshop
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Multiple Access
• Multiple non-cooperative users sharing a common channel have to send information
to a common receiver.
• Fixed/assigned multiple access techniques like:
– Time Division Multiple Access (TDMA)
– Frequency Division Multiple Access (FDMA)
– GSM (hybrid TDMA/FDMA)
are suitable for Constant Bit Rate (CBR) traffic, but not for bursty traffic.
• Bursty traffic:
Peak arrival rate
Average arrival rate
1
Wireless Technologies Workshop
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CDMA: Underlying principles
• Direct Sequence Spread Spectrum System
– Capacity of multiple access channel can be achieved only if users are interfering
with each other and their