26-06-2013, 01:05 PM
Code Division Multiple Access for Wireless Communications
Code Division Multiple.pdf (Size: 259.62 KB / Downloads: 39)
What is Multiple Access?
• Multiple users want to
communicate in a common
geographic area
• Cellular Example: Many people
want to talk on their cell phones.
Each phone must communicate
with a base station.
• Imagine if only one person could
talk on their cell phone at a time!
• Problem: How should we share
our resources so that as many
users as possible can
communicate simultaneously?
Alternative to FDMA and TDMA?
• What if we could allow users to share time and
frequency?
Eliminates need for tight synchronization among many
different users
Eliminates need for expensive analog filters
May have favorable impact on capacity (?)
• But:
How do we separate the users?
Won’t they interfere with each other?
Properties of Walsh Codes
• There are some issues with Walsh Codes
Synchronization of all users is required
In a multipath channel, delayed copies may be received
which are not orthogonal any longer!
Only J codes exist with a bandwidth expansion of J, so as
far as capacity, we are right back where we started with
TDMA and FDMA!
• Advantages relative to TDMA and FDMA
No guard bands or guard times are typically required
No equalizer is typically required, when a RAKE receiver is
used
CDMA – Issues
• So far, CDMA looks like a step backwards:
Tight synchronization is required to use orthogonal codes,
which then break in a multipath channel anyway
Quasi-orthogonal codes cause self-interference, which
dominates the performance in most CDMA systems
Near-far problem is a serious hindrance, requiring fast and
accurate power control (that uses up bits we could otherwise
send information with)
And for all this, the required bandwidth is now J times
larger than it was before, so there doesn’t appear to be a
capacity gain
Voice Activity
• In TDMA and FDMA systems:
If a user doesn’t have anything to send, the time/frequency
slot allocated to them is wasted
It is typically very difficult to dynamically allocate time and
frequency slots
• In CDMA systems:
If a user doesn’t have anything to send, it causes less
interference to other users of the system
Typically, each user needs to transmit less than half the time
Since interference-limited, this doubles the capacity
The Future of CDMA
• CDMA has overcome most cynicism to dominate the
worldwide wireless voice market
• What about data services? Scheduling vs.
Inteference Averaging
• CDMA appears to be an underdog for 4G, but still
may win
• Ongoing research on CDMA
Increase capacity by joint decoding (multiuser detection &
interference cancellation)
Applying CDMA to other applications: optical CDMA, ad
hoc networks, dense wireless LANs
“MultiCDMA”: multiple antenna CDMA, multicarrier
CDMA, multicode CDMA