Shortcomings of Cdma
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Transcript of Shortcomings of Cdma
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M.Saad Ilyas
MSEE-011
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Near-Far Problem
Multipath Fading
Inter Symbol Interference (ISI)
Soft Handoff
Self Jamming
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The receiver must be able to receive all the
required signals within the same channel
bandwidth and it must be able to decode
them For the receiver to be able to decode all the
signals in the channel, they should ideally all
be at the same signal strength
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Users may be received with very different
powers:
Users nearnear the base station are received with
high power
Users farfar from the base station are receivedwith low power
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Solution: owerower ontrolontrol This ensures that all the signals within the
coverage of the base station arrive with same
power at the receiver
DrawbacksDrawbacks
Reduced data capacity: The power control
mechanism requires data to be sent in both
directions across the radio interface. This
utilizes data capacity that could be otherwiseused for carrying revenue earning data
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High power handset power consumption at
cell edges:
In order to be able to maintain the
required signal level at the base station
when the handset is close to the edge of the
cell, it will be required to transmit at a highpower level.
This will reduce battery life. Other cellular
systems might not require such high signal
levels at the base station and may be able toconserve battery power as a result.
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RF transmission is degraded by multipath
Multipath propagation occurs when thereare radio reflective surfaces in the
environment(cliffs, buildings, earth surface) At the Rx antenna the total signal is the sum
of Direct rays
Rays delayed due to several reflections and a
zigzag path
Multipath can cause both fading and
inter-symbol interference (ISI)
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Direct and delayed rays are out of phaseat some locations
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Rake receiver is a radio receiver designed to
counter the effects of multipath fading.
It does this by using several "sub-receivers"
called fingers, that is, several correlators
each assigned to a different multipath fading
.
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Each finger independently decodes a single
multipath component; at a later stage the
contribution of all fingers are combined in
order to make the most use of thedifferent transmission characteristics of each
transmission path
This could very well result in higher signal-
to-noise ratio (or Eb/N0) in a multipathenvironment
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When the multipath delay spread isgreater than about 20% of the digitalsymbol duration, ISI can be a problem.
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Receivers are equipped with an adaptive
equalizer
Adaptive equalizer produces delayed copy/ies
of the received signal waveform and use(s)these copy/ies to cancel the physically delayedradio signals
This equalizer examines the effect ofmultipath delay on the known training
sequence, and then uses this information toundo that effect on the other bits in the cellusing the internally delayed replicas of thesignal
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Higher cost and complexity of base station
(particularly due to soft handoff )
Soft handoff procedure, a mobile makes a
connection to two or more base stationsbefore choosing the one with which to
communicate
It is thus an example of a make before
break operation
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This is more complex than hard handoff used
in FDMA and TDMA schemes
Self Jamming
Arises when the spreading codes used for
different users are not exactly orthogonal
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