Shortcomings of Cdma

8/3/2019 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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Shortcomings of Cdma