CDMA PPT
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Transcript of CDMA PPT
Rituj Chaudhary
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OutlineSpread Spectrum BasicsSpreading CodesIS-95 Features- Transmitter/ReceiverDiversity TechniquesRAKE ReceiverSoft Handoff
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Spread SpectrumA technique in which the transmission
bandwidth W and message bandwidth R are related as
W >> R
Achieves several desirable objectives for e.g. enhanced capacity
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Spread Spectrum Principles
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Does not attempt to allocate disjoint frequency or time slot resources Instead, this approach allocates all resources to
simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR
Each user employs a noise-like wideband signal occupying the entire frequency allocation
Each user contributes to the background noise affecting all users, but to the least extent possible.
SS (cont.)This restriction on interference limits capacity, but
because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system
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Application of Spread Spectrum Systems
AntijammingMultiple accessLow detectabilityMessage PrivacySelective callingIdentificationNavigationMultipath protection
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Types of Spread Spectrum Systems
Frequency HoppingSlow Frequency Hopping - multiple symbols
per hopFast Frequency Hopping - multiple hops per
symbol
Direct Sequence
Time Hopping
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Typical frequency-hopping waveform pattern
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Transmitter side of system
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Receiver side of system
Code Division Multiple Access - CDMAMultiple users occupying the same
band by having different codes is known as a CDMA - Code Division Multiple Access system
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Universal Frequency ReuseObjective of a Wireless Communication System
Deliver desired signal to a designated receiver Minimize the interference that it receives
One way is to use disjoint slots in frequency or time in the same cell as well as adjacent cells - Limited frequency reuse
In spread spectrum, universal frequency reuse applies not only to users in the same cell but also in all other cells
No frequency plan revision as more cells are added
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Spreading CodesIt is desired that each user’s transmitted
signal appears noise like and random. Strictly speaking, the signals should appear as Gaussian noise
Such signals must be constructed from a finite number of randomly preselected stored parameters; to be realizable
The same signal must be generated at the receiver in perfect synchronization
We limit complexity by specifying only one bit per sample i.e. a binary sequence
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PN SequencesA deterministically generated sequence that
nearly satisfies these properties is referred to as a Pseudorandom Sequence (PN)
Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR)
If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence
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PN Sequences (contd…)However, if the feedback connections
satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence.
Thus if r=15, P=32767.
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IS-95 CDMADirect Sequence Spread Spectrum Signaling on Reverse and Forward Links
Each channel occupies 1.25 MHz
Fixed chip rate 1.2288 Mcps19
Reverse CH Forward CH
847.74 MHz 892.74 MHz45 MHz
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I-PN
Seq
Q-PN Seq
Forward Traffic Channel9.6 kbps4.8 kbps2.4 kbps1.2 kbps
Long Code Generator
Convolutional Encoder & repetition
MUX+ +
WiPower Control
bits
19.2 kbps
Decimator x
x
Block Interleaver
Diversity TechniquesFrequency Diversity
Time Diversity
Space Diversity
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RAKE Receiver ConceptMulti-path diversity channelsProblem (to isolate various multi-path signal how to do
this?)If the maximal delay spread (due to multi-path) is
Tm seconds and if the chip rate then individual multi-path signal components can be isolated.
Amplitudes and phases of the multi-path components are found by correlating the received waveform with delayed versions of the signal
Multi-path with delays less than 1/Tc can’t be resolved
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Rake Receiver
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SearcherReceiver
3-Parallel Demod-ulator
DiversityCombiner
(Mobile Station Rake Receiver)
–Search receiver indicates where in time the strongest replicas of the signal can be found
Rake Receiver
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Multipath Arrival of Signals
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Transmittedsymbol
received signal Modified with the combinedat each time delay channel estimate symbol
finger #1
finger #2
finger #3
YY
Handoff in CDMA System
Soft HandoffMobile commences Communication with a new
BS without interrupting communication with old BS
same frequency assignment between old and new BS
provides different site selection diversity
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Soft Handoff Architecture
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MSC
BSC
BTS BTS
BSC
BTSBTS R new lin
k old linkR
R- handoff request sent to the old cell
energy measurements are made at the mobile
R
margin exceeds
T_ADD
T_DROP
Time
Base A
Base B
B added to candidate list
B_Active
Drop timer starts
Drop timer resets
Drop timer expires
Signal levels during Handoff
Eb/Io
Soft Handoff- A unique feature of CDMA Mobile
AdvantagesContact with new base station is made before
the call is switched Diversity combining is used between multiple
cell sites additional resistance to fading
If the new cell is loaded to capacity, handoff can still be performed for a small increase in BER
Neither the mobile nor the base station is required to change frequency
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Advantages of CDMA include:Increased cellular communications security.Simultaneous conversations.Increased efficiency, meaning that the carrier
can serve more subscribers.Smaller phones.Low power requirements and little cell-to-cell
coordination needed by operators.Extended reach - beneficial to rural users
situated far from cells.
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Disadvantages of CDMADue to its proprietary nature, all of CDMA's
flaws are not known to the engineering community.
CDMA is relatively new, and the network is not as mature as GSM.
CDMA cannot offer international roaming, a large GSM advantage.
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