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

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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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