Mobile Communication

•Signals•Spread Spectrum•Routing

Signals

•Physical representation of data •Function of location & time•Periodic signal :Sine wave

g(t) = At sin(2∏ ft + ⱷt)

A[V]

t[S]

ⱷ

•Amplitude (A)•Frequency (f)•Phase Shift (ⱷ)

Representation of Signals

Time DomainFrequency DomainPhase Domain

Time Domain RepresentationA[V]

t[S]

Frequency Domain Representation

A[V]

f[Hz]

Phase Domain Representation

ⱷ

Q = M sin ⱷ

I = M cos ⱷ

Spread Spectrum

User dataBroadband InterferenceNarrowband Interference

dP/df dP/df

dP/df dP/df dP/df

f f

f f f

(1) (2)

(3) (4) (5)

Spreading of the bandwidth to transmit data

Advantage and Disadvantage

•Advantages―Robustness against narrow bandwidth―High security―Background noise

•Disadvantages—Increase complexity of receiver—Large frequency band—Background noise without precaution

Types of Spread Spectrum

Direct Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum(DSSS)

tb

User data

Chipping Sequence

Resulting signal

XOR

=

This system take user data and XOR it with chipping sequence to get spreaded signal

DSSS – Transmitter

X ModulatorTransmit SignalUser data

SpreadSpectrumSignal

Chipping Sequence

Radio Carrier

DSSS – Receiver

X Modulator

Received Signal

Data

LowpassFiltered Signal

Chipping Sequence Radio

Carrier

DecisionDemodulator

Correlator

Products Sampledsums

Frequency Hopping Spread Spectrum(FHSS)

User data

Slow hopping(3 bits/hop)

Fast hopping(3 hops/bit)

f

ff3

f2

f1

f3

f2

f1

f

t

t

t

1 1 100

td

tb

td

FHSS – Transmitter

Modulator Modulator

Frequency synthesizer

User Data

NarrowbandSignal

Transmit Signal

Hopping Sequence

FHSS – Receiver

Demodulator Demodulator

Frequency synthesizer

Data

NarrowbandSignal

Received Signal

Hopping Sequence

DSSS VS FHSSFHSS is Simpler than DSSS to implement for spreading.

FHSS use only part of bandwidth at a time where DSSS uses complete bandwidth

DSSS systems are more resistant to fading and multi-path effect.

DSSS are much harder to detect without spreading code.

Routing One solution: inter satellite links (ISL)• reduced number of gateways needed• forward connections or data packets within the satellite network as

long as possible• only one uplink and one downlink per direction needed for the

connection of two mobile phonesProblems:• more complex focusing of antennas between satellites• high system complexity due to moving routers• higher fuel consumption• thus shorter lifetimeIridium and Teledesic planned with ISLOther systems use gateways and additionally terrestrial networks