Multiplexing
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Multiplexing. The combining of two or more information channels onto a common transmission medium. Basic forms of multiplexing: Frequency-division multiplexing (FDM). Time-division multiplexing (TDM) Code-division multiplexing (CDM) . FDM. Frequency Division Multiplexing

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Multiplexing

Multiplexing

  • The combining of two or more information channels onto a common transmission medium.

  • Basic forms of multiplexing:

    • Frequency-division multiplexing (FDM).

    • Time-division multiplexing (TDM)

    • Code-division multiplexing (CDM)


Multiplexing

FDM

  • Frequency Division Multiplexing

    • The deriving of two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum to each of the individual channels.

  • FDMA (frequency-division multiple access): The use of frequency division to provide multiple and simultaneous transmissions.


Multiplexing

  • Transmission is organized in frequency channels, assigned for an exclusive use by a single user at a time

  • If the channel is not in use, it remains idle and cannot be used by others

  • There are channeling frequency plans elaborated to avoid mutual co-channel and adjacent-channel interference among neighboring stations

  • The use of a radio channel or a group of radio channels requires authorization (license)

    • for each individual station or for group of stations


Fdm frequency division multiplexing

FDM (Frequency Division Multiplexing)

Power

FDMA

Frequency

Frequency

Time

Bc

Bm

Time

Frequency channel

Example: Telephony Bm = 3-9 kHz


Multiplexing

FDD

  • Frequency division duplexing

    • 2 radio frequency channels for each duplex link (1 up-link & 1 down-link or 1 forward link and 1 reverse link)


Transmitter emissions

Output spectrum

Frequency (relative)

Transmitter Emissions

  • Transmitter output components

    • Fundamental (wanted) signal

    • Harmonic emissions

    • Master oscillator (fundamental & harmonics)

    • Non-harmonically related spurious

    • Noise

Ideal

Real


Receiver response

Response

Frequency

Receiver response

  • Fundamental channel

  • Spurious channels

    • Intermediate frequency

    • Image frequency

    • Channels received via LO harmonics

    • Intermodulation channels

Ideal


Intermodulation

Intermodulation

  • 2 or more signals, nonlinear circuit

  • Intermodulation products: Fi = SCkFk

  • {Ck} positive/negative integers or zero

  • {Fk} frequencies of signals applied

  • Order of Intermod. Product = S|Ck|

  • 3rd order (2F1-F2, 2F2-F1), also 5th and 7th


Multiplexing

  • Non-ideal wideband linear systems are frequently treated by expressing the output (Y) of the system as a power series:

  • where X is the total input signal, and the coefficients a are presumed to be real and independent on X.

  • Assume, for simplicity, that input consists of three elementary signals:


Multiplexing

  • Some simple calculations will show that the output of the system Y, in addition to the linearly transposed input signals, contains the following spectral components:

  • 1st order

    Multiplied version of the input signal


Multiplexing

  • 2nd order

  • a)Distorted version of the modulating signals

  • b)2nd harmonics

  • c)Sum and difference


Multiplexing

3rd order

a)Distorted modulating signal

b)3rd harmonics

  • Crossmodulation

    d)Intermodulation


Non essential channels

Non-essential channels

F

Y

X


F d separation concept

F-D Separation Concept


Theoretical cells cell clusters

Theoretical Cells & Cell Clusters

Various combinations possible


Frequency and distance separation

Frequency and Distance Separation

Separation acceptable

L+FDR=

Distance separation

Separation unacceptable

Frequency separation


F d separation 1d line

1

2

1

2

1

2

1

2

3

1

2

3

1

2 zones 3 channels

n zones  (n +1) channels

F-D Separation:1D (Line)

Separation by (reuse distance) 1 zone  2 channels


F d separation 2d surface

F-D Separation:2D (Surface)

Reuse distance = 1 4channels

Reuse distance = 29channels

n zones  (n +1)2 channels

2


Cell clusters

Cell clusters

7

3

Various combinations possible


F d separation 3 d space

9

 4

9

> 8

> 27

 4

9

n zones  (n+1)3 channels

F-D Separation:3D (Space)

1 zone  8 channels

2 zone  27 channels


Ideal lattices

Ideal Lattices

  • Bound-less, regular, plane lattice

  • Each station located at a node

  • All nodes occupied (no "holes")

  • All stations identical (omnidirectional)

  • Uniform propagation (no terrain obstacles)

  • Uniform EM environment

  • One set of channels regularly re-used


Multiplexing

Equipment deficiency: example Spectrum “blocked” by typical UHF-TV terrestrial transmitterdue to receiver’s deficiencies (“FCC Taboos”)

Area * No. of channels

ideal: 1%

co-ch: 23%

other: 77%

Dixon64


Multiplexing

OFDM

F1

F2

FN

Sub-Ch 1

  • Orthogonal Frequency Division Multiplexing (OFDM)

    • The channel is split into a number of sub-channels

    • Each sub-channel transmits a part of the original information

    • Each sub-channel adjusted to its environment (S/N)

    • Reduces multipath & selective fading

    • Allows for higher speeds

    • Requires smart signal processing

    • Used in 802.11a(USA), DTTB(Eu), Hyperplan(Eu), Power Line Coms. standards.

Sub-Ch 2

Demodulation

Signal Processing

Serial-to-Parallel Converter

Serial-to-Parallel Converter

Sub-Ch N

Digital

Modulation

Delogne P, Bellanger M: The Impact of Signal Processing on an Efficient Use of the Spectrum, Radio Science Bulletin June 1999, 23-28

LeFloch B, Alard M, Berrou C: Coded Orthogonal Frequency Division Multiplex, Proc of IEEE June 1995, 982-996


Multiplexing

TDM

  • Time Division Multiplex: A single carrier frequency channel is shared by a number of users, one after another. Transmission is organized in repetitive “time-frames”. Each frame consists of groups of pulses - time slots.

  • Each user is assigned a separate time-slot.

  • TDD – Time Division Duplex provides the forward and reverse links in the same frequency channel.


Multiplexing

TDM

Power density

TDM

Time-frame

Frequency

Time

Frequency

Time

Time slot

Example: DECT (Digital enhanced cordless phone) Frame lasts 10 ms, consists of 24 time slots (each 417s)


Multiplexing

SDM

  • Space Division Multiple Access controls the radiated energy for each user in space using directive antennas

    • Sectorized antennas

    • Adaptive antennas


Cdma or ss

CDMA or SS

  • Code Division Multiple Access or Spread Spectrum communication techniques

    • FH: frequency hoping (frequency synthesizer controlled by pseudo-random sequence of numbers)

    • DS: direct sequence (pseudo-random sequence of pulses used for spreading)

    • TH: time hoping (spreading achieved by randomly spacing transmitted pulses)

    • Other techniques

      • Hybrid combination of the above techniques (radar and other applications)

      • Random noise as carrier


Cdma fh ss

CDMA - FH SS

Power density

Frequency

CDMA

Bm

Bc

Frequency

Time

Transmission is organized in time-frequency “slots”. Each link is assigned a sequence of the slots, according to a specific code. Used e.g. in Bluetooth system

Time

Time-frequency slot


Ds ss communications basics

Spread signal

Spread signal+

DS SS communications basics

Spreading

Original

information

Original signal

Transmission

Propagation effects

Unwanted signals + Noise

De-spreading

Reconstructed

information

Reconstr. signal


Ss basic characteristics

SS: basic characteristics

  • Signal spread over a wide bandwidth >> minimum bandwidth necessary to transmit information

  • Spreading by means of a code independent of the data

  • Data recovered by de-spreading the signal with a synchronous replica of the reference code

    • TR: transmitted reference (separate data-channel and reference-channel, correlation detector)

    • SR: stored reference (independent generation at T & R pseudo-random identical waveforms, synchronization by signal received, correlation detector)

    • Other (MT:T-signal generated by pulsing a matched filter having long, pseudo-randomly controlled impulse response. Signal detection at R by identical filter & correlation computation)


Ds ss transmitter

DS SS: transmitter

Antenna

Modulator

X

[A(t), (t)]

Information

[g1(t)]

Modulated signal

S1(t) = A(t) cos(0t + (t))

band Bm Hz

Spread signal

g1(t)S1(t)

band Bc Hz

Bc >> Bm

Carrier

cos(0t)

gi(t): pseudo-random noise (PN) spreading functions that spreads the energy of S1(t) over a bandwidth considerably wider than that of S1(t): ideally gi(t) gj(t) = 1 if i = j and gi(t) gj(t) = 0 if i j


Multiplexing

DS SS-receiver

antenna

Correlator

&

bandpass

filter

X

To demodulator

Linear

combination

g1(t)S1(t)

g2(t)S2(t)

…….

gn(t)Sn(t)

N(t) (noise)

S’(t)

g1(t) g1(t)S1(t)

g1(t) g2(t)S2(t)

…….

g1(t) gn(t)Sn(t)

g1(t) N(t)

g1(t) S’(t)

Spreading

function

[g1(t)]

S1(t)


Ss receiver s input

SS-receiver’s Input

Unwanted signals

SS s.: g2(t)S2(t); …; gn(t)Sn(t)

Other s. : S’(t)

Noise: N(t)

W/Hz

Wanted (spread) signal: g1(t)S1(t)

Hz

Bc

(S/ I)in = S/ [I()*Bc]

Signal-to-interference ratio

Bc = Input correlator bandwidth

I() = Average spectral power density of unwanted signals in Bc

S = Power of the wanted signal


Multiplexing

SS-correlator/ filter output

Wanted (correlated) signal: de-spread to its original bandwidth

as g1(t) g1(t)S1(t) = S1(t) with g1(t) g1(t) = 1

Bm

Uncorrelated (unwanted) signals

spread & rejected by correlator + noise

g1(t) S’(t); g1(t) N(t); g1(t) gj(t)Sj(t) = 0

as gi(t) gj(t) = 0 for i j

Signal-to-interference ratio

(S/ I)out = S/ [I()*Bm]

Bc = Input correlator bandwidth

Bm = Output filter bandwidth

I() = Average spectral power density of unwanted signals & noise in Bm

S = power of the wanted signal at the correlator output

Bc

Spreading = reducing spectral power density


Ss processing gain

SS Processing Gain =

= [(S/ I)in/ (S/ I)out ] = ~Bc/ Bm

Example: GPS signal

RF bandwidth Bc ~ 2MHzFilter bandwidth Bm ~ 100 Hz

Processing gain ~20’000 (+43 dB)

Input S/N = -20 dB (signal power = 1% of noise power)

Output S/N = +23 dB(signal power = 200 x noise power)

(GPS = Global Positioning System)


Ss systems attributes 1

SS systems attributes (1)

  • Low spectral densityof the signal

    • LPI: low probability of intercept

    • LPPF: low probability of position fix

    • LPSE: low probability of signal exploitation

    • Privacy

    • Covert operations capabilities

    • Low interference potential


Multiplexing

SS systems attributes (2)

  • AJ: anti-jamming/ anti-interference capability

  • Security

  • Natural cryptographic capabilities

  • Multiple-user random access communications with selective addressing (CDMA)

  • High time resolution (~1/B; multi-path suppression)


Summary

Summary

  • To illustrtae the nature of the multiple access techniques consider a number of guests at a cocktail party. The aim is for all the guests to hold an intelligible conversation. In this case the resource available is the house itself

  • FDMA: each guest has a separate room to talk to their partner

  • TDMA: everyone is in a common room and has a limited time slot to hold the conversation

  • FH-CDMA: the guests run from room to room to talk

  • DS-CDMA: everyone is in a common room talkim at the same time, but each pair talks in a different language


Access control to radio resources

Access Control to Radio Resources

  • Distributed wireless networks (e.g. packet radio, ad hoc networks) have no central control.

  • Centralized wireless networks (e.g. WLAN, Cellular) control the use of radio channel; various approaches exist

  • Slotted systems (e.g. TDMA) require wide network synchronization for use of discrete time slots


Packet radio protocols

Packet Radio Protocols


Packet radio

Packet Radio

  • In packet radio access techniques, many user attempt to access a single channel, which may led to collisions.

  • Protocols aim at limiting collisions

  • ALOHA is the oldest, classic protocol, developed in 1970 in Hawaii as an extension of TDMA and FDMA


Aloha

ALOHA

  • If 2 or more users transmit at the same time so that receiver receives more than one packet, the receiver is unable to separate the packets since they are not orthogonal in time (like in TDMA) or in frequency (like in FDMA).

  • The vulnerable period is the time interval during which the packets are susceptible to collisions with transmissions from other users

Transmitter 1

Packet B

Packet C

Transmitter 2

Packet A

t1

T1+2


Multiplexing

  • In pure ALOHA, the vulnerable period is 2 packet durations. A user transmits whenever it has a packet to deliver. If no acknowledgment (ACK) is received, the user waits a random time and retransmit the packet. The throughput is T = Re-2R, R being the normalized channel traffic in Erlangs (Tmax= 0.184 at R = 0.5)

  • In slotted ALOHA, time is divided into equal time slots of length greater than the packet duration. The users have synchronized clocks and transmit messages only at the beginning of a new time slot. This prevent partial collisions where one packet collides with a portion of another. The vulnerable period is only one packet duration. The throughput is T = Re-R (Tmax= 0.368 at R = 1)

  • ALOHA protocols do not listen to the channel before transmission, and do not exploit information about the other users.


Multiplexing

  • Carrier Sense Multiple Access (CSMA) protocols base on monitoring the channel. If the channel is idle (no carrier is detected), then the user is allowed to transmit. Important are detection delay and propagation delay.

  • Reservation protocols – certain packet slots are assigned with priority


References

References

  • Coreira LM, Wireless Flexible Personalized Communications, J Wiley

  • Dunlop J, Smith DG, Telecommunication Engineering, Chapman & Hall

  • Reed JH, Software Radio, Prentice Hall

  • Taub H, Shilling DL, Principles of Communication Systems, McGraw Hill


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