Multiplexing

1
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)

2
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.

3

• 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

4
FDM (Frequency Division Multiplexing)
Power
FDMA
Frequency
Frequency
Time
Bc
Bm
Time
Frequency channel
Example Telephony Bm 3-9 kHz
5
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)

6
Transmitter Emissions

• Transmitter output components
• Fundamental (wanted) signal
• Harmonic emissions
• Master oscillator (fundamental harmonics)
• Non-harmonically related spurious
• Noise

Ideal
Real
7
Receiver response

• Fundamental channel
• Spurious channels
• Intermediate frequency
• Image frequency
• Channels received via LO harmonics
• Intermodulation channels

Ideal
8
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 SCk
• 3rd order (2F1-F2, 2F2-F1), also 5th and 7th

9

• 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

10

• 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

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• 2nd order
• a) Distorted version of the modulating signals
• b) 2nd harmonics
• c) Sum and difference

12

• 3rd order
• a) Distorted modulating signal
• b) 3rd harmonics
• Crossmodulation
• d) Intermodulation

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Non-essential channels
F
Y
X
14
F-D Separation Concept
15
Theoretical Cells Cell Clusters
Various combinations possible
16
Power Transfer
Prec Ptot F(f) F(d) F(Q,F)
F(d) Transmission loss
17
Desired signal
18
Interfering signal
19

20
Emission attenuation mask
21
Receiver selectivity mask
22
LOS propagation
Doc 1B/17/95
23
FDR Calculation process
Spectrum
Substract requ.margin
Margin
Multiply
Integrate
Normalize
Calc. FDR
Selectivity
?f
d
Inverse propagation
FDR
f
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Frequency and Distance Separation
Separation acceptable
LFDR?
Distance separation
Separation unacceptable
Frequency separation
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F-D Separation 1D (Line)
Separation by (reuse distance) 1 zone ? 2 channels
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F-D Separation 2D (Surface)

• Reuse distance 1 ? 4 channels

Reuse distance 2 ? 9 channels
n zones ? (n 1)2 channels
2
27
Cell clusters
7
3
Various combinations possible
28
F-D Separation 3D (Space)
1 zone ? 8 channels
2 zone ? 27 channels
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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

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Frequency-distance separation (2)
H(f)
P(f)
?f
(f) - relative
31
Equipment deficiency example Spectrum
blocked by typical UHF-TV terrestrial
transmitter due to receivers deficiencies (FCC
Taboos)
Area No. of channels ideal
1 co-ch 23 other 77
Dixon64
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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
33
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.

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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)
35
SDM

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

36
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

37
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
38
DS SS communications basics
Spreading
Original information
Original signal
Transmission
Propagation effects
Unwanted signals Noise
Reconstructed information
De-spreading
Reconstr. signal
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SS basic characteristics

• Signal spread over a wide bandwidth gtgt 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)

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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 gtgt 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
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DS SS-receiver
Correlator bandpass filter
antenna
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)
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SS-receivers 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
43
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
44
SS Processing Gain
(S/ I)in/ (S/ I)out Bc/ Bm
Example GPS signal RF bandwidth Bc 2MHz Filter
bandwidth Bm 100 Hz Processing gain 20000
(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)
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SS systems attributes (1)

• Low spectral density of 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

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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)

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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

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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

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Packet Radio Protocols
50
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

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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

Packet B
Packet C
Transmitter 1
Packet A
Transmitter 2
t1
T12?
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• 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.

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• 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

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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