Multiplexing and Multiple access

Mobile & Wireless Communications
- By Dr. Ghanshyam Singh

Course Syllabus
• Unit 1
Wireless Transmission: Frequencies, regulations, Signals, Antennas,
Signal Propagation, path loss, Multi path and other effects.
• Unit 2
Multiplexing: SDM, FDM, TDM, and CDMA Modulation Techniques:
ASK, FSK, PSK, Multi-carrier and Spread Spectrum: their use in
Mobile and Wireless Communication Systems.
• Unit 3
Access Control Mechanisms: SDMA, FDMA, TDMA, CDMA and their
performance evaluation.
• Unit 4
Wireless Networks: Satellite networks, Wireless LAN, Wireless ATM.
OSI, Mobile Network and transport layers, their protocols and
implementation, Wireless Application Protocol.
• Books:
T.S.Rappaport, "Wireless Communications: Principles and Practice,
Second Edition, Pearson Education/ Prentice Hall of India, Third Indian
Reprint 2003.

Multiple Access Techniques for
Wireless Communication
FDMA
TDMA
CDMA
SDMA

Introduction
• many users at same time
• share a finite amount of radio spectrum
• high performance
• duplexing generally required
• frequency domain
• time domain

Frequency division duplexing (FDD)
• two bands of frequencies for every user
• forward band
• reverse band
• duplexer needed
• frequency seperation between forward band
and reverse band is constant
frequency seperation
reverse channel forward channel
f

Time division duplexing (TDD)
• uses time for forward and reverse link
• multiple users share a single radio channel
• forward time slot
• reverse time slot
• no duplexer is required
time seperation
t
forward channelreverse channel

Multiple Access Techniques
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
• Space division multiple access (SDMA)
• grouped as:
• narrowband systems
• wideband systems

Narrowband systems
• large number of narrowband channels
• usually FDD
• Narrowband FDMA
• Narrowband TDMA
• FDMA/FDD
• FDMA/TDD
• TDMA/FDD
• TDMA/TDD

Logical separation FDMA/FDD
f
t
user 1
user n
forward channel
reverse channel
forward channel
reverse channel
...

Logical separation FDMA/TDD
f
t
user 1
user n
forward channel reverse channel
...

Logical separation TDMA/FDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...

Logical separation TDMA/TDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...

Wideband systems
• large number of transmitters on one channel
• TDMA techniques
• CDMA techniques
• FDD or TDD multiplexing techniques
• TDMA/FDD
• TDMA/TDD
• CDMA/FDD
• CDMA/TDD

Logical separation CDMA/FDD
code
f
user 1
user n
...

Logical separation CDMA/TDD
code
t
user 1
user n
...

Multiple Access Techniques in use
Multiple Access
Technique
Advanced Mobile Phone System (AMPS) FDMA/FDD
Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC) TDMA/FDD
Digital European Cordless Telephone (DECT) FDMA/TDD
US Narrowband Spread Spectrum (IS-95) CDMA/FDD
Cellular System

Frequency division multiple access FDMA
• one phone circuit per channel
• idle time causes wasting of resources
• simultaneously and continuously
transmitting
• usually implemented in narrowband systems
• for example: in AMPS is a FDMA
bandwidth of 30 kHz implemented

FDMA compared to TDMA
• fewer bits for synchronization
• fewer bits for framing
• higher cell site system costs
• higher costs for duplexer used in base
station and subscriber units
• FDMA requires RF filtering to minimize
adjacent channel interference

Nonlinear Effects in FDMA
• many channels - same antenna
• for maximum power efficiency operate near
saturation
• near saturation power amplifiers are
nonlinear
• nonlinearities causes signal spreading
• intermodulation frequencies

Nonlinear Effects in FDMA
• IM are undesired harmonics
• interference with other channels in the
FDMA system
• decreases user C/I - decreases performance
• interference outside the mobile radio band:
adjacent-channel interference
• RF filters needed - higher costs

Number of channels in a FDMA system
• N … number of channels
• Bt … total spectrum allocation
• Bguard … guard band
• Bc … channel bandwidth
N=
Bt - Bguard
Bc

Example: Advanced Mobile Phone System
• AMPS
• FDMA/FDD
• analog cellular system
• 12.5 MHz per simplex band - Bt
• Bguard = 10 kHz ; Bc = 30 kHz
N=
12.5E6 - 2*(10E3)
30E3
= 416 channels

Time Division Multiple Access
• time slots
• one user per slot
• buffer and burst method
• noncontinuous transmission
• digital data
• digital modulation

Slot 1 Slot 2 Slot 3 … Slot N
Repeating Frame Structure
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.

Features of TDMA
• a single carrier frequency for several users
• transmission in bursts
• low battery consumption
• handoff process much simpler
• FDD : switch instead of duplexer
• very high transmission rate
• high synchronization overhead
• guard slots necessary

Number of channels in a TDMA system
• N … number of channels
• m … number of TDMA users per radio channel
• Btot … total spectrum allocation
• Bguard … Guard Band
N=
m*(Btot - 2*Bguard)
Bc

Example: Global System for Mobile (GSM)
• TDMA/FDD
• forward link at Btot = 25 MHz
• radio channels of Bc = 200 kHz
• if m = 8 speech channels supported, and
• if no guard band is assumed :
N= 8*25E
6200E3
= 1000 simultaneous users

Efficiency of TDMA
• percentage of transmitted data that contain
information
• frame efficiency f
• usually end user efficiency < f ,
• because of source and channel coding
• How get f ?

Efficiency of TDMA
• bOH … number of overhead bits
• Nr … number of reference bursts per frame
• br … reference bits per reference burst
• Nt … number of traffic bursts per frame
• bp … overhead bits per preamble in each slot
• bg … equivalent bits in each guard time
intervall
bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg

Efficiency of TDMA
bT = Tf * R
• bT … total number of bits per frame
• Tf … frame duration
• R … channel bit rate

Efficiency of TDMA
 f … frame efficiency
• bOH … number of overhead bits per frame
• bT … total number of bits per frame
f = (1-bOH/bT)*100%

Space Division Multiple Access
• Controls radiated energy for each user in space
using spot beam antennas
• base station tracks user when moving
• cover areas with same frequency:
TDMA or CDMA systems
• cover areas with same frequency:
FDMA systems

Space Division Multiple Access
• primitive applications are
“Sectorized antennas”
• in future adaptive
antennas simultaneously
steer energy in the
direction of many users at
once

Reverse link problems
• general problem
• different propagation path from user to base
• dynamic control of transmitting power from
each user to the base station required
• limits by battery consumption of subscriber
units
• possible solution is a filter for each user

Solution by SDMA systems
• adaptive antennas promise to mitigate
reverse link problems
• limiting case of infinitesimal beamwidth
• limiting case of infinitely fast track ability
• thereby unique channel that is free from
interference
• all user communicate at same time using the
same channel

Disadvantage of SDMA
• perfect adaptive antenna system:
infinitely large antenna needed
• compromise needed

SDMA and PDMA in satellites
• INTELSAT IVA
• SDMA dual-beam
receive antenna
• simultaneously access
from two different
regions of the earth

• COMSTAR 1
• PDMA
• separate antennas
• simultaneously
access from same
region

• INTELSAT V
• PDMA and SDMA
• two hemispheric
coverages by SDMA
• two smaller beam
zones by PDMA
• orthogonal
polarization

Capacity of Cellular Systems
• channel capacity: maximum number of users
in a fixed frequency band
• radio capacity : value for spectrum efficiency
• reverse channel interference
• forward channel interference
• How to determine the radio capacity?

Co-Channel Reuse Ratio Q
• Q … co-channel reuse ratio
• D … distance between two co-channel cells
• R … cell radius
Q=D/R

Forward channel interference
• cluster size of 4
• D0 … distance
serving station
to user
• DK … distance
co-channel base
station to user

Carrier-to-interference ratio C/I
• M closest co-channels cells cause first order
interference
C
=
I
D0
-n0
M
k=1
DK
-nk
• n0 … path loss exponent in the desired cell
• nk … path loss exponent to the interfering
base station

Carrier-to-interference ratio C/I
• Assumption:
• just the 6 closest stations interfere
• all these stations have the same distance D
• all have similar path loss exponents to n0
C
I
=
D0
-n
6*D
-n

Worst Case Performance
• maximum interference at D0 = R
• (C/I)min for acceptable signal quality
• following equation must hold:
1/6 * (R/D) (C/I)min=
>
-n

Co-Channel reuse ratio Q
• D … distance of the 6 closest interfering
base stations
• R … cell radius
• (C/I)min … minimum carrier-to-interference
ratio
• n … path loss exponent
Q = D/R = (6*(C/I)min)
1/n

Radio Capacity m
• Bt … total allocated spectrum for the system
• N … number of cells in a complete frequency
reuse cluster
m =
Bt
Bc * N
radio channels/cell

Radio Capacity m
• N is related to the co-channel factor Q by:
Q = (3*N)
1/2
m=
Bt
Bc * (Q²/3)
=
Bt
Bc *
6 C
I3n/2( *( )min
)
2/n

Radio Capacity m for n = 4
• m … number of radio channels per cell
• (C/I)min lower in digital systems compared to
analog systems
• lower (C/I)min imply more capacity
• exact values in real world conditions measured
m =
Bt
Bc * 2/3 * (C/I)min

Compare different Systems
• each digital wireless standard has different
(C/I)min
• to compare them an equivalent (C/I) needed
• keep total spectrum allocation Bt and
number of rario channels per cell m
constant to get (C/I)eq :

Compare different Systems
• Bc … bandwidth of a particular system
• (C/I)min … tolerable value for the same system
• Bc’ … channel bandwidth for a different
system
• (C/I)eq … minimum C/I value for the different
system
C
I
= C
I
Bc
Bc’
( ) ( )
min
)²
eq * (

C/I in digital cellular systems
• Rb … channel bit rate
• Eb … energy per bit
• Rc … rate of the channel code
• Ec … energy per code symbol
C Eb*Rb Ec*Rc
I I I
= =

• combine last two equations:
(C/I) (Ec*Rc)/I Bc’
(C/I)eq (Ec’*Rc’)/I’ Bc
= = ( )²
• The sign ‘ marks compared system
parameters

• Relationship between Rc and Bc is always
linear (Rc/Rc’ = Bc/Bc’ )
• assume that level I is the same for two
different systems ( I’ = I ) :
Ec Bc’
Ec‘ Bc
= ( )³

Compare C/I between FDMA and TDMA
• Assume that multichannel FDMA system
occupies same spectrum as a TDMA system
• FDMA : C = Eb * Rb ; I = I0 * Bc
• TDMA : C’ = Eb * Rb’ ; I’ = I0 * Bc’
• Eb … Energy per bit
• I0 … interference power per Hertz
• Rb … channel bit rate

Example
• A FDMA system has 3 channels , each with
a bandwidth of 10kHz and a transmission
rate of 10 kbps.
• A TDMA system has 3 time slots, a channel
bandwidth of 30kHz and a transmission rate
of 30 kbps.
• What’s the received carrier-to-interference
ratio for a user ?

Example
• In TDMA system C’/I’ be measured in
333.3 ms per second - one time slot
C’ = Eb*Rb’ = 1/3*(Eb*10E4 bits) = 3*Rb*Eb=3*C
I’ = I0*Bc’ = I0*30kHz = 3*I
• In this example FDMA and TDMA have
the same radio capacity (C/I leads to m)

Example
• Peak power of TDMA is 10logk higher then
in FDMA ( k … time slots)
• in practice TDMA have a 3-6 times better
capacity

Capacity of SDMA systems
• one beam each user
• base station tracks each user as it moves
• adaptive antennas most powerful form
• beam pattern G() has maximum gain in the
direction of desired user
• beam is formed by N-element adaptive
array antenna

• G() steered in the horizontal  -plane
through 360°
• G() has no variation in the elevation plane
to account which are near to and far from the
base station
• following picture shows a 60 degree
beamwidth with a 6 dB sideslope level

• reverse link received signal power, from
desired mobiles, is Pr;0
• interfering users i = 1,…,k-1 have received
power Pr;I
• average total interference power I seen by a
single desired user:

Capacity of SDMA
 i … direction of the i-th user in the
horizontal plane
• E … expectation operator
I = E {  G(i) Pr;I}
K-1
i=1

• in case of perfect power control (received
power from each user is the same) :
Pr;I = Pc
• Average interference power seen by user 0:
I = Pc E {  G(i) }
K-1
i=1

• users independently and identically
distributed throughout the cell:
I = Pc *(k -1) * 1/D
• D … directivity of the antenna - given by
max(G())
• D typ. 3dB …10dB

Pb = Q ( )
• Average bit error rate Pb for user 0:
3 D N
K-1
• D … directivity of the antenna
• Q(x) … standard Q-function
• N … spreading factor
• K … number of users in a cell

Multiplexing and Multiple access

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