Multiple Access Techniques-Wireless Communication

Multiple Access Techniques for
Wireless Communications
Dr.C.Helen Sulochana
Professor/ECE
St.Xavier’s Catholic College of Engg, Chunkankadai

Dr.C.Helen Sulochana, Prof/ ECE, SXCCE, Na
gercoil
CO -Design various Multiple Access Techniques for wireless channels
UNIT IV MULTIPLE ACCESS TECHNIQUES
Introduction: Introduction To Multiple
Access- Frequency Division Multiple Access(FDMA)- Time
Division Multiple Access(TDMA)- Spread Spectrum Multiple
Access-Code Division Multiple Access(CDMA)- Space
Division Multiple Access(SDMA)- Capacity Of Cellular
Systems: Capacity Of Cellular CDMA, Capacity Of CDMA
With Multiple Cells.

Multiple Access Techniques
-many mobile users simultaneously share a finite amount of radio
spectrum in the most efficient manner.
Duplexing : talk and listen simultaneously
• Duplexing may be done using frequency or time domain techniques.
1.Frequency division duplexing(FDD)
-consists of two simplex channels, and the device is called a duplexer
- has two distinct bands of frequencies for every user.
-The forward band provides traffic from the base station to the mobile,
and the reverse band provides traffic from the mobile to the base.
FDD
Dr.C.Helen Sulochana, Prof/ ECE, SXCCE,
Nagercoil

2. Time division duplexing(TDD)
- uses time instead of frequency to provide both a forward and
reverse link
-allows communication on a single channel, simplifies the
subscriber equipment
• same frequency is used for transmission and reception
• simplifies the subscriber equipment since a duplexer is not required.
• time latency(delay) in transmission
TDD
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Depends on how the available bandwidth is allocated to the users
Two types of systems
1. Narrowband Systems
2. Wideband systems
1. Narrowband Systems
- bandwidth of a single channel is equal to the coherence bandwidth
of the channel.
-available radio spectrum is divided into a large number of
narrowband channels.
-In narrowband FDMA, a user is assigned a particular channel which is
not shared by other users
-In FDMA/FDD each channel has a forward and reverse link
-Narrowband TDMA, allows users to share the same channel but
allocates a unique time slot to each user in a cyclical fashion
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2. Wideband systems
—bandwidth of a single channel is much larger than the coherence
bandwidth of the channel.
- multipath fading does not greatly affect the received signal
- users are allowed to transmit in a large part(wide band) of the
spectrum.
- A large number of transmitters are allowed to transmit on the
same channel.
Types of Multiple access
1. Frequency Division Multiple Access (FDMA)
2. Time Division Multiple Access (TDMA)
3. Spread Spectrum Multiple Access-Code Division Multiple
Access (CDMA)
4. Space Division Multiple Access(SDMA)-
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Frequency Division Multiple Access (FDMA)
 each user is allocated a unique
frequency band or channel
 channels are assigned on
demand to users. one frequency
for the forward channel, other
frequency for the reverse
channel(FDD).
 During the period of the call, no
other user can share the same
frequency band
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Features of FDMA
-FDMA channel carries only one phone circuit at a time
-After the assignment of a voice channel, the base station and the mobile
transmit simultaneously and continuously.
-requires tight RF filtering to minimize adjacent channel interference.
Disadvantages
1. If an FDMA channel is not in use, and cannot be used by other users to
increase or share capacity. - wasted resource
2. bandwidths of FDMA channels are narrow (30 kHz), each channel supports
only one circuit per carrier- narrowband systems.
Advantages
3. The symbol time(Ts) is larger than average delay spread. This reduces the
intersymbol interference thus, little or no equalization is required
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Comparison of TDMA and FDMA
No. TDMA FDMA
1. More complex( digital
transmission)
Less complex(analog modn.,
2. Discrete transmission Continuous transmission
3. More bits for
overhead
fewer bits are needed for overhead purposes
( synchronization and framing bits)
4. Less cost higher cell site system costs (single channel per
carrier)
costly bandpass filters to eliminate spurious
radiation at the base station
5. No need of dulpexer duplexers are used(since both the transmitter
and receiver operate at the same time).
increases the cost of subscriber units and base
stations.
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Nonlinear Effects in FDMA
• many channels share the same antenna at the base station.
• The power amplifiers operated at saturation( for maximum power
efficiency) are nonlinear.
• The nonlinearities cause signal spreading in the frequency domain
generate intermodulation (IM) frequencies.
• IM is undesired RF radiation which interfere with adjacent
channels in the FDMA systems,
Advanced Mobile Phone System(AMPS)- single channel
FDMA/FDD(analog), 2 duplex channel with 45KHz frequency split
forward channel- base station to mobile unit
reverse channel –mobile unit to the base station.
The number of channels that can be simultaneously supported in a
FDMA system
Bt –total spectrum
Bguard - guard band
B -channel bandwidth
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Problem
Solution
Number of channels available in a FDMA system
Bt = 12.5MHz
Bguard =10KHz
Bc = 30KHz
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Time Division Multiple Access (TDMA)
• divide the radio spectrum into time intervals (slots)
• In each slot only one user is allowed to either transmit or receive.
• Each user is allowed to transmit only within specified time slots.
 user occupies a cyclically repeating
time slot
 particular time slot reoccurs in
every frame
 a frame consists of N time slots
 transmit data in buffer-and-burst
method, thus the transmission for
any user is noncontinuous,
accommodate analog FM, digital
data and digital modulation
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Frame structure of TDMA
 Each frame is made up of a preamble, an information message, and tail bits.
preamble contains the address and synchronization information of base
station and the subscriber
 Guard time is for synchronization of the receivers between different slots and
frames.
 In TDMA/TDD,-half of the time slots for forward channels and other half for
reverse channels.
 In TDMA/FDD systems, similar frame structure for forward and reverse
transmission, but the carrier frequencies of forward and reverse links are
different. Duplexers are not required in the subscriber unit.
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• TDMA shares a single carrier frequency with several users, nonoverlapping
time slots for each user
Advantage
• Data transmission is not continuous, in bursts. Results in low power
consumption, handoff process is simple -mobile assisted handoff (MAHO)
• Duplexers are not required-due to different time slots for transmission and
reception. In FDD a switch is used
• bandwidth can be supplied on demand to different users by allocating different
numbers of time slots per frame to different users.
Disadvantage
• Adaptive equalization is needed, since the transmission rates are high
compared to FDMA .
• High synchronization overhead is required due to burst transmissions.
• To minimize the guard time, the transmitted signal at the edges of a time slot
are suppressed. The transmitted spectrum expand and cause adjacent channel
interference.
.
eatures of TDMA
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Efficiency of TDMA
• is a measure of the percentage of transmitted data that contains
information
• is the percentage of bits per frame which contain transmitted data
Frame efficiency
number of overhead bits per frame
Nr -number of reference bursts per frame
Nt - number of traffic bursts per frame
br - number of overhead bits per reference burst,
bg - number of bits in each guard time interval.
total number of bits per frame
Tf - frame duration
R - channel bit rate.
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Number of channels(users) in TDMA system
N=number of TDMA slots per channel x number of channels available
m -maximum number of TDMA users on each channel
Btot –total spectrum
Bguard - guard band
Bc-channel bandwidth
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Problems
1. Consider Global System for Mobile, which is a TDMA/FDD system
that uses 25 MHz for the forward link, which is broken into radio
channels of 200 kHz. If 8 speech channels are supported on a single
radio channel, and if no guard band is assumed, find the number of
simultaneous users that can be accommodated in GSM.
Solution
The number of simultaneous users that can be accommodated in GSM
N = =1000
m =8
Btot =25MHZ
Bguard =0
Bc = 200KHz
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2. If GSM uses a frame structure where each frame consists of 8 time
slots, and each time slot contains 156.25 bits, and data is transmitted at
270.833 kbps in the channel, find (a) the time duration of a bit, (b) the
time duration of a slot, (c) the time duration of a frame, and (d) how
long must a user occupying a single time slot must wait between two
simultaneous transmissions.
solution
(a) The time duration of a bit, Tb =1/ 270.833 kbps
= 3.692
(b) The time duration of a slot, Tslot= 156.25 x Tb
=156.25 x 3.692 = 0.577 ms.
(c) The time duration of a frame, Tf, = 8 x Tslot
=8x 0.577 ms= 4.615 ms.
(d) A user has to wait 4.615 ms, the arrival time of a new frame, for its
next
transmission.
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Spread Spectrum Multiple Access
• uses signals which have a transmission bandwidth several
times greater than the minimum required RF bandwidth.
• A pseudo-noise (PN) sequence converts a narrowband signal
to a wideband noise-like signal before transmission.
• SSMA provides immunity to multipath interference and
multiple access capability.
• spread spectrum systems become bandwidth efficient in a
multi user environment.
Two types of spread spectrum multiple access techniques;
1.frequency hopped multiple access (FH)
2. direct sequence multiple access (DS).Also called
code division multiple access (CDMA).
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Code Division Multiple Access (CDMA)
- known as spread-spectrum multiple access -(SSMA)
Transmitter
-the narrowband message signal p(t) is multiplied by a very large
bandwidth signal called the spreading signal c(t).
-spreading signal is a pseudo-noise code sequence that has a chip rate
greater than the data rate of the message.
-All users in CDMA system use the same carrier frequency and
transmit simultaneously. Each user has its own pseudorandom
codeword c(t) which is orthogonal to all other codewords.
coded waveform
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c(t)

Receiver
• To detect the message signal, the receiver knows the codeword
used by the transmitter. This is done by acquisition and tracking
system.
• The receiver performs a time correlation operation to detect the
specific desired codeword. All other codewords appear as noise due
to decorrelation.
noise floor(sum of all the noise) is determined from the power of
multiple users at a receiver after decorrelation.
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At
the
Transmitter
At
the
Receiver
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Near-far problem
- when many mobile users share the same channel, the strongest signal(nearby) is received
at a base station.
- stronger received signal levels raise the noise floor of the weaker signals(far away) at the
base station demodulators, thereby decreasing the probability of weak signal reception
To over come near-far problem,
power control is used
- each mobile within the base station provides the same signal level to the base station
receiver.
-Therefore nearby subscriber power at the base station receiver does not exceed the far away
subscriber power.
• Power control is implemented at the base station by sensing the radio signal strength
indicator (RSSI) levels of each mobile and sending a power change command over the
forward radio link .
In FDMA
1. The frequency assigned to a cell are grouped such that frequency in the cells are as far
apart
2. Power control
3. Guard band reduce the interference
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Features of CDMA
1. Many users share the same frequency. TDD or FDD is used
2. CDMA has a soft capacity limit. No limit on the number of users. As the
number of users increases noise floor increases and system performance
decreases.
3. Multipath fading is reduced due to signal spread. If the spread spectrum
bandwidth is greater than the coherence bandwidth of the channel, small
fading effect is minimized.
4. data rates are very high. the symbol (chip) duration is much less than the
channel delay spread.
5. CDMA provide soft handoff. Mobile switching center(MSC) simultaneously
monitor a particular user from two or more base stations, and chose the best
version of the signal at any time without switching frequencies.
• Self-jamming is a problem in CDMA system. spreading sequences of different
users are not exactly orthogonal, hence in the despreading of a particular PN
code, the desired user has minimum contributions to the receiver decision.
• The near-far problem occurs at the CDMA receiver if an undesired user has a
high detected power compared to the desired user.
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Capacity of Cellular Systems
-Channel capacity for a radio system is defined as the maximum
number of channels or users that can be provided in a fixed frequency
band.
-It is a measure of spectrum efficiency
-depends on carrier-to-interference ratio (C/I) and the channel
bandwidth Bc
- reverse channel interference is interference at a base station from
surrounding cell subscriber units.
-forward channel interference is interference at the subscriber unit from
the surrounding co-channel base stations
- co-channel reuse ratio -minimum ratio of D/R required to provide a
tolerable level of co-channel interference
Q = D/R =Dist. Bet. Co channel cells/Radius of cell
Dr.C.Helen Sulochana, Prof/ ECE,
SXCCE, Nagercoil

SXCCE, Nagercoil
Capacity of Cellular CDMA
capacity of CDMA systems is interference limited,
capacity of FDMA and TDMA is bandwidth limited
reduction in the interference will increase the capacity of CDMA system
Increase the capacity of CDMA system by
1. reduce the interference using multisectorized antennas, or
directional antennas results in spatial isolation of users.
2. operate in a discontinuous transmission mode (DTX) - the
transmitter is turned off during the periods of silence in speech
- average capacity is inversely proportional to the duty factor.
-

SXCCE, Nagercoil
single cell system
-large number of mobile users communicating with a base
station within a cell
-The cell-site transmitter has a linear combiner - adds the
spread signals of individual users, with a weighting
factor (equal) for each signal for forward link power control
- A pilot signal in the cell-site transmitter is used by each
mobile to set its own power control for the reverse link.
- Assume same power level is received at the base station.

SXCCE, Nagercoil
Bit energy-to-noise ratio
= signal power / baseband signal bit rate (R)
Let N - the number of users
Demodulator at the cell site receives the desired signal of power S and
(N — I) interfering users with power S
Signal to noise ratio
interference power / total RF bandwidth(W)
SNR at the base station receiver can be represented as
----- (1)

• Considering background thermal noise
Capacity increased by
1.reducing the interference
- by reducing the number of users (equation 1).
- by antenna sectoring . cell site with three antennas, having a
beam width of 1200
, reduced the interference by one-third and
increased the capacity by 3 compared to omni-directional antenna.
2. switched off the transmitter during the periods of no voice activity.
Number of users that can access the system
W/R - processing gain.
background noise determines the cell radius for a given transmitter power

D
using the two techniques
Voice activity is denoted by a factor
capacity (number of users) of the system is
If = 3/8 and three sectors are used,
SNR and capacity increased by 8 compared to an omni-directional
antenna system with no voice activity detection
CDMA Power Control
-capacity is maximized if transmitter power of all mobile
transmitters is controlled.,
- then the total received power at the cell site = average received
power multiplied by number of mobiles users
Weak signal from the user is dropped and strong signal cause
interference

SXCCE, Nagercoil
 CDMA systems has separate forward and reverse links
 neighboring cells share same frequency
 each base station controls the transmit power of its users and unable to
control the power of users in neighboring cells
 users add to the noise floor and decrease capacity on the reverse link
Capacity of CDMA with Multiple Cells
users in CDMA system
frequency reuse factor for a CDMA system on
the reverse link
--------(1)
N0 - total interference power received from the N-1 in-cell users,
Ui-number of users in the ith
adjacent cell,
N0 i - average interference power for a user in the i th adjacent cell
transmit powers of each out-of-cell user is
added to the in-cell interference

SXCCE, Nagercoil
frequency reuse efficiency,
Within the cell, the desired user has same receive power as the N— I
undesired in-cell users due to power control.
Average received power from adjacent cell users
----(2)
Nij- power received at the base station of interest from the jth
user in the
ith
cell.
-Each adjacent cell users(out-of-cell user) will offer a different level of
interference depending on its Tx.ed power and location w.r.t base station
of interest.

concentric circle cellular geometry
-Determine the impact of path loss on frequency reuse of a CDMA
system
by considering the interference from in-cell and out-of-cell
users.
Assume
1. all cells have equal area
2. cell of interest to be circular and located
in the center of all surrounding cells
3. Interfering cells are wedge-shaped and
arranged in layers around the center cell
-all users in the cell of interest are located a distance d ( )
from the base station.
d0 - close-in distance such that all users in the center cell are located
not closer than d0
area A of the center cell =
Let R – radius of interested (center)cell,
d << R
concentric circle cellular geometry

first layer of adjacent interfering cells is found on dist.
It has M1 wedge-shaped cells that each span a particular angle
Area of first surrounding layer
- 8 wedge-shaped cells covering a span of 45°
Second layer is located on ,
i th
interfering layer is located on .
Area of ith
surrounding layer
since each adjacent cell has the same geometry and angular span, it
becomes possible to consider the interference effects of just a single cell
within the surrounding layer.
Weighting Factors
total number of users within the center cell U = KA
K - number of users per unit area
Since all cells have same number of users,
Number of users in the first surrounding layer cell = KA users.

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interfering layers to be broken into two sublayers
1. inner sublayer on distance
-contains a smaller fraction of the area of the cell and contain fewer users
2. outer sublayer on distance
- contains a greater fraction of the area of the cell and wide range of user
weighting factors are used to redistribute users in the inner and outer
sectors of an adjacent cell
So that minimum number of users contribute interference to the desired
cell
Weighting break up the distribution of adjacent cell users between the
inner and outer sectors

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concentric circle geometry in conjunction with a propagation path loss
model is used to determine interference from adjacent cell users.
in-cell interference power
P0 - power received from any one of the U users in the center cell(equal
power due to power control)
Any adjacent cells contain U users at a distance d' from its own base and
receive P0 power from each of its own in-cell users
 path loss models are based on all distances greater than forbidden
distance d0.
 small forbidden zone of width 2d0 exist in all surrounding rings which
is assumed not to contain users.
 forbidden zone is a circle of radius d0 around each adjacent cell

approximation for d’ is
using d and d’, the interference power P0,i,j at the center cell from
the jth
user in the i th
interfering cell is
-----(3)
n -propagation path loss exponent
P0,i,j = the actual transmitter power radiated by the j th
user in the i th
cell,
multiplied by the propagation path loss from that subscriber to the
center base station receiver.
f can range between 0.316 0.707, depending on the path loss exponent
and the distribution of users.
received power from adjacent cell users N0i is calculated from (2) .
Then using (1) frequency factor f is calculated.

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Space Division Multiple Access (SDMA)
Principle :
-The narrow beam of radio waves is aimed at
particular part of space.
 The same channel is reused over the another
narrow beam aimed at another part of the space.
 This division of space in different directions of base
station through highly directional beams or spot
beam antennas is called Space Division Multiple
Access (SDMA).
 SDMA uses physical separation methods that permit the sharing of
wireless channels.
 a single channel may be used simultaneously if the users are
spaced far enough from one another to avoid interference

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•The satellite dish antennas transmit
signals to various zones on earth’s
surface.
• These antennas are highly directional.
• Hence same frequency can be used for
multiple surface zones.
•As shown in Fig, area A1 and area A3 are
physically apart.
•Hence same channel-1 is used to send
signals to A1 and A3 with the help of
highly directional antennas.
• There will be no signal interface
between the signals of areas A1 and A3.

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Features.
• All users can communicate at the same time using the same channel.
SDMA is completely free from interference.
• A single satellite can communicate with more satellites receivers of
the same frequency.
• The directional spot-beam antennas are used and hence the base
station in SDMA, can track a moving user.
• Controls the radiated energy for each user in space.
Advantages
It saves the channel bandwidth.
Improves the utility of bandwidth.
Role of SDMA in wire and Wireless Communications
•SDMA can be used for mobile comm. and satellite comm..

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•satellite based SDMA required careful selection of zones (area) for
each transmitter and precise antenna alignment to avoid co-channel
interference and interchannel interference.
•power of the transmitting antennas is to be controlled to avoid the co-
channel and interchannel interference.
•there are multidirectional horn antennas at the base station (BS). The
base station identifies mobile users by means of their spatial signatures.
•The base station has complete control over the power of all the
transmitted signals on the forward link. The transmitted power from
each mobile user is dynamically controlled to avoid inter channel
interference.
•The base station detects the power level from each mobile user and
connects it. Adaptive antennas are also used.

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Comparison between FDMA, TDMA, CDMA

Capacity of Cellular Systems(FDMA)
-Channel capacity for a radio system is defined as the maximum
number of channels or users that can be provided in a fixed frequency
band.
-It is a measure of spectrum efficiency
-depends on carrier-to-interference ratio (C/I) and the channel
bandwidth Bc
- reverse channel interference is interference at a base station from
surrounding cell subscriber units.
-forward channel interference is interference at the subscriber unit from
the surrounding co-channel base stations
- co-channel reuse ratio -minimum ratio of D/R required to provide a
tolerable level of co-channel
Q = D/R =Dist. Bet. Co channel cells/Radius of cell
SXCCE, Nagercoil

Carrier to interference ratio
Let M closest co-channel cells,
Then Carrier to interference ratio
For N=7, only 6 co-channel cells. If all are at same distance D, same
path loss exponents
maximum interference occurs when the mobile is at edge D0 = R,
C/I for each user to be greater than (C/I) min to provide acceptable signal
quality at the receiver
------------(A)
n0 -path loss exponent in the desired cell
D0 - distance from the desired base station to the mobile
Dk - distance of the kth cell from the mobile
nk -path loss exponent to the kth interfering base station.
SXCCE, Nagercoil

radio capacity of a cellular system -----(B)
Using in (B) ----------(C)
substituting (A) in (C) m ………(D)
is 12 dB for digital systems and 18 dB for analog systems
To compare different systems, an equivalent C/I is used. Bt and m are
kept constant
--------(E)
Bt -total allocated spectrum for the system
Bc -the channel bandwidth,
N -number of cells in a frequency reuse pattern.
Bc-bandwidth of a particular system, Bc
’
-bandwidth for a different system
(C/I) min –minimum C/I for a particular system
(C/I) eq -minimum C/I for the different system

For digital cellular system
-----------(F)
For comparing two digital systems, using (E) and (F)
if the interference level I is same for the two systems, Rc and Bc are
linear
• If Bc is reduced by half, then the energy of symbol increases eight
times.
Rb-channel bit rate,
Eb-energy per bit,
Rc- rate of the symbol
Ec -energy per symbol

FDMA
Bt is divided into M channels, each with bandwidth Bc.
capacity for FDMA is from eqn (D) with n=4
• TDMA systems capacity is 3 to 6 times large compared to analog
cellular radio systems.
SXCCE, Nagercoil

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