Multiple Access Techniques


The transmission from the BS in the downlink can
be heard by each and every mobile user in the cell,
and is referred as broadcasting. Transmission from
the mobile users in the uplink to the BS is many-toone, and is referred to as multiple access.
Multiple access schemes to allow many users to
share simultaneously a finite amount of radio
spectrum resources.


Should not result in severe degradation in the performance
of the system as compared to a single user scenario.
Approaches can be broadly grouped into two categories:
narrowband and wideband.
Multiple Access Techniques

Multiple Accessing Techniques : with possible
conflict and conflict- free





Random access
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Spread spectrum multiple access (SSMA) : an
example is Code division multiple access (CDMA)
Space division multiple access (SDMA)
Multiple Access Techniques -Narrowband Systems


Transmission experiences nonselective fading. This
means that when fades occur, all of the information (i.e.
the whole channel) is affected.
Channel system : generally total spectrum is divided into
a number of relatively narrow radio channels (e.g.
FDMA). Occurrence of call blocking if channels are all
being used. Unused bandwidth in each channel cannot
be used by other users.
Narrowband Systems (Interleaving + coding
techniques are used)





Data generated in source 1 2 3 4
5 … 8 9 10 11 12 … 15 16 17
18 …
Transmitted data 1 8 15 22 29 …
2 9 16 23 … 3 10 17 24 …
Received data 1 8 15 22 29 … 2 9
16 23 30 … 3 10 17 24 31 …
with the shaded data bits undergo
deep fade
Deinterleaved data 1 2 3 4 5 … 8
9 … 15 16 17 18 …
Multiple Access Techniques -Wideband Systems



The main feature of wideband systems is that either
all the spectrum available (e.g. CDMA, TDMA) or a
considerable portion of it is used by each user (e.g.
TDMA+FDMA).
The advantage of wideband systems is that the
transmission bandwidth always exceeds the
coherence bandwidth for which the signal
experiences only selective fading. That is, only a
small fraction of the frequencies composing the
signal is affected by fading.
Signal can be distorted and therefore equalization is
needed but unlikely that a total signal fade will occur.
Duplexing

For voice or data communications, must
assure two way communication (duplexing, it
is possible to talk and listen simultaneously).
Duplexing may be done using frequency or
time domain techniques.


Forward (downlink) band provides traffic from the
BS to the mobile
Reverse (uplink) band provides traffic from the
mobile to the BS.
Frequency division duplexing (FDD)


Provides two distinct bands of frequencies for
every user, one for downlink and one for
uplink.
A large interval between these frequency
bands must be allowed so that interference is
minimized. Reverse
Forward
Channel
Channel
fc,
fc,,
R
F
frequency
Frequency separation
Frequency separation should be carefully decided
Frequency separation is constant
Time division duplexing (TDD)



In TDD communications, both directions of transmission use
one contiguous frequency allocation, but two separate time
slots to provide both a forward and reverse link.
Because transmission from mobile to BS and from BS to
mobile alternates in time, this scheme is also known as “ping
pong”.
As a consequence of the use of the same frequency band, the
communication quality in both directions is the same. This is
different from FDD.
Time division duplexing (TDD)
Slot number
0
1
2
3
4
5
channel
F
R
F
R
F
R
Reverse
Channel
6
F
7
…
R
….
Forward
Channel
Ti+1
Ti
Time separation
time
Random Access -- Aloha systems





Aloha is a packet-switching system. The time interval required to
transmit one packet is called a slot.
When transmissions from two or more users overlap, they
destroy each other, whether it is complete overlap or partial
overlap – collision takes place.
The maximum interval over which two packets can overlap and
destroy each other is called the vulnerable period.
The mode of random access in which users can transmit at
anytime is called pure Aloha. In this case, the vulnerable period
is two slot times.
A version in which users are restricted to transmit only from the
instant corresponding to the slot boundary is referred to as
slotted Aloha. The alignment of transmission to coincide with the
slot boundary means that packets can only experience complete
overlap, so the vulnerable period is only one slot time.
Random Access -- Aloha systems
Throughput of Aloha systems



In the Aloha systems, a transmitting user upon
hearing a collision, backs off for a random delay
interval and transmits again, until success is
achieved. This is known as collision resolution.
We assume that the total traffic generated for
transmission (including re-transmission) obeys a
Poisson distribution.
The transmission is successful when there is no
other packet transmission during the vulnerable
period.
Throughput of Aloha systems
Throughput of Aloha systems


P (success) =P (no other packet transmission occurs
within a vulnerable period)
Let S be the throughput, defined as the successfully
transmitted traffic load, and G be the total offered
channel traffic load. Assuming that traffic generated for
transmission obeys a Poisson distribution, then
P (no other packet transmission occurs) = exp(−τG)
where τ is the vulnerable period.
Hence P (success) = exp (−τG)
By definition P (success) = S /G,
therefore S = G exp (−τG)
Throughput of Aloha systems
Delay


The delay experienced by a packet in the
system is measured from the instant of the
packet’s arrival until the instant the sender
receives confirmation.
It is a function of the number of transmission,
the retransmission delay and the time
required for the sender to receive
confirmation of successful transmission.
Delay

The mean number of transmission until success

The average packet delay for pure Aloha

The average packet delay for slotted Aloha

The mean re-transmission delay depends on the collision
resolution algorithm used.
Delay
FDMA

In FDMA, each user is allocated a unique
frequency band or channel. During the period
of the call, no other user can share the same
frequency band.
FDMA



All channels in a cell are available to all the mobiles. Channel
assignment is carried out on a first-come first- served basis.
The number of channels, given a frequency spectrum BT ,
depends on the modulation technique (hence Bw or Bc ) and
the guard bands between the channels 2Bguard . These
guard bands allow for imperfect filters and oscillators and can
be used to minimize adjacent channel interference.
FDMA is usually implemented in narrowband systems.
Main features




Continuous transmission : the channels, once assigned, are used
on a non-time-sharing basis. This means that both subscriber
and BS can use their corresponding allotted channels
continuously and simultaneously.
Narrow bandwidth : Analog cellular systems use 25-30 kHz.
Digital FDMA systems can make use of low bit rate speech
coding techniques to reduce the channel band even more.
If FDMA channels are not in use, then they sit idle and cannot be
used by other users to increase capacity.
Low ISI : Symbol time is large compared to delay spread. No
equalizer is required (Delay spread is generally less than a few
μs – flat fading).
Main features




Low overhead : Carry overhead messages for control,
synchronization purposes. As the allotted channels can be used
continuously, fewer bits need to be dedicated compared to TDMA
channels.
Simple hardware at mobile unit and BS : (1) no digital processing
needed to combat ISI (2) ease of framing and synchronization.
Use of duplexer since both the transmitter and receiver operate
at the same time. This results in an increase in the cost of mobile
and BSs.
FDMA required tight RF filtering to minimize adjacent channel
interference.
Nonlinear effects in FDMA


In a FDMA system, many channels share the same
antenna at the BS. The power amplifiers or the
power combiners, when operated at or near
saturation are nonlinear.
The nonlinear ties generate inter-modulation
frequencies.


Undesirable harmonics generated outside the mobile radio
band cause interference to adjacent services.
Undesirable harmonics present inside the band cause
interference to other users in the mobile system.
FDMA
TDMA



TDMA systems divide the channel time into frames.
Each frame is further partitioned into time slots. In
each slot only one user is allowed to either transmit
or receive.
Unlike FDMA, only digital data and digital modulation
must be used.
Each user occupies a cyclically repeating time slot,
so a channel may be thought of as a particular time
slot of every frame, where N time slots comprise a
frame.
Features




Multiple channels per carrier or RF channels.
Burst transmission since channels are used
on a timesharing basis. Transmitter can be
turned off during idle periods.
Narrow or wide bandwidth – depends on
factors such as modulation scheme, number
of voice channels per carrier channel.
High ISI – Higher transmission symbol rate,
hence resulting in high ISI. Adaptive equalizer
required.
Features

High framing overhead – A reasonable amount of the
total transmitted bits must be dedicated to
synchronization purposes, channel identification. Also
guard slots are necessary to separate users.
TDMA Frame
One TDMA Frame
Preamble
Slot 1
Guard
Bits
Sync
Bits
Information
Slot 2
Control
Bits
Trail Bits
…
Slot 3
Information
One TDMA Slot
A Frame repeats in time
Slot N
CRC
Features

A guard time between the two time slots must be allowed
in order to avoid interference, especially in the uplink
direction. All mobiles should synchronize with BS to
minimize interference.
Features




The use of digital technology permits the inclusion of
several facilities in the mobile unit, increasing its
complexity. One example is the use of slow
frequency hopping to counteract multi-path fading.
Flexible data rates by assigning multiple time slots
to different users based on their demand.
No duplexer used since uses different time slots for
transmission and reception. Even if FDD is used, a
switch rather than a duplexer is required.
Lower BS cost : the use of multiple channels per
carrier channel provides a proportional reduction of
the equipment at the BS.
Features


Efficient power utilization : FDMA systems require a
3- to 6-dB power back off in order to compensate for
inter-modulation effects.
Efficient handoff : TDMA systems can take
advantage of the fact that the transmitter is switched
off during idle time slots to improve the handoff
procedure. An enhanced link control, such as that
provided by mobile assisted handoff (MAHO) can be
carried out by a subscriber by listening to
neighboring base station during the idle slot of the
TDMA frame.
Efficiency of TDMA

Efficiency of TDMA is a measure of the
percentage of bits per frame which contain
transmitted data. The transmitted data
include source and channel coding bits.

bOH includes all overhead bits such as
preamble, guard bits, etc.
TDMA/TDD and TDMA/FDD


In TDMA/TDD system, half of the time slots in
the frame information message would be
used for the forward link channels and half
would be used for reverse link channels.
Same channel conditions.
In TDMA/FDD systems, same frame structure
can be used for both forward and reverse
transmission but carrier frequencies used are
different.
TDMA/TDD and TDMA/FDD
Cellular Systems and MAC
Cellular System
Multiple Access Technique
AMPS
FDMA/FDD
GSM
TDMA/FDD
USDC (IS-54 and IS-136)
TDMA/FDD
PDC
TDMA/FDD
CT2 Cordless Phone
FDMA/TDD
DECT Cordless Phone
FDMA/TDD
US IS-95
CDMA/FDD
W-CDMA
CDMA/FDD
CDMA/TDD
cdma2000
CDMA/FDD
CDMA/TDD
Synchronous and asynchronous TDMA




Previously we mentioned the same slot of all the frames are
assigned to the same user. The slot assignment can actually be
fixed or dynamic.
If the assigned slot is fixed from frame to frame for the duration of
the connection, the users have to synchronize to their respective
assigned slots. This mode of TDMA is referred to as STDMA.
With packet-switched transmission, it is more efficient to allow a
user to transmit only when it has packets to send. Transmission
slots are dynamically assigned from frame to frame. This mode is
known as ATDMA.
A TDMA is implemented using a reservation access mechanism.
SSMA

Spread spectrum systems : The desired
signal is transmitted over a bandwidth which
is much larger than the Nyquist bandwidth. It
is first developed for military applications for



Security
Undetectability: minimum probability of being
detected
Robust against intentional jammers
Applications





Security
Robust against unintentional interference
It is not bandwidth efficient when used by a single
user but has the capability to overcome narrowband
jamming signals (cannot overcome AWGN or
wideband jamming signal) and multi-path.
Providing multiple access
If many users can share the same spread spectrum
bandwidth without interfering with one another,
bandwidth efficient improved but will affect the
capability to overcome jamming.
Spread Spectrum Access

Two techniques


Frequency Hopped Multiple Access (FHMA)
Direct Sequence Multiple Access (DSMA)

Also called Code Division Multiple Access – CDMA
Frequency Hopping (FHMA)


Digital muliple access technique
A wideband radio channel is used.


Same wideband spectrum is used
The carrier frequency of users are varied in a pseudorandom fashion.
 Each user is using a narrowband channel (spectrum)
at a specific instance of time.
 The random change in frequency make the change of
using the same narrowband channel very low.
Frequency Hopping (FHMA)


The sender receiver change frequency (calling
hopping) using the same pseudo-random
sequence, hence they are synchronized.
Rate of hopping versus Symbol rate


If hopping rate is greather: Called Fast Frequency
Hopping
 One bit transmitted in multiple hops.
If symbol rate is greater: Called Slow Frequency
Hopping
 Multiple bits are transmitted in a hopping period
 GSM and Bluetooth are example systems
Code Division Multiple Access (CDMA)

In CDMA, the narrowband message signal is multiplied
by a very large bandwidth signal called spreading signal
(code) before modulation and transmission over the air.
This is called spreading.

CDMA is also called DSSS (Direct Sequence Spread
Spectrum). DSSS is a more general term.

Message consists of symbols
 Has symbol period and hence, symbol rate
EE 552/452 Spring 2007
Code Division Multiple Access (CDMA)

Spreading signal (code) consists of chips








Has Chip period and and hence, chip rate
Spreading signal use a pseudo-noise (PN) sequence (a
pseudo-random sequence)
PN sequence is called a codeword
Each user has its own cordword
Codewords are orthogonal. (low autocorrelation)
Chip rate is oder of magnitude larger than the symbol rate.
The receiver correlator distinguishes the senders
signal by examining the wideband signal with the
same time-synchronized spreading code
The sent signal is recovered by despreading process
at the receiver.
CDMA
Advantages
Low power
spectral density.

Signal is spread over a larger frequency band
 Other systems suffer less from the transmitter
Interference limited operation
 All frequency spectrum is used
Privacy
 The codeword is known only between the
sender and receiver. Hence other users can
not decode the messages that are in transit
Reduction of multipath affects by using a larger
spectrum




CDMA Advantages

Random access possible







Users can start their transmission at any time
Cell capacity is not concerete fixed like in
TDMA or FDMA systems. Has soft capacity
Higher capacity than TDMA and FDMA
No frequency management
No equalizers needed
No guard time needed
Enables soft handoff
CDMA Principle
Represent bit 1 with +1
Represent bit 0 with -1
One bit period (symbol period)
1
1
Data
0
1
1
1
0
1
0
1
1 1
1
1
0
1
0
1
1
Coded
Signal
Input to the modulator (phase modulation)
Chip period
Processing Gain
Main parameter of CDMA is the processing gain that is
defined as:

Gp 
Bspread
R

Bchip
R
Gp: processing gain
Bspread: PN code rate
Bchip: Chip rate
R: Data rate

IS-95 System (Narrowband CDMA) has a gain of 64. Other
systems have gain between 10 and 100.
 1.228 Mhz chipping rate
 1.25 MHz spread bandwidth
Near Far Problem and Power Control

At a receiver, the signals
may come from various
(multiple sources.
 The strongest signal
usually captures the
modulator. The other
signals are considered
as noise
 Each source may have
different distances to
the base station
B
pr(M)
M
M
M
M
Near Far Problem and Power Control

In CDMA, we want a base station to receive
CDMA coded signals from various mobile users at
the same time.



Therefore the receiver power at the base station for all
mobile users should be close to eacother.
This requires power control at the mobiles.
Power Control: Base station monitors the RSSI
values from different mobiles and then sends
power change commands to the mobiles over a
forward channel. The mobiles then adjust their
transmit power.
DSSS Transmitter
Message
m(t)
sss(t)
Baseband
BPF
+
Transmitted
Signal
p(t)
PN Code
Generator
Oscillator
fc
Chip Clock
2 Es
sss (t ) 
m(t ) p(t ) cos(2f ct   )
Ts
DSSS Receiver
IF Wideband
Filter
sss (t )
Received
DSSS Signal
at IF
s1 (t )
Phase Shift Keying
Demodulator
p(t )
PN Code
Generator
Synchronization
System
2 Es
s1 (t ) 
m(t ) cos(2f ct   )
Ts
m(t )
Received
Data
Spectra
of
Received
Signal
Spectral
Spectral
Density
Interference
Density
Signal
Interference
Signal
Frequency
Output of Wideband filter
Frequency
Output of Correlator after
dispreading,
Input to Demodulator
CDMA Example
R
Data=1011…
A
Transmitter (a mobile)
Codeword=010011
Receiver (a base station)
Data=0010…
B
Transmitter
Codeword=101010
Data transmitted from A and B is multiplexed using CDMA and codeword.
The Receiver de-multiplexes the data using dispreading.
CDMA Example – transmission from two sources
1
A Data
0
1
1
A
Codeword
0 1
0
0
1
1
0 1
0
0
1
1
0
1
0
0
1
1
Data  Code
A Signal
1 0
1
1
0
0
0
0
0 1
1
1
0
1
1
0
0 1
1
0
B Data
0
0
1
0
1
0
0
1
1
1
0
1
0
0
1
0
1
0
1
0
1
0
1
0
1
0 1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1 1
0
1
0
1
0
B
Codeword
Data  Code
B Signal
Transmitted
A+B
Signal
0
CDMA
Example – recovering signal A at the receiver
A+B
Signal
received
A
Codeword
at
receiver
(A  B)  Code
Integrator
Output
Comparator
Output
0
1
0
Take the inverse of this to obtain A
0
CDMA
Example – recovering signal B at the receiver
A+B
Signal
received
B
Codeword
at
receiver
(A  B)  Code
Integrator
Output
Comparator
Output
1
1
0
Take the inverse of this to obtain B
1
CDMA Example – using wrong codeword at the
receiver
A+B
Signal
received
Wrong
Codeword
Used at
receiver
Integrator
Output
Comparator
Output
X
Noise
0
1
1
Wrong codeword will not be able to decode the original data!
Hybrid Spread Spectrum Techniques

FDMA/CDMA


Available wideband spectrum is frequency
divided into number narrowband radio channels.
CDMA is employed inside each channel.
DS/FHMA

The signals are spread using spreading codes
(direct sequence signals are obtained), but these
signal are not transmitted over a constant carrier
frequency; they are transmitted over a frequency
hopping carrier frequency.
Hybrid
Spread Spectrum Techniques
 Time Division CDMA (TCDMA)



Each cell is using a different spreading code (CDMA
employed between cells) that is conveyed to the mobiles
in its range.
Inside each cell (inside a CDMA channel), TDMA is
employed to multiplex multiple users.
Time Division Frequency Hopping


At each time slot, the user is hopped to a new frequency
according to a pseudo-random hopping sequence.
Employed in severe co-interference and multi-path
environments.

Bluetooth and GSM are using this technique.
Capacity of CDMA Systems

Uplink Single-cell System Model
Assumptions
User 2
...
User 1
User k
BTS
.
.
.
...
User Ku
.
.
.
User n
• Total active users Ku
• The intra-cell MAI can be
modeled as AWGN
• Perfect power control
assumed
• Random sequences
is
Capacity
of CDMA
Coarse estimate
of theSystems
reverse link (uplink) capacity



Assumptions:
 Single Cell.
 The interference caused by other users in the cell can be
modeled as AWGN.
 Perfect power control is used, i.e. the received power of each
user at the base station is the same.
If the received power of each user is Ps watts, and the
background noise can be ignored (ex: micro-cells), then the total
interference power (MAI) at the output of the desired user’s
detector is
I  K u  1Ps
where Ku is the total number of equal energy users in the cell.
Suppose each user can operate against Gaussian noise at a bitenergy-to-noise density level of Eb/Io. Let W be the entire spread
bandwidth, then the interference spectral density can be
expressed as:
I
I0 
W
Watts / Hz (one  sided )
Capacity of CDMA Systems
Interference
limited

Ps
Rb
I  W W Rb
I
K u 1   0

Ps E b  R b E b I 0
Also, the bit energy Eb is E b 
Thus,
★Now, if we consider the factors of voice activity (Gv), sectorized
antenna gain (GA), and other-cell interface factor (f), where
Gv  1/v = 2.67
GA (three sectors)  2.4
f = (Interference form other cells)/(Interference from given cell)  0.6
EE 552/452 Spring 2007
Capacity
of
CDMA
Systems
In this case, Ku can be approximated by
Ku 
W R b Gv  GA

E b I 0 1  f 
Ex: If Gv  2.67, GA  2.4, f  0.6
 Ku 
4  W Rb 
 Eb I o 
If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4)
W
Rb
which will be larger than the TDMA or FDMA systems in the cellular
environment.
 Ku 
SDMA

Use spot beam antennas
The different beam area can use TDMA, FDMA,
CDMA
 Sectorized antenna can be thought of as a SDMA
 Adaptive antennas can be used in the future
(simultaneously steer energy
in the direction of many users) spot beam

antenna
Features:



A large number of independently steered
high-gain beams can be formed without any
resulting degradation in SNR ratio.
Beams can be assigned to individual users,
thereby assuring that all links operate with
maximum gain.
Adaptive beam forming can be easily
implemented to improve the system capacity
by suppressing co channel interference.