This document provides an overview of multiple access techniques for wireless communications, including FDMA, TDMA, CDMA, and various spread spectrum techniques. It describes the basic principles of each technique, including how they divide up frequency bands, time slots, or code channels to allow multiple users to access a shared channel simultaneously. Examples of wireless networks that use each technique are also provided.

1. Multiple Access Techniques for
Wireless Communications
By- Gaurav Khanna
13/PIT/082
M.Tech ( WCN)

2. Table of Contents:
• Introduction
• FDMA
• TDMA
• CDMA
• SS (Spread Spectrum)
• FHSS
• DSSS
• Hybrid
• Packet Radio
• Pure ALOHA
• Slotted ALOHA
• CSMA
• Reservation Protocol
• Reservation-ALOHA
• PRMA
• NC-PRMA

3. Introduction:
• Multiple Access:
• Enable many mobile users to share simultaneously
radio spectrum.
• Provide for the sharing of channel capacity
between a number of transmitters at different
locations.
• Aim to share a channel between two or more
signals in such way that each signal can be
received without interference from another.

4. • In conventional telephone systems, it is possible to
talk and listen simultaneously, called duplexing.
• Duplexing
• Allow the possibility of talking and listening
simultaneously.
• Frequency Division Duplex (FDD)
• Provides two distinct bands of frequencies
for every user
• Time Division Duplex (TDD)
• Multiple users share a signal channel by
taking turns in time domain
• Each duplexing channel has both a forward
time slot and a reverse time slot to
facilitate bidirectional communication.

5. Multiple Access Techniques in use
Cellular System 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

6. • Each transmitter is allocated a channel with a particular
bandwidth.
• All transmitters are able to transmit simultaneously.
• Allocation of separate channels to FDMA signals
Frequency Division Multiple Access (FDMA)

7. Time-frequency characteristic of FDMA

8. Features of FDMA
• If an FDMA channel is not in use, then it sits idle and can’t be used by other
users.
• Transmit simultaneously and continuously.
• FDMA is usually implemented in narrowband systems.
• Its symbol time is large as compared to the average delay spread.
• For continuous transmission, fewer bits are needed for overhead purposes
(such as synchronization and framing bits) as compared to TDMA.
• FDMA uses duplexers since both TX and RX operate at the same time.

9. Time Division Multiple Access (TDMA)
• Transmitter share a common channel.
• Only one transmitter is allowed to transmit at a time.
 Synchronous TDMA: access to the channel is restricted to regular.
 Asynchronous TDMA: a station may transmit at any time that the
channel is free.

10. Allocation of time slot in TDMA

11. Time-frequency characteristic of synchronous
TDMA

12. Features of TDMA
 TDMA systems divide the radio spectrum into time slots.
 Each user occupies a cyclically repeating time slot.
 Transmit data in a buffer-and-burst method, thus the transmission for any
user is not continuous.
 TDMA has TDD and FDD modes.
TDMA Frame Structure
 A single carrier frequency for several users.
 Low battery consumption.
 Handoff process much simpler.
 FDD : switch instead of duplexer.
 Very high transmission rate.
 High synchronization overhead.
 Guard slots necessary.

13. TDMA Frame Structure
 In TDMA, the preamble contains the address and synchronization
information that both the base station and the mobiles use to
identify each other.
 Different TDMA standards have different TDMA frame structures.
 Share a single carrier frequency with several users.
 Data transmission is not continuous, but occurs in bursts.
 No duplexers is required since users employ different time slots for
transmission and reception.
 TDMA can allocate different numbers of time slots per frame to
different users, allowing bandwidth be supplied on demand to
different users.

14. Combined use of synchronous TDMA and FDMA

15. Asynchronous TDMA: Carrier-Sense Multiple Access
(CSMA)
Allows a transmitter to access the channel at any time that is not being used
by another transmitter.

16. 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

17. 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.

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

19. Reverse link problems
• 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.

20. Solution by SDMA systems
• Adaptive antennas promise to mitigate reverse link problems.
• Limiting case of infinitesimal beam width.
• 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.

21. Disadvantage of SDMA
• Perfect adaptive antenna system:
infinitely large antenna needed.
• Compromise needed.

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

23. SDMA and PDMA in satellites
• COMSTAR 1
• PDMA
• separate antennas.
• simultaneously access
from same region.

24. SDMA and PDMA in satellites
• INTELSAT V
• PDMA and SDMA
• Two hemispheric coverage
by SDMA.
• Two smaller beam zones by
PDMA.
• orthogonal polarization.

25. Code Division Multiple Access (CDMA)
 Transmitter may transmit at the same time, in the same channel.
 Each signal is modified by spreading it over a large bandwidth.
o This spreading occurs by combining the transmitter signal with a
spreading sequence.

26. Near-far Problem
Users may be received
with very different powers:
 Users near the base station are
received with high power.
 Users far from the base station
are received with low power.
 Nearby users will completely
swamp far away users.

27. Advantages of CDMA
• Capacity is CDMA's biggest asset. It can accommodate more
users per MHz of bandwidth than any other technology.
3 to 5 times more than GSM.
• CDMA has no built-in limit to the number of concurrent users.
• CDMA uses precise clocks that do not limit the distance a
tower can cover.
• CDMA consumes less power and covers large areas so cell size
in CDMA is larger.
• CDMA is able to produce a reasonable call with lower signal
(cell phone reception) levels.
• CDMA uses Soft Handoff, reducing the likelihood of dropped
calls.
• CDMA's variable rate voice coders reduce the rate being
transmitted when speaker is not talking, which allows the
channel to be packed more efficiently.
• Has a well-defined path to higher data rates.

28. Disadvantages of CDMA
• Most technologies are patented and
must be licensed from Qualcomm.
• Breathing of base stations, where
coverage area shrinks under load. As the
number of subscribers using a particular
site goes up, the range of that site goes
down.
• Currently CDMA covers a smaller portion
of the world as compared to GSM which
has more subscribers and is in more
countries overall worldwide.

29. CDMA example
Low-Bandwidth Signal:
High-Bandwidth Spreading Code:
...repeated...

30. CDMA example
Low-Bandwidth Signal:
High-Bandwidth Spreading Code:
Mix is a simple multiply
… and transmit.

31. CDMA example
To Decode / Receive, take the signal:

32. CDMA example
To Decode / Receive, take the signal:
Multiply by the same Spreading Code:
… to get ...
… which we should recognize as...

33. CDMA example
To Decode / Receive, take the signal:
Multiply by the same Spreading Code:
… to get ...

34. What if we use the wrong code?
Take the same signal:
Multiply by the wrong Spreading Code:
… for example, let's just shift the same code left a bit:

35. Take the same signal:
Multiply by the wrong Spreading Code:
… We get ...
… which clearly hasn't recovered the original signal.
Using wrong code is like being off-frequency.
What if we use the wrong code?

36. Features of CDMA
 Many users of a CDMA system share the same
frequency.
 The symbol (chip) duration is very short and
usually much less than the channel delay
spread.
 The near-far problem occurs at a CDMA RX if
an undesired user has a high detected power
as compared to the desired user.

37. Frequency Hopping
 Frequency hopping is a form of FDMA.
 Each transmitter is allocated a group of channels, known as hop set .
 The transmitter transmits data in short bursts, choosing one of these
channels on which to transmit each burst.

38. Time-frequency characteristic of a single
transmitter.

39. Signal received form a pair of frequency-hopping
transmitters.

40. Time Hopping
Each bit is transmitted as a single pulse, with the value of j-th bit
determined by whether it arrives before or after the reference time tj.

41. TH-PPM
   



 




i
Ns
j
i
c
j
iN
f
s
tr
tr d
T
c
jT
iT
t
w
t
S s
1
0


42. Spread Spectrum Multiple Access
 A transmission technique in which a PN code,
independent of information data, is employed as
a modulation waveform to “spread” the signal
energy over a bandwidth much greater than the
signal information bandwidth.
 At the receiver the signal is “de-spread” using a
synchronized replica of the PN code.
 Direct Sequence Spread Spectrum (DSSS)
 Frequency Hopping Spread Spectrum (FHSS)

43. General Model of Spread Spectrum System

44. Pseudo-Noise Sequence/Maximum Length
Sequence/Spreading Sequence
 Noise-like wideband spread-spectrum signals are generated
using PN sequence.
 In DS/SS(direct-sequence spread-spectrum) , a PN spreading
waveform is a time function of a PN sequence.
 In FH/SS(frequency-hopping spread-spectrum), frequency
hopping patterns can be generated from a PN code.
 PN sequences are deterministically generated, however they
almost like random sequences to an observer.
 The time waveform generated from the PN sequences also
seem like random noise.

45.  M-sequence (binary maximal length shift-register sequence)- Generated using
linear feedback shift-register and exclusive OR-gate circuits.
 Linear generator polynomial g(x) of degree m>0
 Recurrence Equation ( gm = g0 =1)
 If gi =1, the corresponding circuit switch is closed, otherwise gi 1, it is open.
 Output of the shift-register circuit is transformed to 1 if it is 0, and –1 if it is 1.

46. Important PN Properties
• Randomness
Uniform distribution
Balance property
Run property
Independence
Correlation property
• Unpredictability

47. Direct Sequence Spread Spectrum (DSSS)
 A carrier is modulated by a digital code in which the code bit rate
is much larger than the information signal bit rate. These systems
are also called pseudo-noise systems.
 Also called code division multiple access (CDMA)
 A short code system uses a PN code length equal to a data symbol.
 Along system uses a PN code length that is much longer than a
data symbol.

48. Basic principle of DSSS
 For BPSK modulation

50. Frequency Hopping Spread Spectrum (FHSS)

51. Frequency Hopping Example:

52. Frequency Hopping Spread Spectrum System
(Transmitter):

53. Frequency Hopping Spread Spectrum System
(Receiver):

54. Frequency Hopping Spread Spectrum (FHSS)
 It divides available bandwidth into N channels and hops between
these channels according to the PN sequence.
1. Fast hopping
2. Slow hopping

55. Slow and Fast FHSS:

56. Slow Frequency Hopping Spread Spectrum
using MFSK (M=4, K=2):

57. Fast Frequency Hopping Spread Spectrum
using MFSK (M=4, K=2):

58. Modulation

61. Performance in the presence of interference
 Narrowband interference
 Wideband interference
 Gaussian noise

62. Narrowband interference

63. Wideband interference

64. Gaussian noise

65. Hybrid FDMA/CDMA (FCDMA):
 The available wideband spectrum is divided into a number of sub-
spectras with smaller bandwidths.
 Each of these smaller sub channels becomes a narrowband CDMA
system having processing gain lower than the original CDMA system.

66. Hybrid Direct Sequence/Frequency Hopped
Multiple Access (DS/FHMA)
 This technique consists of a direct sequence modulated signal
whose center frequency is made to hop periodically in a
pseudorandom fashion.
 Having an advantage in that they avoid the near-far effect.

67. Time Division Frequency Hopping (TDFH)
 The subscriber can hop to a new frequency at the start of a new
TDMA frame.
 Has been adopted in GSM.

68. Packet Radio
 In packet radio (PR) access techniques, many
subscribers attempt to access a single channel in
an uncoordinated (or minimally coordinated
manner.
 Collision from the simultaneous transmissions of
multiple transmitters are detected at the BS, in
which case an ACK or NACK signal is broadcast by
the BS to alert the desired user of received
transmission.
 PR multiple access is very easy to implement but
has low spectral efficiency and may include
delays.
 The subscribers use a contention technique to
transmit on a common channel.

69.  ALOHA protocols, developed for early satellite systems, allow
each subscriber to transmit whenever they have data to sent.
 The transmitting subscribers listen to the acknowledgement
feedback to determine if transmission has been successful or
not.
 If a collision occurs, the subscriber waits a random amount of
time, and then transmits the packet.
 The performance of contention techniques can be evaluated
by throughput (T), which is defined as the average number of
message successfully transmitted per unit time, and the
average delay (D) experienced by a typical message burst

70. Packet Radio Protocols
 Vp , vulnerable period is
defined as the time interval
during which the packets are
susceptible to collisions with
transmission form other user.
 Packet A suffer a collision if
other terminals transmit
packets during the period t1
to t1+ 2τ

71.  Assume that packet transmissions occur with Poisson
distribution having mean arrival rate of λ packets per second
and τ is the packet duration in seconds. The traffic occupancy
or throughput R is given by R= λ τ .
 R is the normalized channel traffic (measured in Erlangs) and if
R > 1, then the packets generated by the users exceed the
maximum transmission rate of the channel. For reasonable
throughput, 0 < R < 1.
 Under normal loading, the throughput T is the same as the
total offered load L.
 The load L is the sum of the newly generated packets and the
retransmitted packets that suffered collisions.
 The normal throughput is given as the total offered load
times the probability of successful transmission, i.e.
Pr[ ] Pr[ ]
T R nocollision nocollision

 

72.  The probability that n packets are generated by the user
population during a given packet duration interval is
assumed to Poisson distributed and is given as:
Pr( )
!
R
e
n
n


 The probability that zero packets are generated (i.e., no collision)
during this interval is given by :
Pr(0) R
e


73. Type of Access
Contention protocols are categorized as:
Random Access: there is no coordination among that users and
the messages are transmitted from the users as they arrive at
the transmitter.
Scheduled Access: based on a coordinated access of users on
the channel and the users transmit messages within allotted
slots or time intervals.
Hybrid Access: a combination of random access and scheduled.

74. Pure ALOHA
 The pure ALOHA protocol is random access protocol used for
data transfer and a user accesses a channel as soon as a
message is ready to be transmitted.
 After a transmission, the user waits for an acknowledgment on
either the same channel or a separate feedback channel.
 In case of collisions, the terminal waits for a random period of
time and retransmits the message.
 For pure ALOHA, the vulnerable period is double the packet
duration:

75. Slotted ALOHA
 In slotted ALOHA, time is divided into equal time slots
of length greater than the packet duration τ .
 The subscribers each have synchronized clocks and
transmit a message only at the beginning of a new time
slot.
 The vulnerable period of slotted ALOHA is only one
packet duration, since partial collisions are prevented
through synchronization.
 The probability that no other packets will be generated
during the vulnerable period is е-R.
 The throughput for the case of slotted ALOHA is thus
given by T=R.е-R .

76. Carrier Sense Multiple Access (CSMA)
 CSMA protocols are based on the fact that each
terminal on the network is able to monitor the
status of the channel before transmitting
information.
 In CSMA, detection delay and propagation delay
are two important parameters.
o Detection delay is a function of he receiver
hardware and is the time required for a
terminal to sense whether or not the channel
is idle.
o Propagation delay is a relative measure of
how fast it takes for a packet to travel from
a BS to a MS.

77. Several variations of the CSMA strategy
 persistent CSMA
 Non-persistent CSMA
 p-persistent CSMA
 CSMA/CD
 Data sense multiple access(DSMA)

78. Reservation Protocols
Reservation ALOHA (R-ALOHA):
 R-ALOHA is a packet scheme based on
time division multiplexing.
 Two phase: contention phase and
transmission phase
 Mobiles contend the channel in
reservation phase (slotted-ALOHA)
 Mobiles that succeed in making
reservation can transmit without
interference
ACK
Frame N
. . .
B-M B-M
M-B M-B
ACK
. . .
Reservation
phase
Transmission
phase

79. PRMA (Packet Reservation Multiple Access)
 A combination of TDMA and reservation
ALOHA
 Ask channel resource in the packet spurt.
 Release channel resource in the silent gap
 Permission probability Effect of voice activity detector
Frame N
B->M ACK B->M B->M
ACK ACK
.
.
.
.
.
.
.
.
.
.
.
M->B M->B .
.
. M->B

80. NC-PRMA (Non-Collision Packet Reservation
Multiple Access)
The existing users inform the BS
about their demands in a non-
collision manner (time-frequency
signaling scheme)
Ii : Information slot
CM i : Control minislot
C : Control slot
UID : Uplink user identifier
DID : Downlink user identifier