Multiple Access Techniques for Wireless Communications

1
Multiple Access Techniques for Wireless
Communications

• 2005/07/06
• Weng Chien-Erh

2
Table of Contents(1)

• Introduction
• FDMA
• TDMA
• CDMA
• SS (Spread Spectrum)
• FHSS
• DSSS
• Hybrid

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Table of Contents(2)

• Packet Radio
• Pure ALOHA
• Slotted ALOHA
• CSMA
• Reservation Protocol
• Reservation-ALOHA
• PRMA
• NC-PRMA

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Introduction (1)

• 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.

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Introduction (2)
6
Introduction (3)

• 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.

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Introduction (4)
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Frequency Division Multiple Access (FDMA) (1)

• Each transmitter is allocated a channel with a
  particular bandwidth.
• All transmitters are able to transmit
  simultaneously.

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Frequency Division Multiple Access (FDMA) (2)

• Allocation of separate channels to FDMA signals

10
Frequency Division Multiple Access (FDMA) (3)

• Time-frequency characteristic of FDMA

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Frequency Division Multiple Access (FDMA) (4)

• Features of FDMA
• If an FDMA channel is not in sue, then it sits
  idle and cant 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.

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Frequency Division Multiple Access (FDMA) (5)

• Features of FDMA (Cont.)
• 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.

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Time Division Multiple Access (TDMA) (1)

• 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.

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Time Division Multiple Access (TDMA) (2)

• Allocation of time slot in TDMA

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Time Division Multiple Access (TDMA) (3)

• Time-frequency characteristic of synchronous TDMA

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Time Division Multiple Access (TDMA) (4)

• Features of TDMA (Cont.)
• 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.

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Time Division Multiple Access (TDMA) (5)

• TDMA Frame Structure

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Time Division Multiple Access (TDMA) (6)

• TDMA Frame Structure (Cont.)
• 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.

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Time Division Multiple Access (TDMA) (7)

• Features of TDMA (Cont.)
• 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.

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Time Division Multiple Access (TDMA) (8)

• Combined used of synchronous TDMA and FDMA

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Time Division Multiple Access (TDMA) (8)

• 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.

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Code Division Multiple Access (CDMA) (1)

• Transmitter may transmit at the same time, in the
  same channel.
• Each signal is modified by spreading it over a
  large bandwidth.
• This spreading occurs by combining the
  transmitter signal with a spreading sequence.

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Code Division Multiple Access (CDMA) (2)
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Code Division Multiple Access (CDMA) (3)

• example

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Code Division Multiple Access (CDMA) (4)

• 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.

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Frequency Hopping (1)

• 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.

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Frequency Hopping (2)

• Time-frequency characteristic of a single
  transmitter.

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Frequency Hopping (3)

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

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Time Hopping (1)

• 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.

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Time Hopping (2)

• TH-PPM

Tc
t
Tf
Ts
User1 C(1)1 0 0 2 d10 User2 C(2)0 1 2
0 d21 User3 C(3)2 2 1 1 d30
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Spread Spectrum Multiple Access (1)

• 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 despread using a
  synchronized replica of the PN code.
• Direct Sequence Spread Spectrum (DSSS)
• Frequency Hopping Spread Spectrum (FHSS)

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Spread Spectrum Multiple Access (2)

• 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.
• A long system uses a PN code length that is much
  longer than a data symbol.

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Spread Spectrum Multiple Access (3)

• Direct Sequence Spread Spectrum (DSSS)

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Spread Spectrum Multiple Access (4)

• Basic principle of DSSS
• For BPSK modulation

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Spread Spectrum Multiple Access (5)

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

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Spread Spectrum Multiple Access (6)

• Frequency Hopping Spread Spectrum (FHSS)

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Spread Spectrum Multiple Access (7)

• Modulation

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Spread Spectrum Multiple Access (8)
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Spread Spectrum Multiple Access (9)
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Spread Spectrum Multiple Access (10)

• Performance in the presence of interference
• Narrowband interference
• Wideband interference
• Gaussian noise

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Spread Spectrum Multiple Access (11)
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Spread Spectrum Multiple Access (12)

• Narrowband interference

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Spread Spectrum Multiple Access (13)

• Wideband interference

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Spread Spectrum Multiple Access (14)

• Gaussian noise

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Spread Spectrum Multiple Access (15)

• Hybrid FDMA/CDMA (FCDMA)
• The available wideband spectrum is divided into a
  number of subspectras with smaller bandwidths.
• Each of these smaller suchannels becomes a
  narrowband CDMA system having processing gain
  lower than the original CDMA system.

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Spread Spectrum Multiple Access (16)

• 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.

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Spread Spectrum Multiple Access (17)

• 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.

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Packet Radio (1)

• 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.

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Packet Radio (2)

• 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.

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Packet Radio (3)

• Packet Radio Protocols
• , 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 to

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Packet Radio (4)

• Assume that packet transmissions occur with
  Poisson distribution having mean arrival rate of
  packets per second and is the packet
  duration in seconds. The traffic occupancy or
  throughput R is given by .
• R is the normalized channel traffic (measured in
  Erlangs) and if R gt 1, then the packets generated
  by the users exceed the maximum transmission rate
  of the channel. For reasonable throughput, 0 lt R
  lt 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.

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Packet Radio (5)

• The normal throughput is given as the total
  offered load times the probability of successful
  transmission, i.e.
• 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
• The probability that zero packets are generated
  (i.e., no collision) during this interval is
  given by

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Packet Radio (6)

• 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.
• Hybird Access a combination of random access and
  scheduled.

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Packet Radio (7)

• 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 ,
  

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Packet Radio (8)

• Slotted ALOHA
• In slotted ALOHA, time is divided into equal time
  slots of length greater than the packet duration
  .
• 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 .
• The throughput for the case of slotted ALOHA is
  thus given by .

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Packet Radio (9)

• 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.
• 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.
• Propagation delay is a relative measure of how
  fast it takes for a packet to travel from a BS to
  a MS.

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Packet Radio (10)

• Several variations of the CSMA strategy
• 1-persistent CSMA
• Non-persistent CSMA
• p-persistent CSMA
• CSMA/CD
• Data sense multiple access(DSMA)

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Packet Radio (11)

• 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

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Packet Radio (12)

• PRMA (Packet Reservation Multiple Access)
• A combination of TDMA and reservation ALOHA
• Ask channel resource in the talkspurt
• Release channel resource in the silent gap
• Permission probability

Effect of voice activity detector
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Packet Radio (13)

• 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)