Multiple Access Techniques

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Multiple Access Techniques
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Duplexing

• Duplexing facilitates communications in both
  directions simultaneously base station to mobile
  and mobile to base station
• Duplexing is done either using frequency or time
  domain techniques
• Frequency division duplexing (FDD)
• Time division duplexing (TDD)
• FDD is suitable for radio communication systems,
  whereas TDD is more suitable for fixed wireless
  systems

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Multiple Access Techniques

• Frequency Division Multiple Access
• Time Division Multiple Access
• Spread Spectrum Multiple Access
• Space Division Multiple Access
• Packet Radio

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Frequency Division Multiple Access (FDMA)

• The frequency spectrum is divided into unique
  frequency bands or channels
• These channels are assigned to users on demand
• Multiple users cannot share a channel
• Users are assigned a channel as a pair of
  frequencies (forward and reverse channels)
• FDMA requires tight RF filtering to reduce
  adjacent channel interference

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FDMA
TIME
Channel-2
Channel-3
Channel-6
Channel-4
Channel-5
Channel-7
Channel-8
Channel-9
FREQUENCY
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Time Division Multiple Access (TDMA)

• TDMA systems divides the radio spectrum into time
  slots, and in each time slot only one use is
  allowed to either transmit or receive
• Transmission for any user is non-continuous
• In each TDMA frame, the preamble contains the
  address and synchronization information
• TDMA shares a single carrier frequency with
  several users
• TDMA could allocate varied number of time slots
  per frame to different users

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TDMA
FREQUENCY
Channel-10
Channel-7
Channel-8
Channel-9
Channel-6
Channel-5
Channel-4
Channel-3
Channel-2
Channel-1
TIME
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Spread Spectrum Multiple Access (SSMA)

• SSMA uses signals that have a transmission
  bandwidth several orders of magnitude greater
  than the minimum required RF bandwidth
• SSMA provides immunity to multipath interference
  and robust multiple access capability
• SSMA is bandwidth efficient in multi-user
  environment
• SSMA techniques
• Frequency hopped (FH) multiple access
• Direct sequence (DS) multiple access
• Also known as code division multiple access
  (CDMA)

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FHMA

• Carrier frequencies of individual users are
  varied in a pseudorandom fashion within a
  wideband channel
• Data of each user is broken into uniform size
  bursts that are transmitted on different channels
  at different time instants based on their
  pseudo-noise (PN) code sequence
• In the FH receiver, a locally generated PN code
  is used to synchronize the receivers
  instantaneous frequency with that of the
  transmitter
• FHMA provides inherent security, and guard
  against erasures through error control coding and
  interleaving
• Use Bluetooth and HomeRF

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CDMA

• The narrowband message signal is multiplied by a
  very large bandwidth signal called spreading
  signal
• The spreading signal has a pseudo-noise code
  sequence that has a chip rate which is orders of
  magnitudes greater than the data rates of the
  message
• All users use the same carrier frequency and can
  transmit simultaneously
• Each use has its own pseudorandom codeword that
  is orthogonal to the others
• The receivers need to know the codeword of the
  corresponding sender
• Power control is used to combat the near-far
  problem
• Near-far problem
• When many mobile users share the same channel,
  the strongest received mobile signal will capture
  the demodulator at a base station. Thus a nearby
  subscriber could overpower the base-station
  receiver by drowning out the signals of far away
  subscribers.

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CDMA Features

• Many users of CDMA share the same frequency
  either TDD or FDD may be used
• CDMA has soft capacity limit system performance
  is inversely proportional to the number of users
• Multipath fading is reduced because of the signal
  spread
• Channel data rates are very high
• Prone to self-jamming and near-far problem
• Self-jamming when the spreading sequences of
  different users are not exactly orthogonal
• Near-far problem occurs at a CDMA receiver if an
  undesired user has a high detected power compared
  to the desired user

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Space Division Multiple Access (SDMA)

• SDMA controls the radiated energy for each user
  and serves different users by using spot beam
  antennas
• Different areas covered by antenna beam may be
  served by same or different frequencies
• Reverse links present difficulty
• Transmitted power from each subscriber must be
  controlled to prevent any single user from
  driving up the interference level
• Transmit power is limited by battery consumption
  at the subscriber unit

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Packet Radio

• In Packet Radio (PR) access techniques, many
  subscribers attempt to access a single channel in
  an uncoordinated manner
• Collisions from simultaneous transmissions from
  multiple transmitters are detected at the base
  station receiver, in which case an ACK or NACK
  signal is broadcasted by the base station to
  alert the user
• PR subscribers use a contention technique to
  transmit on a common channel

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Packet Radio Protocols

• If a packet transmission duration is t, the time
  duration during which it is susceptible to a
  collision (vulnerable period) is t1 2t, where t1
  is the transmission time instant
• Throughput ltPrno collisions
• Contention protocols
• Random access
• Scheduled access
• Hybrid access
• ALOHA protocols pure and slotted

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Carrier Sense Multiple Access Protocols (CSMA)

• In CSMA protocols, each terminal on the network
  is able to monitor the status of the channel
  before transmitting information
• Variations
• 1-persistent CSMA
• non-persistent CSMA
• p-persistent CSMA
• CSMA/CD