Random Access to Wireless Networks

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Random Access to Wireless Networks

• Jean-Paul M.G. Linnartz
• Nat.Lab., Philips Research

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Random Access

• Many terminals communicate to a single base
  station
• Fixed multiple access methods (TDMA, FDMA, CDMA)
  become inefficient when the traffic is bursty.
• Random Access works better for
• many users, where ..
• each user only occasionally sends a message

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Suitable Protocols

• ALOHA
• Carrier Sense
• Inhibit Sense
• Collision Resolution
• Stack Algorithm
• Tree Algorithm
• Reservation methods
• Reservation ALOHA
• Packet Reservation Multiple Access

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ALOHA Protocol

• Any terminal is allowed to transmit without
  considering whether channel is idle or busy
• If packet is received correctly, the base station
  transmits an acknowledgement.
• If no acknowledgement is received by the mobile,
• 1) it assumes the packet to be lost
• 2) it retransmits the packet after waiting a
  random time

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ALOHA Protocol

• Any terminal is allowed to transmit without
  considering whether channel is idle or busy
• If packet is received correctly, the base station
  transmits an acknowledgement.
• If no acknowledgement is received by the mobile,
• 1) it assumes the packet to be lost
• 2) it retransmits the packet after waiting a
  random time, usually with probability Pr in every
  slot.

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ALOHA Protocol

• Unslotted ALOHA transmission may start anytime
• Slotted ALOHA packets are transmitted in time
  slots
• Critical performance issue "How to choose the
  retransmission parameter?"
• Too long leads to excessive delay
• Too short stirs instability
• Instability Number of terminals in backlog grows
  without bounds

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Collision Resolution

• Adaptation of random retransmission
• global control of Pr (by base station)
• doubling the retransmission interval after every
  failure
• Dynamic frame length ALOHA
• Stack algorithm
• Tree algorithm

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Carrier Sense Multiple Access CSMA

• " Listen before talk "
• No new packet transmission is initiated when the
  channel is busy
• This reduces the number of collisions
• Performance is very sensitive to delays in
  Carrier Sense mechanism
• CSMA is useful if channel sensing is much faster
  than packet transmission time
• satellite channel with long roundtrip delay just
  use ALOHA
• Hidden Terminal Problem
• mobile terminal may not be aware of a
  transmission by another (remote) terminal.
• Solution Inhibit Sense Multiple Access (ISMA)
• Decision Problem how to distinguish noise and
  weak transmission?
• Solution Inhibit Sense Multiple Access (ISMA)

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Inhibit Sense Multiple Access ISMABusy Tone
Multiple Access BTMA

• If busy, base station transmits a "busy" signal
  to inhibit all other mobile terminals from
  transmitting
• Collisions still occur, because of
• Signalling delay
• New packet transmissions can start during a delay
  in the broadcasting of the inhibit signal,
• Persistent terminals
• After the termination of transmission, packets
  from persistent terminals, awaiting the channel
  to become idle, can collide.

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Throughput - Offered Traffic (S-G) Relation
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Common Performance Analysis Assumptions

• All packets are of uniform duration,
• unit of time packet duration guard time
• Acknowledgements are never lost
• Steady-state operation (stability)
• Poisson distributed attempts

Steady-state operation Random waiting times need
to be long enough to ensure uncorrelated
interference during the initial and successive
transmission attempts. This is an approximation
dynamic retransmission control is needed in
practice N.B. ALOHA without capture, with
infinite population is always unstable
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WIRELESS RANDOM-ACCESS

• Probability of successful reception depends on
• Receiver capture performance
• Modulation method
• Type of coding
• Signal processing at the receiver (diversity,
  equalization, ...)
• Propagation
• Distance from the central receiver, path loss,
  Shadowing
• Channel fading and dispersion
• Channel noise
• Traffic
• Contending packet traffic (from same cell)
• Interference from co-channel cells
• Initial Access protocol slotted ALOHA, Carrier
  Sense (CSMA) or Inhibit Sense Multiple Access
  (ISMA)
• Retransmission policy

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Capture effect and Near-far effect

• Not all packets in a collision are lost
• Nearby terminals have higher probability of
  success
• Unfairness

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Cellular ALOHA

• Scenario 1 Two separate channels
• Lets assume we do time division
• Total traffic per cell is G packets per slot
• Every available slot has 2G traffic because it
  carries traffic that arrives in two slot periods
• System Throughput
• S 2G exp(-2G)
• NB Receiver is idle 50 of time

• Scenario 2 Common Channel
• Total traffic per cell is G packets per slot
• Every available slot has 2G traffic because there
  are two cells
• System Throughput
• S 2G exp(-2G)
• or better if receivers can catch different
  packets

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Throughput versus offered traffic in wireless
channel
ALOHA (orange), 1-persistent CSMA (green),
non-persistent CSMA (blue)
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Cellular aspects of packet transmission

• Circuit-switching
• Performance criterion is outage at cell boundary
• Packet-switching
• Performance criterion is packet delay
• Collisions from within the same cell, and
• Interference from outside the cell
• Optimum cell reuse C1

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DS-CDMA ALOHA Network

• Under ideal signal separation conditions, DS-CDMA
  can enhance the capacity
• Make a fair comparison!
• Spreading by N in the same transmit bandwidth
  implies slot that are N times longer. The arrival
  rate per slot is N times larger
• Assumption for simple analysis
• All packets in a slot are successful iff the
  number of packets in that slot does not exceed
  the speading gain.

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Intuition

• Compare the ALOHA system with an embarkation quay
• People arrive with Poisson arrival rate l lt 1
  person per unit of time
• Boats of seat capacity N at regular intervals of
  duration N
• Thus total seat capacity is 1 person per unit of
  time
• The boat sinks and the passengers drown if the
  number of people exceeds N

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Intuition and Analysis

• Case I N 1 (ALOHA without spreading)
• Boats arrive very frequently
• Probability of survival is exp-G
• Case II Large N (CDMA)
• Fewer but larger boats arrive
• Average waiting time is N times larger
• Probability of survival is larger, because of the
  law of large numbers
• Probability of success Prob(n N)

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Packet Delay versus Traffic
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Slow Frequency Hopping

• Narrowband (unspread) transmission of each packet
• Receiver threshold remains unchanged
• N parallel channels, each with rate 1/N
• Traffic load per slot remains unchanged
• Advantages
• Frequency diversity
• fading on different carrier uncorrelated
• capture probabilities independent
• improved performance
• Less Intersymbol Interference
• Disadvantages
• longer delay than transmission at high rate

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Conclusions

• Wireless channel affects performance of random
  access scheme
• Packets lost due to fading, inter and intracell
  interference
• Signal capture enhances throughput and stability
• Packet and circuit-switched data should be
  treated differently
• DS-CDMA in ISM bands does not necessarily help

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Bluetooth packet transmission

• 1 Mbit/sec
• Slots of 625 microsecond
• Short packets
• Frequency hopping

0 .. 2745 bits
72
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access code
packet header
payload
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Bluetooth random access scheme

• MULTI-SLOT PACKETS

625 ?s
f(k)
f(k1)
f(k2)
f(k3)
f(k4)
f(k5)
f(k3)
f(k4)
f(k)
f(k5)
f(k)
f(k5)
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Physical Link Definition
SYNCHRONOUS CONNECTION-ORIENTED (SCO) LINK

• circuit switching
• symmetric, synchronous services
• slot reservation at fixed intervals
• packet switching
• (a)symmetric, asynchronous services
• polling access scheme

ASYNCHRONOUS CONNECTION-LESS (ACL) LINK