Random Access to Wireless Networks Jean-Paul M.G. Linnartz Nat.Lab., Philips Research.
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Transcript of Random Access to Wireless Networks Jean-Paul M.G. Linnartz Nat.Lab., Philips Research.
Random Access to Wireless Networks
Jean-Paul M.G. LinnartzNat.Lab., Philips Research
Jean-Paul Linnartz 2
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 3
Radio Resource Management and Random Access
Suitable Protocols
• ALOHA
• Carrier Sense
• Inhibit Sense
• Collision Resolution– Stack Algorithm– Tree Algorithm
• Reservation methods– Reservation ALOHA– Packet Reservation Multiple Access
Jean-Paul Linnartz 4
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 5
Radio Resource Management and Random Access
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.i t b
Jean-Paul Linnartz 6
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 7
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 8
Radio Resource Management and Random Access
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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Radio Resource Management and Random Access
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.
Jean-Paul Linnartz 10
Radio Resource Management and Random Access
Throughput - Offered Traffic (S-G) Relation
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Radio Resource Management and Random Access
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
Jean-Paul Linnartz 12
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 13
Radio Resource Management and Random Access
Capture effect and Near-far effect
• Not all packets in a collision are lost
• Nearby terminals have higher probability of success
• Unfairness
Jean-Paul Linnartz 14
Radio Resource Management and Random Access
Jean-Paul Linnartz 15
Radio Resource Management and Random Access
Cellular ALOHAScenario 1: Two separate
channels
Let’s 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
Jean-Paul Linnartz 16
Radio Resource Management and Random Access
Throughput versus offered traffic in wireless channel
ALOHA (orange), 1-persistent CSMA (green), non-persistent CSMA (blue)
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Radio Resource Management and Random Access
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” C=1
Jean-Paul Linnartz 18
Radio Resource Management and Random Access
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.
•
Jean-Paul Linnartz 19
Radio Resource Management and Random Access
Intuition
• Compare the ALOHA system with an embarkation quay
• People arrive with Poisson arrival rate < 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
6p
1p
Jean-Paul Linnartz 20
Radio Resource Management and Random Access
Intuition and AnalysisCase 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) =
(-NG)n!
)(NG =)(
nN
1=nexpcaptP
Offered Traffic
Th
rou
gh
pu
t
Narrowband
“Ideal” CDMA
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz
Radio Resource Management and Random Access
6p
1p
Jean-Paul Linnartz 29
Radio Resource Management and Random Access
Packet Delay versus Traffic
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Radio Resource Management and Random Access
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
Jean-Paul Linnartz 31
Radio Resource Management and Random Access
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
Jean-Paul Linnartz 32
Radio Resource Management and Random Access
Bluetooth packet transmission• 1 Mbit/sec • Slots of 625 microsecond• Short packets• Frequency hopping
access code packet header payload
72 54 0 .. 2745 bits
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Radio Resource Management and Random Access
Bluetooth random access scheme• MULTI-SLOT PACKETSMULTI-SLOT PACKETS
f(k)
625 s
f(k+1) f(k+2) f(k+3) f(k+4)
f(k+3) f(k+4)f(k)
f(k)
f(k+5)
f(k+5)
f(k+5)
Jean-Paul Linnartz 34
Radio Resource Management and Random Access
Physical Link Definition
circuit switching symmetric, synchronous services slot reservation at fixed intervals
packet switching (a)symmetric, asynchronous services polling access scheme
SYNCHRONOUS CONNECTION-ORIENTED (SCO) LINK
ASYNCHRONOUS CONNECTION-LESS (ACL) LINK