Random Access to Wireless Networks Jean-Paul M.G. Linnartz Nat.Lab., Philips Research

Radio Resource Management and Random Access Jean-Paul Linnartz1 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

Radio Resource Management and Random Access Jean-Paul Linnartz2 Suitable Protocols ALOHA Carrier Sense Inhibit Sense Collision Resolution –Stack Algorithm –Tree Algorithm Reservation methods –Reservation ALOHA –Packet Reservation Multiple Access

Radio Resource Management and Random Access Jean-Paul Linnartz3 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

Radio Resource Management and Random Access Jean-Paul Linnartz4 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. itb

Radio Resource Management and Random Access Jean-Paul Linnartz5 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

Radio Resource Management and Random Access Jean-Paul Linnartz6 Collision Resolution Adaptation of random retransmission –global control of P r (by base station) –doubling the retransmission interval after every failure Dynamic frame length ALOHA Stack algorithm Tree algorithm

Radio Resource Management and Random Access Jean-Paul Linnartz7 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)

Radio Resource Management and Random Access Jean-Paul Linnartz8 Inhibit Sense Multiple Access : ISMA Busy 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.

Radio Resource Management and Random Access Jean-Paul Linnartz9 Throughput - Offered Traffic (S-G) Relation

Radio Resource Management and Random Access Jean-Paul Linnartz10 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

Radio Resource Management and Random Access Jean-Paul Linnartz11 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

Radio Resource Management and Random Access Jean-Paul Linnartz12 Capture effect and Near-far effect Not all packets in a collision are lost Nearby terminals have higher probability of success Unfairness

Radio Resource Management and Random Access Jean-Paul Linnartz13

Radio Resource Management and Random Access Jean-Paul Linnartz14 Cellular ALOHA Scenario 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

Radio Resource Management and Random Access Jean-Paul Linnartz15 Throughput versus offered traffic in wireless channel ALOHA (orange), 1-persistent CSMA (green), non-persistent CSMA (blue)

Radio Resource Management and Random Access Jean-Paul Linnartz16 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

Radio Resource Management and Random Access Jean-Paul Linnartz17 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.

Radio Resource Management and Random Access Jean-Paul Linnartz18 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

Radio Resource Management and Random Access Jean-Paul Linnartz19 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) = Offered Traffic Throughput Narrowband “Ideal” CDMA

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz 6p 1p

Radio Resource Management and Random Access Jean-Paul Linnartz28 Packet Delay versus Traffic

Radio Resource Management and Random Access Jean-Paul Linnartz29 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

Radio Resource Management and Random Access Jean-Paul Linnartz30 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

Radio Resource Management and Random Access Jean-Paul Linnartz31 Bluetooth packet transmission 1 Mbit/sec Slots of 625 microsecond Short packets Frequency hopping access codepacket headerpayload bits

Radio Resource Management and Random Access Jean-Paul Linnartz32 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+5)

Radio Resource Management and Random Access Jean-Paul Linnartz33 Physical Link Definition w circuit switching w symmetric, synchronous services w slot reservation at fixed intervals w packet switching w (a)symmetric, asynchronous services w polling access scheme SYNCHRONOUS CONNECTION-ORIENTED (SCO) LINK ASYNCHRONOUS CONNECTION-LESS (ACL) LINK