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Random Access

Random Access. • ALOHA • Carrier-Sense Techniques • Reservation Protocols • Voice and Data Techniques - PRMA - Adaptive CDMA • Power Control “Channel access algorithms with active link protection for wireless communication networks with power control”: Dan O’neill.

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Random Access

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  1. Random Access • ALOHA • Carrier-Sense Techniques • Reservation Protocols • Voice and Data Techniques - PRMA - Adaptive CDMA • Power Control “Channel access algorithms with active link protection for wireless communication networks with power control”: Dan O’neill 7C29822.038-Cimini-9/97

  2. packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). – packets sent during predefined timeslots – A collision occurs when packets overlap, but there is no partial overlap of packets – Packets received in error are retransmitted after random delay interval. ALOHA • Data is packetized. • Packets occupy a given time interval (TD technique) • Pure ALOHA – send packet whenever data is available – a collision occurs for any partial overlap of • Slotted ALOHA – same as ALOHA but with packet slotting

  3. Throughput* • Throughput – Measures fracture of time channel is used – No power limitations – Doesn’t measure true rate • Assumptions – Normalize slot time to 1 – Retransmission required for any packet overlap – Infinite number of nodes – Poisson packet arrivals at rate l. • Slotted ALOHA – For randomized retransmissions, the sum of new and backlogged packet arrivals is Poisson with rate G>l: • Pure ALOHA *Data Networks, 2nd Ed. Bertsekas and Gallager

  4. – inefficient for heavily loaded systems – capture effect improves efficiency – combining SS with ALOHA reduces collisions Throughput Plot .40 Slotted Aloha S (Throughput per Packet Time) .30 .20 Pure Aloha .10 l 0 0.5 1.0 1.5 2.0 3.0 G(l) (Attempts per Packet TIme) Note that there are two equilibrium points for both slotted and unslotted ALOHA • Comments

  5. .4 .8 Throughput with Link Rates • Aloha Throughput (Abramson’94) • - Assumes power duty cycle is 1/G. • - High efficiency for low traffic and P/N • - Combines info. and queueing theory. Efficiency -10 dB P/N=-20 dB 20 dB 0 dB G

  6. Spread Aloha • One CDMA code assigned to all users • Users separated by time of arrival • Collisions occur when two or more signals arrive simultaneously • Advantages • Simplicity of transmitter/receiver • No code assignment • No limit on number of users for sufficiently wideband signals (UWB) • Disadvantages • Multipath can significantly increase prob. of collisions • RAKE harder to implement. t

  7. Carrier-Sense Techniques • Channel is sensed before transmission to determine if it is occupied. • More efficient than ALOHAÞ fewer retransmissions • Carrier sensing is often combined with collision detection in wired networks (e.g., Ethernet). Þ not possible in a radio environment Busy Tone Wired Network Wireless Network • Collision avoidance is used in current wireless LANs. (WaveLAN, IEEE802.11, Spectral Etiquette) 8C32810.40-Cimini-7/98

  8. Examples • ARDIS – slotted CSMA • RAM Mobile Data – slotted CSMA • CDPD – DSMA/CD - Digital Sense Multiple Access – collisions detected at receiver and transmitted back • WaveLAN – CSMA/CA 8C32810.126-Cimini-7/98

  9. Reservation Protocols – A common reservation channel is used to assign bandwidth on demand – Reservation channel requires extra BW – Offloads the access mechanism from the data channel to the control channel. - Control channel typically uses ALOHA – Very efficient if overhead traffic is a small percentage of the message traffic, and active number of users small – Very inefficient for short messaging - For CDMA, reservation process must assign unique spreading code to transmitter and receiver. 7C29822.041-Cimini-9/97

  10. Common Reservation Protocols • Demand–Based Assignment – a common reservation channel is used to assign bandwidth on demand – reservation channel requires extra bandwidth – very efficient if overhead traffic is a small percentage of the message traffic • Packet Reservation Multiple Access (PRMA) – similar to reservation ALOHA – uses a slotted channel structure – all unreserved slots are open for contention – a successful transmission in an unreserved slot effectively reserves that slot for future transmissions 7C29822.041-Cimini-9/97

  11. Packet Reservation Multiple Access • Time axis organized into slots and frames • All unreserved slots open for contention • Transmit in unreserved slots with prob. p • Data users contend in every slot (Aloha). • For voice users, successful transmission in an unreserved slot reserves slot for future transmissions. Delayed packets dropped. • Takes advantage of voice activity (reservation lost at end of talk spurt). Frame 2 Frame 1 1 1 2 2 3,4

  12. PRMA Analysis • System states modeled as a Markov chain. • Steady state probabilities used to determine blocking probability. • Analysis complexity very high • Equilibrium point analysis (EPA) is alternate technique • Equalizes arrival and departure rate for any state • Used to obtain closed form solutions to dropping probability. • Results match simulations well.

  13. Performance • Reduces voice dropping probability by 1-2 orders of magnitude over Aloha • User mobility • When a mobile changes cells, his reservation is lost. • Delay constraint of voice may be exceeded during recontention • Performance loss negligible • Bit errors • Voice bits received in error discarded. • Header bits received in error cause loss of reservation • Nonnegligible performance impact

  14. Dynamic TDMA • Frames divided into request, voice, and data slots. • Voice slots reserved by voice users using separate control channel. • Data slots dynamically assigned based on pure ALOHA contention in request slots. • Outperforms PRMA under medium to high voice traffic.

  15. Adaptive CDMA • CDMA uplink with synchronized users • Fixed chip rate Rc: spread signals occupy bandwidth W • Voice and data users request service from base station • Users admitted based on current traffic, noise, interference, and type of service request

  16. Adaptive CDMA • SIR Requirements per user • Capacity constraint • W: total spread bandwidth • Rv,Rd: symbol rate for voice,data • gv, gd: SIR requirement for voice,data • Mv,Md: number of users for voice,data • P0: Noise and out-of-cell interference power. • Pt=MvPv+MdPd: total power received at base, where Pv is voice user power and Pd is data user power.

  17. Reservation Strategy • Voice nonadaptive: Pv, Rv, and gv all fixed. • Reserve some fixed number Kv voice channels: maximum number is dictated by capacity equation • Adapt Md, Rd, and gd to maximize data throughput subject to capacity constraint under active voice users.

  18. Rate Control: Data • All data users admitted to the system • Variable rate transmission used to maximize throughput given interference from voice users • Variable rate transmission strategies: • Variable Bit Rate: users vary bit time Td=1/Rd. • Multicode: users assigned multiple spreading codes, each modulated at fixed bit rate Rd. • Variable Constellation Size: users assigned one spreading code that is modulated with variable-size constellations

  19. lv/mv=10 lv/mv=5 lv/mv=1 Voice Occupancy: Kv/M0v Voice Call Blocking Probability • Voice blocking probability derived from voice statistics and Kv – Standard Markov analysis

  20. R0=50 Kbps, R0=100 Kbps VBR/MC VCS/MC lv/mv=1 R0=100 Kbps R0=500 Kbps R0=50 Kbps R0=500 Kbps Voice call blocking probability Average Throughput Comparison

  21. Analysis • Multicode has the worst throughput • Codes interfere with each other • Variable bit rate outperforms variable constellation size • In VBR the bit rate increases linearly with power • In variable-rate MQAM the bit rate increases logarithmicly with power • More efficient to vary the bit rate than to vary the constellation size • Variable bit rate may not be practical

  22. lv/mv=10 Pon=3/8 lv/mv=3 lv/mv=1 10-30 10-25 10-20 10-15 10-10 10-5 100 Voice call blocking probability Throughput Gain with Voice Activity Detection

  23. Power Control • Improves ALOHA efficiency • User with high power can capture a packet even if there is a collision • Used in CDMA to maintain target SIR of voice and data users • Can be used to maintain target SIR for different user classes • Target SIRs must be feasible • Can combine with admission control to maintain SIRs of active users

  24. Main Themes • Retransmissions are power and spectrally inefficient. • ALOHA has poor efficiency and does not work well for data streaming • Reservation protocols are effective for long data spurts but ineffective for short messaging. • Voice and data supported by reserving some channels for voice and using remaining channels for (variable-rate) data • Power control can be used to maintain QOS for all users in system – new users blocked if degrade QOS for existing users 7C29822.042-Cimini-9/97

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