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RANDOM ACCESS TECHNIQUES

RANDOM ACCESS TECHNIQUES. • ALOHA Efficiency • Reservation Protocols • Voice and Data Techniques - PRMA - Variable rate CDMA. 7C29822.038-Cimini-9/97. .40. Pure Aloha. S (Throughput per Packet Time). .30. .20. .10. l. 0. 0.5. 1.0. 1.5. 2.0. 3.0.

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RANDOM ACCESS TECHNIQUES

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  1. RANDOM ACCESS TECHNIQUES •ALOHA Efficiency • Reservation Protocols • Voice and Data Techniques - PRMA - Variable rate CDMA 7C29822.038-Cimini-9/97

  2. .40 Pure Aloha S (Throughput per Packet Time) .30 .20 .10 l 0 0.5 1.0 1.5 2.0 3.0 G(l) (Attempts per Packet TIme) Throughput Plot • - Throughput: Fraction of time channel is used • - No power limitations • - Doesn’t measure true rate

  3. -10 dB P/N=-20 dB 20 dB 0 dB G .4 .8 Efficiency Plot • Aloha Efficiency (Abramson’94) • - Assumes power duty cycle is 1/G. • - High efficiency for low traffic and P/N • - Combines info. and queueing theory. Efficiency

  4. RESERVATION PROTOCOLS – A common reservation channel is used to assign bandwidth on demand – Reservation channel requires extra bandwidth - 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

  5. t 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 • Disadvantages • Multipath can significantly increase prob. of collisions • RAKE harder to implement.

  6. 1 1 2 2 3,4 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).

  7. Performance • Reduces 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

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. Adaptive Strategies • Variable bit rate: • fixed number of data users • each assigned unique code • each user transmits at max rate given voice users • Multicode: • data users assigned multiple codes • each code sends a fixed rate data stream • data rate dictated by capacity • Variable constellation size • each user has one code • constellation size varied

  13. Performance • Voice performance based on voice statistics and Kv • Multicode has the worst performance (self-interference) • Variable bit rate has best performance • more power needed when varying constellation size.

  14. 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 can be effectively combined by reserving some channels for voice and using remaining channels for (variable-rate) data 7C29822.042-Cimini-9/97

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