7C Cimini-9/97 RANDOM ACCESS TECHNIQUES ALOHA Efficiency Reservation Protocols Voice and Data Techniques - PRMA - Variable rate CDMA

Throughput Plot G( ) (Attempts per Packet TIme) S (Throughput per Packet Time) Pure Aloha - Throughput: Fraction of time channel is used - No power limitations - Doesn’t measure true rate

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. P/N=-20 dB -10 dB 0 dB 20 dB.4.8 G Efficiency

– 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. RESERVATION PROTOCOLS 7C Cimini-9/97

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. t

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) ,4

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

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.

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.

Adaptive CDMA SIR Requirements per user Capacity constraint –W: total spread bandwidth –R v,R d : symbol rate for voice,data –  v,  d : SIR requirement for voice,data –M v,M d : number of users for voice,data –P 0 : Noise and out-of-cell interference power. –P t =M v P v +M d P d : total power received at base, where P v is voice user power and Pd is data user power.

Reservation Strategy Voice nonadaptive: P v, R v, and  v all fixed. Reserve some fixed number K v voice channels: maximum number is dictated by capacity equation Adapt M d, R d, and  d to maximize data throughput subject to capacity constraint under active voice users.

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

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

7C Cimini-9/97 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