7C Cimini-9/97 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 Random Access

ALOHA Data is packetized. Pure ALOHA – send packet whenever data is available – a collision occurs for any partial overlap of Slotted ALOHA – 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. packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). Packets occupy a given time interval (TD technique) – same as ALOHA but with packet slotting

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

Throughput Plot Slotted Aloha Pure Aloha Note that there are two equilibrium points for both slotted and unslotted ALOHA Comments – inefficient for heavily loaded systems – capture effect improves efficiency – combining SS with ALOHA reduces collisions S (Throughput per Packet Time) G( ) (Attempts per Packet TIme)

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

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

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

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 8C Cimini-7/98

– 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. Reservation Protocols 7C Cimini-9/97

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 7C Cimini-9/97 Common Reservation Protocols

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 Frame 1 Frame 2

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.

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

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 CDMA uplink with synchronized users Fixed chip rate R c : 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

Adaptive CDMA SIR Requirements per user Capacity constraint W: total spread bandwidth R v,R d : symbol rate for voice,data g v, g 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.

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 T d =1/R d. Multicode: users assigned multiple spreading codes, each modulated at fixed bit rate R d. Variable Constellation Size: users assigned one spreading code that is modulated with variable-size constellations

Voice Call Blocking Probability Voice blocking probability derived  from voice statistics and K v – Standard Markov analysis

Average Throughput Comparison VBR/MC VCS/MC R 0 =50 Kbps, R 0 =100 Kbps R 0 =100 Kbps R 0 =500 Kbps R 0 =50 Kbps v /  v =1 Voice call blocking probability

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

Throughput Gain with Voice Activity Detection v /  v =10 v /  v =3 v /  v = Voice call blocking probability P on =3/8

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

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 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