7C29822.038-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

GGeThroughput

– Retransmission required for any packet overlap – Normalize slot time to 1

*Data Networks, 2nd 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 ALOHAGGeThroughput 2

Throughput Plot

.40

.30

.20

.10

0 0.5 1.0 1.5 2.0 3.0

Slotted Aloha

Pure Aloha

Note that there are two equilibrium pointsfor both slotted and unslotted ALOHA

• Comments

– inefficient for heavily loaded systems – capture effect improves efficiency – combining SS with ALOHA reduces collisions

S (

Th

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gh

pu

t p

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acke

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ime)

G() (Attempts per Packet TIme)

GGe 2

GGe

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.

NPB

GNPBGe

C

Cr

Gu

/1log

)/(1log2

P/N=-20 dB-10 dB

0 dB20 dB

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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)

8C32810.40-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

8C32810.126-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

7C29822.041-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

7C29822.041-Cimini-9/97

Common ReservationProtocols

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).

1 1

2

2

3,4

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 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

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.

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Reservation Strategy

Voice nonadaptive: Pv, Rv, and v all

fixed.

Reserve some fixed number Kv

voice channels: maximum number is dictated by capacity equation

Adapt Md, Rd, and d to maximize

data throughput subject to capacity constraint under active voice users.

vvvv P

P

R

WK 0max 1

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

Voice Occupancy:v/M0v

v/v=1v/v=5

v/v=10

Voice Call Blocking Probability

• Voice blocking probability derived from voice statistics and Kv

– Standard Markov analysis

Average Throughput Comparison

VBR/MCVCS/MC

R0=50 Kbps, R0=100 Kbps

R0=100 Kbps

R0=500 Kbps

R0=500 Kbps

R0=50 Kbps

v/v=1

Voice call blocking probability

Analysis

Multicode has the worst throughputCodes 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=1

10-30 10-25 10-20 10-15 10-10 10-5 100

Voice call blocking probability

Pon=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

7C29822.042-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