Transcript No Slide Title

RANDOM ACCESS TECHNIQUES
• ALOHA Efficiency
• Reservation Protocols
• Voice and Data Techniques
- PRMA
- Variable rate CDMA
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S (Throughput per
Packet Time)
Throughput Plot
.40
Ge2G
.30
Pure Aloha
.20
l
.10
0
0.5
1.0
1.5
2.0
3.0
G(l) (Attempts per Packet TIme)
- Throughput: Fraction of time channel is used
- No power limitations
- Doesn’t measure true rate
Efficiency Plot
Aloha Efficiency (Abramson’94)
Cu Ge2G B log1  P /(GN ) 
r

C
B log1  P / N 
Efficiency
.8
-10 dB
P/N=-20 dB
.4
20 dB
0 dB
G
- Assumes power duty cycle is 1/G.
- High efficiency for low traffic and P/N
- Combines info. and queueing theory.
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.
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Spread Aloha
• One CDMA code assigned to all users
t
• 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.
Packet Reservation Multiple Access
• Time axis organized into slots and frames
1
1
2
2
3,4
• 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).
Performance
• Reduces dropping probability by 12 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
Pu / Ru
 gu,
P0 / W  ( Pt  Pu ) / W
u  v, d
• Capacity constraint
Mv
Md
P0

 1
W
W
Pt  P0
1
1
Rvg v
Rd g d
–
–
–
–
–
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.
Reservation Strategy
• Voice nonadaptive: Pv, Rv, and gv all
fixed.
•
Reserve some fixed number Kv
voice channels: maximum number
is dictated by capacity equation
K
max
v
P0
W

 1
Rvg v
Pv
• Adapt Md, Rd, and gd 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 Kv
• Multicode has the worst
performance (self-interference)
• Variable bit rate has best
performance
– more power needed when
varying constellation size.
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
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