41
Introduction to CDMA ALOHA 3. Access Control Techniques for CDMA ALOHA Takaya Yamazato Center for Information Media Studies, Nagoya University Nagoya 464-01, Japan [email protected]
Introduction to CDMA ALOHA
3. Access Control Techniques for CDMA ALOHA
Takaya Yamazato
Center for Information Media Studies,
Nagoya University
Nagoya 464-01, Japan
CDMA ALOHA
• Random access
• Simultaneous
packet transmission
• High throughput
performance
• Flexible
transmission of
multimedia signal
ALOHA
CDMA ALOHA
• Soft Capacity• High spectral efficiency• Simultaneous transmission• Robustness against interference and distortion• Flexibility for multimedia
• Fundamental protocol• Random access• Simplicity
• Preserving good features of CDMA and ALOHA+ High throughput
CDMA
Why Access Control?
• Improvement of
maximum throughput
• No degradation of
throughput in high
offered traffic load
Packet transmissionaccoding to the informationbroadcasted from the hubstation
Hubstation
CDMAALOHA
ControlSignal
0
0.05
0.1
0.15
0.2
0 0.2 0.4 0.6 0.8 1
N=30 60 100 300
Pure ALOHA
CDMA U-ALOHA
Normalized offered load G/N
Nor
mal
ized
thro
ughp
utS
/N
Eb/No= [dB]L=500 [bits]
X Difficulty of carrier
sensing by each user
due to the low peak
power transmission.
X Fluctuation of
channel load during a
packet transmission
D A little throughput
improvement by
slotted system
How ?
t
Channelload
f
Power spectral density
Hubstation
CDMAALOHA
ControlSignal
Low peak power transmission of CDMA signal
1 Access control protocol
should be based on the
channel load status
observed by hub-station
2 Packet access should be
accomplished in
accordance with control
signal broadcast from
hub-station
3 CDMA Unslotted ALOHA
(CDMA U-ALOHA) is an
appropriate candidate
Access Control for CDMA ALOHA
t
Channelload
Hubstation
CDMAALOHA
ControlSignal
Fluctuationof channelload
Access Timing Delay
• Time difference between
channel load sensing and
associated packet access
timing
• Remarkable in satellite
communication system
• GEO : 0.50 [sec]
• LEO : 0.02 [sec]
Satellite
CDMAALOHA
ControlSignal
Hubstation
Userstation
Channelloadsensing
Controlsignal Packet
transmission
Propagationdelay
Processdelay
Propagationdelay
• Transmission access control protocol
1. Channel load sensing protocol (CLSP)
2. Modified CLSP
• Retransmission control protocol
3. Packet retransmission control (PRC)
• Transmission and retransmission control protocol
4. Optimum access control protocol (OACP)
• CDMA Unslotted ALOHA systems with buffers
Access Control for CDMA ALOHA
System Model
• Centralized single-hop
network
• DS/SS modulated
packet
• Poisson generation of
packet
• Equal power reception
• G : Offered load
L : Fixed packet
length [bit]
• BER -- see (2.1)
Hubstation
CDMA ALOHA
X
PN1
CDMA ALOHA
X
PN1
X
PN1
CDMA ALOHA
X
PN1
+AWGN
Contolsignal
Transmission Control Protocol
• Packet transmission is
controlled by Hub-
station
1. Channel load sensing
protocol (CLSP)
2. Modified CLSPPacket transmissionaccoding to the informationbroadcasted from the hubstation
Hubstation
CDMAALOHA
ControlSignal
Channel Load Sensing Protocol (CLSP)
• Hub-station observes the
channel load, actual
number of on-going packets
• According to the channel
load, the information of
permission or prohibition is
broadcasted.
• Users transmit according to
such information.
• Channel load is always kept
less than or equal to the
threshold, a.
Hubstation
CDMAALOHA
Permissionor
Rejection
t
Channelload
CDMA ALOHA
CDMA ALOHA
CDMA ALOHA
Fluctuation of channel load1
23
threshold a
Throughput of CDMA ALOHA with CLSP
• Significant
improvement
in throughput
• No
degradation
in high
offered load
• Throughput
depends on
threshold, a0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
N=60L=500 [bits]Eb/No=10 [dB]
Th
rou
gh
pu
tS
Offered load G
withoutCLSP
with CLSP=5 7 9 11 13 15
a
Access Timing Delay
• Time difference between
channel load sensing and
associated packet access
timing
• tD : Access timing delay
normalized by packet
duration
• GEO : tD = 9.20
• LEO : tD = 0.26
L=500 [bit] R=9,600 [bps]
Propagation Delay
Process Delay
Propagation Delay
Process Delay
Hub Station User Station
Acc
ess
Tim
ing
Del
ay
Packet Transmission
Packet Transmission
Throughput Degradation of CLSP
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Th
rou
gh
pu
tS
Offered load G
withoutCLSP
N=60L=500 [bits]Eb/No=10 [dB] =9a
tD=0
tD=1
tD=0.1
(no access timing delay)
tD=0.26(LEO)
tD=9.2(GEO)
• Severe
degradation
in the
presence of
access
timing delay
• In the presence of access
timing delay, since the
packet access control is
done by the past
information, the
throughput would degrade
• Access control based not
on instantaneous channel
load but on average
channel load
Throughput Degradation of CLSP
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Th
rou
gh
pu
tS
Offered load G
withoutCLSP
N=60L=500 [bits]Eb/No=10 [dB] =9a
tD=0
tD=1
tD=0.1
(no access timing delay)
tD=0.26(LEO)
tD=9.2(GEO)
Propagation Delay
Process Delay
Propagation Delay
Process Delay
Hub Station User Station
Acc
ess
Tim
ing
Del
ay
Packet Transmission
Packet Transmission
Modified CLSP
• Hub-station estimate the
offered traffic load, G.
• According to the estimated
G, the hub-station broadcast
the probability, Ptr.
• Ptr is obtained so that
actual offered load is set to
the value which gains the
maximum throughput.
• User transmits his packets
according to Ptr
Hubstation
CDMAALOHA
Ptr
User transmit his packet with Ptr,or stops transmitting its packetwith 1-Ptr
Calculation of Ptr
BroadcastPtr
Estimationof G
t
Channelload
CDMA ALOHA
CDMA ALOHA
CDMA ALOHA
Fluctuation of channel load1
23
Throughput of CDMA ALOHA with MCLSP
0
1
2
3
4
5
6
0 5 10 15 20 25 30
Th
rou
gh
pu
tS
Offered load G
N=60L=500 [bits]Eb/No=10 [dB]
tD=0.26 (LEO)tD=9.2 (GEO)
Analysis
Simulation
No accesscontrol
Gmax
ts =10
ts =1
• Maximum
throughput is
the same as
one without
access control.
• Robustness
against access
timing delay
Retransmission Control Protocol
• If packet error occurs,
a user schedules the
packet at a later time
according to a delay
distribution
• This distribution is
calculated and broadcast
by a hub-station
3. Packet retransmission
control (PRC)
Hubstation
Hubstation
Userstation Packet
transmissionDelay
Packeterror
Retransmissioncontrol signal
Packetretransmission
X
Delay
Control signal
Retransmission
Transmission
X
Packet Retransmission Control (PRC)
• Appropriate distribution
of delay is calculated
and broadcast
• The distribution is
obtained by a
observation of channel
load
• PRC is equivalent to a
control of retransmission
offered load, Gr
time
success
fail
failsuccess
retransmissionmode
originatingmode
Transition of users between the originating mode and retransmission mode
G0 Gr
Hubstation
Delay
Control signal
Retransmission
Transmission
X
Throughput of CDMA ALOHA with PRC
0
1
2
3
4
5
6
0 5 10 15 20 25 30
N=60L=500 [bits]Eb/No=10 [dB]
Th
rou
gh
pu
tS
Offered load of new packet Go
PRC
Gr=5
Gr=Go
Gr=25
Gr=15
• Maximum
throughput is
the same as
one without
access control.
• Throughput is
almost same
as MCLSP
• Robustness
against access
timing delay
What is the optimum access control?
Channel Load Sensing
Protocol (CLSP)
• Higher throughput
• Weakness to access
timing delay
Modified CLSP (MCLSP)
• Robustness against
access timing delay
• No gain in maximum
throughput
Packet Retransmission
Control (PRC)
• Robustness against
access timing delay
• No gain in maximum
throughput
?
Optimal Access Control Protocol (OACP)
CLSP :
O Improvement in maximum
throughput
X Weakness against access
timing delay
PRC :
O Robust against access
timing delay
X No improvement in
maximum throughput
CLSP + PRC = Optimum
Hubstation
Retransmission
Transmissioncontrol signal
Retransmissioncontrol signal
Transmission
If packet error occurred
CLSP+PRC
OACP
• Hub-station estimate the
offered traffic load, G.
• If the channel load is below
a, then packet transmission
is allowed. Otherwise, the
users move into retrans-
mission mode (CLSP).
• Backlogged packet is
controlled according to the
retransmission probability
broadcast from the hub-
station (PRC).
Unsuccessful Packets
SuccessfulPackets
Yes
No
RetransmissionControl
New Packets Offered Load Go
RandomRetransmission
Delay
TramsmissionControl
Retransmitted PacketsTransmitted with Pre
CDMA UnslottedALOHA Channel
Is the channel loadless than a ?
Optimal Access Control Protocol (OACP)
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Th
rou
gh
pu
tS
N=60L=500 [bits]Eb/No=10 [dB]
Offered load of new packet Go
tD=0
tD=0.1tD=0.26 (LEO)
OACPCLSP
w/o Access Control
Throughput of CDMA ALOHA with PRC
• Maximum
throughput is
the same as
CLSP if
access timing
delay is
negligible
• Robustness
against access
timing delay
Delay performance of CDMA ALOHA with OACP
0
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30
tD=0=0.1=0.26=1=9.2
N=60L=500 [bits]Eb/No=10 [dB]K=1000
OACP
Offered load of new packet Go
Del
ayD
CDMA Unslotted ALOHA with Buffers
• Each user is equipped with
a certain size of queueing
buffers.
• Retransmission packet can
be managed by each of
users.
• Autonomous control of
packet transmission may be
possible.
CD
MA
ALO
HA
Cha
nnel
Hubstation
. . . .
CDMA ALOHA with Buffer
Busymode
Idle mode
p
Removal
Transmissionto the channel
Retransmission
Correctreception
Incorrectreception
p
Yes
No
Yes
NolBusy?
Rejection
Fullbuffer?
CDMA Unslotted ALOHA with Buffers
• Each user is equipped
with a finite buffer
capacity of B packets.
• Packet arriving at an idle
status is transmitted
immediately.
• Packets are served in a
first-in-first-out (FIFO)
discipline.
• Busy user attempt to
transmit packet with rate
p.
Schematic of packet flow
at each user station
Throughput of CDMA U-ALOHA with Buffers
• The larger buffer
size a user has,
the more rapidly
the throughput is
increasing.
• Throughput is
almost same as
MCLSP or PRC
0
1
2
3
4
5
6
0 5 10 15 20
N=60Eb/No=10dBK=100L=500p=0.1
Offered load G
Th
rou
gh
pu
tS
B=1 2 3 4 5
line:analysisdot:simulation
Rejection Probability
X If a packet
arrives at a user
with a full buffer,
this packet is
rejected
• The larger buffer
size a user has,
the less
rejection
probability is
10
1
0 5 10 15 20
-1
10-2
10-3
10-4
N=60Eb/No=10dBK=100L=500p=0.1
Rej
ecti
on
pro
bab
ility
QR
B=1 2 3 4 5
line:analysisdot:simulation
Offered load of new packet Go
Delay Performance
• The number of
busy user
increases by
increasing the
buffer size.
• Average delay
increases in
compensation for
reduction of the
rejection of packet
transmission.
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20
N=60Eb/No=10dBK=100L=500p=0.1
B=5
B=4
B=3
B=2
B=1
Ave
rag
e d
elay
D
line:analysisdot:simulation
Offered load of new packet Go
CDMA ALOHA for Multimedia signals
• Different media which have
different characteristics are
handled simultaneously
• CDMA is suitable for
handling multimedia signal
• Multi-rate CDMA
• Multi-code CDMA
1 Integrated voice and data
system
2 High and low bit rate data
transmission
User-Station
control signalsignal
data
data
voice
voicedata +voice
data +voice
Hubstation
Access control for
multimedia signals
1. Integrated Voice and Data System
Voice : Real time delivery
Voice users have to reserve the channel before they transmit their signals by sending a reservation packet.
Once they get the reservation, they continue to transmit their signal until voice call ends.
Data : Some tolerance to transmission delay
Circuit switch mode (Reservation mode)
Packet switch mode
Data users transmit thier packets on the CDMA Unslotted ALOHA .
If the number of simultaneous established users reaches to a certain threshold, voice users cannot access to get the reservation.
����
Channel load status seen
at hubstation
• Interference from both
media
• Priority of voice
medium
(continuous voice call
until call ends)
Necessity of traffic
control
Integrated CDMA Voice Signal &
CDMA Data Packet (CDMA unslotted ALOHA)
Number of server = a
time
.
.
.The channels reserved for voice users
The channels ocupied by data pakets
The available channels for additional voice and data users
System Model
HubStation
user 1 (voice)
PN sequence 2
user 2 (data)
user .. (data)
user .. (voice)
user K (voice)
CLSP Control Signal
Random signature
Data packet : CDMA Unslotted ALOHAVoice signal : Circuit Switch
��
Voice signal :
• Poisson generation
• Bit rate : R = 32k [bps]
• Exponential signal length
Length 60.0 [sec]
Silence period 1.7 [sec]
Talk spurt 1.0 [sec]
Data packet :
• Poisson generation
• Bit rate : R = 32k [bps]
• Fixed packet length
L = 500 [bit] (0.01 [sec])
• Total bandwidth
W = 20MHz
• Band expansion factor
N = 312 (=W/R)
The number of simultaneousvoice and data signals, k
a
0
b
•The required signal quality of voice < that of data packet
Why > a ?b
•The signal quality must be guaranteed for voice signals
Threshold of voice signala : b :
voice user can access to the channel
Data packet users can access to the channel
Threshold for data packet
����
Traffic Control for Voice and Data Media
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Threshold b =98 (Pb(b)<10-3)
with V.A
without V.A
Voice Offered Load, Gv
The
mea
n nu
mbe
r of
voi
ce c
alls N =312
Eb/N0= V.A rate = 0.4No data packet Gd=0
Mean Number of Voice Call
• Voice activity (V.A)
improves the
number of
simultaneous call.
• Access control by
counting the
number of talk spurt
of voice signal
Data Throughput
0
10
20
30
40
50
60
0 50 100 150 200 250 300
Consideration of V.A
Data offered load Gd
Dat
a th
roug
hput
S
No consideration of V.A
N =312Eb/N0= V.A rate = 0.4Erlang capacity for voiceEr = 40 (= Maxinum Gv)
0
8
16
24
32
40
48
56
64
0 40 80 120 160 240 280 320Data Offered Load, Gd
Thr
ough
put f
or D
ata
Gv=10
Gv=20
with V.A
no V.A
N =312Eb/N0= V.A rate = 0.4
Without access control With access control
2. High and Low Bit Rate Data Transmission
Hubstation
ControlSignal
Low bit rate
High bit rate
Class II
Class I
Class I : High-bit-rate packet
Class II: Low-bit-rate packet
Multi-rate CDMA system
O Multi-code
• Multi-processing gain
• Multi-modulation
Multi-codeCDMA
CDMAALOHA+
MC-CDMA ALOHA
Source
M.R b
M
Sub-stream (Sub-packet)
M Sub-packets
transmitted
in one sub-packet duration
Transmitter of Class I
Transmitter of Class II:
Conventional CDMA Slotted
ALOHA Transmitter
L [bits]b
Low bit rate
High bit rate
System Model
Class I :
• Poisson generation
• Bit rate MRb [bit/sec]
• Fixed packet length
MLb [bits]
Class II:
• Poisson generation
• Bit rate Rb [bit/sec]
• Fixed packet length
Lb [bits]
Class I has priority over
Class II
One sub-packet
One packetfrom class I (MC-CDMA)
One packetfrom class II (CDMA)
Slot Slot Slot Time
12
M
12
M
Channel Status at Hubstation
0
5
10
15
20
25
0 20 40 60 80 100
Offered load of class I, G1
¥
G2 =10b
Eb/N0 =N=128L =500 bits
Th
rou
gh
pu
t ,
SI
(Single bit rate)When M=1
SI
When M=8
When M=16
SI
SI
I
0
5
10
15
20
25
30
10 20 30 40 50 60 70 80 90 100
¥
G2 =10b
Eb/N0 =N=128L =500 bits
When M=8 with MCLSP
When M=8(without MCLSP)
To
tal t
hro
ug
hp
ut
, Sto
t = S
1 +
S2
Total offered load, Gtot = G1 + G2
S tot
S tot
When M=1(SIngle bit rate )
S tot
Throughput performances
MC-CDMA Slotted ALOHA
(Only class I user)
MC-CDMA Slotted ALOHA
with MCLSP (class I and II)
Conclusions
Fundamentals of CDMA ALOHA
• Throughput analysis of CDMA ALOHA
• CDMA ALOHA v.s. Narrow Band ALOHA
Access control techniques for CDMA ALOHA
• CLSP, MCLSP, PRC, OACP
• CDMA ALOHA with Buffers
Multimedia signal transmission using CDMA ALOHA
• Integrated voice and data system
• Multi-rate transmission using MC-CDMA
