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Cristina Comaniciu and Narayan Mandayam
W INLAB
Rutgers University
DS-CDMA, bandwidth W
Voice rates R v
Data rates Rd
Intolerant to delays
Tolerant to moderate delays
Slotted reverse link
Can tolerate occasional errors
Error intolerant
Normalized Load
time (packet transmission scale)
residual capacity
ON/OFF voice activity model
Normalized Load
time (call arrival scale)
Resources used by voice users
Resources available for data users1 1
ll CDMA Systems are interference limited
1Outage Probability %1≤
Minimum data ratethroughput for data users
Average data ratethroughput for data users
System capacitymaximization
n Imperfections in estimating theresidual capacity
n Imperfections in scheduling thedesired number of data users
n Imperfections in power control
Goal: Elim inate first listed cause foroutage!
n Define outage condition (maps FERrequirements)
n Derive ideal residual capacity expression
n Obtain an estimate for the number of data userswhich are allowed to transmit in the next time slot
n Employ an access procedure to allow the exactpredeterm ined number of data users to transmitin the next time slot
Perfect Power Control - Power Control Feasibility condition:
S(n) = normalized load in the nth time slot;
v(n) and d(n)= # of active voice and data users;
γ i = target SIR for voice/data;
Outage: when
( ) ( ) ( )d−≤+= 1
dv and
anv
nS
1+=
iii R
Wa
g 001.0=d
( ) 1>nS
( ) ( )( )1ˆ −= nvfuncnd
n d(n) = max # of data users in the system s.t.
n Problem: v(n) - random variable d(n) - randomvariable only an estimatecan be derived
n - integer number only data users
can be scheduled for the next time slot
d−≤ 1)(nS
⇒⇒
⇒)(ˆ nd ( ) nd̂
( ) ( ) ( )nvaa
andv
dd −−= d1
NormalizedLoad
residual capacity
1
n Accuracy of approximation:
– sampling frequency fS
– step of modulation ∆∆
n Two types of distortions:
– granular noise
– slope overload distortion
Condition for no slope
overload distortion:
sfts •∆≤•
)(
( )nsns ≤)(ˆ
Steps of MDM algorithm: - first time slot: measure number of active voice usersand initialize
- for : if , then set
if , then set
if , then set
( ) ( )11ˆtt dd =
1>n ( ) ( )11ˆ −>− ndnd tt ( ) ( ) ∆−−= 1ˆˆ ndnd tt
( ) ( )11ˆ −<− ndnd tt ( ) ( ) ∆+−= 1ˆˆ ndnd tt
( ) ( )11ˆ −=− ndnd tt ( ) ( )1ˆˆ −= ndnd tt
Introduce guard margin = step of the modulation (∆)− the function to be approximated becomes:
( ) ( ) ∆−= ndndt
n no slope overload condition
n Max. # of data users allowable in the system a t each timeslot
n cumulative voice activity process - discrete Markov chain(the slot duration very small such that at most change inthe number of active voice users can occur)
n choose
( ) ( ) ( )nvaa
andv
dd −−= d1
1±
v
d
aa
=∆
sfts •∆≤•
)(
n MDM-S (Modified Delta Modulation with Scheduled Access)
– exactly users are scheduled to transm it in thenext time slot in a round-robin fashion
– no outage due to the access control
n MDM-R (Modified Delta Modulation w ith Random Access)
– access probability (ML estimate) - max. prob. thatexactly users will transm it in the next time s lot
– a probability tolerance value is defined to allow the finetuning of system performance
( ) nd̂
( ) nd̂
n Fixed # of adm itted data calls (Kd):
n SMS: Poisson arrival for data with rate :
– rate of adm itted data traffic:
( )( )[ ]
d
p
K
tndnp
−=
ˆ
( )( )[ ]
r
ptndnp
−=
ˆ
( ) ( ) ( )[ ] pa tndnpn −== ˆrr
r
Outage Condition: limited interference
Z(n) = load; and voice and data activity indicators
, = SIRs for voice/data;
= maximum allowed received PSD to background noise PSD
( ) ( ) ( ){ } ( ) ( )( )
01.0var
1,|1)(),(| ≤
−−=−>=
ZZE
QndnvZPP ndnvouth
h
( ) ( ) ( ) ( )hezen −≤+= ∑∑==
1 1
1
00
di
K
ii
d
vi
K
ii
v
dv
nG
nG
nZ
vie d
ie
1
h
( )nin ( )niz
n Two access control schemes are proposed and theperformance is compared with earlier work (W INLAB TR -127, TR-143)
u Both schemes give better performance
u MDM-R - very efficient implementation
u MDM-S - guarantees zero outage for the perfect powercontrol case
n Two data models are considered in the analysis:
u Non-transparent data service (ftp, mail, store andforward facsim ile) - W INLAB TR- 172
u Short Message Service (Poisson arrivals for data; onlyopen loop power control for data) - W INLAB TR inpreparation