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Packet service in UMTS: delay- throughput performance of the downlink shared channel Flaminio...
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Transcript of Packet service in UMTS: delay- throughput performance of the downlink shared channel Flaminio...
Packet service in UMTS: delay-throughput performance of the
downlink shared channel
Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta
2
1. Introduction
In the last ten years: IP applications has pushed the data traffic to grow
quickly 2G cellular systems have heavily changed the
way in which users access the network. The challenge of third generation mobile
communication systems is to provide access for a wide range of multimedia applications and services.
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1. Introduction
UMTS: 3G mobile communication system developed by ETSI, extend the present GSM service to include multimedia.
UMTS provides great flexibility and a variety of different physical and logical channel types. Several user rates and protections are possible
by choosing suitable parameters. More implementation complexity.
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2. UTRA basics (1/3) Two access scheme for the radio interface:
W-CDMA scheme 60MHz for downlink and 60MHz for uplink 3,84 Mchips/s, 5MHz for each channel, QPSK modulation
TD-CDMA scheme 35MHz for downlink and uplink
Physical channels are defined by the associated spreading and scrambling codes. Spreading sequence :
input data XORXOR spreading code XOR XOR scrambling code Spreading code of CDMA: channelization code Scrambling code of CDMA: PN code
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2. UTRA basics (2/3)
Transport channels: Dedicated channels (DCH)
Devoted to the connection between a single mobile station and the UTRA Network.
Mapped into two physical channel :DPDCH, DPCCH Common transport channel
Broadcast channel (BCH), paging channel (PCH) Random access channel (RACH), forward access
channel (FACH): control information or packet Common packet channel (CPCH), downlink shared
channel (DSCH) : packet only
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2. UTRA basics (3/3)
Power control DCH: TPC (Transmit power control) symbols in
each slot carry a command for increasing or decreasing
DSCH: computed on the basis of the power of the DCH
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3. Downlink packet data services (1/3)
Three channels for downlink direction DCH:
assigned to single users through set-up and tear down procedures, subject to closed loop power control and service such as voice.
DSCH No set-up, tear down procedure Doesn’t carry power control signaling, but must have an
associated active DCH FACH
Shared by many users to transmit short bursts of data No DCH must be activated
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3. Downlink packet data services (2/3)
For real-time circuit traffic—DCH Well known results show that CDMA with closed-l
oop power control is very effective.[14] Efficiency can be further enhanced by using powe
rful FEC codes. [15] For packet service—DSCH
Due to the burstiness, the number of interfering channels, its power level, errors can be more efficiently obviated by ARQ than FEC[16][17]
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3. Downlink packet data services (3/3)
it is interesting to investigate whether UMTS achieves the highest data throughput with circuit or packet switching technology For circuit switching, the additional of any further channel
beyond the capacity cannot be accepted since the BER will increase
For packet switching, occasional increases in BER over its target value can be to tolerated Because of the use of ARQ techniques
This paper provide a quantitative evaluation of the performance of the different alternatives.
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4. Simulator description (1/4)
Propagation model The received power Pr is given by
The path loss L is expressed as
Each cell is assigned a signal tree of orthogonal variable spreading factors, so that channels in the same cell are always orthogonal
2 1010r tP P L
10log (128.1 37.6 log )( )L r dB
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4. Simulator description (2/4)
Traffic model The performance of the UMTS downlink heavily
depends on the input traffic characteristics. Users become active according to a Poisson point
process of intensity λ
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4. Simulator description (3/4)
Receiver model Carrier to interference ratio:
Block error rate: Assumes an ideal ARQ procedure
When system operates far from capacity: Increase powerIncrease power on the same channel to maintain SIR RetransmittingRetransmitting the packets.
maximum interference tolerable is attained with a channel traffic G less than 1, and a further increase in retransmission would causes a strong decrease in throughput
int int
r
ra er N
PC
I I I P
1 (1 )lBLER BER
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4. Simulator description (4/4)
Power control model Closed loop power control mechanism
Inner loop: controls the transmitted power to maintain the SIR at target value
Outer loop: controls the SIR to provide a target BLER→provide different qualities to different services.
Each channel cannot exceed a transmitted power of 30 dBm, whereas the overall power transmitted by a BS is limited to 43 dBm. SIR Proportionally reduced
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5. Simulation results (1/4)
Effect of codes-1 Channel codes : Convolutional codes
The encoded bits depend not only on the current k input data bits but also on past input bits
Coding rate:
Decoding strategy for convolutional codes: based on Viterbi algorithm
input information bits
output encoded bitsR
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5. Simulation results (1/4)
Effect of codes-1
Light codes
Heavier code
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5. Simulation results (1/4)
Effect of codes-2
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5. Simulation results (1/4)
Effect of codes-3
Wrongly designed:
Require very low interference
Reach 1, but throughput is limited
G: 0.955~0.97
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5. Simulation results (2/4)
Effect of user traffic on downlink shared channel If the amount of information is lower than the space av
ailable, the efficiency is reduced due to the unused space.
Consider sources that generate an average number of packets Np in the range from 1 to 25…
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5. Simulation results (2/4)
Effect of user traffic on downlink shared channel
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5. Simulation results (2/4)
Effect of user traffic on downlink shared channel
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5. Simulation results (3/4)
Effect of control channels Too many users would reduce the system through
put. Limit the number of DCHs. (User arrived system is qu
eued and wait for DCH availability) Power control commands indicate changes in the
transmitted power level Both the power PDCH and PDSCH must change in the sa
me way. The SIR achieved after despreading on the two chann
els are related as:
, ,DSCH DSCH DCH DSCH
DCH DCH DSCH DCH
SIR SF I P CR R SIR SF
SIR SF I P I
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5. Simulation results (3/4)
Effect of control channels Interference generated by the related DSCH:
(loss-of-orthogonality factor = 0.4 [20])
Assume
0.4DSCHP
DSCH BI I;
( ) 0.4 0.4DSCHDCH DSCH DSCH DSCH
DSCH
SIRI P I
SF
0.4 1 0.4DSCH DSCHDCH B DSCH B DSCH DSCH
DSCH DSCH
SIR SIRI I I I I I
SF SF
;
0.4DSCH DSCH DSCH
DCH DCH
SIR SF SIRR
SIR SF
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5. Simulation results (3/4)
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5. Simulation results (3/4)
Effect of control channels Trade off: interference and multiplexing effect SF, coding rate, =>maximum user number
minimum delay
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5. Simulation results (4/4)
Effect of power control The closed-loop power control: introduced to incre
ase the system capacity The burstiness of data transmission may jeopardize t
he gain achieved. DCH introduce additional interference Thus DSCH is compared with FACH.
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5. Simulation results (4/4)
Effect of power control
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6. Conclusions (1/2)
Present some preliminary results obtained by simulation on the delay-throughput curves Focus on the system parameters and channel
configurations Closed-loop power control mechanism
SIR increases as the speed of the physical channel increases
It has been verified the mechanism is very efficient even with low interference protection
If multiple physical channels is allowed Intra-cell interference is improved by the closed-loop power
control mechanism
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6. Conclusions (2/3)
When the system operates with the highest bearable interference A backoff mechanism is crucial to let the system operate cl
ose to capacity. The use of DCH for power control with DSCH may lead to i
nstability if the number of DCH is not limited.
If low speed users are served A great reduction of capacity may be observed because of
the minimum unit that a single user may use The reduced frame filling degree does not reduce the interf
erence, but yielding a net decrease in throughput
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6. Conclusions (3/3)
In spite of the several limitations of packet switching, due to its intrinsic flexibility, better adapts to interference limited systems than circuit switching.
30
Reference
[14]. Erlang Capacity of a power controlled CDMA system (1993)
[15]. Throughput analysis for Code Division Multiple Access of the Spread Spectrum Channel (1984)
[16]. Channels with block interference
[17]. Retransmissions versus FEC plus interleaving for real-time applications: a comparison between CDMA and MCD-TDMA cellular systems (1999)
[20].3rd Generation Partnership Project, RF system scenarios, 3G TR 25.942, Dic. 1999
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Normalized energy per information:0
1
2bE C
SFN R I