Transcript Floating Point Analysis - University of Ioannina

Performance of VoIP Services over
3GPP WCDMA Networks
Ozcan Ozturk
Qualcomm
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WCDMA Voice Evolution
• The goal is to compare the WCDMA system capacity for Release 99 voice and
Release 6 and 7VoIP by using simulations.
• Only the radio link performance is considered here.
• Release 99 uses Circuit Switched (CS) Voice (Dedicated Channels)
• Release 5-6 provides new features for VoIP
– Smaller TTIs and H-ARQ on both Downlink (DL) and Uplink (UL)
– Lower overhead on DL (F-DPCH instead of A-DPCH)
• Release 7 introduces additional features for VoIP efficiency
– CPC (DTX/DRX): Allows transmitting the pilot only during packet activity
– Enhanced F-DPCH: Reduces code and power usage on DL even further
– HS-SCCH’less mode: Provides savings in HS-SCCH code and power
• VoIP capacity is UL limited in most scenarios. UL Interference Cancellation
(IC) provides significant capacity gain by enabling the system operate at higher
interference levels.
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Simulation Framework
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Simulation model is based on 3GPP TR 25.896 specifications
57 hexagonal cells (19 Node Bs) with wrap-around
Inter-Site-Distance: 1000m
Users (UEs) are dropped uniformly in each cell
Each UE is assigned a channel of PA3, PB3,VA30,VA120 with probabilities
0.3,0.3,0.2,0.2, respectively.
TTI: 2ms [10ms], max number of transmission attempts: 5 [2]
Number of HARQ interlaces: 8 for TTI=2ms, 4 for TTI=10ms
Rake receiver with dual receiver diversity at Node B.
Both Rake and Equalizer w/ and w/o receiver diversity at the UE
DTX Cycle: 8 TTIs for TTI=2ms and 2TTIs for TTI=10ms
• HSDPA Power Overhead: 30%
• CQI Feedback Cycle : 16ms
• Channel estimation errors are modeled
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AMR 12.2 VoIP Traffic Source Model
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Exponential ON-OFF with 50% Voice Activity
3 seconds of average Talk Spurt and Silence durations
A Full Frame (244 bits) is generated every 20ms during Talk Spurt
A SID Frame (39 bits) is generated every 160ms during Silence
Header Overheads
– 12 bits RTP payload header
– 3 bytes of ROHC (RTP/UDP/IP) header
– 1 byte of RLC header
• Transmitted Packet Sizes (with MAC headers, padding and CRC)
– Full Frame
• HSUPA 2ms: 331 bits
• HSUPA 10ms: 341 bits
• HSDPA: 341 bits
– SID Frame
• HSUPA: 144 bits
• HSDPA: 161 bits
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System Capacity Criteria
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One way Radio Link delay bound: 100 ms
– VoIP frames delayed in excess of this bound are dropped at both Node B and UE
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Downlink capacity
– Capacity defined as max number of users where users in outage are less than 5%
– Outage defined by UE’s vocoder frame error rate above 3%
• Packet discard at Node B due to delay (100 ms)
• Late packet arrival to UE (i.e., beyond 100ms)
• Physical layer packet errors
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Uplink capacity
– Noise Rise outage happens when the Noise Rise over 7dB is more than 1%
– Frame Error Rate (FER) outage happens when more than 5% UEs have more than
3% FER due to physical layer errors or delay bound drops
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Overall system capacity determination
– The minimum of Downlink and Uplink capacities
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Uplink Interference Cancellation (IC)
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The amount of energy to be cancelled is determined by the Ec/Nt measured.
At every slot, the pilot energy is canceled for each user.
At every TTI, HSDPCCH is canceled if active.
At every TTI, a decoding attempt is made for a continuing packet transmission.
If successful, the energy of EDPDCH and EDPCCH over the HARQ interlace is
subtracted from the cell Io immediately (also from the other cells in the same
Node B if there is softer-handoff).
• Users are grouped according to the retransmission attempts. Each group is
decoded and cancelled together (using the same waveform).
• The groups are visited in the decreasing order of retransmission attempts and
this is repeated twice.
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System Capacity Comparison
175%
200
190
180
160
97%
136
140
120
49%
100
80
103
69
60
40
20
0
Release 99
Release 6
Release 7
Release 7 + IC
• Rx Diversity assumed
• Release 6 and 7 use 2ms UL TTI
• Release 6 uses FDPCH
• Release 7 uses DTX, E-FDPCH, HS-SCCH-Less
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Release 99 Voice Capacity
AMR
12.2
1Rx
Rake
Downlink
1Rx
2Rx Rake and Uplink
Equalizer Equalizer
51
63
70
69
• The system is power limited on the DL with single Rx
antenna and DL is the bottleneck for the capacity
• Due to the code limitation with receive diversity, the Rake
and Equalizer capacities are equal on DL.
• Equalizer gain is not significant since the spreading factor
is large (SF=128)
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Release 6 and 7 Uplink VoIP Capacity
Rel. 6
TTI=2ms
103
Rel. 6
w/ IC
136
TTI=10ms
95
126
Rel. 7
136
Rel. 7
w/ IC
190
106
142
• In all cases, the system is interference limited
• VoIP capacity is higher for TTI of 2ms than 10ms. The
difference is even more for Release 7 where most of the
gain of the 2ms over 10ms comes from DTX.
• IC provides more than 30% capacity gain
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Release 6 and 7 Downlink VoIP Capacity
Rel. 6
Rake
Rel. 7
Rake
Rel. 7
Equalizer
1 Rx
61
65
83
2 Rx
101
172
232
• Receiver diversity provides significant power savings which translates to
capacity gain.
• The gain of Rel. 7 over Rel. 6 is limited for 1 Rx. This is mostly due to
PA3 which is a single path fading channel. The gain of equalizer in this
case is limited too.
• For both capacity and coverage improvements, receiver diversity is the
preferred option with VoIP.
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Packet Delay
CDF of 98th percentile delay for 190 UEs/cell
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• The figure shows the combined
uplink and downlink over the air
delay at the system capacity.
• This was obtained by summing
up the uplink and downlink
delays.
• Most of the delay happens on the
uplink
0.9
0.8
0.7
CDF
0.6
0.5
0.4
0.3
0.2
0.1
0
60
70
80
90
100
110
Combined uplink and downlink packet delay
120
130
The 98% of the total delay is plotted to capture the effect of the
de-jitter buffer. The graph shows that the total radio-link delay will
satisfy the ITU recommendations for a satisfactory voice call.
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Mixed Data and Voice
Downlink BE Application Layer Throughput vs VoIP Load
• BE traffic is generated by
Full Buffer source.
5000000
RAKE, Rx Div, A-DPCH,
4500000
Throuhgput BPS
Equalizer, Rx Div, A-DPCH,
4000000
RAKE, Rx Div, F-DPCH,
3500000
Equalizer, Rx Div, F-DPCH,
Release 99 Voice
3000000
2500000
2000000
1500000
1000000
• Scheduler is Proportional
Fair for BE.
• Scheduler gives higher
priority to delayed VoIP
users.
• A-DPCH code and power
consumption reduces the BE
throughput.
500000
0
0
20
40
60
80
100 120 140 160 180 200 220 240
VoIP users
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Conclusions
• 3GPP Release 6 and 7 VoIP provides significant capacity gains over Release 99
Voice
• Uplink IC improves the system capacity and performance even further.
• At the VoIP capacity limited by the uplink, substantial best effort traffic can be
served on the downlink.
• On the uplink, the capacity improvements of VoIP are mainly due to:
– Time diversity gain by the use of HARQ
– DTX features which reduce the total transmitted power and interference
– Interference Cancellation
• On the downlink, the high VoIP capacities obtained in Release 7 can be
attributed mainly to the use of:
– Delay and channel sensitive MAC-hs scheduling and resource allocation
– The use of F-DPCH channel
– The presence of advanced receivers (diversity, equalizers) at the UE
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