Performance of VoIP Services over 3GPP WCDMA Networks

1
Performance of VoIP Services over 3GPP WCDMA
Networks

• Ozcan Ozturk
• Qualcomm

2
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-SCCHless 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.

3
Simulation Framework

• 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 TTI2ms, 4 for
  TTI10ms
• 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 TTI2ms and 2TTIs for
  TTI10ms
• HSDPA Power Overhead 30
• CQI Feedback Cycle 16ms
• Channel estimation errors are modeled

4
AMR 12.2 VoIP Traffic Source Model

• 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

5
System Capacity Criteria

• One way Radio Link delay bound 100 ms
• VoIP frames delayed in excess of this bound are
  dropped at both Node B and UE
• Downlink capacity
• Capacity defined as max number of users where
  users in outage are less than 5
• Outage defined by UEs 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
• 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
• Overall system capacity determination
• The minimum of Downlink and Uplink capacities

6
Uplink Interference Cancellation (IC)

• 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.

7
System Capacity Comparison
175

• Rx Diversity assumed
• Release 6 and 7 use 2ms UL TTI
• Release 6 uses FDPCH
• Release 7 uses DTX, E-FDPCH, HS-SCCH-Less

8
Release 99 Voice Capacity

• 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 (SF128)

9
Release 6 and 7 Uplink VoIP Capacity

• 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

10
Release 6 and 7 Downlink VoIP Capacity

• 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.

11
Packet Delay

• 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

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.
12
Mixed Data and Voice

• BE traffic is generated by Full Buffer source.
• Scheduler is Proportional Fair for BE.
• Scheduler gives higher priority to delayed VoIP
  users.
• A-DPCH code and power consumption reduces the BE
  throughput.

13
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