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Performance of VoIP Services over 3GPP WCDMA Networks

Performance of VoIP Services over 3GPP WCDMA Networks. Ozcan Ozturk Qualcomm. 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.

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Performance of VoIP Services over 3GPP WCDMA Networks

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  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 99uses 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.

  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 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

  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 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 • 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 (SF=128)

  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 savingswhich 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

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