High-Throughput Enhancements for 802.11: Features and Performance of QUALCOMM’s Proposal

1. High-Throughput Enhancements for 802.11: Features and Performance of QUALCOMM’s Proposal John Ketchum, Sanjiv Nanda, Rod Walton, Steve Howard, Mark Wallace, Bjorn Bjerke, Irina Medvedev, Santosh Abraham, Arnaud Meylan, Shravan SurineniQUALCOMM, Incorporated9 Damonmill Square, Suite 2AConcord, MA 01742Phone: 781-276-0915Fax: 781-276-0901johnk@qualcomm.com John Ketchum, et al, QUALCOMM

2. Agenda • Introductory remarks • MAC Features • System Performance • PHY Features • Link Performance John Ketchum, et al, QUALCOMM

3. Qualcomm’s Status Assessment • Submitted proposals contain the basis for an excellent solution to HT requirements • Substantial commonality in proposed approaches • MIMO OFDM • Advanced coding • Frame aggregation • Elimination of IFS • Qualcomm is committed to working with TGn to establish rapid convergence to a draft standard • Future proof • Optimized performance/complexity tradeoff • Critical Issues • Informed transmitter operation • Low-overhead feedback • Flexible rates • Minimal feature set for support of low-latency operation John Ketchum, et al, QUALCOMM

4. Main Points • 20 MHz operation • Maximum PHY data rates: • 202 Mbps for stations with two antennas • 404 Mbps for stations with four antennas • Backward compatible modulation, coding and interleaving • Highly reliable, high-performance operation with existing 802.11 convolutional codes used in combination with Eigenvector Steering spatial multiplexing techniques • Fall-back to robust Spatial Spreading waveform for uninformed transmitter • Backward compatible preamble and PLCP with extended SIGNAL field. • Adaptation of rates and spatial multiplexing mode through low overhead asynchronous feedback. Works with TXOPs obtained through EDCA, HCF or ACF. • PHY techniques proven in FPGA-based prototype John Ketchum, et al, QUALCOMM

5. MAC Design Objectives • Objectives • Preserve the simplicity and robustness of distributed coordination • Backward compatible • Enhancements for high throughput, low latency operation • Build on 802.11e, 802.11h feature set: • TXOPs, • Block Ack, Delayed Block Ack, • Direct Link Protocol • Dynamic Frequency Selection • Transmit Power Control John Ketchum, et al, QUALCOMM

6. MAC Feature Summary • Frame aggregation • Length field per encapsulated frame • Max aggregate size is negotiated per flow. • Eliminate immediate ACK • No Immediate ACK for Aggregate Frames in Scheduled or Polled TXOPs • Block Ack Request (BAR) frame does not require Immediate ACK, except in EDCA TXOPs. • Aggregation for multiple STAs • Only AP is permitted to transmit SCHED and establish SCAP • PPDU Aggregation • SCHED: Message indicates Tx and Rx STA, start offset and duration for scheduled TXOPs. Complete information for optimum sleep mode. • Reduced IFS • No IFS for consecutive scheduled transmissions from AP • BIFS (10 us) for consecutive scheduled transmissions from STA • GIFS (800 ns): Guard Period for consecutive scheduled transmissions from multiple STAs. John Ketchum, et al, QUALCOMM

7. MAC Feature Summary • Backward compatible SIGNAL • RATE/Type Field. Overload the legacy RATE field • Legacy STAs will abandon further decoding of the PPDU on seeing unrecognized RATE field • Protection mechanisms • Long NAV • RTS/CTS • Rate Feedback • Extension of SERVICE field • MIMO Training Request and MIMO Rate Feedback Request is implicit. Always included in PPDU • MAC header compression • Compressed Header Formats: Eliminate, TA, RA, Duration/ID fields • Compressed block ACK • Three formats defined • Transmitter option • RRBSS • QoS-capable IBSS with round-robin scheduling John Ketchum, et al, QUALCOMM

8. System Simulation Methodology • The simulator is based on ns2 • Includes physical layer features • TGn Channel Models • PHY Abstraction determines frame loss events • MAC features • EDCA • Adaptive Coordination Function (ACF): SCHED and SCAP • Frame Aggregation • ARQ with Block Ack • Closed Loop Rate Control (DRVF and DRV) • MIMO Modes (ES and SS) • Transport • File Transfer mapped to TCP • QoS Flows mapped to UDP John Ketchum, et al, QUALCOMM

9. Simulation Conditions – Fixed • The following parameters are fixed for all system simulation results. • Bandwidth: 20 MHz. • Frame Aggregation • Fragmentation Threshold: 100 kB • Delayed Block Ack • Adaptive Rate Control • Adaptive Mode Control between ES and SS John Ketchum, et al, QUALCOMM

10. Simulation Conditions – Varied The following parameters are varied. Results are provided for different combinations of these parameters. • Bands: • 2.4 GHz • 5.25 GHz • MIMO: • 2x2: All STAs with 2 antennas • 4x4: All STAs with 4 antennas • Mixed: • Scenario 1: the AP and the HDTV/SDTV displays are assumed to have 4 antennas; all other STAs have 2 antennas. • Scenario 6: AP and all STAs, except VoIP terminals have 4 antennas; VoIP terminals have 2 antennas. • OFDM symbols • Standard: 0.8 μs Guard Interval, 48 data subcarriers • SGI-EXP: 0.4 μs Shortened Guard Interval, 52 data subcarriers • Access Mechanisms • ACF (SCHED) • HCF (Poll) • EDCA with additional AC for Block Ack John Ketchum, et al, QUALCOMM

11. Additional Scenarios • Scenario 1 HT is an extension of Scenario 1: • Additonal FTP flow of up to 130 Mbps at 15.6 m from the AP for 2×2. • Scenario 1 EXT is an extension of Scenario 1: • Additonal FTP flow of up to 130 Mbps at 15.6 m from the AP for 2×2. • Maximum delay requirement for all video/audio streaming flows is decreased from 100/200 ms to 50 ms. • Two HDTV flows are moved from 5 m from the AP, to 25 m from the AP. • Scenario 6 EXT is an extension of Scenario 6: • One FTP flow of 2 Mbps at 31.1 m from the AP is increased up to 80 Mbps for 4x4. John Ketchum, et al, QUALCOMM

12. Summary of Total Throughput Results • Over 100 Mbps BSS throughput in realistic scenarios John Ketchum, et al, QUALCOMM

13. Observations on Total Throughput • ACF provides highest total throughput compared to HCF and EDCA. • ACF satisfies all QoS flows for all Sceanrios when SGI-EXP symbols are used. • Only in the case standard symbols are used (giving reduced throughput) at 5.25 GHz (giving reduced range), the PLR requirement of gaming flows is not satisfied. • No increase in throughput for EDCA with 4x4 compared to 2x2. • Scenario 1 EXT imposes stringent delay (less than 50 ms for streaming) and range (HDTV flows at 25 m) requirements on QoS flows. • When 2x2 is used, one or two QoS flows are not satisfied. • In the Mixed case, by equipping the AP and the HDTV and SDTV displays with 4 antennas, all QoS flows except the gaming flow are satisfied with an almost 50% increase in total throughput compared to 2x2. • In Scenario 4, throughput achieved is over 100 Mbps with 2x2 and almost 200 Mbps with 4x4. • Scenario 6 EXT Mixed case (mixture of 4-antenna and 2-antenna STAs) gives higher TCP throughput than the 4x4 case. • This is because there is more time available for TCP flows due to the reduced training sequence overhead for VoIP STAs with 2 antennas compared to VoIP STAs with 4 antennas. Sceanrio 6 EXT has 30 VoIP flows. John Ketchum, et al, QUALCOMM

14. Summary of MAC Efficiency Results • As defined, MAC Efficiency is meaningful only when the offered load for a scenario exceeds the carried load and there is always backlogged traffic at some flow. In the above table, the MAC Efficiency numbers are shown in red for the cases where the medium is forced idle due to no backlog. These numbers are not meaningful. John Ketchum, et al, QUALCOMM

15. Observations on MAC Efficiency • For 2x2, the MAC Efficiency for ACF is between 0.65-0.7. • For 2x2, the MAC Efficiency for HCF and EDCA is around 0.5. • For 4x4, the MAC Efficiency for HCF and EDCA reduces to 0.4 and 0.2, respectively. ACF manages to sustain a MAC Efficiency around 0.6, even with 4x4. John Ketchum, et al, QUALCOMM

16. Summary of QoS Flows Satisfied John Ketchum, et al, QUALCOMM

17. Observations on QoS Flows • Scenario 1 EXT imposes stringent delay (less than 50 ms for streaming) and range (HDTV flows at 25 m) requirements on QoS flows. • When 2x2 is used, one or two QoS flows are not satisfied. • In the Mixed case, by equipping the AP and the HDTV and SDTV displays with 4 antennas, all QoS flows except the gaming flow are satisfied with an almost 50% increase in total throughput compared to 2x2. • More QoS flows are satisfied with HCF than with EDCA. However, ACF is required to address stringent QoS requirements. • QoS for uplink EDCA VoIP flows is not satisfied. • All QoS Flows are satisfied for Scenario 4. John Ketchum, et al, QUALCOMM

18. Summary of non-QoS Flow Throughput • Scenario 6 EXT Mixed case (mixture of 4-antenna and 2-antenna STAs) gives higher TCP throughput than the 4x4 case. • This is because there is more time available for TCP flows due to the reduced training sequence overhead for VoIP STAs with 2 antennas compared to VoIP STAs with 4 antennas. Sceanrio 6 EXT has 30 VoIP flows. John Ketchum, et al, QUALCOMM

19. Throughput versus Range for Channel Model B John Ketchum, et al, QUALCOMM

20. Throughput versus Range for Channel Model D John Ketchum, et al, QUALCOMM

21. Observations on Throughput versus Range • The plots for Channel Model B and Channel Model D are roughly similar. • Throughput above the MAC of 100 Mbps is achieved at: • 29 m for 2x2, 5.25 GHz • 40 m for 2x2, 2.4 GHz • 47 m for 4x4, 5.25 GHz • 75 m for 4x4, 2.4 GHz John Ketchum, et al, QUALCOMM

22. Qualcomm 802.11n PHY Design • Fully backward compatible with 802.11a/b/g • 20 MHz bandwidth with 802.11a/b/g spectral mask • OFDM based on 802.11a waveform • Optional expanded OFDM symbol (4 add’l data subcarriers) and shortened guard interval • Modulation, coding, interleaving based on 802.11a • Expanded rate set • Scalable MIMO architecture • Supports a maximum of 4 wideband spatial streams • Two forms of spatial processing • Eigenvector Steering (ES): via wideband spatial modes/SVD per subcarrier • Tx and Rx steering • Over the air calibration procedure required • Spatial Spreading (SS): modulation and coding per wideband spatial channel • No calibration required • SNR per wideband spatial stream known at Tx • Use of Eigenvector steering extends the life of low-complexity 802.11 BCC • Sustained high rate operation possible via rate adaptation • low overhead asynchronous feedback. • PHY techniques proven in FPGA-based prototype John Ketchum, et al, QUALCOMM

23. Code Rates and Modulation Notes: 1) short OFDM symbols; 2) expanded OFDM symbols with short guard interval John Ketchum, et al, QUALCOMM

24. Spatial Processing • Two forms of Spatial Processing for data transmission • Eigenvector Steering (ES): Tx attempts to steer optimally to intended Rx • Spatial Spreading (SS): Tx does not attempt to steer optimally to specific Rx • ES operating modes take advantage of channel reciprocity inherent in TDD systems • Full MIMO channel characterization required at Tx • Calibration procedure required • Tx steering using per-bin channel eigenvectors from SVD • Rx steering renders multiple Tx streams orthogonal at receiver, allowing transmission of multiple independent spatial streams • This approach maximizes data rate and range • SS Operation for partially informed transmitter • No explicit knowledge of channel or channel eigenvectors at Tx • Tx has only data rate per wideband spatial channel • Transmit full power regardless of the number of streams Tx’d • Requirement for robust CSMA operation • Maximize diversity per transmitted data stream • Minimize outage probability  maximize throughput • Backwards compatible operation • Spatial spreading of data with simple unitary matrices • Cyclic diversity transmission per Tx antenna to introduce additional diversity John Ketchum, et al, QUALCOMM

25. Spatial Channels and Spatial Streams • ES and SS approaches result in synthesis of spatial channels, or wideband spatial channels. • Also referred to as eigenmodes, or wideband eigenmodes • On MIMO channel between a transmitting STA with NTx antennas and a receiving STA with NRx antennas, maximum of wideband spatial channels available. • Each resulting spatial channel may carry a payload, referred to as a spatial stream. • Number of spatial streams, NS, may not be greater than the Nm John Ketchum, et al, QUALCOMM

26. Over-the-Air Calibration • ES approach requires over-the-air calibration procedure • Compensates for amplitude and phase differences in Tx and Rx chains • Calibration required infrequently– typically on association only • Simple exchange of calibration symbols and measurement information requires little overhead and background processing • Total of ~1000 bytes of calibration data exchanged for 2x2 link • ~2800 bytes for 4x4 link John Ketchum, et al, QUALCOMM

27. Preamble and Training Sequences • Use Standard 802.11a preamble with enhancements • Time and Frequency acquisition and AGC • Last short preamble symbol is inverted to provide improved timing reference • Cyclic delay is applied across Tx antennas • Cyclic delay applied to entire 8 µs short preamble • Cyclic delay applied to entire 8 µs long preamble • Delay increment Tcd=50 ns • Extended SIGNAL field • MIMO training sequence • Orthonormal (in time) cover sequence • Walsh for 2 Tx and 4 Tx • Fourier for 3 Tx • Cyclic shift k*50 ns on Tx antenna k • Combination of orthonormal cover and cyclic shift ensures equal Rx power on all preamble symbols • Number of OFDM symbols = Number of Tx antennas • Supports steered MIMO training for Eigensteered operation John Ketchum, et al, QUALCOMM

28. Feedback for ES and SS Modes • Rate adaptation • Receiving STA determines preferred rates on each of up to four wideband spatial channels • One rate per wideband spatial channel – NO adaptive bit loading • Sends one 4-bit value per spatial channel, differentially encoded, (13 bits total) to inform corresponding STA/AP of rate selections • Corresponding STA/AP uses this info to select Tx rates • Piggy-backed on out-going PPDUs • SS Mode can use single rate across all spatial streams • Channel state information • For ES operation, Tx must have full channel state information • This is obtained through exchange of transmitted training sequences that are part of PLCP header • Very low overhead. • Distributed computation of steering vectors (SVD calculation) • STAs do SVD, send resulting training sequence to AP • For SS operation, unsteered training sequences transmitted in PLCP header to support channel estimation at receiver • Feedback operates with asynchronous MAC transmissions John Ketchum, et al, QUALCOMM

29. Wideband Eigenmodes and OFDM • OFDM chosen so that subcarrier spacing << coherence bandwidth • Find ranked eigenmodes/eigenvalues in each OFDM subcarrier: • Ensemble of eigenmodes of a given rank across OFDM symbol comprise a wideband eigenmode • Highest ranked wideband eigenmodes exhibit very little frequency selectivity • Smallest ranked wideband eigenmode exhibits frequency selectivity of underlying channel John Ketchum, et al, QUALCOMM

30. Wideband Eigenmodes TGn Channel B Power is relative to average total receive power at a single antenna John Ketchum, et al, QUALCOMM

31. Use of Reference for Eigensteering • STAs must be calibrated to use Tx steering • MIMO training sequence part of PLCP preamble for all PPDUs • STA can compute Tx and Rx steering vectors from either steered MIMO training sequence or direct MIMO training sequence • If unsteered MIMO training sequence is used, SVD or similar is required to compute steering vectors from direct channel estimate • One STA in a corresponding pair must compute SVD from direct channel estimate • STA that does SVD sends steered MIMO training sequence in PLCP preamble of PPDU with steered data. Receiving STA uses steered MIMO training sequence to compute Rx and Tx steering • STA not computing SVD must send direct MIMO training sequence to STA computing SVD • Can be part of broadcast message such as SCHED at AP • Can be MIMO training sequence in PLCP preamble • Support of bi-directional steering with SVD calculation distributed to client STAs • Off-loads SVD from AP • Minimal complexity hit to STA John Ketchum, et al, QUALCOMM

32. Simulation of Spatial Multiplexing Using Tx & Rx Eigensteering • Common MIMO Training Sequence broadcast by AP once every SCAP (Scheduled Access Period) (…,t0,t3,…). Forward link (FL) channel coefficients estimated by STA receiver • FL Dedicated MIMO Training Sequence (steered) transmitted by AP at t1=0.5 ms, immediately followed by FL data PPDU • Reverse link (RL) Dedicated MIMO Training Sequence transmitted by STA at t2=1.5 ms, immediately followed by RL data PPDU • Transmit and receive steering vectors derived from most recent channel estimates • Closed-loop rate adaptation: FL and RL data rates determined based on receive SNRs observed in previous frames John Ketchum, et al, QUALCOMM

33. Simulation Parameters • 2x2, 4x2, and 4x4 system configurations • IEEE 802.11n channel models B, D and E • IEEE 802.11n impairment models: • Time-domain channel simulator with 5x oversampling rate (Ts=10 ns) • Rapp nonlinear power amplifier model (IM1): • Total Tx power = 17 dBm; Psat = 25 dBm • 2x2 backoff = 11 dB per PA; 4x4 backoff = 14 dB per PA • Carrier frequency offset : -13.675 PPM (IM2) • Sampling clock frequency offset: -13.675 PPM (IM2) • Phase noise at both transmitter and receiver (IM4) • 100 channel realizations generated for each SNR point • In each channel realization the Doppler process evolves over three SCAPs to allow simulation of channel estimation, closed-loop rate adaptation and FL/RL data transmission in fading conditions • Stopping criterion: 10 packet errors or 400 packets transmitted per channel realization • Targeted packet error rate performance: mean PER <= 1% John Ketchum, et al, QUALCOMM

34. PHY Simulation Results • What we simulated • Standard OFDM symbols • Eigenvector Steering • Spatial Spreading • Expanded OFDM symbols (52 data tones/400ns guard interval: SGI-52) • Eigenvector Steering • Spatial Spreading • PER vs SNR for Fourier channel 1×1, 2×2, 3×3, and 4×4 (CC59) • All above cases • PHY throughput and PER vs SNR; CDFs of throughput and PER • Standard OFDM symbols, ES & SS • 2×2, 4×4, and 4×2 • Channels B, D, and E • SGI-52 OFDM symbols, ES & SS • 2×2, 4×4, and 4×2 • Channel B John Ketchum, et al, QUALCOMM

35. PHY Simulation Results (2) • Average PER vs SNR • Standard OFDM symbols, ES & SS • 2×2, 4×4, and 4×2 • Channels B, D, and E John Ketchum, et al, QUALCOMM

36. Highlights of PHY Simulation Results • Highest PHY throughputs achieved in Eigenvector Steering mode • Eigenvector steering is very effective in combination with 802.11 convolutional codes • 256-QAM contributes substantially to throughput in ES mode. ES array gain overcomes effects of receiver impairments in these cases • Convolutional codes not as effective in Spatial Spreading mode • High SNR variance across subcarriers within an OFDM symbol on an SS spatial channel degrades the performance of convolutional codes • This is particularly pronounced on channel B and on link with 4 Tx and 2 Rx. • Reducing number of streams (NS < min(NTx,NRx)) reduces variance and improves overall performance. • Rate adaptation has clearly demonstrated benefits • Many cases where a given fixed rate has poor performance, but using rate adaptation, higher overall throughput is achieved with lower PER • Part of rate adaptation is controlling the number of streams used John Ketchum, et al, QUALCOMM

37. Highlights of PHY Simulation Results • Use of shortened guard interval and extra data subcarriers contributes to increased throughput • Increased vulnerability to delay spread and ACI. • Improved receiver design should help with this • Optional mode can be turned off in the presence of too much delay spread • Many environments where high rates will be required, such as residential media distribution, have naturally low delay spread. John Ketchum, et al, QUALCOMM

38. PHY Throughput and PER Ch. B, 2×2: Eigenvector Steering John Ketchum, et al, QUALCOMM

39. Average PER w/fixed rates Ch. B, 2×2 : Eigenvector Steering John Ketchum, et al, QUALCOMM

40. PHY Throughput and PER Ch. B, 2×2: Spatial Spreading John Ketchum, et al, QUALCOMM

41. PHY Throughput and PER Ch. B, 2×2: Eigenvector Steering, SGI-52 John Ketchum, et al, QUALCOMM

42. PHY Throughput and PER Ch. B, 4×4 : Eigenvector Steering John Ketchum, et al, QUALCOMM

43. Average PER w/fixed rates Ch. B, 4×4 : Eigenvector Steering John Ketchum, et al, QUALCOMM

44. PHY Throughput and PER Ch. B, 4×4: Eigenvector Steering, SGI-52 John Ketchum, et al, QUALCOMM

45. PHY Throughput and PER Ch. B, 4×2 : Eigenvector Steering John Ketchum, et al, QUALCOMM

46. PHY Throughput and PER Ch. B, 4×2: Spatial Spreading John Ketchum, et al, QUALCOMM

47. Effect of increased latency on Eigensteering: Average Throughput, 2x2, Channel B John Ketchum, et al, QUALCOMM

48. Summary • MIMO PHY design builds on existing 802.11a,g PHY design • Two operating modes provide highly robust operation under a wide range of conditions • Eigenvector Steering provides best rate/range performance • Spatial Spreading • Adaptive rate control through low-overhead rate feedback supports sustained high throughput operation • Low-overhead training sequence exchange supports high-capacity Eigenvector Steered operation for best rate/range performance • Spatial Spreading operation provides robust high throughput operation when Tx does not have sufficiently accurate channel state information • MAC enhancements are required to take full advantage of HT PHY • Required for 100 Mbps throughput in realistic operating environments • QoS-sensitive applications are not satisfied with EDCA John Ketchum, et al, QUALCOMM

49. APPENDIX: MAC Details John Ketchum, et al, QUALCOMM

50. Critical Features for High Throughput Operation • Critical Features for High Data Rates • Adaptation of PHY rates and MIMO transmission mode • Low overhead feedback • Compatible with EDCA or HCCA • Low latency • To support PHY adaptation • To satisfy end-to-end delay requirements of multimedia/interactive applications • High MAC Efficiency, reduced contention overhead • Frame aggregation, Compressed Block ACK • Enhanced Polling • Simplify QoS handling compared to 802.11e • Exploit high data rates of 802.11n John Ketchum, et al, QUALCOMM