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

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

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

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  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 • Proposal guide and main points • MAC Features • System Performance • PHY Features • Link Performance John Ketchum, et al, QUALCOMM

  3. Guide to Qualcomm’s Proposal The complete proposal submitted by QUALCOMM consists of the following four documents: • 11-04-870 High Throughput System Description and Operating Principles. • Section 1 provides an overview of the proposed PHY and MAC enhancements • Section 2 provides a detailed description and proposed text for the MAC and PLCP enhancements. • Section 3 provides a detailed description and proposed text for the PHY enhancements. • Appendix A provides the mathematical background and operating principles for MIMO applicable to the proposal. • 11-04-871 High Throughput Proposal Compliance Statement (this document.) • Section 1 addresses compliance with the functional requirements of 802.11n. • Section 2 addresses compliance with the PAR and Five Criteria of 802.11n. • Section 3 addresses Comparison Criteria of 802.11n. • 11-04-872 Link Level and System Performance Results for High Throughput Enhancements. • Section 1 describes the system simulation methodology • Section 2 provides system performance results for the simulation scenarios defined in the 802.11n usage models document. • Section 3 describes the PHY simulation methodology • Section 4 provides link level simulation results for packet error rate and throughput. • Section 5 defines the link abstraction used to capture the packet error model in system level simulations and also provides model verification results. • Section 6 provides performance results for the modified preamble. • 11-04-873 High Throughput Enhancements Presentation – Features and Performance. Summary presentation of the proposal features and performance results. • PHY Features • MAC Features • Link Performance • System Performance 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 • 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. John Ketchum, et al, QUALCOMM

  5. MAC – Outline • Motivation • MAC Enhancements: Common Features • Scheduled Operation and Adaptive Coordination Function (ACF) • QoS Capable IBSS Operation • Summary John Ketchum, et al, QUALCOMM

  6. 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

  7. 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

  8. Different Operating Environments • Application to different operating regimes • Evolution of current deployments • Solution: Simple enhancements: frame aggregation, closed loop rate control • Low loads: EDCA • High loads: HCCA • Large enterprise networks • Solution: Enhancements to HCCA for deployments with large numbers of STAs • Optimized scheduled operation • Implemented in Enterprise-class AP • Flexible operation modes. See examples. • Small networks with significant QoS traffic • Solution: IBSS with distributed round-robin scheduling John Ketchum, et al, QUALCOMM

  9. Flexible Frame Aggregation • Eliminate MAC throughput bottleneck • Throughput saturates at ~70 Mbps even with 802.11e features • Permits aggregation of encrypted or unencrypted frames • MAC headers in the aggregated frame can be compressed John Ketchum, et al, QUALCOMM

  10. Eliminate Immediate ACK for MIMO Transmissions • Receiver delay for demodulation and decoding of (coded) OFDM transmissions • 802.11a SIFS is 16 us. • 802.11g provides a 6 us OFDM signal extension • MIMO OFDM transmissions impose even greater burden on the receiver • Aggregated frames make matters worse • Inefficient solution • Larger SIFS or longer signal extension • Efficient solution • Eliminate Immediate ACK for MIMO OFDM transmissions • Use 802.11e Block ACK and Delayed Block ACK mechanisms • Reduced IFS for scheduled transmissions • TXOP Bursting with zero IFS (AP transmissions) • Consecutive scheduled STA TXOPs separated by GIFS (800 ns Guard IFS) • TXOP Bursting with BIFS (STA transmissions) John Ketchum, et al, QUALCOMM

  11. Backward Compatible PLCP Header PPDU Type 0000 • Compatible Preamble • Changes described in PHY section • Extended Backward Compatible SIGNAL Field • Set Rate field in current SIGNAL1 field to one of eight unused values. • Indicates presence of SIGNAL2. John Ketchum, et al, QUALCOMM

  12. PPDU Size and LENGTH Fields • LENGTH Field in Legacy SIGNAL field is used by the Receiving STA to parse the received octet stream • To determine location of FCS, length of PAD. • For aggregated frame, need length per encapsulated MAC frame. • Aggregation Header contains LENGTH field for each encapsulated MAC frame. • PPDU Size Field included in Extended SIGNAL • Indicates PPDU Size in number of standard or SGI OFDM symbols. • SIGNAL at 6 Mbps can be decoded by all 802.11n STAs to determine medium time occupied by the PPDU. • Solution: • Replace LENGTH by PPDU Size in Extended SIGNAL. • Include Aggregation Header whenever MIMO PPDU is used. John Ketchum, et al, QUALCOMM

  13. Rate and Mode Adaptation PPDU Type 0000 • Rate vector (DRV) included in Extended SIGNAL field. • Rate and mode feedback (DRVF) included in FEEDBACK field • MIMO OFDM Training symbols inserted as necessary • Fast ramp up to exploit high PHY rates for bursty traffic • Enormous throughput benefit with low overhead • Robustness to interference, shadowing, channel and receiver impairments John Ketchum, et al, QUALCOMM

  14. Compressed Block Ack • Compressed format 1: Do not indicate status of fragments. Shrink BlockAck Frame from 152 to 32 octets. • Compressed format 2: Indicate status of fragments only if there are missing fragments • Compressed format 3: Remove trailing zeroes from Bitmap. John Ketchum, et al, QUALCOMM

  15. Scheduled Operation – SCHED Message • SCHED message and Scheduled Access Period (SCAP) are enhancements of HCCA CAP • 802.11n AP acquires the medium after PIFS (as in the HCCA CAP) and transmits a SCHED message (instead of Poll). • The SCHED message defines the schedule of transmissions for the SCAP. Default values of SCAP: 1.024ms, 2.048 ms, 4 ms. • SCHED is a Multiple Poll Message • Lower overhead, more efficient • Indicates Tx STA and Rx STA for TXOPs => Improved power saving John Ketchum, et al, QUALCOMM

  16. Scheduled Operation – Protection and Recovery • Protection of SCAP • High level procedures to avoid overlapping BSS: Mandatory DFS • CTS-to-Self to clear out NAV for SCAP. For 802.11n STAs set NAV through Duration field in SCHED frame. • Keep SCAP short (< 4 ms) to minimize impact of collisions with legacy STAs during SCAP • No CCA required for transmissions during SCAP John Ketchum, et al, QUALCOMM

  17. Scheduled Operation – Reduced IFS • Reduced IFS • Since, no CCA required for transmissions during SCAP • PPDU Aggregation: IFS and preambles may be eliminated between consecutive Scheduled AP transmissions. • Consecutive Scheduled TXOPs from STAs may be transmitted with GIFS (800 ns) • Optionally, FRACH and Protected EDCA may be scheduled during a SCAP John Ketchum, et al, QUALCOMM

  18. Scheduled Operation – Managed Peer-to-Peer • Managed Peer-to-Peer Operation is an enhancement of DLP • In Scheduled STA-STA TXOPs • PPDU Size in SIGNAL1 is replaced by Request • AP promiscuously decodes Request field in STA-STA transmissions. • STAs indicate SCHED Rate, QoS and requested length for subsequent TXOP. • STAs do closed loop rate control • AP does scheduling PPDU Type 0000 John Ketchum, et al, QUALCOMM

  19. SCHED Frame Format and Fields • SCHED Frame Fields • CTRL0, CTRL1, CTRL2, CTRL3 fields are separately coded and transmitted at 6, 12, 18, 24 Mbps, respectively • Multiple Assignment Elements are included in each CTRLJ • Each Assignment Element specifies: Tx STA (may be AP), Rx STA (may be AP), Start Time, TXOP Duration • MAP field identifies start of FRACH and Protected EDCA within SCAP John Ketchum, et al, QUALCOMM

  20. Summary of HCF Enhancements • Advantages of SCHED over HCF Poll • Reduced overhead: single message instead of multiple Polls, multiple IFS • Efficient encoding of TXOP/RXOP assignments • Improved Power Saving: After decoding the SCHED message, STAs not scheduled for Tx or Rx can sleep for the remaining SCAP • Efficient feedback for ES operation: MIMO OFDM Training symbols attached to SCHED frame permit STAs to estimate the AP-STA channel and achievable rate. • Improved QoS handling: Optimized low-latency operation for 802.11n STAs • Managed peer-to-peer operation • STAs do closed loop rate control. AP does scheduling • Protected Contention Periods to complement scheduling • FRACH • Protected EDCA John Ketchum, et al, QUALCOMM

  21. Operation of Adaptive Coordination Function (ACF) • SCAP is an enhancement of the HCCA CAP • Setting NAV • The Duration field in the SIGNAL field of the SCHED frame sets the NAV for the SCAP at all 802.11n STAs. • To set the NAV for the SCAP at legacy STAs, the AP may transmit a CTS-to-Self prior to the transmission of the SCHED frame. • SCAP Timing • 802.11n STAs respect the SCAP interval so that their transmissions terminate when the SCAP expires. • The AP may schedule back-to-back SCAPs. John Ketchum, et al, QUALCOMM

  22. ACF – Example Operating Mode • Case: No CAP • Legacy STAs, if present, can satisfy their QoS using EDCA • Setting NAV • The Duration field in the SIGNAL field of the SCHED PPDU sets the NAV for the SCAP at all 802.11n STAs. • If only 802.11n STAs are present, there is no need for CTS-to-Self. • Beacon announces CFP to protect most of the Beacon interval to avoid interference from arriving legacy STAs • If medium is shared with legacy STAs, use CTS-to-Self at start of SCAP • Interspersed SCAP and EDCA periods permit “fair” sharing of the medium • 802.11n QoS Flows are served during SCAP • 802.11n non-QoS flows use EDCA periods along with legacy STAs. John Ketchum, et al, QUALCOMM

  23. ACF – Optimized Scheduled Operation • Case: Limited resource required for legacy STAs. • Legacy STAs with non-QoS flows that may be satisfied with only occasional allocations of EDCA periods (CP) • Setting NAV • Beacon sets NAV at legacy STA for CFP. • The Duration field in the SIGNAL field of the SCHED frame sets the NAV for the SCAP at all 802.11n STAs. • Protected EDCA periods for 802.11n STAs included in Scheduled Access Period John Ketchum, et al, QUALCOMM

  24. RRBSS – QoS capable IBSS operation • Provide QoS capability without AP • May also be used by low-end AP • Applicable to usage scenarios with CE devices with high throughput, high QoS needs • Exploit the large PHY data rates of MIMO OFDM to simplify scheduling and QoS management. • Designed for up to 15 STAs • Distributed admission control • Self identification of QoS flows • Distributed Round-Robin Scheduling • Short Beacon Period for low latency John Ketchum, et al, QUALCOMM

  25. RRBSS – Long Token PPDU • Round Robin order for current Beacon period included in Complete RR List • Up to 15 RRIDs • RR Seq indicates changes in RR List • Long Token must be transmitted by each STA if RR Seq changes • Connectivity Vector indicates RRIDs that the STA can hear • Permits clustering of contiguous STAs on RR List • RR Bandwidth Management field permits distributed sharing • Simple standardized rules Long Token PPDU PPDU Type 1010 John Ketchum, et al, QUALCOMM

  26. RRBSS – Short Token PPDU • STA must transmit Long or Short Token PPDU if no data to send. • Explicit Token Passing using Long or Short Token • Implicit Acknowledgment by Next STA • Compact RR List contains • RRID of STA • RRID of Next STA for Token Passing • RRID of Last STA on RR List Short Token PPDU PPDU Type 1000 John Ketchum, et al, QUALCOMM

  27. RRBSS – Robust Operation • Explicit token passing. • Implicit acknowledgment of token passing by Next STA on RR List. • Otherwise STA must pass token to the following STA. • RR List rotates at each Beacon period • No single STA is “designated master” • Last STA in Beacon period n, becomes First STA in Beacon period n+1. • Transmits Beacon and Long Token in Beacon period n+1. • First STA seizes medium at PIFS. • If medium is idle at DIFS, previous First STA transmits Beacon and Long Token. Last STA is dropped from RR List. • Changes in RR List indicated through RR Seq • Each STA must transmit Long Token when RR Seq increments John Ketchum, et al, QUALCOMM

  28. Simplified QoS Handling • In all operating regimes • Exploit high data rates to simplify QoS handling • Simple admission control • Based on simplified TSPEC: Mean data rate, Delay bound • Mean data rate • Mapped to symbols per second for resource allocation • Delay bound • Mapped to scheduling period, and • ARQ operation • Low latency operation is critical • To operate with small buffers. This is critical at high data rates. • To meet low delay guarantees in all operating regimes • EDCA/HCCA with lightly loaded system • RRBSS (with or without AP) • Scheduled operation for heavily loaded system John Ketchum, et al, QUALCOMM

  29. Summary of MAC Enhancements • Detailed design of MAC enhancements for MIMO OFDM • Completely Backward Compatible • Enhancements required for high throughput, low latency operation • Features applicable to different operating regimes • List of proposed features • Frame Aggregation. Aggregation Header. • Eliminate Immediate ACK for MIMO transmissions • Extended SIGNAL field and PPDU Type • Rate and MIMO Mode Adaptation • Compressed Block Ack • SCHED Message, SCAP and Scheduled TXOPs • Reduced IFS between scheduled transmissions • Flexible Operating Modes with ACF • RRBSS – QoS capable IBSS Operation. Token PPDUs. John Ketchum, et al, QUALCOMM

  30. 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

  31. 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

  32. 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

  33. 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

  34. Summary of Total Throughput Results John Ketchum, et al, QUALCOMM

  35. 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

  36. 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

  37. 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

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

  39. 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

  40. 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

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

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

  43. 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

  44. 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 with additional expanded OFDM symbol 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 • Sustained high rate operation possible via rate adaptation John Ketchum, et al, QUALCOMM

  45. Observation • Detailed, up-to-date feedback on channel state is fundamental to achieving high throughput in a TDD MIMO WLAN • The challenge is to achieve this reliably with low overhead • We believe that the design described here achieves this goal John Ketchum, et al, QUALCOMM

  46. OFDM Waveform • Baseline OFDM structure identical to 802.11a/g • 312.5 kHz subcarrier spacing/20 MHz carrier spacing • Same subcarrier structure • 48 subcarriers for data, 4 subcarriers for pilot • “DC” subcarrier empty, 11 subcarriers for guard band • 3.2 µs symbol, 800 ns guard interval • 40% Physical-layer overhead (1 - (48*312.5 kHz/20 MHz)*(3.2 µs/4 µs)) • Expanded OFDM symbol and shortened cyclic prefix introduced • Same subcarrier spacing • 4 additional data subcarriers—52 total • same four pilot subcarriers • Physical-layer overhead < 28% • More vulnerable to time dispersion and ACI John Ketchum, et al, QUALCOMM

  47. Modulation and Coding • Use existing 802.11 constraint length 7, rate ½ convolutional code and punctured rates. • Retain PSK/QAM modulation from 802.11 • Additional rates adopted to provide increased spectral efficiency • 256 QAM modulation gives increased rates and spectral efficiency • Code rates range from ½ bit per modulation symbol to 7 bits per modulation symbol. • Up to four wideband spatial channels supported with separate coding/interleaving for each spatial channel. • Enhanced interleaving over single OFDM symbol for MIMO OFDM • Based on 802.11a/g interleaver • Simple backward compatible mode John Ketchum, et al, QUALCOMM

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

  49. 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 John Ketchum, et al, QUALCOMM

  50. 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

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