WWiSE IEEE n Proposal November 16, 2004

WWiSE IEEE 802.11n Proposal November 16, 2004
Airgo, Broadcom, Buffalo, Conexant, ETRI, Realtek, STMicroelectronics, Texas Instruments, Winbond S. Coffey, et al., WWiSE group

Contributors and contact information Airgo Networks: VK Jones, Broadcom: Jason Trachewsky, Buffalo: Takashi Ishidoshiro, Conexant: Michael Seals, ETRI: Taehyun Jeon, Realtek: Stephan ten Brink, STMicroelectronics: George Vlantis, Texas Instruments: Sean Coffey, Winbond: Jeng-Hong Chen, S. Coffey, et al., WWiSE group

Contents WWiSE approach Overview of key features & updates
Proposal description Physical layer design MAC features Discussion Summary S. Coffey, et al., WWiSE group

The WWiSE approach WWiSE = World Wide Spectrum Efficiency
The partnership was formed to develop a specification for next generation WLAN technology suitable for worldwide deployment Mandatory modes of the WWiSE proposal comply with current requirements in all major regulatory domains: Europe, Asia, Americas Proposal design emphasizes compatibility with existing installed base, building on experience with interoperability in g and previous amendments All modes are compatible with QoS and e Maximal spectral efficiency translates to highest performance and throughput in all modes S. Coffey, et al., WWiSE group

WWiSE proposal development process
Approximately 150 documents submitted Many contributions per topic: short sequence, pilots, laboratory testing results, etc. Many contributions per company Collaborative process Technical selection Modeled after the IEEE process, with supermajority voting Proposal drafting Detailed technical specification in draft format prepared S. Coffey, et al., WWiSE group

Recap WWiSE proposes 2 transmitters in 20 MHz mandatory
Rates 54, 81, 108, 121.5, 135 Mbps Optional extensions to 3 and 4 transmit antennas Optional space-time block codes for longer range Optional 40 MHz counterparts of all 20 MHz modes Optional LDPC code MAC: HTP burst, aggregation, extended Block Ack See r3 for a full description S. Coffey, et al., WWiSE group

Update 20/40 MHz coexistence language strengthened
Devices must perform CCA on secondary channel All space-time block codes are now optional Modifications to rate tables for 2x1-20 MHz, 1x1-40 MHz, and 2x1-40 MHz See r5 S. Coffey, et al., WWiSE group

Old rate plan 2x1, 20 MHz: 1x1, 40 MHz; 2x1, 40 MHz PHY rate, Mbps
Code rate Constellation 6.75 1/2 BPSK 10.125 3/4 13.5 QPSK 20.25 27 16-QAM 40.5 54 2/3 64-QAM 60.75 PHY rate, Mbps Code rate Constellation 54 1/2 16-QAM 81 3/4 108 2/3 64-QAM 121.5 135 5/6 S. Coffey, et al., WWiSE group

New rate plan 2x1, 20 MHz: (optional) 1x1, 40 MHz; 2x1, 40 MHz
PHY rate, Mbps Code rate Constellation 6.75 1/2 BPSK 10.125 3/4 13.5 QPSK 20.25 27 16-QAM 40.5 54 2/3 64-QAM 60.75 67.5 5/6 PHY rate, Mbps Code rate Constellation 13.5 1/2 BPSK 20.25 3/4 27 QPSK 41 54 16-QAM 81 108 2/3 64-QAM 121.5 135 5/6 S. Coffey, et al., WWiSE group

Unified format 1/2 800 BCC, LDPC 16-QAM 3/4 2/3 64-QAM 5/6 Code rate
Cyclic prefix, ns Code Constellation 1/2 800 BCC, LDPC 16-QAM 3/4 2/3 64-QAM 5/6 All combinations of 2, 3, 4 transmit antennas and 20/40 MHz offer exactly these 5 modes All 20 MHz modes have 54 data subcarriers, 2 pilots. All 40 MHz modes have 108 data subcarriers, 4 pilots S. Coffey, et al., WWiSE group

Tone usage in 20 MHz and 40 MHz
48 54 108 Data tones Bandwidth TGnSync WWiSE WWiSE and TGnSync each use 108 data tones at 40 MHz Extra tones via filling gap between 20 MHz channels However WWiSE also uses extra tones in 20 MHz, filling tones out to existing spectral mask 48 S. Coffey, et al., WWiSE group

2 transmitter SDM, 20 MHz (mandatory)
PHY rate Data carriers Pilots Code rate Cyclic prefix, ns Code Constellation 54 Mbps 54 2 1/2 800 BCC 16-QAM 81 Mbps 3/4 108 Mbps 2/3 64-QAM 121.5 Mbps 135 Mbps 5/6 S. Coffey, et al., WWiSE group

Optional data modes 20 MHz: 40 MHz: (all 40 MHz modes optional)
3 Tx space-division multiplexing 4 Tx space division multiplexing 40 MHz: (all 40 MHz modes optional) 1 Tx antenna 2 Tx space division multiplexing 3 Tx space division multiplexing Space-time block codes: (all STBCs optional) 2x1, 3x2, 4x2, 4x3 in 20 MHz and 40 MHz LDPC code option An option in all proposed MIMO configurations and channel bandwidths S. Coffey, et al., WWiSE group

Optional mode data rates
20 MHz: Configuration Rate ½, 16-QAM Rate ¾, 16-QAM Rate 2/3, 64-QAM Rate ¾, 64-QAM Rate 5/6, 64-QAM 3 Tx, 20 MHz 81 121.5 162 182.25 202.5 4 Tx, 20 MHz 108 216 243 270 40 MHz: Configuration Rate ½, 16-QAM Rate ¾, 16-QAM Rate 2/3, 64-QAM Rate ¾, 64-QAM Rate 5/6, 64-QAM 1 Tx, 40 MHz 54 81 108 121.5 135 2 Tx, 40 MHz 162 216 243 270 3 Tx, 40 MHz 324 364.5 405 4 Tx, 40 MHz 364 432 486 540 S. Coffey, et al., WWiSE group

Proposal performance S. Coffey, et al., WWiSE group

Robustness, 135 Mbps, multipath
For 2x2 operation with 64-QAM, rate 5/6 is feasible This rate marks the limit of feasibility ML gains 5 dB over MMSE (1% PER) LDPC gains 2 dB over BCC Similar results for 40 MHz at 2x data rate Curves slightly to left due to frquency diversity S. Coffey, et al., WWiSE group

Interpol., filtering, limiter MIMO interleaver Symbol mapper
Add pilots Insert training FEC encoder, puncturer Interpol., filtering, limiter MIMO interleaver Symbol mapper Upconverter, amplifier IFFT D/A Add cyclic extension (guard) S. Coffey, et al., WWiSE group

MIMO interleaving . Parameterized 802.11a-style interleaver
Bit-cycling across NTX transmitters Parameterized a-style interleaver 5 subcarrier shift, same interleaver . TX 0 interleaved bits Coded bits TX 1 interleaved bits Configuration Idepth 108 tones, 1 Tx, 2x1 12 All others 6 Shift of 5 additional subcarriers for each additional antenna S. Coffey, et al., WWiSE group

Interleaver comparison with TGnSync
Both schemes have 2x2, 64-QAM, rate 3/4 WWiSE: Mbps TGnSync: 108 Mbps WWiSE interleaver has an advantage of 1.25 dB at 2% PER S. Coffey, et al., WWiSE group

108 Mbps modes comparison with TGnSync
Both schemes are 108 Mbps WWiSE: rate 2/3 TGnSync: rate 3/4 WWiSE interleaver has an advantage of 3.75 dB at 2% PER S. Coffey, et al., WWiSE group

Pending comparisons, modes & interleavers
WWiSE 135 Mbps vs. TGnSync 126 Mbps 2x2 rate 5/6 with WWiSE interleaver vs. 2x2 rate 7/8 with TGnSync interleaver WWiSE 135 Mbps vs. TGnSync 144 Mbps 2x2 rate 5/6 64-QAM vs. 2x2 rate 3/4 256-QAM WWiSE 243 Mbps vs. TGnSync MHz Different interleavers S. Coffey, et al., WWiSE group

Space-time block codes and asymmetry
Simple space-time block codes (STBCs) are used to handle asymmetric antenna configurations STBC rate always is an integer - No new PHY rates result from STBC encoding of streams Block size is always two OFDM symbols STBC encoding follows the stream encoding AP STA S. Coffey, et al., WWiSE group

4x2 space-time block code
4x2 gains 4.6 dB over 2x2 (121.5 Mbps); 6.2 dB (135 Mbps) These results for MMSE receiver, BCC S. Coffey, et al., WWiSE group

WWiSE 20 MHz OFDM format FEC encoder, puncturer MIMO interleaver Symbol mapper D/A Interpol., filtering, limiter Upconverter, amplifier IFFT Add cyclic extension (guard) Add pilots Insert training Much of the robustness advantage comes from the WWiSE 20 MHz OFDM format of 56 tones: 54 data, 2 pilots S. Coffey, et al., WWiSE group

52 tones, 3 dB backoff - This is the same 52-tone signal
The PA is operated 3 dB backed off from PO-1 dB compression point The signal is now contained within the spectral mask. - S. Coffey, et al., WWiSE group

56-tone OFDM signals This is a 56 tone OFDM signal with a short trapezoidal window The signal meets spectral mask, with an extra 0.5 dB of PA backoff compared to 52 tone OFDM signal 58 tones fails spectral mask even with an 8 dB backoff 56 tones works well, and is also the feasible limit More details in separate spectral mask presentation - S. Coffey, et al., WWiSE group

Pilots in a frequency selective channel
SISO Rx 1 MIMO Rx 2 S. Coffey, et al., WWiSE group

Pilots in frequency selective channels
Optimal performance depends on total pilot SINR Figure of merit is nr x np Two systems that have same nr x np and correlations will have the same performance In the WWiSE format, correlation in frequency is less than in legacy mode, due to increased pilot separation Correlation in space depends on antenna separation For half-l separation in channel models D and B the net effect is that the WWiSE format has the same distribution function as legacy SISO S. Coffey, et al., WWiSE group

Preambles Mixed mode: “Green field/patch/timeslot”:
Basic preamble, used when there is on-the-air mixing of legacy 11a/g traffic with 11n traffic “Green field/patch/timeslot”: Used (even in a mixed BSS) in time intervals where there is no legacy on-air traffic For example: Within protection mechanisms Within a burst, with initial preamble mixed mode Within a HCCA poll S. Coffey, et al., WWiSE group

Preambles: 2 Tx, mixed mode 20 MHz
Legacy SIG Long - N Short Long SIG-N WWiSE: 32 msec 8 4 SIG-N Short-N Long-N or TGnSync: 44.8 msec 20 8 2.4 7.2 7.2 S. Coffey, et al., WWiSE group

Preambles: 2 Tx, “green time” 20 MHz
Short Long - N SIG-N WWiSE: 20 msec 8 8 4 SIG-N Short-N Long-N or TGnSync: 44.8 msec 20 8 2.4 7.2 7.2 S. Coffey, et al., WWiSE group

Channel estimation performance
The frequency division preamble is double the length of the WWiSE preamble The loss due to lack of smoothing offsets the gain due to doubling the length In this case the two effects balance each other out S. Coffey, et al., WWiSE group

STS vs. LTS Rx power CDF: Cyclic shift on STS and not LTS
25 nsec RMS DS: 50, 100, 200, 400 nsec CDD Large power discontinuity observed S. Coffey, et al., WWiSE group

STS vs. LTS Rx power CDF: Cyclic shift on both STS and LTS
25 nsec RMS DS: 50, 100, 200, 400 nsec CDD Looks well behaved S. Coffey, et al., WWiSE group

Multiplexing across two encoders
Parallel encoders For 40 MHz modes with more than two spatial streams, two parallel BCC encoders are used: Multiplexing across two encoders (round robin) BCC encoder, puncturer To MIMO interleaver Data payload S. Coffey, et al., WWiSE group

Advanced coding option
Rate-compatible LDPC code with the following parameters: Transmitter block diagram as for BCC modes, except symbol interleaver, rate-compatible puncturing, and tail bits are not used Rate Information bits Block length 1/2 972 1944 2/3 1296 1944 3/4 1458 1944 5/6 1620 1944 S. Coffey, et al., WWiSE group

LDPC code, contd. There is no change required to SIFS or to any other system timing parameters when the advanced coding option is used The block size of 1944 reduces or eliminates the need for pad bits at the end of a packet Pad bits are eliminated for 2 transmitter operation in 20 MHz channels, and 2x1 and 1x1 in 40 MHz channels The four parity check matrices are derived from the rate-1/2 matrix via row combining The parity check matrices are structured and based on square-shaped building blocks of size 27x27 The parity check matrices are structured to enable efficient encoding S. Coffey, et al., WWiSE group

MAC features S. Coffey, et al., WWiSE group

MAC features The WWiSE proposal builds on e functionality as much as possible, in particular EDCA, HCCA, and Block Ack Block Ack mandatory WWiSE proposal ensures backwards compatibility Targeted effectiveness - ROI Eliminate the big bottlenecks Avoid schemes which yield relatively small improvement in performance in return for large complexity changes Benefits of simplicity Shorter time to standardization Shorter time to productization Shorter time to interoperability S. Coffey, et al., WWiSE group

WWiSE TGn MAC features WWiSE proposal introduces:
Only ONE new frame subtype not actually a new subtype – uses QOS field reserved bit No new MAC access control functions Re-uses existing DCF/EDCA/HCCA TGE => QOS + Efficiency enhancements EDCA: reduce DCF overhead with continuation TXOP HCCA: reduce EDCA overhead with controlled access WWiSE brings forth three simple efficiency enhancements These achieve high performance, even compared to other proposed enhancements S. Coffey, et al., WWiSE group

The three WWiSE MAC enhancements
MSDU (MAC Layer) Aggregation Removes significant MAC overhead HTP Burst Eliminates major remaining components of MAC / PHY overhead Enhanced Block Ack Allow No-ACK policy Removes significant ACK overhead Block Ack eliminates MAC transmitter turnaround overhead S. Coffey, et al., WWiSE group

The Ideal Protocol t0 = - ∞ t1 = + ∞ All MSDU data bits, all the time
No IFS MSDU = PSDU No MAC Header t0 = - ∞ t1 = + ∞ All MSDU data bits, all the time 100% MAC Efficiency S. Coffey, et al., WWiSE group

MSDU aggregation @ Ideal
No IFS Np Ns MH nMSDU xIFS MAC Header t0 ≠ - ∞ t1 ≠ + ∞ Requires minor change to MAC protocol Need new aggregation subtype Efficiency increases as n increases Efficiency approaches 100% for very large n Flexibility in choice of n allows for per-installation tradeoff of latency vs throughput vs fairness S. Coffey, et al., WWiSE group

WWiSE MSDU aggregation
“New frame subtype” Uses reserved bit of QOS subfield Increased maximum PSDU length, to 8191 octets Impressive improvements in MAC throughput WWiSE simulations use n=8 (with overriding max MPDU size limitation of 8191 Bytes) S. Coffey, et al., WWiSE group

MSDU aggregation shortcomings
Upper limit on n (number of MSDU per aggregate) PHY limitations QOS latency limitations affect aggregate assembly process IFS between aggregates PHY overhead between aggregates Single RA per aggregate Single Rate per aggregate Single TX power value per aggregate Good stuff, but not so ideal due to practical constraints S. Coffey, et al., WWiSE group

HTP Burst more ideal = N-Preamble
No idle gap Optional normal ACK policy Np Ns PSDU Ns PSDU Ns PSDU xIFS Non-normal ACK PSDU can be an aggregate MSDU! = N-Preamble Np Last PSDU bit set Ns = N-Signal robust encoding rate Multiple RA allowed within the burst Block Ack Request and Block Ack frames allowed within burst More IFS eliminated RIFS and ZIFS allowed within burst PHY overhead reduced “Last PSDU” bit indicates receiver should revert to preamble search S. Coffey, et al., WWiSE group

HTP Burst HTP Burst Sequence of MPDUs from same transmitter
IFS between MPDUs not necessary, since same transmitter is used for all MPDUs in the HTP Burst 0 usec IFS if at same Tx power level and PHY configuration 2 usec IFS otherwise (with preamble) IFS value not dependent on RA Preamble also not necessary Optional with ZIFS, required with RIFS PPDUs may be “aggregated” i.e. ZIFS with no preamble between PPDU MPDUs may be of the aggregated-MSDU type MPDUs may have different RAs and Rates S. Coffey, et al., WWiSE group

Block Ack - Ack policy With “normal” block ack
HTP bursts are interrupted by Block Ack Request/response interaction BA/BR exchange includes a change of transmitter => 2*(SIFS + preamble) Efficiency gain of HTP burst is potentially lost WWISE Block Ack REQ/ACK frames have ACK policy bits Allows: Normal ACK behavior per e draft No-ACK behavior other settings reserved Reduces Block Ack overhead by eliminating explicit ACKs to BlockAckRequest and BlockAck by eliminating transmitter changes S. Coffey, et al., WWiSE group

Block Ack - Ack policy, contd.
No-ACK policy allows multiple RA + Block Ack Frames within a single HTP burst if immediate ACK required: HTP burst broken to allow SIFS + ACK HTP burst single RA, ending with Block Ack Request + SIFS + ACK HTP burst single RA, ending with Block Ack Request + Block Ack + ACK without immediate ACK requirement (i.e. using NO-ACK) HTP burst is allowed to continue HTP burst may contain multiple Block Ack Request to multiple RA HTP burst may also contain Block Ack response frames Effectively allows additional, more efficient modes of use for Block Ack mechanism S. Coffey, et al., WWiSE group

WWiSE efficiency 802.11e TXOP Access delay M1 M2 M3 M4 breq ACK ACK M1 M2 M3 M4 back With Aggregation Access delay M1 M2 M3 M4 breq ACK ACK M1 M2 M3 M4 back With HTP Burst Access delay M1 M2 M3 M4 breq ACK ACK M1 M2 M3 M4 back With HTP Burst & No-ACK Block Ack Access delay M1 M2 M3 M4 breq M1 M2 M3 M4 back WWiSE approaches Ideal Protocol by minimizing overheads of MAC and PHY S. Coffey, et al., WWiSE group

WWiSE efficiency 802.11E compared to WWiSE
breq back breq ACK ACK ACK M1 M2 M3 M4 M1 M2 M3 M4 M1 M2 M3 breq back breq M1 M2 M1 breq back 802.11E compared to WWiSE Per-MSDU overhead reduced -> IFS, Pream, Sig, MAChdr ACK overhead reduced using No-ACK Block Ack Transmitter transition overhead reduced using HCCA S. Coffey, et al., WWiSE group

Legacy interaction Legacy remediation N-STA detection/advertisement
Uses proven g signaling and rules Extends ERP information element Identification of TGn and non-TGn devices and BSSs Legacy Protection mechanisms Existing protection mechanisms (extended to N/G case) Set NAV to protect new modulation types E.g. RTS/CTS, CTS2SELF, etc. WWiSE adds PLCP length spoofing as additional tool S. Coffey, et al., WWiSE group

40/20 MHz interaction Pure 802.11n network
Interaction identical to existing, proven g mechanism for sharing with legacy devices When 20-MHz-only n devices associate, AP instructs that all BSS members shall use “20MHz” protection Protective frames are those that the 20-MHz-only STA can understand Protective frames set NAV to cover 20-MHz transmissions Protection may additionally be enabled at AP and STA discretion (same as rules for g protection mechanism) 802.11n mixed with 20 MHz a Method identical to proven g concept Mixing of n-40, n-20, a-20 also works Only occurs in the 5 GHz bands S. Coffey, et al., WWiSE group

Simulations PHY model in MAC simulations
Detailed description in IEEE /877r3 TGn MIMO Channel Models Impairments as specified by FRCC Union bound technique to estimate PHY layer frame errors Performance closely matches full simulations for mandatory phy configurations Binary convolutional coding Executes as MATLAB routine called by MAC simulator in NS S. Coffey, et al., WWiSE group

EDCA simulation parameters e.g., scenario 4
CWMIN (AP/STA) CWMAX AIFSN TXOP LIMIT msec (AP/STA) AC_BK 31/63 511/511 4/4 2.0/1.0 AC_BE 127/255 3.0/2.5 AC_VI 15/15 31/31 3/3 2.5/1.1 AC_VO 7/15 2/2 1.2/1.0 HTP burst and MSDU aggregation employed S. Coffey, et al., WWiSE group

HCCA simulation parameters
HTP burst and MSDU aggregation used Very simple scheduler employed Round robin Same TXOP value granted to all STA, regardless of flow characteristics, number of flows at STA, or priority of flows Each STA polled once per round No real effort put into making an intelligent scheduler Great results Great ROI! S. Coffey, et al., WWiSE group

MAC simulation results
EDCA HCCA CC# Name Scenario 2x2x20 2x2x40 CC3 Goodput 1 58 67 1+ 59 71 86 4 75 124* 4+ 76 127 108 6 54 64 6+ 56 70 79 Green = WWiSE Gray = TGnSync “+” scenarios have all BE traffic offered load increased to 100 mbps (s-1) or 30 mbps (s-4,6) 1 QOS flow missed PLR target S. Coffey, et al., WWiSE group

Power save Re-use TGe mechanisms PHY-independent Legacy PS U-APSD
TIM bit in beacons + PS-Poll exchange U-APSD Trigger frames eliciting multiple downlink frames Increased efficiency over legacy PS ping-pong exchange Lets STA decide when wake periods occur S-APSD STA and AP agree on periodic, scheduled wake times S. Coffey, et al., WWiSE group

Backward compatibility
Fully backward compatible with 11b In a mixed b/g network, the OFDM transmissions are already protected There is no extra effect on 11b devices, as they are unable to distinguish between 11g and 11n, and no further mechanisms to provide At 2.4 GHz, the WWiSE proposal does not provide 40 MHz channels Compatible with 11j, 11h S. Coffey, et al., WWiSE group

Conclusion S. Coffey, et al., WWiSE group

Format One unified format for modes
Used for 2, 3, 4, transmit antennas 20 MHz and 40 MHz channels Used with open-loop space-time block codes BCC, LDPC Code 64-QAM 800 3/4 16-QAM 5/6 1/2 2/3 Constellation Cyclic prefix, ns Code rate S. Coffey, et al., WWiSE group

Summary WWiSE proposes 2 transmitters in 20 MHz mandatory
Rates 54, 81, 108, 121.5, 135 Mbps High performance, maximum robustness for given data rate Optional extensions to 3 and 4 transmit antennas Optional space-time block codes for longer range Optional 40 MHz counterparts of all 20 MHz modes Optional LDPC code MAC: HTP burst, aggregation, extended Block Ack S. Coffey, et al., WWiSE group

References and further information
IEEE / n, “WWiSE group PHY and MAC specification.” IEEE / n, “WWiSE proposal response to functional requirements and comparison criteria.” See also Or send to S. Coffey, et al., WWiSE group

WWiSE IEEE n Proposal November 16, 2004

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