High-Throughput and Enhanced-QoS Technologies in Wireless LAN

Transcription

1 High-Throughput and Enhanced-QoS Technologies in Wireless LAN Sunghyun Choi, Ph.D., Assistant Professor Multimedia & Wireless Networking Lab. (MWNL) School of Electrical Engineering Seoul National University URL:

2 Talk Outline Introduction to IEEE WLAN Baseline MAC of IEEE IEEE e for QoS IEEE n for high throughput Key techniques Overview of.11n proposals Conclusion 2

3 WLAN vs. Other Solutions Outdoor Indoor Mobility Vehicle Walk Fixed Walk Fixed/ Desktop UMTS Wideband Cellular Bluetooth 0.1 WAN b WLAN High performance WLAN a/g n Wired LAN Mbps (Tx Rate) 3

4 Baseline MAC Description

5 Distributed Coordination Function (DCF) Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) similar to IEEE Ethernet CSMA/CD 5

6 Stop-and-Wait ARQ Receiver of a directed frame returns an ACK If ACK not received, sender retransmits after another backoff 6

7 Binary Exponential Backoff Backoff Counter is randomly selected from [0,CW], where CW is contention window For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax) CW 2 (CW+1)-1 Reduces the collision probability CWmin =15 31 Example 63 CWmax=

8 IEEE e EDCA

9 User Priority to Access Category Mapping Priority User Priority (UP - Same as 802.1D User Priority) 802.1D Designation Access Category (AC) Designation (Informative) Lowest Highest 1 BK AC_BK Background 2 - AC_BK Background 0 BE AC_BE Best Effort 3 EE AC_BE Best Effort 4 CL AC_VI Video 5 VI AC_VI Video 6 VO AC_VO Voice 7 NC AC_VO Voice 9

10 Prioritized Channel Access Each channel access function contends with AIFS[AC] (instead of DIFS) and CW[AC] (instead of CW) 10

11 Four Channel Access Functions Channel access function for each AC as a virtual DCF Multiple channel access functions contend independently The winning channel access function transmits a frame Backoff AIFS[0] BC[0] Backoff AIFS[1] BC[1] Backoff AIFS[2] BC[2] Backoff AIFS[3] BC[3] 11

12 EDCA TXOP Within an EDCA TXOP multiple MSDUs from the AC can be transmitted with the limit of TXOPLimit[AC] Ends if a frame transmission fails! TXOP shall not extend across TBTT Should be at least time to transmit 256 byte MPDU at the lowest rate 12

13 IEEE n for High- Throughput

14 802.11n for Higher Throughput To provide higher throughput, i.e., > 100 Mbps, at MAC SAP Enhance both OFDM PHY and MAC Make the current MAC more efficient Add MIMO (SDM, STC, beamforming), channel bonding, etc. to PHY Status: Proposals made in Sept As of March 2005, one pending proposal TGn Sync 14

15 TGn Time Schedule (tentative) Draft proposal Submission 11n down selection 11n Baseline 11n Draft Q Q Q Q Q Milestone Q : 11n proposal presentation Q : 11n down selection process Q : 11n baseline selection Q : 11n Draft Q : 11n 1st Letter Ballot Q : 11n Standard publish 15

16 11n PHY Candidate Techniques Channel bonding E.g., using 40MHz instead of 20MHz (of 11a) Multi-Input Multi-Output (MIMO) Spatial channels of different antenna pairs are often uncorrelated Data rate or reliability can be improved MIMO Processor 16 Source: [Insider04]

17 Why is Legacy MAC limited? (DCF) with lots of overheads related to PHY and MAC Preamble, PHY & MAC headers, backoff, IFSs, and ACK See below for.11a 17

18 Throughput vs. Payload Size Ref: [Yang02] 18.11a & DCF theoretical throughput: ~110 Mbps with max payload=2304 octets & 5400 Mbps TX rate

19 Key Techniques for 11n MAC: e TXOP and Block ACK - Frame Aggregation Ref: [Tinnirello05], [Kim04]

20 802.11e TXOP and Block ACK Transmission Opportunity (TXOP) Multiple MPDUs (or MSDUs) can be transmitted back-to-back per a channel access Block ACK Instead of immediate ACK Block ACK from receiver after a number of MPDUs from transmitter Allowing selective ARQ 20

21 Different Access Modes and ACK Policies BI BB RI RB 21

22 Throughput vs. TXOP Limit Ref: [Tinnirello05] 22.11b 11Mbps, no channel error, 5 STAs

23 Theoretical Throughput Preferred Operation Range 23

24 Packet Size Statistics This statistics is from the measurement taken in IEEE standard meeting in the morning of July 22 nd

25 Frame Formats (Example) Ref: [Kim04] Original With aggregation 25

26 Throughput vs. Payload via Frame Aggregation Ref: [Kim04].11a PHY, no channel error, a single STA 26

27 11n MAC Proposals (mainly TGn Sync proposal unless specified otherwise) Ref: [Sync], [WWiSE] *WWiSE proposal is voted out, but was one of the last two proposals

28 Scalable MAC Architecture LEGACY INTEROP. Long NAV Pairwise Spoofing Single-Ended Spoofing BASELINE MAC Robust Aggregation QoS Support (802.11e) Rx assisted link adapt. ADDITIONAL EFFICIENCY Header Compression Multi-Receiver Aggregation Bi-Directional Data Flow BA Enhancements Robust & Scalable MAC Architecture CHANNEL MANAGEMENT 20/40 MHz Modes 28

29 A-MSDU Aggregation Efficient Structure MSDUs of the same TID can be aggregated MSDUs with differing SA/DA can be aggregated 29

30 A-MPDU Aggregation Robust Structure Aggregation is a purely-mac function PHY has no knowledge of MPDU boundaries Simplest MAC-PHY interface Control and data MPDUs can be aggregated 30

31 Performance evaluation Aggre gation schemes comparision (no channel errors) Throughput, Mbps * Assuming 11a s 54Mbps tx rate 1100 MSDU size, bytes No aggregation A-MPDU A-MSDU A-PPDU

32 32 RX Assisted Link Adaptation

33 Enhanced Block Ack (BA) Initiator Responder MD Aggregation frame D1 D2 D3 D4 SIFS Compressed BA BAR*= Block Ack Request Implicit BAR* BAR can be omitted Compressed BA Support for non-fragmented BA to reduce the bitmap size to 1 bit per MSDU Truncation of the bitmap to reduce the number of MSDUs acknowledged in the bitmap Frame Control Duratio n/id RA TA BA Con trol BA Starting S eq. Control BlockAckBitmap FCS 33 Compressed Non-Frag Num MSDU TID BA Bitmap size is fixed through BA setup.

34 PHY Proposal Comparison Ref: [Sync], [WWiSE]

35 TGn Sync PHY Summary Mandatory Features: 1 or 2 Spatial Streams 20MHz and 40MHz* channelization 1/2, 2/3, 3/4, and 7/8 channel coding rates 400ns & 800ns Guard Interval Full & seamless interoperability with a/b/g Optional Features: Low Density Parity Check (LDPC) Coding 3 or 4 spatial streams *Not required in regulatory domains where prohibited. 140Mbps in 20MHz 243Mbps in 40MHz 35

36 WWiSE PHY Summary Mandatory Features: 2 transmitters in 20 MHz 1/2, 2/3, 3/4, and 5/6 channel coding rates Rates 54, 81, 108, 121.5, 135 Mbps Optional Features: 3 and 4 transmit antennas Space-time block codes for longer range 40 MHz counterparts of all 20 MHz modes LDPC code 36

37 Throughput Enhancement Features TGnSync WWiSE Bandwidth extension 20MHz 40MHz mode (M) 20MHz mode (M) 40MHz, whenever regulatory domain permits this extension (M) (O) 20 MHz mode 40 MHz mode MIMO-OFDM-SDM (M) 2 spatial 20MHz mode (M) 2 spatial 20MHz mode Guard interval (GI) shorten ing ( 0.8us 0.4us ) (M) (N) Higher code rate (R) (M) R= ½, 2/3, ¾, 7/8 (M) R= ½, 2/3, ¾, 5/6 Higher order modulation scheme (O) 256 QAM (ABF-MIMO mode) (N) Adaptive modulation (O) Bit loading (+ 256 QAM) + power weighting (ABF-MIMO) (N) Reserve more data tones (M) 48 (4 20MHz (M) 108 (6 40MHz (M) 54 (2 20MHz (O) 108 (4 40MHz (M) Mandatory (O) Optional (N) Not available 37

38 Tx Rate Comparison (Maximum achievable uncoded data 64QAM) TGnSync 20 MHz BW + 2 Tx (M) 108 (R=3/4) (M) 140 (R=7/8 with ½ GI) WWiSE (M) (R=3/4) (M) 135 (R=5/6 ) 20 MHz BW + 4 Tx (O) 280 (R=7/8 with ½ GI) (O) 270 (R=5/6 ) 40 MHz BW + 2 Tx (M) 243 (R=3/4 ) (M) 315 (R=7/8 with ½ GI) (O) 243 (R=3/4) (O) 270 (R=5/6) 40 MHz BW + 4 TX (O) 630 (R= 7/8 with ½ GI) (O) 540 (R=5/6 ) (M) Mandatory (O) Optional Observations: (1) T > W, between R=7/8 with ½ GI for T and R=5/6 for W (2) W >= T, at R=3/4 & 2Tx 38

39 Conclusion IEEE is evolving today Up to v on-going Overviewed e and 11n core techniques and proposals Close interworking with other air interfaces, e.g., Wibro, should be one of key issues in the future! 39

40 References [Yang02] Yang Xiao and Jon Rosdahl, Throughput and delay limits of IEEE , IEEE Com. Letters, Aug [Tinnirello05] Ilenia Tinnirello and Sunghyun Choi, "Efficiency Analysis of Burst Transmissions with Block ACK in Contention- Based e WLANs," to appear in Proc. IEEE ICC'2005, May [Kim04] Youngsoo Kim, Sunghyun Choi, Kyunghun Jang, and Hyosun Hwang, "Throughput Enhancement of IEEE WLAN via Frame Aggregation," in Proc. IEEE VTC'04-Fall, Sept [Samsung] Kunghun Jang et al. "SAMSUNG MAC Proposal Technical Specification," IEEE /918r2, August [Sync] Syed Aon Mujtaba, TGn Sync Proposal Technical Specification, IEEE /889r2, Jan [WWiSE] Manoneet Singh et al., WWiSE Proposal: High throughput extension to the Standard, IEEE /886r6, Jan