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MAC Partial Proposal for TGn

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  1. MAC Partial Proposal for TGn Nokia Yousuf Saifullah Naveen Kakani Srinivas Sreemanthula Nico van Waes Nico van Waes, Nokia

  2. Introduction • MAC efficiency is an important aspect of the goal of achieving 100 Mbps at the MAC SAP in a robust, economically attractive fashion. • Power Efficiency is a critical aspect of making 802.11n suitable for the handset market. • The following MAC features are proposed for achieving these goals: • Multi data rate frame aggregation • Power Efficiency in aggregation • MAC Header Compression • Aggregate ACK Nico van Waes, Nokia

  3. Multi Data Rate Frame Aggregation • STAs use different data rates due to different radio environment. Performing aggregation on multi rate increases the usability of aggregation. Thus, increases the data throughput. • Experiment results show improvement is Channel Efficiency (Data Throughput), Channel Occupancy, and Power Savings, over Single Rate Aggregation. • Introduce Num Rates, RATE #x, and Offset RATE-x in Aggregation Control Header (ACH). Could be introduced in the PHY or MAC Header. Nico van Waes, Nokia

  4. Num Rates=3 RATE #1 OffsetRATE-1 # ofSTAs # ofSTAs RATE #2 OffsetRATE-2 OffsetRATE-3 # ofSTAs FCS RATE #3 STAInfo STAInfo STAInfo MRate-ACH MPDU1 MPDU2 Midamble MPDU3 MPDU4 Midamble MPDU5 Multi Data Rate Frame Aggregation • Codecs need to be emptied and reset between two different MCS. This interruption (one OFDM symbol) is filled with single symbol Midamble for improvement of channel estimation. • Proposed structure allows flexible insertion of different size preamble (including full preamble copy) in rare cases when MCS change requires this. Nico van Waes, Nokia

  5. Exp No Rate (Mbps) M-Rate Scenario App1 users App2 Users % Ch Efficiency Improv. App3 Users % Ch Occupancy Improv. Total Users % Power Saving 126 1 0 1 2 1 11111100000 37.68 27.37 70.07 108 0 1 1 2 2 10011100110 39.98 28.56 57.09 96 0 1 1 2 3 00011111111 29.27 22.64 53.20 72 1 1 0 2 63 0 1 0 1 4 00011111000 30.90 23.60 60.02 54 0 0 1 1 5 11100001111 21.22 17.51 50.36 48 1 0 0 1 6 10101010101 40.00 24.24 47.39 36 0 0 1 1 7 11101010111 33.47 25.08 54.74 24 0 1 0 1 12 1 0 0 1 8 11100110011 24.12 19.43 51.27 6 1 0 0 1 9 11111111100 36.16 26.55 74.41 10 00111111100 36.67 25.75 67.46 M-Rate Aggregation Performance Results • Experiments are run with a a mix of users at different rates and different application traffic mix • Table-1: Appl User 1, MSDU=50; Appl User 2, MSDU=120; Appl User 3, MSDU=1500 bytes; • Table-2 shows an M-Rate scenario with results. Each bit under M-Rate Scenario indicates the inclusion (1) or exclusion (0) of the data rate rows in Table-1. Top row is indicated by MSB. Nico van Waes, Nokia

  6. Power Efficiency • Power efficiency is important for small handheld devices. These devices will be an important segment of WLAN High Throughput products. • Power efficiency should not be compromised in Frame Aggregation. • Provide power efficiency by placing MPDU lengths along with the receiving STA’s MAC address in the ACH. Doesn’t compromise MAC throughput efficiency • A STA reads ACH determines the position of its MPDUs and reads them only without reading MPDUs of other STAs. Nico van Waes, Nokia

  7. MAC Header Compression • With the High Throughput need and new applications (e.g. VoIP), MAC header (36 bytes) is becoming a significant overhead • MAC Header Compression Procedure • An AP creates a mapping between 1 byte unique Compression ID (CID) and the set of addresses in the MAC Header (Addr 1 to 4). • AP establishes the same CID in the non-AP STA by introducing “CID Association” procedure. For example: AP and STA exchange a CID Association Request followed by ACK. • The CID is established prior to exchanging any data frames • AP and STA start exchanging Compressed Header (CH) MPDU • MAC HC procedure is equally applicable for adhoc mode Nico van Waes, Nokia

  8. FrameBody Seq Control QoSControl FrameControl Duration /ID Addr 3 Addr 4 Addr 2 Addr 1 FCS Advantages & Format • Starting compression from the very first MPDU • No overhead in transmitting full header MPDUs during data transfer • Making long lived (life of association) compression context to increase the compression efficiency • Applicable to MPDUs in Frame Aggregation (FA) or outside of FA • Applicable to management frames also. Existing MAC Header Octets: 2 2 6 6 6 2 6 2 n 4 CH-MPDU Octets: 2 1 1 2 2 2 n 4 FrameBody Duration /ID QoSControl Seq Control FrameControl FCS Rsrvd CID Nico van Waes, Nokia

  9. Applications with Gain in MAC Throughput • Application with MSDU size =< 512 bytes show most gain • One application for each MSDU size is selected from TGn Usage Model document. Also, TCP ACK is selected. • Typical case of MAC header with 3 addresses are assumed, instead of max 4 addresses. Simple HC is used for compressing only the addresses. Nico van Waes, Nokia

  10. ACK Aggregation • Frame Aggregation (FA) feature sends multiple MPDUs together. For the MPDUs needing ACK, the receiving STAs could send either Block ACK or Normal ACK in the reverse link. • There is significant redundancy in using both, even if FA is used in the reverse link. • Normal ACK adds considerable overhead in the traffic, even in an aggregated frame, since an ACK frame (14 bytes) is sent for each MPDU • A Block ACK frame size is 152 bytes. Block ACK is defined on a TID basis. An aggregated frame may contain MPDUs with different TIDs for a STA. This would still result into multiple Block ACK Req and Block ACK per STA. Nico van Waes, Nokia

  11. Solution: Aggregate ACK • A-ACK is an enhancement on BA • Differences from BA • A-ACK is sent, by the receiver of an aggregated frame, without any BA Request. • It doesn’t have any issue of receiving BAR and responding it within SIFS • A-ACK is sent indicating status of each received frame only in the aggregation • One A-ACK frame contains status of frames across TIDs. Thus, no need for sending multiple BA per TID. • BA Bitmap is compressed. 128 bytes of bitmap is a significant overhead. • In summary, a responder utilizes one compressed A-ACK frame to acknowledge all the frames received in an aggregation Nico van Waes, Nokia

  12. A-ACK Format X octets 2 octets 2 octets 6 octets 6 octets 2*Num TIDs octets 2 octets 4 octets Frame Control Duration RA TA BA Control Starting Sequence Control BA Bitmap FCS Num MSDUs NumTIDs TID Rsrvd Bits : 4 4 2 6 Repeat “Num TIDs” times. Subsequent BA Control will have 4Rsrvd bits instead of Num TIDs. • “Num TIDs” indicates number of TIDs aggregated in one A-ACK frame • “Num MSDUs” indicates number of MSDUs represented in the BA Bitmap. This is used in calculating BA Bitmap size as follows: • No Fragmentation: “Num MSDUs” indicates number of valid BA MSDU Bitmap bits. BA bitmap is sent at byte boundary, & rest of the bits are don’t care, e.g. a value of 6 indicates a byte of BA MSDU bitmap with only first 7 bits valid. X = (Num MSDU/8+1) octets • Fragmentation: Each MSDU has two bytes of “BA Bitmap”, indicating the status of all possible 16 MPDUs, e.g. a value of 6 indicates 14 bytes of BA MPDU Bitmap. X = (Num MSDU +1) * 2 octets • “BA Bitmap” can take two definitions depending on the use of fragmentation • No Fragmentation: Each bit indicates the status of one MSDU with sequence number= Starting Sequence Number + bit position. • Fragmentation: In this case, it has the same definition as in 802.11e spec. Each MSDU has two bytes of bitmap, indicating the status of all possible 16 MPDUs. Nico van Waes, Nokia

  13. A-ACK Format Considerations • It is likely that MSDU fragmentation will not be used in aggregation. However, a generic A-ACK structure is defined that is used for both fragmentation and/or no-fragmentation case. • No explicit need for negotiating fragmentation/no-fragmentation structure between receiver and originator. Based on fragmented/no-fragmented frames sent/received in aggregation, the originator/receiver decode/encode 1 bit/2 bytes per “Num MSDU” value. • The originator and receiver both know the “Num of TIDs”, “Num of MSDUs” per TID, and fragmentation/no-fragmentation received in an aggregated frame. Thus, can calculate the exact size of A-ACK from receiver, this helps in setting NAV accurately. Nico van Waes, Nokia

  14. Aggregate ACK Benefit Analysis • Assuming • no-fragmentation • 24 MSDUs equally divided in 3 TIDS • A-ACK Overhead • Frame size of an A-ACK = 36 bytes • Normal ACK Overhead • Frame size of on ACK = 14 bytes • For 24 MSDUs, Frame size = 24*14 = 336 • Block ACK Overhead • Overhead due to 3 Block ACK Req/Block ACK = 3 (24 + 152) = 528 • A-ACK saves 89% over Normal ACK • A-ACK saves 93% over Block ACK Nico van Waes, Nokia

  15. Conclusion • The proposed MAC features substantially improve MAC throughput, as well as power efficiency, which is critical for handset applications • The features can be introduced easily by modifying/enhancing the existing procedures and frame structures • Analysis has been provided to show the benefit Nico van Waes, Nokia

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