Patent Publication Number: US-2007097930-A1

Title: Method of implementing the Multi-MCS-Multi-Receiver Aggregation&#39;&#39; scheme in IEEE 802.11n standard

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to data communication, and more particularly, to data communication in wireless local area network (WLAN) environment.  
     BACKGROUND OF THE INVENTION  
      The IEEE 802.11n standard provides for data communication, and more particularly, to data communication in WLAN environments. In IEEE 802.11n development process, a Multi-MCS-Multi-Receiver Aggregation (MMRA) mechanism has been proposed to increase the wireless LAN (WLAN) efficiency.  
      Implementing Multi-MCS-Multi-Receiver Aggregation (MMRA) requires a modification in the physical layer (PHY) aggregation. Compared to MRA, the MMRA technique can aggregate multiple packets with different transmission rates (MCS). MRA always uses the same MCS to transmit the aggregated packet to multiple receivers.  
       FIG. 1  shows the PHY Protocol Data Unit (PPDU) format for MIMO transmission, wherein the HT-SIG-MMRA (High Throughput Signal Field MMRA) signaling part is placed before the HT-STF (High Throughput Short Training Field). This required a fixed number of bits for the MCS (modulation and coding scheme), and a variable number of bits for the length of MMRA signaling part in HT-SIG 1 . Since HT-SIG field is running out of bits, it is difficult to add more bits in it unless the length of HT-SIG field is increased. However, as HT-SIG is to be transmitted in the most robust data rate, i.e., 6 Mbps, increasing the length of HT-SIG will largely increase the overhead of existing PPDU.  
     BRIEF SUMMARY OF THE INVENTION  
      In one embodiment the present invention provides a method of communicating data over a wireless local area network according to an IEEE 802.11n proposal for Multi-MCS-Multi-Receiver Aggregation (MMRA), the improvement comprising providing a modified PPDU format with MMRA signaling wherein the HT-SIG format is preserved.  
      Instead of increasing the length of HT-SIG, a modified PPDU format with MMRA signaling is provided where the HT-SIG-MMRA is placed at the end of HT-LTF. In this manner, the original HT-SIG format described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE.802.11, 11-04-889r1, November 2004 (incorporated herein by reference) is preserved, while accommodating the variable length of HT-SIG-MMRA.  
      Further, in a modified HT-SIG structure according to an embodiment of the present invention, the Length and MCS of the HT-SIG-MMRA signaling part is placed in the LENGTH field and MCS field of HT-SIG 1  (4 us), respectively, without changing the length of existing HT-SIG (8 us).  
      Further, two bits in HT-SIG 2  (4 us) indicate aggregation: AGGREGATION and MRA bits.  
      These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a PPDU format for basic MIMO transmission.  
       FIG. 2  shows an example modified PPDU format with MMRA signaling according to an embodiment of the present invention.  
       FIG. 3  shows an example HT-SIG format in the PPDU format of  FIG. 2  according to an embodiment of the present invention.  
       FIG. 4  shows a HT-SIG-MMRA Part transmission format. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Initial 802.11 stations typically send frames in the order they are received. For throughput purposes, it is highly desirable to reorder frames so that they can coalesce into larger aggregated frames. Aggregation in TGnSync is a MAC-layer function that bundles several MAC frames into a single PLCP (Physical Layer Convergence Protocol) frame for transmission.  
      A single physical-layer frame contains several MAC layer frames. Several MAC frames are put into the same PLCP frame, with an appropriate delimiter between them. The delimiter has a small reserved field, a length field for the following MAC frame, a CRC to protect the delimiter, and a unique pattern to assist in recovering individual frames from the aggregate. MAC frames are put into the aggregate without modification, and contain the full header and MAC CRC. Even if one frame out of an aggregate is lost, it may be possible to successfully receive all the remaining frames.  
      However, the benefits of aggregation in TGnSync are not confined to pairs. Single-receiver aggregation is required; an optional extension allows aggregate frames to contain MAC frames for multiple receivers, in which case they are called Multiple Receiver Aggregate (MRA) frames. Inside the single rate transmitted aggregate frame, there are multiple Initiator Access Control frames. Each IAC specifies an offset to transmit the response to the aggregated frames, which will usually be a block acknowledgment response. To distinguish multiple receiver aggregate frames from single-receiver aggregate frames, multiple-receiver frames start with a control item called the Multiple Receiver Aggregate Descriptor (MRAD). The initiator&#39;s aggregate frame starts with the aggregate descriptor, and is followed by the aggregated frames for each destination. (802.11 Wireless Networks: The Definitive Guide, Second Edition”, by Matthew Gast, Second Edition, Ch. 15, April 2005 ISBN: 0-596-10052-3).  
      Implementing Multi-MCS-Multi-Receiver Aggregation (MMRA) requires a modification in the physical layer (PHY) aggregation. In IEEE 802.11n development process, a Multi-MCS-Multi-Receiver Aggregation (MMRA) mechanism has been proposed to increase the wireless LAN (WLAN) efficiency, as described in the TGnSync Technical Proposal in the attached Appendix. As noted,  FIG. 1  shows the PHY Protocol Data Unit (PPDU) format  100 , wherein the HT-SIG-MMPA (High Throughput Signal Field MMRA) signaling part  104  of the HT-SIG field  102  is placed before the HT-STF (High Throughput Short Training Field)  106 . This required a fixed number of bits for the MCS (modulation and coding scheme), and a variable number of bits for the length of MMRA signaling part in HT-SIG 1  field  108 . Since HT-SIG field  102  is running out of bits, it is difficult to add more bits in it unless the length of HT-SIG field  102  is increased. However, as HT-SIG  102  is to be transmitted in the most robust data rate, i.e., 6 Mbps, increasing the length of HT-SIG  102  will largely increase the overhead of existing PPDU.  
      According to one embodiment of the present invention shown by example in  FIG. 2 , instead of increasing the length of HT-SIG  102 , a modified PPDU format  200  with MMRA signaling is provided where the HT-SIG-MMRA field  202  is placed at the end of HT-LTF  204 . In this manner, the original HT-SIG format  206  (described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE.802.11, 11-04-889r1, November 2004 (incorporated herein by reference) is preserved, while accommodating the variable length of HT-SIG-MMRA  202 .  
      After moving the HT-SIG-MMRA  202  to the location after the HT-LTF  204 , the HT-SIG-MMRA  202  can be transmitted in higher data rate, compared with the lowest data rate transmission in conventional systems. The modified format  200  ( FIG. 2 ) according to an embodiment of the present invention requires less time to be transmitted than the format  100  ( FIG. 1 ) where HT-SIG-MMRA  104  was in front of HT-LTF  110 , which can only be transmitted through the lowest data rate of 6 Mbps.  
      Further, referring to  FIG. 3 , showing a modified HT-SIG structure  300  including fields HT-SIG 1  and HT-SIG 2 , according to an embodiment of the present invention, the Length and MCS of the HT-SIG-MMRA signaling part is placed in the LENGTH field  302  and MCS field  304  of HT-SIG 1  (4 us), respectively, without changing the length of existing HT-SIG (8 us).  
      Two bits in HT-SIG 2  (4 us) indicate aggregation i.e. SMRA (Single MCS Multiple Receiver Aggregation), and MMRA, wherein: the AGGREGATION bit  306  indicates weather it is an aggregated packet and one more bit to indicate MMRA (e.g., the spare bit from ADV coding  308  can be used).  
      With AGGREGATION and MRA bits, the HT-SIG 1  field can be interpreted by a receiver as:  
      AGGREGATION=0: Without aggregation, LENGTH and MCS fields in HT-SIG 1  are interpreted as the length and MCS of the transmitting PSDU (PHY Service Data Unit), respectively.  
      AGGREGATION=1: With aggregation, MRA=0-&gt;SMRA, such that when MRA=0, the transmitting packet is a SMRA packet.  
      LENGTH and MCS fields  302 ,  304 , in HT-SIG 1  are interpreted by a receiver as the length and MCS of the transmitting PSDU, respectively. The LENGTH indicates the whole aggregated packet length. For example, two packet aggregates to a single PSDU, their length are l 1  and l 2  byte respectively. The LENGTH field in this case is l 1 +l 2  bytes.  
      AGGREGATION=1: With aggregation, MRA=1-&gt;MMRA, such that when MRA=1, the transmitting packet is a MMRA packet.  
      LENGTH and MCS fields in HT-SIG 1  are interpreted by a receiver as the Length and MCS of HT-SIG-MMRA, respectively. The Length and MCS for each aggregated packet are indicated in HT-SIG-MMRA. In one example, two packets are to be aggregated. One packet has length of  11  bytes and should be transmitted with MCS 1 . The other packet has length of l 2  bytes and should be transmitted with MCS 2 . The value of l 1 , MCS 1  and l 2 , MCS 2  are placed into HT-SIG-MMRA  400  as shown in  FIG. 4  which should be transmitted at rate MCS 3 . LENGTH and MCS fields in HT-SIG 1  will be l 3  and MCS 3 , respectively.  
      The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.