1. Compressed MAC PDU Overhead
Document Number:
IEEE S802.16maint-08/011r2
Date Submitted:
Source:
Xiao Xu and Hua Xu Voice:
Motorola Inc.
Venue:
Working group letter ballot recirc #26a, IEEE P802.16Rev2/D2
Base Contribution:
IEEE C802.16maint-08/011r3
Purpose:
For review and adapt.
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2. Background and Problem
Currently for each MAC PDU, 6 bytes are used for generic MAC header (GMH) and 2 bytes are used for fragmentation subheader (FSH).
These MAC overheads significantly reduces system capacity, especially for applications such as VOIP, where the payload is relatively small.

3. Current Generic MAC Header Structure

4. Proposed Compressed GMH (3 bytes)

5. Proposed Compressed GMH (3 bytes)
Removed 1 bit ‘Reserved’.
Removed EC and CI fields. These information will be exchanged during connection establishment process only and will not be transmitted in every MAC PDU.
‘Type’ field reduced from 6 bits to 3 bits:
Bit # 5 Reserved is removed, assuming initial deployments will be PMP
Bit # 4 ARQ feedback is removed, assuming non-ARQ (HARQ only) connections
Bit # 3 Extended type is removed, assuming ‘Not extended’ to save overhead.
Reduced 8-bit HCS to 6 bits.

6. Proposed Compressed GMH (3 bytes)
LEN field is reduced from 11 bits to 7 bits. This represents packet sizes of up to 127 bytes. It is sufficient for small packet size applications such as VOIP.
Changed the ‘CID’ field to ‘CID index’ field. The BS knows to which MS the data region belongs so there is no need to carry the 16-bit CID in every packet. All that is needed is to carry a CID index (0-15, representing maximum of 16 flows associated with the MS). The BS maintains a table which maps the index to the 16-bit CID associated with the MS.

7. Current FSH (2 bytes)

8. Proposed compressed FSH (1 byte)
For non-ARQ (HARQ) connections, remove 3 bits ‘reserved’, combine them with the 3 bits FSN for ‘non-extended’ to form a 6-bit FSN field.
Assume ‘Not extended’ in order to save overhead.

9. Current PSH (3 bytes)

10. Proposed compressed PSH (2 bytes)
For non-ARQ (HARQ) and non-extended connections, use 6 bits for the FSN field.
Reduce the ‘Length’ field from 11 bits down to 8 bits.

11. Benefits of MAC Overhead Reduction
MAC overhead went down from 8 bytes to 4 bytes, a savings of 50%.
Assuming VOIP payload size of 33bytes, the overhead percentage went down from 13.6% to 6.8%.
Simulation shows system capacity improvements of 7%.