1. Considerations on short packet transmission overhead
Month Year
doc.: IEEE yy/xxxxr0
Considerations on short packet transmission overhead
Date: September 18, 2011
Authors:
Name
Company
Address
Phone
George Calcev
Huawei Technologies Co., Ltd.
3601 Algonquin Road, Suite 1000
Rolling Meadows,
IL 60008,  USA
Bin Chen
Lin Cai
John Doe, Some Company

2. Objective
This contribution presents some considerations on small packet overhead in ah

3. 802.11ah Requirements
Operate under 1GHz bands; in US it will operate at MHz with output power limit of 30dBm for less than 6dBi antenna gain and uniformly spread over more than 500KHz bandwidth
Provide a mode of operation with low rate and high range (100kbps, 1Km)
Allow for more than 2007 STAs to be associated with an AP
Operate in indoor as well as in outdoor environments

4. Short Transmissions Overhead
In sensor networks the packet length is mostly between bytes
Overhead
PHY overhead – 8 symbols
MAC overhead –MAC header with at least 3 addresses (Table 8-18) , FC(2)+Duration(2)+RA(6)+TA(6)+DA(6)+SC(2) + FCS(4) = 28 octets=224 bits
Protocol overhead – ACK
Channel access overhead – at least SIFS for polled access

5. SIFS, CCA durations (for 1.25 MHz)
aRxTxSwitchTime << 1 μs
aCCATime ≤ 64 μs (scaled up from 4us for 20 MHz to 1.25 MHz, see Table 17-17)
aRxTxTurnaroundTime ≤ 2 μs
aAirPropagationTime < 4 us
aMACProcessingDelay ≤ 2 μs
aSlotTime =: aCCATime + aRxTxTurnaroundTime + aAirPropagationTime+ aMACProcessingDelay=72 us (scaled)
aSIFSTime =: aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime = (16 us in 20 MHz, scales to 256us, see Table 17-7)

6. Short Transmissions Overhead
Each symbol can carry 24 bits in 52 subcarriers , with BPSK, ½ (Table 17-4)
Data bytes about symbols necessary
MAC overhead =9.33 symbols
PHY overhead =8 symbols (Greenfield HT), 10 symbols in 11ac, and 10 symbols in 11n mixed (Figure 19-1)
SIFS overhead = 4 symbols (16us scales up based Table 17-7)
ACK overhead =13 symbols (14 bytes MAC, 14*8/24=4.6 symbols +8 PHY)
The overhead is ( )/33.3= 103% or 34.33/66.7 =51.5%
If the extra SIFS before the transmission is considered the overhead increases
This overhead gets potentially higher for higher data rates when data takes less symbols

7. Conclusions
The actual overhead is very high for short packet communications
The actual aggregation is not useful in UL transmissions for smart sensors networks (it could add large delays)
802.11ah needs better solutions for handling short packets

8. References
802.11REVmb_D8.0
P802.11ah_D1.0