1. Synchronization Requirements and Solutions for 802.11n
Month 2002
doc.: IEEE /xxxr0
July 2004
Synchronization Requirements and Solutions for n
Morgan H. Miki, John M. Kowalski
Sharp
Morgan H. Miki, John M. Kowalski, Sharp
John Doe, His Company

2. July 2004
Problem:
Consumer electronic devices figure prominently in n usage models.
CE devices however, require tight synchronization to maintain high quality audio if multicasting is done.
This presentation presents some results on the state of the art for synchronization in 11a, and recommendations to improve it for n.
Without some solution in this regard, the user experience of CE over n may be compromised.
Morgan H. Miki, John M. Kowalski

3. Clock Synchronization
July 2004 1
Clock difference jitter
Clock difference jitter: Depends on clock precision and
how often TSs are distributed
Tx clock:
20MHz ± 20ppm
Rx clock:
20MHz ± 20ppm
TS: Time Stamp
TSF: Timing synchronization function
TSF timer
TSF timer TS
Channel TS RX TX
Packet detection jitter 3
Packet detection jitter: Depends on synchronization algorithm, PHY rate and PER
Channel jitter 2
Channel jitter: Depends on the movement of stations (1ns jitter for 30cm)
Clock synchronization is affected by
Morgan H. Miki, John M. Kowalski

4. Clock Synchronization requirements
July 2004
Clock Synchronization requirements
Desired for stereo audio (professional level) [1]
Acceptable for one receiver
Acceptable for stereo audio[1]
Synchronization jitter
10ms
1ms
100us
10us
1us
100ns
10ns
1ns
100ps
802.11a
MPEG2: 500ns
1394: app 40ns
Why improve synchronization?
HDTV/STDV/audio streams are n applications.
The jitter generated by wireless link should be reduced at wireless level, to minimize overall delay.
The effect is to make CSMA/CA behave more truly like a discrete time system.
Technology has improved anyway since 11a was introduced.
Morgan H. Miki, John M. Kowalski

5. Possible solution:
July 2004
Add 8 byte 10ns unit Time Stamp in every QoS packet (Option)
Example of implementation may use DLL (Delay locked loop) eP eQ ef
TX TSF
Timer
TimeStamp
+ PHY/MAC
processing
time
1 / T KP eI KI 1
fRX
Z-1
fTX
Loop Filter
RX TSF
Timer rf
Tx/Rx timer
synchronization
error jitter
PHY/MAC processing time is the time between packet detection and
MAC to output the Time Stamp for DLL
Morgan H. Miki, John M. Kowalski

6. Simulation conditions
July 2004
Simulation conditions
Tx and Rx clock precision: 20 ppm
Packet detection with legacy 11a preambles and PLCP.
App. 10% 16QAM ¾ coding rate. This results in packet detection jitter of ±100ns. (video applications)
Simulations with Channel B/D/E
Channel jitter considered as 0ns i.e. no station mobility (1ns jitter/30cm).
Interval between packets: 500us (max 30ms at high error rate region)
Morgan H. Miki, John M. Kowalski

7. Simulation results (channel B)
July 2004
Synchronization error jitter (ns)
Jitter: < ±25ns
Convergence time: 40s
Time(s)
Similar results for channel D/E
Morgan H. Miki, John M. Kowalski

8. Comparison Mode Time to convergence Synchronization error jitter --
July 2004
Mode
Time to convergence
Synchronization error jitter
1us TS, TS in beacons,
no DLL, 20ppm oscillator(11a) --
±10us
with DLL, 20ppm oscillator(11a)
20 min
±250ns
10ns TS, TS in beacons,
with DLL, 20ppm oscillator
20min
±50ns
10ns TS, TS in every QoS packets, with DLL, 20ppm oscillator
40s
±25ns
10ns TS, TS in every QoS packets, with DLL, 1ppm oscillator
60s
±10ns
Morgan H. Miki, John M. Kowalski

9. July 2004
Conclusions
The use of 8 byte 10ns unit time stamp in all QoS packets should be considered as option.
A great improvement from legacy (10us to 25ns) can be achieved.
Synchronization can be further improved to acceptable stereo audio level by using higher precision oscillators (10ns)
Presently investigating the effects of synchronization when MIMO preambles, other information, other preamble formats, etc. are used.
Morgan H. Miki, John M. Kowalski

10. July 2004
References
[1] Julian Dunn: “Sample clock jitter and real-time audio over the IEEE1394 high performance serial bus”, Nanophon Limited
Morgan H. Miki, John M. Kowalski