1. Date Submitted: [03 July, 2008]
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Enhancement to with Adaptive Frequency Diversity]
Date Submitted: [03 July, 2008]
Source: [Qin Wang / Peng Zeng], Company [University of Science & Technology Beijing/ Shenyang Institute of Automation-Chinese Academy of Sciences /]
Address [ 30 Xueyuan Road, Beijing , China]
Phone: [ ], FAX: [ ], [
Re: [IEEE P e Call For proposal]
Abstract: [This document proposes an enhancement to IEEE MAC Layer with Adaptive Frequency Diversity for the Reliability and Deterministic Aware Applications]
Purpose: [This document is a response to Item a) better support the industrial markets in IEEE P SG4e Call for Application on 14 November, 2007]
Notice: This document has been prepared to assist the IEEE P It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P
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2. Enhancement to 802.15.4-2006 with Adaptive Frequency Diversity
Qin Wang1) , Peng Zeng2)
1) University of Science & Technology Beijing
2) Shenyang Institute of Automation-Chinese Academy of Sciences
03 July 2008
In order to increase the reliability more, we develop a kind of adaptive frequency diversity (AFD). The technology can be used in our previous proposal, but not limited in that.
The proposal is also given by Peng Zeng and I.
18 July Qin Wang /Peng Zeng
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3. Components. Mesh+Star topology Extended superframe
CSMA/TDMA+FDMA during active period
TDMA+FH/AFD during scheduled period
Two-stage allocation
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4. TDMA+FH in the Inter cluster period and manger period
scheduled period
CH1
CH2
CH3
CH4
CH5
CH6
CH7
Intra-cluster
Sleeping
Collect data from SNs
Inter cluster
& Manager period
sleeping
TDMA+
FDMA+AFD
*=>R1
* retry
* retry
R2=>R1
Similar with the previous proposed scheme, the other parts of scheduled period are also arranged by system manager, while the AFD replaces FH during the intra-cluster period.
R1=>R2
R1=>R3
R3 =>R1
R2 =>R1
The Inter cluster period and manager period are scheduled by the system manager.
18 July Qin Wang /Peng Zeng
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5. E-Superframe MAC
IEEE
Proposed Scheme
MAC Protocol
CAP
CSMA + FDMA
CFP
TDMA + FDMA
Inactive
Intra-cluster period
+ Adaptive Frequency Diversity (AFD)
Inter-cluster period
Manager period
TDMA+FH
Note: Difference between AFD with Frequency Hopping (FH)
FH: Change communication channel according to a scheduled frequency hopping pattern, regardless how the real channel condition is.
AFD: Change communication channel according to the real channel condition. In another word, bad channel condition, which can be measured with packet drop rate or resend time, triggers the operation of changing channel.
Then, the table shows where the AFD is used. In the intra-cluster communication period, instead of FH, a kind of Adaptive Frequency Diversity is used.
Both of AFD and FH are based on a fact, i.e. individual channel experience different levels of fading and interference. But they are different approach. The difference between AFD and FH used here is,
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6. Successful transmission rate over time (7 hours)
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7. Successful transmission rate over time (3 days)
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8. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
Measurement
Denote the interval of channel blocked in number of slot as ICB
Before present the AFD scheme, I would like to describe our observation in the experiments and analysis.
Usually, people measure the packet loss rate to observe the link quality, and according to the feature of link quality to develop technology for increasing reliability. However, we found another metrics is also very important, i.e. the interval of channel blocked in number of slot, denoted as ICB. The figure shows the probability distribution of ICB under different packet loss rate. For example, while packet loss rate less than 10%, the probability of ICB=1 is above 95%.
On the other side, it indicated that from the distribution of ICB, instead of packet loss rate, the device can figure out the channel condition.
Probability of ICB=1,2,3,4,5 vs. Packet loss rate in 0-30%
18 July Qin Wang /Peng Zeng
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9. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
Measurement
Denote the interval of channel blocked in number of slot as ICB
The figure shows the relationship between probability of each ICB value and the packet loss rate. The x-axis is the packet loss rate, and the y-axis is the probability of ICB equals to 1,2,3,4,5 and greater than 5. It shows, while packet loss rate is less than 10%, the probability of ICB=1 is dominated, while the packet loss rate is greater than 85%, the probability of ICB>5 is dominated.
Usually, the packet loss rate in the real field of factory is mostly less than 15%. Thus, ICB=1 is dominated.
Probability of ICB=1,2,3,4,5, >5 vs. Packet loss rate
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10. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
Analysis: a b
considering two kinds of slot assignment:
a: two continuous transmission opportunities
b: two discontinuous transmission opportunities
define:
A: transmission failure in the first opportunity;
B: transmission failure in the second opportunity;
For a certain link on a fix channel , if a and b all fail,
P(a) = P(B|A)= P(AB)/P(A)
P(b) = (P(A)+P(B))/2
In a similar way, for a, b
Define:
C: transmitting fail in the first channel,
D: transmitting fail in the second channel
For a certain link, if a and b all fail,
P(a) = P(CD)/P(C)
P(b) = (P(C)+P(D))/2
conclusion
If P(a)<P(b),
for a fixed channel, retransmitting immediately will bring high link reliability than retrying later.
For a fixed link, changing channel when channel failed brings high reliability than fixed channel changing.
From the conditional probability theory, we know that if the probability of ICB=1 is dominated, doing retry just after one time of packet loss will be successful with higher probability, comparing with doing retry later without condition. We call such kind of retry as immediate retry.
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11. TDMA+FDMA+AFD in the Intr-cluster periods
Scheduled period of a E-superframe
Scheduled period of a E-superframe CH 1
Intra Cluster_1
inter cluster & manager period
sleeping
Intra Cluster_1
inter cluster & manager period 2
Intra Cluster_2
inter cluster & manager period
sleeping
Intra Cluster_2
inter cluster & manager period 3
Intra Cluster_3
inter cluster & manager period
sleeping
Intra Cluster_3
inter cluster & manager period 4
Intra Cluster_4
inter cluster & manager period
sleeping
Intra Cluster_4
inter cluster & manager period 5
Intra Cluster_5
inter cluster & manager period
sleeping
Intra Cluster_5
inter cluster & manager period 6
Intra Cluster_6
inter cluster & manager period
sleeping
Intra Cluster_6
inter cluster & manager period 7
Intra Cluster_7
inter cluster & manager period
sleeping
Intra Cluster_7
inter cluster & manager period 1
Intra Cluster_8
inter cluster & manager period
sleeping
Intra Cluster_8
inter cluster & manager period
Now, we are going to present the scheme used during intra cluster period.
During intra-cluster communication period, each router, i.e. cluster head, is assigned more than one channel, e.g. two channel, denoted as CH_i and CH_j.
In the slide, we presents some criteria and a process to assign channel to a cluster head. That is,
Channel allocation: (1) choose a channel from {ch1..ch8} as CH_i; (2) choose a channel from {ch9..ch16} as CH_j, while init or re-init.
Criteria: (1) haven’t been chosen by neighbors; (2) with best SNR for communication
The channel used in the contention period could be different with CH_i and CH_j
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12. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
Communication intra cluster-i. Each slot have m minislot, m can be sent by system manager. In each minislot, a node can send a message and receive an ACK. With REQ, cluster head announce which communication is successful.
Inter-cluster
Retry period
CAP
Inter-cluster
Retry period 1 2 Rx
ACK Rx
ACK Rx
ACK
REQ Rx
ACK Rx
ACK Rx
ACK Rx
ACK Rx
ACK
REQ Ri Rx
ACK Rx
ACK Rx
ACK Rx
ACK Tx Rx Tx Rx N1 Tx Tx N2 Tx Rx Tx Tx N3 Tx Rx Tx Rx Tx N4 Tx Rx Tx Tx Rx
The figure shows a example of AFD during intra cluster period. Assume there are five sensor nodes in the cluster of Ri. The sensor nodes transmit measured data at CH-i sequentially based TDMA. In each timeslot, there are two mini-slots. In each mini-slot sensor node can transmit a data packet and receive a ACK. The sensor node recognize FAIL by losing ACK.
The intra cluster period is divided into data collect period and frequency diversity period. During data collect period, cluster head and sensor nodes work on CH-I channel, and during the frequency diversity period, cluster head and part of sensor nodes, who are failed in the data collect period, work on the CH-j channel.
As shown in the figure, every sensor node wakeup, communicate with Ri, and then go to sleep regardless successful or fail. For example, N1 transmits data and receives ACK. N2 transmits data but not receives ACK, so do immediate retry, and then fail again. But, in fact, the Ri receive data correctly. N5 transmits data, fail, and do immediate retry, and fail again. But different from N3, Ri didn’t receive the data from N5 correctly. From the view of Ri, only the communication between N5 and Ri is failed, while the communication between N3 and Ri is successful.
At the beginning of frequency diversity period, N3 and N5 wake up, switch to CH-j and listening; at same time Ri switch to CH-j and broadcast REQ, and allocate N5 a timeslot, but does not assign N3 a timeslot. From the REQ, N3 knows it is unnecessary to transmit data again, and thus go to sleep. N5 finds the timeslot assigned to it, thus transmit data again during the frequency diversity period.
There are two points need to mention.
CH-I and CH-j are assigned by system manager, and thus, can be re-assigned by system manager according to the channel condition.
The CAP & CFP is also opportunity to re-send data while the communication in both data collect period and frequency diversity period are fail. N5 Tx Rx Tx Rx Tx Tx
1st channel
2nd channel
Contention period channel
Invited retry
Content retry 2 1
18 July Qin Wang /Peng Zeng
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13. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
Algorithm
A slot is divided into m mini-slots, m is set by system manager. A mini-slot is used to finish a communication. Thus, there are m communication opportunities in a slot.
The communication rules are as follows:
1) Employ ch_i to communicate in the first mini-slot
2) if the first communication fails, employ ch_i to retry
3) Form the third mini-slot to the mth mini-slot, employ ch_i to retry, or retry in a defined alternative channel according to configuration.
If the communication failed in the m mini-slots, the device will sleep until the first intra-cluster retry slot. While waking up, the device changes to the defined alternative channel, and listens to the broadcast from the cluster-head. The broadcast from cluster-head has the status of the communications on each link, success or fail. For the link on which communication failed, a timeslot is assigned in the broadcast message.
For the devices who didn’t receive ACK, if cluster-head has received the data but the device lost ACK, the device will know the its package has been received successfully, thus, will not retry again. Otherwise, the device transmits packets in the assigned time slot.
After the device retry, it changes back to the former channel.
If the device retry failed, it can retry during the next CAP.
If the packet loss rate of a link is too high during a period, cluster head will notice the device to change to a new channel in the manager period.
Here is the algorithm in detail
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14. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
The figure shows the number of retry for 99.99% reliability. It indicates that immediate retry is a big contributor to increase reliability.
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15. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
With immediate retry, but without channel diversity. Five cluster, about 70 nodes in a cluster. <15% packet loss rate.
The figure shows that while the packet loss rate is less than 20%, with immediate retry, the reliability can reach to 95%.
18 July Qin Wang /Peng Zeng
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16. TDMA+FDMA+AFD in the Intra-cluster periods (cont.)
With immediate retry, but with channel diversity. Five cluster, about 70 nodes in a cluster. <15% packet loss rate.
The figure shows that besides immediate retry, a frequency diversity while failing in retry will increase the reliability to 99%.
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17. Summary
Besides packet loss rate, the interval of channel blocked in number of slot is very important metrics for increasing reliability of communication
Immediate retry in MAC layer is a efficient way to increase reliability.
Frequency diversity is a effective way to increase reliability, while it is secondary to immediate retry .
Let’s summary what we found from experiments and analysis.
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18. AFD: immediate-retry, backup-channel
Conclusion from 0503 & 0504
Mesh+Star topology
Extended superframe
CSMA/TDMA+FDMA during active period
TDMA+FH/AFD during scheduled period
Two-stage allocation
AFD: immediate-retry, backup-channel
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