1. On Robust Neighbor Discovery in Mobile Wireless Networks
ACM CoNEXT 2015
On Robust Neighbor Discovery in Mobile Wireless Networks
Tong Meng1, Fan Wu1, Aijing Li2, Guihai Chen1, Nitin H. Vaidya3
1Shanghai Jiao Tong University, China
2PLA University of Science and Technology, China
3University of Illinois at Urbana-Champaign, US
Two key words

2. Keyword 1: Neighbor Discovery
Prerequisite: Smart Devices
Rising User Population
Increasing Device Capabilities
User population -> existence of neighboring devices
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

3. Keyword 1: Neighbor Discovery
Prerequisite: Smart Devices
Rising User Population
Increasing Device Capabilities
Internet
Social Media
Music
Games
Phones s
TV/Film
Texting
Books
Camera
Entertaining usages -> proximity-based applications
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

4. Neighbor Discovery: Convenience
Demand: Proximity-Based Applications
Entertainment (e.g., FireChat)
Thanks for Greeting!
Hi! How You Doing
Communicate with nearby users
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

5. Neighbor Discovery: Convenience
Demand: Proximity-Based Applications
User-Provided Connection (e.g., OpenGarden)
Connect to nearby users to enjoy Internet, which is called UPC
All such applications rely on neighbor discovery ability.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

6. Previous Focus Requirement: Neighbor Discovery Protocol
Energy Efficiency: Battery Power
Time Efficiency: Mobile Users
Active Slot
Sleeping Slot
Till now, previous works mainly focus on designing ND protocols.
They typically use a time-slotted model.
Active/sleep model
Deterministic active-sleep schedule
Exchange beacons to discover neighboring nodes.
Beacons
(MAC)
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

7. Keyword 2: Robustness Beacon Decoding Bottleneck
Cause: Interfering Background Signals
WiFi AP
Have to decode beacons, impeded by the interference
WiFi Extender
WiFi AP
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

8. Searchlight [Mobicom’12]
Existing Drawbacks
Beacon Decoding Bottleneck
Problem: Longer Latency
Problem: No Successful Discovery Guarantee
Long Tail
Median Increment ≈ 20%
> 70%
Unfortunately, such decoding bottleneck can significantly prolong the discovery latency. …
Therefore, the decoding bottleneck seriously restrict the robustness of ND.
Searchlight [Mobicom’12]
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

9. Why Decoding Purpose of Beacon Decoding
MAC Address: Distinguish Neighbors
So Many
Names …
Record
the Voice
How do We
Distinguish Strangers
in Practice?
Makes us think that what is the purpose of such decoding process?
But is that necessary?
Let’s imagine another case: how people distinguish strangers in real life?
So, we can increase the robustness of ND by designing a similar discovery mechanism w/o decoding.
Decoding MACs(Names): not Necessary
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

10. Design Challenges Decoding-Free Mechanism Resist Strong Interference
Detect Neighbor Discovery Messages
Distinguish Neighbor Identities
Resist Strong Interference
Discovery Technique?
Message Structure?
That will involve these main challenges.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

11. Technique: Correlation
Pseudo-Random Sequence: “𝒔” (L bits)
Received Symbol Sequence: “𝒚”
𝐶 𝒔, 𝒚, ∆ = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑦 𝑖+∆ ) = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑛 𝑖+∆ )
Receive Interference/Noise
In this work, we utilize the technique of correlation, which is a common technique in wireless communication to detect a pre-defined sequence.
Say, there is a fixed well-designed pseudo-random sequence s.
When we only receive some interference and noise, the correlation with s only produce some low magnitudes.
Low Magnitude
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

12. Technique: Correlation
Pseudo-Random Sequence: “𝒔” (L bits)
Received Symbol Sequence: “𝒚”
𝐶 𝒔, 𝒚, ∆ = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑦 𝑖+∆ ) = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑛 𝑖+∆ ) + 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑠 𝑖 )
Receive “s”
Peak
Only when an actual copy of sequence s is received, there will be a peak in the correlation magnitude,
Which denotes the appearance of s.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

13. Technique: Correlation
Pseudo-Random Sequence: “𝒔” (L bits)
Received Symbol Sequence: “𝒚”
𝐶 𝒔, 𝒚, ∆ = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑦 𝑖+∆ ) = 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑛 𝑖+∆ ) + 𝑖=1 𝐿 ( 𝑠 𝑖 ∗ ∙ 𝑠 𝑖 )
Resist Low SINR
Moreover, such correlation technique can resist very low SINR,
For example, As low as -6dB shown here.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

14. ReCorder: Message Detection
RCover Preamble: Pre-Defined, Pseudo-Random
Received
Symbols *
Preamble
(LC-bit)
Considering such correlation process, we design a new neighbor discovery mechanism named ReCorder.
In ReCorder, we implement a message structure that is specific to ND.
First, we fix a pseudo-random sequence as an RCover preamble for neighbor discovery detection.
Each node keeps calculating the correlation between the RCover preamble and the received symbols.
The peak of the correlation magnitude represents the starting point of a discovery message.
Correlation
Position =
Appearance of Discovery Message
Correlation Magnitude
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

15. ReCorder: Neighbor Recognition
ReCord Signature: Two-Level Identity
Level-1: Pseudo-Random Sequence (L1-bit)
Then, we design a unique ReCord signature for each node.
It contains two levels of identity.
On the first level, we use a randomly shifted pseudo-random sequence.
Random Cyclic Shift
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

16. ReCorder: Neighbor Recognition
ReCord Signature: Two-Level Identity
Level-1: Pseudo-Random Sequence (L1-bit)
Level-2: Randomly Generated (L2-bit) + 1 1 1 1 1
The second level is a totally random sequences.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

17. ReCorder: Neighbor Recognition
ReCord Signature: Two-Level Identity
Level-1: Pseudo-Random Sequence (L1-bit)
Level-2: Randomly Generated (L2-bit)
Actual Shift Offset
Then, the recognition of leve-1 identity is to identify the shift offset of the received signature,
For that purpose, a node need to calculate its correlation with the pseudo-random sequence for level-1 identity generation on all possible shift offsets.
As shown here, the offset that generates the highest correlation value is the actual shift offset.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

18. ReCorder: Neighbor Recognition
ReCord Signature: Two-Level Identity
Level-1: Pseudo-Random Sequence (L1-bit)
Level-2: Randomly Generated (L2-bit)
Match with Stored Level-2’s 1 1 1
Guarantee Uniqueness
Clearly, the number of different level-1 identities is restricted by the code length.
That’s why we include the level-2 identity, which can guarantee the uniqueness of different signatures.
In the case of level-1 name collisions, a node will need to match a new identity with the stored ones.
Such level-2 identity is for secondary usage, only necessary in case of level-1 name collision.
Secondary Usage
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

19. Correlation Threshold
Signal Strength Estimation
Moving Average of Correlation Magnitude
Correlation “minus” Average
Received Energy Level
Self-Correlation of Received Symbols
As for the signal strength, we estimate that using the difference between the instant and average correlation value.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

20. Continuous OFDM Interference Source
Implementation
USRP-N210 Testbed
Neighbor Discovery
Continuous OFDM Interference Source
In the following, to evaluate all the above designs, we use a USRP testbed.
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

21. Feasibility Benchmark (RCover)
RCover Detection
Few False Positives in All 500 Samples
Increased
Overhead
Increased Robustness
127-bit, -6 dB, 3.5% False Negative
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

22. Feasibility Benchmark (ReCord)
ReCord Recognition
Even Lower False Possibility
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

23. Feasibility Benchmark (ReCord)
ReCord Recognition
Even Lower False Possibility
Below 2%
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

24. Feasibility Benchmark (ReCord)
ReCord Recognition
Even Lower False Possibility
-5 dB, 5% False Possibility
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

25. Increased Robustness ReCorder vs. OFDM Beacon-Decoding
10 dB Increment in Robustness
10 dB More Robust
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

26. Application in Protocols
ReCorder (-5dB) vs. Beacon-Decoding (4dB)
5% duty cycle, 4 neighbors, 200 runs
Long-Tail
55.2% Worst-Case Gain
11.9% Median Gain
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

27. Application in Protocols
ReCorder (-5dB) vs. Beacon-Decoding (4dB)
5% duty cycle, 4 neighbors, 200 runs
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

28. Co-Existence with OFDM
Continuous ReCorder Message Transmission
No Degradation to Low Bit-Rate OFDM Packets
Management/Control
Frames
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

29. Co-Existence with OFDM
Low Duty-Cycle ReCorder
Only Cause Random/Occasional OFDM Packet Loss
E.g., 20ms Time Slot for ReCorder, 1500-Byte OFDM Packets
ReCorder
Duty Cycle
UDP Throughput (Mbps)
Degradation
w/o ReCorder
w/ ReCorder 1% 2
1.994
0.3% 5%
1.97
1.5% 5
4.994
0.12%
4.97
0.6%
* Similar for TCP (PCC [NSDI’15])
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

30. Conclusion
Decoding-Based Discovery Mechanism in Existing Neighbor Discovery Protocols Lacks Robustness
ReCorder Message Structure (RCover Preamble + ReCord two-level Signature) Eliminates Decoding
ReCorder has 10 dB gain in Robustness and Promising Co-Existence with Background Signals
Meng, Tong: On Robust Neighbor Discovery in Mobile Wireless Networks

31. THANK YOU ! Contact: mengtong.sjtu@gmail.com
You’ll Never Walk Alone. – as a KOP