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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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