PHY + MAC: The Whole is Greater than the Sum Romit Roy Choudhury Associate Professor 1

Two Research Threads PHY MAC / Link Network Transport Security Application Wireless Networking (bottom up) Wireless Networking (bottom up) Mobile Computing (top down) Mobile Computing (top down)

Our Research PHY MAC / Link Network Transport Security Application Localization CSMA/CN P2P Gaming Activity / Gestures Smart Content Energy Management Wireless Networking (bottom up) Wireless Networking (bottom up) Mobile Computing (top down) Mobile Computing (top down) Rate Control Freq. Backoff Telescope Interference Cancellation

Motivation  Significant leaps in PHY layer link capacity  MIMO, OFDM, Coding, Beamforming …  Significant leaps in PHY layer link capacity  MIMO, OFDM, Coding, Beamforming …  Advancement in distributed algorithms, protocols  Scheduling, coordination, fairness, synchronization…  Advancement in distributed algorithms, protocols  Scheduling, coordination, fairness, synchronization… 4

… Nevertheless 5 PHY MAC

… Nevertheless 6 PHY MAC 1.Lack of integrated experimentation platform  Difficult to build cross-layer research prototypes 2.Protocol designers untrained in communications  Past cross layer research mostly MAC and above 1.Lack of integrated experimentation platform  Difficult to build cross-layer research prototypes 2.Protocol designers untrained in communications  Past cross layer research mostly MAC and above Why ?

Software Radios  Software defined radios  Changing landscape of wireless systems Transparent PHY layer enabling fostering unconventional ideas … Transparent PHY layer enabling fostering unconventional ideas … We intend to contribute here 7

1. Back2F: Backing off in the Frequency Domain 2. AccuRate: Constellation based Rate Selection 3. CSMA/CN: Making Wireless MAC like Ethernet 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 8 Some Ongoing Projects

1. Back2F: Backing off in the Frequency Domain 2. CSMA/CN: Making Wireless MAC like Ethernet 3. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 9 Some Ongoing Projects

 Distributed contention resolution  ALOHA 1972  introduced notion of randomized backoff Backoff 10

 Distributed contention resolution  ALOHA 1972  introduced notion of randomized backoff Backoff AP1 Tx AP2 Waits ACKAP1 Waits AP2 TxACK 11 AP1’s Backoff = 9 AP2’s Backoff = 15 … … Wastage

Fundamentally, backoff is not a time domain operation … its implementation has been in the time domain 12

Fundamentally, backoff is not a time domain operation … its implementation has been in the time domain We intend to break away, and implement backoff on the frequency domain by taking advantage of the PHY layer 13

Frequency Domain  a/g PHY adopts OFDM  Wideband frequency channel divided into 48 narrow sub-carriers  Copes better with fast, frequency selective fading  Purely a PHY layer motivation  MAC Opportunity  Pretend OFDM subcarriers are integers  Emulate randomized backoff Frequency Subcarriers: … 48 14

Back2F: Main Idea  Pick random backoff, say 6  Transmit signal on 6 th subcarrier AP1 Backoff = 6 AP2 Backoff = 18 15

Listen Antenna Listen Antenna 6 18  Pick random backoff, say 6  Transmit signal on 6 th subcarrier Back2F: Main Idea AP1 Backoff = 6 AP2 Backoff = 18 16

Subcarrier Second Round What if Collision?  Introduce a second round of contention  Winners of first go to second Subcarrier First Round Winner 17

Why beneficial? Avg. temporal backoff ~ 100 micro sec. Frequency backoff = 1 OFDM symbol = 4 micro sec 2 rounds of backoff = 8 micro sec. 18

Subcarrier Second Round Creating a Queue Subcarrier First Round Winner Rank 2 19

Subcarrier Second Round Creating a Queue Subcarrier First Round Rank 1Rank 2Rank 3 Enabling TDMA Enabling TDMA 20

Improved Channel Utilization Data WiFi: Contention per packet T2F: OFDM contention per TDMA schedule TDMA 21

1. What happens in multiple collision domains? 2. What if subcarriers are misdetected? 3. What happens when new client join, other clients leave? 4. Can we support legacy APs? … 22 Several Other Challenges

Performance  10 USRP Testbed  Deployed in Duke  Quantify  Reliable subcarrier detection  Collision probability  Net throughput gain over WiFi 23

Subcarrier Detection SNR in dB FFT Number Reliable subcarrier detection at 12dB 24

Throughput Gain Throughput Gain increases with higher bitrates 25

Closing Thoughts  Contention is not fundamentally a time domain operation  Back2F shows feasibility in frequency domain  Long standing overheads of backoff can be alleviated 26

1. Back2F: Backing off in the Frequency Domain 1. CSMA/CN: Making Wireless MAC like Ethernet 2. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 27 Some Ongoing Projects

Collision in Wireless Networks T1RT2 Retransmit time ACK Timeout Collision

Collision in Wireless Networks T1RT2 Collision Retransmit Not Efficient! Better if T1 stops right after collision Better if T1 stops right after collision ACK Timeout

Ethernet is Efficient Called Collision Detection (CSMA/CD) Collision Transmitter detects a collision, and immediately aborts transmission. Unfortunately, CSMA/CD not feasible in wireless networks … 30

We ask: Can we emulate CSMA/CD in wireless networks i.e., abort collisions right when they occur 31

MAC PHY MAC PHY TxRx Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx Check for Collision Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Cross Layer Data Transmission (S1) S=S1 TxRx Check for Collision Search for Abort Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx Search for Abort Cross Layer Check for Collision CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx Collision Detected Search for Abort Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1+S2 TxRx Collision. Send Abort Search for Abort Abort Signal (S2) Cross Layer CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx Abort signal detected Abort Signal (S2) S=S1+S2 Cross Layer Collision. Send Abort CSMA/CN: Basic Idea

MAC PHY MAC PHY Data Transmission (S1) S=S1 TxRx ABORT Abort Signal (S2) S=S1+S2 Cross Layer Collision. Send Abort CSMA/CN: Basic Idea

Correlation is the Key But works when notification is no weaker than 18dB of self-signal Correlation spikes whenever notification arrives

Wired Wireless Interference Cancellation  Need to detect very weak notification signals  Opportunity  Pass the Tx signal over wire  Listen antenna has 2 copies of the Tx signal  Both copies have same filter and frequency offset effects  Align the two signals using sampling offset information  Subtract the wired signal from wireless  Correlate residue with collision notification 42

Collision Detection at Rx  Receiver detects collision within 20 bytes  Total turnaround time for CN signature 18us  Quicker turnaround  Faster Tx abortion  Throughput gain over PPR MAC PHY Median gain = 25% 43

1. Back2F: Backing off in the Frequency Domain 2. CSMA/CN: Making Wireless MAC like Ethernet 3. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 44 Some Ongoing Projects

✦ Recently PHY-based: ✦ SoftRate [SIGCOMM ’09] Uses a BER heuristic to estimate bit rate BER accurately identifies when to increase/decrease rate However, may not be able to jump to optimal rate Current Wireless Rate Selection Data ACK History Info. Data SNR Frame BasedSNR Based SampleRate, RRAARBAR, CHARM We dive deeper into PHY … jump to the optimal rate 45

01 0 Strong Channel Moderate Channel Weak Channel 0111 In General 46

01 0 Strong Channel Moderate Channel Weak Channel Mbps 24 Mbps 36 Mbps Smaller dispersion permits higher rate In General 47

AccuRate Hypothesis: Symbol dispersion is independent of modulation Hypothesis: Symbol dispersion is independent of modulation Observation: Dispersion conveys the optimal rate that should have been used for that packet Observation: Dispersion conveys the optimal rate that should have been used for that packet 48

Hypothesis Verification Tx 4QAM Tx 16QAM Channel Rx QPSK Rx16QAM 49

McKinley et. al., 2004, “EVM calculation for broadband modulated signals” Hypothesis Verification 50

Observe symbol dispersion and select optimal modulation Observe symbol dispersion and select optimal modulation Given that symbol dispersion is independent of modulation Given that symbol dispersion is independent of modulation 51

Data BPSK 4QAM 16QAM 52

Data BPSK 4QAM 16QAM 53

BPSK 4QAM 16QAM Data We call this Virtual Channel Replay 54

Channel Replay Vector d1d1 Vector V = {d 1, d 2,...., d n } d2d2 55

Receiver Demodulator Packet Best Rate BPSK Channel Replay Demodulator CRC Check 4QAM Channel Replay Demodulator CRC Check 16QAM Channel Replay Demodulator CRC Check 56

Optimal modulation ≠ Optimal rate Bit-rate is a function of both modulation and coding Need to find the optimal for a received packet? Need to find the optimal for a received packet? 57

Receiver Demodulator Data BPSK Channel Replay 1/2 Demodulator CRC Check Decoder BPSK Channel Replay 3/4 Demodulator CRC Check Decoder QAM4 Channel Replay 1/2 Demodulator CRC Check Decoder QAM4 Channel Replay 3/4 Demodulator CRC Check Decoder 18 Mbps Decoder 6 Mbps 9 Mbps 12 Mbps QAM64 Channel Replay 3/4 Demodulator CRC Check Decoder Best Rate 54 Mbps 58

AccuRate achieves 87% of the optimal throughput Testbed Throughput at Walking Speeds 59

1. Back2F: Backing off in the Frequency Domain 2. CSMA/CN: Making Wireless MAC like Ethernet 3. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 60 Some Ongoing Projects

Sensor Assisted Wireless Communication Synergy between sensing and wireless - Out-of-band contexts can provide useful cues - Useful for optimizing wireless PHY/MAC (e.g., switch WiFi channel on sensing microwave hum … modify rate control based on accelerometer … turn off WiFi when in subway train …)

1. Back2F: Backing off in the Frequency Domain 2. CSMA/CN: Making Wireless MAC like Ethernet 3. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 62 Some Ongoing Projects

Thank You Visit our Systems Networking Research Group (SyNRG) 63

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1. Back2F: Backing off in the Frequency Domain 2. CSMA/CN: Making Wireless MAC like Ethernet 3. AccuRate: Constellation based Rate Selection 1. SAWC: Sensor Assisted Wireless Communication 2. Uncollide: Is SIC worth it? 3. SleepWell: WiFi Energy Management 65 Some Ongoing Projects

2. Distinct receivers

Case: Two links with distinct receivers Situations much less favorable to SIC

Main Concern: T1 will transmit at best possible bit rate to R1 R2 has to decode T1’s signal at this bit rate … despite the presence of T2’s signal T1 R2 T2 R1 Case: Two links with distinct receivers

Thus, necessary (but not sufficient) conditions: 1.R2’s interferer (T1) must be closer than its own transmitter (T2) 2.T1’s own receiver (R1) must be further than interered receiver (R2) T1 R2 T2 R1 Case: Two links with distinct receivers

Gains available when all conditions hold: T1 R2 T2 R1 How often do these SIC permissible topologies occur?

T1 R2 T2 R1 Enterprise WLANs: Clients likely to associate with stronger AP Such scenarios unlikely Enterprise WLANs: Clients likely to associate with stronger AP Such scenarios unlikely Residential WLANs: Neighbors AP may be stronger Some SIC scenarios possible Residential WLANs: Neighbors AP may be stronger Some SIC scenarios possible

Gain with SIC in less than 10% of the cases Monte Carlo Simulations (AP Transmit Range)

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Collision Probability Small collision probability in dense networks Benefit of second round 74